Geologic Field-Trip Guide to Lassen Volcanic 

National Park and Vicinity, California

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2015–5067

Mount

Conard

Andesite of

Raker Peak

Hill

8283 Crescent 

Crater

Bumpass

Mountain

Mount

Helen

Andesite flows

from Prospect Peak

Remnant of 

May 19–20,1915 

dacite flow

Painted Dunes

Fantastic Lava Beds

Cinder Cone

Lassen Peak Chaos Crags domes

B D E F

Pumiceous pyroclastic-flow

and fall deposits

from Chaos Crags

Reading

Peak

Hat

Mountain

Fairfield

Peak

Rhyodacite flow

of Kings Creek

Hill

6975
COVER

Photograph looking west-southwest from Sunrise Peak (just east of Butte Lake) towards the Lassen domefield (the 

youngest part of the Lassen Volcanic Center). In the foreground are Cinder Cone (1666 C.E.) and its related flows 

(Painted Dunes and Fantastic Lava Beds). Cinder Cone, Fairfield Peak (82±14 ka), and Hat Mountain (~50–40 ka) are 

units in the younger Twin Lakes sequence of the Lassen domefield. Hill 6975 is the vent for the andesite and basaltic 

andesite of Cluster Lakes (300–250 ka). Both it and the andesite of Raker Peak (270±18 ka) are units in the older Twin 

Lakes sequence. Lassen Peak (27±1 ka) and Chaos Crags (1,103±13 B.P.) are domes of the Eagle Peak sequence of the 

Lassen domefield. Reading Peak (212±5 ka), Bumpass Mountain (232±8 ka), Mount Helen (249±12 ka), Crescent Crater 

(236±1 ka), and Hill 8283 (261±5 ka) are domes of the Bumpass sequence. At the far left, Mount Conard is an erosional 

remnant of Brokeoff Volcano (590–385 ka), an older part of the Lassen Volcanic Center. Flows from Prospect Peak 

(247±56 ka) are not part of the Lassen Volcanic Center, but instead are part of the regional calc-alkaline suite. 
Geologic Field-Trip Guide to Lassen 

Volcanic National Park and Vicinity, 

California

By L.J. Patrick Muffler and Michael A. Clynne

Scientific Investigations Report 2015–5067

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015

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Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

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Suggested citation:

Muffler, L.J.P., and Clynne, M.A., 2015, Geologic field-trip guide to Lassen Volcanic National Park and vicinity, 

California: U.S. Geological Survey Scientific Investigations Report 2015–5067, 67 p., http://dx.doi.org/10.3133/

sir20155067.

ISSN 2328-031X (print) 

ISSN 2328-0328 (online)

ISBN 978-1-4113-3926-2

http://dx.doi.org/10.3133/sir20156067
iii

Acknowledgments

Our work in the Lassen region builds on that of previous geologists and would have been 

considerably more difficult without the context they provided. The mapping and publications 

of Howel Williams were, by far, the most insightful in this regard. His “Geology of the Lassen 

Volcanic National Park, California” (Williams, 1932) stands as a classic work in interpretation 

of the geologic history of the Lassen area and the Cascade Range. 

A project of the scope and size of our Lassen investigations could not possibly be 

completed without the assistance and consultation of colleagues. We particularly acknowledge 

the significant collaborations of Robert Christiansen and Marianne Guffanti. We were ably 

assisted over the course of geologic mapping by a number of geologists, primarily Lars 

Borg, Marianne Guffanti, Gordon Keating (deceased), Deborah Trimble, Jacob Madden, 

Charles Melancon, and Miranda Fram. We are also grateful to Tracey Felger, Joel Robinson, 

David Ramsey, Dillon Dutton, Ellen Lougee, and Peggy Bruggman for providing assistance, 

instruction, and collaboration with GIS compilation.

Geochronologic data are essential to the satisfactory completion of any modern geologic 

map in volcanic terrain. The volcanic stratigraphy presented herein reflects the major 

contributions of Brent Dalrymple, Marvin Lanphere, Brent Turrin, Deborah Trimble, Andrew 

Calvert, and Paul Renne, each of whom contributed age dates on a variety of materials. 

Potassium-argon (K-Ar) and argon-argon (40Ar/39Ar) ages given in the text and figure captions 

of this guide can be found in Clynne and Muffler (2010) unless otherwise noted. Paleomagnetic 

data and correlations contributed by Duane Champion played a significant role in refining 

volcanic stratigraphy in some areas. 

Reviews of the manuscript by Russell Evarts, Richard Conrey, and Stephen Self improved 

both the content and presentation of this guide. The guide also benefited greatly from the 

careful and constructive editing provided by Peter Stauffer and Claire Landowski.
iv

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Geological Overview .....................................................................................................................................3

Tectonic Setting ....................................................................................................................................3

Regional Volcanism..............................................................................................................................3

Calc-Alkaline Volcanoes.............................................................................................................4

Low-Potassium Olivine Tholeiite...............................................................................................6

Parental Magmas and Evolution of the Regional Suite ........................................................6

Volcanic Centers...................................................................................................................................8

Eruptive History of the Lassen Area ................................................................................................10

Regional Volcanic Rocks ..........................................................................................................10

Yana and Latour Volcanic Centers and the Tuscan Formation ..........................................10

Dittmar Volcanic Center............................................................................................................11

Maidu Volcanic Center .............................................................................................................11

Lassen Volcanic Center............................................................................................................12

Rockland Caldera Complex .............................................................................................12

Brokeoff Volcano ..............................................................................................................12

Lassen Domefield .............................................................................................................14

Holocene Volcanic Activity .............................................................................................15

Caribou Volcanic Field ..............................................................................................................16

Glacial Geology ...................................................................................................................................16

Hydrothermal Features ......................................................................................................................16

Volcano-Related Hazards..................................................................................................................16

Road Log........................................................................................................................................................20

Section A: Red Bluff to Intersection of Calif. Hwys 36 and 89.....................................................20

Section B: Chester to Intersection of Calif. Hwys 36 and 89 .......................................................24

Section C: Chico to Intersection of Calif. Hwy 32 with Calif. Hwys 36 and 89...........................26

Section D: Intersection of Calif. Hwys 36 and 89 North Through Lassen Volcanic 

National Park to the Northwest Park Entrance................................................................29

Section E: Intersection of Calif. Hwys 44 and 89 North to Hat Creek Rim.................................45

Section F: Hat Creek Rim Overlook to Westwood and Chester...................................................47

Section G: Alternate Route from Hat Creek Rim Overlook to Cinder Cone and 

Poison Lake Chain.................................................................................................................51

Section H: Intersection of Calif. Hwys 44 and 89 at the Northwest Entrance to 

Lassen Volcanic National Park West to Anderson..........................................................54

Section I: Old Station to Burney .......................................................................................................57

References Cited..........................................................................................................................................62
v

Figures

 1. Index map of Lassen Volcanic National Park and surrounding region.......................................2

 2. Map showing regional tectonic setting of area from Sacramento Valley to CaliforniaNevada border, including Lassen Volcanic National Park and vicinity, California............4

 3. Map of Lassen Volcanic National Park and vicinity emphasizing calc-alkaline and 

tholeiitic volcanic rocks that surround the Lassen Volcanic Center...................................5

 4. Graph of K2Ocontent against SiO2 content of regional volcanic rocks in the Lassen area.....7

 5. Multielement spider diagram showing concentrations of trace elements in primitive 

basalts of the Lassen region.......................................................................................................7

 6. Diagrammatic cross-section showing magmagenesis model for the Lassen region ......................8

 7. Map and chronostratigraphic chart of volcanic centers in the Lassen region.........................9

 8. Generalized geologic map and chronostratigraphic chart of Lassen Volcanic National 

Park and vicinity, California, emphasizing the location, extent, and stratigraphy 

of the Lassen Volcanic Center..................................................................................................13

 9. Graph of SiO2 content versus K2O content for 250 analyses of volcanic rocks from 

the Lassen Volcanic Center grouped by eruptive sequence...............................................14

10. Generalized geologic map of eastern part of Lassen Volcanic National Park and 

adjacent areas, emphasizing the location and extent of volcanic sequences 

and chains in the Caribou Volcanic Field .....................................................................................17

11. Chart showing correlation of glacial terminology, ages, and depiction of glacial 

deposits in the Lassen area by various authors .................................................................18

12. Diagrammatic cross sections of the Lassen hydrothermal system...........................................19

13. Map showing thermal areas in and adjacent to Lassen Volcanic National Park ...................20

14. Photograph looking along Calif. Hwy 36 between Red Bluff and Dales, showing 

features along the Battle Creek Fault scarp and the Cascade Range north of the 

Lassen Volcanic Center..........................................................................................................21

15. Photograph looking along Calif. Hwy 36 between Red Bluff and Dales showing 

volcanic features of the southern Cascade Range...............................................................22

16. Panoramic photograph of volcanic features of the Lassen region on Calif. Hwy 36 

northeast of the turnoff to Paynes Creek...................................................................................... 23

17. Panoramic photograph looking west down Deer Creek...........................................................27

18. Panoramic photograph looking northeast up Deer Creek........................................................28

19. Photograph looking west at andesite flow exposed in the face of Bluff Falls quarry ............. 29

20. Photograph showing landslides in hydrothermally altered volcanic rocks .............................30

21. Annotated photograph showing changes of hydrothermal features at Sulphur Works........31

22. Photograph taken in November 2011 of mudpot north of Calif. Hwy 89 at Sulphur Works....31

23. Photograph of Brokeoff Mountain looking west from turnout on Calif. Hwy 89 ......................32

24. Photograph of the May 22, 1915, crater at the summit of Lassen Peak ....................................35

25. Photograph of the northeast flank of Lassen Peak taken in June 1913, before the 

1914–1917 eruption .....................................................................................................................36
vi

26. Photograph of the Devastated Area taken on the morning of May 22, 1915, before 

the Plinian eruption of ~4:00 pm on May 22, 1915 .................................................................37

27. Photograph showing the Devastated Area after the eruption of May 22, 1915............................37

28. Chart giving a summary of events in the 1914–1917 eruptions of Lassen Peak.......................38

29. Generalized geologic map of deposits from the May 1915 eruptions at the summit 

of Lassen Peak ............................................................................................................................39

30. Low-angle oblique aerial photograph looking southwest at Lassen Peak and the 

proximal Devastated Area...............................................................................................................40

31. Telephoto view northeast from summit of Brokeoff Mountain looking at Lassen Peak 

 and the west lobe of the dacite flow of May 19–20, 1915 ....................................................41

32. Photographs of hand specimens of four rock types of very different appearance 

that were produced over a week-long period during the May 1915 eruption

from the summit of Lassen Peak ..............................................................................................42

33. Chart giving a summary of events in the formation of Chaos Crags..........................................42

34. Photograph of Chaos Crags and Chaos Jumbles looking southeast from Calif. Hwy 89 .......44

35. Graph of SiO2 content versus K2O content for the quenched mafic inclusions and 

host rocks in Chaos Crags.........................................................................................................44

36. Photograph of quenched mafic inclusion in dacite/rhyodacite of Chaos Crags.....................45

37. Panoramic photograph looking southwest to northwest from Parhams Point across 

the Hat Creek Graben and the Hat Creek Basalt towards the crest of the Cascade 

Range north of Lassen Volcanic National Park.....................................................................47

38. Photograph looking southwest across Silver Lake, illustrating the glaciated 

physiography of the southern part of the Caribou Volcanic Field ......................................50

39. Graph showing variation of SiO2 versus height in a composite ash section at 

Cinder Cone .................................................................................................................................53

40. Harker diagram showing variation of TiO2 versus SiO2 for the array of compositions 

at Cinder Cone.............................................................................................................................54

41. Photograph of roadcut exposing complex volcanic stratigraphy west of 

Manzanita Lake...........................................................................................................................55

42. Generalized geologic map of the Hat Creek Basalt......................................................................58

43. Oblique satellite image of the northern part of the Hat Creek Fault ..........................................59

44. Annotated photograph looking south along the Active Scarp of the Hat Creek Fault............60

45. Photograph looking north across Calif. Hwy 299 northeast of Johnson Park..........................61
Geologic Field-Trip Guide to Lassen Volcanic National 

Park and Vicinity, California

By L.J. Patrick Muffler and Michael A. Clynne

Abstract

This field-trip guide provides an overview of Quaternary 

volcanism in and around Lassen Volcanic National Park in northern 

California. The guide begins with a comprehensive overview 

of the geologic framework and the stratigraphic terminology 

of the Lassen region, based primarily on the “Geologic map of 

Lassen Volcanic National Park and vicinity” (Clynne and Muffler, 

2010). The geologic overview is then followed by detailed road 

logs describing the volcanic features that can readily be seen in 

the park and its periphery. Twenty-one designated stops provide 

detailed explanations of important volcanic features. The guide 

also includes mileage logs along the highways leading into the 

park from the major nearby communities. The field-trip guide is 

intended to be a flexible document that can be adapted to the needs 

of a visitor approaching the park from any direction. 

Introduction

This field-trip guide provides an overview of Quaternary 

volcanism in and around Lassen Volcanic National Park, California, 

emphasizing the stratigraphy of the Lassen Volcanic Center. 

Additional stops focus on the 1915 eruption at Lassen Peak, the 

Lassen geothermal system, and regional tectonism and volcanism. 

This guide builds on two previous guides: one written for the 1989 

meeting of the International Geological Congress in San Francisco, 

California, and one written for the International Association of 

Volcanology and Chemistry of the Earth’s Interior in Santa Fe, 

New Mexico (Clynne and Muffler, 1989). A revision of the latter 

guide was prepared in 2000 for the Workshop on Volcanism in 

National Parks held by the U.S. Geological Survey (USGS) and 

the National Park Service in Redding, California, and in Lassen 

Volcanic National Park (Clynne and others, 2000c). The present 

guide expands and updates the previous guides, drawing heavily 

on the introductory text for the “Geologic map of Lassen Volcanic 

National Park and vicinity” (Clynne and Muffler, 2010). The trip 

is designed to be self-guided and focuses on geologic features and 

stratigraphy that can be seen easily from the road network through 

and surrounding Lassen Volcanic National Park. Discussions of 

geochemistry and petrology are limited to generalities; users who 

desire more comprehensive information on these subjects should 

consult the works in the reference list provided. 

The field-trip guide is designed to be supplemented by four 

USGS Fact Sheets that present more detailed information about 

aspects of young volcanism and active hydrothermal features of 

Lassen Volcanic National Park (LVNP): 

1. Fact Sheet 022–00: “Volcano hazards of the Lassen Volcanic National Park area, California” (Clynne and others, 

2000b; http://pubs.usgs.gov/fs/2000/fs022-00/)

2. Fact Sheet 2014–3119: “A sight ‘fearfully grand’—Eruptions of Lassen Peak, California, 1914 to 1917” (Clynne 

and others, 2014; http://pubs.usgs.gov/fs/2014/3119/) 

3. Fact Sheet 023–00: “How old is ‘Cinder Cone’?—Solving 

a mystery in Lassen Volcanic National Park, California” 

(Clynne and others, 2000a; http://pubs.usgs.gov/fs/2000/

fs023-00/).

4. Fact Sheet 101–02: “Hot water in Lassen Volcanic 

National Park—Fumaroles, steaming ground and boiling 

mudpots” (Clynne and others, 2003; http://pubs.usgs.gov/

fs/2002/fs101-02/). 

Another useful supplement to this field-trip guidebook is 

the set of 48 annotated photographs in the CD-ROM (Muffler 

and others, 2010) accompanying the geologic map of Clynne 

and Muffler (2010). To access the photos from the CD-ROM, 

click on “site” and then the photos folder. The annotated photos 

also are at http://pubs.usgs.gov/sim/2899/database.html (click on 

“photographs”). 

The core of the road log (Section D) traverses LVNP 

northward on Calif. Hwy 89 to its intersection with Calif. Hwy 

44 near Manzanita Lake (fig. 1). It can be completed in one day. 

Three introductory sections guide one to the beginning of Section 

D from different starting points: Section A from Red Bluff,

Section B from Chester, and Section C from Chico. Section E 

guides one from the end of Section D north to the Hat Creek Rim.

Section F guides one from the Hat Creek Rim clockwise around 

the Park to Westwood and Chester. Section G is an alternate loop 

on unpaved roads from the Hat Creek Rim to the trail that leads 

to Cinder Cone. Section H guides one from the end of Section 

D west to Shingletown and Anderson. Section I guides one from 

Old Station north to Burney. To follow all these sections would 

take three days, particularly if one walks to Bumpass Hell, Lassen 

Peak, and Cinder Cone. The road logs are thus designed to allow 

the visitor to pick and choose according to his or her interests, 

direction of approach to LVNP, or available time. 
2 Geologic Field-Trip Guide to Lassen Volcanic National Park and Vicinity, California

Lassen Volcanic National Park comprises 430 km2 of 

scenic volcanic features, glacially sculpted terrain, and the 

most spectacular array of thermal features in the Cascade 

Range. Interest in preserving the scenic wonders of the Lassen 

area arose in the early 1900s and led to the establishment in 

1907 of two small National Monuments centered on Lassen 

Peak and Cinder Cone (Strong, 1989). The eruptions of Lassen 

Peak in 1914–15 were the first in the Cascade Range since 

widespread European settling of the American West in the 

late 1800s. Through the print media, the eruptions aroused 

considerable public interest and inspired renewed efforts to 

establish a National Park. In 1916, Lassen Volcanic National 

Park was established by combining the previously established 

National Monuments with adjacent Federal lands. 

The southernmost Cascade Range is bounded on the west 

by the Sacramento Valley and the Klamath Mountains, on the 

south by the Sierra Nevada, and on the east by the Basin and 

Range. The Lassen region, which comprises much of the Lake 

Almanor and Burney 1:100,000 topographic maps, is underlain 

by volcanic rocks ranging in age from 3.5million years (Ma) 

to 100 years. A profound unconformity separates these late 

Cenozoic volcanic rocks from underlying sedimentary rocks of 

Cretaceous age and metamorphic and igneous rocks of Paleozoic 

and Mesozoic age. These much older rocks are exposed only 

along Section C of the field-trip guide, but they crop out in the 

mountains south and southeast of Lake Almanor, in the canyons 

of Mill Creek and Deer Creek ~20 miles south-southwest 

of LVNP, and in the area of Montgomery Creek ~16 miles 

northwest of LVNP.

Diller (1895) recognized the young volcanic geology of 

the region and made the first geologic map of the Lassen area. 

A 1:48,000-scale map of LVNP produced in one remarkable 

field season by Williams (1932) was extended by later geologic 

mapping, notably by Wilson (1961) and by Macdonald (1963, 

1964, 1965). Investigations by the USGS over the past 38 years 

have led to a detailed 1:24,000 “Geologic map of Lassen Peak 

and Chaos Crags” (Christiansen and others, 2002) and a 1:50,000 

“Geologic map of Lassen Volcanic National Park and vicinity” 

(Clynne and Muffler, 2010). The latter map is accompanied by 

a CD-ROM (Muffler and others, 2010) that contains ArcGIScompatible data files used to create the 1:50,000-scale geologic 

map, both geologic and topographic data and their associated 

Figure 1. Index map of Lassen Volcanic National Park and surrounding region. Field-trip routes are shown in heavy colored lines. 

Stops are shown by colored circles. Letters in boxes refer to sections of the road log (see text). Stop C-1 is ~10 miles south of map 

boundary, 7.6 miles north of Chico.

CARIBOU 

WILDERNESS

CALIFORNIA

NEVADA

Hat Creek

LASSEN COUNTY

PLUMAS COUNTY

SHASTA COUNTY

TEHAMA COUNTY

Cascade Range

S i e r r a N e v a d a

BUTTE

COUNTY from Chico

A

B

C

E G F

H

D

I-1

I-2

I

A-1 B-1

H-1

C-2

C-3

D-2

D-1

D-3

D-4

D-5 D-6

E-2

G-1

G-2

G-3

E-1

F-1

F-2

139

273

299

299

32

36

36

A21

36

44

44

70

70

89

89

89

99

395

395

17

LASSEN

VOLCANIC 

NATIONAL 

 PARK

5

5

5

Old Station

Los Molinos

Herlong

Chester

Greenville

Shingletown

Manton

Viola

Mineral

Fall River

Mills

Cassel

Burney

Corning Quincy

Westwood

Anderson

Red Bluff

Susanville

Redding

0 10

0 10 20 30 MILES

20 30 KILOMETERS

122° 121° 120°

41°

40°

Butte Creek

Pine Creek

Susan River

Sacramento River

Pit River

Pit River

Middle Fork

Battle Creek

Butte Creek

Mill Creek

Deer Creek

North Fork

Feather River

Honey Lake

Eagle 

Lake

Lake

Almanor

Shasta

Lake
Geological Overview 3

metadata files, printable versions of the geologic map and 

pamphlet as PDF formatted files, and 1:24,000-scale geologic 

maps of individual quadrangles used to compile the 1:50,000 

map. It also contains ancillary data that support the map, 

including locations of rock samples selected for chemical 

analysis and radiometric dating, links to related data or Web 

sites, and 48 annotated photographs of geologic features. Data 

contained in the CD-ROM are also available at http://pubs.usgs.

gov/sim/2899/. Chemical analyses and locations of samples are 

given in Clynne and others (2008). 

A generalized geologic map at 1:100,000 scale of the Lake 

Almanor sheet, which includes the southern two-thirds of the 

Lassen 1:50,000 map, is in preparation by Clynne and Muffler 

and serves as the source of many of the geological descriptions 

outside the boundaries of the 1:50,000-scale “Geologic map 

of Lassen Volcanic National Park and vicinity” (Clynne and 

Muffler, 2010, http://pubs.usgs.gov/sim/2899/). 

A thoughtful discussion and overview of the entire Cascade 

magmatic arc was presented by Hildreth (2007).

Geological Overview

Tectonic Setting

Figure 2 (from Clynne and Muffler, 2010) illustrates the 

tectonic setting of the Lassen segment (Guffanti and Weaver, 

1988) of the Cascade volcanic arc. Volcanism in the Lassen 

segment is a result of oblique subduction of the oceanic Juan 

de Fuca Plate eastward beneath the continental North American 

Plate. From 12 million years ago (12 Ma) to the present, 

the axis of the Lassen segment migrated westward from the 

California-Nevada border to its present position (Guffanti and 

others, 1990). Simultaneously, the width of the arc narrowed, 

probably because of slab steepening. The southern terminus of 

active volcanism retreated roughly parallel to the northward 

extension of the San Andreas Fault System and migration of 

the Mendocino Triple Junction off the northern California 

coast. At 12 Ma, the southern limit of Cascade volcanism was 

in the Sierran block south of Lake Tahoe, 110 miles (180 km) 

southeast of the Lassen area (Busby and others, 2008; Busby, 

2013). At 3 Ma, the southern limit of active volcanism was in 

the area of the Yana Volcanic Center, 18 miles (30 km) south of 

Lassen Peak. At present, the southern limit of active volcanism 

approximately corresponds to the south boundary of LVNP, 

and the youngest dated regional volcano in the area is Sifford 

Mountain (fig. 3) at about 170 ka. These relations suggest a 

long-term northerly migration rate for the southern terminus of 

Cascade Arc volcanism of 15 to 20 km per million years (m.y.; 

1.5 to 2 cm per yr) and a rate for at least the past 3 m.y. of 

about 10 km per m.y. (1 cm per yr). Detailed calculations of the 

extrusion rate for the Lassen segment of the arc have not been 

made, but the available data (Sherrod and Smith, 1990) suggest 

that the extrusion rate was significantly higher at 3–2 Ma than 

from 2 Ma to the present. 

Two independent tectonic regimes affect the Cascade 

volcanic arc in the Lassen area: the Basin and Range 

Province and the Walker Lane. The extensional Basin and 

Range Province is expanding westward into the Cascade Arc 

(Guffanti and others, 1990), and the active Hat Creek Graben 

and Lake Almanor Graben are the westernmost major Basin 

and Range structures in the area (fig. 2). This impingement 

is manifested in numerous normal faults, many of which 

provide pathways for magma to reach the surface. Some 

magmas traverse the crust rapidly, and relatively primitive 

mafic lavas are widespread, albeit not abundant (Borg 

and others, 1997). Many regional volcanoes are aligned 

along fault traces and partially to completely bury them. 

The Lassen area also sits on the northwest projection of 

the Walker Lane, a broad linear zone of distributed strikeslip faulting, parallel to and inboard of the San Andreas 

Fault System, that accommodates about 20 percent of the 

relative motion between the North American and Pacific 

Plates (Unruh, 1995; Blakely and others, 1997). This regime 

generally manifests itself as a small strike-slip component 

on the dominantly normal faults. Although the Hat Creek 

Graben–Lake Almanor Graben structure crosses the eastern 

half of LVNP (Clynne and Muffler, 2010), few faults are 

mapped there. Expression of the grabens is obscured by 

a combination of young volcanism and glaciation, and 

additional faults are likely to be concealed in the subsurface. 

Regional Volcanism

On a regional scale, Quaternary volcanism in the 

southernmost Cascade Range has built a broad platform of 

overlapping and intercalated mafic to intermediate volcanoes. 

This platform comprises hundreds of coalescing small- to 

medium-size volcanoes (as large as a few km3) consisting 

predominantly of basalt, basaltic andesite, and andesite. We 

refer to these as regional volcanoes and (or) regional lavas. 

A seismic-refraction experiment (Berge and Stauber, 1987) 

indicates that the young volcanic platform is ~4 km thick in 

the active southernmost Cascade Arc. 

Two distinct parental magmas and their derivatives 

contribute to the suite of regional lavas in the Lassen area: 

calc-alkaline basalt and low-potassium olivine tholeiite 

basalt (fig. 3; Borg, 1995; Borg and others, 1997; Clynne 

and Borg, 1997; Borg and others, 2002). Edifices containing 

both magma types are rare. Calc-alkaline compositions are 

related to Cascade Arc magmatism and dominate the regional 

suite in both volume and abundance. Small calc-alkaline 

volcanoes are scoria cones with fields of lava flows, whereas 

large calc-alkaline volcanoes are steep-sided cones or shields 

with gentle slopes. Low-potassium olivine tholeiite (LKOT; 

sometimes termed high-alumina olivine tholeiite or HAOT) 

basalt is a volumetrically minor but widespread component 

of the regional volcanism and is related to Basin and Range 

extension. LKOT basalt erupts from fissures between the 

shield volcanoes, and the fluid lavas flood intervening 

valleys. 
4 Geologic Field-Trip Guide to Lassen Volcanic National Park and Vicinity, California

Calc-Alkaline Volcanoes

Regional calc-alkaline volcanoes have a wide range of 

sizes and forms. They comprise a continuum from scoria 

cones (commonly and locally called cinder cones) through 

steep-sided lava cones to broad shield volcanoes. Typically, 

lava flows of regional calc-alkaline volcanoes have blocky 

vesicular lava surfaces, although ‘a‘ā surfaces are also 

observed. Interiors have joint patterns that are thickly 

slabby to thinly platy. Aphanitic textures are typical, and 

90 km

130 km

Honey Lake

Eagle Lake

41°

0 10 20 30 MILES

0 10 20 30 KIL0METERS

122° 121° 120°

40°

0 Ma

3 Ma

D

Y

M

Ln

C

Lt

CALIFORNIA

NEVADA

10–12 Ma

Cascade Arc at 0 Ma

3 Ma

Lake

Almanor

BASIN AND RANGE

SIERRA

NEVADA

KLAMATH

MOUNTAINS

Hat Creek Graben

Walker Lane

Lake Almanor Graben

SACRAMENTO VALLEY

LASSEN

VOLCANIC

NATIONAL

PARK Outline of

figure 7

Outline of

figure 3

holocrystalline textures are generally confined to thick or 

ponded flows. 

The smallest and most abundant volcanoes are scoria 

cones, typically 50–200 m in height. Scoria cones are 

composed of bedded scoria and bombs piled up around 

a vent. Typically they have an associated lava flow that 

erupted from the base of the cone. Scoria cones have a single 

period of activity (that is, they are monogenetic). In this 

region they typically erupt small-volume, blocky to ‘a‘ā lava 

flows. Scoria cones are usually homogeneous in chemical 

Figure 2. Map showing regional tectonic setting of area from Sacramento Valley to California-Nevada border, including Lassen Volcanic 

National Park and vicinity, California. Light-yellow background indicates late Tertiary to Holocene volcanic rocks of the Cascade Arc and 

the Basin and Range. Green background indicates Paleozoic to Mesozoic metamorphic-plutonic basement of the Sierra Nevada and 

Klamath Provinces (small outliers of volcanic rocks not shown). Orange dots indicate the location of volcanic vents younger than about 7 

Ma (older vents not shown). The large majority of vents are regional calc-alkaline volcanoes. Thin violet lines show the location of faults. 

Dashed red lines show the approximate position of the arc axis at the times indicated, and dashed black lines indicate the approximate 

southern terminus of arc volcanism at the times indicated. At ~12 Ma, the axis of the Lassen segment of the arc was near the CaliforniaNevada border. Since then, the arc axis has migrated westward to its present position. Simultaneously, the width of the arc narrowed 

(Guffanti and others, 1990). Thick blue lines indicate the estimated present-day depth to the subducted Gorda Plate. C, Caribou Volcanic 

Field; D, Dittmar Volcanic Center; Ln, Lassen Volcanic Center; Lt, Latour Volcanic Center; M, Maidu Volcanic Center; Y, Yana Volcanic 

Center. Map taken from figure 5 of Clynne and Muffler (2010), as updated from Guffanti and others (1990).
Geological Overview 5

3.5

0.5

1.0

1.5

2.0

2.5

3.0

0

Regional calc-alkaline

volcanic rocks

Age (Ma)

Red Lake

Mountain

Bogard

Buttes

West

Prospect 

Peak

Table

Mountain

Hootman

Ranch

Badger

Mountain Prospect 

Peak

Sifford 

Mountain

Mount

Harkness

Cal

Mountain

Poison Lake chain

Mud Creek Rim

Hat Creek Rim

C a r i b o u c h a i n

Red Cinder

chain

Bidwell 

Spring

chain

Sugarloaf

chain

Tumble 

 Buttes

 chain

Grays

Peak

Huckleberry

Lake

Latour

Volcanic

Center

Dittmar

Volcanic

Center

Lassen Volcanic Center

Maidu

Volcanic

Center

0 5 10

0 5 10

KILOMETERS

MILES

LASSEN VOLCANIC NATIONAL PARK

121°37.5' 121°7.5'

40°40'

40°20'

40°30'

Tuscan

Formation

Glacial and 

sedimentary

deposits

Regional 

low-K

olivine 

tholeiites

e

d

c

b

a

Latour

Volcanic

Center

Dittmar

Volcanic

Center

Maidu

Volcanic

Center

Lassen

Volcanic

Center

Figure 3. Map of Lassen 

Volcanic National Park and vicinity 

emphasizing calc-alkaline and 

tholeiitic volcanic rocks that 

surround the Lassen Volcanic 

Center. Regional calc-alkaline 

rocks are divided into five age 

groups (from youngest to oldest, 

a–e) with age boundaries at 100, 

450, 788, and 1,810 ka. Prominent 

calc-alkaline edifices and chains 

are labeled. Light blue areas are 

lakes. Map adapted from figure 4B

of Clynne and Muffler (2010).
6 Geologic Field-Trip Guide to Lassen Volcanic National Park and Vicinity, California

composition and mineralogy, and they tend to occur in 

groups of similar age and lithology. 

Lava cones and shield volcanoes are bigger (typically 

300–800 m in height) than scoria cones and are more or less 

circular in plan view. Lava cones and shield volcanoes are 

constructed by eruptions from a central vent or flank vents 

high on the edifice that together build a steep-sided lava 

cone or a flatter shield volcano. These are commonly capped 

by a scoria cone, but they generally lack widely distributed 

fragmental deposits. They have a single period of activity 

(monogenetic) or a longer period of intermittent activity as 

long as a few tens of millennia (polygenetic), and they build a 

variety of edifice sizes as much as a few km3 in volume. Lavas 

from the longer-lived, larger regional volcanoes (shields or 

lava cones) have a single chemical composition or display 

small compositional ranges throughout the lifetime of the 

edifice. If compositionally heterogeneous, they typically 

evolve with time to higher silica and crystal contents with 

accompanying changes in phenocryst assemblage. 

The calc-alkaline regional suite is a continuum of olivine 

basalt, olivine-augite basaltic andesite, and two-pyroxene 

andesite; rocks more silicic than andesite are rare (fig. 4). 

Phenocryst assemblages are relatively simple. Primitive basalts 

contain olivine, commonly without a second phase. Evolved 

basalts add augite and plagioclase to the assemblage, and most 

basaltic andesites contain a combination of olivine, augite, and 

plagioclase phenocrysts. Complex zoning patterns, multiple 

populations of phenocrysts, and resorption or reaction features 

in phenocrysts are generally absent in mafic calc-alkaline 

rocks, but they are present in some andesites. Hypersthene 

replaces olivine in the fractionating assemblage in the range 

56–58 percent SiO2, and most andesites contain both augite and 

hypersthene. Glomeroporphyritic clots of crystals, generally of 

the same phases as are present as phenocrysts in the rock, are 

common in evolved rocks of the calc-alkaline suite. 

