National Park Service

U.S. Department of the Interior

Natural Resource Stewardship and Science

El Malpais National Monument

Geologic Resources Inventory Report

Natural Resource Report NPS/NRSS/GRD/NRR—2012/578 
ON THE COVER

The 3,900-year-old McCartys flow is juxtaposed 

against the 160-million-year-old Zuni Sandstone, 

which is capped by 100-million-year-old Dakota 

Sandstone at the Sandstone Bluffs Overlook. The 

bluffs consist of gently dipping sandstone that 

crops out along the eastern flank of the Cebollita 

Mesa on the eastern margin of El Malpais National 

Monument. Photograph by Katie KellerLynn

(Colorado State University).

THIS PAGE

The McCartys flow is the youngest lava flow in 

New Mexico and still exhibits a glassy rind in 

many locations. Photograph by Katie KellerLynn 

(Colorado State University). 
El Malpais National Monument

Geologic Resources Inventory Report

Natural Resource Report NPS/NRSS/GRD/NRR—2012/578 

National Park Service

Geologic Resources Division

PO Box 25287 

Denver, CO 80225 

September 2012

U.S. Department of the Interior

National Park Service 

Natural Resource Stewardship and Science

Fort Collins, Colorado
ii NPS Geologic Resources Division

The National Park Service, Natural Resource Stewardship and Science office in Fort 

Collins, Colorado publishes a range of reports that address natural resource topics of 

interest and applicability to a broad audience in the National Park Service and others in 

natural resource management, including scientists, conservation and environmental 

constituencies, and the public. 

The Natural Resource Report Series is used to disseminate high-priority, current natural 

resource management information with managerial application. The series targets a 

general, diverse audience, and may contain NPS policy considerations or address 

sensitive issues of management applicability.

All manuscripts in the series receive the appropriate level of peer review to ensure that 

the information is scientifically credible, technically accurate, appropriately written for 

the intended audience, and designed and published in a professional manner. This 

report received informal peer review by subject-matter experts who were not directly 

involved in the collection, analysis, or reporting of the data.

Views, statements, findings, conclusions, recommendations, and data in this report do 

not necessarily reflect views and policies of the National Park Service, U.S. Department 

of the Interior. Mention of trade names or commercial products does not constitute 

endorsement or recommendation for use by the U.S. Government.

Printed copies of this report are produced in a limited quantity and they are only 

available as long as the supply lasts. This report is available from the Geologic Resources 

Inventory website (http://www.nature.nps.gov/geology/inventory/

gre_publications.cfm) and the Natural Resource Publications Management website 

(http://www.nature.nps.gov/publications/nrpm/). 

Please cite this publication as: 

KellerLynn, K. 2012. El Malpais National Monument: geologic resources inventory 

report. Natural Resource Report NPS/NRSS/GRD/NRR—2012/578. National Park 

Service, Fort Collins, Colorado. 

NPS 103/117008, September 2012
ELMA Geologic Resources Inventory Report iii

Contents

List of Figures................................................................................................................ iv

List of Tables ................................................................................................................. iv

Executive Summary ....................................................................................................... v

Acknowledgements..................................................................................................... vii

Credits ....................................................................................................................................................................... vii

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

Geologic Resources Inventory Program......................................................................................................................... 1

Regional Geologic Setting............................................................................................................................................ 1

Geologic Issues .............................................................................................................. 9

Cave Management ...................................................................................................................................................... 9

Mass Wasting............................................................................................................................................................11

Erosion ......................................................................................................................................................................12

Flooding ....................................................................................................................................................................14

Disturbed Lands.........................................................................................................................................................14

Recent and Potential Volcanism ................................................................................................................................. 16

Seismic Activity ..........................................................................................................................................................17

Geochronology, Future Research, and Interpretation..................................................................................................17

Geologic Features and Processes................................................................................ 19

Volcanic Features and Processes................................................................................................................................. 19

Vents .........................................................................................................................................................................20

Lava Flows................................................................................................................................................................. 22

Lava Tubes................................................................................................................................................................. 26

Ice Caves ...................................................................................................................................................................29

Eolian Features and Processes ....................................................................................................................................30

Biologic and Geologic Connections ............................................................................................................................30

Paleontological Resources..........................................................................................................................................32

Sandstone Features....................................................................................................................................................34

Geologic History .......................................................................................................... 37

Precambrian Crystalline Rocks....................................................................................................................................37

Sedimentary Rocks.....................................................................................................................................................37

Volcanic Rocks...........................................................................................................................................................41

Unconsolidated Deposits............................................................................................................................................42

Geologic Map Data...................................................................................................... 43

Geologic Maps ..........................................................................................................................................................43

Source Maps..............................................................................................................................................................43

Geologic GIS Data .....................................................................................................................................................43

Geologic Map Overview Graphic................................................................................................................................44

Map Unit Properties Table..........................................................................................................................................44

Use Constraints..........................................................................................................................................................44

Glossary........................................................................................................................ 45

Literature Cited............................................................................................................ 51

Additional References ................................................................................................. 59

Geology of National Park Service Areas......................................................................................................................59

NPS Resource Management Guidance and Documents...............................................................................................59

Geological Surveys and Societies................................................................................................................................59

U.S. Geological Survey Reference Tools......................................................................................................................59

Appendix: Scoping Meeting Participants................................................................... 61

2006 Scoping Meeting Participants............................................................................................................................61

2011 Conference Call Participants..............................................................................................................................61
iv NPS Geologic Resources Division

List of Figures

Figure 1 Map of El Malpais National Monument........................................................................................................... 2

Figure 2. Colorado Plateau........................................................................................................................................... 3

Figure 3. Geologic time scale. ...................................................................................................................................... 4

Figure 4. General stratigraphic column for El Malpais National Monument................................................................. 5-6 

Figure 5. Major volcanic fields in New Mexico.. ............................................................................................................ 7

Figure 6. Entrance to Braided Cave. ............................................................................................................................10

Figure 7. Bat Cave......................................................................................................................................................10

Figure 8. Skylight and garden.....................................................................................................................................11

Figure 9. Mass wasting. .............................................................................................................................................12

Figure 10. Variables of arroyo formation. ...................................................................................................................13

Figure 11. Bandera Crater. .........................................................................................................................................14

Figure 12. McCartys cinder cone. ...............................................................................................................................15

Figure 13. Junction Cave............................................................................................................................................16

Figure 14. Horace Mesa. ............................................................................................................................................20

Figure 15. Paxton Springs Crater. ...............................................................................................................................20

Figure 16. Aerial image of El Malpais. ........................................................................................................................21

Figure 17. Hornito. ....................................................................................................................................................22

Figure 18. Pahoehoe lava...........................................................................................................................................22

Figure 19. Aa lava......................................................................................................................................................22

Figure 20. Vesicular basalt..........................................................................................................................................22

Figure 21. Kipuka. .....................................................................................................................................................23

Figure 22. Crust of a basalt flow. ...............................................................................................................................23

Figure 23. Inflation pits and mounds. .........................................................................................................................24

Figure 24. Squeeze-out..............................................................................................................................................24

Figure 25. Bandera flows. ..........................................................................................................................................24

Figure 26. El Calderon................................................................................................................................................26

Figure 27. Lava bridge................................................................................................................................................26

Figure 28. Breakdown................................................................................................................................................27

Figure 29. Skylight on the McCartys flow. ..................................................................................................................27

Figure 30. Ice crystals.................................................................................................................................................29

Figure 31. Perennial ice..............................................................................................................................................30

Figure 32. Tinaja........................................................................................................................................................32

Figure 33. Tree molds. ...............................................................................................................................................33

Figure 34. Sandstone and lava. ..................................................................................................................................34

Figure 35. Sandstone Bluffs Overlook.........................................................................................................................35

Figure 36. La Vieja .....................................................................................................................................................35

Figure 37. Sandstone spires........................................................................................................................................36

Figure 38. La Ventana Natural Arch............................................................................................................................36

Figure 39. El Malpais rock types. ................................................................................................................................38

Figure 40. Paleogeographic maps for El Malpais National Monument. ........................................................................39

Figure 41. Mount Taylor and the McCartys lava flow. ................................................................................................. 41

List of Tables 

Table 1. Volcanism within and near El Malpais National Monument............................................................................19

Table 2. Cave minerals...............................................................................................................................................35

Table 3. Geology data layers in the El Malpais National Monument GIS data...............................................................44
ELMA Geologic Resources Inventory Report v 

Executive Summary

This report accompanies the digital geologic map data for El Malpais National Monument 

in New Mexico, produced by the Geologic Resources Division in collaboration with its 

partners. It contains information relevant to resource management and scientific research. 

This report was prepared using available published and unpublished geologic information. 

The Geologic Resources Division did not conduct any new fieldwork in association with this 

report.

El Malpais National Monument is part of the Zuni–

Bandera volcanic field in west-central New Mexico. The 

relatively recent volcanism, combined with the arid New 

Mexico climate, make El Malpais an ideal place in the 

continental United States to see Hawaiian-style volcanic 

deposits, extensive lava-tube systems, and associated 

lava-tube caves. 

The Zuni–Bandera volcanic field began erupting about 

700,000 years ago, but the El Malpais episode occurred 

less than 60,000 years ago. The youngest lava flow at the 

national monument is 3,900 years old and represents the 

most recent volcanic eruption in New Mexico; this 

eruption produced the McCartys flow. El Malpais 

National Monument preserves a portion of this young 

flow, as well as its vent, the McCartys cinder cone, which 

is the youngest volcano in New Mexico.

Early Spanish explorers who encountered the extremely 

rough lava-flow surfaces named the area “El Malpais,” 

meaning “The Badlands.” The geologic importance of 

the area has been recognized since at least the 1930s, 

when El Malpais was first proposed as a national 

monument. El Malpais National Monument and 

adjacent El Malpais National Conservation Area were 

formally established in 1987 and are managed by the 

National Park Service and the Bureau of Land 

Management, respectively.

Maxwell (1986) is the primary source for the digital 

geologic map covering the area of El Malpais National 

Monument. The digital data set also includes Maxwell 

(1977), which covers the McCartys quadrangle northeast 

of the national monument and includes the portion of 

the McCartys lava flow outside the national monument 

boundaries. 

The rock units mapped by Maxwell (1986) can be 

divided into three main types: (1) very old metamorphic 

and igneous rocks of Precambrian age (approximately 

1.4 billion years old); (2) sedimentary rocks of Permian, 

Triassic, Jurassic, and Cretaceous ages (299 million–65.5 

million years old); and (3) young volcanic rocks, most of 

which erupted in the past 700,000 years. The deposition 

of unconsolidated materials, such as alluvium, colluvium, 

landslide deposits, and windblown silt, is also part of 

recent geologic history at the national monument. A 

geologic time scale showing the eras, periods, and epochs 

of geologic time is included in this report.

The Precambrian rocks at El Malpais National 

Monument are fragments of oceanic and continental 

crust that accreted to the western edge of the North 

American continent. Over time, these rocks became the 

stable North American craton. These ancient rocks can 

be seen in the El Calderon area north of Highway 53. 

The “middle-aged” sedimentary rocks at the national 

monument accumulated over millions of years as shallow 

seas advanced and withdrew from the area from the 

Permian through the Upper Cretaceous periods. 

Representatives of these rocks are exposed at the 

Sandstone Bluffs Overlook. 

The young volcanic rocks, which dominate the landscape

at the national monument, erupted over the last 60,000 

years. Older pulses of volcanism occurred 150,000 and 

700,000 years ago and cover the Chain of Craters and 

Hole-in-the-Wall areas, respectively. 

This Geologic Resources Inventory (GRI) report was 

written for resource managers to assist in resource 

management and science-based decision making, but it 

may also be useful for interpretation. The report 

discusses geologic issues facing resource managers at El 

Malpais National Monument, distinctive geologic 

features and processes within the national monument, 

and the geologic history leading to the present-day 

landscape. The Geologic Map Overview Graphic

illustrates the geologic data; the Map Unit Properties 

Table summarizes the main features, characteristics, and 

potential management issues for all rocks and 

unconsolidated deposits on the digital geologic map for 

El Malpais National Monument. This report also 

provides a glossary that contains explanations of 

technical, geologic terms, including terms on the Map 

Unit Properties Table. The geologic time scale shows the 

chronologic arrangement of major geologic events, with 

the oldest events and time units at the bottom and the 

youngest at the top. 

Geologic issues of particular significance for resource 

management at El Malpais National Monument were 

identified during a 2006 GRI scoping meeting and a 2011 

conference call. They include the following:

• Cave Management. The top resource-management 

priority at El Malpais National Monument is to 

complete a cave management plan. The most 

immediate threat to cave resources at the national 
vi NPS Geologic Resources Division

monument is the potential spread of white-nose 

syndrome, which is responsible for millions of bat 

deaths across the eastern and central United States. 

Other threats include pollution, garbage dumping, 

physical impacts such as trampling, and impacts to the

fragile moss gardens at cave entrances and under 

skylights. The loss of cave ice as a result of multiannual 

drought is also a concern.

• Mass Wasting. Within El Malpais National 

Monument, the primary mass-wasting (gravity-driven) 

process is rockfall off the sandstone bluffs. Maxwell 

(1986) mapped extensive landslides east of the national 

monument. 

• Erosion. Although erosion is a natural feature on the 

landscape, a legacy of grazing and other agricultural 

practices has exacerbated this process in parts of the 

national monument, particularly in the Sandstone 

Bluffs Overlook area.

• Flooding. Streams in El Malpais National Monument

are ephemeral and flow only during flash-flooding 

events. Storm-water runoff is a concern for 

management because stream channels originating in 

agricultural and grazed areas outside the national 

monument transport pesticides, herbicides, fertilizers, 

and increased amounts of sediment into the lava-tube 

system within the national monument, thereby 

introducing contaminants and impacting cave biota. 

On the east side of the national monument, flooding 

causes erosion between the sandstone bluffs and the 

McCartys flow, affecting an area in which

archeological sites are highly concentrated. Flooding 

on the east side also periodically washes out Highway 

117. 

• Disturbed Lands. Disturbances on the El Malpais 

landscape have resulted from channel diversions, 

grazing and agricultural practices, logging, mining, and 

military activities. 

