U.S. Department of the Interior

U.S. Geological Survey

Pamphlet to accompany

Scientific Investigations Map 3405

Prepared in cooperation with the National Park Service and the 

Bureau of Land Management

Geologic Map of the North Half of the Lake Walcott 

30'×60' Quadrangle, Idaho
Cover Photographs.

Left—Vent area for Wapi Park lava field. Photograph taken July 11, 2002.

Top right—Split Butte. Photograph taken July 15, 2004. 

Bottom right—Kings Bowl lava field. Photograph taken July 11, 2002. Photographs by Susan Sakimoto (Space Science Institute and University 

at Buffalo) and Scott Hughes, pilot (Dept. of Geosciences Idaho State University). Used with permission.
Geologic Map of the North Half of the 

Lake Walcott 30'×60' Quadrangle, Idaho

By Mel A. Kuntz, Duane E. Champion, Brent R. Turrin, Phillip B. Gans, 

Harry R. Covington, and D. Paco VanSistine

Prepared in cooperation with the National Park Service and the 

Bureau of Land Management

Pamphlet to accompany

Scientific Investigations Map 3405

U.S. Department of the Interior

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

RYAN K. ZINKE, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2018

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

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U.S. Government.

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

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Kuntz, M.A., Champion, D.E., Turrin, B.R., Gans, P.B., Covington, H.R., and VanSistine, D.P., 2018, Geologic map of 

the north half of the Lake Walcott 30'×60' quadrangle, Idaho: U.S. Geological Survey Scientific Investigations Report 

3405, pamphlet 25 p., scale 1:100,000, https://doi.org/10.3133/sim3405.

ScienceBase data release GIS and other files that support this report are available at https://doi.org/10.5066/F7VQ30VZ

ISSN 2329-132X (online)
iii

Contents

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

Pleistocene and Holocene Basaltic Volcanism in the Eastern Snake River Plain and the 

Area of this Map......................................................................................................................1

Evolution, Purpose, and Use of this Map...................................................................................................3

Rock Nomenclature, Petrologic Descriptions, and Notes on Rock Names, Descriptions, and 

Thickness of Lava Flows .................................................................................................................3

Notes on Names and Unit Labels Given to Lava Fields in this Map.............................................3

Subdivisions of Holocene and Pleistocene Time Used for Basaltic Lava Fields 

for this Map ..............................................................................................................................5

Geochronology...............................................................................................................................................6

Radiocarbon Ages of Lava Flows of the Craters of the Moon Lava Field ...................................6

40Ar/39Ar Studies of Basaltic Lava Fields in the North Half of the Lake Walcott 30'×60' 

Quadrangle...............................................................................................................................6

Methods.........................................................................................................................................6

Results ...........................................................................................................................................6

Whole-Rock Potassium-Argon Studies and Results ......................................................................7

Paleomagnetic Studies........................................................................................................................7

Methods.........................................................................................................................................7

Results and Interpretations........................................................................................................7

Description of Map Units............................................................................................................................12

Unconsolidated Surficial Deposits ..................................................................................................12

Holocene and Pleistocene Basaltic Lava Flows and Related Near-Vent Pyroclastic and 

Eruptive-Fissure Deposits of the Snake River Group ......................................................12

Eruptive Period A .......................................................................................................................12

Eruptive Period B .......................................................................................................................13

Eruptive Period E........................................................................................................................13

Eruptive Period H .......................................................................................................................13

Eruptive Period B .......................................................................................................................14

Claasen Vent Complex (late Pleistocene)..............................................................................15

Inferno Chasm Vent Complex (late Pleistocene)..................................................................16

Eruptive Period C .......................................................................................................................18

Flat Top Vent Complex (late middle Pleistocene)..................................................................19

Eruptive Period D .......................................................................................................................20

Eruptive Period E........................................................................................................................22

References Cited..........................................................................................................................................24
iv

Figures

1. Regional index map with the location of the study area and showing prominent

features ..........................................................................................................................................1

2. Craters of the Moon National Park and Preserve administrative boundaries with

U.S. Geological Survey quadrangles.........................................................................................2

3. Digital orthophotograph of the Craters of the Moon, Kings Bowl, and Wapi lava

fields showing the location of the Craters of the Moon 30'×60' quadrangle and the

north half of the Lake Walcott 30'×60' quadrangle .................................................................4

Tables

1. 40Ar/39Ar and K-Ar ages for basaltic lava fields on or extending into the north half

of the Lake Walcott 30'×60' quadrangle, Idaho .......................................................................5

2. Paleomagnetic data for the north half of the Lake Walcott 30'×60' quadrangle, Idaho...8
v

Conversion Factors

International System of Units to U.S. customary units

Multiply By To obtain

Length

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

square kilometer (km2

) 0.3861 square mile (mi2)

Volume

cubic kilometer (km3

) 0.2399 cubic mile (mi3) 

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as

°F = (1.8 × °C) + 32.

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as

°C = (°F – 32) / 1.8.

Datum

Vertical coordinate information is referenced to the North American Vertical Datum of 1929 

(NAVD 29).

Horizontal coordinate information is referenced to the North American Datum of 1927(NAD 27).

Altitude, as used in this report, refers to distance above the vertical datum.

Divisions of Quaternary, Neogene, and Paleogene time 

used in this report1

Period or

subperiod Epoch Age(sidereal years)

Quaternary

Holocene 0–11.7 ka

Pleistocene

late 11.7–128 ka

late middle 128–400 ka

early middle 400–780 ka

early 780 ka–1.8 Ma

1

Ages of time boundaries are those of the U.S. Geological Survey Geologic Names Committee (2010) 

except those for the Pleistocene. Age boundaries for the Pleistocene are as defined by the authors for the 

purposes of this report. Ages are expressed in in ka for kilo-annum (thousand years), and Ma for megaannum (million years). Yr B.P., years before present (before A.D. 1950)

Geologic Map of the North Half of the Lake Walcott 

30'×60' Quadrangle, Idaho

By Mel A. Kuntz,1 Duane E. Champion,1 Brent R. Turrin,2 Phillip B. Gans,3 Harry R. Covington,1 and 

D. Paco VanSistine1

1

U.S. Geological Survey

2

Department of Earth and Planetary Sciences, Rutgers University, Piscataway, N.J.

3

Department of Earth Science, University of California, Santa Barbara, Calif.

Introduction

Pleistocene and Holocene Basaltic Volcanism 

in the Eastern Snake River Plain and the Area of 

this Map

This map depicts the volcanic geology of the north half 

of the U.S. Geological Survey (USGS) Lake Walcott 30'×60' 

quadrangle in the Snake River Plain in southern Idaho. The 

map also contains a part of the Craters of the Moon National 

Monument and Preserve (fig. 1). The Great Rift volcanic rift 

zone is present here; an 85-kilometer (km)-long, 2- to 15-km 

wide belt of lava fields of various sizes, tephra cones, lava 

cones, shield volcanoes, eruptive fissures and associated 

tephra deposits, and non-eruptive fissures. Basaltic volcanism 

was a dominant geologic process in the eastern Snake River 

Plain in Holocene time, as attested by eight dominantly Holocene (<15 thousand year old [ka]) lava fields covering about 

13 percent of the eastern Snake River Plain (Kuntz and others, 

1992). Three of the lava fields, Craters of the Moon, Kings 

Bowl, and Wapi, are located along the Great Rift volcanic rift 

zone. The Wapi and Kings Bowl lava fields are depicted in this 

map. The Craters of the Moon lava field and associated structures along the Great Rift volcanic rift zone are depicted in the 

geologic map of the Craters of the Moon 30'×60' quadrangle, 

Idaho (Kuntz and others, 2007).

President Coolidge established the Craters of the Moon 

National Monument on May 2, 1924. Since 1924, the Monument has been expanded through five presidential proclamations. The most recent and largest expansion of the Monument 

occurred November 9, 2000, when President Clinton signed 

a proclamation enlarging the Monument 13-fold, from 222 

square kilometers (km2

) that was managed as Craters of the 

Moon National Monument, to 2,893 km2 and now includes 

lands managed by the Bureau of Land Management (BLM) 

and the National Park Service as the Craters of the Moon 

National Monument and Preserve (fig. 2). The expanded 

Monument and Preserve encompasses the entire Great Rift 

volcanic rift zone and the entire Craters of the Moon, Wapi, 

and Kings Bowl lava fields and most of the Horse Butte-Bear 

Trap lava-tube system (Kuntz and others, 2007).

The Geographic Information System (GIS) data and 

related data files are available as a USGS data release in 

ScienceBase at https://doi.org/10.5066/F7VQ30VZ (Kuntz 

and VanSistine, 2018)

Map area

IDAHO

North half of the Lake Walcott 30’ x 60’ quadrangle

117° 116° 115° 114° 113° 112° 111°

45°

44°

43°

42°

0 50 100 KILOMETERS

0 50 100 MILES

Boise

Pocatello

Idaho

Falls

Twin Falls

OREGON

WESTERN

 SNAKE

 RIVER

 PLAIN

WAPI LAVA FIELD

MONTANA

CRATERS OF THE MOON

LAVA FIELD

EASTERN

SNAKE RIVER

PLAIN

Study area 

Study

area 

IDAHO

GREAT

RIFT

KINGS BOWL

LAVA FIELD

CRATERS OF THE MOON

NATIONAL MONUMENT

Figure 1. Regional index map with the location of the study area 

and showing prominent features. Modified from Kuntz and others 

(2007). Yellow area is the Snake River Plain.
Map

area

IDAHO

EXPLANATION

Craters of the Moon National Monument Private

Idaho State lands National Park Service (NPS)

Bureau of Land Management

(BLM)

0 4 8 KILOMETERS

0 4 8 MILES

North half of the Lake Walcott 30' x 60' quadrangle

Craters of the Moon 30' x 60' quadrangle

BAUGH 

CREEK

LITTLE WOOD

RIVER

RESERVOIR

PICABO

SPLIT TOP ANTELOPE 

LAKE

BEAR PARK

EAST

BEAR PARK

WEST

BEAR DEN

BUTTE

QUAKING

ASPEN BUTTE

BIG

SOUTHERN

BUTTE

FINGERS

BUTTE

THE 

WATCHMAN

INFERNO

CORE

SERVICEBERRY

BUTTE SUNSET LAKE PRATTBUTTE

FISSURE

BUTTE

NORTH

LAIDLAW

BUTTE

LITTLE PARK PADDELFORD

FLAT

MOSBY BUTTE MOSBY WELL

PILLAR BUTTE

NE

PILLAR BUTTE

SE

RATTLESNAKE

BUTTE

SCHODDE

WELL

PILLAR BUTTE BEAR TRAP

CAVE

TIKURA

PAGARI

OWINZA

OWINZA

BUTTE

EDEN NE BURLEY NW BURLEY NE RUPERT NW ACEQUIA

LAKE

WALCOTT

WEST

LAKE

WALCOTT

EAST

GIFFORD

SPRING

REGISTER

ROCK

KIMAMA

BUTTE KIMAMA NORLAND MAX MINIDOKA

SENTER SHALE BUTTE COMMUNITY

LAKE

LARKSPUR

PARK

BRIGHAM

POINT

WAGON 

BUTTE

LAIDLAW 

BUTTE PAGARI WELL

BLACK RIDGE

CRATER

HALFWAY

LAKE

LAIDLAW

LAKE

BOTTLENECK

LAKE

BEAR PARK

SW MULE BUTTE

CARY

MULDOON

LAKE HILLS

TRAIL 

CREEK

FISH CREEK

RESERVOIR

BLIZZAERD 

MTN. SOUTH

BLIZZARD 

MOUNTAIN 

NORTH

CHAMPAGNE 

CREEK

NICHOLS 

RESERVOIR ARCO SOUTH BUTTE CITY ARCO HILLS SE

114°00'

43°15'

43°30'

43°00'

42°45'

113°15' 113°00'

2 Geologic Map of the North Half of the Lake Walcott 30'×60' Quadrangle, Idaho

Figure 2. Craters of the Moon National Park and Preserve administrative boundaries with U.S. 

Geological Survey 7.5 quadrangles. In 2000, a proclamation was signed enlarging the monument 

from 222 square kilometers (km2

, purple area) to now include lands managed by the Bureau of 

Land Management and the National Park Service as the Craters of the Moon National Monument 

and Preserve for a total of 2,893 km2

 (green and gray areas, respectively). Figure modified from 

National Park Service quadrangle map accessed on October 25, 2017, at https://www.nps.gov/crmo/

planyourvisit/upload/quads.pdf.
Evolution, Purpose, and Use of this Map

With the expansion of the Craters of the Moon National 

Monument and Preserve, funds became available from the 

National Park Service (NPS) and the U.S. Bureau of Land 

Management (BLM). This funding enabled the completion 

of geologic mapping, detailed paleomagnetic studies, and 

40Ar/39Ar dating of the pre-Holocene lava fields of the northern 

part of the USGS Lake Walcott quadrangle.

A preliminary geologic map in the area of the Wapi and 

Kings Bowl lava fields produced by Covington (1977) provided details of the geology of that area.