Low-Potassium Olivine Tholeiite

LKOT lava flows form widespread sheets that have 

a characteristic surface morphology. The upper and lower 

surfaces of flow units contain abundant spherical vesicles, and 

flow tops are often ropy (pāhoehoe) and have a characteristic 

joint pattern of crudely hexagonal blocks. Interiors of flow units 

are generally holocrystalline and often diktytaxitic; the flows are 

massive with a pattern of widely spaced blocky joints. Upper 

flow surfaces have little relief except for common to abundant 

tumuli. These characteristics are rapidly obscured by weathering 

and soil-forming processes, so that, within about 400–500k.y., 

LKOT flows in the Lassen area become flat surfaces with 

thick soils. Despite being volumetrically subordinate overall to 

calc-alkaline volcanic products, volumes of individual LKOT 

flows are relatively large; volumes of as much as a few km3 

are typical. LKOT flows were erupted from fissures that built 

spatter cones and ramparts; scoria cones are rare. These vents 

are inconspicuous in the geologic record because they are 

composed of glassy scoria that is easily weathered and eroded, 

and the low-relief vent profiles are often buried by younger 

volcanic rocks. Vents for LKOT flows older than a few hundred 

thousand years are difficult to identify. 

The LKOT suite displays remarkable chemical and petrographic homogeneity—almost all analyses contain 48–50

percent SiO2 (fig. 4). LKOT typically contains sparse olivine 

phenocrysts, accompanied by plagioclase in slightly more 

evolved examples. Augite phenocrysts and multiple phenocryst 

populations are rare, as are phenocrysts with strong zoning or 

resorption textures. Glomeroporphyritic clots of crystals like 

those found in calc-alkaline rocks are uncommon and generally 

are confined to near-vent parts of lava flows. Borg and others 

(2002) summarized the lithologic, petrographic, and distinctive 

geochemical characteristics of LKOT. 

Parental Magmas and Evolution of the Regional 

Suite

A diverse array of mafic compositions is found in the 

Lassen region. These have been extensively studied and 

modeled, and only a brief summary of magma compositions 

and sources is given here. For additional discussion see the 

references cited in Clynne and Muffler (2010).

The most striking feature of LKOT from the Lassen area is 

the limited range in major- and trace-element contents displayed 

by these lavas (Clynne, 1993; Bacon and others, 1997; Clynne 

and others, 2008). Distinctive features of LKOT from the Lassen 

region are low SiO2 (~48–50 weight percent), low alkali content, 

especially K2O (~0.1–0.4 weight percent), and low volatile 

content (Sisson and Layne, 1993). In general, their trace-element 

characteristics are like those of the mid-ocean ridge basalts that 

are enriched in light rare-earth elements (E-MORB) (fig. 5). Sr 

and Nd and other isotopic systematics of LKOT are distinct from 

those of calc-alkaline basalts (Bacon and others, 1997; Borg and 

others, 1997; Borg and others, 2000; Borg and others, 2002). 

In contrast to LKOT, the calc-alkaline suite forms a 

compositional (fig. 4) and isotopic continuum. The calc-alkaline 

lava suite has geochemical characteristics typical of arc magmas 

erupted through continental crust, for example, in the Japan 

and Kamchatka Arcs (Gill, 1981). Geochemical diversity is 

greatest in the most primitive compositions, and fractionation 

reduces the range of geochemical diversity in more evolved 

rocks through interaction with the lower crust and mixing 

within the suite (Clynne, 1990; Bullen and Clynne, 1990; Hart 

and others, 2002). Evolution of mafic magmas is dominated by 

crystal fractionation, but addition of small-degree partial melts 

of mafic lower crust is also an important process (Clynne, 1993). 

Assimilation of upper crust also plays a role in determining the 

geochemical composition of some andesitic regional volcanoes. 

For the volcanic centers in the Lassen region, calc-alkaline basalt 

is the parental magma. 

The Lassen region contains primitive (unevolved) calc-alkaline basalts and magnesian andesites that display a wide range 

of compositions with systematic variation across the arc. At one 

end of the spectrum are low-K (0.3–0.8 weight percent) basalts 

with low contents of incompatible elements and a strong arc 
Geological Overview 7

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76

K2O, in weight percent

SiO2, in weight percent

Basaltic

andesite

Basalt Andesite Dacite Rhyodacite Rhyolite

LKOT suite

Calc-alkaline suite

EXPLANATION

1 

10

5 

50

2 

20

100

Sr Cs Rb Ba Th U Nb Ta K La Pb P Sm Hf Ti Tb 

EXPLANATION

Y Ce Nd Tm Lu Zr Eu Gd Dy Er Yb 

LKOT

low-K calc-alkaline basalt

high-K calc-alkaline basalt

Concentration, normalized to primitive mantle values

Figure 4. Graph of K2Ocontent 

against SiO2 content of regional 

volcanic rocks in the Lassen 

area, showing ~600 analyses of 

calc-alkaline rocks and lowpotassium olivine tholeiite (LKOT) 

basalts less than about 3 million 

years old. The calc-alkaline 

rocks are dominantly mediumK2

O as defined by Gill (1981), 

and most are basalt to andesite; 

rocks with more than about 

63 percent K2O are sparse in the 

regional suite. The LKOT suite 

is chemically homogeneous at 

48–50 percent SiO2.

Figure 5. Multielement spider diagram showing concentrations of trace elements in primitive basalts of the Lassen region. Diagram 

shows the abundances of trace elements in the average compositions that produce the array of primitive basalts in the Lassen region, 

normalized to primitive mantle values (vertical axis) of Sun and McDonough (1989). Magnesian andesites (not shown) are essentially 

equivalent to the low-K calc-alkaline basalts, but have SiO2> 53 percent, FeO*/MgO< 1, and slightly steeper patterns than low-K calcalkaline basalts. Low-potassium olivine tholeiites (LKOT) have lower overall abundances of incompatible elements, a smaller slab 

component, and higher concentrations of heavy rare-earth elements (HREE).
8 Geologic Field-Trip Guide to Lassen Volcanic National Park and Vicinity, California

geochemical and MORB-like isotopic signature (Clynne, 1993; 

Bacon and others, 1997; Borg and others, 1997) that are present 

in the arc axis. The most extreme of these are the primitive magnesian andesites, present mostly in the forearc. The other end of 

the spectrum is characterized by high-K (0.8–2.0weight percent) 

basalts with higher contents of incompatible elements, a weaker 

arc geochemical signature, and isotopic compositions similar to 

ocean-island basalts (Clynne, 1993; Bacon and others, 1997; Borg 

and others, 1997) that are found mostly in the backarc. The differences in basalt compositions lead to a model for magmagenesis in 

the Lassen region that explains the mineralogical, compositional, 

and spatial systematics of the lavas (fig. 6).

Volcanic Centers

Intercalated within the regional volcanic rocks are a few 

voluminous (as much as a few hundred km3) and long-lived 

(0.5–1 m.y. or longer) foci of volcanism called volcanic centers. 

Volcanic centers erupt only calc-alkaline magmas but are 

compositionally diverse. The fundamental differences between 

regional calc-alkaline volcanoes and volcanic centers are the 

latters’ longevity, volume, and range of compositions in a single 

edifice or geographic locale. Five volcanic centers younger than 

about 3.5 Ma have been recognized along the Cascade axis in the 

Lassen area (fig. 7). Although each volcanic center experienced 

a distinct evolution, that evolution generally consisted of an 

initial phase of silicic volcanism followed by construction of a 

large composite cone dominated by andesite, which is flanked 

by a variety of younger, more silicic rocks. Late in the evolution 

of each volcanic center, an acidic hydrothermal system, driven 

by heat from cooling subvolcanic silicic magma bodies, caused 

alteration of the permeable rocks of the composite cone. 

Subsequent glacial and fluvial erosion of the altered interiors 

of the composite cones resulted in bowl-shaped depressions 

surrounded by more resistant rims of thick, unaltered, flank lava 

flows. The Yana, Maidu, Dittmar, and Latour Volcanic Centers 

have reached this stage, and their hydrothermal systems are 

extinct. The Lassen Volcanic Center (LVC) hosts continuing 

silicic volcanism and an active hydrothermal system (Muffler and 

others, 1982; Muffler and others, 1983; Ingebritsen and Sorey, 

1985; Clynne and others, 2003; Janik and McLaren, 2010). 

Volcanic centers contain a diverse assemblage of rock types 

from basalt to rhyolite, but they are dominated by andesite and 

High

K

Arc

axis

Calc-alkaline magmas

Low

K LKOT LKOT

Cascades

Basin and Range

WEST

Slab mantle

Slab component decreases towards the east

 decreases towards the west

Fertility of mantle component

Upwelling mantle

asthenosphere

Asthenosphere

Metasomatized mantle

lithosphere Altered mantle

EAST

Decompression melting

minimal slab flux

Flux 

melting

Continental crust

Slab crust

Figure 6. Diagrammatic cross-section showing magmagenesis model for the Lassen region. The arc geochemical signature decreases from 

west to east across the arc, whereas the fertility of the subarc mantle decreases from east to west. Calc-alkaline arc magmas in the east have 

a weak slab signature and are generated by decompression melting of relatively fertile lherzolite. In contrast, calc-alkaline basalt magmas 

from the arc axis have a stronger slab signature and are generated by flux melting of relatively depleted lherzolite. Calc-alkaline magnesian 

andesites in the forearc have the strongest slab signature and are generated by flux melting of harzburgite. This model is consistent with 

mineralogy, major- and trace-element composition, isotope geochemistry, and spatial systematics of magmas in the Lassen area. Note that lowpotassium olivine tholeiite (LKOT) comes from a much shallower depth, probably in the mantle lithosphere, than does the calc-alkaline basalt.
Geological Overview 9

121°45'

40°20'

40°30'

122°

121°37.5' 121°

40°00'

40°40'

Pre-Tertiary Rocks

Tuscan Formation

Maidu Volcanic Center

Lassen Volcanic Center

Latour Volcanic Center

Yana Volcanic Center

Dittmar

Volcanic

Center

Caribou

Volcanic

Field

Lake

Almanor

Tuscan

Formation

Glacial and

sedimentary

deposits

Maidu

Volcanic

Center

Dittmar

Volcanic

Center

Yana

Volcanic

Center Latour

Volcanic

Center

Regional

low-K

olivine

tholeiites

Lassen

Volcanic

Center

Regional

calc-alkaline

volcanic rocks

Caribou

Volcanic

Field

Age (Ma)

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0

0 5 10 KILOMETERS

5 10 MILES

Outline of

figure 3

121°7.5'

Outline of figure 10

Figure 7. Map and 

chronostratigraphic 

chart of volcanic 

centers in the Lassen 

region. Generalized 

regional geologic 

map emphasizes the 

location, extent, and 

ages of large volcanic 

centers, including those 

in Lassen Volcanic 

National Park. Blue 

areas are lakes. Map 

adapted from figure 4A

of Clynne and Muffler 

(2010).
10 Geologic Field-Trip Guide to Lassen Volcanic National Park and Vicinity, California

dacite. Andesites in volcanic centers evolve from regional calcalkaline mafic parental compositions by crystal fractionation 

and addition of a small crustal melt component (Clynne, 1990; 

Bullen and Clynne, 1990). Petrography of volcanic-center 

andesites is much more complex than that of rocks from regional 

volcanoes, and geochemical and petrographic evidence for 

magma mixing is abundant. Phenocrysts commonly are larger 

and more abundant than in regional lavas of equivalent silica 

content. Multiple phenocryst populations, reverse and strong 

normal zoning, and abrupt changes in phenocryst compositions 

are typical of volcanic-center rocks. Glomeroporphyritic clots 

of crystals, interpreted to be derived from bodies of cumulate 

material fractionated from parental and evolved mafic magmas, 

are abundant in volcanic-center andesites and attest to efficient 

recycling of crystals in the magmatic system. Dacites are 

characterized by very pronounced disequilibrium phenocryst 

assemblages. Coexisting populations of resorbed and unresorbed 

phenocrysts and unstable assemblages (for example, magnesian 

olivine plus quartz) are common, and quenched mafic magmatic 

inclusions are abundant. 

Volcanic centers result from a local increase in the amount 

of basaltic magma intruded into the crust from the mantle relative 

to the background calc-alkaline regional volcanism (Guffanti 

and others, 1996). Increased basaltic intrusion heats the crust and 

promotes interaction between mafic magma and crustal rocks. A 

complex, vertically zoned magmatic system with localized bodies 

of magma develops in the crust beneath the volcanic center. 

Magma mixing among andesitic magma batches is common, 

and eventually the enhanced heat causes crustal melting beneath 

volcanic centers and interaction between mafic and silicic 

magma bodies, leading to eruption of intermediate and silicic 

lavas (Borg and Clynne, 1998). Geochemical diversity of rocks 

decreases with time in volcanic centers, attesting to the increasing 

importance of homogenization in the magmatic systems during 

their evolution (Clynne, 1990; Bullen and Clynne, 1990). 

Volcanic centers are the surface expression of a magmatic 

cycle driven by a local increase in regional magmatic flux 

that waxes as the crust is heated and wanes as the easily 

fusible component of the lower crust is depleted. During the 

andesitic phase of volcanic centers, magmatic fractionation is 

dominated by crystal fractionation of basalt with a relatively 

small contribution from the crust. As the evolution of volcanic 

centers proceeds, the crustal component becomes increasingly 

important until, in the later stages, it dominates the magmatic 

system. The origin of the increased magmatic flux that initiates 

volcanic centers is unclear; it may be caused by structures that 

focus magma at the base of the crust or within the lower crust 

(Blakely and others, 1997). Alternatively, it could be related to 

inhomogeneities of magma production in the mantle wedge or 

even to heterogeneities at the slab-wedge interface. Volcanic 

centers do not significantly overlap in space, possibly as a 

result of the depletion of the easily fusible component in the 

lower crust that is required for production of large volumes of 

silicic magma. Volcanism in volcanic centers has been nearly 

continuous somewhere in the Lassen area over the past 3.5 m.y. 

Typically, a single volcanic center becomes extinct before or as 

the next center becomes active. During the early Pleistocene, 

however, the life spans of the Maidu and Dittmar Volcanic 

Centers coincided (fig. 7). 

Eruptive History of the Lassen Area

Regional Volcanic Rocks

Over the past 3.5 m.y., regional calc-alkaline volcanism 

in the Lassen segment of the Cascade Arc constructed a broad 

platform of volcanic rocks in a band straddling the present 

arc axis (figs. 2 and 3). A variety of these older lavas, erupted 

from shield volcanoes, lava cones, and scoria cones, are 

exposed in the mostly unglaciated terrane surrounding the 

LVC. North and east of the LVC, these older rocks commonly 

are broken into linear blocks bounded by faults. The volume 

of calc-alkaline regional volcanic rocks diminishes with 

distance both east and west from the arc axis, so that there 

are lesser volumes of fore-arc and back-arc volcanism. On a 

time scale of 50–200 thousand years (k.y.), regional volcanism 

is concentrated in local areas. Regional volcanism does not 

occur within the boundaries of active volcanic centers. There, 

rising mafic magmas are intercepted and processed, and 

thus they contribute to the magmatic system of the volcanic 

center. Return of regional volcanism within the boundaries 

of a volcanic center indicates solidification of that part of the 

center’s magmatic system. 

Within the regional lavas of the Lassen area are many 

young alignments of monogenetic cones and flows that share 

spatial, chemical, and lithologic characteristics. These are 

illustrated on this field trip by the Tumble Buttes, Sugarloaf, 

and Poison Lake chains (fig. 3). 

LKOT is related to Basin and Range tectonics (Clynne, 

1993), and it erupted across the Lassen region in a wave 

progressing from east to west that approximately coincides 

with progressive impingement of Basin and Range extension 

on the Cascade Arc (Guffanti and others, 1990). Although they 

produce relatively large volumes of lava, LKOT eruptions are 

relatively infrequent (Clynne and others, 2004), and, overall, 

LKOT is volumetrically subordinate to the dominant calcalkaline volcanism. Because LKOT lavas are readily buried by 

younger volcanism, some are likely hidden in the subsurface.

Yana and Latour Volcanic Centers and the Tuscan 

Formation

The oldest volcanic rocks in the vicinity of the present-day 

axis of the Cascade Range in the Lassen area make up the Yana 

and Latour Volcanic Centers. Rocks of the Yana Volcanic Center 

(~3.4–2.4 Ma) dominate the area southwest of Lake Almanor, 

whereas rocks of the Latour Volcanic Center (>3 Ma) are present 

northwest of LVNP. The evolution of the Yana Volcanic Center 

is understood in only a general sense, and even less is known 
Geological Overview 11

about the Latour Volcanic Center. The Yana and Latour Volcanic 

Centers are dominantly andesitic, and the lithologies are similar 

to the better-known volcanic centers described below. 

The Yana and Latour Volcanic Centers are important, 

however, as the primary sources of the Tuscan Formation, a 

broad volcanic and volcaniclastic wedge of fragmental material 

emplaced on the west slope of the southernmost Cascade Range 

(Anderson, 1933; Lydon, 1968; Helley and others, 1981; Helley 

and Harwood, 1985). The Tuscan Formation consists primarily 

of volcanic debris-flow deposits, conglomerate, sandstone, and 

siltstone, with minor silicic ash-flow and air-fall tuffs and sparse 

basaltic to andesitic lava flows. The Tuscan Formation originally 

covered about 5,000km2 and had a volume of 1,200 km3 (Lydon, 

1968). Adjacent to the Cascade Range, the Tuscan Formation is 

~500m thick (Lydon, 1968) where it interfingers with proximal deposits of the Latour and Yana Volcanic Centers. It thins 

dramatically in distal areas to the west in the Sacramento Valley, 

where volcanic debris was reworked by fluvial processes (Harwood and others, 1981) and intercalated with fluvial sediments. 

The Tuscan Formation ranges in age from ~3.5Ma (Helley and 

others, 1981; Helley and Harwood, 1985) to perhaps as young 

as ~2.5 Ma. In some places, the uppermost part of the Tuscan 

Formation may be derived from the Dittmar Volcanic Center. 

The Nomlaki Tuff Member of the Tuscan Formation is a 

dacitic ash flow tuff of regional extent found intercalated with the 

lower part of the Tuscan Formation in the northern Sacramento 

Valley. The Nomlaki Tuff Member was dated at about 3.3 Ma 

by Evernden and others (1964). Distal Nomlaki Tuff Member 

is widely found in the western United States, and its volume is 

probably in the range of 75–100 km3. The Nomlaki Tuff Member 

thickens dramatically towards the Latour Volcanic Center, its 

likely source, but no caldera has yet been located. 

Dittmar Volcanic Center

The Dittmar Volcanic Center (2.4–1.3 Ma) lies at the 

northwest end of the Lake Almanor Graben (figs. 2 and 7). The 

volcanic center has been chopped into a number of fault blocks, 

and detailed stratigraphy cannot generally be traced across the 

faults or correlated among blocks. Consequently, the details of 

the center’s evolution are poorly known. The Dittmar Volcanic 

Center hosted a hydrothermal system in its waning stages of 

activity; thus many of the early Dittmar rocks are hydrothermally 

altered. Glaciation of weak, hydrothermally altered rocks has 

deeply eroded the central area, and rocks are generally poorly 

exposed. Deposits of the Dittmar Volcanic Center are grouped 

into three stratigraphic stages (Clynne and Muffler, 2010): Stage 

1, early growth of the composite cone; Stage 2, later growth 

of the composite cone; and Stage 3, late silicic volcanism. The 

durations and ages of all three stages are poorly constrained.

Stage 1 consists of generally thin lava flows and 

interbedded fragmental deposits ranging from flow breccias, 

lithic fall deposits, and block-and-ash flow deposits to lahars. 

These lavas and volcanic deposits erupted from a central 

vent whose exact location has been obscured by erosion 

and other surficial processes. Olivine-bearing basaltic 

andesite and hypersthene-augite andesite dominate Stage 1; 

pyroxene±hornblende dacite to rhyolite are subordinate. Three 

K-Ar ages from unaltered parts of the stratigraphic section 

are 2,315±29 ka from near the bottom and 1,785±35 ka and 

1,650±35 ka from near the top. An ash-flow eruption probably 

produced a small caldera (Ingebritsen and Rojstaczer, 1983) 

during the andesitic part of Stage 1. An exploratory well 

~3 miles south of LVNP intersected more than 150 m of 

ponded ash-flow tuff (authors’ unpublished data). This tuff is 

dacitic and may be the same as an ash-flow tuff interbedded 

with Stage 1 andesites and seen as fragments in the debrisavalanche deposit at Stop A-1. Small erosional remnants of 

this tuff are found in the area of the Yana Volcanic Center. 

Stage 2 consists of generally thick lava flows with 

sparse interbedded fragmental deposits. Lavas of Stage 2 

erupted from flank vents whose locations are not preserved. 

Hypersthene-augite andesite dominates Stage 2; olivinebearing andesite and pyroxene±hornblende dacite are 

subordinate. A single K-Ar age is 1,398±42 ka.

Stage 3 consists of a few lava flows and domes of 

hornblende-biotite rhyodacite and rhyolite. No vents for these 

units are known. A single K-Ar age is 1,273±7 ka. 

Maidu Volcanic Center

The Maidu Volcanic Center (2.4–1.2 Ma) is located 

southwest of LVNP (fig. 7), where it unconformably overlies 

the Tuscan Formation and rocks of the Yana Volcanic Center 

(Clynne and Muffler, 2010). The summit and vent areas have 

been completely obscured by a combination of hydrothermal 

alteration and glacial and fluvial erosion. Although deeply 

eroded, the center is, in general, structurally intact. The total 

volume erupted was in excess of 200 km3. Deposits of the Maidu 

Volcanic Center are grouped into three stratigraphic stages: Stage 

1, early growth of the composite cone; Stage 2, later growth of 

the composite cone; and Stage 3, late silicic volcanism. 

Stage 1 is composed mostly of thin lava flows and 

fragmental deposits erupted from a central vent. Hyperstheneaugite andesite dominates; olivine-bearing basaltic andesite 

and pyroxene±hornblende dacite are subordinate. Stage 1 

contains considerable intercalated fragmental deposits ranging 

from fallout to block-and-ash flow deposits; lahar deposits are 

common. The permeable fragmental deposits and brecciated 

flow carapaces are particularly susceptible to hydrothermal 

alteration. Stage 1 was active from~2.4 to ~2.15 Ma.

Stage 2 is dominated by a small number of thick 

porphyritic augite-hypersthene dacite lava flows erupted 

from flank vents. These lava flows generally lack 

interbedded fragmental deposits. Stage 2 was active from 

~2.15 to ~2.05 Ma. 

Stage 3 consists of five large, very extensive, and very 

thick felsic lava flows erupted between ~1.3 and ~1.2 Ma. One 

is composed of porphyritic rhyodacite, and the other four are 

sparsely phyric rhyolite. These flows are some of the largest 

silicic lava flows in the Cascade Range, with an aggregate 

volume of ~37 km3.
12 Geologic Field-Trip Guide to Lassen Volcanic National Park and Vicinity, California

Lassen Volcanic Center

Recently acquired geochronology (Lanphere and others, 

1999, 2004) and consequent reinterpretation of rock units 

surrounding Brokeoff Volcano demanded reorganization of 

the three-stage stratigraphic description of the Lassen Volcanic 

Center (LVC) presented in our earlier publications (Clynne, 

1984, 1990; Bullen and Clynne, 1990; Clynne and Muffler, 

1989). The major stratigraphic divisions of the LVC are now 

designated as the Rockland caldera complex, Brokeoff Volcano, 

and the Lassen domefield (fig. 8; Clynne and Muffler, 2010). 

These eruptive stages are contiguous periods of lithologically 

correlated volcanism distinctly different from one another but 

linked by a common magmatic system. The Rockland caldera 

complex comprises the Rockland tephra (previously considered 

to be younger than Brokeoff Volcano) and a number of silicic 

rock units previously assigned to the Maidu and Dittmar Volcanic 

Centers (Clynne, 1984). Brokeoff Volcano was previously called 

Stages 1 and 2 of the LVC. The Lassen domefield consists of the 

dacite domefield and surrounding hybrid andesite lava flows on 

the central plateau area of LVNP and surrounding areas that we 

previously called Stage 3 of the LVC. 

Rocks of the Lassen Volcanic Center range from basaltic 

andesite to rhyolite but are dominated by andesite and dacite 

(fig. 9). 

The rate of eruption for the Lassen domefield appears to 

be significantly less than for the Rockland caldera complex 

and Brokeoff volcano. The Rockland caldera complex 

erupted ~80 km3 of magma over its ~215-k.y. lifetime, a 

long-term average of ~0.4 km3/k.y. Brokeoff Volcano also 

erupted ~80 km3 of magma over its ~205-k.y. lifetime, again 

a long-term average of ~0.4 km3/k.y. In contrast, the Lassen 

domefield erupted only ~40 km3 over its 315-k.y. lifetime, a 

long-term average of ~<0.15 km3/k.y. This output, however, 

was interrupted by a hiatus of 100 k.y. between 190 ka and 

90 ka. The rate for the Lassen domefield both before and 

after the hiatus was ~0.2 km3/k.y. All of these estimates are 

unavoidably approximate.

Inspection of the well-preserved youngest volcanic 

activity of the Lassen domefield, however, suggests that 

volcanic activity probably occurs in short episodes, so that 

the short-term eruptive rate is variable. For example, the three 

eruptions at LVC in the Holocene (the past 11,700 years) total 

~2 km3, giving an eruptive rate of ~ 0.17 km3/k.y., similar 

to the long-term rate of the Lassen domefield. All three 

eruptions, however, occurred within the past 1,100 years, 

making the eruptive rate during this short period much higher 

(~1.8 km3/k.y).

Rockland Caldera Complex

The Rockland caldera complex consists of the Rockland 

tephra and a group of dacite to rhyolite domes and flows 

ranging in age from ~825 to ~600 ka. Extrusion of these 

domes and flows culminated in eruption of the Rockland 

tephra at 609±7 ka (Lanphere and others, 1999, 2004). 

The Rockland tephra was first recognized by Wilson 

(1961) as the “Rockland rhyolite pumice tuff breccia.” A 

coeval distal ash-fall deposit is widely distributed in northern 

and central California, northeast to Idaho, and in deep-sea 

cores taken off the northern California coast (Sarna-Wojcicki 

and others, 1985). 

Wilson (1961), Helley and others (1981), and Meyer 

and others (1991) attributed the Rockland deposits to the 

Maidu Volcanic Center; indeed, Helley and others (1981) 

called the unit the “ash of Mount Maidu.” Clynne (1984) 

recognized that the Rockland deposits were younger than the 

Maidu Volcanic Center and, on the basis of a fission-track 

age of ~400 ka (Meyer and others, 1980, 1991), attributed 

the Rockland tephra to the LVC. This fission-track age was 

younger than Brokeoff Volcano, and, thus, the Rockland 

tephra was thought possibly to be the initial eruption of the 

Lassen domefield of the LVC. Subsequent precise dating 

determined the Rockland tephra to be 609±7 ka (Lanphere 

and others, 1999, 2004), slightly older than Brokeoff Volcano. 

The calculated volume of the Rockland tephra is ~50 km3 

(dense rock equivalent; Sarna-Wojcicki and others, 1985), 

which is similar to that of the Mazama ash from Crater Lake 

(Bacon, 1983). An eruption of this magnitude must have 

formed a collapse caldera. Projection of the outcrop pattern 

of proximal deposits points towards Brokeoff Volcano, and it 

seems likely that eruption of the Rockland tephra produced 

a small caldera that has subsequently been filled by Brokeoff 

Volcano and younger products of the LVC. 

Clynne (1984) also recognized a number of silicic rock 

units older than Brokeoff Volcano and the Rockland tephra, 

and he assigned them to the Maidu and Dittmar Volcanic 

Centers. Subsequent radiometric dating and improved 

understanding of the evolution of the Maidu and Dittmar 

Volcanic Centers showed these silicic rock units to be 

unrelated to those centers. Instead, these silicic rock units 

form a ring around the inferred source of the Rockland tephra 

and, thus, are now grouped with the Rockland tephra as the 

Rockland caldera complex (Clynne and Muffler, 2010). Lithic 

fragments from several of these silicic rock units are present 

in the Rockland tephra. These relations support our hypothesis 

that the Rockland tephra was erupted from a caldera located at 

the present site of Brokeoff Volcano. 

Brokeoff Volcano

Brokeoff Volcano, also designated “Mount Tehama” by the 

National Park Service, consists of a large, 80-km3, composite

volcano. Almost immediately after eruption of the Rockland 

tephra, its caldera began to fill as renewed activity formed 

Brokeoff Volcano. Clynne and Muffler (2010) describe the 

stratigraphy of Brokeoff Volcano as two sequences of deposits: 

the Mill Canyon sequence and the Diller sequence.

The Mill Canyon sequence consists of dozens of smallvolume basaltic andesite to dacite lava flows and interbedded 

fragmental deposits erupted from a central vent between about 

590 and 470 ka. The sequence is dominated by packages of 
Geological Overview 13

Age (Ma)

3.5

0.5

1.0

1.5

2.0

2.5

3.0

 0

Glacial

and 

sedimentary

deposits

Lassen

Volcanic

Center

Regional 

calcalkaline

volcanic

rocks

Latour

Volcanic

Center

0.1

0.2

0.3

0.4

0.5

 0 

Eagle Peak

sequence

0.6

0.7

0.8 

Mill Canyon

sequence

Lassen domefield Brokeoff Volcano

Diller sequence

Twin Lakes

sequence

Bumpass

sequence older

younger

Lassen Peak

LASSEN VOLCANIC NATIONAL PARK

0

0 5 KILOMETERS

5 MILES

Area of figure 10

121°37.5' 121°7.5'

40°40'

40°30'

40°20'

Tuscan

Formation

Maidu

Volcanic

Center

Dittmar

Volcanic

Center

Regional 

low-K

olivine 

tholeiites

Caribou

Volcanic

Field

Rockland caldera

complex

Age

(Ma)

Figure 8. Generalized geologic map and chronostratigraphic 

chart of Lassen Volcanic National Park and vicinity, California, 

emphasizing the location, extent, and stratigraphy of the Lassen 

Volcanic Center. Blue areas are lakes. See figures 3 and 7 for areas 

beyond the Lassen Volcanic Center. Map and chart taken from 

figure 4 of Clynne and Muffler (2010).
14 Geologic Field-Trip Guide to Lassen Volcanic National Park and Vicinity, California

thin andesite lava flows and monolithologic breccias. Adjacent 

flow packages are of different lithologies, and a wide variety of 

porphyritic olivine and pyroxene basaltic andesites and andesites 

to pyroxene-hornblende dacites are present. The Mill Canyon 

sequence is capped by thick dacite lava flows erupted at 470 ka 

(the dacite of Twin Meadows). This dacite is the only rock unit 

of significant lateral extent in Brokeoff Volcano and provides 

a convenient stratigraphic boundary between the Mill Canyon 

sequence and the overlying Diller sequence. 

The Diller sequence consists primarily of six thick, 

large-volume, lithologically similar lava flows that erupted 

from flank vents between 470 and 385 ka. These flows consist 

of porphyritic augite-hypersthene andesite to silicic andesite and generally lack interbedded pyroclastic deposits. The 

lithologic, petrographic, and geochemical characteristics of 

the Mill Canyon and Diller sequences indicate a change in the 

Brokeoff Volcano magmatic system between those periods. 

During Mill Canyon time, evolving magma batches were 

small and temporally or physically isolated so that a diversity 

of lithologies was erupted. In contrast, during Diller time, the 

magmatic system was more integrated, and more voluminous 

lavas of relatively constant composition were erupted.

Williams (1932) proposed that the eroded core of Brokeoff Volcano was a collapsed caldera. Our mapping, however, 

has failed to reveal either caldera-bounding faults or any 

pyroclastic deposit that could be related to a caldera. The eviscerated core of Brokeoff Volcano appears to be the product 

of glacial and fluvial erosion of weak, hydrothermally altered 

rock, and much of the core area now consists of material 

SiO2, in weight percent

K2O, in weight percent

50

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

52 54 56 58 60 62 64 66 68 70 72 74 76

Basalt Andesite Dacite Rhyodacite Rhyolite

Basaltic

andesite

Brokeoff Volcano, Mill Canyon sequence

Rockland caldera complex

Brokeoff Volcano, Diller sequence

Lassen domefield, Bumpass sequence

Lassen domefield, Eagle Peak sequence

Lassen domefield, Twin Lakes sequence

Inclusions

EXPLANATION

Figure 9. Graph of SiO2

content versus K2O content 

for 250 analyses of volcanic 

rocks from the Lassen 

Volcanic Center grouped 

by eruptive sequence. The 

Rockland caldera complex 

is dominated by dacite to 

rhyolite, Brokeoff Volcano 

is dominated by andesite, 

and the Lassen domefield 

(Bumpass, Eagle Peak, and 

Twin Lakes sequences) 

is dominated by dacite to 

rhyodacite. Basalt input is 

expressed as quenched mafic 

inclusions in the Bumpass 

and Eagle Lake sequences 

and as andesitic hybrid lavas 

of the Twin Lakes sequence 

(Clynne, 1990; Bullen and 

Clynne, 1990).

emplaced by landslides (see figure 19 of Clynne and others, 

2012). The largest remnants of Brokeoff Volcano (Brokeoff 

Mountain and Mount Diller) were protected from erosion by 

thick, unaltered, lava flows of the Diller sequence on their 

outer flanks.

Lassen Domefield

No volcanism is recognized in the LVC between about 

385 and 315 ka. After this hiatus, the character and locus of 

volcanism in the LVC changed dramatically from the andesitic 

stratocone to the Lassen domefield, which consists of a core of 

dacite domes surrounded by an arc of hybrid andesite flows. 