• Recent and Potential Volcanism. A 1% chance exists 

that some type of volcanic eruption will occur in New 

Mexico in the next 100 years, and a 10% chance exists 

that an eruption will occur in the next 1,000 years. In 

probabilistic terms, 100 eruptions will occur in the 

next 1 million years. Future eruptions are most likely 

to occur along the Jemez lineament and the Rio 

Grande rift, particularly where the Rio Grande rift 

intersects crosscutting lineaments, such as in the Jemez 

Mountains and near Socorro, New Mexico. The 

eruption will likely be basaltic, which has been the 

dominant mode of volcanism in New Mexico over the 

past 5 million years.

• Seismic Activity. New Mexico has a recorded history 

of seismic activity since 1869. This long record allows 

researchers to evaluate seismic hazards and make 

predictions concerning the likely locations and 

magnitudes of future earthquakes. A disproportionate 

number of earthquakes in New Mexico are centered in 

the Rio Grande Valley near Socorro, which is about 80 

km (50 mi) southeast of El Malpais National 

Monument. The intact nature of lava flows at the

national monument indicates that seismic activity has 

not occurred recently. The most recent “felt quake” 

was in the 1970s.

• Geochronology, Future Research, and Interpretation. 

Although a few gaps exist, the chronology of volcanic 

events at El Malpais National Monument has been 

well defined through relative and radiometric dating 

methods. Researchers continue to use the national 

monument as a field area, in particular for testing 

various dating techniques on volcanic rocks and 

applying cross-dating methods for comparative 

analysis. Dating of some of the older flows would give 

a more complete picture of the sequence of volcanic 

events. The quality of preservation of the volcanic 

features and accessible location make the national 

monument an ideal location for field trips and public 

education.

Geologic features of particular significance for resource 

management at El Malpais National Monument include 

the following:

• Volcanic Features and Processes. Geologists have 

grouped the volcanic features at El Malpais National 

Monument into two main categories: flows and vents. 

Pahoehoe (smooth and “ropey”) and aa (rough and 

“blocky”) lava flows are present at El Malpais. These 

flows extruded from numerous vents, which are 

primarily cinder cones, but also shield volcanoes, at El 

Malpais. As part of the main El Malpais event, 

Maxwell (1986) mapped the McCartys flow and cinder 

cone (map unit symbols Qbm and Qcm); Bandera 

flows, crater, and cinder field (Qbb, Qcb, and Qvb); 

Hoya de Cibola flows and shield volcano (Qbw and 

Qvw); Twin Craters flows, cones, and mixed 

pyroclastics (Qbt, Qct, and Qvt); and El Calderon 

flows, crater, and cinder field (Qbc, Qcc, and Qvc).

Maxwell (1977) used map symbols Qbd for the 

McCartys flow and Qba for the oldest Zuni basalt 

flow, which are mapped in the McCartys quadrangle

northeast of the national monument.

• Lava Tubes. Lava tubes are conduits for molten lava 

flowing from a vent, which later become cavernous 

passages that remain after the lava has ceased flowing. 

The Bandera flows, with a flow length of 37 km (23 mi) 

contains remnants of what had been a very extensive 

lave tube system at El Malpais. This system today 

includes 28 km (17 mi) of small segments of intact lava 

tubes and longer segments of trenches left over from 

the roof collapse of the original lava tube. If all intact, 

this tube system would the longest known lava tube 

system in North America.

• Ice Caves. Seasonal and perennial accumulations of ice

are present in many of the lava tubes at El Malpais 

National Monument. Some of the ice is more than 

3,100 years old.

• Eolian Features and Processes. Eolian deposits in the 

El Malpais area are recent accumulations of 

windblown silt and sand that form sand sheets, dune 

fields, longitudinal dunes, and smaller deposits. The 

friable Zuni Sandstone and walls of kipukas, 

composed of sandstone and limestone, are sources of 
ELMA Geologic Resources Inventory Report vii

eolian sediment. Eolian processes influence the lava 

flows by infilling vesicles and cracks with windblown 

dust. Infilling is a significant part of landscape 

evolution and is the first stage in soil development on 

the lava flows. 

• Biologic and Geologic Connections. Many 

connections can be made between the biologic and 

geologic resources at El Malpais National Monument. 

In particular, lava flows, kipukas, cinder cones, lava 

tubes, tinajas, playas, and eolian deposits create special 

habitats. Moreover, soil crusts are an important and 

widespread component of the ecosystem and improve 

soil quality.

• Paleontological Resources. Tree molds, such as those 

on the surface of the Bandera flows (Qbb), are the 

most noteworthy paleontological resource at the 

national monument. Fossils, primarily marine 

organisms, are known from Permian and Cretaceous 

rocks within El Malpais National Monument. 

Additionally, Quaternary plant debris, packrat 

middens, and bones occur in lava-tube caves. 

• Sandstone Features. Sandstone figures prominently in 

the landscape of El Malpais National Monument. 

Rock units of the Cretaceous Dakota and Jurassic Zuni 

sandstones are exposed at the national monument. 

The ridgetop at Sandstone Bluffs Overlook is 

composed of Dakota Sandstone. Zuni Sandstone lines 

the Highway 117 corridor on the eastern side of the 

national monument. Maxwell (1977, 1986) divided the 

Dakota Sandstone into many subunits called 

“tongues,” which are listed in the Map Unit Properties 

Table. Mesas, ridges, arches, and hoodoos are 

characteristic landforms of these sandstone units.

Acknowledgements

The Geologic Resources Inventory (GRI) is one of 12 inventories funded by the National 

Park Service Inventory and Monitoring Program. The GRI is administered by the Geologic 

Resources Division of the Natural Resource Stewardship and Science Directorate. 

The Geologic Resources Division relies heavily on 

partnerships with institutions such as the U.S. Geological 

Survey, Colorado State University, state geologic surveys, 

local museums, and universities in developing GRI 

products.

The GRI team would like to thank the participants at the 

scoping meeting in 2006, who are listed in the Appendix. 

In addition, Ken Mabery, former superintendent at El 

Malpais National Monument, provided input during a 

pre-scoping conference call in 2006. He is currently 

superintendent at Scotts Bluff National Monument. 

Dana Sullivan (chief ranger) and Dave Hays (resource 

manager) at El Malpais National Monument participated 

in a conference call in December 2011 and provided

much post-scoping guidance on issues related to the 

national monument’s geologic resources. Dale 

Dombrowski (writer/editor) at El Malpais National 

Monument assisted with collecting photographs used in 

this report. Larry Crumpler (research curator) at the 

New Mexico Museum of Natural History and Science 

provided photographs and granted permission for their

use in this report. Gina D’Ambrosio (production editor) 

at the New Mexico Bureau of Geology and Mineral 

Resources provided graphics originally drafted for use in 

Bureau publications. Dale Pate (Cave and Karst Program 

Coordinator) at the NPS Geologic Resources Division 

reviewed the lava tube sections of the report.

Credits

Author

Katie KellerLynn (Colorado State University)

Review

Nelia Dunbar (New Mexico Bureau of Geology and 

Mineral Resources)

Jason Kenworthy (NPS Geologic Resources Division) 

Editing

Jennifer Piehl Martinez (Write Science Right)

Report Layout, Formatting, and Production

Philip Reiker (NPS Geologic Resources Division)

Digital Geologic Data Production

Anne Poole (NPS Intermountain GIS Center)

Stephanie O’Meara (Colorado State University)

Derek Witt (Colorado State University intern) 

Geologic Map Overview Graphic

Layout and Design

Ian Hageman(Colorado State University intern)

Georgia Hybels (NPS Geologic Resources Division)

Review

Georgia Hybels (NPS Geologic Resources Division)

Rebecca Port (Colorado State University)

Jason Kenworthy (NPS Geologic Resources Division)

ELMA Geologic Resources Inventory Report 1 

Introduction

This section briefly describes the National Park Service Geologic Resources Inventory 

Program and the regional geologic setting of El Malpais National Monument. 

Geologic Resources Inventory Program

The Geologic Resources Inventory (GRI) is one of 12 

baseline natural resource inventories funded by the 

National Park Service (NPS) Inventory and Monitoring 

Program. The Geologic Resources Division of the NPS 

Natural Resource Stewardship and Science Directorate 

administers the GRI.

The compilation and use of natural resource information 

by park managers is called for in the 1998 National Parks 

Omnibus Management Act (section 204), the 2006 NPS 

Management Policies, and in the Natural Resources 

Inventory and Monitoring Guideline (NPS-75). Refer to 

the “Additional References” section for links to these 

and other resource management documents.

The objectives of the GRI are to provide geologic map 

data and pertinent geologic information to support 

resource management and science-based decision 

making in more than 270 natural resource parks 

throughout the National Park System. To realize these 

objectives, the GRI team undertakes three tasks for each 

natural resource park: (1) conduct a scoping meeting and 

provide a scoping summary, (2) provide digital geologic 

map data in a geographic information system (GIS) 

format, and (3) provide a GRI report. These products are 

designed and written for nongeoscientists. Scoping 

meetings bring together park staff and geologic experts 

to review available geologic maps, develop a geologic 

mapping plan, and discuss geologic issues, features, and 

processes that should be included in the GRI report. 

Following the scoping meeting, the GRI map team 

converts the geologic maps identified in the mapping 

plan into digital geologic map data in accordance with 

their data model. Refer to the “Geologic Map Data” 

section for additional map information. After the map is 

completed, the GRI report team uses these data, as well 

as the scoping summary and additional research, to 

prepare the geologic report. This geologic report assists 

park managers in the use of the map, and provides an 

overview of the park geology, including geologic 

resource management issues, geologic features and 

process, and the geologic history leading to the park’s 

present-day landscape. 

For additional information regarding the GRI, including 

contact information, please refer to the Geologic 

Resources Inventory website 

(http://www.nature.nps.gov/geology/inventory/). The 

current status and projected completion dates for GRI 

products are available on the GRI status website 

(http://www.nature.nps.gov/geology/GRI_DB/Scoping/

Quick_Status.aspx).

Regional Geologic Setting

El Malpais National Monument (fig. 1) is part of the 

Zuni–Bandera volcanic field, which features more than 

100 vents and associated cinder cones, shield volcanoes, 

and lava flows (Luedke and Smith 1979). The Zuni–

Bandera volcanic field is on the southeastern edge of the 

Colorado Plateau in the “Four Corners” area of New 

Mexico, Arizona, Colorado, and Utah (fig. 2). Generally 

speaking, the Colorado Plateau is characterized by 

uplifted horizontal or slightly tilted sedimentary strata 

and a lack of volcanism. However, the Zuni–Bandera 

volcanic field and several other areas of volcanism, 

including Sunset Crater Volcano National Monument in 

Arizona (Thornberry-Ehrlich 2005), are located along 

the edge of the plateau. The vents of the Zuni–Bandera 

volcanic field erupted in a transition zone between the 

Colorado Plateau and the Rio Grande rift, which is 

characterized by crustal extension and thinning (Dunbar 

2010).

The Zuni–Bandera volcanic field began erupting about 

700,000 years ago, with the El Malpais event beginning 

less than 60,000 years ago (fig. 3). The primary El Malpais 

flows mapped by Maxwell (1986) are the McCartys (map 

unit symbol Qbm), Bandera (Qbb), Hoya de Cibola 

(Qbw), Twin Craters (Qbt), and El Calderon (Qbc) flows. 

Maxwell (1977) mapped the McCartys flow as unit Qbd. 

The Paxton Springs (Qbp), Zuni (Qbz and Qba), and Oso 

Ridge (Qbo) flows originated north of El Malpais 

National Monument in the Zuni Mountains, and erupted 

during the same period as the El Malpais flows (Maxwell 

1977, 1986). Of these three, only a portion of the Oso 

Ridge flows occurs within the national monument. 

Maxwell (1986) also mapped nearby cinder cones (Qc), 

volcanic debris (Qv), and basalt flows (Qbu) typical of 

the Chain of Craters area west of the national monument 

(fig. 4). Old basalt flows (Qb) that represent initial

volcanism of the Zuni–Bandera volcanic field also occur 

within the national monument. Old basalt flows underlie 

Hole-in-the-Wall and the area west of the national 

monument (figs. 4 and 16). Volcanic features at the 

national monument also include lava-tube caves such as 

the 27-km- (17-mi-) long system in the Bandera flows. 

The Zuni–Bandera volcanic field is situated along a zone 

of crustal weakness called the Jemez lineament, which 

contains numerous volcanic fields (fig. 5). El Malpais 

National Monument lies near the center of the 

lineament, which extends from central Arizona to 

northeastern New Mexico. The major volcanic fields on 

the Jemez lineament in New Mexico are, from west to 

east, the Zuni–Bandera volcanic field, the Mount Taylor 

volcanic field, the Valles Caldera, the Ocaté volcanic 

field, and the Raton–Clayton volcanic field. The National 

Park System is well represented along the Jemez 
2 NPS Geologic Resources Division

lineament: El Malpais and El Morro national monuments 

are part of the Zuni–Bandera volcanic field (KellerLynn 

2012a), Fort Union National Monument is surrounded 

by the Ocaté volcanic field (KellerLynn 2012b), Capulin 

Volcano National Monument is part of the Raton–

Clayton volcanic field (KellerLynn 2011), and Bandelier 

National Monument is in the Valles Caldera (National 

Park Service 2005). Petroglyph National Monument lies 

on the Rio Grande rift (National Park Service 2006c), 

which was a center of volcanic activity during an earlier 

episode of volcanism (40 million–20 million years ago)

and intersects the Jemez lineament near Valles Caldera.

Figure 1 Map of El Malpais National Monument. Along with adjacent El Malpais National Conservation Area, administered by the Bureau of

Land Management, El Malpais National Monument was established in 1987. El Malpais is situated in the Zuni–Bandera volcanic field south of 

Grants, New Mexico. The Chain of Craters, Hole-in-the-Wall, and Cebollita Mesa are distinctive landscape-scale features in the vicinity of the 

national monument. National Park Service graphic.
ELMA Geologic Resources Inventory Report 3 

Figure 2. Colorado Plateau. The Colorado Plateau is roughly centered on the Four Corners area of Utah, Colorado, Arizona, and New Mexico. 