The distal parts of the Craters of the Moon lava field 

extend southward into the Lake Walcott quadrangle; thus, 

some of the field, paleomagnetic, radiocarbon, and 40Ar/39Ar 

data from Kuntz and others (2007) have been incorporated in 

this geologic map (fig. 3). These data give additional details 

of the volcanic history of the Holocene lava fields along the 

Great Rift volcanic rift zone, and older lava fields and their 

vent complexes near the Great Rift and nearby areas in the 

Lake Walcott quadrangle. For vent complexes, a half dozen or 

so vents are aerially contiguous and distributed in a seemingly 

random spatial pattern, but paleomagnetic data for the vents 

are identical, suggesting that these vents erupted simultaneously or within a short span of time (<100 years). Kuntz and 

others (2007) noted that the radiocarbon and 40Ar/39Ar data 

enabled a detailed analysis of the time scales of basaltic volcanism along the Great Rift volcanic rift zone in the geologic 

map of the Craters of the Moon 30'×60' quadrangle. The additional radiocarbon and 40Ar/39Ar data depicted on this map can 

be used to determine the “pulse rate” for basaltic volcanism 

for the entire length of the Great Rift volcanic rift zone and 

for a very large area that spans nearly the entire width of the 

eastern Snake River Plain.

In addition to the pulse rate, the geologic mapping, 

paleomagnetic, and geochronologic studies of the north half 

of the Lake Walcott quadrangle have provided detail about 

additional volcanic rift zones in the map area. A volcanic 

rift zone parallel to (about N.10°W.) but approximately 2.5 

km east of the Great Rift volcanic rift zone includes vents 

at Horse Butte (Qsbb25), Cottrells Blowout (Qwfb14), 

Inferno Chasm (Qwfb12), and Grandview Crater (Qwfb3). 

Eruptions from the first three vents occurred about 73±8 ka; 

eruptions from Grandview Crater occurred about 38±12 ka 

(table 1). This volcanic rift zone has previously been called 

the “Inferno Chasm” volcanic rift zone (Greeley, 1982). 

Another volcanic rift zone, having an orientation of N.15° E., 

is defined by the vents of Wapi Park, a kipuka vent in the 

Wapi lava field about 4 km southwest of Pillar Butte, and the 

Higgins Blowout along the southwest margin of the Wapi lava 

field. 40Ar/39Ar ages along this volcanic rift zone are 87±9 ka 

for the Wapi Park lava field (Qwfb15) and 289±5 ka (Qwfc3)

for the Higgins Blowout lava field (table 1), indicating a 

long eruptive history. The young (19±2 ka; table 1) Shale 

Butte lava field (Qwfb1) lies on a southward extension of the 

Borkum Rift volcanic rift zone in the Craters of the Moon 

quadrangle that includes north-south-aligned vents at Sand 

Butte (Qsbb6; 34±29 ka) (table 1) and Broken Top Butte 

(<50 ka) (Kuntz and others, 2007).

Rock Nomenclature, Petrologic 

Descriptions, and Notes on Rock 

Names, Descriptions, and Thickness 

of Lava Flows

The chemical and petrographic classification of lava 

fields in the eastern Snake River Plain and the Craters of the 

Moon lava field can be found in Kuntz and others (1992), 

Kuntz and others (2007), and Putirka and others (2009). 

Detailed chemical analyses of rocks and petrographic studies of the lava flows were not completed for the north half of 

the Lake Walcott quadrangle area due to project priorities. 

We therefore classify lava fields of the north half of the Lake 

Walcott quadrangle as simply “basaltic” rocks.

In the map area, there is little if any erosion of lava 

flows, thus the thickness of buried lava flows is not revealed 

through any erosional remnants. In addition, there are few 

logged core holes within the map area for which the thickness 

of lava flows may be determined. For these reasons, thickness 

of lava flows and lava fields are not given in the Description 

of Map Units. However, from the study of core holes at the 

Idaho National Laboratory (INL), it has been determined that 

pahoehoe flows average about 20 m thick and a'a flows are 

typically 15–30 m thick (Kuntz, Champion, and others, 1986; 

Champion and others, 2002). From the analysis of topographic 

maps, it can be determined that lava flows may accumulate 

to thicknesses of as much as 200 m around a vent and on top 

of underlying flows. However, a more common thickness of 

flows in vent areas is about 30–50 m. The area of individual 

lava fields given in the Description of Map Units was determined from USGS 1:24,000-scale topographic maps. Volumes 

of lava fields were estimated assuming the lava field has the 

shape of a cone. The basal area of the cone is equal to the 

lava-field area and the height of the cone as determined from 

the topographic maps.

Notes on Names and Unit Labels Given to Lava 

Fields in this Map

A discussion of the naming of lava flows of the Craters 

of the Moon lava field, some of which flow into the northern 

part of this map, is given in Kuntz and others (2007). The map 

unit labels for those lava flows of the Craters of the Moon 

lava field that are present in the northern part of this map are 

retained for this map. For Craters of the Moon flows, “Q” 

refers to Quaternary age, and the second letter, “c,” references 

flows of the Craters of the Moon lava field. The third letter, 

for example, “f,” refers to the type of deposit (f, flow), and 

Rock Nomenclature, Petrologic Descriptions, Notes on Rock Names, Descriptions, Thickness of Lava Flows 3
4 Geologic Map of the North Half of the Lake Walcott 30'×60' Quadrangle, Idaho

Figure 3. Digital orthophotograph of the Craters of the Moon, Kings Bowl, and Wapi lava fields showing the 

location of the Craters of the Moon 30'×60' quadrangle and the north half of the Lake Walcott 30'×60' quadrangle. 

Yellow arrows indicate the locations of photographs of selected features: 1, Split Butte; 2, Higgins Blowout; 3, Wapi 

Park; 4, Kings Bowl; 5, Pillar Butte; 6, Cottrells Blowout; 7, Bear Trap cave.

114°00' 113°00'

43°30'

43°00'

86

93

20

93

26

20

93

20

0 5 10 KILOMETERS

Base from U.S. Department of Agriculture, National Agriculture Imagery Program (NAIP), accessed in 2015 0 5 10 MILES

Universal Transverse Mercator (UTM) zone 12. North American Datum 1983 (NAD 83). Craters of the Moon 

National Monument and Preserve boundaries from the National Park Service accessed on October 25, 2017, 

at https://catalog.data.gov/dataset/administrative-boundariesof-national-park-system-units-9-30-2017-national-geospatial-data.

Lake Walcott

Snake River

Snake

River

Big Southern Butte

Carey

Arco

CRATERS OF THE MOON

NATIONAL PRESERVE

North half of the Lake Walcott 30’x60’quadrangle

Craters of the Moon 30 x60’ quadrangle

CRATERS OF

THE MOON

NATIONAL

MONUMENT

WAPI

LAVA FIELD

CRATERS OF THE MOON

LAVA FIELD

Big Southern Butte

KINGS BOWL

LAVA FIELD

Carey

Arco

CRATERS OF THE MOON

NATIONAL PRESERVE

North half of the Lake Walcott 30’x60’quadrangle

Craters of the Moon 30’x60’ quadrangle

CRATERS OF

THE MOON

NATIONAL

MONUMENT

1

2

3

4

7 6

5
Rock Nomenclature, Petrologic Descriptions, Notes on Rock Names, Descriptions, Thickness of Lava Flows 5

the fourth letter refers to the eruptive period (A through H). 

The last number refers to the stratigraphic order of deposits 

assigned to an eruptive period.

For lava fields in this map, a slightly different scheme for 

unit labels has been employed. The descriptive names for some 

of the lava fields on this map are taken from prominent named 

vents and buttes and spot elevations near vents, as they appear 

on USGS 1:24,000-scale topographic map quadrangles that 

cover parts of this map. In the absence of named landmarks, 

spot elevations, mostly at high points at vents or nearby buttes, 

are used to identify various basaltic lava fields. The map unit 

symbols, as they appear in the Description of Map Units and on 

the map, consist of letters and numbers arranged in the following system: “Q” refers to the general age (Quaternary) of the 

unit. The second letter “w” refers to flows that have source vents 

within the north half of the Lake Walcott quadrangle. The third 

letter “f” refers to lava flows. The fourth letter, for example, “b” 

refers to arbitrary subdivisions of Pleistocene time, as described 

in the subsequent section. The number suffix identifies lava 

fields in lava-field age units, based on radiometric and stratigraphic data where available. However, because radiometric 

ages are available for only a few lava fields, the numbers do not 

imply absolute age relations of lava fields.

Subdivisions of Holocene and Pleistocene Time 

Used for Basaltic Lava Fields for this Map

For the purposes of this map, Holocene time is the 

period from the present to 10,000 14C yr or 11,700 calibrated or sidereal yr (USGS, 2010). The Kimama flow of 

the Craters of the Moon Lava field, having a radiocarbon 

age of 15,100±160 14C yr (Kuntz, Spiker, and others, 1986), 

is included in the age class “predominantly Holocene” used 

in the Craters of the Moon quadrangle (Kuntz and others, 

2007). For two Holocene lava fields in this map (Kings Bowl 

Table 1. 40Ar/39Ar and K/Ar ages for basaltic lava fields on or extending into the North Half of the Lake Walcott 30'×60' quadrangle, Idaho.

40Ar/39Ar data for basaltic lava fields of the Snake River Group

[--, indicates no data; ka, age in thousands of years. Analytical laboratories: UCSB, University of California at Santa Barbara, Santa Barbara, Calif., Dr. Phil 

Gans; RU, Rutgers University, Dr. Brent Turrin; DEN, U.S. Geogical Survey, Denver, Colo., Dr. Dan Miggins. All errors given for our estimated (preferred) 

ages, as reported throughout the text and here, are ±1 one sigma (σ)]

Geographic location Analytical 

Laboratory

Field

number

Weighted

mean plateau

age (ka)

lsochron

age (ka)

Total

fusion

age (ka)

Preferred

age (±1σ) ka

Map

unit

symbol

Shale Butte UCSB 03K45 11±6 1±1 2±7 ~19 Qwfb1

Grandview Crater UCSB 03K19 38±12 25±28 29±11 38±12 Qwfb3

West Wildhorse Butte1 UCSB SRP–105 -- -- -- <50 Qsbb8

Claasen vent complex (Vent 4801) RU Vent 4801 68±8 106±19 44±9 68±8 Qwfb7

Horse Butte1

 and Bear Trap lava-tube system UCSB SRP–111 73±8 86±41 66±8 73±8 Qsbb26

Wapi Park vent and lava-tube system UCSB 03K18 87±9 53±19 64±11 87±9 Qwfb15

Kimama Butte RU 03K50 87±11 70±20 43±19 87±11 Qwfb17

Split Butte UCSB 03K26 138±6 145±13 122±7 138±6 Qwfc1

Higgins Blowout DEN 03K51 300±60 289±5 -- 289±5 Qwfc3

Adelaide Butte RU 03K34 471±18 475±38 394±54 475±38 Qwfd3

Vent 4640 RU 03K13 607±13 690±50 885±15 690±50 Qwfd1

Sid Butte UCSB 03K22 778±8 772±15 754±9 778±8 Qwfe1

1

Located on geologic map of the Craters of the Moon 30’×60’ quadrangle.

K-Ar data

[K2O, potassium oxide; 40Arrad, 40Ar radiogenic; mol/g, moles per gram; ka, age in thousands of years. Analytical laboratories: Menlo Park, U.S. Geological 

Survey laboratory located in Menlo Park, Calif. Analyses by M.A. Kuntz, G.Brent Dalrymple, and J.C. von Essen, and K2O analyses by S.T. Prebble are mean 

and standard deviation of four measurements]

Geographic location Analytical 

Laboratory

Field

number

K2

O weight 

percent

40Arrad

(10–13 mol/g)

40Arrad

(percent)

Calculated

age (ka)

Map

unit

symbol

Sand Butte1 Menlo Park 92KS5 0.841±0.005 0.415 0.518 34±292, 3 Qsbb6

1

Located on geologic map of the Craters of the Moon 30'×60' quadrangle.

2

Pooled mean age of two argon measurements.

3

λε = 0.581 x 10−10/yr; λB = 4.962 x 10−10/yr; 40K/K = 1.167 x 10−4 mol/mol. Errors are estimates of standard deviation of analytical precision.λε
6 Geologic Map of the North Half of the Lake Walcott 30'×60' Quadrangle, Idaho

[Qwfa1] and Wapi [Qwfa2]), the first four letters of their 

unit labels are “Qwfa.” Lava fields having radiometric or 

stratigraphically assumed ages between 11,680 14C yr and 

128,000 sidereal yr are designated late Pleistocene in age, the 

first four letters of their unit labels are “Qwfb.” Lava fields 

having radiometric ages or stratigraphically assumed ages 

between 128,000 and 400,000 sidereal years are designated 

as late middle Pleistocene in age; the first four letters of their 

unit labels are “Qwfc.” Lava fields having radiometric ages or 

stratigraphically assumed ages between 400,000 and 780,000 

sidereal yr are designated as early middle Pleistocene in age; 

the first four letters of their unit labels are “Qwfd.” Lava fields 

having radiometric ages greater than 780,000 sidereal yr and 

reversed magnetic polarity are designated as early Pleistocene 

in age; the first four letters of their unit labels are “Qwfe.”

Geochronology

Radiocarbon Ages of Lava Flows of the Craters 

of the Moon Lava Field

Radiocarbon dating has been used to delineate the evolution of the Holocene Craters of the Moon lava field. Those 

data are summarized in Kuntz, Spiker, and others (1986) and 

Kuntz and others (2007) and are not discussed further here. 