The dacite domes erupted along the northern flank of Brokeoff 

Volcano and are divided on the basis of age into the Bumpass 

(~300–190 ka) and Eagle Peak (~70–0 ka) sequences. The 

hybrid andesite units erupted in two groups called the older 

(~315–~240 ka) Twin Lakes sequence and the younger 

(~90–0ka) Twin Lakes sequence, which are contemporaneous 

with the Bumpass and Eagle Peak sequences, respectively. No 

volcanism is known in the LVC during the period 190–90 ka. 

The Bumpass sequence is a group of 15 dacite to 

rhyodacite lava domes and flows emplaced between ~300ka 

and ~190 ka as a broad curvilinear array in the south part 

of the Lassen domefield. Repeated glaciation has removed 

glassy dome carapaces and most of the fragmental deposits 

associated with domes and flows; only a single lithic 

pyroclastic-flow deposit is preserved. Lavas of the Bumpass 

sequence are mostly porphyritic pyroxene and hornblende 
Geological Overview 15

dacite; quartz and biotite are generally sparse. A few lavas are 

porphyritic biotite-hornblende rhyodacite. Quenched mafic 

inclusions (enclaves) are sparse to abundant. The Bumpass 

sequence is characterized by disequilibrium phenocryst 

assemblages that originated by mixing of a small proportion 

of regional mafic magma with rhyodacite of the Lassen 

domefield magmatic system. 

The Eagle Peak sequence comprises seven units of dacite 

and rhyodacite lava domes and flows and their associated 

pyroclastic deposits that erupted in the north part of Lassen 

domefield. These include the two most prominent young 

volcanic features in LVNP: Lassen Peak (27±1 ka) and Chaos 

Crags (1,103±13 ka; ~850 C.E.), as well as the products of 

five older eruptions. All units of the Eagle Peak sequence 

excepting Chaos Crags are glaciated, but the degree of glacial 

erosion is much less than on domes of the Bumpass sequence, 

and significant portions of glassy carapaces, primary hot talus, 

and pyroclastic deposits are preserved. Lavas of the Eagle 

Peak sequence are porphyritic hornblende and biotite dacite 

and rhyodacite; some also contain quartz. Like the Bumpass 

sequence, the Eagle Peak sequence is characterized by 

disequilibrium phenocryst assemblages and quenched mafic 

inclusions that originate by mixing of a small proportion of 

regional mafic magma with rhyodacite of the active Lassen 

domefield magmatic system. 

The Twin Lakes sequence consists of two groups of mostly 

andesite and basaltic andesite lava flows and agglutinate cones 

erupted around the periphery of the Lassen domefield, but 

primarily on the central plateau of LVNP. The older group of 

lava flows is correlative with the Bumpass sequence, and the 

younger group is correlative with the Eagle Peak sequence. 

The two youngest eruptions in LVNP, the basaltic andesites of 

Cinder Cone and deposits of the 1914–17 eruption of Lassen 

Peak, are parts of the younger Twin Lakes sequence. Rocks 

of the Twin Lakes sequence have pronounced disequilibrium 

phenocryst assemblages that are characterized by coexistence 

of magnesian olivine and quartz. Except for Cinder Cone (see 

detailed description at Stop G-3), they originate by mixing of 

subequal proportions of regional mafic magma and rhyodacite 

of the Lassen domefield magmatic system. 

The Bumpass, Eagle Peak, and Twin Lakes sequences 

belong to a suite of lavas formed by a continuum of magmamixing processes between a basaltic component equivalent 

to the regional basaltic volcanism and a rhyolitic component 

generated by melting of solidified basalt underplated or 

intruded into the lower crust (Borg and Clynne, 1998; Feeley 

and others, 2008a). Compositional variation of the Bumpass 

sequence (and the earlier Rockland caldera complex) is 

dominated by the felsic melt. The youngest Lassen dacites 

(Eagle Peak Sequence) evolved by introduction of regional 

mafic magma into a reservoir containing residual felsic melt 

and phenocrysts that is probably too cool and viscous to 

erupt. Mixing of hot mafic magma and its phenocrysts with 

cool crystal mush, partial resorption of phenocrysts in the 

host dacite, quenching of the resultant mixed magma to form 

undercooled inclusions, and circulation and disaggregation 

of inclusions back into the host dacite created complex 

disequilibrium phenocryst assemblages (Clynne, 1999). 

Phenocryst abundance, character, and even assemblage can 

vary within individual dacite domes, and intradome variability 

can be as large as interdome variability. The two fundamental 

factors controlling the character of the erupted magma are the 

relative proportions of mafic and silicic components in the 

mixture and the length of time between the mixing event(s) 

and eruption. When the relative proportions of mafic and 

silicic components approach unity, inclusions are no longer 

formed, and the magmas mix homogeneously to form hybrid 

magmas. Hybrids (Twin Lakes sequence) are typically 

andesitic in bulk composition and are usually characterized 

by strongly resorbed phenocrysts and the coexistence of 

magnesian olivine and quartz as phenocrysts. 

Holocene Volcanic Activity

The LVC is still active, and three eruptions have occurred 

in the Holocene: Chaos Crags, Cinder Cone, and the 1914–17 

eruption at the summit of Lassen Peak. Chaos Crags is part 

of the Eagle Peak sequence, whereas Cinder Cone and the 

eruption at the summit of Lassen Peak are part of the younger 

Twin Lakes sequence. No other Holocene eruptions have been 

documented in the Lassen region. 

Chaos Crags, located northwest of Lassen Peak, erupted 

at 1,103±13 B.P. (~850 C.E.; Clynne and others, 2002) and is 

the youngest unit of the Eagle Peak sequence. As mapped by 

Christiansen and others (2002), the eruptive sequence consists 

of a group of six rhyodacite lava domes and associated 

pyroclastic deposits that define a typical silicic eruption in the 

LVC. Chaos Crags lavas are porphyritic hornblende-biotite 

rhyodacite that contains a complex disequilibrium phenocryst 

assemblage characteristic of Lassen dacites (Clynne, 1999). 

Quenched mafic inclusions are sparse to rare in the early 

rhyodacite lavas (generally less than 1 percent to a few 

percent), whereas they form as much as 20 percent of the late 

rhyodacite lavas. 

Cinder Cone, located in the northern part of LVNP, is the 

youngest mafic volcano in the Lassen region and the second 

youngest eruption in the Twin Lakes sequence. Radiocarbon, 

stratigraphic, paleomagnetic, and dendrochronologic data 

(Clynne and others, 2000a, 2002; Sheppard and others, 

2009) indicate that Cinder Cone erupted in 1666 C.E., not in 

1851C.E. as cited in earlier literature going back to Harkness 

(1875). Cinder Cone consists of five lava flows of basaltic 

andesite to andesite composition, two nested scoria cones, and 

a widespread mafic ash-fall deposit. 

The 1914–17 eruption of Lassen Peak is the most 

recent eruption in the Twin Lakes sequence. It comprises 

a complex eruptive sequence consisting of a dacite dome 

and lava flow, dacite pyroclastic flow and fall deposit, and 

phreatic, avalanche, debris-flow, and flood deposits (Clynne 

and others, 2014; Christiansen and others, 2002; Clynne, 

1999). The eruptive sequence began on May 30, 1914, with a 

phreatic explosion at the summit of Lassen Peak. By mid-May 
16 Geologic Field-Trip Guide to Lassen Volcanic National Park and Vicinity, California

of 1915, more than 180 steam explosions had blasted out 

a 300-m-long crater at the summit. Magmatic activity was 

confined to the period from about May 14 to May 22, 1915, 

and affected primarily the northeast flank and slope of Lassen 

Peak (now called the Devastated Area) and the valleys of Lost 

and Hat Creeks as far as about 30 miles (50 km) downstream. 

Rock types range from olivine andesite to several varieties 

of hornblende-biotite dacite with abundant quenched mafic 

inclusions and complex mineralogy. Spectacularly banded 

pumice blocks, erupted on May 22, serve to distinguish the 

May 22 deposits from those of May 14–19. 

Caribou Volcanic Field

The Caribou Volcanic Field (CVF) is an area of intense, 

young regional volcanism located 20–30 km east of the LVC 

(figs. 7 and 10). The field is composed of monogenetic scoria 

cones, flows, and small- to medium-size lava cones, generally 

of mafic composition, and is distinguished from older regional 

mafic lavas around its periphery by substantial constructional 

morphology and the absence of significant structural 

disruption. The CVF (~425–0 ka) is contemporaneous with the 

later part of the LVC and is the result of a higher flux of magma 

from the mantle compared to the Lassen segment of the arc as a 

whole, but not as high as that at the LVC (Guffanti and others, 

1996). The CVF may represent an incipient volcanic center 

that is not quite vigorous enough to initiate crustal melting 

and silicic volcanism. No single edifice has a long history of 

activity. CVF volcanoes erupted in nine arrays discriminated 

by spatial, temporal, and compositional characteristics. Some 

arrays were active for only a few thousand years (for example, 

the Poison Lake chain), whereas several others (such as the 

Caribou chain) were active for periods as long as 100–200 k.y. 

Emplacement of arrays is generally structurally controlled; 

that is, linear arrays of vents bury active or inactive faults that 

provided pathways exploited by ascending magma and acted to 

focus volcanism at the surface.

The older part of the CVF consists of the Caribou chain 

(425–275 ka)—a large, extensively glaciated massif that 

dominates the Caribou Wilderness, just east of LVNP—and 4 

contemporaneous groups of volcanoes peripheral to the Caribou 

chain (Cone Lake chain, Sunrise Peak sequence, Beauty Lake 

sequence, and Island Lake sequence) that range in age from 

450–200 ka. The younger part of the CVF consists of 4 chains 

of volcanoes <110 ka. Most prominent is the weakly glaciated 

Red Cinder chain (~100 to ~25 ka), located on the border of 

LVNP and the Caribou Wilderness. To the northeast, the Poison 

Lake chain is unglaciated, and the constructional forms of its 

39 young basalt and basaltic andesite scoria cones and lava 

flows (110–100 ka) are well preserved (Muffler and others, 

2011). Margins of the flows of the Bidwell Spring chain (one 

date of 46.3±3.4 ka) are slightly overlapped by till, particularly 

on the west side, but most of the prominent scoria cones are 

unglaciated. The CVF also contains two chains of tuyas (flattopped subglacial volcanic constructs); the older is either 130–

120 ka or ~60 ka, and the younger is 35–17 ka.

Glacial Geology

At least five episodes of Pleistocene glaciation are 

recognized in the Lassen area (Crandell, 1972; Kane, 1982; 

Colman and Pierce, 1992; Turrin and others, 1998). A 

comparison of different workers’ depictions of the Lassen 

area glacial geology is presented in figure 11.

In the Lost, Hat, and Manzanita Creek drainages, 

Christiansen and others (2002) recognized and mapped six 

ages of glacial deposits intercalated with volcanic rocks 

of the Eagle Peak sequence. Use of this detailed glacial 

unit terminology was partially extended in collaboration 

with Robert Christiansen to the Bailey and Digger Creek 

drainages (Clynne and Muffler, 2010). In all areas outside of 

these drainages, Clynne and Muffler (2010) divided glacial 

deposits simply into older and younger units. Extension 

of the detailed glacial stratigraphy beyond the Lost, Hat, 

and Manzanita Creek drainages represents an important 

remaining geologic mapping problem in the Lassen region. 

Hydrothermal Features

The Lassen geothermal system (fig. 12) consists of a 

central vapor-dominated reservoir underlain by a reservoir 

of hot water at a temperature of 240 °C (Muffler and others, 

1982; Ingebritsen and Sorey, 1985; Clynne and others, 

2003). Morgan Hot Springs and Growler Hot Spring are 

located in the canyon of Mill Creek 3,200 ft (975 m) below 

Bumpass Hell, at the contact between Brokeoff Volcano and 

the underlying Maidu Volcanic Center (fig. 12, top panel; 

fig. 13). These springs mark the lateral discharge of nearneutral, chloride-bearing hot water from the deep hot-water 

reservoir under Bumpass Hell. In contrast to the acid-sulfate 

alteration at Bumpass Hell, Morgan Hot Springs and Growler 

Hot Spring deposit silica sinter. Deep hot water also was 

encountered in a commercial geothermal well at Terminal 

Geyser, in an area subsequently incorporated into Lassen 

Volcanic National Park. This deep hot water may be from a 

separate geothermal upflow centered at either Devils Kitchen 

or Boiling Springs Lake in the southeast part of the park 

(fig. 13; fig. 12, bottom panel; Janik and McLaren, 2010). 

Heat loss from the thermal areas of Bumpass Hell, Sulphur 

Works, Pilot Pinnacle, and Little Hot Springs Valley totals 

56 megawatts (MW), whereas the heat loss from Devils 

Kitchen and Boiling Springs Lake totals 49 MW (Sorey and 

Ingebritsen, 1995). 

Volcano-Related Hazards

 Clynne and others (2012) comprehensively evaluated 

five classes of volcano-related hazards in the Lassen region: 

(1) regional mafic lava flows and ash, (2) silicic lava domes, 

flows, and pyroclastic flows, (3) lahars, (4) hydrothermal 

hazards, and (5) landside and rockfall hazards. Robinson 

and Clynne (2012) presented details and modeling of 
Geological Overview 17

LASSEN 

VOLCANIC

NATIONAL

PARK

121°5'

40°22.5'

40°40'

121°22.5'

Island

Lake

sequence Beauty

Lake

sequence

Red

Cinder

chain

Cone

Lake

chain

Bidwell

Spring

chain

Poison

Lake

chain

Sunrise

Peak

sequence

Tuya

chains Caribou

chain

0

0 5 KILOMETERS

5 MILES

Figure 10. Generalized geologic map of eastern part of Lassen Volcanic National 

Park and adjacent areas, emphasizing the location and extent of volcanic sequences 

and chains in the Caribou Volcanic Field. Lighter colors are units not part of Caribou 

Volcanic Field (see fig. 7). Map adapted from figure 6 of Clynne and Muffler (2010).
18 Geologic Field-Trip Guide to Lassen Volcanic National Park and Vicinity, California

Crandell (1972) Kane (1982) Turrin and others (1998)

Christiansen and others (2002) 

and detailed areas of 

Clynne and Muffler (2010)

Additional generalized 

units of Clynne and 

Muffler (2010)

Estimated 

age

[Not indicated] [Not indicated] Till of Anklin Meadows A4 [Not indicated] [Not indicated] 200–300 

yr

Moraines of 

late Tioga age Late Tioga till

Till of Anklin Meadows A3

Till or protalus-rampart debris 

of early Holocene age (the)

Till of younger 

glaciations (Qty)

8–12 ka

Late till of Anklin Meadows (tal)

Till of Anklin Meadows A2

Till of Anklin Meadows A (ta) 17–25 ka

Tioga Drift Middle Tioga

till Till of Anklin Meadows A1

Tahoe Drift

Early Tioga till “Early Tioga” till

Post-maximum till of Raker 

Peak, consisting of Lassen Peak 

avalanche debris (trl)

~27 ka

Till of Raker Peak (tr) 27–35 ka

Tahoe till Till of Badger Mountain B2

Till of Badger Mountain (tb) Till of older 

glaciations (Qto) Till 60–70 ka of Badger Mountain B1

Pre-Tahoe drift Pre-Tahoe till [Not indicated] [Not indicated] >130 ka

Figure 11. Chart showing correlation of glacial terminology, ages, and depiction of glacial deposits in the Lassen area by various 

authors. From table 1 of Clynne and Muffler (2010). In most parts of the Geologic Map of Lassen Volcanic National Park and Vicinity 

(Clynne and Muffler, 2010), glacial deposits are divided simply into younger and older tills. In general, till of younger glaciations (Qty) 

has well-preserved moraines, whereas till of older glaciations (Qto) has only moderately to poorly preserved moraines. The boundary 

between the two tills corresponds to the boundary between middle Wisconsin and early Wisconsin glacial stages as defined 

by Colman and Pierce (1992), or approximately to the Tioga and Tahoe glacial advances as these terms are used by Kane (1982). 

Estimated ages in the last column should be applied neither to the terms used by Crandell (1972) and Kane (1982) nor to the terms 

“Tahoe” and “Tioga” in other regions. Furthermore, the interpretations of Clynne and Muffler (2010) do not always agree with those of 

Crandell (1972) and Kane (1982). For example, in the Bailey Creek drainage northwest of Brokeoff Mountain, the contact of Clynne and 

Muffler (2010) lies between that shown by Crandell (1972) and that shown by Kane (1982).
Geological Overview 19

Figure 12. Diagrammatic 

cross sections of the Lassen 

hydrothermal system. The 

upper, north-south cross 

section illustrates circulation 

in cell 1 of Janik and 

McLaren (2010), whereas 

the lower, east-west cross 

section illustrates circulation 

in their cell 2. In both cells, 

meteoric water percolates 

to depth and is heated 

by hot rock overlying a 

body or bodies of magma 

(shown generalized and 

diagrammatically). Hot 

water rises convectively 

and boils in response to 

decreasing pressure to form 

a vapor-dominated zone 

that releases steam and 

gases to surface fumaroles 

and acidic hot springs. The 

residual thermal water flows 

laterally and emerges as hot 

springs south of the park 

(cell 1) and was encountered 

in a now-abandoned 

geothermal well at Terminal 

Geyser (cell 2). Approximate 

depths indicated are below 

ground surface. Taken from 

figure 14 of KellerLynn (2014), 

in turn based on Janik and 

McLaren (2010), Clynne and 

others (2003), and Muffler 

and others (1982).

Magma 

>8 km (5 mi)

3

Thermal cracking

Bumpass

Mountain

Bumpass Hell

Mount

Conard

E A R T H Q U A K E S

Growler

Hot 

Spring

Morgan

Hot 

Springs

Lassen

Peak

Meteoric 

water

Recharge

Gas-depleted thermal water

Vapor-dominated zone

Liquid-dominated zone

NORTH SOUTH

Fumaroles

and acid sulfate

hot springs

plume

Outflow

5 km 

(3 mi)

7–10 km 

(4–6 mi)

 235 °C–270 °C

(455 °F–520 °F)

Rises convectively

Boils

CELL 1

Magma >8 km (5 mi)

3

Thermal cracking

Lassen

Peak

Meteoric 

water

Recharge

Vapor-dominated zone

Liquid-dominated zone

WEST CELL 2 EAST

Outflow

plume

180–610 m 

(590–2,000 ft)

5 km 

(3 mi)

7–10 km 

(4–6 mi)

 220 °C–240 °C

(430 °F–460 °F)

Rises convectively

Boils

Reading

Peak

Devils Kitchen

Boiling

Springs

Lake

Terminal

Geyser

Gas-depleted thermal water

E A R T H Q U A K E S

A

B

Hot rock >350 °C (660 °F)

Hot rock >350 °C (660 °F)
20 Geologic Field-Trip Guide to Lassen Volcanic National Park and Vicinity, California

lahar hazards, and Nathenson and others (2012) analyzed 

eruption probabilities. The average recurrence interval 

for regional mafic volcanism is 1,550 years, whereas the 

average recurrence interval for the Lassen Volcanic Center is 

7,150years. Hydrothermal hazards and landslide hazards are 

ongoing, particularly in the erosional amphitheater in Brokeoff 

Volcano (figure 19 of Clynne and others, 2012). Lahar hazards 

are primarily in the drainages heading in the high-elevation 

terrain around Lassen Peak, in particular Lost Creek, Hat 

Creek, and Manzanita Creek (Robinson and Clynne, 2012; 

figure 10 and plate 1 of Clynne and others, 2012). 

Road Log

Reading the “Geological Overview” section above before 

launching into the road log will acquaint the user with the 

geologic framework and the stratigraphic terminology of the 

Lassen area, thus providing a context for specific discussions 

in the road log.

Numbers at left are distances in miles from starting point. 

Numbers in bold at the end of each entry are the distance 

in miles to the next entry. View directions are indicated by 

reference to a clock face—12:00 means straight ahead, 9:00 

means 90° to left, 3:00 means 90° to right.

Section A: Red Bluff to Intersection of Calif. 

Hwys 36 and 89

 0.0 At Red Bluff, head east from Interstate 5 on Calif. 

Hwys 36 and 99. Bluffs at 12:00 are Tuscan 

Formation, with Lassen Peak on the skyline. Tuscan 

Buttes are at 11:30. 2.1

 2.1 Turn left on Calif. Hwy 36 towards Susanville and 

Lassen Volcanic National Park. Highway is on the 

flood plain of the Sacramento River. 0.8

 2.9 At 2:00, view over Tuscan Formation towards Tuscan 

Buttes. 0.9

 3.8 Roadcuts in Pleistocene terrace gravels related to the 

Sacramento River. 0.4

 4.2 Large roadcuts on both sides of road in Pleistocene 

terrace gravels. 0.5

 4.7 Contact of Pleistocene terrace gravels with the underlying 

Tuscan Formation (~3.5 to ~2.5 Ma). Weathering and 

erosion of the debris-flow matrix of the Tuscan Formation 

produces a characteristic “stony plain.” 0.4

 5.1 Mount Shasta visible in distance at 10:30. 1.9

 7.0 Boundary of Red Bluff East quadrangle with Bend 

quadrangle to the north. 0.4

 7.3 On left is BLM parking area for Sacramento River 

Bend, Iron Canyon Trailhead. To west at 9:00 across 

the Sacramento River are the northern Coast Ranges. 

7600

8400

8000

8000

8800

7200

6800

6000

6000

6800

6400

6000

6000

6800

5200

7200

6800

89

121°35’

40°27.5’

40°22.5’

40°25’

121°30’ 121°25’

Boiling Springs Lake

Cold Boiling Lake

Terminal Geyser

Devils Kitchen Drakesbad

Growler Hot Spring

Sulphur Works

Brokeoff

Mountain

Mount

Conard

Mount Diller

Pilot Pinnacle

Lassen Peak

Pilot Pinnacle

Morgan Hot Springs

Little Hot

Springs Valley

Bumpass

Hell

LASSEN

VOLCANIC

NATIONAL

PARK

Mill Canyon

0

0 1 2 3 KILOMETERS

1 2 3 MILES

Hydrothermal areas

Volcanic feature or remnant

Outline of Brokeoff Volcano

EXPLANATION

Contour interval 400 feet

Figure 13. Map showing thermal areas in and adjacent to Lassen Volcanic National Park. From 

figure 8 of Clynne and others (2012).
Road Log 21

Burney

Mountain

Freaner

Peak

Tuscan Formation

Tholeiitic basalt of Coleman Forebay

Tholeiitic basalt of Coleman Forebay

CALIF. HWY 36

Dales (4.4 miles)

Red Bluff (8.9 miles)

Crater

Peak

Latour

Butte

Soap Butte

Clover

Mountain

Snow

Mountain

Black Butte

Battle Creek Fault scarp

Figure 14. Photograph looking at azimuth 025° from mile 8.9 on Calif. Hwy 36 between Red Bluff and Dales, showing features along 

the Battle Creek Fault scarp and the Cascade Range north of the Lassen Volcanic Center. The Tuscan Formation (~3.5 to ~2.5 Ma) in 

the foreground is overlain by the tholeiitic basalts of Coleman Forebay (950–750 ka). Extending across the photograph is the east-weststriking scarp of the Battle Creek Fault (down to the south). The tholeiitic basalts of Coleman Forebay and the tholeiitic basalt of Eagle 

Canyon (199±22 ka) drape this fault scarp with minimal offset; basaltic andesite of Hootman Ranch (562±12 ka) flows over the scarp 

and along its base with little offset. A flow from Black Butte, the ~60-ka cinder cone at the top of the scarp, is unfaulted. South of the 

fault scarp is Soap Butte, a 426±26-ka calc-alkaline basalt (K-Ar date by G.B. Dalrymple, written commun., 1994). On the skyline are 

volcanoes of the Cascade axis: Burney Mountain (280±6 ka; 40Ar/39Ar date by M.A. Lanphere, written commun., 1998), Freaner Peak, 

and Magee volcano (210±120 ka; the highest point is Crater Peak). Between the Battle Creek Fault scarp and the Cascade axis lie the 

Snow Mountain Volcanic Center (>1.4 Ma) and the Latour Volcanic Center (>3.0 Ma), centered at Latour Butte.

East of the highway at 1:00 across the stony plain of 

the Tuscan Formation are the Tuscan Buttes. 1.6

 8.9 In the distance ahead is the scarp of the east-weststriking Battle Creek Fault (fig. 14), mapped and 

described by Helley and others (1981), Helley and 

Harwood, (1985), Harwood (1987), and Blake and 

others (1999). Age data from Marvin Lanphere 

(written commun., 1997) indicate that the 950–750-ka 

lava flows draping the Battle Creek Fault scarp are 

progressively less offset with decreasing age. A 562-ka 

lava flow ponded at the base of the scarp is barely 

offset, and a ~60-ka lava flow from Black Butte (Page 

and Renne, 1994) that crosses the fault is not offset. 

For the next several miles, Calif. Hwy 36 is on 

Tuscan Formation, but the oak-covered low bluffs 

on both sides of the road are a thin lava flow of the 

overlying tholeiitic basalts of Coleman Forebay. 

In the distance ahead is Magee Volcano, with Soap 

Butte in the middle ground. 0.2

 9.1 Boundary of Bend quadrangle with Dales quadrangle 

to the east. 0.4

 9.5 On left is BLM parking area for Hog Lake Plateau. 

A thin (~1 m) basalt flow of the tholeiitic basalts 

of Coleman Forebay is exposed in the highway 

roadcuts, overlying a baked zone in the Tuscan 

Formation. On right is the service road to microwave 

and cell towers on Tuscan Buttes. The Tuscan Buttes 

are hornblende andesite domes erupted onto the 

underlying Tuscan Formation (Helley and Harwood, 

1985; Lydon, 1967). They are the northernmost of 

at least 34 unusual domes, lava flows, and shallow 

intrusions emplaced into and on top of the Tuscan 

Formation. See discussion at Stop C-2. 0.8

 10.3 Straight stretch headed towards Black Butte. 0.8

 11.1 As highway curves to right, one gets an excellent view 

of the Cascade Range (fig. 15). 0.2

 11.3 Cross Palmer Gulch. Still on Tuscan Formation. 0.6

 11.9 Cross Supan Gulch. 1.0

Road Log, Section A
22 Geologic Field-Trip Guide to Lassen Volcanic National Park and Vicinity, California

 12.9 Cross Sheep Gulch. 0.4

 13.3 The settlement of Dales and the intersection with 

Tehama County Road A16 to the left towards 

Manton; continue straight on Calif. Hwy 36. Brief 

views of Black Butte at 9:30, Soap Butte at 10:00, 

and Inskip Hill at 12:00. 2.9 

 16.2 Pass from Tuscan Formation onto basalt of Inskip 

Hill. 0.4

 16.6 Cross Paynes Creek. 0.4

 17.0 Good exposures of the basalt of Inskip Hill. Roadcuts 

for the next 5 miles alternate between Tuscan 

Formation and basaltic lava flows from Inskip Hill, 

a late Pleistocene shield volcano located to the west 

(trenchward) of the Cascade Arc. Flows from Inskip 

Hill fill present-day drainages and are estimated to be 

about 50 ka. 0.2

 17.2 Pass back onto Tuscan Formation. 0.1

 17.3 Boundary of Dales quadrangle and Inskip Hill 

quadrangle to the east. 0.2

 17.5 Good views of Tuscan Formation at 12:00 to left of 

highway. 1.2

Little

Inskip Hill Inskip Hill

Hampton

Butte

Turner

Mountain

Latour

Butte

CALIF. HWY 36

Paynes Creek

Dales (1.2 miles)

South

Fork

Battle

Creek

Tholeiitic basalt of Coleman Forebay

Red Bluff (11.1 miles)

Chaos

Crags

D B

Red Lake

Mountain

Table

Mountain Grays

Peak

Soap Butte

Tuscan Formation

Crater

Peak

Lassen

Peak

Lassen Volcanic Center Maidu Volcanic Center

Black Butte and 

Clover Mountain

just out of picture 

to left

 18.7 Pass from Tuscan Formation onto basalt of Inskip 

Hill. 0.3

 19.0 Pass from basalt of Inskip Hill onto Tuscan 

Formation. 0.3

 19.3 Pass from Tuscan Formation onto basalt of Inskip 

Hill. 0.1

 19.4 Pass from basalt of Inskip Hill onto Tuscan 

Formation. 0.4

 19.8 Stop A-1: Debris-avalanche deposit from Dittmar 

Volcanic Center. Pull off on broad turnout on right 

side of road; the debris avalanche is exposed in the 

roadcut directly opposite the turnout. Beware of 

high-speed vehicles, particularly trucks, when 

crossing the highway. 

The debris-avalanche deposit consists of megablocks 

of various volcanic rock types in a matrix of broken 

material. Notable features are the heterogeneity of the 

matrix composition, the invasion of matrix material into 

fractures and joint surfaces of lava blocks, the jigsaw fit of 

some blocks, and features indicating laminar flow of matrix 

material beneath the megablocks at the base of the deposit. 

Figure 15. Photograph looking at azimuth 062° from mile 11.1 on Calif. Hwy 36 between Red Bluff and Dales showing volcanic features 

of the southern Cascade Range. Foreground and banks of Paynes Creek are Tuscan Formation (~3.5 to ~2.5 Ma). At left-center, in front 

of the Battle Creek Fault scarp, is Soap Butte, a 426±26-ka calc-alkaline basalt. At the right-center are Little Inskip Hill and Inskip Hill 

(~50 ka); both are olivine basalt. Behind Soap Butte are Latour Butte in the Latour Volcanic Center (>3.0 Ma) and Crater Peak (summit 

of Magee Volcano; 210±120 ka). Behind Little Inskip Hill and Inskip Hill are Chaos Crags domes B and D (1,103±13 B.P.; ~850 C.E.) and 

Lassen Peak (27±1 ka), units in the Lassen Volcanic Center. Red Lake Mountain (~75 ka), Table Mountain (~700 ka), and Grays Peak 

(1,080±26 ka) are andesite and basaltic andesite volcanoes of the regional calc-alkaline suite. Further to the right is the Maidu Volcanic 

Center (2.4–1.2 Ma), cut by the canyon of South Fork Battle Creek.
Road Log 23

The three most abundant rock types (dark-gray 

ash-flow tuff with black pumice and sporadic fragments 

of obsidian; light-gray ash-flow tuff; andesite rich in 

large green augite phenocrysts) can be traced to their 

source in the Dittmar Volcanic Center, approximately 

30 mi (48 km) east. The age of this debris-avalanche 

deposit is poorly known. It postdates deposition of most 

of the Tuscan Formation in the Inskip Hill area but 

is probably older than most of the intervening Maidu 

Volcanic Center; it may be ~2.4 Ma. The uncertainties 

in interpretation of this exposure and its relationship to 

the Tuscan Formation illustrate the need for systematic 

mapping and petrologic study of the Tuscan Formation.

In the roadcut 150 m west, the megabreccia is 

underlain by an ash-flow tuff identical to the ash-flow 

tuff blocks in the megabreccia. Between the ash-flow 

tuff and the megabreccia is a discontinuous, poorly 

sorted, channel-filling alluvial deposit. 

The roadcut 150 m east exposes thin olivine basalt 

flows from Inskip Hill, 1.7 mi (2.7 km) northeast. 

Vineyards to the south in the valley of Paynes Creek are 

on this basalt. 0.2

 20.0 Pass from Tuscan Formation onto basalt of Inskip Hill. 2.1

 22.1 To right is Plum Creek Road leading to the community 

of Paynes Creek. 0.3

 22.4 Pass from basalt of Inskip Hill onto Tuscan Formation. 0.1

 22.5 To left is Lanes Valley Road. 0.6

 23.1 For the next 1.5 miles, Calif. Hwy 36 is on a wide 

bench of Tuscan Formation with excellent views 

(fig.16). 1.5

CALIF. HWY 36

Paynes Creek (1 mile)

Tuscan Formation

Blue Ridge

Lassen Peak

Brokeoff Volcano

Chaos

Crags

Red

Rock

Mountain

Digger

Butte

Crater

Peak

Latour

Butte

Hill

7241

Table

Mountain

Turner

Mountain Hampton

Butte

Hill

6584

Brokeoff

Mountain

Mount

Diller

Hill

8198

Magee

Volcano

Latour 

Volcanic

Center

Lassen Volcanic Center

Maidu Volcanic Center

Figure 16. Panoramic photograph of volcanic features of the Lassen region taken at azimuth 060° from mile 23.1 (E 0593164, N 4465942) on 

Calif. Hwy 36 approximately 1 mile northeast of the turnoff to Paynes Creek. At the left are the Latour Volcanic Center (>3.0 Ma) and Magee 

Volcano (210±120 ka). In the center are peaks of the Lassen Volcanic Center: hill 7241 (vent for the rhyodacite of Manzanita Chute; 297±1ka), 

Chaos Crags (1,103±13 yr B.P.; ~850 C.E.), the dome of Red Rock Mountain (672±20 ka; part of the Rockland caldera complex), Lassen Peak 

(27±1 ka), and three erosional remnants of Brokeoff Volcano (590–385 ka). Digger Butte and Table Mountain (~700 ka) are, respectively, 

basaltic andesite and andesite volcanoes of the regional calc-alkaline suite. Farther to the right are peaks of the Maidu Volcanic Center (2.4–

1.2 Ma). Blue Ridge (directly ahead along the road) is a giant (~15 km3) rhyolite lava flow erupted at ~1.2 Ma as part of Stage 3 of the Maidu 

Volcanic Center. To the left (off the panorama) one can see Inskip Hill at 7:00, Black Butte at 8:30, and Mount Shasta in the far distance at 10:00.