The plateau is bounded on the west by the Basin and Range physiographic province. A primary feature of the Basin and Range in New 

Mexico is the Rio Grande rift. Portions of the Great Plains and Rocky Mountain physiographic provinces also occur in New Mexico. Green 

areas on the figure represent the locations of National Park System units, some of which are labelled. Map by Philip Reiker (NPS Geologic 

Resources Division) and Rebecca Port (Colorado State University).
4 NPS Geologic Resources Division

Figure 3. Geologic time scale. The divisions of the geologic time scale are organized stratigraphically, with the oldest at the bottom and 

youngest at the top. Boundary ages are in millions of years (Ma). Major life history and tectonic events occurring on the North American 

continent are included. Compass directions in parentheses indicate the regional location of individual geologic events. Red lines indicate 

major boundaries between eras. Graphic design by Trista Thornberry-Ehrlich (Colorado State University), adapted from geologic time scales 

published by the U.S. Geological Survey (http://pubs.usgs.gov/fs/2010/3059/) and the International Commission on Stratigraphy 

(http://www.stratigraphy.org/ics%20chart/09_2010/StratChart2010.pdf).
ELMA Geologic Resources Inventory Report 5 

Era Period Epoch Age* Rock/Sediment Unit Description Cenozoic

Quaternary 

Holocene 0.01–

present

Eolian deposits

(Qe)

Windblown sand 

and silt

Holocene 

and 

Pleistocene

2.6–

present

Alluvium

(Qal) Sand and pebbles

Landslide deposits

(Ql)

Blocks of 

sandstone and 

shale

Terrace alluvium and gravel

([Qtg]) 

Pebbles, cobbles,

and boulders of 

volcanic and 

Precambrian rocks 

in a matrix of silt 

and sand

Alluvium, colluvium, and soil

(Qac) Soil, silt, and sand

Colluvium of McCartys Mesa

([Qc]) 

Colluvium, soil, 

caliche, terrace 

sand and gravel, 

alluvium, and 

windblown sand

Pyroclastic debris and basalt flows

Qbm, [Qbd], Qcm (McCartys);

Qbb, Qcb, Qvb (Bandera);

Qbw, Qvw (Hoya de Cibola);

Qbp, Qcp, Qvp (Paxton Springs);

Qbz, [Qba] (Zuni);

Qbo, Qvo (Oso Ridge);

Qbt, Qct, Qvt (Twin Craters);

Qbc, Qcc, Qvc (El Calderon);

Qbu, Qc, Qv (basalt flows, cinder 

cones, volcanic debris);

Qb (old basalt flows)

Quaternary (Q)

Middle letter in 

symbol:

b = basalt flows 

c = craters/cinder 

cones 

v = volcanic 

debris/shield 

volcanoes/cinder 

fields

Tertiary 

Neogene Pliocene 5.3–2.6

Basalt flows on Cebollita Mesa, 

Mesa Negro, and Horace Mesa

(Tb)

Tertiary basalt 

flows

Diabase dikes

([Td]) 

Intrusive igneous 

rock

Miocene

65.5–

2.6

Units of this age are absent in the El Malpais National 

Paleogene Monument area.

Oligocene

Eocene

Paleocene

NOTE: Figure continues on next page. *Age is in millions of years before present and indicates the time spanned by associated epoch or 

period. Rock/sediment units associated with those epochs or periods may not encompass the entire age range. Map unit symbols in brackets 

indicate units mapped in the McCartys quadrangle by Maxwell (1977).

Figure 4. General stratigraphic column for El Malpais National Monument. The rocks in and around El Malpais National Monument include 

very old metamorphic and igneous rocks of Precambrian age (approximately 1.4 billion years old); sedimentary rocks of Permian, Triassic, 

Jurassic, and Cretaceous ages (299 million–65.5 million years old); and young (Quaternary) volcanic rocks, most of which erupted in the past 

700,000 years. Unconsolidated deposits top the sequence. Colors are standard colors approved by the U.S. Geological Survey to indicate 

different time periods on geologic maps; they also correspond to the colors on the Map Unit Properties Table. See the Map Unit Properties 

Table for more detail. Figure continues on next page.
6 NPS Geologic Resources Division

Era Period Epoch Age* Rock/Sediment Unit Description Mesozoic

Cretaceous 

Upper 100–

65.5

Crevasse Canyon Formation

(Kcc, [Kgc, Kcd, Kcs, and Kcdi])

Sandstone, 

siltstone, shale, 

and coal beds

Gallup Formation

(Kg, [Kgm, Kgu, and Kgi]) Silty shale

Mancos Shale

(Kmd, Kmr, Kmw, [Km, Kmm, 

Kmwc, and Kmc]) 

Shale with some 

limestone and thinbedded sandstone

Tres Hermanos Sandstone

(Kth)

Sandstone, shale, 

coal beds, and 

siltstone

Dakota Sandstone

(Kdt, Kdp, Kdo) Sandstone

Lower 145–

100

Units of this age are absent in the El Malpais National 

Monument area.

Jurassic

Upper 161–

145

Morrison Formation

(Jm)

Mudstone, 

siltstone, and 

sandstone

Middle 175–

161

Zuni Sandstone and Wanakah 

Formation 

(Jz)

Sandstone, 

mudstone, and 

claystone

Todilto Limestone Member of the 

Wanakah Formation

(Jt) 

Limestone

Entrada Formation

(Je) Sandstone

Lower 200–

175

Units of this age are absent in the El Malpais National 

Monument area.

Triassic

Upper 228–

200

Chinle and Moenkopi formations 

(TRcm)

Claystone and 

siltstone Middle 245–

228

Lower 251–

245

Paleozoic

Permian

Lopingian 

(Upper) 

260–

251 Units of this age are absent in the El Malpais National 

Guadalupian Monument area.

(Middle)

270–

260

Cisuralian 

(Lower)

299–

270

San Andres Limestone

(Psa) Limestone

Glorieta Sandstone

(Pg)

Sandstone and 

siltstone

Yeso Formation

(Py)

Sandstone, 

siltstone, and 

gypsum beds

Abo Formation

(Pa)

Sandstone and 

siltstone

Precambrian

Proterozoic Eon

Mesoproterozoic Era 1,600–

1,000

Igneous and metamorphic rocks

(PChb, PCpa, PCap, PCpg, PCag, 

PCgg, PCmg, PCqt, PChg, PCig, 

PCsc)

Hornblendite, 

gabbro, intrusive 

basalt, aplite, 

granite, gneiss, 

quartzite, and 

schist

*Age is in millions of years before present and indicates the time spanned by associated epoch or period. Rock/sediment units associated with 

those epochs or periods may not encompass the entire age range. Map unit symbols in brackets indicate units mapped in the McCartys 

quadrangle by Maxwell (1977).

Figure 4 (continued). General stratigraphic column for El Malpais National Monument. Caption on previous page.
ELMA Geologic Resources Inventory Report 7 

Figure 5. Major volcanic fields in New Mexico. El Malpais National Monument is part of the Zuni–Bandera volcanic field in west-central New 

Mexico. This and many other volcanic fields lie on the Jemez lineament, a zone of crustal weakness and concentrated volcanism that stretches 

from central Arizona to northeastern New Mexico. The National Park System is well represented along the Jemez lineament. El Malpais 

National Monument and El Morro National Monument are part of the Zuni–Bandera volcanic field. Bandelier National Monument is part of 

the Valles Caldera. Fort Union National Monument is surrounded by the Ocaté volcanic field. Capulin Volcano National Monument is in the 

Raton-Clayton volcanic field. The north–south-trending Rio Grande rift—a center of volcanic activity during an earlier episode of volcanism 

(40 million–20 million years ago)—intersects the Jemez lineament near Valles Caldera. Petroglyph National Monument lies on the Rio Grande 

rift. New Mexico Bureau of Geology and Mineral Resources graphic, modified by Philip Reiker (NPS Geologic Resources Division).

ELMA Geologic Resources Inventory Report 9 

Geologic Issues 

The Geologic Resources Division held a Geologic Resources Inventory scoping meeting for El 

Malpais National Monument on 30 March 2006 and a follow-up conference call on 9 

December 2011, to discuss geologic resources, address the status of geologic mapping, and 

assess resource management issues and needs. This section synthesizes those discussions and 

highlights particular issues that may require attention from resource managers. Contact the 

Geologic Resources Division for technical assistance. 

Discussions during a pre-scoping conference call (16 

March 2006), the GRI scoping meeting (30 March 2006), 

and a post-scoping conference call (9 December 2011) 

identified the following geologic issues of management 

concern at El Malpais National Monument: 

• Cave Management

• Mass Wasting

• Erosion

• Flooding

• Disturbed Lands

• Recent and Potential Volcanism

• Seismic Activity

• Geochronology, Research, and Interpretation

Resource managers may find Geological Monitoring

(Young and Norby 2009) useful for addressing these 

geologic resource management issues. The manual 

provides guidance for monitoring vital signs—

measurable parameters of the overall condition of 

natural resources. The NPS Geologic Resources Division 

initiated and funded the development of Geological 

Monitoring to provide guidance for resource managers 

seeking to establish the status and trends of geologic 

resources and processes within the National Park 

System, and to advance the understanding of how 

geologic processes impact ecosystem dynamics. Each 

chapter of the manual covers a different geologic 

resource and includes detailed recommendations for 

resource managers, including expertise, personnel, and 

equipment needed, as well as the approximate cost and 

labor intensity of each method. 

Cave Management

El Malpais National Monument contains more than 290 

caves in lava flows, referred to as “lava tubes” or “lavatube caves” (see “Lava Tubes” section). The top resource

management priority at the national monument is to 

complete a management plan for these cave resources 

(Dave Hays, resource manager, El Malpais NM and El 

Morro NM, conference call, 9 December 2011). During a 

conference call on 9 December 2011, staff from the 

Geologic Resources Division suggested that El Malpais 

managers contact Dale Pate (NPS cave specialist) for 

guidance in preparing a cave management plan.

Toomey (2009)—the chapter in Geological Monitoring 

about caves and associated landscapes—describes 

methods for inventorying and monitoring cave-related 

vital signs, including the following: (1) cave meteorology, 

such as microclimate and air composition; (2) airborne 

sedimentation, including dust and lint; (3) direct visitor 

impacts, such as breakage of cave formations, trail use in 

caves, graffiti, and artificial cave lighting; (4) permanent 

or seasonal ice; (5) cave drip and pool water, including 

drip locations, drip rate, drip volume, drip water 

chemistry, pool microbiology, and temperature; (6) cave 

microbiology; (7) stability associated with breakdown, 

rockfall, and partings; (8) mineral growth of 

speleothems, such as stalagmites and stalactites; (9) 

surface expressions and processes that link the surface 

and the cave environment, including springs, sinkholes, 

and cracks; (10) regional groundwater levels and 

quantity; and (11) fluvial processes, including 

underground streams and rivers. This information may 

be useful for managers at El Malpais as they develop a 

cave management plan for the national monument.

As part of cave management, resource managers at El 

Malpais National Monument will continue to maintain a 

database of information about the lava-tube caves. The 

data include electronic files, hard-copy materials, and 

institutional knowledge. As part of the cave management 

plan, resource managers are examining and synthesizing 

existing information and proposing a solution for 

modernizing the system (Dave Hays, resource manager, 

El Malpais NM and El Morro NM, conference call, 9 

December 2011). 

Many caves at El Malpais National Monument contain 

seasonal or perennial accumulations of ice (see “Ice 

Caves” section). Compared with periodic observations 

made since the establishment of the national monument, 

present-day accumulations of perennial ice appear to be 

at an “all-time low” (Dana Sullivan, chief ranger, and 

Dave Hays, resource manager, El Malpais NM and El 

Morro NM, conference call, 9 December 2011). The 

National Park Service is not currently monitoring the ice 

accumulations at El Malpais National Monument. 

However, monitoring will likely be part of the future 

cave management plan (Dave Hays, resource manager, El 

Malpais NM and El Morro NM, conference call, 9 

December 2011). Dickfoss et al. (1997) discussed the use 

of cave ice as a climate proxy and provided a history of 

ice accumulation at Ice Cave. This quantitative analysis 

may be useful for monitoring ice accumulations in the 
10 NPS Geologic Resources Division

future. Toomey (2009) highlighted methods for 

monitoring permanent and seasonal ice in caves.

Investigators have studied the major lava-tube systems in 

El Malpais National Monument. This research is 

discussed in Natural History of El Malpais National 

Monument (Mabery 1997). However, the secondary cave 

systems have not been extensively studied (Ken Mabery, 

superintendent, Scotts Bluff NM, conference call, 16 

March 2006). Most of what is known about the 

secondary systems, such as that at Hoya Volcano, has 

been interpreted during overflights, for instance during 

fires (Ken Mabery, conference call, 16 March 2006). 

El Malpais National Monument has a memorandum of 

understanding with the Sandia Grotto—the local chapter 

of the National Speleological Society—for conducting 

basic inventory and cartography. Previously unknown 

locations of caves are often discovered during the 

inventory process. A Student Conservation Association

intern found 30 new caves in 2001–2002 (National Park 

Service 2006b).

The 1990 El Malpais general management plan identified 

four caves as open to recreational use: Junction, Four

Windows, Big Skylight, and Braided caves (fig. 6). In 

addition, the general management plan noted the special 

case of El Calderon Bat Cave. This cave is closed to 

recreational exploration, but the trail to the cave is a 

prime location for observing bat flights (fig. 7). As of 6 

December 2010, all caves at El Malpais National 

Monument are closed to recreational use. The closure 

was prompted by the westward progression of whitenose syndrome, which is decimating bat populations in 

the eastern United States.

Figure 6. Entrance to Braided Cave. Braided Cave was traditionally 

one of five recreational caves at El Malpais National Monument, 

where visitors were able to obtain a “cave experience.” All caves are 

presently closed to recreation as a result of the spread of white-nose 

syndrome, which has decimated bat populations in the eastern 

United States. National Park Service photograph.

Figure 7. Bat Cave. Even before the 2010 closure of all caves within 

El Malpais National Monument, Bat Cave was not open for

recreation. However, the trail to the cave is a prime location for 

observing bat flights. National Park Service photograph.

The closure of the caves at El Malpais brought the 

national monument into compliance with NPS policy, 

much of which is derived from the Federal Cave 

Resources Protection Act of 1988. Guidelines for the 

protection of bat populations seek to limit the humancaused spread of this fungus through actions such as 

screening visitors and employees before they enter caves, 

and by restricting access if screening is not feasible. The 

caves at El Malpais are spread over 46,100 ha (114,000 

ac), with no existing control points, such as entrance 

stations or visitor centers through which people must 

pass on their way to the caves. Hence, guidance for El 

Malpais National Monument and other National Park 

System units with similar situations recommends the

closure of caves to protect bat populations until 

permitting and management strategies are implemented 

to mitigate risks to bat populations. Research is 

underway to determine whether the fungus is present in 

caves at El Malpais National Monument and whether 

hibernating bats already have the disease (Cook Collins 

2011).