Radiocarbon ages of lava flows of the Craters of the Moon 

lava field that extend into the Lake Walcott quadrangle are 

shown on the Correlation of Map Units and are discussed in 

the Descriptions of Map Units.

40Ar/39Ar Studies of Basaltic Lava Fields in 

the North Half of the Lake Walcott 30'×60' 

Quadrangle

Methods

All samples chosen for 40Ar/39Ar dating were examined 

petrographically to determine if they were sufficiently fresh 

and had minimal amounts of glass in the groundmass. Those 

deemed not altered and containing minimal amounts of glass 

were selected for dating. The samples were crushed and sieved 

to varying size fractions (100–200 mesh to 300–500 mesh, 

depending on the samples), and cleaned in an ultrasonic device 

in ionized water.

The 40Ar/39Ar studies were carried out in two laboratories, with differing specific dating protocols: the University 

of California, Santa Barbara, Calif. (UCSB), and at Rutgers University (RU), Piscataway, N.J. At UCSB, standard 

magnetic separation techniques and handpicking were used 

to amass groundmass concentrates. At RU, the entire rock 

was separated into at least three magnetic separates, thus 

every part of the sample was analyzed. Splits of each sample, 

ranging from 50 to 200 milligrams (mg), were then either 

encapsulated in copper packets interspersed with packaged 

flux monitors (UCSB), or loaded into pitted aluminum disks 

with flux monitors (RU), and then placed into a sealed quartz 

vial. Vials were irradiated in cadmium-lined receptacles for 

15–20 minutes at either the TRIGA reactor at the USGS in 

Denver or at the TRIGA reactor at Oregon State University. 

Samples were then analyzed either by incremental heating 

in a Staudacher-type resistance furnace (UCSB; Gans, 1997) 

or by stepwise laser heating (RU). For samples analyzed at 

UCSB, only groundmass concentrates of samples were analyzed, At RU, splits of three or more magnetic separates were 

independently run. From 6 to 15 step-heating experiments 

were carried out for each sample. Twelve new age determinations are given in table 1. All errors given for our estimated (preferred) ages, as reported throughout the text and 

in table 1, are ±1 one sigma (σ). The flux monitors used for 

neutron irradiations were sanidine, from either Taylor Creek 

Rhyolite (UCSB) with an assigned age of 27.92 Ma, or the 

Alder Creek Rhyolite (RU) with an assigned age of 1.196 Ma 

(Dalrymple and Duffield, 1988).

At Rutgers University, active measurement of spectrometer mass discrimination is carried out throughout 

the unknown sample incremental data collection interval, 

through the use of an online automated air pipette (Turrin 

and others, 2010). The olivine basalt samples analyzed in 

this study are relatively young in age, uniformly low in 

potassium oxide (K2

O), and have low 40Arrad (radiogenic) 

concentrations. Such samples are particularly vulnerable 

to uncertainty in spectrometer mass discrimination. Realtime measurement of an air sample with zero age every 3 to 

4 hours, while incrementally heating unknown samples, is 

the best guarantee that the significant corrections to those 

analyses for atmosphere argon, and for 37Ar and 36Ar generated by neutron irradiation, are done properly.

Virtually all of the samples collected for 40Ar/39Ar dating 

yielded consistent, moderately well-behaved 40Ar/39Ar data 

with interpretable ages and uncertainties (table 1). Most of 

the samples yielded fairly flat age spectra having well-defined 

plateaus, as described by Fleck and others (1977). These 

spectra are readily explainable in terms of the combined 

effects and variable contributions of reactor-induced recoil, 

low-temperature argon loss, and a non-atmospheric “trapped” 

component (that is, excess argon). In general, the flattest and 

most reliable parts of the individual spectra were associated 

with the gas released at lower to intermediate temperatures. 

Typical confidence estimates range from 6 to 50 ka.

Results

The order of laboratory-determined ages of the rocks 

agreed with stratigraphic order of the rocks determined independently by geologic mapping and field relations.

The new 40Ar/39Ar geochronologic data reveal an apparent bimodal age distribution of basaltic volcanism during the 

time span of the Brunhes Normal Polarity Chron (0 to 780 ka) 

(table 1). Most ages are younger than about 150 ka, with two 

units older than about 700 ka (Vent 4640 [Qwfd3], and Sid 
Butte [Qwfe1]). Two ages lie between these values (Higgins 

Blowout lava field, Qwfc3, 289±5 ka; and Adelaide Butte, 

475±38 ka; table 1). Numerous map units of modest areal 

extent and of intermediate age appearance were not analyzed 

by either 40Ar/39Ar or potassium-argon (K-Ar) methods. These 

lava fields may have erupted at intermediate times between 

about 750 ka and about 150 ka.

Most of the younger basalt eruptions (<150 ka) in the 

north half of the Lake Walcott quadrangle have occurred 

along the Great Rift volcanic rift zone. However, two of 

the younger eruptions are not located near the Great Rift; 

they are located in the western third of this quadrangle. 

Kimama Butte (Qwfb17), a basalt shield volcano covering 

about 250 km2, is dated at 87±11 ka (table 1). Shale Butte 

(Qwfb1), a complex of a'a and pahoehoe flows located in 

the northwest corner of the quadrangle, is dated at 19±2 ka 

(table 1). Due to an odd direction of remanent magnetization 

recorded in Shale Butte lava flows, we believe that it erupted 

during a time interval of aberrant geomagnetic field dated 

near 19±2 ka (table 1) and called the Hilina Pali Excursion 

(Coe and others, 1978). This age assignment is slightly older 

than one standard deviation of the weighted mean plateau 

age from the 40Ar/39Ar studies.

Whole-Rock Potassium-Argon Studies and 

Results

One sample of pre-Holocene lava flows in this quadrangle was dated by the K-Ar method (Sand Butte, table 1). K-Ar 

dating of the pre-Holocene lava flows was hampered by the 

low K2O contents of the flows (typically less than 0.8 percent), 

the relative youth of the flows, and mass spectrometric and 

electronic limitations. The one experiment was successful on 

a sample from Sand Butte (Qsbb6; 34±29 ka, table 1). The 

Sand Butte vent is located just north of the northern boundary 

of the Lake Walcott quadrangle in the Craters of the Moon 

30'×60' quadrangle.

Paleomagnetic Studies

Methods

Magnetic polarity and remanent inclination and declination directions (table 2) were determined for basaltic lava 

flows from surface samples. Paleomagnetic site-average data 

(table 2) are ordered by lava field location north to south, 

within three west-to-east corridors: (1) lava fields west of the 

Great Rift volcanic rift zone, (2) lava fields in the Great Rift 

volcanic rift zone, and (3) lava fields east of the Great Rift volcanic rift zone. Eleven site averages included in table 2 are not 

located within the boundaries of north half of the Lake Walcott 

30'×60' quadrangle; they are included in the paleomagnetic 

data for completeness, and to emphasize that several of the 

lava fields on this map extend northward into the Craters of 

the Moon 30'×60' quadrangle (Kuntz and others, 2007).

Results and Interpretations

All lava flows except two, Sid Butte (Qwfe1) and 

Vent 4559 (Qwfe2) in this quadrangle, have normal polarity and were emplaced during the Brunhes Normal Polarity 

Chron, and are younger than 780,000 yrs. Secular variation 

of the geomagnetic field occurs at a geologically rapid rate 

(about 4° per century; Champion and Shoemaker, 1977), 

which allows grouping of lava flows having similar paleomagnetic directions. Conversely, dissimilar paleomagnetic 

directions indicate that two lava flows erupted at different 

times. With these two ideas as guides and by incorporating the new 40Ar/39Ar and K-Ar ages, we used directions of 

remanent magnetization to correlate and approximately date 

lava fields for this map. The paleomagnetic field studies were 

highly successful in identifying the full extent of two basalt 

shield and (or) lava-tube eruptions in the center part of the 

Lake Walcott quadrangle. The older eruption from vents in 

the Wapi Park area (Qwfb15; 87±9 ka, table 1) formed an 

extensive lava-tube system that enabled lava to move in a 

broad flow field as much as 24 km to the west. This flow field 

is partly overlain by the Holocene Wapi lava field (Qwfa2; 

2,270±50 14C yr before present [B.P.], Kuntz, Spiker, and 

others, 1986); therefore, its full extent cannot be determined. 

The younger eruption (73±8 ka, table 1) was from aligned 

vents in the Inferno Chasm volcanic rift zone, including 

Horse Butte (Qsbb25), Cottrells Blowout (Qwfb14), and 

Inferno Chasm (Qwfb12). A double lava-tube system, called 

the Bear Trap lava-tube system (Greeley, 1982; King, 1982) 

facilitated the movement of lava as far as 42 km westward 

across the north-central part of the Lake Walcott quadrangle. 

This lava field is significantly masked by younger, mostly 

Holocene lava flows of the Craters of the Moon lava field, 

and also by the younger lava flows of the Grandview Crater 

lava field (Qwfb3; 38±12 ka, table 1); thus, its full extent can 

only be inferred.

The extents of other lava fields have been identified 

through paleomagnetic field studies. Split Butte (Qwfc1; 

138±6 ka, table 1), a maar volcano located on the western margin of the Holocene Wapi lava field (Qwfa2; 2,270±50 14C yr 

B.P., Kuntz, Spiker, and others, 1986) has an odd direction of

remanent magnetization. A correlative area of lava flows was

identified more than 11 km to the southwest near the southern

boundary of this quadrangle.

Several aerially contiguous lava fields are correlative 

because they have identical or similar paleomagnetic directions. For example, the Inferno Chasm vent complex has five 

correlative lava fields, the Claasen vent complex has seven 

correlative lava fields, and the Flat Top vent complex has six 

correlative lava fields.

Geochronology 7
Table 2. Paleomagnetic data for the north half of the Lake Walcott 30'×60' quadrangle, Idaho.—Continued [Site label, alphanumeric identifier of location; latitude and longitude in degrees north and east in North American Datum of 1927 (NAD 27) of the site location; N/No, number the number originally taken at the site; Exp., strength of the peak alternating demagnetizing field in milliTeslas (mT); NRM, means no alternating-field demagnitization was donessary; Li, line site solution; Pl, plane solution; Mx, mixed line and plane site solution; I and D, remanent inclination and declination in degrees; α95, 95 percent confidence limidegrees; k, estimate of the Fisherian precision parameter; R, length of the resultant vector; and P latitude, P longitude, location in degrees north and east of the virtual geomagnethe site mean direction; Averaged lat/long data for mean of the site, means is required to calculate averaged VGP position;--, no data] Site name Map unit symbol Site label Latitude Longitude N/No Exp. I D α95 k 

Vents west of the Great Rift volcanic rift zone Black Ridge Crater lava field Qsbb14 554B21 43.025 246.044 8/8 30+ 63.0 346.8 2.4 522 7.9

551B41 42.900 245.941 7/8 Pl 59.2 348.7 3.1 352 -- 

226B7 42.982 246.080 8/8 20 63.4 347.2 1.5 1,346 7.9

43.0 246.0 3/3 61.9 347.6 3.6 1,172 2.9

Sand Butte lava field Qsbb6 610B21 43.177 246.148 8/8 30 66.0 7.1 2.0 768 7.9

626B21 43.084 246.181 7/8 30+ 66.2 358.6 1.5 1,664 6.9

43.1 246.16 2/2 -- 66.2 2.9 7.5 1,105 1.9

West Wildhorse Butte lava field Qsbb8 594B21 43.071 246.257 8/8 20 57.8 20.1 2.0 807 7.9

605B3 42.954 246.251 7/8 30+ 62.4 18.7 2.3 719 6.9

43.02 246.25 2/2 -- 60.1 19.5 10.2 607 1.9

Shale Butte lava field Qwfb1 995B4 42.988 246.195 8/8 20 36.9 25.6 1.1 2,435 7.9

234B7 42.974 246.144 3/3 30 39.3 26.8 10.9 129 2.9

42.98 246.17 2/2 -- 38.1 26.2 5.6 1,974 1.9

Vent 4371' lava field Qwfd7 597B3 42.933 246.293 7/8 Pl 59.2 352.4 3.8 328 -- 

Adelaide Butte lava field Qwfd3 757B3 42.823 246.233 7/8 30 40.0 0.3 1.9 997 6.9

Sid Butte lava field Qwfe1 773B3 42.875 246.021 7/8 30 –51.3 176.5 2.2 740 6.9

Vent 4373' lava field Qwfe3

971B4 42.815 246.020 7/8 30+ –57.6 175.9 4.7 164 6.9

42.85 246.020 2/2 -- –54.5 176.2 13.8 330 1.9

Vent 4559 lava field Qwfe2 573B3 42.874 246.427 7/8 50 –65.6 167.8 3.3 336 6.9

Vent 4401' lava field Qwfd5 557B3 42.837 246.436 8/8 20 56.7 19.6 2.1 724 7.9

Vent 4429' lava field Qwfd6 549B3 42.822 246.435 7/8 Pl 51.3 348.6 7.9 84 -- 
Table 2. Paleomagnetic data for the north half of the Lake Walcott 30'×60' quadrangle, Idaho.—Continued[Site label, alphanumeric identifier of location; latitude and longitude in degrees north and east in North American Datum of 1927 (NAD 27) of the site location; N/No, number the number originally taken at the site; Exp., strength of the peak alternating demagnetizing field in milliTeslas (mT); NRM, means no alternating-field demagnitization was donessary; Li, line site solution; Pl, plane solution; Mx, mixed line and plane site solution; I and D, remanent inclination and declination in degrees; α95, 95 percent confidence limidegrees; k, estimate of the Fisherian precision parameter; R, length of the resultant vector; and P latitude, P longitude, location in degrees north and east of the virtual geomagnethe site mean direction; Averaged lat/long data for mean of the site, means is required to calculate averaged VGP position;--, no data]