 24.6 Boundary of Inskip Hill quadrangle with Finley Butte 

quadrangle to the east. 1.4

 26.0 Large roadcuts for the next 0.7 mile display chaotic 

breccias of the Tuscan Formation. 0.7

 26.7 For the next 4 miles, the highway is on another wide 

bench of Tuscan Formation. 3.9

 30.6 Battle Creek Vista Point. Burned trees are from the 

fire of August 2012. View of the Tuscan Formation 

overlain by the rhyolite of Blue Ridge (~1.2 Ma), 

a single 15- km3 flow from the Maidu Volcanic 

Center, Stage 3. This lava flow covers 75 km2 and 

averages 200m thick. Similar huge flows form Lost 

Creek Plateau, Mill Creek Plateau, and North Stover 

Mountain on the eastern flank of the Maidu Volcanic 

Center. These rhyolite flows are the largest in the 

Cascade Range and are similar in size to the huge 

lava flows filling the Yellowstone caldera. 0.4

 31.0 White deposits of Rockland tephra with orange 

soil are exposed in low roadcuts for the next 2.5 

miles. Eruption of the Rockland tephra at 609±7 ka 

produced a small caldera that has subsequently been 

filled by Brokeoff Volcano and younger products 

of the Lassen Volcanic Center. Bedrock beneath the 

veneer of Rockland tephra consists of andesites of the 

Maidu Volcanic Center, Stage 1. 0.3

31.3 Road to right leads to Ponderosa Sky Ranch. 0.8

 32.1 Boundary of Finley Butte quadrangle with Lyonsville 

quadrangle to the east. 0.4

32.5 Pass from Rockland tephra onto andesites and dacites 

of the Maidu Volcanic Center, Stage 2. 0.5

Road Log, Section A
24 Geologic Field-Trip Guide to Lassen Volcanic National Park and Vicinity, California

 33.0 On right, Little Giant Mill Road to Lyonsville. Pass 

onto another patch of Rockland tephra. 0.4

 33.4 Highway climbs onto a thick flow in andesites and 

dacites of the Maidu Volcanic Center, Stage 2. From 

here east to the town of Mineral, the highway traverses 

the northwest flank of the Maidu Volcanic Center, going 

downsection towards the core of the Maidu Volcanic 

Center near Mineral. 0.9

 34.3 California Department of Forestry and Fire Protection 

(CDF) fire station on right. 0.1

 34.4 Lassen Lodge on left. For the next mile, gaps between 

the trees give sporadic views of andesite and dacites of 

the Maidu Volcanic Center, Stage 2, on the north wall of 

the deep canyon of South Fork Battle Creek. 0.4

 34.8 The highway passes from dacites of the Maidu Volcanic 

Center, Stage 2, downsection onto andesites of the 

Maidu Volcanic Center, Stage 1. 1.3

 36.1 Roadcuts of andesitic flows and pyroclastic rocks on the 

flank of the Maidu Volcanic Center. 0.6

 36.7 Road to left into the canyon of South Fork Battle Creek. 

Road to right (termed “A line”) is the primary access 

to the Maidu Volcanic Center. Calif. Hwy 36 continues 

east on colluvium that is derived from the andesites of 

the Maidu Volcanic Center, Stage 1, to the south. A few 

poor exposures along the highway display the basalt of 

Cold Creek Butte (100–75 ka). This calc-alkaline flow 

extends >5 miles west down the canyon of the South 

Fork Battle Creek. 1.6

 38.3 Quarry in the scoria cone of Cold Creek Butte. The 

layered internal structure of the scoria cone is exposed 

by the quarrying operation. Flows from this vent 

dammed South Fork Battle Creek to create Battle 

Creek Meadows to the east. 0.9

 39.2 Bare rubble to the right is a basalt flow from Cold Creek 

Butte. 0.2

 39.4 Boundary of Lyonsville quadrangle with Mineral 

quadrangle to the east. Cross South Fork Battle 

Creek into Battle Creek Meadows, the center of the 

hydrothermally altered and deeply eroded core of the 

andesitic composite cone of Maidu Volcanic Center; 

andesites of the composite cone are exposed in the cliffs 

north and south of the meadows. Maidu Volcanic Center 

was active from about 2.4 to 1.2Ma (fig. 7). To the left 

is U.S. Forest Service Road 17, which skirts the west 

side of the Lassen Volcanic Center to an intersection 

with Calif. Hwy 44 west of Manzanita Lake. 1.6

41.0 Intersection with Calif. Hwy 172 in the town of Mineral. 

Hwy 172 climbs southeast over the resistant rim of the 

andesitic composite cone of the Maidu Volcanic Center. 

Continue northeast on Calif. Hwy 36. 2.5

 43.5 White roadcuts for the next two miles are in altered 

dacite of Christie Hill, one of several dacite domes 

emplaced at ~800 ka as part of the Rockland caldera 

complex of the Lassen Volcanic Center. 2.0

 45.5 Turn left on Hwy 89 north to Lassen Volcanic National 

Park. Refer to road log through the park (Section D) and 

reset odometer to 0.0. 

Section B: Chester to Intersection of Calif. Hwys 

36 and 89

0.0 Intersection at Fire Station with Plumas County Road 

318 to Juniper Lake and Road 312 to Warner Valley 

and Drakesbad, in the southeast corner of LVNP. A 

trail leads from the warm springs at Drakesbad through 

beautiful meadows and virgin forest to Boiling Springs 

Lake and Devils Kitchen, a high-temperature fumarolic 

area (Muffler and others, 1982, 1983).

Drive southwest along Calif. Hwy 36 towards 

Red Bluff. The highway is on outwash gravels of the 

younger glaciation (35–17 ka). 1.5

 1.5 Chester Airport on left. 0.2

 1.7 U.S. Forest Service Ranger Station on right. 0.6

 2.3 Boundary of Chester quadrangle with Stover 

Mountain quadrangle to the west. 0.1

 2.4 Cross Chester flood control channel, the diversion for 

the North Fork Feather River. 0.2

 2.6 Highway 89 to left goes down the west side of Lake 

Almanor to Canyondam, traversing the tholeiitic basalt 

of Warner Valley (610±22 ka) and the tholeiitic basalt 

of Rock Creek (989±49 ka). These lithologically nearly 

identical basalts can most easily be distinguished by 

their different magnetic polarities (the tholeiitic basalt 

of Rock Creek has reversed polarity). The contact 

between the two units was mapped using a flux-gate 

magnetometer. 0.2

 2.8 Calif. Hwys 36 and 89 climb up onto the tholeiitic 

basalt of Warner Valley (610±22 ka). This large LKOT 

flow erupted from vents in vicinity of Mount Harkness 

in the southern part of LVNP and flowed southward at 

least 36 miles through the Lake Almanor Graben and 

into the canyon of the North Fork Feather River. 0.6

 3.4 Contact between the tholeiitic basalt of Warner Valley 

and the basaltic andesite of Stover Mountain, a small 

shield volcano with a basal diameter of about 5 miles. 

Stover Mountain is typical of the regional calc-alkaline 

sequence. It consists of a 1,500-foot-thick sequence 

of sparsely porphyritic olivine basaltic andesite lava 

flows. The volcano is old and eroded, has no preserved 

vent areas, and is covered by a thick soil. Stover 

Mountain has a K-Ar age of 1,937±39 ka (M.A. 

Lanphere, written commun., 1997). 0.5

 3.9 Roadcuts in the basaltic andesite of Stover Mountain. 

On the left for the next few miles, through the trees, 

one can glimpse andesites of the northern part of the 

Yana Volcanic Center (3.4–2.4 Ma). 5.0

 8.9 Tehama County line. Highway passes onto the 

tholeiitic basalt of Ice Cave Mountain (~65 ka). The 

vent for this young LKOT flow is 6 miles to the north 

at Ice Cave Mountain. The flow went south down the 

valley of Lost Creek to this point and then split into 

two lobes, one flowing southeast for another 3 miles 

to Soldier Meadows and the other flowing west for 
Road Log 25

3.5 miles to Deer Creek Meadows. The tholeiitic basalt 

of Ice Cave Mountain overlies outwash gravel of the 

older glaciations (>130 to 60 ka) and underlies outwash 

gravels of the younger glaciation (35–17 ka). 0.9

 9.8 Saint Bernard. 0.1

 9.9 Boundary of Stover Mountain quadrangle with Childs 

Meadow quadrangle to the west. 0.5

 10.4 Roadcut in the distal toe of the rhyodacite of Gurnsey 

Creek (~1.3 Ma). This thick lava flow of coarsely 

porphyritic augite-hypersthene rhyodacite erupted from 

a vent several miles to the northwest and flowed east 

from the east flank of Maidu Volcanic Center. 0.5

 10.9 Deer Creek restaurant. The highway continues 

northwest along the contact between outwash gravels of 

the older glaciations to the north and the tholeiitic basalt 

of Ice Cave Mountain to the south. 2.0

 12.9 At left is Calif. Hwy 32, which leads south along 

Deer Creek between the Yana Volcanic Center to the 

east and the Maidu Volcanic Center to the west (see 

Section C). Inconspicuous outcrops at the junction and 

along Gurnsey Creek 0.5 mi west and south expose the 

Rockland tephra, part of the Rockland caldera complex 

of the Lassen Volcanic Center. The Rockland tephra 

was erupted at 609±7 ka (Lanphere and others, 1999, 

2004) from a small caldera now filled by younger rocks 

of the Lassen Volcanic Center (fig. 8). 0.3

 13.2 View at 12:00 of the rhyolite of Lost Creek Plateau. For 

the next 7 miles to the north, the highway is in a broad 

valley cut in the rhyodacite of Gurnsey Creek (volume 

~2 km3). Overlying the rhyodacite of Gurnsey Creek 

on the right (east) is the rhyolite of Lost Creek Plateau 

(~5km3; 1,305±10 ka) and on the left (west) the rhyolite 

of Mill Creek Plateau (~7 km3). These huge silicic 

flows, along with the rhyolite of North Stover Mountain 

(~8.5km3) and the rhyolite of Blue Ridge (15km3), 

comprise the youngest part of the Maidu Volcanic 

Center and have an aggregate volume of ~37km3. 0.6

 13.8 Bridge over Gurnsey Creek. To the left of the highway 

are exposures of one of the tholeiitic basalts of Mill Creek 

Plateau (140–100 ka). This basalt flow erupted from a 

vent 4 miles to the northwest, flowed northeast off the 

Mill Creek Plateau, and then south along Gurnsey Creek. 

Highway is on top of this flow for the next 1.3 mi. 0.8

 14.6 Fire Mountain Lodge. 0.5

 15.1 Bridge over Gurnsey Creek. Gurnsey Creek Campground 

to the right. In the bed of the creek just northwest of the 

bridge are outcrops of the tholeiitic basalts of Mill Creek 

Plateau. The valley of Gurnsey Creek for the next 5 miles 

is choked with glacial outwash and with colluvium from 

the friable rhyolite and rhyodacite flows that form the 

bluffs to the east and west. 1.1

 16.2 View at 12:00 of the light-colored rhyolite of Mill 

Creek Plateau. 2.1

 18.3 View at 11:30 of Brokeoff Mountain, part of the west 

rim of Brokeoff Volcano. This volcano (~590–385 ka) 

is part of the Lassen Volcanic Center. 0.4

 18.7 To the right is U.S. Forest Service Road 769 to Wilson 

Lake. 1.8

 20.5 Stop B-1: Childs Meadows. Pull off on the right at the 

Childs Meadows Resort. View to the north up Mill Creek 

into the core of Brokeoff Volcano, composed of the Mill 

Canyon sequence (590–470 ka) of the Lassen Volcanic 

Center. The prominent peak in the left distance is 

Brokeoff Mountain (9,235 ft; 2,816 m), which forms the 

southwest rim of Brokeoff Volcano. Brokeoff Mountain 

consists primarily of the dacite of Twin Meadows 

(470±10 ka), the uppermost unit of the Mill Canyon 

sequence. On the dip slope just to the left of the summit 

is the overlying andesite of Mount Diller (387±10 ka), 

one of six thick, large-volume, resistant lava flows that 

erupted from flank vents between 470 and 385 ka. 

In the right distance is Mount Conard, which 

forms the southeast rim of Brokeoff Volcano. Mount 

Conard consists of flows and breccias of the Mill 

Canyon sequence. On the west flank of Mount Conard 

is the rhyodacite of Mount Conard (298±9 ka), part of 

the Bumpass sequence of the Lassen domefield. 

To the left of Mill Canyon in the middle ground 

is Morgan Mountain, one of several dacite domes 

emplaced at ~800 ka as part of the Rockland caldera 

complex of the Lassen Volcanic Center. 

The cliffs to the east of Childs Meadows consist 

of the andesite of Wild Cattle Mountain (~2.0–1.6Ma), 

part of Stage 1 of the Dittmar Volcanic Center. The 

mountain south-southwest of Childs Meadows consists 

of the andesite of Doe Mountain (1.6–1.4 Ma; part of 

Stage 2 of the Dittmar Volcanic Center). 0.4

 20.9 Highway passes through a small, geologically complex 

area containing andesites of Maidu Volcanic Center, 

Stage 1, the andesite of Doe Mountain, and a sliver of 

the andesite of Wild Cattle Mountain. 0.1

 21.0 Boundary of Childs Meadow quadrangle with Mineral 

quadrangle to the west. 0.1

 21.1 Cross Mill Creek. 2.5 miles to the north up Mill Creek 

on private land are Morgan Hot Springs and Growler 

Hot Spring, both characterized by near-neutral-pH, 

chloride-bearing boiling water that deposits siliceous 

sinter and pyrite. These springs represent outflow from 

the deep, 240 °C hot-water reservoir beneath the vapordominated system that feeds the fumarole areas such as 

Sulphur Works and Bumpass Hell in Lassen Volcanic 

National Park (Muffler and others, 1982; Ingebritsen 

and Sorey, 1985; Clynne and others, 2003; Janik and 

McLaren, 2010; Clynne and others, 2012). 

Mill Creek and Deer Creek are the two major 

remaining free-running, salmon-spawning streams 

flowing into the Sacramento Valley. 0.4

 21.5 Junction with Calif. Hwy 172, which loops south to 

the community of Mill Creek and then northwest to 

rejoin Calif. Hwy 36 at the town of Mineral. West of 

the junction, Calif. Hwys 36 and 89 climb up the south 

flank of Morgan Mountain and cross onto a large patch 

Road Log, Section B
26 Geologic Field-Trip Guide to Lassen Volcanic National Park and Vicinity, California

of till of older glaciations (>130–60 ka). 1.9

 23.4 Passing lane. 0.2

 23.6 Pass from till of older glaciations onto the andesites of 

Maidu Volcanic Center, Stage 1 (~2.4 to ~2.15 Ma). 0.3

 23.9 Pass onto white, hydrothermally altered dacites of the 

Maidu Volcanic Center, Stage 2 (~2.15 to ~2.05Ma). 

This alteration was produced by a shallow fossil 

acid-sulfate magmatic-hydrothermal system (John and 

others, 2005). 0.8

 24.7 Till of older glaciations. 0.2

 24.9 Rest area at Morgan Summit (5,750 feet). 0.1

 25.0 Intersection of Calif. Hwy 36 to Red Bluff and Calif. 

Hwy 89 north to Lassen Volcanic National Park. Refer 

to road log through the Park (Section D) and reset 

odometer to 0.0. 

Section C: Chico to Intersection of Calif. Hwy 32 

with Calif. Hwys 36 and 89

 0.0 U.S. Hwy 99E at Calif. Hwy 32 in Chico. 0.8

 0.8 Forest Avenue. 0.3

 1.1 Stoplight at El Monte Avenue. 0.4

 1.5 Stoplight at Bruce Road. Contact of Sacramento River 

deposits with Tuscan Formation to the east. 0.7

 2.2 Chico city limit sign. 0.6

 2.8 Roadcuts in Tuscan Formation. 0.5

 3.3 Boundary of Chico quadrangle with Richardson 

Springs quadrangle to the north. Microwave tower 

north of highway. 0.3

 3.6 Power line. 0.2

 3.8 Boundary of Richardson Springs quadrangle with 

Paradise West quadrangle to the east. 0.3

 4.1 Roadcuts in Tuscan Formation. 0.9

 5.0 Humboldt Road to east. 2.1

 7.1 View back to right at 5:00 of Sutter Buttes (geologic 

description in Hausback and others, 2011). For next 

10miles, Calif. Hwy 32 is on a narrow bench of Tuscan 

Formation rising gently to the north between Big Chico 

Creek to the west and Little Chico Creek to the east. 0.5

 7.6 Stop C-1: View of Tuscan Formation in Big Chico 

Creek. Large pullout on left with excellent view west 

across Big Chico Creek. The creek is cut into the 

Tuscan Formation, a broad volcanic and volcaniclastic 

wedge of fragmental material emplaced on the west 

slope of the southernmost Cascade Range (Anderson, 

1933; Lydon, 1968; Helley and others, 1981; Helley 

and Harwood, 1985). The Tuscan Formation (~3.5 

to ~2.5Ma), consists primarily of volcanic debris 

flows (lahars), conglomerate, sandstone, and siltstone, 

with minor silicic ash-flow and air-fall tuffs and 

sparse basaltic to andesitic lava flows. For the next 

20.7 miles, Calif. Hwy 32 is mostly on the Tuscan 

Formation. 0.2

 7.8 Santos Road to east. 0.1

 7.9 Small pullout on left with excellent view. 0.2

 8.1 Good view at large pullout on the left. 1.3

 9.4 Dirt road on left leads to some good views of Big 

Chico Creek. 1.4

10.8 Large pullout on left. One can walk down through 

trees and brush for view to west across Big Chico 

Creek. 0.5

11.3 Roadcuts in Tuscan Formation. 0.3

11.6 Historical marker for 14-Mile House. 2.1

13.7 Nopel Ave (main road to community of ForestRanch). 0.2

13.9 Boundary of Paradise West quadrangle with Cohasset 

quadrangle to the north. 1.0

14.9 Schott Road. School to the right. 3.1

18.0 Platt Mountain Road. Forest Ranch CDF fire station 

on right. Good roadcuts in Tuscan Formation for next 

0.5mile. 3.3

21.3 Calaveras Formation in roadcut to left. The Calaveras 

Formation in this region consists of phyllite and 

metachert with discontinuous limestone beds and is 

Carboniferous in age (Hietanen, 1973). 0.6

21.9 Carpenter Ridge Road (130G to west; F-line to east). 1.0

22.9 Good roadcuts in Tuscan Formation. Deeply weathered 

red zone overlies fresher gray Tuscan Formation. 0.8

23.7 Boundary of Cohasset quadrangle with Devils Parade 

Ground quadrangle to the north. 2.2

25.9 Boundary of Devils Parade Ground quadrangle with 

Butte Meadows quadrangle to the east. Good roadcuts 

in Tuscan Formation. 0.8

26.7 Lomo on left. 0.1

26.8 Humboldt Road (U.S. Forest Service Road 27N43) to 

right leads to the community of Butte Meadows and 

then deep into the Yana Volcanic Center, ending at 

Lake Almanor. 0.3

27.1 Small pullout on left with good view to west over 

Tuscan Formation. 0.4

27.5 Highway is along contact of Tuscan Formation to the 

right and Calaveras Formation to the left. For the next 

2.2 miles, the highway follows the eastern contact of a 

window into the Calaveras Formation in the valley of 

Big Chico Creek, just to the west. 0.3

27.8 Tuscan Formation on both sides of highway. Large 

roadcuts in Tuscan Formation for the next 0.5 mile. 0.5

28.3 Pass onto Calaveras Formation. Large roadcuts of 

Calaveras Formation for next 0.6 mile. 1.6

29.9 Leave north end of window of Calaveras Formation 

and pass onto Tuscan Formation. 1.0

30.9 Conspicuous road junction with road to right leading 

to campground at Soda Springs. 0.2

31.1 Pass from Tuscan Formation onto a dome composed 

of andesite of Barkley Mountain. Vent is above and 

to left. This dome is one of a series of andesite and 

basaltic andesite domes along a lineament trending 

310°. The domes were emplaced into and on top of 

the Tuscan Formation at 1.6–1.4 Ma. See discussion at 

mile 38.2, Stop C-2. 0.5

31.6 Pass from andesite of Barkley Mountain back onto 

Tuscan Formation. 2.6
Road Log 27

34.2 Boundary of Butte Meadows quadrangle with Onion 

Butte quadrangle to the north. 0.2

34.4 Big Smoky Creek. 0.8

35.2 Transfer (an old place name). Pass from Tuscan 

Formation onto the basaltic andesite of Transfer, a 

regional calc-alkaline lava that flowed northwest for 

four miles from a vent 1.25 mi to the east. A K-Ar date 

on the basaltic andesite of Transfer is 1,777±124ka 

(M.A. Lanphere, written commun., 1997). 1.8

37.0 Pass from basaltic andesite of Transfer onto Tuscan 

Formation; start descent into valley of Deer Creek. 

For the next 8 miles, there are many good roadcuts in 

massive volcanic breccias of the Tuscan Formation, 

proximal to its source in the Yana Volcanic Center to 

the east. 1.2

38.2 Stop C-2: Overlook of Deer Creek. Turnoff to left 

for views looking up and down Deer Creek (figs. 17 

and 18). 

This stop gives a good view of several of at 

least 34 dacite to basaltic andesite domes, lava 

flows, and shallow intrusions that were emplaced 

into and on top of the Tuscan Formation during the 

early Pleistocene. The features appear to fall on two 

lineaments. The northern lineament, trending 290° 

and including Onion Butte (fig. 18), is composed 

primarily of hornblende dacites with K-Ar dates 

by Marvin Lanphere (written commun., 1997) of 

2,593±29 ka (Onion Butte itself), 2,570±32 ka, and 

2,505±39 ka. The southern lineament, trending 310° 

and including the andesites and basaltic andesites 

of Barkley Mountain (fig. 17), erupted about 

1,600–1,400ka (K-Ar dates of 1,562±50 ka and 

1,440±54ka by M.A. Lanphere, written commun., 

1997). All 34 domes are nearly aphyric or sparsely 

porphyritic; most contain hornblende but lack 

plagioclase, in contrast to most rocks in the Lassen 

area or the Cascade Range. 

Domes of the northern (Onion Butte) lineament 

have high Sr and Sr/Y and MORB-like 87Sr/86Sr values 

(C.M. Stout, written commun., 2015), thus, they are 

adakitic and may be slab or lower-crustal melts.

Domes of the southern (Barkley Mountain) 

lineament have low Fe/O*/MgO and major 

incompatible-element abundances similar to 

basaltic rocks found in the forearc area, but with 

higher 87Sr/86Sr (C.M. Stout, written commun., 

2015), which suggests that these domes are derived 

from a source different from the basaltic rocks or 

that the dome magmas have assimilated crust during 

differentiation. 

The localization of chemically distinct domes of 

different ages on two lineaments suggests tectonic 

control. In the Lassen region, the tectonic stress 

regime is dominated by impingement of Basin and 

Range tectonism on the Cascade Arc, and faults and 

most vent alignments are approximately north-south. 

Mesozoic or Paleozoic

Mesozoic or Paleozoic metamorphic rocks metamorphic rocks

Basaltic andesite Basaltic andesite of Transfer

of Transfer

Andesite and basaltic andesite

domes of Barkley Mountain

Tuscan Formation

Tuscan Formation

Tuscan Formation

Tuscan

Formation

Tuscan Formation Basalt of Deer Creek Basalt of Deer Creek

Basalt of Deer Creek

Basalt of

Deer Creek

Thick flank flows of

Maidu Volcanic Center,

Stage 2

Deer Creek

Figure 17. Panoramic photograph looking at azimuth 290° down Deer Creek from viewpoint just west of Calif. Hwy 32 at mile 38.2. 

Skyline to the north of Deer Creek consists of thick flank flows of Stage 2 of the Maidu Volcanic Center (2,150–2,050 ka). Skyline to the 

south of Deer Creek consists of domes of the andesite and basaltic andesite of Barkley Mountain (1,600–1,400 ka; see text), overlying 

the basaltic andesite of Transfer (1,777±124 ka; K-Ar date by M.A. Lanphere; written commun., 1997). All these units overlie the 

Tuscan Formation (~3.5–2.5 Ma), which in turn rests unconformably on Paleozoic or Mesozoic metamorphic rocks (>65 Ma). Ancestral 

Deer Creek was filled at least three times by flows of tholeiitic basalt, at ~1,800 ka, 1,561±10 ka (40Ar/39Ar age by P.R. Renne, written 

commun.), and ~1,400 ka. At least one of these flows vented from the Onion Butte area (off the photograph to the right; see figure 18). 

Subsequent incision of Deer Creek has left benches of these basalts perched on both sides of the canyon.

Road Log, Section C
28 Geologic Field-Trip Guide to Lassen Volcanic National Park and Vicinity, California

Deer Creek

Onion Butte

Basalt of Deer Creek Basalt of Deer Creek

Basaltic andesite of Onion Butte

Dacite of Onion Butte Dacite of Onion Butte

Maidu Volcanic Center

Stage 1

Maidu Volcanic Center, Stage 2

Tuscan

Formation

Tuscan

Formation

Tuscan

Formation

Tuscan

Formation

Tuscan

Formation

Tuscan

Formation

Tuscan

Formation

Mesozoic or

Paleozoic 

metamorphic 

rocks

Hill

3970 Yana Volcanic Center

Stage 3

Yana Volcanic Center

Hill 5082 Stage 2

Figure 18. Panoramic photograph looking at azimuth 060° up Deer Creek from viewpoint to west of Calif. Hwy 32 at mile 38.2. Skyline 

to the left is dominated by domes of the dacite of Onion Butte (2,600 ka), underlain by the basaltic andesite of Onion Butte, and overlain 

by andesitic flows of the Maidu Volcanic Center, Stage 2 (2,150–2,050 ka). All three units overlie the Tuscan Formation (~3.5–2.5 Ma), 

exposed in the canyon of Deer Creek. Skyline in the center displays flows of Stage 1 of the Maidu Volcanic Center (2,400–2,150 ka; 

vents behind Onion Butte). Skyline on the right displays flows of Stage 2 and Stage 3 of the Yana Volcanic Center (3.4–2.4 Ma; vents out 

of picture to the right).

The lineaments here, however, are oriented at ~60° to 

the arc (approximately parallel to the southern edge 

of the subducted Gorda Plate), and their timing is 

broadly consistent with the passing of the Mendocino 

Triple Junction. In detail, however, such a control is 

not certain because the northern lineament is older than 

the southern lineament. Still, it seems likely that these 

rocks are somehow related to passage of the edge of the 

subducted Gorda Plate beneath the Lassen area. 1.1

39.3 Graded road up Cow Creek to the southeast provides 

access to the Yana Volcanic Center. 1.0

40.3 Bridge to west bank of Deer Creek. 1.7

42.0 Potato Patch Campground on right. 1.5

43.5 Deer Creek Falls. 0.6

44.1 Bridge to east bank of Deer Creek. 0.2

44.3 Boundary of Onion Butte quadrangle with Humboldt 

Peak quadrangle to the east. 0.4

44.7 Bridge to west bank of Deer Creek at Ladder Creek 

Campground. 0.4

45.1 Round Valley Creek on left. 0.2

45.3 Pass from Tuscan Formation onto flows and breccias 

of the Yana Volcanic Center, Stage 3. 2.5

47.8 On left is Slate Creek and its tributary to the north, 

Swamp Creek. 0.3 mi up creek, andesites of Maidu 

Volcanic Center, Stage 1, overlie a sequence of lithic 

pyroclastic flows, lithic and scoriaceous tephra, 

debris-flow deposits, and sediments. The sequence 

is intercalated between the Yana and Maidu Volcanic 

Centers. 0.35 mi further north up Swamp Creek are the 

valley-filling tholeiitic basalts of Mill Creek Plateau 

(140–100 ka), erupted from a vent at the summit of the 

plateau 3 miles to the north-northwest. 0.9

48.7 Entrance on left to Elam Creek Campground. For the 

next 1.3 miles, roadcuts on the right provide excellent 

exposures of the sequence of lithic pyroclastic flows, 

lithic and scoriaceous tephra, lahars, and sediments 

noted at mile 47.8. This sequence lies unconformably 

on dacites of the Yana Volcanic Center, Stage 3, and 

is overlain by andesitic lava flows of Maidu Volcanic 

Center, Stage 1. 0.1

48.8 Stop C-3: Volcaniclastic sequence at base of 

Maidu Volcanic Center. Elam Creek picnic area 

on left. Be very careful to watch for fast-moving 

trucks and cars. Walk north on the road to view a 

large roadcut of the lithic pyroclastic deposits noted 

above, here covered by a veneer of much younger 

gravel conglomerate deposited by ancestral Deer 

Creek. The highway for the next 2.3 miles to the 

north has very narrow shoulders with almost no 

opportunity for parking. Several roadcuts expose the 

volcaniclastic sequence overlain by an andesitic lava 

flow of the Maidu Volcanic Center, Stage 1. 0.2

49.0 Boundary between Humboldt Peak quadrangle and 

Childs Meadows quadrangle to the north. Small pullout 

on left with room for only a few cars. Be very careful 

to watch for fast-moving trucks and cars. Section 
Road Log 29

exposed in roadcut on right consists of monolithologic 

scoria layers, lithic air-fall material, lahars, and 

a monolithologic lithic pyroclastic flow. The 

weathered top of the section is baked and reddened 

by the overlying andesitic lava flow from the Maidu 

Volcanic Center. The flow displays an excellent basal 

breccia overlain by a homogeneous core. 1.1

50.1 Bridge to west bank of Deer Creek. Bluff ahead at 

12:00 to 3:00 consists of the rhyolite of the Mill 

Creek Plateau (1,300–1,200 ka), a thick, 7-km3 flow 

in the Maidu Volcanic Center, Stage 3. Forested 

slopes to the right (east) comprise dacites of the Yana 

Volcanic Center, Stage 3. 1.1

51.2 To left is main logging road to the west side of Deer 

Creek. 0.1

51.3 Bridge across Gurnsey Creek. 0.2

51.5 Low roadcut on left exposes the Rockland tephra, 

part of the Rockland caldera complex of the Lassen 

Volcanic Center. The Rockland tephra, a hypersthenehornblende rhyolite, was erupted at 609±7 ka 

(Lanphere and others, 1999, 2004) from a small 

caldera now filled by younger rocks of the Lassen 

Volcanic Center (fig. 8). 0.1

51.6 Pass from Rockland tephra onto colluvium underlain 

by Rockland tephra. 0.5

52.1 Intersection of Calif. Hwy 32 with Calif. Hwy 36. 

Exposures of Rockland tephra. Turn left onto Calif. 

Hwy 32 and follow the road log of Section B from 

mile 12.4 to the intersection with Sections A and D 

at mile 24.5. Then reset odometer to 0.0 and follow 

Section D north through Lassen Volcanic National 

Park. 

Talus and rockfall

Massive jointing

Thin platy jointing

Thin platy jointing

Crude columnar

 jointing

Glassy and vesicular 

flow top

Section D: Intersection of Calif. Hwys 36 and 89 

North Through Lassen Volcanic National Park to 

the Northwest Park Entrance

0.0 Both Section A (from Red Bluff) and Section B 

(from Chester) end here. Follow Calif. Hwy 89 north 

towards Lassen Volcanic National Park. To the right 

for the next 1.4 miles is the dacite dome of Morgan 

Mountain, one of the dacite domes of the Rockland 

caldera complex. The valley to the left between 

Christie Hill and Morgan Mountain is filled by the 

basaltic andesite of Huckleberry Lake (~300ka), 

which erupted from a vent high on the flank of the 

extinct Brokeoff Volcano. This flow represents a 

return of the regional mafic volcanism after the 

extinction of Brokeoff Volcano about 385 ka. 0.6

0.6 Boundary of Mineral quadrangle with Lassen Peak 

quadrangle to the north. 1.7

2.3 Roadcut to left exposes till of Anklin Meadows 

(~25–17 ka). See “Introduction” and figure 7 for 

glacial chronology. 1.6

3.9	 Stop D-1: Bluff Falls quarry (fig. 19). To visit this 

quarry, park on right, walk 300 ft north, cross the 

highway carefully, and climb up the slope just north 

of the small stream. 

Figure 19. Photograph looking west at andesite flow exposed 

in the face of Bluff Falls quarry, just west of Calif. Hwy 89, 0.8 mile 

south of the south entrance to Lassen Volcanic National Park. The 

glacially carved cliff, modified by quarrying, exposes nearly the entire 

thickness of the andesite of Bluff Falls quarry (467±10 ka), a typical 

silicic andesite flow from the Diller Sequence of Brokeoff Volcano. 

The base of the flow (not exposed here) consists of a thin, reddened, 

scoriaceous flow breccia that is overlain by a massively jointed, faintly 

flow-banded glass-rich zone. This zone grades upward into a thick 

zone of thin platy jointing. In the flow interior, platy jointing is nearly 

horizontal to slightly wavy, but at flow edges it can be ramped upward 

or highly contorted. On the glaciated flanks of Brokeoff Volcano, the 

orientation of platy jointing can be used to map flow edges and flow 

direction. The platy-jointed interior of the flow grades upward into a 

zone of crude columnar joints superimposed on the platy jointing. The 

upper surface of the flow is glassy and vesicular. The color change 

halfway up the cliff marks the base of the natural exposure. Large 

blocks at bottom of the photograph are 1–2 m; cliff is ~50 m high. 

Photograph 12 of Clynne and Muffler (2010).