Although white-nose syndrome is the most immediate 

threat to cave resources at El Malpais National 

Monument, investigators have identified other threats 

(Northup and Welbourn 1997). Perhaps the most 

publicized threat to cave environments in general has 

been pollution (Northup and Welbourn 1997). Surface 

runoff of pesticides, nitrates, and herbicides from 

agricultural lands and petrochemicals from roads pose 

major problems for karst waters and cave biota (Northup 

and Welbourn 1997). Contamination of cave waters by 

gasoline and septic leakage can kill aquatic species. 

Surface modifications, such as the construction of 

parking lots and campgrounds, alter the input of organic 

matter and water from the surface and may introduce 

additional pollutants (Northup and Welbourn 1997). 

The remote locations of many caves, as well as protection 

as part of the National Park System, have helped to 

protect the lava-tube caves at El Malpais National 

Monument from many common threats (Northup and 

Welbourn 1997). However, awareness of these threats is 
ELMA Geologic Resources Inventory Report 11

significant for park planning and development of a cave 

management plan. 

In addition to pollution, the physical actions of humans 

in caves can be detrimental to cave biota. Physical 

trampling by cave visitors can kill invertebrates, which 

are not noticed or are living under the rocks on which 

visitors walk. Cave entrances are especially vulnerable to 

physical disturbances due to the greater number and 

types of species and the amount of activity that occurs 

there. Furthermore, sunlit areas, including entrances and 

areas under skylights (fig. 8), are known for their moss 

gardens, which are important habitats. In the 1990s, El 

Malpais National Monument staff took steps to 

implement protection by installing educational signs 

about the importance of these gardens (Northup and 

Welbourn 1997). 

Figure 8. Skylight and garden. Light shines through a skylight in 

Braided Cave. Luxuriant gardens grow beneath skylights in the lava 

tubes at El Malpais National Monument. National Park Service 

photograph.

Other potential impacts include the dumping of garbage 

in collapsed lava tubes (see “Lava Tubes” section), which 

introduces harmful substances into the cave 

environment (Northup and Welbourn 1997). On a lesser 

scale, visitors to the lava tubes occasionally leave their 

trash behind. Chapman (1993) noted that the 

composition of cave fauna changed in an area containing 

food trash. Northup and Welbourn (1997) documented 

food trash in Bat and Four Windows caves, and human 

feces in Big Skylight Cave. In the past, the dumping of 

carbide from lamps had a notable impact (Peck 1969; 

Lavoie 1980) because carbide is toxic to invertebrates 

(Northup and Welbourn 1997). Carbide lamps have been 

largely replaced by high-intensity LED illumination. 

Cigarette smoking in caves is a threat to cave biota 

because nicotine is an insecticide (Northup and 

Welbourn 1997). The introduction of foreign organic 

matter, such as firewood, also threatens the natural cave 

environment. Such materials equate to food sources in 

what was formerly a nutrient-poor system, potentially 

leading to “boom-and-bust” cycles in native 

invertebrates (National Park Service 1994). Additionally, 

fires within caves may be damaging or fatal to bats and 

invertebrates (Northup and Welbourn 1997). 

Mass Wasting

Mass wasting—the downslope movement of soil and 

rock material—occurs as rockfall, slumping, debris flows, 

and landslides. Within El Malpais National Monument, 

the primary type of mass wasting is rockfall off the 

sandstone bluffs, most likely as a result of freeze-thaw 

processes (Dave Hays, resource manager, El Malpais 

NM and El Morro NM, conference call, 9 December 

2011). Mass wasting is generally a low-priority 

management concern at the national monument because 

areas prone to rockfall do not coincide with 

infrastructure or trails, and thus pose minimal hazards 

for visitors (National Park Service 2006b). 

The eastern margin of El Malpais National Monument 

follows cliffs composed of Zuni Sandstone (map unit

symbol Jz). The steep cliffs adjacent to Highway 117 near 

the Sandstone Bluffs Overlook illustrate the process of 

mass wasting (Crumpler et al. 2003). The sandstone is 

being undercut, resulting in cliff retreat. The cause may 

be groundwater seepage from springs, which is 

disintegrating the sandstone near the cliff base and 

ultimately causing massive vertical spalls and slumps 

(Crumpler et al. 2003). An example of such a slump is 

located approximately 10 km (6 mi) south of the 

Sandstone Bluffs Overlook along Highway 117 

(Crumpler et al. 2003) (fig. 9). 

Maxwell (1986) mapped extensive landslides east of El 

Malpais National Monument. These deposits are 

composed mostly of rotational slides—referred to as 

“toreva-block slides”—of Tertiary basalt (Tb) and 

Cretaceous sandstone and shale that have slid over soft 

shale units (Maxwell 1986). The deposits also contain 

some rock and mudflow slides and talus and fan 

accumulations, some of which are derived from outcrops 

and others from landslide debris (Maxwell 1986). The 

landslides are generally older than the alluvium (Qal) that 

covers the valley floors. Flash flooding, which is 

characteristic of present-day arid conditions, produces 

deep gullies in the alluvium, sometimes exhuming older 

landslide blocks (Maxwell 1986).
12 NPS Geologic Resources Division

Figure 9. Mass wasting. The steep cliffs of massive Zuni Sandstone illustrate the process of mass wasting within the Highway-117 corridor 

along the eastern edge of El Malpais National Monument. National Park Service photograph by Dale Dombrowski (El Malpais National 

Monument).

Erosion

The “deep gullies” identified by Maxwell (1986) are 

generally referred to as “arroyos,” the Spanish word for 

“stream.” In the Southwest, the term is applied to 

streambeds that are dry most of the time, except during 

flash floods. Arroyos are commonly seen as “a symptom 

that something is wrong” (Love and Gellis 2008). 

However, according to Love and Gellis (2008), the 

natural function of arroyos is complicated and depends 

on external influences, such as land use and climatic 

conditions, and internal adjustments in surface 

hydrology, stream morphology, changes in sediment 

availability and type, and biological response of native 

and invasive species (fig. 10). 

Managers at El Malpais National Monument have linked 

arroyo formation within the national monument to past 

land-use practices, in particular overgrazing of sheep and 

cattle (Dana Sullivan, chief ranger, and Dave Hays,

resource manager, El Malpais NM and El Morro NM, 

conference call, 9 December 2011). Within the national 

monument, all grazing ceased after 1997 in accordance 

with provisions in the 1987 enabling legislation. 

However, a checkerboard of lands—grazed in the past 

and at present—surrounds the national monument. Staff 

members at El Malpais survey and mend fenced areas of 

the national monument’s boundary to control sheep and 

cattle trespass (National Park Service 2006b). 

In 2001, an investigator from the NPS Geologic 

Resources Division found that erosion rates in the area 

of Sandstone Bluffs Overlook have likely been 

exacerbated by overgrazing (Greco 2001). Erosion in this 

area has also been accelerated by changes in natural 

drainage patterns. The natural hydrology of the area has 

been greatly altered by channel diversion (Greco 2001). 

Even under natural conditions, eolian deposits (map unit 

symbol Qe) found in the area are highly susceptible to 

erosion. The wash in the upper reaches of the Sandstone 

Bluffs Overlook area has downcut through this surficial 

material into the underlying, more-resistant sandstone of 

the Clay Mesa Tongue of Mancos Shale and lower part 

of Dakota Sandstone (Kdo). Greco (2001) recommended 

that any approach to reducing erosional impacts to the 

Sandstone Bluffs Road should consider natural drainage 

patterns. Culverts should be configured to match the 

natural path of the wash. Furthermore, to prevent 

blockage of culverts in the event of a 100-year or greater 

flood, culvert pipes should be sized to carry sediment, as 

well as floodwaters. Moreover, culvert installation 

should include armoring the inlet and outlets by “keying 

in” (securing/fitting) riprap to dissipate energy and 

reduce erosional forces (Greco 2001). Another 

significant aspect of restoration in this area is the 

revegetation of heavily grazed areas, which will help

stabilize the soils and retard erosion (Greco 2001).
ELMA Geologic Resources Inventory Report 13

Figure 10. Variables of arroyo formation. The function of arroyos is complicated by and dependent upon antecedent conditions and external 

influences. Antecedent conditions at El Malpais National Monument include landscapes covered in eolian deposits (Qe). External influences 

include land-use practices such as cattle and sheep grazing, and changes in the natural drainage system caused by channel diversions and 

culverts. Graphic from Love and Gellis (2008), modified by Trista Thornberry-Ehrlich (Colorado State University).
14 NPS Geologic Resources Division

Flooding

Streams in El Malpais National Monument are 

ephemeral and flow only as a result of flash-flooding 

events (National Park Service 2006b). In general, surface 

water percolates into the groundwater system within a 

few weeks after a flash flood. Windmills within El 

Malpais attest to past extraction of groundwater for 

livestock use. Upon establishment of the national 

monument, the National Park Service removed many 

windmills from national monument lands, but some 

wells were retained for their historical value and use by 

wildlife (National Park Service 2006b). 

Storm-water runoff—and associated pesticides, 

herbicides, and fertilizers from agricultural lands, as well 

as excessive runoff and sedimentation from grazed 

areas—is a resource management concern at El Malpais 

National Monument. Stream channels originate outside 

the monument but flow into the lava-tube systems, 

potentially introducing contaminants and impacting cave 

biota. Source areas of concern are in the northwest 

corner of the national monument near Bandera Crater, 

and Pueblo (tribal) and Bureau of Land Management 

(BLM) lands on the southeast side of the national 

monument. In the northwest corner, pesticides are 

applied and grazing occurs on small ranches and other 

private lands adjacent to national monument lands. On 

the southeast, grazing occurs on Pueblo and BLM lands 

bordering the national monument (National Park Service 

2006b).

Flooding also causes erosion between the sandstone 

bluffs and the McCartys flow on the east side of El 

Malpais National Monument, where archeological sites

are highly concentrated. Furthermore, Highway 117 on 

the east side of the national monument is periodically 

washed out during flooding events (National Park 

Service 2006b).

Disturbed Lands

Disturbed lands within the National Park System include

abandoned roads, dams, canals, railroads, grazed areas, 

campgrounds, mines, and other abandoned sites. Some 

of these features may be of historical significance, but 

most are not. Non-historic features can be mitigated to 

restore natural conditions. The NPS Geologic Resources 

Division assists with disturbed lands restoration, 

including abandoned mineral lands, within the National 

Park System. Disturbances at El Malpais National 

Monument include the following:

• Abandoned Mineral Lands

• Mining and Military Activities

• Logging

• Grazing and Agricultural Practices

• Channel Diversions

Abandoned Mineral Lands

Past mining activities impacted parts of the national 

monument. As of September 2012, the abandoned 

mineral land (AML) database maintained by the 

Geologic Resources Division included 28 AML features 

at six sites within El Malpais National Monument. “Lava 

rock” (basalt), cinders, sandstone, and road-base 

aggregate were extracted from the sites. Staff members at 

the national monument are aware of four additional sites 

for which they are collecting geographic positioning 

system (GPS) data that they will provide to the AML 

Program (Dana Sullivan, chief ranger, and Dave Hays, 

resource manager, El Malpais NM and El Morro NM, 

conference call, 9 December 2011). 

 

Figure 11. Bandera Crater. Bandera Crater exemplifies the typical 

steep-sided and symmetrical form of a cinder cone. The cone exhibits 

a breach from which lava erupted. Today, a lava-tube system starts 

at the breach. Note the Bandera cinder quarry behind and to the 

right of the crater rim in the photograph. The Bandera cinder quarry 

is privately owned, but its location adjacent to the national 

monument and within the monument’s viewshed makes it highly 

visible to visitors. Photograph by Larry Crumpler (New Mexico 

Museum of Natural History and Science).

At present, the two main mining-related issues for 

managers at El Malpais National Monument are (1) a 

highly visible cinder quarry in the northwest corner of 

the national monument, near Bandera Crater (fig. 11), 

and (2) an abandoned cinder quarry on the side of El 

Calderon. Cinders from the Bandera quarry were sold by 

the owner and used as road base by the state highway 

department. Although the quarry is privately owned, it is 

adjacent to NPS lands and within the national 

monument’s viewshed. The most recent mining activity 

at this site ceased in 2010 (Dana Sullivan, chief ranger, 

and Dave Hays, resource manager, El Malpais NM and 

El Morro NM, conference call, 9 December 2011).

The El Calderon quarry site is owned by the National 

Park Service and is no longer used. However, the site is 

difficult to reclaim because re-contouring cinders is 

challenging. The National Park Service is considering 

simply leaving the site in its present state and interpreting 

its mining history for visitors (National Park Service 

2006b). Glass bottles, tin cans, and tools are occasionally 

found in the loose cinders and are evidence of past 

mining activities (National Park Service 2004a).

In the Lava Falls area, Dakota Sandstone (Kdo) was 

quarried. This quarry has been reclaimed and is no 

longer an issue (National Park Service 2006b). The New 

Mexico Department of Transportation used sandstone 

blocks from this quarry to build culvert headwalls within 
ELMA Geologic Resources Inventory Report 15

the national monument and along Highway 117 between 

Grants and Pie Town. In 1996, the National Park Service 

revegetated the quarry to restore natural drainage at the 

site.

Finally, some evidence of guano mining remain in lava 

tubes. Bat guano is an excellent source of fertilizer 

because of its high nitrate content. Remnants of a simple 

mining operation are apparent in Bat Cave (National 

Park Service 2004a), although this activity has now 

ceased. 

Mining and Military Activities

With the advent of World War II, fluorspar (calcium 

fluoride) and pumice were mined near Grants, New 

Mexico. The Navajo Fluorspar Company opened three 

mines near the commercially operated Ice Cave, which is

adjacent to what is now El Malpais National Monument 

(Mangum 1997). Fluorspar was used in the manufacture 

and hardening of steel, in paints, and in acids. The 

fluorspar mines remained active until 1952, when foreign 

competition drove down the price of this material 

(Mangum 1997). 

The source rocks were Precambrian granites and 

gneisses, which contain veins of fluorite (Laughlin et al. 

1993a). According to Maxwell (1986), the veins range in 

width from a few centimeters to 5 m (16 ft), and are filled 

with coarsely crystalline, bright-green fluorite that is

generally brecciated and locally mixed with wall-rock 

fragments. The breccia is locally cemented by finegrained, purple fluorite. Associated minerals are calcite, 

barite, and quartz (Maxwell 1986). Fluorite veins are 

indicated on the digital geologic map for El Malpais 

National Monument; one vein is mapped within the 

national monument (see “Geologic Map Data” section). 