Site name Map unit symbol Site label Latitude Longitude N/No Exp. I D α95 k 

The Crater lava field Qwfd4 979B4 42.791 246.197 8/8 Pl 51.6 9.5 2.0 728 -- Kimama Butte lava field Qwfb17 765B3 42.84 246.202 8/8 30 40.0 0.3 1.3 1,858 7.9Knoll Vent lava field Qwfd9 527B41 42.73 246.010 8/8 Pl 63.0 355.0 2.6 535 -- 

Vent 4354' lava field Qwfc13

670B41 42.744 246.559 9/9 30 58.1 18.6 3.5 219 8.9

978B4 42.778 246.945 7/8 30 56.8 15.6 2.1 868 6.9

42.76 246.53 2/2 -- 57.5 17.1 4.5 3,057 2.0

Vents in and adjacent to the Great Rift volcanic rift zone

Horse Butte lava field Qsbb25

530B21 43.028 246.769 8/8 40+ 48.9 4.7 1.5 1,296 7.9501B31 43.082 246.762 8/8 20 51.5 2.0 1.0 2,906 7.9

589B3 42.951 246.364 8/8 20 48.7 2.2 1.9 841 7.9581B3 42.948 246.467 8/8 30 47.5 0.5 2.4 551 7.9939B4 42.908 246.508 8/8 20 48.5 1.8 1.0 3,205 7.9525B3 42.966 246.657 8/8 20 47.1 3.2 1.2 2,124 7.9533B3 42.993 246.671 8/8 30 48.2 0.7 1.2 2,187 7.9

947B41 43.008 246.367 8/8 20 49.3 3.2 1.1 2,366 7.9

Vent 5094' & Vent 5108' lava fields Qsbb24 741B3 42.961 246.793 8/8 30 44.2 3.9 1.7 1,070 7.9Cottrells Blowout lava field Qwfb14 733B3 42.990 246.817 8/8 30 47.1 1.4 2.2 630 7.9Inferno Chasm lava field Qwfb12

725B3 42.959 246.800 7/8 Pl 46.4 1.6 2.6 460 -- 717B3 42.964 246.749 6/8 30+ 48.7 0.6 4.1 263 5.9

East Inferno Chasm lava field Qwfb13

661B3 42.964 246.876 8/8 30+ 49.7 1.7 1.6 1,153 7.9

42.97 246.6 13/13 -- 48.2 2.2 1.0 1,694 12.9
Table 2. Paleomagnetic data for the north half of the Lake Walcott 30'×60' quadrangle, Idaho.—Continued [Site label, alphanumeric identifier of location; latitude and longitude in degrees north and east in North American Datum of 1927 (NAD 27) of the site location; N/No, number the number originally taken at the site; Exp., strength of the peak alternating demagnetizing field in milliTeslas (mT); NRM, means no alternating-field demagnitization was donessary; Li, line site solution; Pl, plane solution; Mx, mixed line and plane site solution; I and D, remanent inclination and declination in degrees; α95, 95 percent confidence limidegrees; k, estimate of the Fisherian precision parameter; R, length of the resultant vector; and P latitude, P longitude, location in degrees north and east of the virtual geomagnethe site mean direction; Averaged lat/long data for mean of the site, means is required to calculate averaged VGP position;--, no data] Site name Map unit symbol Site label Latitude Longitude N/No Exp. I D α95 k 

Grandview Crater lava field Qwfb3 693B3 42.909 246.870 8/8 30 77.8 340.2 3.2 310 6.9

701B3 42.928 246.820 8/8 20 72.7 335.9 4.6 148 7.9

42.92 246.85 2/2 -- 75.3 337.7 11.4 483 1.9

Wapi Park lava field Qwfb15 509B3 42.913 246.614 6/8 30 54.2 351.6 2.2 963 5.9

517B3 42.884 246.635 8/8 30 55.1 351.7 1.7 1,118 7.9

565B3 42.849 246.448 8/8 40 53.3 349.7 1.8 1,006 7.9709B3 42.919 246.793 8/8 30 56.0 350.4 1.0 2,933 7.9

695B4 42.804 246.659 8/8 30 52.7 354.4 1.6 1,229 7.9

703B4 42.845 246.645 8/8 30+ 54.6 354.2 1.4 1,518 7.9

931B4 42.898 246.508 8/8 30 54.8 353.4 1.1 2,802 7.9

42.9 246.6 7/7 -- 54.4 352.2 1.2 2,743 6.9

Split Butte lava field Qwfc1 921B4 42.897 246.626 10/10 Pl 26.2 334.9 2.5 461 -- 

711B4 42.782 246.549 6/8 Pl 28.1 336.8 7.8 90 -- 

42.8 246.6 2/2 -- 27.2 335.8 5.6 2,031 2.0

Wapi lava field Qwfa2 WapI 1⁄2 42.804 246.881 13/13 NRM 62.8 12.5 1.2 1,142 12.9

WapI 3⁄4 42.9 246.767 12/12 NRM 62.8 13.9 1.5 827 11.9

Kings Bowl lava field Qwfa1

KBII 42.949 246.785 24/24 NRM 61.9 13.2 1.0 906 23.9

42.9 246.8 3/3 -- 62.5 13.2 0.9 18,936 3.0

Higgins Blowout Qwfc3

541B3 42.768 246.622 6/8 30 57.5 331.20 1.4 2,190 5.9679B4 42.775 246.707 8/8 Pl 57.2 328.00 3.1 269 -- 

687B4 42.775 246.686 8/8 30 58.5 323.90 1.7 1,132 7.9

42.77 246.66 3/3 -- 57.8 327.70 3.1 1,547 2.9

Vents east of the Great Rift volcanic rift zone

Vent 5270' lava field Qsbc60

645B3 42.983 246.896 8/8 40+ 61.7 335.4 1.9 835 7.9

43.0 246.9 3/3 -- 63.7 336.3 2.9 1,790 2.9
Table 2. Paleomagnetic data for the north half of the Lake Walcott 30'×60' quadrangle, Idaho.—Continued[Site label, alphanumeric identifier of location; latitude and longitude in degrees north and east in North American Datum of 1927 (NAD 27) of the site location; N/No, number the number originally taken at the site; Exp., strength of the peak alternating demagnetizing field in milliTeslas (mT); NRM, means no alternating-field demagnitization was donessary; Li, line site solution; Pl, plane solution; Mx, mixed line and plane site solution; I and D, remanent inclination and declination in degrees; α95, 95 percent confidence limidegrees; k, estimate of the Fisherian precision parameter; R, length of the resultant vector; and P latitude, P longitude, location in degrees north and east of the virtual geomagnethe site mean direction; Averaged lat/long data for mean of the site, means is required to calculate averaged VGP position;--, no data]

Site name Map unit symbol Site label Latitude Longitude N/No Exp. I D α95 k 

Flat Top lava field Qwfc4

621B3 42.995 246.934 7/8 30 73.6 19.8 2.1 838 6.9629B3 42.982 246.935 7/8 50 75.7 17.0 1.9 986 6.9

Streifling lava field Qsbc59 613B3 42.995 246.959 8/8 30 74.0 37.1 2.0 754 7.9Vent 5026' lava field Qwfc5

637B3 42.972 246.934 8/8 Pl 72.3 22.2 4.6 140 -- 662B4 42.950 246.924 8/8 Pl 75.9 7.5 3.5 282 -- 

Vent 4822' lava field Qwfc6

669B3 42.921 246.949 7/8 40+ 77.2 18.9 2.2 772 6.9

42.98 246.94 6/6 -- 75.0 20.8 2.6 689 5.9

Vent 5329 lava field Qsbb34 653B3 42.968 246.837 8/8 30 62.1 345.6 1.1 2,421 7.9Claasen lava field Qwfb4 4380B 42.849 246.914 8/8 Li 59.7 34.2 2.5 504 7.9Vent 4829' lava field Qwfb6 4300B 42.849 246.930 8/8 Mx 64.7 30.5 1.5 1,385 7.9Vent 4801' lava field Qwfb7 677B3 42.839 246.899 8/8 30 56.8 32.8 2.5 487 7.9Vent 4862' lava field Qwfb8 4460B 42.860 246.914 8/8 Mx 59.3 27.4 2.5 567 7.9Winters Blowout Lava field Qwfb5

685B3 42.895 246.895 7/8 30 59.5 26.9 2.6 539 6.9

42.86 246.91 5/5 -- 60.0 30.4 3.1 600 4.9

Vent 4640' lava field Qwfd1 4540B 42.806 246.942 7/8 Mx 69.4 349.9 2.0 1,183 6.9“Bower” vent E of vent 4640' Qwfd1

4620B1 42.835 247.032 7/8 Li 71.2 345.2 1.3 2,036 6.9

42.82 246.99 2/2 -- 70.3 347.7 5.2 2,286 2.0

Vent 4645' lava field Qwfb19 749B3 42.762 246.843 8/8 Pl 62.6 331.6 1.3 1,739 -- 

1Indicates site is not located within the boundary of the north half of the Lake Walcott 30'×60' quadrangle.
12 Geologic Map of the North Half of the Lake Walcott 30'×60' Quadrangle, Idaho

DESCRIPTION OF MAP UNITS

[Description of rock types in the Description of Map Units follows common terms used in field 

descriptions of basaltic volcanic rocks. Minerals in lava flows and mineral dimensions were identified 

by typical hand-specimen investigation using a hand lens and millimeter (mm) scale. Rock colors were 

assigned by field inspection. The description of mineralogy and textures of rocks are given for samples 

collected locally; the descriptions may not reflect crystal sizes, proportions, or textures that may be 

obtained for a sample of the same lava field collected elsewhere. Where measured, normal magnetic 

polarity indicates age younger than 780 ka and reversed magnetic polarity indicates age older than 780 ka]

UNCONSOLIDATED SURFICIAL DEPOSITS

Qes Eolian sand (Holocene to early middle Pleistocene)—Fine to medium sand, well sorted. 

Typically forms barchan and longitudinal dunes. Much of source material derived from 

alluvial sediment deposited along Snake River southwest of map area in the vicinity of 

Lake Walcott, chiefly during the Bonneville Flood about 14,500 yr ago (Malde, 1968; 

O'Connor, 1993). Thickness 0–15 m

Qp Playa deposits (Holocene to early middle Pleistocene)—Silty sand to clayey silt; minor 

gravel and scattered boulders of basalt along margins. Fills depressions along contacts 

between lava flows. Deposited along low-gradient ephemeral streams and in ephemeral 

lakes. Thickness 0–10 m

HOLOCENE AND PLEISTOCENE BASALTIC LAVA FLOWS AND 

RELATED NEAR-VENT PYROCLASTIC AND ERUPTIVE-FISSURE 

DEPOSITS OF THE SNAKE RIVER GROUP

ERUPTIVE PERIOD A

Qwfa1 Surface- and fountain-fed, proximal, medial, and distal basaltic pahoehoe lava flows 

of the Kings Bowl lava field (Holocene)—The Kings Bowl lava field is characterized 

by thin, fissure-fed flows adjacent to the 7-km-long set of eruptive fissures of the Great 

Rift volcanic rift zone. The eruptive fissures consist of linear, discontinuous, en echelon 

cracks that are 2–3 m wide and locally filled with feeder dikes and breccia. The eruptive 

fissures are flanked by parallel, non-eruptive fissures that are 600 meters (m) to as 

much as 1.1 kilometers (km) from the main fissure. Thin, fissure-fed, shelly-pahoehoe 

flows, lava ponds, low lake levees, spatter ramparts, small spatter cones, and a large 

explosion pit characterize the lava field adjacent to the eruptive fissures. Spatter ramparts and spatter cones are prominent in the vicinity of Creons Cave and South Grotto. 

Kings Bowl is an 85-m-long, 30 m-wide, and 30-m-deep explosion pit. A lava pond 

surrounded the site of Kings Bowl prior to the explosion. Prominent basalt mounds, 

believed to be the remnants of levees, define the limits of the lava pond. Blocks as 

large as 10 m in diameter were hurled westward from Kings Bowl for distances of up 

to 245 m; many of the large blocks broke through the crust of the lava pond. A lightcolored blanket of tephra spread eastward from the explosion pit; tephra about 1 mm 

in diameter occurs as far east as 1.2 km from Kings Bowl. The tephra blanket has an 

area of 0.15 km2. Calculations by King (1977) show that the volume of ejected tephra 

falls far short of that needed to fill the cavity at Kings Bowl. This indicates collapse 

of the vent area subsequent to the explosive event, or shattering of the material originally in the cavity to dimensions much smaller than 1 mm and dispersal of the finegrained material far beyond the vicinity of Kings Bowl. Crystal Ice Cave is a locality 

on the eruptive fissure north of Kings Bowl in which ice stalagmites and stalactites 

occur. The path to the ice features has been closed by the NPS and BLM because they 

cannot maintain the trail to the ice features. In addition, if access to the ice features 

were to remain open, the ice would melt away due to ingress of relatively hot surface 

air. The Kings Bowl field is small, consisting of about 0.005 cubic kilometers (km3

) 

of pahoehoe lava flows that cover an area of about 3.3 km2. Kuntz and others (1992) 
Description of Map Units 13

surmised that the Kings Bowl lava field formed within a matter of a few hours or a few 

days. A radiocarbon age of 2,130±130 14C yr before the present (B.P.) has been determined for the lava field (Prinz, 1970). The precise location of the sample is unknown, 

so it is not plotted on the map.