Road Log, Section D
30 Geologic Field-Trip Guide to Lassen Volcanic National Park and Vicinity, California

This glacially carved cliff exposes nearly the 

entire thickness of the andesite of Bluff Falls quarry 

(467±10 ka), a typical andesite lava flow of the Diller 

Sequence of Brokeoff Volcano. The upper part of 

Brokeoff Volcano, called the Diller Sequence (470–

385ka), is composed of six thick, laterally extensive 

lava flows, generally lacking intercalated fragmental 

deposits. Although different in detail, each unit of the 

Diller Sequence is a porphyritic augite-hypersthene 

andesite containing about 30–40percent phenocrysts, 

dominantly plagioclase, and is difficult to distinguish 

from other units of the Diller Sequence. Sparse olivine 

is commonly present and xenocrystic amphibole rarely 

so. Flows of the Diller Sequence are characterized by 

abundant mm- to cm-size glomeroporphyritic clots 

of the phenocryst minerals, but with slightly different 

compositions. The clots commonly display solid-state 

recrystallization of orthopyroxene from olivine as well 

as exsolution features in pyroxenes; the clots have been 

interpreted as cognate crystals derived from cumulates 

(Clynne, 1990). Larger inclusions with gabbroic bulk 

compositions are sparse, and quenched magmatic 

inclusions like those in Lassen domefield rocks are rare. 

The major-element compositions of lavas in the 

Diller Sequence range from about 58 to 63 percent 

SiO2 (fig. 9) and can be nicely modeled by crystal 

fractionation of average calc-alkaline basalt (Clynne, 

Diamond Peak Landslides in

Mill Creek

Landslides composed of hydrothermally altered rock

Figure 20. Photograph showing landslides in hydrothermally altered volcanic rocks. View looking southeast over active landslides in 

the area south of Pilot Pinnacle (off of the photograph to the left) and northwest of Diamond Peak. Most of the central area of Brokeoff 

Volcano is covered by these young landslides, which form in clay-rich, hydrothermally altered volcanic rock and are most active during 

the annual spring snowmelt. From figure 18 of Clynne and others (2012).

1990) of the Lassen area. Incompatible trace-element 

abundances, however, increase much more rapidly in 

the models than they do in the lava suite, and addition 

of a high-silica component low in incompatible 

elements is required for the models to be viable. 

About 10 percent of a rhyolite melt remarkably 

similar to rhyolite lava flows found in the Lassen area 

satisfies this requirement. 1.6

5.5 Entrance station for Lassen Volcanic National Park. 

Just beyond is the Kohm Yah-mah-nee Visitor Center, 

with exhibits, gift shop, restaurant, and rest rooms. 

The entrance station and the visitor center are located 

on a large landslide that, 3,310±55 years ago, moved 

7 km down Mill Creek from the cliff at the base of 

Brokeoff Mountain. Reactivation of a small part 

of this landslide on January 2, 1997, temporarily 

blocked the park road just south of the entrance 

station. 1.0

6.5 Stop D-2: Sulphur Works. Turn left into the parking 

lot and walk east along Calif. Hwy 89. Sulphur Works 

is an area of fumaroles, boiling pools, mudpots, and 

steaming ground; it is the most accessible of the 

thermal areas in LVNP. Expansion of the thermal area 

has forced removal of a boardwalk that used to lead 

north across the thermal area. Visitors can, however, 

take the first 300 feet of the trail to Ridge Lakes for 

several good overlooks of the thermal area. Most of 
Road Log 31

Mudpot P

Figure 21. Annotated photograph showing changes of hydrothermal features at Sulphur Works from 2004 to 2005, 

2008, and 2009. Photograph taken August 16, 2004, looking at 300° azimuth across Calif. Hwy 89. Dotted white lines 

denote vents that were either enlarged (C) or new (L and M) as of October 12, 2005. Short-dashed black line represents 

the boundary of the large vent resulting from the collapse of the area between vent M and vent C, sometime between 

October 16, 2005, and June 20, 2008, probably in the winter of 2007–2008. Vents A, B, D, and E were essentially 

unchanged from August 2004 to June 2008. By April 2009, the large vent of 2008 had expanded further (solid blue line) 

and had developed into a vigorous mudpot just north of the fence and sidewalk (see fig. 22). Red dashed line represents 

north-south alignment of vents in 2009. 

Figure 22. Photograph taken in 

November 2011 of mudpot P (see fig. 21) 

north of Calif. Hwy 89 at Sulphur Works. 

The fence shown in figure 21 was just to 

the left of the curb at the lower right of 

the photograph.

D

E

B

L

October 12, 2005

and June 20, 2008

M

October 12, 2005

C

August 16, 2004

A

Peak 8662

Tree fallen on

June 20, 2008

Tree fallen on 

June 20, 2008

Tree leaning 

to left on

June 20, 2008

and April 21–22, 2009

N–S alignment

of vents

Outline of composite vent

April 21–22, 2009

Outline of composite vent 

June 20, 2008

Steaming

October 12, 2005

C

October 12, 2005

Mudpot P

April 21–22, 2009

Road Log, Section D
32 Geologic Field-Trip Guide to Lassen Volcanic National Park and Vicinity, California

the area above Sulphur Works north to Mount Diller 

comprises dozens of old landslides of hydrothermally 

altered lava flows and pyroclastic rocks (fig. 20).

Temporal and spatial variation of fumaroles has 

been observed in all thermal areas of Lassen Volcanic 

Park, but it has been most systematically documented 

in recent years at Sulphur Works. From 2004 to 2011 

(fig. 21), fumarolic activity north of Calif. Hwy 

89 migrated south towards the road and became 

concentrated in a large boiling mudpot adjacent to 

the sidewalk curb (fig. 22). In addition, borings into 

the roadbed by the Federal Highway Administration 

in April 2009 and November 2011 demonstrated 

that boiling temperatures have risen to a depth of 

15–20 ft (5–6 m) under Calif. Hwy 89, compromising 

the integrity of the roadbed. This situation led to 

extensive road repairs in the spring of 2013, the most 

recent of several road reconstructions across this 

treacherous area over the past 75 years. 1.4

7.9 Stop D-3: Diamond Peak. Pull far off the road into 

the parking area. Be careful crossing the road to 

examine the roadcut because visibility is restricted 

on this corner. 

West Sulphur Creek

Andesite of Glassburner Meadows Andesites of Mill Canyon

Hydrothermally altered

andesites of Mill Canyon

Thick breccia unit

Dacite of 

Twin Meadows

Brokeoff Mountain

Figure 23. Photograph of Brokeoff Mountain looking west from turnout on Calif. Hwy 89 south-southwest of Diamond Peak, 

approximately at the center of Brokeoff Volcano. Foreground to middle ground consists of incipiently to strongly altered lava flows and 

fragmental deposits of the Mill Canyon sequence, with many postglacial (<15 ka) landslides. Brokeoff Mountain forms the southwest rim 

of Brokeoff Volcano and consists primarily of the dacite of Twin Meadows (470±10 ka), the uppermost unit of the Mill Canyon sequence. 

The overlying andesite of Mount Diller (387±10 ka), part of the Diller sequence, crops out just beyond the summit of Brokeoff Mountain 

and is not visible in this photograph. The andesite of Glassburner Meadows (~450 ka) is also part of the Diller sequence. Photograph 6 of 

Muffler and others (2010).

This viewpoint is near the center of Brokeoff 

Volcano. Virtually all the rocks that can be seen 

(fig.23) are at least incipiently altered, and most 

areas in the core of the volcano are strongly to 

totally altered to propylitic and argillic assemblages 

related to fossil magmatic-hydrothermal systems at 

temperatures of 200 to >300°C (Crowley and others, 

2004; John and others, 2006). Unaltered rock is 

preserved only in dense, glassy, basal parts of lava 

flows. Flows and breccias on Diamond Peak dip 

steeply eastward, indicating that the summit vent 

of Brokeoff Volcano at the time of their eruption 

was nearby to the west. To the south, in the glacial 

valley of Mill Creek, the base of Brokeoff Volcano 

is exposed where it overlies rocks of Maidu Volcanic 

Center. Mill Canyon and the flanks of Brokeoff 

Mountain expose rocks representing nearly the 

complete history of Brokeoff Volcano. Thick flank 

flows of olivine-augite and hypersthene-augite 

andesite can be correlated across Mill Canyon with 

no offset by faulting. The triangle-shaped peak of 

Brokeoff Mountain to the west is resistant to erosion 

by virtue of the thick flow of pyroxene-hornblende 
Road Log 33

dacite (the dacite of Twin Meadows; 470±10 ka) that 

ended the Mill Canyon sequence (590–470 ka). To 

the southeast, flows and breccias of the Mill Canyon 

sequence form most of Mount Conard. The cliff 

forming the prominent shoulder above Mill Canyon is 

a thick hornblende-biotite rhyodacite lava flow of the 

Bumpass sequence of the Lassen domefield. This flow 

followed the course of a canyon cut in rocks of the 

Mill Canyon sequence. The rhyodacite flow is dated 

at 298±9 ka and illustrates the amount of erosion in 

the core of Brokeoff Volcano since that time. 

The exposure in this roadcut is typical of the 

Mill Canyon sequence (590–470 ka), the lower part 

of Brokeoff Volcano. The Mill Canyon Sequence 

is composed of packages of thin lava flows and 

intercalated fragmental deposits, including agglomerates 

(like this exposure), breccias, and lahars. Adjacent 

packages of flows can have very different phenocryst 

abundances, phenocryst assemblages, and bulk 

composition. The lavas are mostly a diverse array 

of porphyritic andesites. Phenocryst assemblages 

are generally plagioclase-dominated and include 

combinations of olivine, augite, and hypersthene. 

Sparse porphyritic dacites have augite-hypersthene 

or hypersthene-hornblende assemblages. Abundant 

evidence for magma mixing includes resorption of 

phenocrysts, as well as reverse zoning, overgrowth rims, 

and compatible-element spikes in mafic phenocrysts. 

Small glomeroporphyritic clots of the phenocryst 

minerals are present in some Mill Canyon lavas, but 

are never as abundant as in the Diller Sequence. Larger 

inclusions with gabbroic bulk compositions are sparse, 

and quenched magmatic inclusions like those in Lassen 

domefield rocks are absent. 

The major-element compositions of lavas of 

the Mill Canyon sequence range from about 54 

to 68percent SiO2 (fig. 4), although most are 56–

60percent SiO2. As with the Diller Sequence, they can 

be nicely modeled by crystal fractionation of average 

Lassen area calc-alkaline basalt (Clynne, 1990) (see 

discussion at Stop D-1). The lavas and fragmental 

deposits of the Mill Canyon sequence evolved in a 

magmatic system that processed small increments of 

magma and thus erupted a diverse array of lithologies. 

Compared to the Mill Canyon Sequence, the largervolume eruptions, less heterogeneous lithologies, and 

less evidence for mineralogic disequilibrium argue 

that the Diller Sequence erupted from a magmatic 

system that was more mature and better integrated 

than during Mill Canyon time. 

Pullouts over the next 2 miles provide excellent 

views of the east wall of Little Hot Springs Valley. 

Prominent are thin to thick lava flows of the andesites 

of Mill Canyon, part of the Mill Canyon sequence 

in the interior of Brokeoff Volcano. Permeable 

fragmental deposits are preferentially altered to light 

colors compared to the dark cores of lava flows. 

Overlying the andesites of Mill Canyon is the lessaltered andesite of Mount Diller (387±10 ka, part of 

the Diller sequence). Steam vents and acid springs are 

abundant in landslide deposits along the floor of Little 

Hot Springs Valley. 2.6

10.5 White, kaolinite-rich altered andesite of the Mill 

Canyon sequence of Brokeoff Volcano at sharp bend 

in road. 0.6

11.1 Just before the south end of Emerald Lake, the road 

crosses the contact between the uppermost preserved 

rocks of Brokeoff Volcano and the augite-hornblende 

dacite dome of Ski Heil Peak (244±10 ka; part of the 

Bumpass sequence of the Lassen domefield). Straight 

ahead, Eagle Peak is a hornblende-biotite rhyodacite 

dome (66±4 ka; part of the Eagle Peak sequence of 

the Lassen domefield) that was preceded by a lava 

flow (the cliffs ringing the base of the dome) and a 

pyroclastic flow (which mantles Ski Heil Peak and is 

exposed distally just northwest of LVNP). 0.4

11.5 Stop D-4: Bumpass Hell parking lot. Walk out to 

the south end of the parking lot for excellent views of 

Brokeoff Volcano and the Bumpass and Eagle Peak 

sequences of the Lassen domefield. The parking lot is 

built on the contact between lava flows of the Diller 

sequence of Brokeoff Volcano and overlying dacites 

of the Bumpass sequence. Note the glacial striations 

and glacial boulders; the prominent 4-m boulder is 

hornblende-augite dacite from Bumpass Mountain. 

To the south is the eroded core of Brokeoff 

Volcano, with the Maidu Volcanic Center in the 

distance. To the east is the hornblende-augite dacite of 

Bumpass Mountain (232±8 ka). All the dacite domes 

of the Bumpass sequence have suffered extensive 

glacial erosion. Although the overall shapes of the 

domes have not been greatly modified, no trace of an 

outer glassy or pumiceous carapace or talus mantle 

is preserved. The view to the north is dominated by 

the glaciated dacite dome of Lassen Peak (27±1 ka; 

Turrin and others, 1998). 

A gentle, level, 1.2-mi (2 km) trail winds 

southeast from the parking lot to Bumpass Hell, the 

major thermal area in this part of the park. The trail 

lies along the contact between the Diller sequence 

and the Bumpass sequence. Shortly after leaving 

the parking lot, one can see well-preserved glacial 

striae and polish on the massive dacite of Bumpass 

Mountain dome. The middle part of the trail has been 

blasted from the massively jointed devitrified interior 

of the dome. At the viewpoint where the trail turns 

east, 0.55 mi (0.85 km) from the parking lot, andesite 

of the Diller sequence is exposed, and a Park Service 

exhibit shows a model of Brokeoff Volcano before 

erosion. Farther along the trail, explosion breccia 

related to emplacement of the dacite of Bumpass 

Mountain is prominently exposed. 

Road Log, Section D
34 Geologic Field-Trip Guide to Lassen Volcanic National Park and Vicinity, California

Bumpass Hell (see photograph on page 1 of 

Clynne and others, 2003) marks a major thermal 

upflow (37±5megawatts; Sorey and Ingebritsen, 

1995) of the Lassen geothermal system. Bumpass 

Hell is located at the vent for the dacite of Bumpass 

Mountain along the contact with the Diller sequence 

of Brokeoff Volcano. 

Bumpass Hell contains numerous superheated 

fumaroles (Muffler and others, 1982), one of which 

had a temperature in the summer of 1988 of 161.4 °C 

(Clynne and others, 2003). Boiling of the 240 °C hot 

water in the reservoir provides the steam that feeds the 

surface features. Approximately 75 major fumaroles, 

acid-sulfate hot springs, and mudpots, plus myriad 

smaller features, occur in an area of approximately 

0.13km2 that is intensely altered to an aggregate of 

opal and kaolinite. Much of the surface of the active 

part of Bumpass Hell is covered with orange and 

yellow sulfates; many steam vents are lined with 

bright-yellow native sulfur. Pyrite is common as 

linings of the vents and discharge channels, as scum 

floating on the surface of pools, and as dispersions in 

gray or black mudpots. 

The acid-sulfate water from Bumpass Hell 

is typical of hot springs related to a vapordominated reservoir in having low pH, high sulfate 

concentrations, and no significant Cl. Most other 

thermal areas in LVNP have thermal features and 

chemistry similar to Bumpass Hell and are also 

manifestations of the vapor-dominated reservoir. 

Some springs just below Sulphur Works (see 

photograph on page 4 of Clynne and others, 2003), 

in upper Little Hot Springs Valley, and at Drakesbad 

are relatively rich in HCO3

- (bicarbonate) and deposit 

travertine (calcium carbonate) or a mixture of silica 

and travertine. These springs are interpreted to be 

surface discharge from the zone of steam condensate 

that overlies the vapor-dominated reservoir (Muffler 

and others, 1982). 0.6

12.1 For the next 0.4 mile the road passes through the 

dacite of Mount Helen (249±12 ka), a lava dome of 

the Bumpass Sequence. This glaciated dome contains 

very abundant quenched mafic inclusions (Feeley 

and others, 2008b). See Stop D-6 (Chaos Crags) for a 

discussion of the origin and significance of quenched 

mafic inclusions. To the west is Lake Helen, a cirque 

lake of Anklin Meadows age (see fig. 11). 0.7

12.8 Lassen Peak parking lot. The large parking lot marks 

the start of an excellent trail that gains 1,950 ft 

(600m) in elevation as it switchbacks for 2.5 mi (4 

km) to the summit of 27±1-ka Lassen Peak (Eagle 

Peak sequence of the Lassen domefield) at 10,457 

ft (3,187 m). Although outer, pumiceous material 

has been removed by glaciation, Lassen Peak is still 

an impressively large dacite dome with a volume 

approaching 2km3. The dacite of Lassen Peak has 

an unusual and distinctive phenocryst assemblage 

consisting of plagioclase, hornblende, biotite, quartz, 

augite, and olivine. The sparse augite and olivine 

were derived from disaggregation of the abundant 

quenched mafic inclusions in the dacite (see petrologic 

discussion of quenched mafic inclusions at Stop D-6). 

Paleomagnetic work by Duane Champion (Turrin and 

others, 1998) indicates that Lassen Peak was emplaced 

over a short period of time, perhaps as little as 5 years. 

At the summit of Lassen Peak (fig. 24), one can 

observe the 1915–17 eruption craters and deposits 

(see 1:2,500-scale geologic map of the summit area 

in Christiansen and others, 2002) and on a clear day 

have good views of the Sierra Nevada, Sacramento 

Valley, Coast Ranges, Klamath Mountains, Mount 

Shasta, and the Medicine Lake Highlands. To the 

west of the parking lot is the hornblende-augite dacite 

dome of Lassen Peak parking lot (260–250 ka), part 

of the Bumpass sequence. Glacial erosion has exposed 

intricate flow banding in the margin of the dome. On 

the east side of the parking lot, a small tuff cone marks 

the vent for the hornblende-biotite rhyodacite lava flow 

and pyroclastic flows of Kings Creek (35±1 ka), part of 

the Eagle Peak sequence. 0.2

 13.0 This pass at 8,511 ft (2,594 m) is the crest of the Cascade 

Range in this area. To the right is the glaciated biotitehornblende dacite dome of Mount Helen (249±12 ka), 

rich in quenched mafic inclusions. On the left is the 

dacite flow of Kings Creek. Much of the pumiceous 

glassy carapace and structure of the flow is preserved 

because it has not been heavily glaciated. Directly 

ahead, the next roadcut exposes thick breccia at the 

base of the flow. To the right beyond Mount Helen is 

the hornblende-augite dacite dome and flow complex of 

Bumpass Mountain (232±8 ka) and a short stubby biotitehornblende dacite lava flow from Mount Helen. 0.05

 13.05 Boundary of Lassen Peak quadrangle with Reading 

Peak quadrangle to the east. 0.65

13.7 View of Reading Peak, a deeply glaciated, dacite dome 

complex (212±5 ka) of the Bumpass sequence. Before 

being eroded by glaciers, the Reading Peak complex 

was probably bigger than Lassen Peak. To the right 

and in the distance behind Reading Peak are Mount 

Harkness (a late Pleistocene basaltic andesite shield 

volcano), Kelly Mountain (a remnant of the 2.4–1.3Ma 

Dittmar Volcanic Center), and Sifford Mountain, a 

basalt to basaltic andesite shield volcano (172±23 ka 

and 167±4 ka). Sifford Mountain is the southernmost 

young regional volcano in the Cascade arc and marks 

the approximate southern limit of active volcanism in 

the Lassen area. Over the past 2.5–3 m.y., the southern 

limit of active volcanism has migrated 27km to the 

north from the vicinity of the Yana Volcanic Center 

(about 1 cm/yr). The broad flat plateau to the east of 

Sifford Mountain is a thick lava flow that is part of the 

dacite of Bumpass Mountain (232±8 ka). 0.7
Road Log 35

May 22, 1915 tephra

Person

for scale

May 19–20, 1915 flow filling May 19, 1915 explosion crater

Dacite of 

Lassen Peak Dacite of 

Lassen Peak

North-northwest rim

of 1917 crater

May 22, 1915 crater

Remnant of

May 14–19, 1915

lava dome

Remnant of

May 14–19, 1915

lava dome

Figure 24. Photograph of the May 22, 1915, crater at the summit of Lassen Peak. View looking north-northwest from the summit of 

Lassen Peak at deposits from the summit eruption of 1914–1917 (Clynne and others, 2014; Christiansen and others, 2002). Black crags on 

left and right skyline are remnants of the dacite dome erupted May 14–19, 1915, filling the May 30, 1914–May 14, 1915, crater. Craggy area 

in the foreground to middle ground is the May 19–20, 1915, lava flow that filled the crater created when the lava dome was disrupted on 

the late evening of May 19, 1915. Crater (now snow-filled) in the middle to upper ground resulted from the pyroclastic eruption of May 22, 

1915. Troughs (also snow-filled) in the lava flow resulted from partial slumping of the still-hot lava flow into the May 22 crater. Light-gray 

patches on the lava flow are tephra from the May 22, 1915, eruption. Gray walls of the May 22 crater are formed of weakly hydrothermally 

altered dacite of Lassen Peak (27±1 ka) and are partially covered by yellow sulfur deposits related to the 1915 eruption. Rim of crater 

formed by phreatic eruptions in 1917 visible on the center skyline. Modified from photograph 23 of Muffler and others (2010).

 14.4 A set of linear cracks extends from here northwest 

across the Kings Creek lava flow, but cannot be 

traced into adjacent rock units. The en echelon 

segments define a zone of extension 1.1 km long. 

Segment widths range from a few meters to a 

few tens of meters, and there is no vertical offset. 

Opening took place after the Chaos Crags eruption 

at 1,103±13 B.P. (~850 C.E.) and before the 1915 

eruption of Lassen Peak. The most likely origin of 

the cracks is extension above a propagating dike that 

did not quite reach the surface. The cracks might be 

related to the 1915 eruption of Lassen Peak. 1.8

 16.2 Excellent view of Lassen Peak at 12:00, Mount 

Helen at 10:30, and Bumpass Mountain at 9:30. 

The shoulder to the left of Bumpass Mountain is the 

Diller sequence of Brokeoff Volcano. Further to the 

left is Mount Conard, composed of andesites of the 

Mill Canyon sequence. 2.0

 18.2 Cross Kings Creek. To the left is the 212±5 ka dacite 

dome of Reading Peak. To the right is the large dacite 

flow (232±8 ka) that vented at Bumpass Mountain. 

The trail to Kings Creek Falls begins here and 

winds through delightful meadows and past beautiful cascades as Kings Creek descends through the 

thick dacite flow. Kings Creek Falls is just beneath 

the dacite flow, where the creek plunges over a thick 

lava flow of the augite-hypersthene andesite of Rice 

Creek (485±12ka and 477±14 ka), part of the Diller 

Sequence of Brokeoff Volcano. 0.8

 19.0 The skyline to the east, visible through the trees for 

the next 2.7 miles, is the Caribou Volcanic Field, an 

intensely glaciated area of over a hundred, mostly 

small, late Pleistocene basalt to andesite volcanoes. 

Volcanism in the Caribou Volcanic Field began at 

~425 ka and was especially active from 400 to 300 ka 

and from 110 ka to the present. Conspicuous volcanic 

peaks in the Caribou Volcanic Field are Red Cinder, 

North Caribou, and South Caribou. 1.3

 20.3 Elevation 7,000 ft (2,134 m). For the next 0.75 mile, 

volcanic vents of the Central Plateau are visible to 

the right through the trees. Rocks of the Central 

Plateau are all hybrid andesites of the Twin Lakes 

Road Log, Section D
36 Geologic Field-Trip Guide to Lassen Volcanic National Park and Vicinity, California

Lassen Peak

Crescent

Crater

Forest stripped

 by avalanche of

 May 19–20, 1915

Forest stripped

 by pyroclastic flow of

 May 22, 1915

Figure 25. Photograph of the northeast flank of Lassen Peak taken by B.F. Loomis in June 1913, before the 1914–1917 

eruption. The photograph looks southwest and shows the forest subsequently stripped by the eruptions of May 1915 

(see fig. 19). Annotations by L.J.P. Muffler and M.A. Clynne.

sequence that erupted from vents on the periphery of 

the more central Bumpass and Eagle Peak sequences. 

The hybrid andesite magmas formed by mixing 

approximately equal amounts of dacite magma from 

beneath the domefield and calc-alkaline basaltic 

magma responsible for the regional volcanism. 

Hybrid andesite lava flows are thick, cover large 

areas, and are characterized by the coexistence of 

olivine and quartz. These rocks were called quartz 

basalts by Finch and Anderson (1930) and by 

Williams (1932). The two youngest volcanic events 

in Lassen Volcanic National Park, Cinder Cone 

(1666 C.E.) and the 1915 eruption at the summit of 

Lassen Peak, are hybrid andesites of the Twin Lakes 

sequence. At 12:00 is Hat Mountain, a ~50–40 ka 

hybrid andesite. Most of the Central Plateau visible 

from here consists of the thick lava flows from Hat 

Mountain. Exposed along the road is monolithologic 

till derived from Reading Peak. 1.5

 21.8 Summit Lake. 0.5

 22.3 Ranger Station on East Fork Hat Creek. For the next 

2.5 miles, the road passes through till of younger 

glaciations (35–17 ka) derived from Reading Peak, 

with sporadic outcrops of a 193±11 ka lava flow, the 

dacite of Dersch Meadows, which is the youngest 

dacite of the Bumpass sequence. 1.5 

 23.8 Boundary of Reading Peak quadrangle with West 

Prospect Peak quadrangle to the north. 0.9

 24.7 West Fork Hat Creek. 0.4

 25.1 Stop D-5: Devastated Area parking lot. From the 

Devastated Area parking lot, first look southwest 

towards Lassen Peak. The May 1915 eruptions at 

the summit of Lassen Peak destroyed a 3-mi2 (8 km2) 

area now called the Devastated Area (Clynne and 

others, 2014). The mature conifer forest that covered 

the area (fig. 25) was completely destroyed by the 

eruptions of May 1915 (figs. 26 and 27). Compare 

figure 27 with the vigorously growing trees that now 

cover the area. 

Next, follow the gentle trail to the north of the 

parking lot to observe the deposits from the May 

1915 eruptions. Note that the large “Hot Rock” in the 

Loomis photographs (figs. 26 and 27) can still be seen 

adjacent to the trail, but views of the Devastated Area 

are now obscured by trees. 

Modern field and laboratory studies (Clynne, 

1999; Christiansen and others, 2002) have produced a 

revised scenario of the May 1915 eruptions of Lassen 

Peak (fig. 28). 

The eruptive sequence began on May 30, 1914, 

with a phreatic explosion at the top of Lassen Peak. 

By mid-May of 1915, more than 180 steam explosions 
Road Log 37

May 19–20 debris flow

Lassen Peak

Crescent 

Crater

“HOT ROCK”

Forest stripped by 

avalanche of May 19–20, 1915

May 19–20

lava flow 

Avalanche and debris-flow deposits 

of May 19–20, 1915

Figure 26. Photograph of the Devastated Area taken by B.F Loomis on the morning of May 22, 1915, before the 

Plinian eruption of ~4:00 pm on May 22, 1915. The photograph shows the path of the avalanche and debris flow of 

May 19–20. Note that the northeast lobe of the May 19–20 lava flow at the summit of Lassen Peak is still in place. 

Annotations by L.J.P. Muffler and M.A. Clynne.

 Remnant of May 19–20 lava flow 

Pyroclastic flow

Avalanche and debris-flow 

deposits of May 19–20 

“HOT ROCK”

Figure 27. Photograph showing the Devastated 

Area after the eruption of May 22, 1915. Taken 

by B.F. Loomis in June 1915, the photograph 

shows the path of the pyroclastic flow produced 

by the Plinian eruption of ~4:00 pm on May 22, 

1915. Note the much wider zone of devastation 

compared to that of May 19–20 (fig. 26). Also, 

the May 19–20 lava flow high on the northeast 

flank of Lassen Peak is almost entirely gone. 

Annotations by L.J.P. Muffler and M.A. Clynne.

Road Log, Section D
38 Geologic Field-Trip Guide to Lassen Volcanic National Park and Vicinity, California

Spring of 1917

•	 Steam blast activity excavates a new 

crater at the summit of Lassen Peak

May 22, 1915

•	 Small debris flows on all flanks of Lassen 

Peak

•	 Explosive eruption of dacite pumice 

generates a pyroclastic flow on the 

northeast flank of Lassen Peak; melting of 

snow transforms the pyroclastic flow into a 

fluid debris flow that goes down Lost Creek 

and generates a second flood in Hat Creek 

Valley; ash falls as far east as Elko, Nevada

May 19–20, 1915

•	 Dacite lava flow emplaced at the summit of 

Lassen Peak

•	 Explosive disruption of the dome generates 

an avalanche of hot dacite and snow 

down the northeast flank of Lassen Peak; 

melting of snow transforms the avalanche 

to a debris flow that goes north along Lost 

Creek and causes a flood in the Hat Creek 

Valley

May 14–19, 1915

•	 Black lava dome fills crater

May 30, 1914 to mid May, 1915

•	 More than 180 steam blast eruptions 

excavate a crater ~300 m (1,000 ft) in length 

at the summit of Lassen Peak

Figure 28. Chart giving a summary of events 

in the 1914–1917 eruptions of Lassen Peak. In 

stratigraphic order: earliest event at bottom; 

latest event at top. From Clynne and others (2014).

had blasted out a 300-m-long crater at the summit. 

In the week before May 19, 1915, a small dacite lava 

dome filled the crater. On the late evening of May 19, 

1915, the growing dacite dome was disrupted by a 

large explosion. Hot blocks of lava thrown onto the 

snow-covered upper flanks and summit of Lassen Peak 

initiated an avalanche of snow and rock that swept 

down the steep northeast face of Lassen Peak, over the 

low till ridge northeast of Emigrant Pass, and into Hat 

Creek (fig. 29). The avalanche was quickly followed 

by a debris flow generated by melting of snow by the 

hot dacite lava blocks and incorporation of underlying 

loose rock debris. The debris flow followed the same 

path as the avalanche until it encountered the low till 

ridge northeast of Emigrant Pass, where it was deflected 

west into Lost Creek, and flowed another 10km to 

about 3 km west of Twin Bridges (the confluence of Hat 

and Lost Creeks). During the early morning hours of 

May 20, 1915, the debris-flow and avalanche deposits 

released large volumes of water that caused a flood 

along Hat Creek north of Old Station. Also in the late 

evening and early morning of May 19–20, 1915, dacite 

lava, which was less viscous than that erupted during 

the previous week, welled up into and filled the newly 

excavated crater. It spilled over two low spots on the 

rim and emplaced two short lava flows on the steep 

west and northeast flanks of Lassen Peak. 

The eruptive sequence culminated two days later at 

approximately 4:00 p.m. on May 22, 1915. A vertically 

directed column of pumice and gas blasted through the 

May 19–20 lava flow, created a new crater, and rose to 

30,000 ft (6 mi; 9 km). Partial collapse of the eruption 

column initiated a pyroclastic flow on the northeast 

slope of Lassen Peak, which rapidly incorporated and 

melted snow in its path. By the time the pyroclastic flow 

reached the lower Devastated Area, it had transformed 

into a fluid debris flow that traveled down Lost Creek 

to Twin Bridges, where it released water and caused 

a second flood along Hat Creek north of Old Station. 

Fallout from the eruption cloud emplaced a pumice 

deposit on the upper slopes of Lassen Peak and a thin 

fallout lobe directed northeast towards Cinder Cone. 

Ash fall was recorded as far away as Elko, Nevada, 280 

mi (450 km) to the east. Pumice that fell on remaining 

snow-covered areas high on Lassen Peak generated six 

viscous debris flows that were emplaced on the west, 

north, and east flanks of Lassen Peak. The May 19–20 

lava flow on the northeast flank of Lassen Peak was 

removed in this eruption and incorporated into May 22 

deposits, and the still-hot lava at the summit partially 

slumped back into the May 22 crater (fig. 24). Only a 

small, dark fragment of this flow can be seen from the 

Devastated Area parking lot in the notch at the top of 

Lassen Peak (fig. 30). In contrast, the entire May 19–20 

lava flow on the west flank of Lassen Peak can be seen 

today (fig. 31). 
Road Log 39

0 MILE

0

1

1 KILOMETER

Lassen 

Peak

Lassen Peak 

Trail

Chaos 

Crags 

domes

Sunflower 

Flat domes

Limit of May 22,

1915, lahar

Approximate limit of

May 19–20, 1915 lahars

Approximate

limit of May

19, 1915 

avalanche

Eagle

Peak 

DEVASTATED

AREA 

DEVASTATED

AREA 

89

89

44

Emigrant

Pass 

Hot Rock 

Raker 

Peak

Twin 

Bridges

Old Station

May 22, 1915 Deposits

Pyroclastic flow and 

 related lahars

May 19-20, 1915 Deposits

Lava flows and dome

Avalanche and lahar

Lassen Volcanic 

 National Park 

 boundary

Lassen Peak lava dome

California state

 highway

27,000 years old

Tree blow-down limits

May 22, 1915

May 19–20, 1915

EXPLANATION

Lost Creek

Hat Creek

Hat Creek

West Fork Hat Creek

89

89

Area

of

map

NATIONAL PARK

BOUNDARY

LASSEN

VOLCANIC

N

Figure 29. Generalized 

geologic map of deposits 

from the May 1915 

eruptions at the summit 

of Lassen Peak. Map 

adapted from Clynne 

and others (2014), in turn 

taken from plate 1 of 

Christiansen and others 

(2002) and plate 1 of 

Clynne and Muffler (2010).