Uranium was another mineral resource that affected El 

Malpais National Monument. The uranium boom of the 

1950s postponed establishment of a national monument 

until 1987, although the inclusion of the El Malpais area

in the National Park System was proposed as early as the 

1930s (National Park Service 2006b). Uranium was first 

discovered in this region in 1950 in the Jurassic Todilto 

Formation, which is exposed in Haystack Butte west of 

Grants, New Mexico. In the 1970s, the uranium boom 

ended and local mines closed.

Another war-effort disturbance is the use of the 

McCartys lava flow and cinder cone as a bombing range 

(Mangum 1997). The U.S. Army used the 23-km2 (9-mi2

) 

area of rugged lava terrain for target practice, in 

particular the McCartys cinder cone (fig. 12). Although 

the military conducted cleanup, ordnance is periodically 

found on the McCartys flow (Dana Sullivan, chief ranger, 

and Dave Hays, resource manager, El Malpais NM and 

El Morro NM, conference call, 9 December 2011). El 

Malpais was also included on the short list for selection 

as the test detonation site of the first atomic bomb (Dana 

Sullivan and Dave Hays, conference call, 9 December 

2011). Ultimately, the Trinity Site near White Sands 

National Monument in southern New Mexico was 

selected (see fig. 2). Today, the Trinity Site is a national 

historical landmark located on White Sands Missile 

Range, which surrounds White Sands National 

Monument (KellerLynn 2012c).

Logging

Logging activities in the 1970s altered ecosystems in what 

is now El Malpais National Monument. In areas not 

subject to logging, the lava flows support extremely old 

specimens of Douglas-fir (Pseudotsuga menziesii 

menziesii; see “Biologic and Geologic Connections”

section). 

The National Park Service is protecting some of the 

sawmill sites associated with past logging activities due to 

their value as cultural resources. Park managers do not 

plan to restore old logging roads within the national 

monument, but these roads in the wilderness area are 

now closed to motorized vehicles (National Park Service 

2006b). Remaining slash that was cut during logging 

operations is now being removed during prescribed

burns (National Park Service 2006b).

Figure 12. McCartys cinder cone. The McCartys vent is a small cinder cone that is approximately 8 m (26 ft) tall (note person on rim of cone for 

scale). The cone is approximately 90 m (295 ft) in diameter. It rests on a small shield approximately 300 m (980 ft) across. Photograph by Larry 

Crumpler (New Mexico Museum of Natural History and Science)
16 NPS Geologic Resources Division

Grazing and Agricultural Practices

During the period of BLM management, cattle heavily 

grazed the El Malpais National Monument area, which 

likely amplified the effects of wind erosion and the 

occurrence of dust storms (National Park Service 2006b). 

Grazing and other agricultural practices surrounding

Grants, New Mexico, probably contributed material to 

eolian deposits. In the 1940s and 1950s, large-scale 

farming developed in Grants, with carrots as the primary 

cash crop. Grants was known as the “carrot capitol of the 

world” and this carrot industry flourished until the late 

1950s, when California produce put New Mexico 

growers out of business (Mangum 1997). In general, dust 

from the Grants area continues to be an issue for the 

national monument in the form of periodic dust storms 

(Dave Hays, resource manager, El Malpais NM and El 

Morro NM, e-mail communication, 27 December 2011).

Channel Diversions

Prior to the establishment of El Malpais National 

Monument, ranching practices diverted water from 

natural channels into ditches, dikes, and stock ponds. 

Diversion also occurred in conjunction with the 

construction of highways, such as Highway 53 near the 

El Calderon area. The manipulation of stream channels 

for ranching and highway construction has negatively 

impacted Junction Cave in the El Calderon area (fig. 13). 

During heavy rainfall, this cave becomes a storm-drain 

system; flooding occurred in 1987 and 1999 (National 

Park Service 2006b). In the 1960s, or perhaps earlier, 

diversion projects changed the natural flow path of Agua 

Fria Creek, in turn impacting Junction Cave (Pranger 

2001). An artificial dike diverts water away from the 

native Agua Fria Creek channel into a diversion channel 

that passes through a stock pond and past the El 

Calderon parking lot. The flow splits at the El Calderon 

parking lot, with some high flow entering a collapsed lava 

tube and the rest flowing past the parking lot and into 

Junction Cave (Pranger 2001).

In 2001, the Geologic Resources Division responded to a 

technical assistance request from managers at El Malpais 

National Monument to assess fluvial features in relation 

to lava-tube caves within the Agua Fria Creek channel, 

and also to inspect and assess the impacts of disturbed 

lands along the creek resulting from the development of 

ditches, dikes, and stock tanks (Pranger 2001). This 

assessment identified specific areas that could be 

successfully restored and recommended the removal of 

two stock ponds and diversion channels and the 

reconstruction of a segment of the Agua Fria Creek 

channel (Pranger 2001). The project is currently on hold 

until the provenance of the diversion features is 

determined. These features may have been constructed 

as Works Progress Administration/Civilian Conservation 

Corps–era dams, in which case their preservation as 

cultural resources may be warranted (Dana Sullivan, 

chief ranger, and Dave Hays, resource manager, El 

Malpais NM and El Morro NM, conference call, 9 

December 2011).

Figure 13. Junction Cave. Junction Cave is situated near the El 

Calderon parking lot. Approximately 49 m (160 ft) away, an eroded 

diversion ditch allows storm-water runoff to enter this lava-tube 

cave. National Park Service photograph.

Recent and Potential Volcanism

Limburg (1990) evaluated the 700 volcanic events that 

have occurred in New Mexico over the past 5 million 

years. During this time period, basaltic volcanism has 

been the dominant mode of volcanism in New Mexico 

(Limburg 1990). Basaltic volcanism is characterized by 

effusive, fairly continuous eruptions, with many vents 

erupting simultaneously within a volcanic field. If 

basaltic volcanism were to recur in New Mexico today, 

erupting basalt would change the face of the landscape, 

filling low-lying areas and disrupting drainages. Such 

modifications would take place relatively close to 

volcanic vents (Dunbar 2005). However, lava flowing

down a river bed would affect river behavior. For 

instance, damming of a river by a lava flow could cause 

an upstream lake to form, which could result in 

catastrophic flooding when the natural dam fails (Fenton 

et al. 2004).

Limburg (1990) estimated a 1% chance that some type of 

volcanic eruption will occur in New Mexico by 2090, and 

a 10% chance that an eruption will occur in the next 

1,000 years. In probabilistic terms, 100 eruptions will 

occur in the next 1 million years.

Future eruptions are most likely to occur along the Jemez 

lineament and the Rio Grande rift (Chapin et al. 2004; 

Dunbar 2005). The most vulnerable areas occur where 

the Rio Grande rift intersects crosscutting lineaments, 

such as in the Jemez Mountains and near Socorro, New 

Mexico (Chapin et al. 2004). An active magma chamber 

called the Socorro magma body has been recognized in 

the area around Bernardo, New Mexico (Sanford et al. 

1977; Reilinger et al. 1980). This magma body is located 

19 km (12 mi) below the surface and covers an estimated 

area of 3,400 km2 (1,300 mi2

) (Balch et al. 1997). The area 

around the magma body has been the location of 

elevated levels of seismic activity (Sanford et al. 1977; 

Fialko and Simons 2001) (see “Seismic Activity” section), 

likely as a result of migration of magma, or instability, 

within the chamber. These combined factors suggest that 

the Socorro magma body is a possible source of future 

volcanism (Dunbar and Phillips 2004).
ELMA Geologic Resources Inventory Report 17

Although renewed volcanism at El Malpais National 

Monument is not imminent and resource managers are 

not currently monitoring volcanic activity, Walkup 

(2012)—an inventory of volcanic features and hazards in 

the National Park System—identified the following 

potential hazards in the event of renewed volcanism at 

the national monument: ash/tephra fall, earthquakes, gas, 

lava eruptions, and volcanic projectiles (e.g., lava bombs). 

Empirical observations and monitoring can be used to 

predict volcanic eruptions. Methods include geophysical 

(seismic), geodetic (tilt), and geochemical monitoring, as 

well as observations of readily detectible phenomena by 

casual observers, such as felt earthquakes, tremors, and 

noises; bulging or cracking of the ground surface; 

changes in color, temperature, and sediment content of 

streams; death or sickness of vegetation; and changes in 

usual emissions from the vents (Banks et al. 1989). Such 

data are qualitative, but may be useful in deciding the 

type and deployment location of monitoring equipment 

(Limburg 1990).

Two chapters in Geological Monitoring may be useful for 

anticipating future volcanic activity. Smith et al. (2009) 

presented the following methods and vital signs for 

monitoring volcanoes: (1) earthquake activity, (2) ground 

deformation, (3) emissions at ground level, (4) emission 

of gas plume and ash clouds, (4) hydrologic activity, and 

(5) slope instability. Braile (2009) provided information 

about seismic monitoring, which is discussed in the 

following section. Widespread seismic monitoring 

around New Mexico would forewarn of future 

volcanism and enable prediction of the potential location 

of an eruptive event (Dunbar 2005).

Seismic Activity

New Mexico has a recorded history of seismic activity 

since 1869 (Sanford et al. 2002, 2006; U.S. Geological 

Survey 2009b). This long record aids researchers in 

evaluating seismic hazards and forecasting the location 

and magnitude of future earthquakes (Sanford et al. 

2002). The largest earthquake in New Mexico occurred 

on 15 November 1906 in the Socorro area. The 

earthquake was felt over most of New Mexico and in 

parts of Arizona and Texas (U.S. Geological Survey 

2009a). This historic quake occurred before the Richter 

scale came into use and was later assigned a Modified 

Mercalli intensity level VII (on a scale of I to XII), 

indicating little damage to well-designed and wellconstructed buildings, but considerable damage to 

poorly built or poorly designed structures, as well as 

breakage of some chimneys. Indeed, four chimneys were 

shaken off the Socorro County Courthouse, and two 

others were cracked severely. Plaster fell at the 

courthouse, and a cornice on the northwest corner of the 

two-story adobe Masonic Temple was thrown onto its 

first floor. Plaster was shaken from walls in Santa Fe, 

about 200 km (120 mi) from the epicenter. 

A disproportionate number of earthquakes in New 

Mexico occurred in the Rio Grande Valley near Socorro, 

forming a tight cluster of earthquakes known as the 

Socorro seismic anomaly. Enhanced seismic activity in 

this region is believed to be the result of the stretching of 

Earth’s crust over a large body of magma that exists at a 

depth (see “Recent and Potential Volcanism” section). 

This magma is associated with the Rio Grande rift, but 

surprisingly, with the exception of the Socorro seismic 

anomaly, this rift is not characterized by earthquake 

activity (Sanford et al. 2006). The Socorro seismic 

anomaly occupies less than 1% of New Mexico and 

bordering areas, but accounts for 23% of earthquakes of 

magnitude 2.0 or greater during 1962–1998 (Sanford et 

al. 2002), and 15% during 1999–2004 (Sanford et al. 

2006).

The intact nature of the lava flows at El Malpais National 

Monument indicates that seismic activity has not 

occurred recently. The most recent “felt quake” was in 

the 1970s (National Park Service 2006b). However, earth 

cracks south of Point of Malpais in the BLM 

conservation area may be an indicator of seismic activity 

in the region (National Park Service 2006b). Maxwell 

(1986) mapped surface faults on the McCartys flow, but 

investigators have not subsequently reverified these 

features (National Park Service 2006b). These surface 

faults may line up as a series of spatter cones, indicating

extrusion of lava along a fault, but this possibility has not 

been confirmed in the field (National Park Service 

2006b).

Braile (2009)—the chapter in Geological Monitoring

about earthquakes and seismic activity—described the 

following methods and vital signs for understanding 

earthquakes and monitoring seismic activity: (1) 

monitoring earthquakes, (2) analysis and statistics of 

earthquake activity, (3) analysis of historic and 

prehistoric earthquake activity, (4) earthquake risk 

estimation, (5) geodetic monitoring and ground 

deformation, and (6) geomorphic and geologic 

indications of active tectonics. Braile (2009) provided a 

summary of these methods, including the expertise, 

special equipment, cost, and personnel required, and the 

labor intensity. 

Geochronology, Future Research, and Interpretation

El Malpais National Monument has served as a natural 

laboratory for testing various isotopic methods of dating 

the ages of lava flows, including potassium-argon (K-Ar), 

argon-argon (40Ar39Ar), carbon-14 (14C), helium-3 

(

3

He), chlorine-36 (36Cl), and uranium (U) series. 

Laughlin and WoldeGabriel (1997) provided a summary 

of these six analytical methods, which have been applied 

to the basalt flows of the Zuni–Bandera volcanic field, 

including the very young flows in El Malpais National 

Monument. The national monument has been 

considered an ideal site for the inter-calibration of 

techniques because it contains a large number of wellexposed and well-mapped late-Quaternary lava flows 

(Laughlin et al. 1994b). The plethora of dating methods 

used on specific flows has provided the opportunity for 

cross dating and comparative analysis (National Park 

Service 2006b). In several cases in which multiple 

methods have been applied to the same flow, agreement 

among methods has been excellent. In other cases, 

however, the dates have not agreed (Laughlin and 

WoldeGabriel 1997). For example, the dating of very 
18 NPS Geologic Resources Division

young (<100,000 years old) basalts using the K-Ar 

method has been problematic because the amount of 

argon accumulated via decay during this time period is so

small and difficult to measure. Researchers continue to 

use the national monument as a field area (National Park 

Service 2006b). 

Starting with Nichols (1946) and Maxwell (1986), the 

geochronology of El Malpais first came to light through 

relative dating of lava flows. Investigators used standard 

geologic methods of relative dating, including the 

principle of superposition (younger layers are on top of 

older layers) and morphology. More recently, work by 

Laughlin et al. (1993a, 1993b, 1994a, 1994b), Dunbar and 

Phillips (1994, 2004), McIntosh (1994), Laughlin and 

WoldeGabriel (1997), and Cascadden et al. (1997a, 

1997b) used radiometric dating methods to refine the 

geochronology.

Innovations in basaltic lava flow dating continue to be 

developed, as shown by a recent study using gypsum 

crusts (Dillion et al. 2009). Gypsum crusts have been 

preserved in the drier parts of the lava-tube caves at El 

Malpais National Monument. U-series dating of this type 

of crust yields ages of lava tube formation, and therefore 

eruption ages. These gypsum crusts were likely formed 

by the percolation of sulfuric acid within the host basalt. 