Qwfa2 Proximal, medial, and distal, chiefly tube-fed, basaltic pahoehoe lava flows and 

near-vent deposits of the Wapi lava field (Holocene)—The Wapi lava field is a 

monogenetic, shield-type lava field that covers about 325 km2 and has a volume of 

about 5 km3 of lava flows. Paleomagnetic studies by Champion and Shoemaker (1977) 

show that all areas of the lava field have the same paleomagnetic direction, suggesting that the entire lava field formed within a few months, years, or decades. The nearly 

identical paleomagnetic directions of the Wapi and Kings Bowl lava fields suggest that 

they are part of the same eruptive event. The slopes of the lava field are typically less 

than 1 degree, a consequence of very fluid pahoehoe lava flows and high rates of lava 

effusion. Steeper slopes occur at Pillar Butte, the summit region of the lava field, where 

slopes are as high as 7°. Pillar Butte is a mass of agglutinated, shelly pahoehoe flows, 

possibly injected by dikes. The vent region consists of 11 eruptive centers indented 

by pit craters; there is abundant evidence of former lava ponds. Near-vent flows are 

typically a few hundred meters long, contain many small lava tubes and channels that 

distributed surface-fed flows. Many flows in the vent area are pahoehoe near the vent 

but transition into slab pahoehoe flows near their termini. Dashed contact enclose area 

of slab pahoehoe and a'a flows that surrounds vent area. Medial and distal pahoehoe 

flows are typified by flow ridges and collapse depressions, whereas proximal flows are 

leveed-channel a'a and slab pahoehoe flows and pahoehoe flows that consist largely 

of surface-fed pahoehoe toes. The age of the Wapi lava field is 2,270 ±50 14C yr B.P. 

(Kuntz, Spiker, and others, 1986)

ERUPTIVE PERIOD B

Qcfb4 Distal pahoehoe basalt lava flows of the Minidoka lava field (Holocene)—Tube- and 

surface-fed pahoehoe basalt flows having hummocky, billowy surfaces. Lava-tube 

systems carried lava as far as 35 km southwest from obscure source vents in the Great 

Rift volcanic rift zone near New Butte in the Craters of the Moon quadrangle (Kuntz 

and others, 2007). Distal parts of the flow are present along the northern part of this 

quadrangle about 12 to 25 km west of the Kings Bowl lava field (Qwfa1). Radiocarbon 

age of unit is 3,590±70 14C yr B.P. (Kuntz, Spiker, and others, 1986)

Qcfb5 Distal pahoehoe basalt lava flows of the Larkspur Park lava field (Holocene)—Surfaceand tube (?)-fed pahoehoe flows having a hummocky, billowy surface. Field relations, 

petrographic similarities, and paleomagnetic data indicate that the unit may be an early 

phase of the Minidoka lava field (Qcfb4). Distal parts of lava field cross the northern 

boundary of this quadrangle about 32 km west of the Kings Bowl lava field (Qwfa1)

ERUPTIVE PERIOD E

Qcfe2 Distal pahoehoe and a'a basalt lava flows of the Laidlaw Lake lava field (Holocene)—

Chiefly surface and tube (?)-fed pahoehoe basalt flows possibly from source vents at 

Grassy Cone in the Craters of the Moon quadrangle (Kuntz and others, 2007). Pahoehoe is hummocky and has pressure plateaus, collapse depressions, and flow ridges. 

Distal parts of lava field cross the northern boundary of this quadrangle about 40 km 

west of the Kings Bowl lava field (Qwfa1). Radiocarbon age of unit is 7,470±80 14C yr 

B.P. (Kuntz, Spiker, and others, 1986)

ERUPTIVE PERIOD H

Qcfh1 Distal pahoehoe basalt lava flows of the Kimama lava field (late Pleistocene)—Surfaceand tube-fed hummocky, pahoehoe basalt flow. Only distal part of flow is exposed, 

source vent is unknown. Flow is covered by thin, discontinuous mantle of loess and 
14 Geologic Map of the North Half of the Lake Walcott 30'×60' Quadrangle, Idaho

eolian sand. Distal parts of lava field cross the northern boundary of this quadrangle 

about 35 km west of the Kings Bowl lava field (Qwfa1). Radiocarbon age of flow is 

15,100±160 14C yr B.P. (Kuntz, Spiker, and others, 1986)

ERUPTIVE PERIOD B

Qwfb1 Near-vent pyroclastic deposits and proximal, medial, and distal pahoehoe and a'a 

basaltic lava flows of the Shale Butte lava field (late Pleistocene)—Chiefly older, 

surface-fed pahoehoe flows and younger, surface-fed a'a flows. Vent area is about 

30 m long, and consists of three nested depressions in a small lava pond. The vent 

depression is surrounded by shelly pahoehoe levees. Dashed contact within unit 

separates older pahoehoe flows in the distal parts of the lava field from younger a'a 

flows in the medial and proximal parts of the lava field. A'a flows contain many 

lobes having steep fronts, flow ridges that are both perpendicular and convex toward 

the direction of flow movement, and also longitudinal furrows and cracks that are 

roughly parallel to flow movement. Shale Butte is the only known source vent in 

the Snake River Plain that erupted both pahoehoe and a'a flows. A'a is gray, dense, 

and all crystals are less than 1 mm long; pahoehoe is dark gray, coarse grained; 

plagioclase crystals are ≤10 mm, olivine crystals are ≤2 mm, and glomerophyric 

clots of plagioclase+olivine as large as 15 mm are common. Distal pahoehoe flows 

on the western side of lava field are mantled by thin, discontinuous loess and eolian 

sand. Sample for dating contained essentially no radiogenic argon, thus total fusion 

and isochron ages have high uncertainties. Age assigned (19±2 ka, table 1) is based 

on paleomagnetic evidence that the Shale Butte eruption occurred during a time 

interval of aberrant geomagnetic field dated about 19±2 ka and called the Hilina Pali 

Excursion (Coe and others, 1978)

Qwfb2 Near-vent, basaltic pahoehoe lava flows of the Vent 4319 lava field (late Pleistocene)—

Small vent aligned with vent at Shale Butte, about 4 km north-northwest. The rock is 

gray, medium grained, and contains phenocrysts of plagioclase <8 mm, olivine crystals 

≤1.5 mm, and glomerophyric clots of olivine+plagioclase crystals that are as large as 

12 mm. Vent 4319 flows believed to have been erupted early in the Vent 4319-Shale 

Butte eruption sequence and later surrounded by pahoehoe flows from the vent at 

Shale Butte (Qwfb1)

Qwfb3 Pahoehoe basaltic lava flows of the Grandview Crater lava field (late Pleistocene)—

Vent area is a north-northwest-elongated lava dome, about 20 m high, having a shallow 

circular vent depression about 100 m wide and ≤5 m deep. Flow surfaces are hummocky, containing many pressure ridges and tumuli; local relief as much as 5 m. Rock 

is porphyritic, containing phenocrysts of plagioclase as long as 3 mm and olivine as 

much as 4 mm in diameter set in a matrix of the same minerals plus clinopyroxene 

and glass that are all ≤1 mm. Rock also contains glomerophyric clots of olivine and 

olivine+plagioclase that are as much as 5 mm in longest dimension. 40Ar/39Ar age is 

38±12 ka (table 1)

Qsbb5 Distal basaltic pahoehoe flows of Broken Top lava field (late Pleistocene)—Surface-, 

channel-, and tube-fed, dark gray to medium-gray pahoehoe basalt flows. Flows erupted 

from a north-south trending, 830-m-long, 47-m deep slot-shaped vent at Broken Top 

Butte in the Craters of the Moon quadrangle, about 17 km north of the northern boundary of this map. Distal flow areas in this map are characterized by rough surfaces; lava 

mounds, tumuli cut by cracks, and tilted plates of lava. Rock is porphyritic, containing 

moderately elongated to elongated laths of plagioclase as long as 3 mm, and subhedral, rounded, mostly equant olivine crystals that are ≤1.0 mm. Cumulophyric clots 

of plagioclase+plagioclase and plagioclase+olivine are common, as is waist texture. 

Phenocrysts are set in a fine, intergranular matrix of plagioclase crystals, equant olivine 

crystals, intergranular clots of clinopyroxene intergrown with needle-like ilmenite crystals, intersertal yellow glass, and ubiquitous small crystals of opaque minerals. Flows 

covered by little or no loess or eolian sand. 40Ar/39Ar age is <50 ka (Kuntz and others, 

2007)
Description of Map Units 15

Qsbb6 Distal pahoehoe basaltic lava flows of the Sand Butte lava field (late Pleistocene)—

Distal, tube-fed, hummocky pahoehoe flows of the Sand Butte lava field are present 

in the northwestern part of this quadrangle north of Shale Butte. Flows contain many 

tumuli and broken slabs of pahoehoe, local relief is ≤5 m. Rock is dark gray, weakly 

porphyritic and contains crystals of plagioclase and olivine typically ≤2 mm in a matrix 

of plagioclase, olivine, clinopyroxene, and glass ≤0.5 mm. The source vent, Sand 

Butte, is located about 15 km north of the northern border of this quadrangle (Kuntz 

and others, 2007). Sand Butte is a tuff cone formed by the phreatomagmatic interaction 

of groundwater and basaltic magma (Womer and others, 1982). Geographic alignment 

with Broken Top Butte and similarity in paleomagnetic directions with Spud Butte 

suggest contemporaneity. K-Ar age is 34±29 ka (table 1)

Qsbb8 Basaltic pahoehoe lava flows and near-vent pyroclastic deposits of West Wildhorse 

Butte lava field (late Pleistocene)—Distal flows of the West Wildhorse Butte lava 

field occur in the northwest part of this quadrangle east of Shale Butte. Flows are 

characterized by many tumuli and pressure plateaus. Rock is coarse grained, containing plagioclase phenocrysts as long as 4 mm and olivine phenocrysts as long as 2 mm. 

Glomerophyric clots of olivine, plagioclase, and plagioclase+olivine are as large as 

3.5 mm diameter. Syntaxial bundles of as many as 10 plagioclase crystals are common. 

40Ar/39Ar age is <50 ka (Kuntz and others, 2007)

CLAASEN VENT COMPLEX (LATE PLEISTOCENE)

The Claasen vent complex contains seven individual lava fields. Flows from five of these vents have 

the same paleomagnetic direction, indicating eruptions occurred within a short period of time, perhaps 

spanning only a few hundred years. Age relationships of overlapping flows are ambiguous; thus, a definitive sequence of eruptions in the vent complex is difficult to determine. 40Ar/39Ar age of Vent 4801 lava 

field (Qwfb7) in the Claasen vent complex is 68±8 ka (table 1). Because of nearly identical paleomagnetic 

directions for all lava fields in the Claasen vent complex, this age applies to all of them

Qwfb4 Basaltic, near-vent, pyroclastic deposits and proximal and medial pahoehoe lava flows 

of Claasen lava field—Vent area for Claasen lava field consists of a north-south elongated, shallow depression about 400 m long. Proximal flows are pahoehoe; pyroclastic 

deposits are absent. A rootless vent about 600 m south of main vent depression consists 

of reddish, oxidized cinders. Medial and distal flows are farmed; loess and eolian sand 

cover is typically ≤2 m, but greater than plow depth. Rock has a fine-grained matrix in 

which all crystals are ≤1 mm; larger crystals of plagioclase ≤2 mm and olivine ≤1 mm 

are present, along with glomerophyric clots of olivine+plagioclase as large as 6 mm

Qwfb5 Basaltic pahoehoe lava flows of the Winters Blowout lava field—Vent area consists of a 

circular depression about 600 m diameter and 20 m deep. Only about 1 km2 of proximal 

flows exposed; source vent surrounded by flows from relatively younger lava fields. 

The rock is gray and has a dense matrix; it contains rare plagioclase crystals ≤2 mm and 

glomerophyric clots of olivine as much as 4 mm diameter. No paleomagnetic data. Part 

of Claasen vent complex, correlation based on location and similar depth of loess and 

eolian sand

Qwfb6 Basaltic pahoehoe lava flows and near-vent pyroclastic deposits of the Vent 4829 lava 

field—Vent area is a N.10°W.-oriented elongated slot about 3 m deep and 100 m long. 