Road Log, Section D
40 Geologic Field-Trip Guide to Lassen Volcanic National Park and Vicinity, California

For several years after the 1915 eruption, spring 

snowmelt percolating into Lassen Peak encountered 

hot rock and triggered steam explosions. Particularly 

vigorous phreatic explosions in May 1917 blasted out 

the northwestern of the two craters at the summit of 

Lassen Peak and emplaced phreatic deposits high on 

the west flank of Lassen Peak. The craters continued 

to emit substantial amounts of steam into the 1950s. 

Today, on cold days one can still find a few weakly 

steaming vents. Despite the widespread effects of the 

1915 eruption, the total volume erupted was small, 

about 0.03 km3.

In the May 1915 eruptions, four types of volcanic 

rock were produced: 

• a black dacite (64–65 percent SiO2) erupted 

as a lava dome between May 14 and 19 

and as a lava flow (64–65percent SiO2) 

emplaced on May 19–20 (fig. 32A)

• unbanded light-colored pumice (64–68 

percent SiO2) erupted on May 22 (fig. 32B)

Figure 30. Low-angle oblique aerial photograph looking southwest at Lassen Peak and the proximal Devastated Area. Terrain with sparse 

trees marks the paths of avalanche and debris-flow deposits of May 19–20, 1915, and pyroclastic-flow and fluid debris-flow deposits of 

May 22, 1915 (Clynne and others, 2014; Christiansen and others, 2002). Small dark crags just to right of the summit are remnants of the May 

19–20, 1915, lava flow. The composite dacite dome of Lassen Peak (27±1 ka) dominates the upper part of the view. Lithic pyroclastic-flow 

deposit from partial collapse of the dome of Lassen Peak is exposed in the canyon of the headwaters of Lost Creek in center of view. Ridges 

flanking central area are glacial moraines thinly covered by deposits of the 1915 eruption of Lassen Peak (Christiansen and others, 2002). A 

small permanent snowfield is seen on the left lower slope of Lassen Peak. Area to left of the snowfield is the rhyodacite lava flow of Kings 

Creek (35±1 ka, part of the Eagle Peak sequence). Dacite domes of Bumpass Mountain (232±8 ka), Crescent Crater (236±1 ka), hill 8283 

(261±5 ka) and Loomis Peak (~300 ka) are part of the Bumpass sequence. Photograph 15 of Muffler and others (2010).

Lost Creek

Remnant of May 19–20, 1915 lava flow

Rhyodacite flow Lassen Peak

of Kings Creek

Proximal Devastated Area

Lithic pyroclastic flow 

from partial collapse of 

the dome of Lassen Peak

Moraine thinly mantled 

by 1915 deposits

Moraine thinly mantled 

by 1915 deposits

Coast Ranges

Northern

Sacramento 

Valley

Crescent Crater

Loomis Peak Bumpass 

Mountain

Hill 8283

• compositionally banded pumice with 

dark (60–61percent SiO2) and light bands 

(64–68 percent SiO2) erupted on May 22 

(fig. 32C)

• quenched inclusions of andesitic magma 

(57–60percent SiO2) that are abundant 

in the black lavas and sparse in the light 

pumice (fig. 32D). 

The light bands in the banded pumice are 

generally compositionally equivalent to the black 

dacite, and the dark bands are compositionally 

equivalent to the quenched andesitic magma. 

The 1915 rock types contain a complex mineral 

assemblage that indicates mixing between two 

compositionally distinct magmas: hornblende-biotite 

rhyodacite (approaching 70 percent SiO2) and olivine 

basaltic andesite (~54 percent SiO2). Each of the 

1915 rock types is formed from different proportions 

of these two magmas through a complex series of 

interactions (Clynne, 1999). Intrusion of basaltic 
Road Log 41

Figure 31. Telephoto view northeast from summit of Brokeoff Mountain looking at Lassen Peak (27±1 ka) and the west lobe of the dacite 

flow of May 19–20, 1915. The dacite dome of Eagle Peak (66±4 ka) is part of the Eagle Peak sequence. The dacite dome of Ski Heil Peak 

(244±10 ka, part of the Bumpass sequence) is mantled by tephra from the eruption of Eagle Peak. Ridge in the foreground is composed of 

the altered andesite of Mount Diller (387±10 ka), part of the Diller sequence of Brokeoff Volcano. Photograph 21 of Muffler and others (2010).

Lassen

Peak

Dacite of Lassen Peak Dacite of Lassen Peak

Dacite of Lassen Peak

Andesite of Mount Diller

Dacite of Ski Heil Peak

May 19–20, 1915

flow

Rhyodacite of Eagle Peak

Tephra from

Eagle Peak

andesite heated and mobilized rhyodacitic hornblendebiotite crystal mush. Formation and disaggregation 

of partially crystalline quenched inclusions of the 

hydridized basaltic andesite played a critical role 

in forming the array of compositions erupted. It is 

likely that the intrusion and magma-mixing event that 

produced these rock types also provoked the eruptions. 

As one leaves the Devastated Area parking lot, 

the cliff ahead and to the right is the rhyolite of Raker 

Peak (588±69 ka), a dome of the Rockland caldera 

complex. The front face of the dome is glaciated, 

exposing the internal flow structure of the dome. A 

much younger hybrid andesite volcano, Raker Peak 

(older Twin Lakes sequence), was built on top of the 

rhyolite dome at 270±18 ka. 

For the next 1.8 miles, the road crosses 1915 

debris flows, then a recessional moraine of the till of 

Anklin Meadows (25–17 ka), and then drops down 

onto the 1915 debris flows again. 1.4

26.5 To the left, 300 feet down a locked service road, the 

Lost Creek sand pit exposes the two lower pyroclastic 

flows of Chaos Crags. The sand pit also provides 

excellent views to the south of Lassen Peak (behind 

Crescent Crater), several highly glaciated domes of the 

Bumpass Sequence, and four of the six Chaos Crags 

domes (behind a large lateral moraine of the till of 

Anklin Meadows). 

The Chaos Crags eruption at 1,103±13 B.P. 

(~850C.E.) is the third youngest eruption in Lassen 

Volcanic National Park. This eruption is typical of 

the type of volcanic activity to be expected in the 

Lassen domefield in the future. Detailed geologic 

mapping (Christiansen and others, 2002) has added 

significant new detail to the general stratigraphy of 

the Chaos Crags given by Crandell and others (1974). 

The Chaos Crags eruptive sequence consists of three 

pumiceous pyroclastic flows, six domes, and two 

dome-collapse lithic pyroclastic flows (fig. 33). The 

volume of the Chaos Crags deposits is approximately 

1.2 km3, about 15percent as pyroclastic flows (not 

including distal ash). 

The initial event of the Chaos Crags eruptive 

sequence was the eruption of pumice and lithic debris 

and the formation of a tuff cone. Eruption of the 

lower and middle pyroclastic flows quickly followed. 

These were confined to the valleys of Lost and 

Manzanita Creeks, where they flowed to a distance of 5 

km. The most accessible exposures of these pyroclastic 

flows are at the Lost Creek sand pit. Whole tree trunks 

converted to charcoal are often found in the basal zone 

Road Log, Section D
42 Geologic Field-Trip Guide to Lassen Volcanic National Park and Vicinity, California

A

B

C

D

Figure 32. Photographs of hand specimens of four rock types of very 

different appearance that were produced over a week-long period during 

the May 1915 eruption from the summit of Lassen Peak. Black dacite (A) 

erupted as the lava dome during approximately May 14–19 and as the 

lava flows on the late evening to early morning of May 19–20. Light dacite 

pumice (B) and banded pumice (C) were erupted during the pyroclastic 

eruption on May 22. The composition of light dacite pumice is similar to 

that of the light bands in the banded pumice (C) and to that of the black 

dacite (A). The composition of the dark bands in the banded pumice (C) 

is similar to that of blobs of andesite found as quenched inclusions (D) in 

both the black dacite and the light dacite pumice. All four rock types are 

the products of incomplete mixing between two magmas with contrasting 

compositions. The mixing process was interrupted by eruption of the 

partially mixed magmas (Clynne, 1999).

•	 Three cold rockfall avalanches from dome C formed 

Chaos Jumbles. Radiocarbon age is 278±28 years B.P. 

(weighted average of 3 samples)

Hiatus of approximately 800 years

•	 Emplacement of dome F

•	 Hot dome-collapse lithic pyroclastic avalanche from 

dome E

•	 Emplacement of dome E

•	 Hot dome-collapse lithic pyroclastic avalanche from 

dome D

•	 Emplacement of dome D

•	 Emplacement of dome C

Possible short hiatus

•	 Emplacement of dome B

•	 Explosive disruption of dome A by eruption of the 

upper pyroclastic flow, accompanied by airfall pumice 

lobe and formation of a tuff cone. Radiocarbon age is 

1,103±13 years B.P. (weighted average of 16 samples 

from upper, middle and lower pyroclastic flows)

•	 Emplacement of dome A

•	 Eruption of pumiceous lower and middle pyroclastic 

flows and formation of a tuff cone

•	 Initial vent opening, air-fall pumice and lithic deposit

Figure 33. Chart giving a summary of events in the formation 

of Chaos Crags. In stratigraphic order: earliest event at bottom; 

latest event at top. From Clynne and others (2002).
Road Log 43

of the lower pyroclastic flow, indicating that it flowed 

through and destroyed mature Jeffrey pine forest. The 

middle pyroclastic flow apparently followed the same 

path and rarely contains charcoal. At Lost Creek sand 

pit, tar from distillation of a tree trunk crosses the contact 

between the two pyroclastic flows and indicates that 

the middle pyroclastic flow was erupted and emplaced 

before the lower pyroclastic flow had completely cooled. 

Although the best exposure in the sand pit was destroyed 

by bulldozing in 2008, there are other locations in the pit 

where these relationships can still be seen. 

Shortly after eruption of the lower and middle 

pyroclastic flows, the vent of the tuff cone was plugged 

by growth of a small dome, designated dome A. Dome 

A was mostly destroyed by a large pyroclastic event 

that emplaced the widespread upper pyroclastic flow. At 

3 km from the vent, the upper pyroclastic flow became 

confined to the drainages of Lost Creek and Manzanita 

Creek, where it flowed to distances of 18 km and 28 km, 

respectively. The eruption also built a second tuff cone and 

deposited a lobe of air-fall pumice that can be traced northeast for 25 mi (40 km). Near the vent, the upper pyroclastic 

flow contains blocks of dome A up to 1 m in size. No evidence of erosion has been found between the pyroclastic 

flows, and all three are closely similar in age. A weighted 

average of 16 uncalibrated 14C ages from all three pyroclastic flows is 1,103±13 B.P. (Clynne and others, 2002). 

Subsequent eruption of domes B–F is discussed at 

Stop D-6. 1.1

 27.6 Lost Creek. 1915 debris flows overlie pyroclastic flows 

from the Chaos Crags eruptions of 1,103±13 B.P. 

(~850C.E.). For the next 2 miles, the road passes through 

till of Anklin Meadows (25–17 ka) that is composed 

primarily of dacite boulders from Lassen Peak. 0.9

28.5 Boundary of West Prospect Peak quadrangle with Manzanita Lake quadrangle to the west. 1.2

29.7 To the left are the Sunflower Flat domes, a group of 

eight 41±1 ka, hornblende-biotite rhyodacite domes 

that were preceded by eruption of air-fall pumice and a 

pyroclastic flow (the flat over which the road passes). 

Mature forest obscures the relatively youthful, unglaciated morphology of the domes. 0.6

 30.3 On the right is the bouldery block-lava flow surface of 

the two-pyroxene andesite of Table Mountain, a shield 

volcano (~700 ka) related to regional volcanism outside 

the Lassen Volcanic Center. 1.4

 31.7 Talus on left is from the northernmost of the Sunflower 

Flat rhyodacite domes; the road is once again on 

pyroclastic flow of Sunflower Flat. 0.8

 32.5 Edge of Chaos Jumbles. 0.6

 33.1 Stop D-6: Chaos Crags, Chaos Jumbles, and 

quenched mafic inclusions. The initial events of 

the Chaos Crags eruptive sequence 1,103±13 B.P. 

(~850C.E.) were discussed at the Lost Creek sand pit 

(mile 26.5). Subsequent to eruption of the upper pyroclastic flow, a series of domes was emplaced, probably 

in rapid succession (fig. 34). The domes are designated 

in chronological order, B through F. Domes D and E had 

hot collapse events that produced short lithic pyroclastic 

flows. Approximately 350 years ago (mid 1600s C.E.), 

the Chaos Jumbles were formed when dome C partially 

collapsed in three cold rockfall avalanches (see below). 

All of the Chaos Crags units are porphyritic 

hornblende-biotite rhyodacites or dacites containing 

67–70 percent SiO2 (fig. 35). Dense rocks contain about 

40 percent crystals of plagioclase, biotite, hornblende, 

quartz, and xenocrystic olivine and augite. Two 

lithologic groups are recognized: Group 1 comprises 

the three pyroclastic flows plus domes A and B. This 

group has a narrow range of composition (69–70 

percent SiO2), contains sparse quenched inclusions (see 

below), and displays little evidence of disaggregated 

inclusions. Group 2 comprises domes C–F and 

contains 67–69percent SiO2, abundant inclusions 

(sometimes approaching 20 percent of the rock), and 

abundant evidence of disaggregated inclusions. The 

compositional variation in the Chaos Crags is due to 

disaggregation of inclusions (Tepley and others, 1999). 

Quenched inclusions (“blobs”) of mafic magma 

occur in all the Chaos Crags rocks but are especially 

abundant in the later domes. The inclusions form when 

blobs of hot mafic magma are injected into cooler silicic 

magma and are thus chilled (fig. 36). The inclusions display a variety of textures from aphyric (with a relatively 

coarse-grained groundmass) to porphyritic (with a relatively fine-grained groundmass). The phenocrysts in the 

porphyritic variety are resorbed felsic crystals derived 

from mixing with the silicic host magma at the time of 

inclusion formation. Where both aphyric and porphyritic 

types occur in the same inclusion, the finer-grained porphyritic material concentrically surrounds the coarsergrained aphyric material. Most quenched inclusions also 

contain olivine, calcic plagioclase, and augite phenocrysts inherited from their basaltic parent. The magmamixing events associated with the intrusion of basalt and 

the formation and disaggregation of quenched inclusions 

play an important role in maintaining the magmatic 

system at the Lassen Volcanic Center by adding heat and 

mass to mostly crystalline magma, and in some cases 

these events provoked eruptions (Guffanti and others, 

1996; Clynne, 1999; Klemetti and Clynne, 2014). 

The Chaos Jumbles comprise three separate rockfall 

avalanches covering 2.5 mi2 (6.8 km2) that can be distinguished on the basis of flow margins and distribution 

of block sizes. The deposits consist of a monolithologic 

breccia of Chaos Crags dacite blocks in a matrix of pulverized dacite. The deposits have steep distal and lateral 

margins 1–5 m in height. The first rockfall avalanche 

was the largest and traveled ~3 mi (~5 km) with a vertical drop of 2,200 ft (670 m) from the breakaway scar on 

dome C of Chaos Crags. The other avalanches were successively smaller and shorter, but thicker. The avalanche 

Road Log, Section D
44 Geologic Field-Trip Guide to Lassen Volcanic National Park and Vicinity, California

CALIF. HWY 89

Lassen Peak

Dome B

covered by talus Dome B

Chaos Jumbles

Chaos Jumbles

Chaos Jumbles

Dome C

Dome D

Dome E

Dome F

Uplifted

andesites of Diller

sequence, undivided

0.5

1.0

1.5

2.0

2.5

3.0

52 54 56 58 60 62 64 66 68 70 72

SiO2, in weight percent

Group 2 rhyodacites

Group 2 inclusions

Group 1 rhyodacites

Group 1 inclusions

K2O, in weight percent

EXPLANATION

Resident rhyodacite

magma

Intruded basalt magma

Mixing line

Figure 34. Photograph of Chaos Crags and Chaos Jumbles looking southeast from Calif. Hwy 89. Hummocky surface of the avalanche 

deposits of Chaos Jumbles (278±28 B.P.; mid 1600s C.E.) is in the foreground and middle ground. The oval-shaped massive rock cliff 

(dome C of Chaos Crags) in the center of the view is the source of the Chaos Jumbles deposits. The highest peak is dome D, and the 

high ground above and to the left of dome C is dome E. Small peak on the left is dome F. The sparsely tree-covered area (below the 

large snow bank) on the right flank of dome D is dome B, which extends beneath talus from younger domes to the sharp talus-covered 

ridge beneath dome D. Lassen Peak (27±1 ka) is on the extreme right at the skyline. Forested area on right below the domes is underlain 

by young glacial moraines. Light-colored smooth surface above dome C and outcrops to right of dome C are andesite of the Diller

Sequence (part of Brokeoff Volcano) uplifted during emplacement of the dome. Photograph 29 of Muffler and others (2010).

Figure 35. Graph of SiO2 content versus 

K2O content for the quenched mafic 

inclusions and host rocks in Chaos 

Crags. The composition of the intruded 

basalt magma was approximately 

52 percent SiO2 and 0.7 percent K2O, 

and the composition of the resident 

rhyodacite magma was approximately 

70 percent SiO2 and 2.8 percent K2O. 

The basalt magma was variably 

hybridized by mixing with rhyodacite 

before quenching to form inclusions. 

Convection of the host rhyodacite 

magma caused disaggregation of the 

inclusions (Clynne, 1999; Tepley and 

others, 1999), thus also hybridizing the 

rhyodacite.
Road Log 45

paths were controlled by existing topography. The initial 

direction of each avalanche was west-northwest towards 

Table Mountain. The first avalanche deposit rode up 

nearly 400 ft (120 m) onto Table Mountain before being 

deflected to the west. Groovelike linear features on the 

surface of the deposit were interpreted by Eppler and 

others (1987) to be strike-slip faults caused by compression during flow as the avalanche was deflected by Table 

Mountain. Regularly spaced surface ridges are oriented 

perpendicular (transverse) to the direction of flow. The 

avalanches apparently were emplaced as material of high 

yield-strength capable of deforming and shearing rather 

than as a plug of nondeforming material being carried on 

a deforming basal layer (Eppler and others, 1987). 

Steep-sided volcanic domes are clearly unstable 

and prone to collapse, but the trigger for the Chaos 

Jumbles collapse is unknown. A number of mechanisms 

have been proposed, including an explosion at the base 

of dome C (Williams, 1932) and renewed volcanic 

activity on dome D, which was reported by the Whitney 

Survey to have been steaming in 1857 (Brewer, 1930). 

However, there is no evidence to support either of these 

ideas, and the most likely explanation is triggering by a 

large earthquake, possibly on the Hat Creek Fault 20 mi 

to the north (see Section I). 1.7

 34.8 To the left across Manzanita Lake can be seen Loomis 

Peak, a rhyodacite lava flow (~300 ka) of the Bumpass 

sequence. To the east is the 66±4 ka rhyodacite dome 

of Eagle Peak. To the right, Reflection Lake is a closed 

depression on the Chaos Jumbles rockfall avalanche. The 

oldest of the three Chaos Jumbles rockfall avalanches 

crossed and dammed Manzanita Creek to form Manzanita Lake. Radiocarbon ages of wood samples from trees 

drowned in the lake that formed behind the dam (Clynne 

and others, 2002) are consistent with cores taken from 

trees on the oldest avalanche (Heath, 1959) and confirm 

the conclusion of Crandell and others (1974) that all 

three avalanches occurred in quick succession about 

350years ago. 0.7

 35.5 Intersection. To the left, Calif. Hwy 44 leads west to 

Redding (see Section H). To the right, Calif. Hwy 44 

and 89 lead north to Old Station and the Hat Creek 

Rim. (See Section E).

Section E: Intersection of Calif. Hwys 44 and 89 

North to Hat Creek Rim

 0.0 Proceed north on Calif. Hwys 89 and 44. Roadcuts are 

in rhyodacite pyroclastic-flow deposits from Eagle Peak 

(66±4 ka). Pass out of the Lassen Volcanic Center and 

onto rocks related to regional volcanism. 1.3

 1.3 Eskimo Summit is the crest of the Cascade Range in 

this area. Scoria cones to the left are vents for basaltic 

andesite and andesite of Red Lake Mountain (~75 ka). 

For the next 6 miles, the road lies on slightly older 

flows from Red Mountain. The northern flank of Table 

Mountain (~700 ka) is on the right. 4.6

 5.9 Boulders on the flat to the right were deposited by the 

May 19–20, 1915, debris flow from Lassen Peak. The 

May 22 debris flow also left a thin deposit in this area, 

but that debris flow was too fluid to carry large boulders 

this far. The debris flows lie on the upper pyroclastic 

flow from Chaos Crags (1,103±13 B.P.; ~850 C.E.). 1.0

 6.9 To left is the block-lava flow front of a 35-ka augiteolivine andesite from Bear Wallow Butte, which is 

the southernmost of a group of 10 scoria cones and 

lava flows of the Tumble Buttes chain. The youngest 

of these, the andesite of Devils Rock Garden, may be 

Holocene (less than 11.7 ka), and most of the units 

are less than ~50 ka. This 15-km-long line of scoria 

cones and lava flows is oriented at azimuth 345°, 

Figure 36. Photograph of quenched mafic 

inclusion in dacite/rhyodacite of Chaos Crags 

(width of view 30 cm). This inclusion shows 

some of the features that can be observed in 

the suite of inclusions from Chaos Crags. The 

inclusion has a fine-grained quenched rim with 

a convolute boundary against the host, whereas 

its core is more coarsely crystalline. Fingers 

of the inclusion demonstrate how the host 

silicic magma is pinched off and crystals are 

“captured” by the mafic inclusion. This inclusion 

has relatively few host crystals, but a few can 

be seen even in the core of the inclusion. Brown 

spots at the lower left are lichen.

Road Log, Section E
46 Geologic Field-Trip Guide to Lassen Volcanic National Park and Vicinity, California

parallel to regional faults, and illustrates fault control 

of the patterns of vents in the regional volcanism. The 

most likely future volcanic event in the Lassen area is 

eruption of mafic magma leading to the formation of 

a scoria cone and lava flows like those in the Tumble 

Buttes chain (Clynne and others, 2012). 0.3

 7.2 Boundary of Manzanita Peak quadrangle with West 

Prospect Peak quadrangle to the east. 0.6

 7.8 The flat plateau at 12:00 is the Hat Creek Rim, the upthrown block of a normal fault bounding the Hat Creek 

Valley on the east (Muffler and others, 1994). Two rocks 

from the top of the fault scarp gave ages of 1,645±35 ka 

and 924±24 ka (see description at mile 15.1). The rounded shield volcano on the skyline is Blacks Mountain, a 

nearly aphyric olivine andesite (K-Ar date 2,403±19 ka; 

G.B. Dalrymple, written commun., 1991). Good views of 

Lassen Peak and Chaos Crags to the south. 0.7

8.5 Boundary of West Prospect Peak quadrangle with Old 

Station quadrangle to the north. On right is a U.S. Forest 

Service exhibit entitled “Panoramic Point” that once had 

an excellent view of Lassen Peak to the south. Unfortunately, uncontrolled growth of the forest since it was 

planted in 1961 has (as of 2015) completely obscured the 

view. Restrooms and picnic tables are available. 1.4

 9.9 Big Springs, the source of much of the water in Hat 

Creek, discharge along a prominent fault uplifting the 

tholeiitic basalt of Twin Bridges (780–730 ka), which is 

exposed in roadcuts for the next 0.7 mile. 0.7

10.6 Old Station Post Office is built on glacial outwash of 

Anklin Meadows (~17 ka), which contains cobbles 

derived from the dacite of Lassen Peak. 1.5

12.1 Stop E-1: Vents of Hat Creek Basalt. Parking area 

for the trail to the vent area of the Hat Creek Basalt, 

a 24±6 ka (Turrin and others, 2007) low-K olivine 

tholeiitic (LKOT) basalt lava flow that is characteristic 

of volcanism in the Basin and Range Province of the 

western United States. This type of lava erupts from 

a fissure and builds a low edifice capped by spatter 

cones. The lava forms an extensive but thin sheet that 

is distributed by a system of lava tubes. The Hat Creek 

Basalt flowed for 20 miles to the north in Hat Creek 

Valley (Anderson, 1940). The rock has sparse olivine 

phenocrysts in a completely crystalline (holocrystalline) 

groundmass of small plagioclase, olivine, and augite 

crystals projecting into abundant tiny vesicles. This 

texture, termed diktytaxitic, is characteristic of LKOT 

lava flows. For details of petrology and geochemistry of 

the Hat Creek Basalt see Anderson (1971), Anderson and 

others, (1982), and Anderson and Gottfried (1971). 

A 1.5-mile hike (round trip) along this trail 

takes one to the vent area for the Hat Creek Basalt. 

The trail begins in an old calc-alkaline lava flow 

and then winds along the margin of the Hat Creek 

Basalt before ascending onto the flow. The flow 

locally displays a pāhoehoe surface and is broken by 

tumuli. White hornblende-biotite pumice lapilli and 

ash scattered on the ground are from the eruption of 

Chaos Crags at 1,103±13 B.P.; ~850 C.E.) The vent 

area is a 1.75-mile-long rampart dotted with small 

spatter cones. Lava was transported away from the 

vent alignment in tubes. 1.6

13.7 At the junction of Calif. Hwys 89 and 44, turn right 

onto Calif. Hwy 44 towards Susanville. Hwy 89 

continues north down the Hat Creek Valley towards 

Burney. 0.3 miles north on Calif. Hwy 89 is the turnoff east to Subway Cave, a U.S. Forest Service exhibit 

that allows exploration of a part of a lava tube in the 

Hat Creek Basalt. Note that no services are available 

on Hwy 44 until the city of Susanville (51 miles). 1.4

15.1 Cross the Hat Creek Fault and climb up the eroded 

1,000-foot-high fault scarp of the Hat Creek Rim. 

The 29-mile-long Hat Creek Fault is contiguous 

with faults to the south in the eastern part of Lassen 

Volcanic National Park that also join with faults 

bounding the Lake Almanor graben, a total distance of 

more than 50miles. Together these structures define 

the westernmost major graben of the Basin and Range 

Province. The Basin and Range structural regime 

is expanding to the west and impinging upon the 

Cascade volcanic arc in the Lassen area. Older faults 

(for example, the Butte Creek Rim) occur farther to 

the east. 

Rocks exposed on the eroded fault scarp of the Hat 

Creek Rim are regional basalts and andesites similar to 

the volcanic rocks on the west side of Hat Creek Valley. 

Two rocks from the top of the fault scarp gave ages 

of 1,645±35 ka and 924±24 ka (Clynne and Muffler, 

2010). Total vertical offset has been at least 1,000 feet 

in the past million years. The valley bottom has been 

filled with younger basalt flows, so the observed offset 

is a minimum amount. 

Recent movement on the Hat Creek Fault is 

demonstrated by offset of lava ponded against the 

fault scarp. The most recent movement occurred 

since ~15 ka and offsets the Hat Creek Basalt by as 

much as 30 meters (Muffler and others, 1994; Walker, 

2008; Walker and Kattenhorn, 2008; Blakeslee and 

Kattenhorn, 2013). A single event that large would 

have an earthquake magnitude of at least 8; thus it is 

likely that the movement occurred in more than one 

event. The existence of such a prominent, young fault 

with no associated modern seismicity suggests that 

movement on the Hat Creek Fault is episodic over 

periods of perhaps hundreds or thousands of years. 

Despite the lack of historical activity, the Hat Creek 

Fault is clearly an active structure capable of large 

earthquakes (Wills, 1991). Walker and Kattenhorn 

(2008) and Blakeslee and Kattenhorn (2013) 

estimate that the fault has the potential to produce 

an earthquake of moment magnitude of 6.7 with a 

recurrence interval of 667±167 yr. 1.4

16.5 Turn left onto side road to Hat Creek Rim Overlook. 0.4
Road Log 47

16.9 Stop E-2: Hat Creek Rim Overlook

From this vantage point at the top of the Hat Creek 

Fault scarp, the crest of the Cascade Range from the 

Lassen Volcanic Center to Mount Shasta is visible. To 

the south, in the Lassen Volcanic Center, prominent 

visible features include Lassen Peak, Chaos Crags, and 

Raker Peak, which is a 270±18 ka hybrid andesite of 

the Twin Lakes sequence. Regional volcanoes include 

Table Mountain (~700ka) and Badger Mountain, a 

faulted 708±21 ka andesite shield. 

Volcanoes of the Cascade Range north of Lassen 

Peak are identified in an annotated panorama of the 

Cascade Range (fig. 37) taken from Parhams Point, 

1.85 miles south-southeast (see Stop G-1 of Section 

G). These volcanoes are typical of Cascade Range 

volcanoes outside volcanic centers or away from 

large volcanoes. The flat, brush-covered area in the 

foreground and valley floor is the 24±6 ka tholeiitic 

Hat Creek Basalt (Turrin and others, 2007), and the 

low mound just to the south of Old Station is its vent 

area. West of the vent area of the Hat Creek Basalt is 

the north-south-trending Sugarloaf chain of sparsely 

porphyritic olivine and pyroxene basaltic andesite and 

andesite volcanoes (all calc-alkaline), consisting of 

Sugarloaf Peak (46±7 ka), Little Potato Butte (67±4 

ka), Potato Butte (77±11 ka), and several smaller 

vents (Turrin and others, 2007). Together the Potato 

Buttes, the Hat Creek Basalt, and Sugarloaf Peak 

obscure the western boundary fault of the Hat Creek 

Valley that can be seen as a scarp farther north. 

In the distance, to the left of Sugarloaf Peak, 

the cones in the Red Lake Mountain group and the 

southern end of the Tumble Buttes chain (Bear Wallow 

Butte) can be discerned. Hidden behind Sugarloaf Peak 

is deeply glaciated Magee Volcano (Borg, 1989), the 

upper part of which has a K-Ar date of 210±120 ka. 

Older volcanoes and flows in this area are cut by many 

small faults. The prominent cone-shaped mountain to 

the north is Burney Mountain. This volcano comprises 

six pyroxene dacite lava domes emplaced on top of 

each other; the oldest dome has a 40Ar/39Ar age of 

280±6 ka (M.A. Lanphere, written commun., 1998). 

Sugarloaf

Peak Burney

Mountain

Mount

Shasta

Magee

Volcano

Old Station

Hat Creek Basalt

Lassen

Peak Badger

Mountain Chaos

Crags

Potato Butte

Little Potato Butte

Vents of Hat Creek Basalt

Bear

Wallow 

Butte

CALIF. HWY 44

Hat

Mountain

West

Prospect

Peak Flow from Potato Butte

Logan

Mountain Wilcox 

Peak Freaner

Peak Brown 

Butte

Cornaz 

Peak

Red Lake

Table Mountain

Mountain Devils Rock

Garden

Ashpan

Butte

Figure 37. Panoramic photograph looking southwest (left) to northwest (right) from Parhams Point across the Hat Creek Graben and 

the Hat Creek Basalt (24±6 ka) towards the crest of the Cascade Range north of Lassen Volcanic National Park.

The area east of Burney Mountain contains a number of 

young scoria cones and lava flows, some of which may 

be postglacial. 

Mount Shasta can be seen in the distance. With 

a volume of >350 km3, it is the largest composite 

volcano in the High Cascade Range (Calvert and 

Christiansen, 2011). Its elevation is 14,162 feet 

(4,318 m), nearly 4,000 feet (1,130 m) higher than 

Lassen Peak. At >300 ka, Mount Shasta experienced 

an enormous debris avalanche that nearly destroyed 

the ancestral Sand Flat volcano (Crandell and others, 

1984; Crandell, 1989), dated at ~700–600 ka (Calvert 

and Christiansen, 2011). The present edifice comprises 

four overlapping cones (Christiansen and Miller, 

1989; Calvert and Christiansen, 2011): Sargents Ridge 

(~300–100 ka), Misery Hill (50–35ka), Shastina (~11 

ka), and Hotlum (8.8–3.1 ka). The last activity of 

Mount Shasta may have been a very small eruption 

in 1786 C.E. (Finch, 1930). Although this specific 

eruption is questionable, there is evidence of mildly 

explosive activity at Mount Shasta during the last 

few millennia, but probably no magmatic eruptions 

since 3.1 ka (R.L. Christiansen and A.T. Calvert, oral 

commun., 2015). 

Section F: Hat Creek Rim Overlook to Westwood 

and Chester

0.0 Return to Calif. Hwy 44. 0.4 

 0.4 Turn left onto Calif. Hwy 44 and proceed north and 

then east towards Susanville. 1.6

 2.0 On right is U.S. Forest Service Road 33N56. An 

excellent view of the Cascade Range (fig. 37) can be 

obtained by detouring 1.3 mi south on this unpaved 

road (see Stop G-1). 0.4 

 2.4 U.S. Forest Service Road 33N20 to right. 0.1

 2.5 Boundary of Old Station quadrangle with Swains Hole 

quadrangle to the east. 0.5

 3.0 U.S. Forest Service Road 33N54 to left. 0.5

 3.5 Summit Lake on the left is in a small fault-bounded 

depression. 0.7

Road Log, Section F
48 Geologic Field-Trip Guide to Lassen Volcanic National Park and Vicinity, California

 4.2 U.S. Forest Service Road 33N56 to right. 0.3

 4.5 Roadcut in basaltic andesite of Signal Butte (~1,000–

900ka), which erupted from the large scoria cone 

0.7mi at 1:30. This lava flow is cut by small faults 

oriented parallel to the Hat Creek Fault and can be 

considered as the youngest unit of the Hat Creek 

Rim. 0.7

 5.2 Contact with the tholeiitic basalt of Calif. Hwy 44 

(190±18 ka). 0.1

 5.3 Roadcuts in the tholeiitic basalt of Calif. Hwy 44. This 

thin lava flow of sparsely porphyritic olivine basalt 

(49.6 percent SiO2) erupted from a fissure at hill 5855, 

1.75 mi to the south-southwest. This extensive lava 

flow was fed by lava tubes and flowed north along the 

base of Butte Creek Rim for >9 mi (15 km). It also 

flowed to the west and barely spilled over the Hat 

Creek Rim. The surface of the flow is relatively flat and 

has low relief, although punctuated by widely spaced 

tumuli. A40Ar/39Ar age of 190±18 ka is consistent 

with the flow’s excursional magnetic direction (D.E. 