Gypsum-crust speleothems have been found to form in 

lava tubes immediately after the cessation of caveforming volcanic activity, making them potentially very 

useful for dating lava flows (Polyak 2009). 

Although much is now known about the sequence of 

events at El Malpais National Monument (see “Volcanic 

Features and Processes” and “Geologic History”

sections), a few gaps prohibit a full understanding of the 

volcanic story. In particular, the Hoya de Cibola flows 

(Qbw) have presented challenges in efforts to decipher 

the geochronology of eruptions at El Malpais. Maxwell 

(1986) mapped the Hoya de Cibola flows (Qbw) as 

younger than the Twin Craters flows (Qbt). However, 

investigations since 1986 have brought this interpretation 

into question. The difficulty arises because the Hoya de 

Cibola (Qbw) and Twin Craters (Qbt) flows do not come 

into contact; they are separated by the Bandera flows 

(Qbb). Hence, the principle of superposition cannot be 

applied in this circumstance, and no reliable ages have 

been produced. Confusion also arises because Maxwell 

(1986) used the map symbol Qbw for the Hoya de Cibola 

flows. Since 1986, however, the Bluewater flow west of 

Grants, New Mexico, which was not mapped by 

Maxwell (1986), has been recognized to represent a 

significant eruptive event in the chronology of the Zuni–

Bandera volcanic field. Dunbar and Phillips (2004) used 

the symbol Qbw for the Bluewater flow, and substituted 

Qh for the Hoya de Cibola flows. In addition, the exact 

ages of various flows in the Zuni Mountains north of El 

Malpais are unknown, including the Zuni (Qbz) and Oso 

Ridge (Qbo) flows mapped by Maxwell (1986). 

Some areas of El Malpais could benefit from more 

detailed mapping, for instance, the area around Lost 

Woman Crater in the northwestern part of the national 

monument, which Maxwell (1986) mapped as part of the 

Twin Craters cinder cones (Qct). Original mapping (from 

aerial photographs) missed features in this area because 

it is heavily forested (Ken Mabery, superintendent, 

Scotts Bluff NM, conference call, 16 March 2006). 

Preliminary ground reconnaissance revealed a cinder 

cone and vent that had not been mapped by Maxwell 

(1986). Findings of this field reconnaissance were not 

published (Ken Mabery, , e-mail communication, 14 May 

2012), but the existence of previously unidentified 

features suggests that additional field work is warranted. 

Work by Cascadden et al. (1997b) improved the

understanding of this area, but much remains unknown 

and new discoveries are likely. 

A full understanding of the history of the Bandera flows 

(Qbb) also requires further research. This unit is huge, 

approximately 37 km (23 mi) long, and is mostly 

composed of aa lava, which contrasts with other 

pahoehoe events (e.g., Hoya de Cibola) in the national 

monument. 

Finally, unanswered research questions remain regarding 

the national monument’s youngest volcanic event, 

represented by the McCartys flow (Qbm and Qbd): Why 

was this flow so long, extruding from such a small vent? 

Why was the flow so fluid, while its chemical 

composition does not differ widely from those of other 

flows in the monument? Why did aa flows occur in a 

“sea” of pahoehoe?

Larry Crumpler (New Mexico Museum of Natural 

History) is studying the physical characteristics of the 

McCartys flow. Using differential GPS measurements 

across several transects, Crumpler will map surface 

textures to determine the sequences of lava inflation and 

flow breakouts. He is expected to complete his research 

in 2012. Peter Reiners (University of Arizona) submitted

a still-pending grant proposal to the National Science 

Foundation (NSF) for a light detection and ranging

(LiDAR) project across the western half of the national 

monument to characterize the emplacement and 

structure of the older flows. More recent volcanic data 

can be integrated into the national monument’s GIS. 

Nelia Dunbar (New Mexico Bureau of Geology and 

Mineral Resources) can provide updated information 

and guidance.

The geologic features at El Malpais National Monument 

are also a valuable resource for public education. Their 

accessibility and level of preservation makes them ideal 

field-trip stops. For example, El Malpais was highlighted 

in the Rockin’ Around New Mexico Workshop led by 

the New Mexico Bureau of Geology and Mineral 

Resources in 2000 (New Mexico Bureau of Geology and 

Mineral Resources 2009). Additionally, many examples 

of volcanic features from El Malpais are included in the 

popular video Lava Flows and Lava Tubes: What They 

Are, How They Form (Volcano Video Productions 2004).
ELMA Geologic Resources Inventory Report 19

Geologic Features and Processes 

Geologic resources underlie park ecosystems and geologic processes shape the landscape of 

every park. This section describes the most prominent and distinctive geologic features and 

processes in El Malpais National Monument. 

Discussions during a pre-scoping conference call (16 

March 2006), the GRI scoping meeting (30 March 2006), 

and a post-scoping conference call (9 December 2011) 

developed a list of distinctive geologic features and 

processes in El Malpais National Monument: 

• Volcanic Features and Processes

• Vents

• Lava Flows

• Lava Tubes 

• Ice Caves

• Eolian Features and Processes

• Biologic and Geologic Connections

• Paleontological Resources 

• Sandstone Features

Volcanic Features and Processes

The Zuni–Bandera volcanic field, of which El Malpais

National Monument is a part, was produced by 

Hawaiian-style volcanism. The field began erupting 

about 700,000 years ago. However, the lava flows and 

other volcanic features within the national monument 

are part of an episode of eruption that occurred within 

the past 60,000 years, starting with the eruption of El 

Calderon and ending with the McCartys flow (table 1). 

Tertiary basalt flows of the Mount Taylor volcanic field 

east of El Malpais erupted before the Zuni–Bandera 

volcanic field. Maxwell (1986) mapped these flows as 

basalt flows on Cebollita Mesa, Mesa Negro, and Horace 

Mesa (may unit symbol Tb). The distinctive Horace 

Mesa formed during this eruption (fig. 14). At the same 

time, diabase dikes (Td) intruded the sedimentary rocks 

in the area, including Gallup Sandstone and Mancos 

Shale (Maxwell 1977).

Characteristic features of Hawaiian-style volcanism are 

cinder cones, spatter cones, and lava flows. Fire 

fountains, which can spray molten lava 900 m (3,000 ft) 

into the air, likely produced many of the volcanic 

features at El Malpais National Monument (National 

Park Service 2006b; Dunbar 2010). During this type of 

eruption, much spatter and scoria (see descriptions

below) would have piled into cones and mounds along 

vents (see descriptions below). In addition, great 

quantities of fluid, basaltic lava would have poured out of 

fissures. 

Geologists have grouped the volcanic features at El 

Malpais National Monument into the two main 

categories of vents and lava flows. A third category is lava 

tubes. Each of these categories is discussed in following 

sections (see “Vents,” “Lava Flows,” and “Lava Tubes”

sections). Maxwell (1986) also mapped “pyroclastic 

debris” as a primary volcanic feature at El Malpais. 

According to Maxwell (1986), pyroclastic debris is 

composed largely of basalt flows mixed with and covered 

by cinders, bombs, scoria blocks, and flow breccia (see 

descriptions below). Around Bandera Crater, the 

deposits of pyroclastic debris, most of which are cinders, 

cover sedimentary rocks (Maxwell 1986). 

Walkup (2012) completed an inventory of volcanic 

features throughout the National Park System, including 

El Malpais National Monument. This volcanic inventory 

complements the broader GRI and may be of interest to 

resource managers at El Malpais National Monument.

Table 1. Volcanism within and near El Malpais

National Monument

Flow Age (years ago)

McCartys (Qbm and Qbd) 3,900

Bandera (Qbb) 11,000

Hoya de Cibola (Qbw) Exact age unknown, but 

older than Bandera

Paxton Springs, south (Qbp) 15,000

Zuni (Qbz and Qba) Exact age unknown, but 

overlie Oso Ridge flows in 

Zuni Canyon

Oso Ridge (Qbo) Exact age unknown, but 

younger than Twin Craters

Twin Craters (Qbt) 18,000

Paxton Springs, north (Qbp) 20,700

El Calderon (Qbc) <60,000

Basalt flows (Qbu) 

“Chain of Craters” ~150,000

Old basalt flows (Qb), e.g., 

Hole-in-the-Wall ~700,000

Basalt flows on Cebollita 

Mesa, Mesa Negro, and 

Horace Mesa (Tb) 

“Mount Taylor”

3.73 million to 1.57 million

Sources: Maxwell (1977, 1986), Laughlin et al. (1993b), Mabery et al. 

(1999), Dunbar and Phillips (2004), and Dunbar (2010).
20 NPS Geologic Resources Division

Figure 14. Horace Mesa. Maxwell (1986) mapped the Horace Mesa as 

composed of Tertiary basalt flows. These flows are part of the 

Mount Taylor volcanic field. Horace Mesa displays distinctive 

columnar basalt that is broken by parallel, prismatic columns that 

formed as a result of contraction during cooling. The McCartys flow 

is in the foreground. Photograph by Larry Crumpler (New Mexico 

Museum of Natural History and Science).

The following terms describe volcanic features found at

El Malpais National Monument. These terms were used 

by Maxwell (1986) and other investigators, and are used 

throughout this report. They are presented here for 

convenience and as an introduction to the geology of 

volcanic features. Other less frequently used terms are 

included in the glossary.

• Basalt—a dark-colored igneous rock, commonly 

extrusive, composed primarily of calcic (calcium-rich) 

plagioclase and pyroxene minerals.

• Bomb—rock ejected in a viscous state and shaped 

while in flight. By definition, a bomb is larger than 64 

mm (2.5 in) in diameter, and may be vesicular (bubbly) 

or even hollow inside. The actual shapes of bombs vary

greatly, and are used in descriptions such as 

“rotational bomb” or “spindle bomb” (Neuendorf et 

al. 2005).

• Cinder—a vitric (glassy), vesicular (bubbly) fragment 

of rock that falls to the ground in an essentially solid 

condition.

• Cinder cone—a conical hill primarily formed by the 

accumulation of cinders around a vent. The steepness 

of the cone is normally greater than 10° (Neuendorf et 

al. 2005) but depends on factors such as the coarseness 

of the ejected material, height of the eruption, and 

wind velocity. 

• Fissures—elongated cracks that break the surface of a 

lava flow. During eruptions, fissures develop and serve 

as conduits for lava, which erupts onto the surface as 

spatter, cinders, and flows. Hawaiian-style fissure 

eruptions typically start as a curtain of fire, which may 

extend discontinuously for several miles (Mabery et al. 

1999). Most fissure eruptions eventually become 

concentrated at a single vent or group of closely 

spaced vents (Mabery et al. 1999). 

• Flow breccia—rock composed of angular, broken 

fragments that is formed contemporaneously with the 

movement of a lava flow. The cooling crust becomes 

fragmented while the flow is in motion (Neuendorf et 

al. 2005). 

• Lava flow—a lateral, surficial outpouring of molten 

lava from a vent or a fissure. Also, the solidified body 

of rock that is so formed. 

• Scoria—vesicular, cindery material formed by the 

escape of gases before solidification.

• Spatter—an accumulation of initially very fluid 

material that usually becomes agglutinated (welded 

together) and coats the surface around a vent.

• Vent—an opening in Earth’s crust through which 

volcanic materials extrude.

Vents

The primary type of vent at El Malpais National 

Monument is a cinder cone, which is composed of 

basaltic ash, cinders, bombs, large angular blocks of 

scoria, and minor flows of glassy lava (Maxwell 1986). 

Cinder cones at the national monument range in height 

from 1 m (3 ft) to more than 170 m (560 ft); many are 1 

km (0.6 mi) or more in diameter. A few cones are 

symmetrical, but most have been breached on one side 

by lava flows and have a crescent shape. Breaches in the 

cones fed the lava flows. Examples of cinder cones are El 

Calderon (Qcc), Lost Woman Crater (part of unit Qct), 

Twin Craters (Qct), Bandera Crater (Qcb), Cerro 

Candelaria (Qc), Cerro Bandera (Qc), and Paxton 

Springs Crater (Qcp; fig. 15). 

The shape of a cinder cone is controlled by the wind, 

which directs the exploding, airborne cinders during an 

eruption. As such, shape is an indicator of paleowind 

direction. The resulting shape of a cone is often 

asymmetrical, making symmetrical cones like those at El 

Malpais distinctive. Cone morphology changes through 

time via erosion.

 

Figure 15. Paxton Springs Crater. Two eruption events separated by 

about 5,000 years emanated from the vent at Paxton Springs. The 

Paxton Springs South flow is 15,000 years old; the Paxton Springs 

North flow is 20,700 years old. Photograph by Larry Crumpler (New 

Mexico Museum of Natural History and Science).
ELMA Geologic Resources Inventory Report 21

The eruption duration of a cinder cone is relatively short, 

generally no more than 50 years (National Park Service 

2006b). Bedding (layers of volcanic rock), which builds 

up during the life of the cone, documents the progression 

of eruptions and cone evolution. 

Directly to the west of El Malpais National Monument, 

the Chain of Craters area displays an interesting feature 

of the Zuni–Bandera volcanic field; many of the cinder 

cones within the field are aligned (fig. 16). This alignment 

is a result of a zone of weakness in Earth’s crust, which 

allowed magma to rise to the surface. This type of linear 

alignment is observed in many Hawaiian-style volcanic 

fields, including that at Hawaii Volcanoes National Park 

(Thornberry-Ehrlich 2009). 

Shield volcanoes also occur in the vicinity of El Malpais 

National Monument. The characteristic feature of this 

type of vent is its shape, which, as the name implies, 

resembles a shield. The Hawaiian Islands are enormous 

shield volcanoes, and GRI reports have been produced

for six National Park System units in Hawaii 

(Thornberry-Ehrlich 2009, 2010, 2011a, 2011b, 2011c, 

2011d). These reports may be of interest to resource 

managers and interpreters at El Malpais National 

Monument. Shields form by progressive, very fluid lava 

on low-gradient slopes, building up almost entirely by 

lava flows, with little or no spatter or cinders. Cerro 

Rendija is a good example of a shield at El Malpais. 