Vent rimmed by reddish, oxidized pyroclastic deposits. A rootless (?) satellite vent surrounded by pyroclastic deposits is located about 600 m east of the main vent. Medial 

and distal flows mantled by ≤2 m loess and eolian sand, which is farmed. Rock is 

dark gray, fine grained, and contains plagioclase phenocrysts ≤2 mm, olivine crystals 

≤1 mm, and glomerophyric clots of olivine crystals as large as 6 mm diameter. Part of 

Claasen vent complex, based on paleomagnetic correlation

Qwfb7 Basaltic near-vent pyroclastic deposits and medial and distal pahoehoe lava flows of 

the Vent 4801 lava field—Vent area is eruptive fissure about 600 m long bordered 

by reddish, oxidized, bedded cinders and scoria exposed in road cut. Medial and 

distal flows are mantled by ≤2 m loess, which is farmed. Rock is dark gray, has a 

fine- to medium-grained matrix, and contains plagioclase ≤4 mm, olivine ≤2mm, and 
16 Geologic Map of the North Half of the Lake Walcott 30'×60' Quadrangle, Idaho

glomerophyric clots of plagioclase+olivine ≤6 mm diameter. 40Ar/39Ar age is 68±8 ka. 

Part of Claasen vent complex

Qwfb8 Basaltic near-vent pyroclastic deposits and medial and distal pahoehoe lava flows 

of the Vent 4862 lava field—Vent area is a high-standing (12 m above surrounding 

terrane), north-south elongated ridge having two source vents about 400 m apart. 

Pyroclastic materials present but in small volume near vents. Medial and distal flows 

mantled by ≤2 m loess and eolian sand, but farmed. Rock is dark gray, has a fine- to 

medium-grained matrix, and phenocrysts of plagioclase ≤5 mm, olivine ≤2 mm, and 

moderately abundant glomerophyric clots of olivine ≤10 mm diameter. Part of Claasen 

vent complex based on paleomagnetic correlation

Qwfb9 Basaltic near-vent pyroclastic deposits and medial and distal pahoehoe lava flows of 

the Vent 4835 lava field—Vent area consists of a single circular depression 100 m 

diameter. Pyroclastic materials present but in small volume near vent. Medial and distal 

flows mantled by ≤2 m loess and eolian sand, but farmed. Rock is dark gray, fine- to 

medium-grained, and contains distinctive glomerophyric clots of olivine+plagioclase 

≤7 mm. Part of Claasen vent complex based on paleomagnetic correlation

Qwfb10 Basaltic near-vent pyroclastic deposits and proximal pahoehoe lava flows of the Vent 

4569 lava field—Vent area surrounded by flows of younger lava fields. Mantled by 

loess ≤3 m, but farmed. Part of Claasen vent complex; correlation based on location and 

similar depth of loess and eolian sand

INFERNO CHASM VENT COMPLEX (LATE PLEISTOCENE)

The Inferno Chasm vent complex is an eruptive-fissure system that includes, from north to south, 

the Horse Butte (Qsbb25), Cottrells Blowout (Qwfb14), Inferno Chasm (Qwfb12), East Inferno Chasm 

(Qwfb13), and Vent 5094 (Qsbb24) lava fields. All lava fields in this eruptive-fissure system have the 

same paleomagnetic direction (table 1), indicating that they all formed in the same eruptive pulse about 

73±8 ka (see description for Horse Butte lava field (Qsbb25) for age). Overlapping relationships among 

flows indicate that eruptions began at Cottrells Blowout, migrated north to Horse Butte, then south to the 

Inferno Chasm vents, and ended at Vent 5094

Qsbb24 Basaltic pahoehoe lava flows and minor near-vent pyroclastic deposits of the Vent 5094 

lava field—Vent area for Vent 5094 is a north-south elongated, 500 m-long, eruptivefissure. Two small vent depressions about 100 m northwest and southwest of hill 5108 

are believed to be bocca vents to the main eruptive fissure. The largest volume of the 

Vent 5094 eruption occurred at these two vents. Two lava ponds, each about 1.5 km 

long and 500 m wide extend, northwest and southwest from the vents at hill 5108. The 

lava ponds have steep leveed margins as high as 10 m. These lava ponds are known 

informally as the Papadakis perched lava ponds. Reddish, oxidized pyroclastic deposits 

occur both on hill 5094 and hill 5108. Pahoehoe flows from the bocca vents traveled 

westward and lapped onto two high-standing areas of lava-tubes of the Horse Butte 

lava field (Qsbb25). Medial and distal flows are rough and contain jumbled slabs of 

pahoehoe and steep flow fronts as high as 8 m. Relief on medial and distal flows is 

≤15 m. Rock is dark gray, contains small phenocrysts of plagioclase ≤3 mm and olivine 

≤1.5 mm, and glomerophyric clots of plagioclase+olivine about 5 mm diameter. 

Qwfb12 Basaltic, pahoehoe lava flows and near-vent deposits of the Inferno Chasm lava field—

Main vent is a roughly circular depression about 200 m diameter, 30 m deep, having 

steep to vertical walls. A lava channel about 100 m wide and as much as 20 m deep 

has steep walls. The lava channel, called Inferno Chasm, extends about 1.5 km west 

and southwest of the vent area. At its terminus, the channel is overlapped by flows 

from Vent 5094 (Qsbb24) lava field, indicating relative age relationships. Proximal 

deposits are mainly thin, shelly pahoehoe flows, medial flows are relatively smooth 

pahoehoe. Rock is gray, has a fine-grained matrix, and contains abundant phenocrysts 

of plagioclase ≤3 mm and olivine ≤3 mm, and abundant glomerophyric clots of 

olivine+plagioclase crystals as large as 5 mm diameter

Qwfb13 Basaltic pahoehoe lava flows and near-vent pyroclastic deposits of the East Inferno 

Chasm lava field—Vent area consists of two, roughly circular, shallow depressions, 
Description of Map Units 17

each about 25 m diameter. Vents aligned about N.10° W. Vent area has smooth slopes 

consisting of oxidized pyroclastic deposits. Medial and distal flows are relatively 

smooth pahoehoe. Flows lap onto flows of Horse Butte lava field (Qsbb25) about 

1.5 km northeast of the northernmost vent. Rock is dark gray, and contains moderately 

abundant phenocrysts of plagioclase ≤4 mm and olivine ≤1.5 mm. Glomerophyric clots 

of olivine+plagioclase are ≤4 mm

Qsbb25 Basaltic pahoehoe lava flows and near-vent pyroclastic deposits of the Horse Butte lava 

field—Vent area for flows at Horse Butte is located about 2 km north of Cottrells Blowout and 1 km north of the northern boundary of this quadrangle. The vent area consists 

of a north-south trending 1.1-km long, 700-m wide, 90-m deep crater at the top of a large 

shield volcano. Vent crater is one of the largest and deepest in the eastern Snake River 

Plain. Crater contains several nearly horizontal terraces, indicating that piston-like filling 

and draining and lava-lake activity occurred during the later stages of vent development. 

A tube-skylight system in Horse Butte flows, called the Bear Trap lava tube system, 

extends across the northern margin of this map for a distance of about 35 km. Many 

skylights of the tube system are accessible by roads. Medial flows contain many tumuli, 

pressure ridges, and pressure plateaus. Flows from rootless vents along the tube system 

are extremely smooth; local relief is ≤0.5 m. Dashed contact in unit separates flows from 

two adjacent lava-tube systems. Map units Qsbb36 and Qsbc22 that occur as kipukas 

in the Craters of the Moon lava field of the geologic map of the Craters of the Moon 

quadrangle (Kuntz and others, 2007) are now believed to be parts of the Horse Butte 

lava field (Qsbb25). Basaltic lava flows of the Horse Butte lava field cover an estimated 

area of about 500 km2

 in this quadrangle and about 180 km2 in the Craters of the Moon 

quadrangle to the north. The total volume of the Horse Butte lava field is estimated to be 

about 6 km3

. Proximal flows are typically dark gray, contain phenocrysts of plagioclase 

≤6 mm and olivine ≤2 mm in a matrix of minerals all ≤0.5 mm. Glomerophyric clots of 

plagioclase and plagioclase+olivine are typically ≤5 mm. Flows mantled by ≤2 m loess 

and eolian sand. 40Ar/39Ar age is 73±8 ka (table 2)

Qwfb14 Near-vent pyroclastic deposits and proximal pahoehoe lava flows of the Cottrells Blowout lava field—Slot-shaped vent trending about N.15°

W. is one of the most spectacular 

in terms of depth and length in the eastern Snake River Plain. Vent is 600 m long, about 

200 m wide, and as deep as 50 m. Walls of vent are steep to vertical. Flows exposed in 

vent walls are mainly shelly pahoehoe. Flows to east and south of vent were fed from a 

lava channel. Proximal flows mantled by thin (≤1 m) pyroclastic deposits from younger, 

nearby vents. Flows from Cottrells Blowout have limited distribution; they are lapped 

onto by flows from Horse Butte (Qsbb25) and Vent 5094 (Qsbb24). Rock is gray, has 

a fine- to medium-grained matrix where crystals are ≤0.5 mm and contains moderately 

abundant phenocrysts of plagioclase ≤4 mm and olivine ≤1.5 mm. Glomerophyric clots 

of olivine+plagioclase are ≤4 mm

Qwfb15 Basaltic, near-vent and proximal, medial, and distal pahoehoe lava flows of the Wapi 

Park lava field (late Pleistocene)—Elongated vent area is a set of about 10, roughly 

circular depressions oriented about N. 15°

E. Each depression is about 200–400 m 

diameter. The vent depressions occur over a distance of 2 km in Wapi Park. A complex 

lava pond having numerous irregular embayments is present at the northeast end of the 

vent alignment. Southwest part of vent alignment overlapped by flows of the Wapi lava 

field (Qwfa2; 2,270±50 14C yr B.P., Kuntz, Spiker, and others, 1986). Vent area stands 

about 30–35 m above surrounding proximal flows. Proximal pahoehoe flows are bulbous, having local relief of ≤3 m; pyroclastic deposits are rare in the vent area. Medial 

flows are covered by flows of the Wapi lava field (Qwfa2; 2,270±50 14C yr B.P., Kuntz, 

Spiker, and others, 1986). A prominent lava-tube system with numerous skylights, 

perched lava ponds, and irregular depressions extends west and southwest for a distance 

of about 13 km from where the flows of the Wapi Park lava field emerge from beneath 

the Wapi lava field (Qwfa1).

Pahoehoe flows in the tube-fed lobe have smooth surfaces; local relief is 

≤0.5 m. Distal flows of the Wapi Park lava field extend about 30 km southwest of the 

source vents in Wapi Park. Distal flows range from dense rock in which all crystals 
18 Geologic Map of the North Half of the Lake Walcott 30'×60' Quadrangle, Idaho

are ≤0.5 mm to fine-grained rock containing larger crystals of plagioclase and olivine 

≤1.5 mm in a dense matrix. Vents in Wapi Park are believed to be at the northeast end 

of a N.15°E.-N.30°E.-trending volcanic rift zone that extends southwest through Vent 

4901 (Qwfb16) to Higgins Blowout (Qwfc3; 289±5 ka, table 1) along the southwest 

margin of the Wapi lava field. 40Ar/39Ar age is 87±9 ka (table 1)

Qwfb16 Near-vent, basaltic pyroclastic deposits of the Vent 4901 lava field (late Pleistocene)—

Vent area is a kipuka surrounded by pahoehoe flows of the Wapi lava field (Qwfa2; 

2,270±50 14C yr B.P., Kuntz, Spiker, and others, 1986). Unit not studied in the field; 

examination of air photos and topographic map shows two circular vent depressions 

aligned N.15oE., on strike with vents in Wapi Park. Southernmost depression filled 

by pahoehoe flows of Wapi lava field. Age assignment based on vent alignment and 

assumption that Vent 4901 lava field formed simultaneously with Wapi Park lava field

Qwfb17 Basaltic near-vent pyroclastic deposits and pahoehoe lava flows of the Kimama Butte 

lava field—Lava field is a large shield volcano rising about 160 m above surrounding 

lava fields. Vent area consists of a north-south, 700-m long, 200-m wide depression 

surrounded by basalt parapets as high as 50 m on east, south, and west sides. A circular pit crater, about 200 m diameter and 15 m deep, is at the northern end of the vent 

area. Vent area mantled chiefly by shelly pahoehoe flows, pyroclastic deposits are rare. 

Medial and distal flows are mainly tube-fed pahoehoe having local relief of ≤2 m. Lava 

field has an area of about 250 km2 and contains about 8 km3 of flows. Flows mantled by 

loess and eolian sand ≤2 m only locally. Not farmed in most areas. Rock is light gray, 

diktytaxitic, and ranges from very fine (all crystals ≤1 mm) to medium grained in which 

plagioclase and olivine crystals are ≤2 mm. 40Ar/39Ar age is 87±11 ka (table 1)

Qsbb14 Basaltic medial and distal pahoehoe lava flows of the Black Ridge Crater lava field (late 

Pleistocene)—Vent area is located in Craters of the Moon map area about 3 km northeast 

of the northwest corner of this quadrangle (Kuntz and others, 2007). Medial and distal 

flows of the southern part of lava field exposed in northwest corner of this quadrangle. 

Medial and distal pahoehoe flows have prominent lava-tube and skylight systems. Flows 

characterized by many tumuli, pressure ridges and pressure plateaus, having local relief of 

≤5 m. Flows are mantled locally by ≤2 m loess and eolian sand; most areas free of loess 

and sand. Rock is moderately coarse grained, containing plagioclase and olivine phenocrysts ≤5 mm in a matrix of minerals ≤1 mm. Relative age inferred from stratigraphic relations with dated, nearby lava fields, estimated age about 120 ka (Kuntz and others, 2007)

Qsbb34 Basaltic near-vent pyroclastic deposits and medial and distal pahoehoe lava flows of 

the Vent 5329 lava field (late Pleistocene)—Vent area is 200-m long, roughly circular, shallow depression. A major lava tube extends east and southeast of the vent. 