Champion, written commun., 2006). 0.9

 6.2 Lassen County line. 1.6

 7.8 View back to the right at 3:00 to 4:30 of West Prospect 

Peak (400–300 ka) and Prospect Peak (247±56 ka). 

These andesite and basaltic andesite shield volcanoes 

are typical of regional calc-alkaline edifices. 

At 2:00 is Sunrise Peak, the highly glaciated 

vent for the basalts of Sunrise Peak (393±4 ka), at the 

western edge of the Caribou Volcanic Field just east of 

Butte Lake. 

In the distance at 9:30 is the shield volcano of 

Blacks Mountain, a nearly aphyric olivine andesite 

(K-Ar date 2,403±19 ka; G.B. Dalrymple, written 

commun., 1991). This unusual regional calc-alkaline 

rock has a silica content of 59 percent SiO2 but 

contains only a few, tiny olivine phenocrysts. 

Highway in this area passes over glacial outwash 

brought from the volcanic highlands to the south along 

Butte Creek. 0.8

 8.6 At right is U.S. Forest Service Road 32N21, a 

washboard unpaved road that goes ~6 miles south into 

LVNP. From the end of the road at Butte Lake (Section 

G, Stop G-3) is a 1.8-mile trail that goes to the top 

of Cinder Cone, which, at 1666 C.E., is the youngest 

mafic volcano in the Lassen region (Sheppard and 

others, 2009). The products of the Cinder Cone 

eruption consist of five lava flows, two nested scoria 

cones and a widespread mafic ash-fall deposit (Clynne 

and others, 2000a; Clynne and others, 2002). Cinder 

Cone is part of the younger Twin Lakes sequence of 

the Lassen domefield. A scramble from Butte Lake 

up the west flank of Sunrise Peak east of Butte Lake 

to a bluff at approximately UTM 10T E0645650 

N4490680 (NAD27) is rewarded by a spectacular view 

of Cinder Cone, Lassen Peak, and Chaos Crags (cover; 

photograph 31 of Muffler and others, 2010). 1.6

10.2 Boundary of Swains Hole quadrangle with Poison Lake 

quadrangle to the east. 0.1

 10.3 U.S. Forest Service Road 32N26 goes south towards 

the young Bidwell Spring chain of the Caribou Volcanic 

Field. 0.5 

 10.8 View at 12:00 of scoria cones of the Poison Lake chain, 

a line of 39 small, monogenetic calc-alkaline basaltic 

volcanoes in a zone 14 km long and 2 km wide trending 

north-northwest parallel to and partially burying nearby 

Quaternary normal faults. 1.9

 12.7 Stop F-1: Poison Lake chain. On the left is Poison 

Lake. Visible from this point are several prominent 

scoria and agglutinate cones that mark vents for 

volcanoes of the Poison Lake chain, part of the 

Caribou Volcanic Field. The northern half of the 

Caribou Volcanic Field, including the Poison Lake 

chain, has not been glaciated, and constructional forms 

are well preserved. The 39 units of the Poison Lake 

chain fall into 9 distinct groups based on stratigraphy, 

petrography, and major-element geochemistry (Muffler 

and others, 2011). Many units contain only sparse 

olivine; a few units are distinctly porphyritic with 

olivine, clinopyroxene, and plagioclase. MgO-SiO2, 

K2O-SiO2, and TiO2-SiO2 variation diagrams illustrate 

clear differences in compatible and incompatible 

elements among the groups. Limited trace-element 

and 87Sr/86Sr data suggest little if any interaction with 

the upper crust. Precise 40Ar/39Ar determinations show 

that the lavas erupted at 110–100 ka. Paleomagnetic 

remanent directions suggest that the entire Poison 

Lake chain could represent three short-lived episodes 

of volcanism within a period as brief as 500 years. 

Collectively, the data indicate that each of the 9 groups 

represents a small, discrete magma batch generated 

in the mantle and stored briefly in the lower crust. 

Episodic movement along a north-northwest normal 

fault zone provided transitory conduits for these 

batches of magma to ascend to the surface and erupt as 

distinct volcanic groups, each aligned along a segment 

of the Poison Lake chain. 

At 10:00 is Cal Mountain, a regional calc-alkaline 

shield volcano of olivine andesite (2,345±49ka). In the 

distance at 9:00 is Blacks Mountain (2,403±19 ka). 2.3

 15.0 Road to Pittville on left. For the next 8 miles, Calif. 

Hwy 44 traverses a flat valley underlain by the tholeiitic 

basalt of Grays Valley, a thin, extensive sheet of nearly 

aphyric, tholeiitic, olivine basalt (48.5 percent SiO2) 

erupted at ~600 ka from a spatter rampart east of 

Bogard Buttes (at 1:00). Multiple flow units of tube-fed 

pāhoehoe flowed north and filled the valley between 

Bogard Buttes, Cal Mountain, Cone Mountain (at 9:00), 

and Crater Mountain (at 11:00), all members of the 

regional calc-alkaline suite. See Stop F-2 for further 

discussion of Bogard Buttes and Crater Mountain.

To the left (north of the highway) are the tracks 

of the Burlington Northern Santa Fe (BNSF) Railroad, 
Road Log 49

originally the Western Pacific Railroad. From here to 

mile 44 just north of Westwood, this railroad parallels 

the field-trip route to the east. 1.4

 16.4 To the right of Calif. Hwy 44, a grove of aspen marks 

Robbers Spring, at the toe of a flow of the basalts of 

Robbers Spring (101.5±8.7 ka), part of the Poison Lake 

chain (Muffler and others, 2011). 0.9

 17.3 Boundary of Poison Lake quadrangle with Bogard 

Buttes quadrangle to the south. 0.5

 17.8 Boundary of Bogard Buttes quadrangle with Pine 

Creek Valley quadrangle to the east. 0.7

 18.5 Mountains to the right of Calif. Hwy 44 are underlain 

by the dacite of Bogard Buttes (2,350±70 to 

2,225±49 ka). 1.7

 20.2 Inconspicuous vents for the tholeiitic basalt of Grays 

Valley are in the woods to the right. 0.7

 20.9 Turn right on U.S. Forest Service Road 32N09 to 

southwest (right). 0.1

21.0 Turn right at entrance to Bogard Buttes Rest Area. 0.2

 21.2 Bogard Buttes Rest Area. Views from the rest area itself 

are obscured by trees.

Return to the intersection of U.S. Forest Service 

Road 32N09 and Calif. Hwy 44 for a good view to the 

northeast. 0.3

 21.5 Stop F-2: Calc-alkaline volcanoes and intervening 

LKOT basalts. The topography of the Bogard Buttes 

area is typical of that located behind the active Cascade 

Arc (see Borg, 1995). East of the Poison Lake chain, 

most of the volcanoes are 2–3 million years old, 

although a few are younger. The landscape is dotted 

with shield volcanoes, and the intervening valleys are 

flooded with widespread sheets of LKOT basalt lava. 

From this vantage point a number of shield volcanoes 

can be seen, including Crater Mountain, Bogard Buttes, 

Campbell Mountain, Logan Mountain, and Antelope 

Mountain. The shield volcanoes are composed of 

basaltic andesite to andesite lavas that are generally 

porphyritic, but without large or complexly zoned 

crystals. Each edifice is different; most are composed 

of a single rock type, but some have a narrow range of 

compositions. The density of faults on each volcano 

gives a relative indication of its age. 

The large shield volcano to the northeast is Crater 

Mountain. Its surface is covered by forested block lava 

flows of augite-hypersthene andesite, with abundant 

small phenocrysts. A second, slightly more mafic 

olivine andesite lies mostly buried by the main edifice. 

The volcano is relatively large, with a nearly 5-mile 

diameter, rises about 1,750 ft (530 m) above the valley 

and has a volume of about 10 km3. Crater Mountain 

is undated, but it is little eroded, has a thin soil, and 

is unfaulted. Its summit is weakly glaciated but still 

has a partially preserved scoria cone. It has a normal 

paleomagnetic direction and an age of ~500 ka. The 

lake at the summit is probably a cirque dammed by 

glacial deposits. U.S. Forest Service Road 32N08 is a 

Road Log, Section F

well-maintained gravel road and makes a nice side trip 

to the summit of Crater Mountain. The road leaves the 

highway just northwest of the entrance to the rest area. 

Bogard Buttes, the forested shield volcano to the 

northwest of the rest area, is composed of a single type 

of porphyritic augite-hypersthene dacite. It has a basal 

diameter of about 3 miles, rises 1,900 feet (580m) 

above the valley floor, and has a volume of about 4 

km3. The edifice has a thick soil, is cut by several 

conspicuous faults (not visible from this vantage point) 

and has no preserved vent area or scoria. Two K-Ar 

ages for Bogard Buttes are 2,350±70 and 2,225±49 ka. 

The rest area is constructed on the proximal high 

ground of the tholeiitic basalt of Grays Valley. This 

flow vented from the area of the inconspicuous low 

hills 0.25–0.5 mile west of the rest area and flowed 

northward between Crater Mountain and Bogard 

Buttes into Grays Valley. The tholeiitic basalt of Grays 

Valley is a nearly aphyric, diktytaxitic, olivine basalt. 

The valley floor to the east of the rest area is flooded 

with another tholeiitic basalt lava flow, the basalt 

of Pine Creek Valley, which vented from an inconspicuous hill just above the fault scarp about 2 miles 

south-southeast of the rest area. The tholeiitic basalt of 

Pine Creek Valley is also a nearly aphyric, diktytaxitic, 

olivine basalt, but the two flows have subtle chemical differences. Neither of these basalt flows is dated, 

but both have normal polarity, thin soils, are relatively 

uneroded, and probably about 600 ka in age. 

This part of Calif. Hwy 44 marks the route of both 

the Lassen Trail and the Nobles Trail to California 

from Missouri (Strong, 1989). The Lassen Trail was 

first traversed in 1847 by a party led by Peter Lassen. 

Coming from Goose Lake north of Alturas, the trail 

led down the Pit River and then south to what is now 

Calif. Hwy 44. The trail then went southeast and south, 

roughly paralleling the field-trip route, to Westwood 

and Big Meadows (now Lake Almanor) and thence 

down the ridge between Mill Creek and Deer Creek 

to the Sacramento Valley. The Nobles Trail, charted in 

1851 by William Nobles, came west from Susanville 

along this section of Calif. Hwy 44, went south between Cinder Cone and Prospect Peak, crossed into the 

Hat Creek drainage, and then went down Shingletown 

Ridge to the Sacramento Valley. Along the parts of 

these trails near Calif. Hwy 44, emigrants to California 

in the early 1850s going southeast on the Lassen trail 

were passing emigrants on the Nobles Trail travelling 

in the opposite, northwest direction! 1.0

22.5 Calif. Hwy 44 crosses over a nearly imperceptible contact 

between the tholeiitic basalt of Grays Valley and the tholeiitic basalt of Pine Creek Valley (also ~600 ka). Vents 

for this LKOT are in the hills to the south at 1:30. 0.7

23.2 To the right is U.S. Forest Service Road 31N26 leading 

west to Bogard Campground. The prominent scoria cone 

2 miles away at 1:30 is the vent for the calc-alkaline 
50 Geologic Field-Trip Guide to Lassen Volcanic National Park and Vicinity, California

basalt of Round Valley Butte (K-Ar date 477±25 ka; 

G.B. Dalrymple, written commun., 1994). This basalt is 

separated from the tholeiitic basalt of Grays Valley by a 

series of discontinuous, en echelon normal faults (up to 

the southwest) that cut both units. 2.1

25.3 Roadcuts in the tholeiitic basalt of Pine Creek Valley. 0.7

26.0 Turn right on Lassen County Road A21 south towards 

Westwood. [If one is not following the field trip guide 

to Westwood, he or she can continue east on Calif. 

Hwy 44 another 17.5 miles to a junction with Calif. 

Hwy 36. From there, Calif. Hwy 36 continues another 

6 miles to the town of Susanville, where food, lodging, 

and gas services are available. A 1:100,000-scale 

geologic map of the Susanville Quadrangle (Grose and 

others, 2013) shows the geology along the route.] 0.2

26.2 Road curves right up the fault scarp onto the basalt of 

Round Valley Butte. 2.6

28.8 Road curves sharply left as it passes off the basalt of 

Round Valley Butte. 0.7

29.5 Boundary of Pine Creek Valley quadrangle with 

Swain Mountain quadrangle to the south. 0.1

29.6 Road passes from the basalt of Round Valley Butte onto 

the basaltic andesites of Susan River. These old calcalkaline rocks (>2,580 ka) show little constructional 

morphology and are thickly mantled by soil and by till of 

older glaciations (70–60 ka or older; see fig. 11). 0.7

30.3 To the right, Lassen County Road 110 on the north side of 

the Susan River leads to Silver Lake on the east boundary 

of the Caribou Wilderness. The Caribou Wilderness 

comprises the highly glaciated, southern part of the 

Caribou Volcanic Field (fig. 38; see also fig. 10). 0.2

30.5 To the right, U.S. Forest Service Road 30N21 on the 

south side of the Susan River also leads to the Caribou 

Wilderness. 0.3

Figure 38. Photograph looking southwest 

across Silver Lake, illustrating the glaciated 

physiography of the southern part of the 

Caribou Volcanic Field. The cliffs on the far 

side of Silver Lake represent the glaciated 

flow margin of the basaltic andesites of 

Silver Lake (400±14 ka), stripped of surficial 

vesicular blocks to expose the dense 

core of the lava flow. The summit of North 

Caribou is the glaciated agglutinate vent 

of the basaltic andesites of North Caribou 

(~350 ka). Photograph by Carl E. Mortensen. 

Units generalized from photograph 36 of 

Muffler and others (2010).

Basaltic andesites

of North Caribou

Talus Talus

Silver Lake

Basaltic andesites of Silver Lake

North

Caribou

30.8 Through the trees to the left, one can catch a quick 

glimpse of the scoria cone of Bennett Knoll, a calcalkaline basalt. Hidden in the trees south of Bennett 

Knoll is Clover Butte, also a calc-alkaline scoria 

cone (K-Ar date 688±31 ka; G.B. Dalrymple, written 

commun., 1994). 0.2

31.0 Road passes from the andesites of Susan River onto 

till of older glaciations (60 to >130 ka. 0.6

31.6 To the right, U.S. Forest Service Road 30N81 goes to 

the west along the northern flank of Swain Mountain, 

a large shield volcano composed of porphyritic olivine 

basaltic andesite with a K-Ar age of 1,570±35ka 

(G.B. Dalrymple, written commun., 1994). 0.1

31.7 Lassen County Road A21 passes onto the tholeiitic 

basalt of Westwood (401±7 ka; weighted average of 

five 40Ar/39Ar determinations by P.R. Renne and one 

by B.D. Turrin, written commun.). The inconspicuous 

vent for this very extensive basalt flow is hidden in the 

trees at 9:00, just south of Clover Butte. The tholeiitic 

basalt of Westwood flows at least 23 miles to the 

south and southwest into the canyon of the North Fork 

Feather River, 2.5 miles south of Lake Almanor. 1.0

32.7 View at 12:00 of the andesite of Brush Hill, a calcalkaline edifice probably of late Pliocene age. 0.7

33.4 To right is the road to a snowmobile park. 1.0

34.4 Flow surface of the tholeiitic basalt of Westwood. 

Note the intermittently preserved surface morphology 

of this LKOT lava flow. 0.9

35.3 View at 12:00 of the andesite of Brush Hill. 2.6

37.9 Tumuli on surface of tholeiitic basalt of Westwood. 1.8

39.7 Boundary of Swain Mountain quadrangle with 

Westwood West quadrangle to the south. 1.3

41.0 Boundary of Westwood West quadrangle with 

Westwood East quadrangle to the east. 0.1
Road Log 51

41.1 Lassen County Road A21 crosses Burlington Northern 

Santa Fe (originally Western Pacific) Railroad and 

Bizz Johnson National Recreation Trail (to east 

towards Susanville; see http://www.traillink.com/trail/

bizz-johnson-national-recreation-trail.aspx). 0.2

41.3 Mountains in distance at 12:00 are composed of 

Paleozoic metamorphic rocks in the northernmost 

Sierra Nevada. 2.8

44.1 Junction with Calif. Hwy 36 at Westwood. Turn right 

towards Chester and Red Bluff. 0.3

44.4 Calif. Hwy 36 crosses Burlington Northern Santa Fe 

Railroad. 0.1

44.5 Boundary of Westwood East quadrangle with 

Westwood West quadrangle to the west. 2.5

47.0 On left, Calif. Hwy 147 to Clear Creek. 0.5

47.5 Young normal fault (down to west) cutting tholeiitic 

basalt of Westwood. Lassen Trail crosses the highway 

at the historical marker just west of the fault. 0.2

47.7 Plumas County Line. 1.4

49.1 Pass from tholeiitic basalt of Westwood onto 

tholeiitic basalt of Rock Creek (989±49 ka). This 

older basalt has more subdued flow topography and 

reversed magnetic polarity. 2.1

51.2 Roadcut in tholeiitic basalt of Rock Creek. Boundary 

of Westwood West quadrangle with Chester 

quadrangle to the west. 0.2

51.4 View at 12:00 of Butt Mountain, to the west of Lake 

Almanor. Butt Mountain is composed of andesitic 

rocks related to the Yana Volcanic Center and has a 

40Ar/39Ar age of 3,320±80 ka (M.A. Lanphere, written 

commun., 1996). 0.8

52.2 At left is Plumas County Road A13 leading southeast 

to Hamilton Branch and the Almanor Peninsula. 

Continue west on Calif. Hwy 36. 0.1

52.3 Contact with the basalts and basaltic andesites of 

Mud Creek Rim. This thick (>400 m) section of 

calc-alkaline lava flows is exposed primarily in fault 

scarps that form the east boundary of the north part 

of the Lake Almanor graben. Four K-Ar ages range 

from 2,785±16 ka to 2,200±49 ka. 0.7

 53.0 Roadcut in basalts and basaltic andesites of Mud 

Creek Rim. 0.2

 53.2 Rest area on left side of highway. Views across Lake 

Almanor to the Lassen and Maidu Volcanic Centers 

can be obtained by scrambling through the trees 

below the rest area. Beyond the rest area, Calif. Hwy 

36 angles down a major fault scarp that forms the east 

boundary of the Lake Almanor Graben. 0.2

 53.4 Large roadcut exposes a lava flow with a baked zone 

at the base and a silicic ash flow. 1.3

 54.7 Large roadcut exposes basalts and basaltic andesites 

of Mud Creek Rim. 0.4

 55.1 Turnoffs to North Shore and Last Chance 

campgrounds. 0.4

55.5 East shore of Lake Almanor. Across the northern 

arm of Lake Almanor is a beautiful view of Brokeoff 

Road Log, Section G

Volcano and the Bumpass and Eagle Peak sequences 

of the Lassen domefield. A few cars can pull off onto 

the right shoulder of the highway, but be very aware 

of the fast-moving traffic coming down the fault

scarp from the east. Much better parking is on the 

left (south) side of the highway. Safest access to this 

parking is to drive west across the causeway into 

Chester, make a U turn, and drive back to the safe 

parking on the south side of Calif. Hwy 36. 1.6

 57.1 Intersection to right at Fire Station with Plumas 

County Roads 318 (to Juniper Lake) and 312 (to 

Warner Valley and Drakesbad, in the southeast 

corner of LVNP). A trail leads from the warm 

springs at Drakesbad through beautiful meadows and 

virgin forest to Devils Kitchen, a high-temperature 

fumarolic area (Muffler and others, 1982, 1983).

Section G: Alternate Route from Hat Creek Rim 

Overlook to Cinder Cone and Poison Lake Chain

If one walks into Cinder Cone from Butte Lake, section 

G could take a full day; if one just drives it, a few hours 

will suffice. From mile 2.0 on, the road log follows graded 

gravel and dirt roads, sometimes washboard and rough. Not 

recommended for buses or cars with low clearance. 

0.0 Return to Calif. Hwy 44. 0.4 

0.4 Turn left onto Calif. Hwy 44 and proceed north and 

east towards Susanville. 1.6

2.0 Turn right onto U.S. Forest Service Road 33N56. 1.4

3.4 Stop G-1: Parhams Point. This informally named 

overlook on the Hat Creek Rim presents an excellent 

view of the Cascade Range (fig. 37). Scramble ~30 

feet down the scarp to several rocky outcrops below 

the Pacific Crest trail. Watch out for poison oak and 

rattlesnakes. View is best in the morning. The rock 

here (best seen on the east side of the road) is a very 

porphyritic olivine-augite basalt, not the tholeiitic 

“Burney Basalt” shown on Macdonald (1964). 

Continue southeast on U.S. Forest Service Road 

33N56 away from the Hat Creek Rim. 0.6

 4.0 Turn right (south) on U.S. Forest Service Road 

33N20. 0.2

 4.2 Junction with U.S. Forest Service Road 33N20A 

leading right to Baker Lake. Continue south on U.S. 

Forest Service Road 33N20 passing onto the basaltic 

andesite of Signal Butte (1,000 to ~900 ka). 0.8

 5.0 Contact with the tholeiitic basalt of Calif. Hwy 44. 

This thin lava flow of sparsely porphyritic olivine 

basalt (49.6 percent SiO2) erupted from a fissure at hill 

5855, 2.6 mi to the southeast. This extensive lava flow 

was fed by lava tubes and flowed north along the base 

of Butte Creek Rim for>9 mi (15 km). It also flowed 

to the west and barely spilled over the Hat Creek Rim. 

The surface of the flow is relatively flat and has low 
52 Geologic Field-Trip Guide to Lassen Volcanic National Park and Vicinity, California

relief, although punctuated by widely spaced tumuli. 

A40Ar/39Ar age of 190±18 ka is consistent with the 

flow’s excursional magnetic direction (D.E. Champion, 

written commun., 2006). 0.2

 5.2 Boundary of Old Station quadrangle with Swains Hole 

quadrangle to the east. 0.8

 6.0 Complex road junction. Bear 30° to right on U.S. 

Forest Service Road 32N56. 2.3

 8.3 On left is a dirt road leading north 0.3 mile to a small 

“cinder pit” in the tholeiitic basalt of Calif. Hwy 44. 

The low hills north of the cinder pit comprise the 

partially eroded spatter rampart that is the vent for this 

basalt. 0.2

 8.5 Stop G-2: Andesite of hill 7416. Pull off on right 

at spur Road 32N56C. Walk back 100 m to the west 

along Road 32N56 to inspect flow front of the andesite 

of hill 7416, part of the younger Twin Lakes sequence 

of the Lassen domefield. This thick (as much as 50 m) 

block-lava flow of porphyritic olivine-augite andesite 

(57.5 percent SiO2) erupted from a vent marked by a 

scoria cone on the upper east flank of West Prospect 

Peak. Lava flowed northward down the crease between 

Prospect Peak (247±56 ka) and West Prospect Peak 

(400–300 ka) and partially buried the trace of the 

Hat Creek Fault on the north flank of Prospect Peak. 

The flow has not been glaciated, is unforested, and is 

completely lacking in soil; its original block surface 

is well preserved. Climb to the top of the flow front 

for a view of the top surface of the flow and a distant 

view of the vent. The lava contains a disequilibrium 

phenocryst assemblage consisting of strongly reacted 

sodic plagioclase, quartz, hornblende, and biotite(?) 

and unreacted olivine, augite, and calcic plagioclase. 

The unit is not dated, but its age is well constrained 

by stratigraphy. The flow is older than the pumice 

of Chaos Crags (1,103±13 B.P.) but is not offset by 

the Hat Creek Fault, which offsets 17–15 ka glacial 

deposits farther north (Muffler and others, 1994). The 

flow may be of Holocene age, but is more likely to be 

about 15–12 ka. 0.1

 8.6 Unimproved dirt road leads left (north) 0.5 mile to 

another part of the inconspicuous, agglutinated spatter 

rampart in the vent area of the tholeiitic basalt of Calif. 

Hwy 44. 0.4

 9.0 Cross over yellow cattle guard and turn right 

(southeast) on U.S. Forest Service Road 32N92Y. 0.2

 9.2 Cross from tholeiitic basalt of Calif. Hwy 44 onto 

block-lava flows of basaltic andesites of Prospect 

Peak (~57 percent K2O content). Some of these flows 

contain olivine, augite, and orthopyroxene, thus 

capturing the reaction of olivine and melt to form 

orthopyroxene. This reaction generally occurs in the 

Lassen suite at about 56 percent SiO2. 0.2

 9.4 Boundary of Swains Hole quadrangle with Prospect 

Peak quadrangle to the south. 0.7

10.1 Good exposure of basaltic andesites of Prospect Peak. 0.4

10.5 Intersection with U.S. Forest Service Roads 32N82Y 

(to left) and 32N44 (to right). 0.5

11.0 Pass from basaltic andesite of Prospect Peak onto 

the unit “till, younger glaciations” (fig. 11) mantled 

by ash from Cinder Cone (1666 C.E.). This young 

till was deposited by a lobe of ice extending north 

from Lassen Volcanic National Park and the Caribou 

Wilderness area down the valley of Butte Creek. 0.4

11.4 Turn right on U.S. Forest Service Road 32N21 south 

towards Butte Lake and Lassen Volcanic National 

Park. The road junction is on the young till just 

west of Butte Creek; the till extends ~0.7 km north 

of the junction. As one proceeds south on this road 

towards LVNP, the ground surface is covered by an 

increasingly thick layer of dark-gray ash erupted from 

Cinder Cone in 1666 C.E. 1.4

12.8 Junction to left with U.S. Forest Service Road 32N09 

that leads to Pole Springs Road. Continue south on 

U.S. Forest Service Road 32N21. The road is on 

young till, but just to the right of the road are heavily 

glaciated outcrops of the basalt of Bathtub Lake 

(~400 ka), a unit of the Sunrise Peak sequence of the 

Caribou Volcanic Field. Similar outcrops overlain by 

till are seen on both sides of the road from here south 

to Butte Lake. 0.3

13.1 Boundary of Lassen Volcanic National Park. The ash 

from Cinder Cone is about 30 cm thick in this area. 1.8

14.9 Stop G-3: Campground and picnic ground at 

Butte Lake in Lassen Volcanic National Park.

Immediately across the lake is the flow margin of the 

Fantastic Lava Beds flow; its vent, at Cinder Cone, is 

out of sight through the trees to the right. The summit 

of Cinder Cone can be accessed via a 1.8-mile trail 

starting at the west side of the parking lot. From this 

trail, one can see the flow margin of the Fantastic 

Lava Beds flow and patches of white diatomite that 

underlie the flow. Continuing on the trail provides 

views of the Old Bench and Painted Dunes lava 

flows. Ascending the trail up Cinder Cone provides 

spectacular views of the Painted Dunes and much of 

the rest of LVNP.

Cinder Cone, part of the younger Twin Lakes 

sequence of the Lassen domefield, has received 

considerable attention in the literature since its initial 

description by Harkness (1875) and the careful 

study by Diller (1891) and has been the subject 

of considerable controversy concerning its age 

and eruptive history (see, for example, Finch and 

Anderson, 1930; Finch, 1937). Miners claimed to 

have witnessed an eruption in the winter of 1850–51, 

and Harkness (1875) described the eruption and 

promoted that date. Diller (1891) attributed the 

deposits to two periods of eruption, one at about 

1675 C.E. and the second before about 1840 C.E. 

Using tree-ring studies, Finch (1937) advocated 

five eruptions (1567, 1666, 1720, 1785, and 1850). 
Road Log 53

Recent work by Clynne and others (2000a) and a 

dendrochronologic age reported by Sheppard and 

others (2009) demonstrate that Cinder Cone had only 

a single short eruption in 1666 C.E. Cinder Cone is 

the youngest mafic volcano in the Lassen region. 

The Cinder Cone eruption produced five lava 

flows of basaltic andesite to andesite composition, 

two scoria cones, and a widespread ash-fall deposit 

(Clynne and others, 2000a, 2002; Clynne and 

Muffler, 2010). The ash, divided into three units 

by Heiken (1978), covers an area of >200 km2 and 

accounts for 20 percent of the 0.36 km3 total volume 

of the Cinder Cone eruption. The change in ash 

composition (illustrated by SiO2 content) with time 

(proxied by height in the composite section) is shown 

in figure 39. The eruption began with construction 

of the first scoria cone, deposition of ash unit 1, and 

emplacement of the Old Bench flow, followed by 

deposition of ash unit 2. Painted Dunes 1 and Painted 

Dunes 2 flows erupted during the waning phases of 

ash unit 2; the first scoria cone was mostly destroyed 

by effusion of these flows. Ash unit 3 constructed 

Cinder Cone itself; the Fantastic Lava Beds were 

erupted at the same time from the south base of the 

cone. Ash units 2 and 3 fell on the Old Bench flow, 

and ash unit 3 fell on the Painted Dunes flows. Both 

of these flows were still hot enough to oxidize the 

overlying ash to form the colorful Painted Dunes, 

thus demonstrating that the entire Cinder Cone 

sequence was produced in a single eruptive event. 

Diatomite deposits bulldozed up by the flows 

demonstrate that the lavas flowed into and partially 

filled ancient Lake Bidwell (now called Butte Lake). 

The ash and the flows are sparsely porphyritic 

basaltic andesite to andesite. The most abundant 

54 55 56 57 58 59

320

280

240

200

160

120

80

40

0

Painted Dunes 2 flow

Painted Dunes 1 flow

Fantastic Lava Beds 1 flow

Fantastic Lava Beds 2 flow

Old Bench flow

Ash unit 1

Ash unit 2

Ash unit 3

Tephra sample height, in cm

SiO2, in weight percent

EXPLANATION

Figure 39. Graph showing 

variation of SiO2 versus height (a 

proxy for time) in a composite ash 

section at Cinder Cone. The Old 

Bench flow was emplaced following 

ash unit 1. The Painted Dunes 

flows were emplaced during the 

waning stages of ash unit 2. The 

Fantastic Lava Beds flows erupted 

concurrently with ash unit 3. Lines 

indicate the stratigraphic position 

of the lava flows in the ash section 

and the range of SiO2 in the flows.

phenocryst is magnesian olivine (Fo88–90) with 

inclusions of chromian spinel, accompanied by 

sparse plagioclase and augite. Chemical analyses 

document three compositional trends (two simple; 

one complex) from silicic to mafic compositions (fig. 

39). Ash unit 1 ranges from 56 to 53.5 percent SiO2, 

ash unit 2 ranges from 56 to 53 percent and back to 

nearly 60 percent SiO2, and ash unit 3 ranges from 

58 to 55 percent SiO2. Four types of inclusions are 

present: (1) sparse, mostly melted, granitic rocks 

derived from Sierran basement, (2) abundant single 

crystals of quartz derived from granitic rocks, (3) 

multicrystalline quartz derived from metamorphic 

veins, and (4) rare magmatic enclaves of olivine 

basalt with rapid growth textures. 

Compositional variation of the early magmas 

is modeled by assimilation of granitic xenoliths 

(fig.40). The compositional reversal at the boundary 

between ash units 2 and 3 and the subsequent 

decrease in SiO2 indicate mixing with a new mafic 

magma having the composition of the high-TiO2

quenched magmatic inclusions. 

The mountains to the southeast of Stop G-3 

are glaciated flows of the Caribou Volcanic Field, 

including the basalts of Sunrise Peak (393±4 ka). 

From Stop G-3, a cross-country scramble up the 

west flank of Sunrise Peak to the prominent, bare 

bluff (at approximately UTM 10T E0645650 

N4490680 NAD27) produces a spectacular view of 

Cinder Cone, Lassen Peak, and Chaos Crags (cover;

photograph 31 of Muffler and others, 2010).