Rootless vents, such as hornitos and spatter cones, also 

occur in the national monument. Some good examples of 

spatter cones occur along the trail to Bandera Crater and 

near the Big Tubes Trail. The term “rootless” is used to 

describe a volcanic feature that formed with no direct 

connection to the subterranean magma reservoir feeding 

the eruptive event (Mabery et al. 1999). “Hornito” is the 

diminutive of the Spanish word “horno,” meaning 

“oven.” This volcanic form resembles the old-fashioned, 

wood-fired ovens used in outdoor cooking. Spatter is 

ejected through a hole on the surface of the flow, 

forming mounds that generally range from 1.5 m (5 ft) to 

9 m (30 ft) high. Hornitos are common in a few places on 

the McCartys flow, (fig. 17) and typically formed over

Figure 16. Aerial image of El Malpais. Aerial imagery reveals many distinctive features on the El Malpais landscape, including the long 

McCartys flow that spans the eastern boundary of El Malpais National Monument. The lava flow that erupted from the vent has a low 

gradient, but is quite long. Some of the lava flowed southward for about 8 km (5 mi), but the majority flowed northward about 40 km (25 mi) 

before turning eastward and flowing for 10 km (6 mi). The vent for the flow was the McCartys cinder cone in the southern part of the 

national monument. Another distinctive feature in aerial view is Hole-in-the-Wall, which is a kipuka—a Hawaiian term that refers to an 

exposure of older rock not covered by a younger lava flow (also see fig. 21). This triangular-shaped kipuka is underlain by old basalt flows 

(Qb) and surrounded by younger flows. The McCartys flow (Qbm) forms the lower/east wall of the kipuka, the Bandera flows (Qbb) form the 

upper/north wall, and the Hoya de Cibola flows (Qbw) forms the left/west wall. Also apparent in aerial view is Chain of Craters west of El 

Malpais National Monument. The cinder cones are aligned along a zone of crustal weakness where magma flowed to the surface. The green 

outlines on the figure mark National Park System areas. Graphic by Rebecca Port (Colorado State University) and Philip Reiker (NPS Geologic 

Resources Division).
22 NPS Geologic Resources Division

cracks that extended to sufficient depths to tap the stillmolten lava beneath the solidified surface. The molten 

lava rose through the cracks and spewed spatter and 

small flows, building these oven-shaped mounds over 

short periods of time (Mabery et al. 1999). A spatter cone 

is formed by the ejection of primarily plastic bombs of 

lava that fuse together after impact into a small edifice. 

Spatter cones may form over a fissure or vent, or may be 

rootless. 

Figure 17. Hornito. As lava ejected through a hole in the Bandera 

flows, this hornito formed from lava spatter about 11,000 years ago. 

National Park Service photograph.

Lava Flows

Basaltic flows at El Malpais National Monument consist 

of pahoehoe and aa lava. Lava flows may contain both 

lava types as a result of a gradient change or later pulse of 

different viscosity lava. Pahoehoe, a Hawaiian term 

pronounced “pah-hoy-hoy,” produces ropey flow tops

(fig. 18), with multiple ropes occurring at different scales. 

Figure 18. Pahoehoe lava. Hot, fluid lava is responsible for the ropey 

texture of the surface of the McCartys flow. Photograph by Larry 

Crumpler (New Mexico Museum of Natural History and Science).

The texture is typical of low-viscosity, fluid flows. 

Pahoehoe flows can expand as more lava collects under a 

solidified crust through a process called inflation. The 

McCartys flow (map unit symbols Qbm and Qba) 

exemplifies the pahoehoe type (see “McCartys Flow”

section). Aa, pronounced “ah-ah,” is characterized by 

rough, jagged flow tops (fig. 19). Aa flows have higher 

viscosity than pahoehoe and move like bulldozers, 

creating blocky, rubbly, or clinkery deposits. Features of 

aa flows include rafting of lava blocks within flows.

Figure 19. Aa lava. Aa lava is much more viscous than pahoehoe, 

creating blocky, rubbly, or clinkery deposits. The Bandera flows are 

known for aa, although the surface of the McCartys flow is shown in 

this photograph. Lava flows may contain pahoehoe and aa due to a 

gradient change or a later pulse of different-viscosity lava. 

Photograph by Katie KellerLynn (Colorado State University).

With higher viscosity than pahoehoe, aa is commonly 

quite vesicular, with much evidence of trapped air 

bubbles in solidified form (fig. 20). The Bandera flows 

(Qbb) exemplify the aa type (see “Bandera Crater and 

Flows” section). 

Figure 20. Vesicular basalt. As gas bubbles escaped from the cooling 

McCartys flow, vesicular basalt formed. The small holes in the 

solidified lava are vesicles that once held volcanic gases. Photograph 

by Larry Crumpler (New Mexico Museum of Natural History and 

Science).
ELMA Geologic Resources Inventory Report 23

Maxwell (1986) divided the basalt flows of El Malpais 

into two informal groups: (1) the younger Malpais and 

(2) an older group. The older group is deeply weathered 

and largely covered by a more or less well-developed 

soil. Old basalt flows (Qb) locally are overlain by slightly 

younger flows (Qbu) and by pyroclastic debris (Qv) and 

cinder cones (Qc). Notably, old basalt flows (Qb) 

underlie the Hole-in-the-Wall area (fig. 16). The Holein-the-Wall is an example of a volcanic feature called a 

“kipuka,” a Hawaiian term that refers to an exposure of 

older rock not covered by an overlying lava flow. The 

younger flow forms walls, enclosing the kipuka (fig. 21). 

Figure 21. Kipuka. Lava surrounds an outcrop of sandstone, creating 

a kipuka in the McCartys flow. The outcrop is about 100 m (300 ft) 

long. Refer to figure 16 for location. Graphic by Philip Reiker (NPS 

Geologic Resources Division) utilizing ESRI Arc GIS-Bing Maps layer.

In the case of Hole-in-the-Wall, three flows surround the 

triangular-shaped kipuka—Bandera (Qbb) on the north, 

McCartys (Qbm) on the southeast, and Hoya de Cibola 

(Qbw) on the west (fig. 16). Hole-in-the-Wall is the 

largest kipuka at El Malpais National Monument, but 

other kipukas occur in the Hoya de Cibola (Qbw), 

Bandera (Qbb), and McCartys (Qbm) flows. Not all 

kipukas are underlain by old basalt flows (Qb).

The “younger Malpais” group of Maxwell (1986) 

includes the McCartys flow and cinder cone (Qbm and 

Qcm); Bandera flows, crater, and cinder field (Qbb, Qcb, 

and Qvb); Hoya de Cibola flows and shield volcano 

(Qbw and Qvw); Twin Craters flows, cones, and mixed 

pyroclastics (Qbt, Qct, and Qvt); and El Calderon flows, 

crater, and cinder field (Qbc, Qcc, and Qvc). 

McCartys Flow

Short of a trip to Hawaii, the McCartys lava flow is one of 

the best places in the United States to see young, basaltic, 

pahoehoe volcanism. Interested visitors can view the 

flow from their car windows on Interstate 40, up close at 

several places along Highway 117, and from the 

Sandstone Bluffs Overlook (see “Sandstone Features”

section). The McCartys flow overlies older basalts of the 

Zuni–Bandera volcanic field, as well as Holocene 

alluvium (Qal) (Laughlin et al. 1993a). Dunbar and 

Phillips (2004) estimated the cosmogenic chlorine-36 

(

36Cl) age of the flow to be 3,900 years, making it the 

youngest lava flow in New Mexico. 

The McCartys flow is beautifully preserved because of 

the arid conditions in the region. The flow follows 

topography, and although it has a low gradient, it is quite 

long. The source is a low shield volcano about 40 km (25 

mi) south of the intersection of Interstate 40 and 

Highway 117 (fig. 16). A small, 8-m- (26-ft-) high cinder 

cone sits on top of this broad shield. Although some lava 

flowed southward for about 8 km (5 mi), most followed 

the existing drainage and flowed northward about 40 km 

(25 mi) before turning eastward and flowing 10 km (6 mi) 

down the Rio San Jose Valley (Laughlin et al. 1993a). 

Maxwell (1986) mapped the portion of the McCartys 

flow (Qbm) within El Malpais National Monument; 

Maxwell (1977) mapped the portion of the McCartys 

flow (Qbd) in the McCartys quadrangle north of the 

national monument.

An indicator of the youthfulness of a flow is the presence 

of a glassy rind, which forms on basaltic flows as they 

move across Earth’s surface (Dunbar and Phillips 2004). 

The outer skin of the flow becomes quenched and forms 

glass (fig. 22 and inside front cover). Just-erupted basaltic 

flows, such as those in Hawaii, exhibit glassy rinds. The 

glassy part of the rind, which is very fragile compared 

with the rest of the flow, can be several centimeters thick 

on a fresh flow. The glassy rind is initially intact, but 

fractures develop in the glass during cooling. Because of 

its delicate and fractured nature, this outer rind may be 

stripped by mechanical weathering processes.

Figure 22. Crust of a basalt flow. The gray zone at the uppermost 

part of the vesicular basalt shown in this photograph represents the 

“chilled crust” of the McCartys flow. The crust of a lava flow initially 

exhibits a glassy rind, but weathering of the flow progressively 

strips away the glass over time. Some areas of the McCartys flow 

retain a glassy rind (see inside cover photograph). Photograph by 

Larry Crumpler (New Mexico Museum of Natural History and 

Science).

In the case of the McCartys flow, much of the glass has 

been stripped away in the last 3,900 years, but some rind 

remains. Dunbar and Phillips (2004) suggested that the 

glassy rind on flows is stripped progressively through 

time, rather than being completely removed shortly after 

flow emplacement.
24 NPS Geologic Resources Division

Other features of the McCartys flow include the 

following:

• Inflation features—areas of pahoehoe flows that have

swelled as a result of lava injection beneath the surface 

crust. Examples of inflation features occur along the 

Lava Falls Trail (Mabery et al. 1999), which crosses 2.4 

km (1.5 mi) of the McCartys flow. Classic inflation 

features include inflation pits and mounds (fig. 23); 

most mounds have distinctive cracks, which formed as 

the brittle crust cracked as additional lava inflated the 

mound from below. 

• Squeeze-ups and squeeze-outs—squeeze-ups form 

when the brittle crust of a lava flow is broken or 

cracked and the still-liquid lava is forced upward 

through the crack. Squeeze-outs form by the same 

process when the lava is squeezed laterally, rather than 

vertically. Squeeze-outs are present in the sides of 

inflation pits within the McCartys flow (Larry 

Crumpler, research curator, New Mexico Museum of 

Natural History and Science, telephone 

communication, 2 August 2012) (fig. 24). 

• Vesicular basalt—vesicular basalt contains many small 

cavities that formed by the expansion of gas during 

lava solidification (fig. 20). 

Figure 23. Inflation pits and mounds. Pits and mounds created 

through a process called inflation characterize the McCartys flow. As 

molten lava pushed up from beneath the hardened crust of the 

McCartys flow, inflation mounds formed, cracking the brittle 

(cooled) crust. The area in the figure is about 400 m (1200 ft) across. 

Refer to figure 16 for location. Graphic by Philip Reiker (NPS 

Geologic Resources Division) utilizing ESRI Arc GIS-Bing Maps layer.

Figure 24. Squeeze-out. Molten lava squeezed out of a crack in the 

walls surrounding an inflation pit on the McCartys flow. The orange 

field notebook rests on the squeeze-out in the photograph. 

Photograph by Larry Crumpler (New Mexico Museum of Natural 

History and Science).

Bandera Crater and Flows

The second youngest volcanic event at El Malpais 

yielded the Bandera flows (Qbb). Various isotopic age 

dates for the flows are in good agreement and cluster 

around 11,000 years: the carbon-14 (14C) method 

produced an age of 10,990 years before present 

(Laughlin et al. 1993b), the helium-3 (3

He) method 

produced an age of 10,400 years before present 

(Laughlin et al. 1993b), and the chlorine-36 (

36Cl) 

method produced an age of 11,200 years before present 

(Dunbar and Phillips 2004).

The Bandera flows (Qbb) originated from the Bandera 

Crater (Qcb), a cinder cone about 1 km (0.6 mi) in 

diameter and 150 m (490 ft) high (Laughlin et al. 1993a). 

The cone is adjacent to Highway 53. The crater is 

privately owned and open to the public. The Ice Caves 

Trading Company, which owns the crater, encourages 

visitors to “Go climb a volcano!” in a lunar-like 

landscape.

Bandera Crater is an ideal example of a breached cinder 

cone (fig. 11). The cone has a central depression 180 m 

(600 ft) below the rim and 80 m (260 ft) below the breach 

on the southwest side (Maxwell 1986). Aa and pahoehoe 

surfaces are common on the Bandera flows (Laughlin et 

al. 1993a). These flows originated at the breach in the 

south wall of the cinder cone, and extend 37 km (23 mi) 

from the vent (fig. 25). 

One of the most remarkable features of the Bandera 

flows is the lengthy, highly developed system of lava 

tubes (see “Lava Tubes” section). These tubes form the 

longest identified system in North America (Mabery et 

al. 1999). At least 27 km (17 mi) of tubes have been 

discovered and explored (Mabery et al. 1999).

Figure 25. Bandera flows. The Bandera Crater erupted about 11,000 

years ago, depositing extensive flows (Qbb) and a cinder field (Qvb). 

Refer to figure 16 for location. Graphic by Philip Reiker (NPS 

Geologic Resources Division) utilizing ESRI Arc GIS Bing Maps layer 

and GRI digital geologic data. 
ELMA Geologic Resources Inventory Report 25

Hoya de Cibola Flows

The exact age of the Hoya de Cibola flows (Qbw) is 

unknown, but their relative position indicates that they 

are older than the Bandera flows (Qbb) (Dunbar and 

Philips 2004). The source of the Hoya de Cibola flows is 

Cerro Hoya (Qvw), a large shield volcano about 2.4 km 

(1.5 mi) in diameter and 90 m (300 ft) high, located near 

the western edge of El Malpais National Monument. 

Flows traveled at least 53 km (33 mi) south, east, and 

probably northeast from the vent. The flows are covered 

by the Bandera and McCartys flows (Maxwell 1986). 

Mabery et al. (1999) estimated that the volume of the 

Hoya de Cibola flows was 7.5 km3 (1.8 mi3). A notable 

feature of the Hoya de Cibola flows is the lava-tube 

system, which includes Braided Cave (see “Lava Tubes” 

section).

Paxton Springs Flows

North of El Malpais National Monument in Zuni 

Canyon, Maxwell (1986) mapped the Paxton Springs 

flows (Qbp). The flow originated from a volcanic center 

(fig. 15) in the Precambrian core of the Zuni Mountains 

(Maxwell 1986). The cone is near the historic settlement 

of Paxton Springs, which is about 6 km (4 mi) northnortheast of Bandera Crater (Mabery et al. 1999). 