A secondary vent is located about 1.5 km southeast of main vent; it may be a rootless 

vent or a vent aligned with the main vent. If the latter, the two vents define a N.30°W.-

trending eruptive-fissure system. The two vents in this quadrangle are aligned with two 

vents several kilometers farther north in the Craters of the Moon quadrangle. Rock not 

studied in the field. Age approximately 210 ka (Kuntz and others, 2007)

ERUPTIVE PERIOD C

Qwfc1 Near-vent pyroclastic deposits and distal pahoehoe lava flows of the Split Butte lava 

field (late middle Pleistocene)—Split Butte is a tuff cone or maar formed by a phreatomagmatic eruption. The vent area is roughly circular, 600 m diameter, and consists of a 

tephra ring containing an inner lava pond. Tuff cone consists chiefly of reddish brown, 

olive green, and black vitric ash. The margins of the lava pond are preserved as a narrow 

shelf of basalt; the central part of the lava pond has subsided forming a pit crater 400 m 

diameter and 20 m deep. Partial palagonitization of sideromelane, near lack of vesicles, 

accretionary lapilli, plastic deformation of layers under ejecta blocks, and the presence 

of abundant secondary minerals such as calcite and zeolites all suggest phreatomagmatic 

origin for Split Butte. Definitive study of Split Butte is by Womer and others (1982). Area 

of pahoehoe flows (Qwfc2) about 15 km southwest is tentatively correlated to source vent 

at Split Butte by paleomagnetic data (table 2). 40Ar/39Ar age is 138±6 ka (table 1)
Description of Map Units 19

Qwfc2 Basaltic, distal pahoehoe flows (late middle Pleistocene)—Flows are smooth and consist 

of dense, fine-grained rock in which all crystals are ≤1 mm. May be correlated to source 

vent at Split Butte on the basis of similarity in paleomagnetic directions

Qsbc22 Basaltic pahoehoe flows of the Vent 4645 lava field (late middle Pleistocene)—Surface-, 

channel-, and tube(?)-fed, dark-gray and medium-gray, pahoehoe basalt flows, and red, 

oxidized, and black, near-vent pyroclastic deposits. Vent area is located about 7 km 

north of the northern boundary of this map in the Craters of the Moon 30'×60' quadrangle (Kuntz and others, 2007). Vent area is a N.5°W.-trending, fissure-dominated, 

1-km-long, slot-shaped, shallow (≤15 m deep) depression atop a broad lava cone that 

is about 50 m above surrounding, younger flows. Distal and medial flows have smooth 

surfaces; local relief ≤2 m. Rock examined is unique: olivine phenocrysts are larger 

(≤3 mm) than plagioclase phenocrysts (≤1.5 mm). Phenocrysts are set in a fine matrix 

of crystals ≤0.5 mm. Cumulophyric clots of as many as 4 olivine crystals and clots of as 

many as 20 plagioclase and 20 olivine crystals are common. Most large olivine crystals are euhedral, ≤1.5 mm, and many are skeletal, containing vacuoles filled by glass. 

Flows are mantled by ≤2 m of loess and eolian sand

Qsbc52 Distal pahoehoe flows of the Mosby Butte lava field (late middle Pleistocene)—Very 

small area (<1 km2

) of distal pahoehoe lava flows of the Mosby Butte lava field (Kuntz 

and others, 2007) at northern border of this map. Flows characterized by many tumuli, 

pressure plateaus, and pressure ridges. Rock is porphyritic containing plagioclase 

(≤5mm) and olivine (≤1.5 mm) crystals in a matrix in which crystals are ≤0.3 mm. 

Cumulophyric clots of as many as 20 olivine crystals and olivine+plagioclase crystals 

are common. 40Ar/39Ar age is 265±30 ka (Kuntz and others, 2007)

Qwfc3 Basaltic, near-vent pyroclastic deposits and proximal and medial pahoehoe lava flows 

of the Higgins Blowout lava field (late middle Pleistocene)—Vent is a 600-m-long, 

200-m-wide, slot-shaped depression oriented about N.40°E., roughly parallel to and on 

trend with the vent system in Wapi Park lava field (Qwfb15). Slot is as deep as 50 m. 

Vent walls are steep and consist of shelly pahoehoe and minor amounts of pyroclastic 

material. A prominent, 400-m-wide, 1.5-km-long, leveed channel extends southwest from 

the vent area. Levees are about 5 m high. Proximal pahoehoe flows are very rough surfaced and hummocky, verging on a'a. Rock is dark gray, fine grained, and contains small 

crystals of plagioclase and olivine both ≤2 mm in longest dimension. Glomerophyric clots 

of olivine+plagioclase as large as 8 mm are rare. K-Ar age is 289±5 ka (table 1)

FLAT TOP VENT COMPLEX (LATE MIDDLE PLEISTOCENE)

The Flat Top vent complex consists of five contiguous lava fields that extend from the vent for the 

Rock Lake lava field in the Craters of the Moon quadrangle south to the Vent 4822 lava field (Qwfc6). All 

have the same paleomagnetic direction, suggesting that they all formed within a short period of time, perhaps a hundred years or less. 40Ar/39Ar age of the Rock Lake Lava field, part of the Flat Top Vent complex, 

is 290±50 ka (Kuntz and others, 2007). This age applies to all lava fields of the Flat Top vent complex

Qwfc4 Basaltic, near-vent pyroclastic and proximal, medial, and distal pahoehoe flows of the 

Flat Top lava field—Flat Top is a broad lava dome, having a vent area largely covered 

by shelly pahoehoe and reddish, oxidized cinders. The vent is marked by a shallow 

(≤5 m), roughly circular depression that is about 100 m wide. A roughly circular lava 

pond, about 0.5 km in longest dimension, is about 0.5 km southeast of the vent area. A 

second lava pond, 0.2 km diameter, is 1.1 km southeast of the vent area. Rock is dark 

gray, has a fine-grained matrix, contains crystals of plagioclase ≤5 mm, olivine crystals 

≤2 mm, and glomerophyric clots of plagioclase+olivine as large as 10 mm diameter

Qwfc5 Basaltic, near-vent pyroclastic and proximal, medial, and distal pahoehoe lava flows 

of the Vent 5026 lava field—Vent area is a northeast-elongated ridge having a 

shallow vent depression. Vent area surrounded by shelly pahoehoe flows and minor 

pyroclastic deposits. Medial and distal flows are smooth pahoehoe. Rock is dark gray, 

has a fine-grained matrix, and contains plagioclase crystals ≤3 mm, olivine crystals 

≤1 mm, and rare glomerophyric clots of plagioclase+olivine crystals ≤6 mm. Part of

Flat Top vent complex
20 Geologic Map of the North Half of the Lake Walcott 30'×60' Quadrangle, Idaho

Qwfc6 Basaltic, near-vent pyroclastic and proximal, medial, and distal pahoehoe lava flows 

of the Vent 4822 lava field—Vent area is a low mound about 400 m across. Shelly 

pahoehoe is prominent at vent, pyroclastic deposits are minor. Medial and distal flows 

are smooth pahoehoe. Rock is dark gray, has a fine-grained matrix, contains plagioclase 

crystals ≤3 mm, all olivine crystals are ≤1 mm, and contains rare glomerophyric clots of 

plagioclase+olivine ≤3 mm. Part of Flat Top vent complex

Qsbc53 Distal basaltic pahoehoe flows of the Vent 5083 lava field—Distal pahoehoe basalt flows 

from Vent 5083, approximately 1 km north of northern boundary of this quadrangle, in 

Craters of the Moon quadrangle. Flows mostly covered by alluvial deposits, loess, and 

eolian sand. Part of Flat Top vent complex

Qsbc59 Basaltic, near-vent pyroclastic and proximal, medial, and distal pahoehoe lava flows of 

the Streifling lava field—Surface-, channel-, and tube (?)-fed, dark-gray and medium 

gray, pahoehoe flows, shelly pahoehoe, and red, oxidized, and black, near-vent pyroclastic deposits. Vent area is a nearly circular crater approximately 180 m in diameter. Crater walls are shallow; vent is ≤10 m deep. Near-vent pyroclastic deposits are 

exposed locally around vent area. Flows have relatively smooth surfaces; local relief is 

≤2.5 m. Flows mantled by ≤3 m of loess and eolian sand. Part of Flat Top vent complex

The following five map units may be part of a vent complex, but this has not been verified by paleomagnetic studies. Supposition of vent complex status is supported by close proximity of vents and similar 

thickness of loess cover, suggesting close or similar ages for the five map units.

Qwfc7 Basaltic, near-vent pyroclastic and proximal, medial, and distal pahoehoe lava flows of 

the Vent 4753 lava field (late middle Pleistocene)—Vent area mantled by relatively 

thick loess and eolian sand ≤5 m thick. Identified mainly by topographic signature. 

Lava field almost entirely covered by farmed land. Not studied in field

Qwfc8 Basaltic, near-vent pyroclastic and proximal, medial, and distal pahoehoe lava flows of 

the Vent 4754 lava field (late middle Pleistocene)—Vent area mantled by relatively 

thick loess and eolian sand ≤5 m thick; identified mainly by topographic signature. 

Rock is gray, matrix contains crystals ≤1 mm, plagioclase crystals ≤2 mm, olivine crystals ≤1 mm, and rare glomerophyric clots of plagioclase+olivine ≤3 mm diameter. Lava 

field almost entirely covered by farmed land

Qwfc9 Basaltic, near-vent pyroclastic and proximal, medial, and distal pahoehoe lava flows of 

the Vent 4705 lava field (late middle Pleistocene)—Vent area mantled by relatively 

thick loess and eolian sand ≤5 m thick; identified mainly by topographic signature. 

Lava field almost entirely covered by farmed land. Not studied in field

Qwfc10 Basaltic, near-vent pyroclastic and proximal, medial, and distal pahoehoe lava flows of 

the Vent 4644 lava field (late middle Pleistocene)—Vent area mantled by relatively 

thick loess and eolian sand ≤5 m thick; identified mainly by topographic signature. 

Lava field almost entirely covered by farmed land. Not studied in field

Qwfc11 Basaltic, near-vent pyroclastic deposits and proximal, medial, and distal pahoehoe 

lava flows of the Vent 4539 lava field (late middle Pleistocene)—Vent area mantled 

by relatively thick loess and eolian sand <5 m thick; identified mainly by topographic 

signature. Surrounded by flows of the Wapi lava field (Qwfa2; 2,270±50 14C yr B.P., 

Kuntz, Spiker, and others, 1986) on west, south, and east sides. Lava field almost 

entirely covered by farmed land. Not studied in field

ERUPTIVE PERIOD D

Qwfd1 Basaltic near-vent pyroclastic deposits, proximal, medial and distal pahoehoe lava 

flows of the Adelaide Butte lava field (early middle Pleistocene)—The Adelaide 

Butte lava field is a large shield volcano covering an area of about 315 km2

 and has a 

volume of about 3–5 km3. About 250 km2 of the lava field lie within the boundaries 

of this map. The vent is a shallow, slightly elongated, N.10°W.-trending depression 

about 500 m long and 10 m deep. Black cinders, agglomerate, and welded spatter are 

scattered near the vent. The vent crown lies about 75 m above the surrounding plateau 

of pahoehoe flows. Two prominent lava-tube systems having numerous rootless vents 
Description of Map Units 21

extend north and east from the vent depression. Rock in distal flows is dark gray, dense; 

plagioclase crystals are ≤1 mm, and olivine crystals are ≤3 mm. The dense matrix and 

near lack of phenocrysts in distal flows suggest rapid transport of lava through lava 

tubes. Medial and distal flows covered by ≤7 m loess; region is extensively farmed. 

40Ar/39Ar age is 475±38 ka (table 1)

Qwfd2 Basaltic proximal, medial and distal pahoehoe lava flows of the Vent 4659 lava field 

(early middle Pleistocene)—Vent is roughly circular depression about 500 m diameter 

and 7 m deep. Vent lies at northern part of a 6-km-long, 3 to 5 km-wide lava ridge; only 

western half of ridge exposed in this quadrangle. Vent area and pahoehoe flows covered 

by thick loess ≤7 m deep; region heavily farmed. Rock is dark gray, has a fine, granular 

matrix, all plagioclase crystals are ≤1 mm, olivine crystals are ≤3 mm, and glomerophyric clots of olivine are ≤6 mm. Age assumed to be same as Vent 4640 (Qwfd3) lava 

field because the two fields appear to be part of the same lava ridge that formed from a 

single eruptive fissure

Qwfd3 Basaltic proximal, medial and distal pahoehoe lava flows of the Vent 4640 lava field 

(early middle Pleistocene)—Roughly circular depression about 500 m diameter and 

10 m deep marks vent for lava field. Vent area and pahoehoe flows covered by thick 

loess ≤7 m deep; region heavily farmed. Rock is dark gray, matrix is dense, contains 

abundant olivine crystals ≤2 mm, and glomerophyric clots of olivine crystals <6 mm. 