Return north along U.S. Forest Service Road 

32N21. 2.1

17.0 Turn right (east) on U.S. Forest Service Road 32N09. 

The road for the next 2.1 miles is on till of younger 

Road Log, Section G
54 Geologic Field-Trip Guide to Lassen Volcanic National Park and Vicinity, California

glaciations that was deposited by the lobe of ice 

extending north from Lassen Volcanic National Park 

and the Caribou Wilderness area. 1.3

18.3 Pole Spring is in the marshy area to left of the road. 0.8

19.1 Turn left (north) on U.S. Forest Service Road 32N26. 

Road is on the right lateral moraine banked to the east 

against the unglaciated basalt of Twin Buttes (46.3±3.4 

ka), part of the Bidwell Spring chain in the northern 

(unglaciated) part of the Caribou Volcanic Field. 0.3

19.4 Road climbs from till of younger glaciations onto the 

basalt of Twin Buttes. Although this large lava flow 

is bounded on the west by till of younger glaciations, 

its flow top is not glaciated. The vents for the basalt 

of Twin Buttes are marked by two large scoria cones 

(Twin Buttes) 2 mi to the southeast. 0.5 

19.9 Road climbs onto the overlying basaltic andesite of 

section 36, a very young (~40–25 ka) flow in the 

Bidwell Spring chain of the northern Caribou Volcanic Field. The vents for this flow are two scoria cones 

0.5 mi to the southeast. 0.6 

20.5 Junction with U.S. Forest Service Road 32N35 to the 

right. Continue north on U.S. Forest Service Road 

32N26. To the left for the next 0.5 mile are the scoria 

cones of the basaltic andesite of Pole Spring Road, 

another very young (~45–25 ka) unglaciated flow of 

the Bidwell Spring Chain of the northern Caribou 

Volcanic Field. This unit overlies the lava flow of the 

basalt of Twin Buttes (on the right of the road). 0.5

21.0 For the next 0.6 mile, the road is on the basalt of hill 

2109, a calc-alkaline basalt erupted from two large scoria 

cones 1.5–2.0 miles to the southeast. This basalt, part of 

the Cone Lake chain, is much older than the flows of the 

Bidwell Spring chain, probably 250–225 ka. 0.3

21.3 To the left is the very young flow margin of the 

basaltic andesite of Pole Spring Road. 0.3

21.6 Road descends onto the tholeiitic basalt of hill 1879 

(~190 ka). This short flow, vented from a scoria 

cone ~1 mile to the southeast, is unusually small for 

tholeiitic basalt in the Lassen area. 0.9

22.5 Boundary of Prospect Peak quadrangle and Swains 

Hole quadrangle to the north. 0.1

22.6 Road is on colluvium overlying low, obscure outcrops 

of the basalts and basaltic andesites of Swains Hole. 

This unit is poorly exposed here, but is well exposed to 

the north in the Butte Creek Rim. The age of the unit is 

poorly constrained, probably ~725–600 ka. 0.7

23.3 Boundary of Swains Hole quadrangle and Poison 

Lake quadrangle to the east. 0.7

24.0 Intersection of U.S. Forest Service Road 32N26 with 

Calif. Hwy 44 at mile 10.3 of road log from Hat 

Creek Rim overlook to Chester (Section F). 

Section H: Intersection of Calif. Hwys 44 and 89 

at the northwest entrance to Lassen Volcanic 

National Park west to Anderson

0.0 Section H starts at the end of Section D and the beginning of Section E. Roadcuts at the intersection of Calif. 

Hwys 44 and 89 expose the pumiceous pyroclasticflow deposits derived from Eagle Peak (66±4 ka). 

Drive west on Calif. Hwy 44 towards Redding. The 

highway heads down the ancestral drainage of Manzanita Creek, which contains the upper pyroclastic flow 

of Chaos Crags. This ancestral drainage was blocked 

by the landslide of Chaos Jumbles ~350years ago. 

Figure 40. Harker diagram 

showing variation of TiO2 versus 

SiO2 for the array of compositions 

at Cinder Cone. The mixing line 

between the composition of the 

Old Bench flow and ash unit 1 and 

the composition of the granitic 

inclusions indicates that variation 

is caused primarily by assimilation 

of up to 30 percent of granite. The 

composition of Devonian granite 

exposed 20 km to the south is a 

proxy for the granite composition. 

Quenched inclusions with higher 

TiO2 indicate that a second mafic 

magma also interacted with 

granite to produce the array of 

Fantastic Lava Beds and ash unit 

3 compositions. 0

0.2

0.4

0.6

0.8

1.0

1.2

50 55 60 65 70 75 80

EXPLANATION

Fantastic Lava Beds 2 flow

Fantastic Lava Beds 1 flow

Ash unit 3

Quenched inclusion

Painted Dunes 2 flow

Painted Dunes 1 flow

Ash unit 2

Old Bench flow

Ash unit 1

Granitic inclusion

Devonian granite

SiO2, in weight percent

Ti

O2, in weight percent
Road Log 55

Manzanita Creek now flows in a channel approximately 0.5 mile to the south of the road. 0.3

0.3 Roadcuts along the next few miles expose a complex 

stratigraphy of rocks from the Lassen Volcanic 

Center and regional volcanoes. The bouldery deposit 

on the right is the avalanche deposit from the dacite 

dome of hill 8283, located just northwest of Lassen 

Peak. The dacite of hill 8283 (261±5 ka) is part of 

the Bumpass sequence. The avalanche occurred 

some time between 260 and 200 ka. 0.1

0.4 Roadcuts, primarily on the right, expose a thick and 

widespread two-pyroxene andesite lava flow, the 

450–425 ka andesite of Manzanita Creek, part of 

the Diller sequence of Brokeoff Volcano. The vent 

for this andesite is marked by a scoria cone just 

southeast of Manzanita Lake. 0.7

1.1 The gravel road to the left (U.S. Forest Service 

Road 17) leads south to the town of Mineral, 

approximately following the western boundary of 

the Lassen Volcanic Center. Many good exposures 

of rocks related to Brokeoff Volcano, the Maidu 

Volcanic Center, and glacial deposits can be seen 

along this route. In addition to the rocks described 

at mile 0.3, the pyroclastic flow from Eagle Peak 

(66±4 ka), a tuff deposit of lithic material from the 

formation of the nearby phreatic explosion crater 

called Deep Hole (~200ka), and another rhyodacite 

pyroclastic flow whose origins are obscure are all 

exposed in the first roadcut, approximately 50 m 

south of Calif. Hwy 44. 0.3

1.4 Roadcuts in augite-olivine basaltic andesite lava 

flows from Red Lake Mountain (~75 ka). 0.8

Andesite of Manzanita Creek

450–425 ka

Lithic tuff deposit

Surge deposits

Surge deposits

Basaltic andesite of Red Lake Mountain

~75 ka

Flow-top breccia

Basal breccia

Flow core

Andesite of Manzanita Creek

2.2 Stop H-1: Roadcut exposing complex volcanic 

stratigraphy west of Manzanita Lake. At the 

base of this new roadcut is the 450–425-ka, twopyroxene andesite of Manzanita Creek (discussed 

at mile 0.4). The lava flow is deeply weathered and 

has a thick soil. Locally preserved on top of the soil 

is the lithic tuff deposit from the formation of Deep 

Hole (discussed at mile 1.1). The lithic tuff in this 

new roadcut is only a few cm thick and consists of 

lithic lapilli, mostly <5 mm, of andesite of Viola. The 

overlying surge deposits consist of dark, crystal-rich, 

bedded tuff containing abundant augite and olivine 

and are about 50 cm thick in this roadcut. The surge 

deposits are slightly older than the lava flows from 

Red Lake Mountain (~75 ka) but younger than the 

formation of Deep Hole. Their source is unknown but 

must have been close by and might be related to the 

Red Lake Mountain volcano. Directly overlying the 

surge deposits are lava flows of the basaltic andesite 

of Red Lake Mountain. Basal and flow-top breccias 

enclosing a dense, massively jointed flow core are 

well exposed (fig. 41). 0.2

 2.4 The flat on both sides of the highway is underlain 

by the avalanche deposit from the dacite dome of 

hill 8283 (261±5 ka) and by pyroclastic flows from 

Eagle Peak (66±4 ka). Manzanita Creek rejoins its 

ancestral channel on the south side of this flat area. 

White pumice from the upper pyroclastic flow of 

Chaos Crags (1,103±13 B.P.) covers the surface of 

the flat. 0.6

 3.0 Boundary of Manzanita Lake quadrangle with Viola 

quadrangle to the west. 0.2

Figure 41. Photograph of roadcut 

exposing complex volcanic 

stratigraphy west of Manzanita 

Lake at mile 2.2 of Section H. Highly 

weathered andesite of Manzanita 

Creek (450–425 ka) is overlain by the 

lithic tuff deposit from the formation 

of Deep Hole (see inset). This 

thin, discontinuous unit is overlain 

by about 50 cm of bedded surge 

deposits, possibly associated with 

the eruption of the basaltic andesite 

of Red Lake Mountain (~75 ka). Top 

of roadcut exposes both the flow-top 

and basal breccias of the basaltic 

andesite of Red Lake Mountain as 

well as its dense flow core.

Road Log, Section H
56 Geologic Field-Trip Guide to Lassen Volcanic National Park and Vicinity, California

3.2 Cliffs above the highway on the right are lava flows 

from Red Lake Mountain. Highway is on the andesite 

of Viola, a 313±8 ka lava flow of the older Twin Lakes 

sequence of the Lassen Volcanic Center. This flow is the 

oldest of the hybrid andesite lava flows of the Lassen 

domefield. Its vent is 3 miles southeast. The upper pyroclastic flow of Chaos Crags crops out sporadically along 

Manzanita Creek, just to the left of the highway. 0.8

 4.0 Roadcuts on the right are in andesite of Viola. 0.5

 4.5 To the right of the highway in the Manzanita Creek 

drainage, bulldozing has exposed a thin, white veneer 

of the upper pyroclastic flow of Chaos Crags. This 

unit can be traced west along Manzanita Creek for at 

least 3.5 miles to McCumber Flat, the last 2 miles as 

a lahar. For the next 1.3 miles, Calif. Hwy 44 is on 

the upper surface of the andesite of Viola. 1.3

 5.8 For the next 0.3 mile, the highway lies on andesites 

of the Diller sequence of Brokeoff Volcano. 0.3

 6.1 Slope to left of highway is the toe of the andesite of 

Viola. 0.3

 6.4 Lassen Pines Christian Camp at the junction with 

the road southeast to Brokeoff Meadows and the 

road south to Mineral. Settlement of Viola is just 

south of the junction. The flat for the next few miles 

along Calif. Hwy 44 is formed by glacial outwash 

that consists of material transported by streams from 

the terminal moraine complexes of glaciers in the 

Manzanita and Bailey Creek drainages. 0.4

 6.8 Deer Flat Road to right. 0.8

 7.6 Good view back towards the east of Brokeoff 

Mountain and Mount Diller, the major remnants of 

the western flank of Brokeoff Volcano. 1.0

 8.6 Rock Creek Road to left leads to the community of 

Manton. 0.3

 8.9 Long Hay Flat Road to left also leads to Manton. 1.2

10.1 Boundary of Viola quadrangle with Hagaman Gulch 

quadrangle to the west. 0.1

10.2 North Fork Battle Creek. For the next several miles 

the highway travels over the tholeiitic basalt of Eagle 

Canyon. This LKOT lava flow erupted from a vent near 

Manzanita Lake, and its age is 199±22 ka. The lava 

flowed south over the Battle Creek Fault scarp but is not 

offset, indicating that the fault has been inactive for at 

least 200 ka, and probably closer to 500 ka. 0.1

10.3 Lake McCumber Road. 2.0

12.3 Satellite Pines. Just ahead is a good roadcut in 

tholeiitic basalt of Eagle Canyon. 0.5

12.8 Ritts Mill Road. 0.6

13.4 Airport Way. 1.0

14.4 Mt. Lassen Woods. 0.2

14.6 Rest Stop. 0.4

15.0 For the next 0.5 mile, Calif. Hwy 44 descends a 

gentle slope that marks the western edge of the 

tholeiitic basalt of Eagle Canyon. 0.5

15.5 Millseat Creek marks the contact between tholeiitic 

basalt of Eagle Canyon and older tholeiitic basalt 

flows collectively called the basalt of Coleman 

Forebay (Blake and others, 1999). 40Ar/39Ar dates on 

these basalts are 949±22, 948±28, and 778±43ka 

(M.A. Lanphere, written commun., 1997). Vent 

locations are to the northeast, probably buried by 

younger lavas. The soil on the basalt of Coleman 

Forebay is several meters thick, whereas the soil on 

the basalt of Eagle Canyon (199±22 ka) is only a few 

tens of centimeters thick. Millseat Creek, in great part, 

is fed by springs that emerge from the base of the 

basalt of Eagle Canyon. Boundary of Hagaman Gulch 

quadrangle with Manton quadrangle to the south. 1.5

17.0 Boundary of Manton quadrangle with Shingletown 

quadrangle to the west. 0.6

17.6 Town of Shingletown. To left, Wilson Hill road leads 

down over the Battle Creek Fault scarp and connects 

with roads to the community of Manton. 0.7 

18.3 Pass from basalt of Coleman Forebay onto older 

regional calc-alkaline andesites and basaltic 

andesites. These rocks form a platform of normally 

faulted and eroded lava flows erupted from numerous 

small vents. Ages are poorly known but probably 

range from 1 Ma to possibly as old as 3.5 Ma. 0.4

18.7 Pass from regional calc-alkaline andesites and basaltic 

andesites back onto the basalt of Coleman Forebay. 0.4

19.1 Shasta Forest Drive. Pass from the basalt of Coleman 

Forebay back onto regional calc-alkaline andesites 

and basaltic andesites. 0.5

19.6 Boundary of Shingletown quadrangle with Inwood 

quadrangle to the north. 0.4

20.0 Roadcuts on both sides of the highway expose regional 

andesites and basaltic andesites. Over the next 11.5 

miles, the highway passes over more of these andesites 

as well as the basalt of Coleman Forebay. 0.3

20.3 Basalt of Coleman Forebay. 0.6

20.9 To the north (right) along Ponderosa Way are found 

outcrops of the Tuscan Formation (3.5–2.5 Ma), 

in this region probably derived from the Latour 

Volcanic Center and (or) other unknown sources to 

the northeast. Within the Tuscan Formation is the 

Nomlaki Tuff Member (~3.3Ma), a thick dacitic 

ash-flow tuff of regional extent that may also have 

erupted from the Latour Volcanic Center. One can 

follow Ponderosa Way 2.1 mi north and then turn left 

(west) for 0.8 mile to Bear Creek Falls, where the 

Nomlaki Tuff is densely welded and >100 m thick; it 

probably was ponded in a canyon. 0.5

21.4 Basalt of Coleman Forebay along road, with hills to left 

being older regional andesites and basaltic andesites. 0.1

21.5 Power line. 1.5

23.0 Pine Bluff Road. Pass from basalt of Coleman 

Forebay onto Tuscan Formation. Boundary of Inwood 

quadrangle with Shingletown quadrangle to the 

south, just south of settlement of Midway. 0.2

23.2 Pass from Tuscan Formation onto basalt of Coleman 

Forebay. 0.4
Road Log 57

23.6 Boundary of Shingletown quadrangle with Inwood 

quadrangle to the north. Still basalt of Coleman 

Forebay. 0.6

24.2 Black Butte Road. 0.5

24.7 Boundary of Inwood quadrangle with Clough Gulch 

quadrangle to the west. 0.1

24.8 Inwood Road. 1.5

26.3 Turn left from Calif. Hwy 44 onto Dersch Road. 1.2

27.5 Boundary of Clough Gulch quadrangle with Tuscan 

Buttes NE quadrangle to the south. For the next 

5.6miles, the road traverses a series of poorly understood 

tholeiitic basalts, mapped by Blake and others (1999) 

as the basalt flows of Shingletown Ridge but probably 

equivalent to the basalt of Coleman Forebay. 1.9

29.4 Bridge over Lack Creek. 0.8

30.2 To the left is Ash Creek Road. Bear right and 

continue west on Dersch Road. 2.5

32.7 Boundary of Tuscan Buttes NE quadrangle with Balls 

Ferry quadrangle to the west. 0.5

33.2 Bridge over Lack Creek. Drop down from basalt 

onto Tuscan Formation. Hills to the right are Tuscan 

Formation. Flats to the left are gravels of the 

Pleistocene Riverbank Formation. 0.9

34.1 Bridge across Bear Creek. Bluffs on west side of creek 

are tholeiitic basalts, mapped by Blake and others 

(1999) as the basalt flows of Shingletown Ridge. 0.2

34.3 For the next 2 miles, the road is just south of the Bear 

Creek Fault, a minor east-trending fault that uplifts 

Tuscan Formation and Riverbank Formation to the 

north. The Riverbank Formation consists of gravel, 

sand, silt, and clay of Pleistocene age (Blake and 

others, 1999). 2.0

36.3 Parkville Road to left. 0.8

37.1 Bridge over Dry Creek. West of Dry Creek, the 

road climbs onto a bench of Red Bluff Formation 

and overlying Riverbank Formation, both underlain 

by the much older Tuscan Formation. Here, the 

Red Bluff Formation is a coarse gravel with a 

characteristic red color (Blake and others, 1999). 0.9

38.0 Road drops through roadcuts of Red Bluff and 

Riverbank Formations to Cow Creek. 0.3

38.3 Bridge over Cow Creek. From here to the end of the 

road log at I-5, the road traverses various Pleistocene 

and Holocene sediments of the northern Sacramento 

Valley. 0.7

39.0 Four-way stop at blinking red light. Turn left onto 

Deschutes Road. 1.0

40.0 Bridge over Sacramento River. 0.8

40.8 Boundary of Balls Ferry quadrangle with Cottonwood quadrangle to the west. 1.4

42.2 Sharp right bend at sawmill. 0.9

43.1 Pass under Interstate 5. 0.3

43.4 Stoplight. Turn left onto Calif. Hwy 273 (old Calif. 

Hwy 99), which leads south to Interstate 5. 0.2

43.6 Stoplight. Continue straight. 0.5

44.1 Merge with I-5 south to Red Bluff.

Section I: Old Station to Burney

 0.0 Junction of Calif. Hwys 89 and 44. Proceed north on 

Calif. Hwy 89. 0.3

 0.3 Turnoff to right leads to Subway Cave, a U.S. Forest 

Service exhibit that allows exploration of part of a 

lava tube in the Hat Creek Basalt. For the next 17 

miles, Calif. Hwy 89 lies primarily on the Hat Creek 

Basalt, at 24±6ka the youngest volcanic rock in 

the north-trending Hat Creek Graben (fig. 42). This 

LKOT lava erupted from a 3-km-long spatter rampart 

(see Section E, Stop E-1) located just south of Old 

Station (fig. 42) and flowed in lava tubes for nearly 

20 miles (32 km) to the north. The flow covered 

~100km2, is as thick as 50 m along the axis of the 

Hat Creek Graben, and has a volume of ~2.5km3.

To the right is the Hat Creek Fault (see Stop 

E-2). According to Blakeslee and Kattenhorn (2013), 

the Hat Creek Fault is composed of three subparallel 

systems of scarps of different ages that accrued 

a cumulative throw in excess of 600 m (see also 

Muffler and others, 1994, and Walker, 2008). The 

oldest and largest system of scarps, referred to as 

the Rim Scarp, has as much as 350 m of throw and 

defines the easternmost extent of the fault system. 

A lava flow at the top of the footwall has a K-Ar 

age of 924±24ka (Clynne and Muffler, 2010), thus 

constraining the maximum age of the fault system 

(Blakeslee and Kattenhorn, 2013). The fault scarps of 

intermediate age west of the Rim Scarp are referred 

to by Blakeslee and Kattenhorn (2013) as the Pali 

Scarp and have accrued as much as ~175 m of 

throw (Walker, 2008). The youngest system of scarp 

segments, referred to by Blakeslee and Kattenhorn 

(2013) as the Active Scarp, has a maximum throw of 

56 m and exhibits evidence of repeated earthquake 

activity since the late Pleistocene (fig. 43). 

To the left is the rugged edifice of the andesite of 

Sugarloaf Peak (46±7 ka; Turrin and others, 2007), 

the most prominent volcano of the Sugarloaf chain 

(fig.42). Units of the Sugarloaf chain bury young faults 

on the west boundary of the Hat Creek Graben. 3.6

3.9 Calif. Hwy 89 crosses Hat Creek and passes from the Hat 

Creek Basalt onto an outlier of the older andesitic rocks 

that form the west rim of the Hat Creek Graben. 1.2

 5.1 Rocks to the left of the road for the next 1.1 miles are 

units of the Sugarloaf chain. The cones adjacent to the 

road are the andesite of hill 4041 and the andesite of 

Calif. Hwy 89, whereas the large volcano to the south 

is the andesite of Sugarloaf Peak. 1.6

 6.7 After crossing a small patch of alluvium, the highway 

returns to the Hat Creek Basalt. In the right distance 

is the valley of Lost Creek, which cuts a prominent 

canyon through the Hat Creek Rim. The water in 

Lost Creek is derived by underground flow from Butte 

Creek (east of the Hat Creek Rim) and ultimately from 

Road Log, Section I
58 Geologic Field-Trip Guide to Lassen Volcanic National Park and Vicinity, California

44

44

89

89

89

22

22

 Old Station

Sugarloaf 

 Peak

Cinder 

Butte

Potato 

Butte

Little 

 Potato

 Butte

H AT

C R E E K

V A L L E Y

Retreated ~35° west-dipping 

 scarps of Hat Creek fault system

Young fault scarps cutting 

 Hat Creek Basalt and basaltic

 andesite of Cinder Butte

Fault scarps marking west 

 side of Hat Creek Valley 

 (dotted where concealed)

Tholeiitic vent

Calc-alkaline vent

Flow direction

Andesites and basaltic andesites of 

 Sugarloaf chain: Sugarloaf Peak 

 (46±7 ka), Potato Butte (77±11 ka), 

 Little Potato Butte (67±4 ka), and 

 related vents 

Hat Creek Basalt (24±6 ka)

Outwash gravels related to 

 recession of last major glaciation 

 (~15 ka)

Flowage deposits from 1915 Lassen 

 Peak and 1,103±13 B.P. Chaos Crags 

 eruptions

Basaltic andesite of Cinder Butte 

 (38±7 ka)

Older volcanic rocks (mostly early 

 Pleistocene)

EXPLANATION

89 California State highway

National Forest road 22

0 1 2 KILOMETERS

0 1 2 MILES

STOP I-1

40°52.5'

121°22.5'

40°37.5'

121°30'

40°45'

Alluvium (Holocene)

Figure 42. Generalized geologic map of 

the northward-flowing Hat Creek Basalt. 

Adapted from Turrin and others (2007).
Road Log 59

the eastern part of Lassen Volcanic National Park. In 

this area, the Hat Creek Basalt is overlain by ~15-ka 

gravel deposits that are related to the end of the last 

major glaciation. These gravels are displaced ~20 m 

across the Active Scarp of the Hat Creek Fault (Muffler 

and others, 1994). Despite this conspicuous evidence of 

young faulting, modern seismicity in this area is low and 

diffuse (LaForge and Hawkins, 1986; Waldhauser and 

Schaff, 2008; see also figure 3 of Blakeslee and Kattenhorn, 2013). The Hat Creek Fault has produced no large 

historical earthquakes (Blakeslee and Kattenhorn, 2013). 

The ridge to the left (west) of the highway consists 

of older andesitic rocks and separates the Hat Creek 

Basalt from a north-trending belt of quite young 

volcanic rocks, as yet undated. 0.2

 6.9 Boundary of Old Station quadrangle with Murken 

Bench quadrangle to the north. 1.3

 8.2 Boundary of Murken Bench quadrangle with Burney 

Mountain East quadrangle to the west. 2.7

 10.9 Turn right (east) onto U.S. Forest Service Road 6R200 

(Doty Road), still on Hat Creek Basalt. 0.9

 11.8 Cross Hat Creek. 0.4

 12.2 Turn right onto U.S. Forest Service Road 6R201 (also 

known as U.S. Forest Service Road 22 and Bidwell 

Figure 43. Oblique satellite image with perspective view down and to the northeast of the northern part of the Hat Creek 

Fault. The Active Scarp is shown in red, the Pali Scarp in green, and the Rim Scarp in blue; fault lines are drawn at the top of 

the scarps. Dashed orange line is the margin of the basalt west of Six Mile Hill, dashed yellow line is the margin of the basaltic 

andesite of Cinder Butte, and dashed white line is the margin of the Hat Creek Basalt. Figure is adapted from figure 5 of Blakeslee 

and Kattenhorn (2013).

Road Log, Section I

Road) and continue east across Hat Creek Basalt. 0.1

 12.3 Boundary of Burney Mountain East quadrangle with 

Murken Bench quadrangle to the east. 1.7

 14.0 Road to right (south) leads to the University of 

California Radio Observatory. 0.5

 14.5 At T junction, turn left. 0.3

 14.8 Stop I-1: Active Scarp of the Hat Creek Fault. 

See figures 42 and 43 for location, figure 44 for 

photographic view. The Active Scarp of the Hat Creek 

Fault is not just a simple disruption of the Hat Creek 

Basalt along a vertical fault. A succinct summary of the 

near-surface fault features is presented by Blakeslee 

and Kattenhorn (2013, p. 1402; see also their figure 6): 

The Active Scarp is also characterized 

by a hanging wall fault-trace monocline, 

representing the near-surface flexing of 

the Hat Creek Basalt above an upwardpropagating fault tip prior to initial surface 

breaching by the fault. 

Along the Active Scarp, the monocline 

accounts for as much as 33 m of throw 

(Walker, 2008), implying numerous fault slip 

events prior to breaching of the surface along 

38±7 ka

24±6 ka

Basalt west of

Six Mile Hill

53.5±2 ka

CALIF. HWY 89

Basaltic andesite of

Cinder Butte

Hat Creek Basalt

Hat Creek Rim

Pit River

Fall River

Mills

Flow

direction

Stop I-1

Bald Mountain

~2,500 ka 

Basalt of Popcorn Cave

~50 ka

Maximum throw on southern

segments of Active Scarp (56 m) Maximum throw on

Pali Scarp (175 m)

Maximum throw on

Rim Scarp (350 m)

Maximum throw on northern

segments of Active Scarp (30 m)

Active Scarp

Pali Scarp

Rim Scarp

EXPLANATION
60 Geologic Field-Trip Guide to Lassen Volcanic National Park and Vicinity, California

the upper hinge line of the fold. Once the 

surface was breached by the fault, monocline 

growth ceased, and all subsequent surface 

throw accumulation was directed along the 

vertical scarp. 

Progressive disaggregation of the monocline 

occurred along the Active Scarp in response to 

the local effects of repeated fault rupture, with 

the final stage of monocline history being its 

complete collapse, rendering the monocline 

to a pile of rubble...commonly composed of 

intact columns of basalt...

Comprehensive descriptions of the various surface 

features of the Active Scarp are presented by Walker 

(2008), along with detailed maps of the fault traces 

prepared by Marie Jackson for several parts of the fault 

trace. 

Turn around and retrace route to mile 12.2. 2.5

 17.3 At T-junction, turn right on U.S. Forest Service Road 

6R200 (Doty Road). 0.7

 18.0 Cross Hat Creek. 0.2

 18.2 At T junction, turn right (north) on Calif. Hwy 89. 

Highway is on the Hat Creek Basalt, but steep hills just 

to the left (west) of the highway are much older (early 

to middle Pleistocene?) andesites. The prominent, 

rugged volcanic edifice to the northeast is the basaltic 

andesite of Cinder Butte (40Ar/39Ar age of 38±7 ka; 

Turrin and others, 2007). 1.7

 19.9 Highway crosses Hat Creek. 1.6

 21.5 Highway crosses Hat Creek and passes along the 

contact between alluvium to the right and the toe of 

the late Pleistocene basalt of Doyles Corner, one of the 

units in the belt of the young lava flows that lies west of 

the early to middle Pleistocene(?) andesites that form 

the west rim of the Hat Creek Graben. The vent for the 

basalt of Doyles Corner is 2 miles to the southwest. 0.5

 22.0 Boundary of Burney Mountain East quadrangle with 

Cassel quadrangle to the north. 0.3

 22.3 Doyles Corner. Road 7R01 (Cassel Road) to the right 

leads along Hat Creek to the town of Cassel. Rising 

River, a large tributary of Hat Creek at the toe of the 

Hat Creek Basalt, is 2.5 miles northeast of Doyles 

Corner. Rising River is fed primarily by water flowing 

from beneath and within the Hat Creek Basalt. 0.3

 22.6 Highway passes from alluvium across a fault scarp onto 

the tholeiitic basalt of Rocky Ledge (197±8 ka; weighted 

average of three 40Ar/39Ar determinations by A.T. Calvert, 

written commun.). This unit is one of three tholeiitic 

basalts that are monotonously similar in appearance 

and cannot be distinguished readily one from another 

in the field or petrographically. They are, however, 

quite distinctive in age, paleomagnetic direction, and 

major-element composition (Muffler and others, 2012), 

allowing robust discrimination of the tholeiitic basalt 

of Rocky Ledge, the tholeiitic basalt of Rock Spring 

(545.7±6.7 ka), and the tholeiitic basalt of Sam Wolfin 

Spring (647.3±21.7ka) (40Ar/39Ar determinations by 

Figure 44.

Annotated 

photograph from 

Stop I-1 looking 

south along the 

Active Scarp of 

the Hat Creek 

Fault.

Prospect Peak

West

Prospect Peak

Older Pleistocene lavas

of the Pali Scarp

Sugarloaf Peak

Vents of

Hat Creek Basalt

(behind Sugarloaf Peak)

Hat Creek Basalt

Veneer of alluvium

over Hat Creek Basalt

(downthrown)

Talus on the 

Active Scarp cutting

24-ka Hat Creek Basalt

Talus on retreated ~35° segments of the Pali Scarp

Depositional contact

of Hat Creek Basalt

on talus of the Pali Scarp
Road Log 61

A.T. Calvert, written commun.). These tholeiitic units 

are overlain to the east by the calc-alkaline basalt west 

of Six Mile Hill (53.5±2.0ka; 40Ar/39Ar determination 

by A.T. Calvert, written commun.) and to the south by 

the tholeiitic Hat Creek Basalt (24±6ka) and the calcalkaline basaltic andesite of Cinder Butte (38±7

ka; both 

ages from Turrin and others, 2007). 

The tholeiitic basalts in the northern part of the 

Hat Creek Graben inundated low topography between 

higher-elevation remnants of >1-Ma calc-alkaline 

volcanoes. Tholeiitic basalts have a wider areal extent 

than calc-alkaline edifices and thus serve as important 

regional stratigraphic markers. Their upper surfaces, 

having little relief, also provide strain markers useful 

for measuring fault offsets and estimating times 

between significant earthquakes. 0.2.

 22.8 Intersection on left with a gravel road (Mountain 

View Road) that leads south for a mile and then west 

five miles to the town of Burney. 

Calif. Hwy 89 continues northwest on the flat upper 

surface of the tholeiitic basalt of Rocky Ledge. 2.3

 25.1 The mountain to the right, Brush Mountain, is a 

complex, faulted calc-alkaline volcano probably 

~2.5Ma in age. Surprisingly, the summit of the 

mountain consists of a much younger basalt plug and 

basaltic andesite cinder cone, perhaps ~50 ka in age. 2.0

 27.1 Four-way stop at a busy, major highway intersection. 

Calif. Hwy 89 continues straight across the tholeiitic 

basalt of Rocky Ledge to McArthur-Burney Falls 

Memorial State Park (6 miles), across the Pit River, 

and eventually to Interstate 5 at the city of Mt. Shasta. 

Calif. Hwy 299 to the right leads northeast across the 

Figure 45. Photograph at Stop I-2 

looking north across Calif. Hwy 299 0.5 

mile northeast of Johnson Park.

Johnson Park

and Burney

CALIF. HWY 299

Kipuka of old basalt

(2,740±110 ka)

Tholeiitic basalt of 

Rocky Ledge (197±8 ka)

Talus and colluvium from 

kipuka of old basalt, overlain

by tholeiitic basalt of Rocky Ledge

Road Log, Section H

Pit River to Fall River Mills and eventually to Alturas. 

 Turn left on Calif. Hwy 299 towards Burney. 0.7

 27.8 Calif. Hwy 299 crosses the prominent scarp of the 

Rocky Ledge Fault, which displaces the tholeiitic basalt 

of Rocky Ledge ~100 feet vertically. The northeast end 

of the roadcut consists of the tholeiitic basalt of Rocky 

Ledge, whereas the southwest part of the roadcut consists 

of much older underlying calc-alkaline basalt. 0.3

 28.1 Stop I-2: Tholeiitic basalt of Rocky Ledge 

overlying much older calc-alkaline basaltic 

andesite. Pulloff on the right allows safe parking for 

several vehicles (do not block the chain barrier).

Cross Calif. Hwy 299 with very great care 

to avoid high-speed vehicles, particularly trucks.

Walk ~300 feet through brush to the east lip of the 

roadcut of Calif. Hwy 299, where one can see the 

contact (fig. 45) between the tholeiitic basalt of Rocky 

Ledge (197±8 ka) and the underlying kipuka of calcalkaline basalt (2,740±110 ka; 40Ar/39Ar determination 

by P.R. Renne, written commun., 1993). 

Walk back to Calif. Hwy 299 and, once again, be 

very aware of fast-moving traffic when crossing the

highway. Continue southwest on Calif. Hwy 299. 0.5

 28.6 Pass through community of Johnson Park. 0.2

 28.8 Boundary of Cassel quadrangle with Burney 

quadrangle to the west. 2.2

 31.0 Black Ranch Road (7P200) to the right. 0.7

 31.7 Prominent stop light in the town of Burney. To the 

left is Mountain View Road, which after 1.6 miles 

becomes a gravel road leading east 4 miles farther 

to an intersection with Calif. Hwy 89, 0.5 miles 

northwest of Doyles Corner. 
62 Geologic Field-Trip Guide to Lassen Volcanic National Park and Vicinity, California

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Menlo Park Publishing Service Center, California

Manuscript approved for publication April 30, 2015

Design and layout by Jeanne S. DiLeo
Muffler and Clynne—Geologic Field-Trip Guide to Lassen Volcanic National Park and Vicinity, California—Scientific Investigations Report 2015–5067

ISSN 2328-031X (print) 

ISSN 2328-0328 (online)

ISBN 978-1-4113-3926-2

http://dx.doi.org/10.3133/sir20156067