Vent lies in central part of a 6-km-long, 3 to 5 km-wide lava ridge; only western half of 

ridge exposed in this quadrangle. 40Ar/39Ar age is 690±50 ka (table 2)

Qwfd4 Basaltic near-vent pyroclastic deposits and proximal pahoehoe lava flows of The 

Crater lava field (early middle Pleistocene)—The Crater is a large lava cone that 

rises about 100 m above surrounding, younger lava flows. The vent is a circular 

depression about 500 m wide and 30 m deep. The lava cone is mantled by scoriaceous flows, welded spatter and cinders; the interior of the vent displays numerous, 

thin, shelly pahoehoe flows. Rock exposed in crater walls is light gray, diktytaxitic, 

fine grained, and has plagioclase ≤1.5 mm, olivine ≤1.5 mm, and glomerophyric clots 

of olivine+plagioclase as large as 6 mm. A thin strip of proximal and medial pahoehoe flows exposed between younger enclosing flows of the Kimama Butte lava field 

extends southeast from the base of the lava cone for about 4 km to near the southern 

boundary of this map. Medial pahoehoe flows are dark gray, dense, all plagioclase 

is ≤1 mm, and conspicuous olivine phenocrysts are ≤3 mm. Lava flows have normal 

magnetic polarity (table 2) so are younger than 780 ka but are older than overlying lava 

flows of the Kimama Butte lava field (Qwfb17; 87±11 ka ) (table 1)

Qwfd5 Basaltic near-vent pyroclastic deposits and proximal pahoehoe lava flows of the Vent 

4401 lava field (early middle Pleistocene)—Low lava cone that rises about 30 m 

above surrounding flows. Vent area is a shallow depression about 700 m across, about 

10 m deep, and open to the northwest. Vent area and flows mantled by ≤7 m loess, 

extensively farmed. Rock is dark gray, fine- to medium-grained, contains moderately 

abundant plagioclase ≤6 mm, rare olivine ≤2 mm, and glomerophyric clots of plagioclase ≤10 mm. Normal magnetic polarity (table 2). Overlapped by flows from Wapi 

Park (Qwfb15, 87±9 ka) and Adelaide Butte (Qwfd1, 475±38 ka) lava fields 

Qwfd6 Near-vent pyroclastic deposits and proximal and medial pahoehoe lava flows of the 

Vent 4429 lava field (early middle Pleistocene)—Vent area is a roughly circular 

depression about 300 m diameter, about 10 m deep, open to the west. Vent area and 

flows mantled by ≤7 m loess, and extensively farmed. Rock is dark gray, fine- to 

medium-grained, contains abundant plagioclase ≤6 mm, olivine crystals ≤2 mm, and 

glomerophyric clots of plagioclase+olivine. Lava flows have normal magnetic polarity 

(table 2) thus younger than 780 ka and are overlapped by flows of Adelaide Butte lava 

field (Qwfd1, 475±38 ka, table 1)

Qwfd7 Basaltic near-vent pyroclastic deposits and proximal pahoehoe lava flows of the Vent 

4371 lava field (early middle Pleistocene)—Vent area is low lava cone of pyroclastic 

materials about 20 m high. Pahoehoe flows covered by loess ≤3 m thick. Lava cone 

overlapped on east by flows of Horse Butte lava field (Qsbb25, 73±8 ka, table 1) and 

on south by flows of Adelaide Butte lava field (Qwfd1, 475±38 ka, table 1). Rock 
22 Geologic Map of the North Half of the Lake Walcott 30'×60' Quadrangle, Idaho

is dark gray, fine grained, and contains plagioclase crystals ≤3 mm, olivine crystals 

≤2 mm, and rare glomerophyric clots of olivine+plagioclase. Lava flows have normal 

magnetic polarity (table 2) thus are younger than 780 ka and are overlapped by flows of 

Adelaide Butte lava field (475±38 ka)

Qwfd8 Basaltic, near-vent pyroclastic deposits and proximal pahoehoe lava flows of the Vent 

4267 lava field (early middle Pleistocene)—Vent area is a low lava cone that rises 

about 20 m above surrounding lava fields. Reddish, oxidized cinders at vent area. 

Proximal flows mantled by loess ≤5 m thick. Rock has reddish-gray, fine-grained 

matrix, olivine crystals ≤2 mm, plagioclase crystals ≤7 mm, and glomerophyric clots of 

olivine+plagioclase ≤10 mm. Covered by flows from Adelaide Butte lava field (Qwfd1, 

475±38 ka, table 1) on the south. No paleomagnetic data

Qwfd9 Basaltic, near-vent pyroclastic and proximal and medial pahoehoe lava flows of the 

Knoll Vent lava field (early middle Pleistocene)—Vent area is a steep-sided shield 

volcano having a local relief of about 100 m. Vent depression is oriented north-northwest, is about 200 m across, and about 10 m deep. Vent area thickly mantled by loess 

and eolian sand ≤10 m thick, extensively farmed. Lava field surrounded by flows of 

Kimama Butte lava field (Qwfb17, 87±11 ka, table 1) on eastern and northern margins. 

Rock is dark gray, has a fine-grained matrix in which all crystals are ≤1 mm, rare plagioclase crystals ≤2 mm, rare olivine crystals ≤1 mm, and conspicuous glomerophyric 

clots of olivine+plagioclase ≤8 mm diameter. Normal magnetic polarity (table 2). Thick 

loess cover may indicate older age assignment than that given

ERUPTIVE PERIOD E

Qwfe1 Near-vent pyroclastic deposits and proximal, medial, and distal pahoehoe lava flows 

of the Sid Butte lava field (early Pleistocene)—Sid Butte is a moderate-sized shield 

volcano that covers an area of about 75 km2 in this quadrangle and about 50 km2 in the 

Twin Falls 1:100,000-scale quadrangle to the west. The vent rises about 180 m above 

younger lava flows from Kimama Butte 3 km to the southeast. Vent is a 400-m long, 

shallow depression ≤5 m deep. Vent is elongated N.20°W. Vent area mantled by significant amounts of reddish, oxidized cinders, welded spatter, and shelly pahoehoe flows. 

Medial and distal flows are deeply mantled by loess ≤5m thick and extensively farmed. 

Rock near vent is light gray, fine-grained, and contains plagioclase and olivine crystals 

≤2 mm, and glomerophyric clots of plagioclase+olivine ≤3 mm. Distal flows are dark 

gray and have the same texture and crystal sizes. 40Ar/39Ar age is 778±8 ka (table 1). 

Reversed magnetic polarity

Qwfe2 Near-vent pyroclastic deposits and proximal pahoehoe lava flows of the Vent 4559 

lava field (early Pleistocene)—Lava field is a broad lava shield that rises about 100 m 

above surrounding lava fields. Vent is a roughly circular depression about 1,500 m 

diameter, less than 5 m deep, and open to the northwest. Vent area is filled and nearvent flows are obscured by ≤10 m loess, extensively farmed. Medial pahoehoe flows 

are fine- to medium-grained and contain moderate amounts of plagioclase crystals 

≤9 mm. Overlapped by flows of Wapi Park (Qwfb15, 87±9 ka) and Adelaide Butte 

(Qwfd1,475±38 ka) lava fields. Reversed magnetic polarity

Qwfe3 Near-vent pyroclastic deposits and proximal pahoehoe lava flows of the Vent 4373 

lava field (early Pleistocene)—Lava cone about 20 m high; vent depression, about 

3 m deep and 500 m long, is oriented north-south. The vent area and proximal and 

medial flows mantled by loess <8 m deep; few flows are exposed. Extensively 

farmed. Rock is dark gray, fine-grained, contains rare olivine crystals ≤1 mm, rare 

plagioclase crystals ≤2 mm, and glomerophyric clots of plagioclase crystals ≤3 mm. 

Remanent magnetic direction for unit is similar to that of Sid Butte (Qwfe1), which 

suggests they are the same age, 778±8 ka, table 1

Qwfe4 Proximal pahoehoe lava flows of the Vent 4373 lava field (early Pleistocene)—Area 

of deeply (≤8 m), loess-mantled flows; vent area not evident. Overlapped by flows of 

Kimama Butte lava field (Qwfb17) on north, east, and south sides
Description of Map Units 23

None of the following map units have radiometric ages that would determine their absolute age. Some 

of the units have stratigraphic relations with dated lava fields; thus, their relative ages can be inferred. 

Where applicable, depth of loess and eolian sand has been used to determine relative age. Arrows in 

Correlation of Map Units show inferred age range of units

Qwfb18 Basaltic near-vent deposits of the Rattlesnake Butte lava field (late Pleistocene)—Vent 

area is a 300-m-long, 100-m-wide, north-south trending depression about 30 m deep. 

Only north half of vent exposed in this quadrangle. Vent area and adjacent flows 

mantled by eolian sand that is locally ≤10 m thick. Normal magnetic polarity

Qwfb19 Basaltic near-vent deposits and proximal pahoehoe lava flows of the Vent 4645 lava 

field (late Pleistocene)—Vent area is a slot-shaped depression about 200 m long and 

50 m wide. Circular depression about 30 m diameter indents southern part of slotshaped depression. Only minor pyroclastic deposits at vent area; thin pahoehoe flows 

predominate. A second vent is located about 150 m south of the slot shaped depression. 

A 600-m-long, tear-drop shaped lava pond extends south from the second vent. A poorly 

defined lava tube having skylights and perched lava ponds is located about 2 km east of 

the lava pond. Vents of Vent 4645 and Rattlesnake Butte lava field (Qwfb18) are aligned, 

suggesting they may be part of the same eruptive sequence. Flows display a rough, jagged 

surface composed of plates of jumbled pahoehoe. Local relief is ≤5 m. Flows mantled by 

discontinuous patches of loess and eolian sand ≤1 m thick. Near-vent pahoehoe flows are 

coarse grained, containing abundant plagioclase and olivine crystals ≤5 mm. Discontinuous nature of loess and eolian sand deposits suggests a late Pleistocene age, but may be 

much older. Age assumption also applies to Rattlesnake Butte lava field (Qwfb18)

Qwfb20 Near-vent pyroclastic deposits and pahoehoe lava flows of the Vent 4757 lava field (late 

Pleistocene)—Vent is a kipuka pyroclastic cone about 300 m diameter in Wapi lava 

field (Qwfa2; 2,270±50 14C yr B.P., Kuntz, Spiker, and others, 1986). Air photo interpretation suggests significant mantle by loess and eolian sand. Not studied in field

Qwfc12 Basaltic, near-vent pyroclastic deposits of the Vent 4811 lava field (late middle 

Pleistocene)—Vent area is a N.15o

W.-trending edifice having vent-wall deposits exposed 

on north, northwest, southwest, and southeast flanks. Eastern part of vent area covered by 

flows from Kimama Butte lava field (Qwfb17; 87±11 ka, table 1). Vent walls are as high 

as 30 m above the central vent depression. Only near-vent pyroclastic deposits exposed; 

all flows covered by Kimama Butte flows in this quadrangle. Near-vent deposits consist 

of well-bedded cinders, ash, and thin, shelly pahoehoe flows. Glomerophyric clots of plagioclase and olivine are as large as 10 mm diameter. Covered by ≤7 m loess; extensively 

farmed. Deep loess cover suggests early Pleistocene age. No paleomagnetic data

Qwfc13 Basaltic near-vent pyroclastic deposits of the Vent 4354 lava field (late middle and 

early middle Pleistocene)—Large slot-shaped vent approximately 1.8 km long, 600 m 

wide, and 40 m deep. Only northern third of vent exposed in this quadrangle. Reddish 

pyroclastic deposits and mostly shelly pahoehoe flows exposed in crater walls. Rock 

is medium gray, has a dense matrix, contains plagioclase crystals ≤4 mm, and olivine 

≤1 mm. No paleomagnetic data

Qwfc14 Basaltic near-vent pyroclastic deposits of the Vent 4384 lava field (late middle and 

early middle Pleistocene)—Small cinder cone about 700 m diameter and 25 m high. 

Rock at vent area consists of reddish oxidized cinders that mostly lack crystals. Cone 

may be part of vent system for Vent 4354 (Qwfc13) lava field. No paleomagnetic data

Qsbc60 Basaltic, near-vent pyroclastic and proximal, medial, and distal pahoehoe flows of the 

Vent 5270 lava field (late middle Pleistocene)—Vent 5270 lava field is a small shield 

volcano having a low vent depression about 300 m in diameter and 15-m deep at the 

summit. Proximal flows are mostly shelly pahoehoe, medial and distal flows and are 

mostly smooth pahoehoe having ≤10 m of relief. Two prominent lava channels are on 

the southwest flanks of the volcano. Most flows moved west, south and east of the vent, 

and only marginally to the north. Rock has medium-grained texture containing moderate amounts of plagioclase ≤3 mm and olivine+plagioclase clots of ≤8 mm.
24 Geologic Map of the North Half of the Lake Walcott 30'×60' Quadrangle, Idaho

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For more information concerning this publication, contact:

Center Director, USGS Geosciences and Environmental Change 

Science Center

Box 25046, Mail Stop 980

Denver, CO 80225

(303) 236-5344
Kuntz and others—Geologic Map of the North Half of the Lake Walcott 30’×60’ Quadrangle, Idaho—Scientific Investigations Map 3405

ISSN 2329-132X (online)

https://doi.org/10.3133/sim3405