According to Maxwell (1986), part of the lava flowed 3 

km (2 mi) down Agua Fria Creek, and another part 

flowed northeast through Zuni Canyon for 27 km (17 mi) 

to the vicinity of Grants (Maxwell 1986). 

Since the time of Maxwell’s mapping, investigators have 

dated these two segments and determined that they 

actually represent two events, separated in time by about 

5,000 years. Lava that flowed south comprises the 

younger (cosmogenic chlorine-36 dating, about 15,000 

years old) Paxton Springs South flow, and lava that 

flowed north of the cinder cone forms the older (20,700 

years old) Paxton Springs North flow (Dunbar and 

Phillips 2004). These flows are separated in time by the 

Twin Craters (Qbt) flow event, which occurred 18,000 

years ago (Dunbar and Phillips 2004) (see “Twin Craters 

Flows” section). Morphologically, all three of these flows 

appear similar in age; they lack glass on their surfaces and 

have lost much of the ropey tops of the original 

pahoehoe texture (Dunbar and Phillips 2004). 

Zuni Flows

The Zuni flows erupted in the Zuni Mountains, north of 

the “main” El Malpais flows. Maxwell (1986) mapped 

these flows as unit Qbz; Maxwell (1977) mapped the 

Zuni flows in the McCartys quadrangle as unit Qba. The 

Zuni flows originated from a volcanic vent about 1.7 km 

(1 mi) north of the El Malpais National Monument 

boundary (Maxwell 1986). These flows have not been 

dated using absolute methods. However, Laughlin and 

WoldeGabriel (1997) placed them in a position younger 

than the Oso Ridge flows (Qbo) in a relative chronology 

of Zuni–Bandera lava flows. 

Oso Ridge Flows

Like the Zuni flows (Qbz and Qba), the Oso Ridge flows 

(Qbo) erupted in the Zuni Mountains, although not as 

far north. These flows originated from a lava cone (Qvo) 

on Oso Ridge about 5 km (3 mi) northwest of Bandera 

Crater (Qcb). The lava cone is composed largely of 

breccia, welded pyroclastic debris, and glassy flows 

(Maxwell 1986). Basalt from the Oso Ridge lava cone 

flowed eastward to Agua Fria Creek and southward 

almost to the Bandera cinder field (Qvb). A small portion 

of the flow is within the national monument. The exact 

age of these flows is not known, but Maxwell (1986) 

mapped them as younger than the Twin Craters flows 

(Qbt), which are now known to be about 18,000 years old 

(Dunbar and Phillips 2004).

Twin Craters Flows

The Twin Craters flows (Qbt) are composed of several 

overlapping flows from different vents, which Maxwell 

(1986) described as “difficult or impractical to separate.” 

This map unit covers many prominent landmarks 

between Bandera Crater (Qcb) and El Calderon (Qcc), 

including La Tetra, Twin Craters, and Lost Woman. 

Maxwell (1986) mapped these three cinder cones (Qct) 

and surrounding mixed pyroclastics (Qvt) as part of this 

unit. The map of Dunbar and Phillips (2004) identified 

these flows as “flows from La Tetra area,” with the map 

symbol Qt. Cascadden et al. (1997b) included Cerro 

Candelaria and Lava Crater, which are on private lands 

and closed to the public, within this group. Cerro 

Candelaria is 0.6 km (0.4 mi) east of Lava Crater, which is 

labeled on the park map (fig. 1).

Flows from these vents traveled at least 26 km (16 mi) 

east and northeast. Mabery et al. (1999) estimated that 

the total volume of the eruptions was 1.0 km3 (0.25 mi3

). 

The similarity of the flows—for example, in abundance 

and size of olivine phenocrysts within the basalts—made 

distinguishing relative ages of the flows challenging for 

early workers (Mabery et al. 1999). However, since initial 

mapping, Cascadden et al. (1997b) analyzed 

paleomagnetic variations in the flows and mapped the 

geology around these cones in considerable detail, 

thereby discovering the relationships in this “family of 

volcanoes.” Approximately 18,000 years ago (Dunbar 

and Phillips 2004), each of these cones erupted within a 

few hundred years of one another: Cerro Candelaria 

erupted first, followed by Twin Craters, Lost Woman 

Crater, and Lava Crater (Cascadden et al. 1997b). The 

entire eruption event lasted less than 1,000 years 

(Cascadden et al. 1997b).

El Calderon Flows

The El Calderon flows (Qbc)—referred to as the “Laguna 

flow” by some investigators (e.g., Dunbar and Phillips 

2004)—issued from a volcano (Qcc) east of the Twin 

Craters cluster of volcanoes. Following Maxwell (1986) 

and the suggestion provided in Crumpler et al. (2003), 

this report uses “El Calderon” rather than “Laguna” 

because the former is more descriptive and avoids 

confusion with a lava flow located near Laguna, New 

Mexico, farther east along Interstate 40 (Crumpler et al. 

2003). 
26 NPS Geologic Resources Division

Although the El Calderon volcano was once thought to 

have erupted about 115,000 years ago, more recent 

dating assigns it an age of about 54,000 years (Cascadden 

et al. 1997b; Laughlin and WoldeGabriel 1997). Dunbar 

(2010) suggested that El Calderon may have erupted 

between 40,000 and 30,000 years ago, but certainly less 

than 60,000 years ago. 

Figure 26. El Calderon. The El Calderon area contains the most 

developed trail in El Malpais National Monument. A 5-km (3-mi) 

loop guides hikers past lava flows and the El Calderon volcano. 

National Park Service photograph.

At the time of eruption, fire fountains would have issued 

high into the air from the center of the cone, raining 

down cinders in layers that built up the conical shape

(fig. 26). Original bedding (layers of cinders) is visible

where erosion has cut into the cone’s side (Dunbar 

2010). In places, the El Calderon flows (Qbc) are covered 

by the Twin Craters (Qbt) and Bandera (Qbb) flows and 

appear small and fragmented in map view. However, 

investigators traced the El Calderon flows (Qbc) from 

the crater (Qcc) to a termination point east of the 

intersection of Highway 117 and Interstate 40—a 

distance of about 35 km (22 mi) (Mabery et al. 1999). 

Investigators estimated the volume of the El Calderon 

flows to be at least 4 km3 (1 mi3

) (Mabery et al. 1999). 

Lava Tubes

The third main category of volcanic feature at El Malpais 

National Monument is lava tubes. The term “lava tube” 

has a dual meaning. First, lava tubes are conduits for 

molten lava flowing from a vent to a depositional site, 

generally the advancing front of a lava flow. Second, the 

term refers to the cavernous segment of the lava conduit 

that remains after the lava has ceased to flow (Larson 

1990). These tubular cavities are also referred to as “lavatube caves” (Rogers and Mosch 1997a). 

Lava-tube systems are the principal means by which 

pahoehoe flows spread widely and thinly (Larson 1990). 

Most lava tubes at El Malpais represent the major 

conduits of lava along the length of a lava flow. Smaller 

distributary tubes diverged downslope from these master 

tubes and dispersed the lava along a broad front (Rogers 

and Mosch 1997a). However, these smaller tubes are 

seldom preserved at El Malpais National Monument 

because of low surface gradients, relatively short 

eruption lengths, and low discharge rates that allowed 

flowing lava to slow and congeal within the tubes, 

thereby blocking them (Rogers and Mosch 1997a). 

El Malpais National Monument contains 15 major lava 

tubes (Marinakis 1997). Laid end-to-end, they would 

stretch more than 100 km (60 mi) (Marinakis 1997). The 

most extensive lava tube at El Malpais—and, if it were all 

still intact, the longest identified system in North 

America (Mabery et al. 1999)—is the Bandera lava tube 

(Marinakis 1997). This lava tube occurs in the Bandera 

flows (Qbb), which are 37 km (23 mi) long and contain 

28 km (17 mi) of identifiable tube, most of which is now 

collapsed trench (Rogers 1991a). The Bandera lava tube 

begins at the breach in the cinder cone (fig. 11), and its

first 800-m (0.5-mi) segment is totally collapsed 

(Maxwell 1986). The Bandera flows contain dozens of 

individual caves ranging from 300 to 400 m (980 to 1,310

ft) in length and systems with lengths exceeding 1 km 

(0.6 mi) (Rogers 1991a, 1991b). Many tubes also contain 

natural bridges, which can be 50 m (164 ft) long (Rogers 

1991a). Notable examples of bridges occur in the Big 

Tubes area of the national monument and include 

Natural Bridge, bridges in the Seven Bridges collapse 

trench, and bridges near Four Windows Cave (fig. 27).

Figure 27. Lava bridge. Lava bridges form between sections of 

collapsed lava tubes. The bridge shown in the photograph is located 

in the Big Tubes area of El Malpais National Monument. Note person 

for scale. National Park Service photograph.

Collapse Features

Among the common features in lava tubes at El Malpais 

National Monument are abundant piles of debris and 

rubble called breakdown (fig. 28). This material is 

indicative of collapse that occurred when the tube 

cooled and contracted. For instance, the floor of Big 

Skylight Cave is littered with rubble that was once part of 

the ceiling. Caterpillar Collapse and Seven Bridges were 

once lava tubes, but their roofs have subsequently 

collapsed. The roof of Caterpillar Collapse crumbled 

slowly and completely, leaving the long, winding trench 

seen today (National Park Service 2003a).
ELMA Geologic Resources Inventory Report 27

Figure 28. Breakdown. The floor of the entrance to Junction Cave is 

covered in breakdown. U.S. Geological Survey photograph, available 

at http://3dparks.wr.usgs.gov/elma/html2/elma0074.htm (accessed 4 

May 2012). 

Blocks broken from the ceiling or walls of a lava tube 

may be carried along with the moving lava flow. They 

may be partly melted and moved by rafting on top of the 

flow or by rolling within it. Blocks that are too large to 

melt are carried by the tremendous viscous drag of the 

moving lava. As part of the flow, blocks may become 

partially melted, abraded, rounded, and sometimes 

coated with lava; such transformed blocks are known as 

“lava balls.” In several caves, such as Xenolith and 

Junction caves, blocks became jammed between the floor 

and ceiling and cemented in place by the still-flowing 

lava, nearly closing off the lava-tube passage (Rogers and 

Mosch 1997a).

Figure 29. Skylight on the McCartys flow. A skylight on the McCartys 

flow allows sunlight into the lava-tube cave below. Skylights are 

important during eruption because they allow atmospheric gases to 

mix with superheated gases in the lava tube and combust, which 

may assist with remelting of the basalt lining, thereby creating 

distinctive cave features. Photograph by Larry Crumpler (New 

Mexico Museum of Natural History and Science).

When the extent of collapse is large, breakdown may 

constrict or completely block the tube. Vertically 

extensive collapse may break through the ground surface 

to form a skylight (fig. 29). Skylights are important at the 

time of eruption because they allow atmospheric gases to 

mix with the superheated tube gases and combust. 

Rogers and Mosch (1997a) speculated that this burning 

of hydrogen-rich gas may heat the interior of the tube 

and thereby assist with remelting of the basalt lining it 

(see “Cave Features” section). Subsequent lava flows can 

enter the tubes through these skylights, occasionally 

filling them. 

Cave Features

As lava tubes form, the gases within them can be hotter 

than the lava coursing over the floor or forming the walls 

(Rogers and Mosch 1997b). For example, gas 

temperatures of 1,150°C–1,555°C (2,102°F–2,831°F) have 

been measured in the tubes of Hawaii (Swanson 1973). 

Interior surfaces melt under these conditions, forming

cave features that are often compared to the well-known 

speleothems (mineral deposits) found in limestone caves 

(Rogers and Mosch 1997b). In contrast to limestone 

caves, where speleothems are most commonly composed 

of calcite (CaCO3), the El Malpais lava-tube caves 

contain features that are most commonly formed of 

basalt, and are thus some shade of black (Rogers and 

Mosch 1997b). Furthermore, whereas true speleothems

form by the action of water and thus grow rather slowly 

in limestone caves, lava speleothems grow rapidly by the 

flowing, dripping, splashing, accreting, and pulling apart 

of molten or partially consolidated lava, and by the 

interaction of associated gases in the lava tube (Hill and 

Forti 1997).

Lava flows and melting lava created the following 

features in the lava-tube caves at El Malpais National 

Monument: 

• Draperies and ribs—thin deposits of basalt that 

developed where trails of lava trickled down the walls 

of a lava tube. These features are fairly common in the 

lava tubes of El Malpais. They often have white or 

ochre-colored coatings of other minerals that

precipitated much later in the history of the cave

(Rogers and Mosch 1997b). 

• Floors—where not obscured by breakdown, the floors 

of lava tubes often show features indicative of flow 

history. When all lava flowing within a tube merely 

stopped moving and quickly cooled, a relatively 

smooth floor with ropey pahoehoe textures formed. 

However, changes in viscosity and the rate of shear 

strain can cause tube-floor textures characteristic of

aa. The centers of floors are often textured with ropey 

coils of pahoehoe, while the edges of the flow are 

clinkered fragments of aa. Gradations between these 

extremes, including cauliflower lava, are usually well 

displayed wherever floors are preserved and not 

covered by breakdown (Rogers and Mosch 1997a).

• Flowstone—masses of solidified lava that mimic calcite 

flowstone found in limestone caves. These features 

were named for their resemblance to a frozen waterfall 

or river. 

• Helictites—deposits of lava that twist and turn in 

gravity-defying contortions. Lava helictites are 

sparingly found in lava tubes with intact ceiling or wall 

linings. Their stems are usually 0.2–0.5 cm (0.08–0.2 in) 
28 NPS Geologic Resources Division

in diameter. Some have beaded surfaces and may be 

partially hollow near their tips. 

• Linings—relatively thin shells of lava plastered against 

the inside surfaces of a lava tube. Linings form in active 

tubes as an additional layer of lava or a melted crust 

(Rogers and Mosch 1997a). Where linings do not 

reach the ceiling of the lava tube, their upper surfaces 

may mark the upper limits of secondary flows. In cross 

section, multiple linings may look like layers of a cut 

onion. 

Where the entire wall has melted to a depth of about 1

cm (0.4 in), the lining may slump or partially sag down 

the wall. Should this sagged lining fail along a discrete 

crack, causing the lower panel to sag farther, the lower 

panel would stretch the lava apart, leaving a line of 

taffy-like strings and threads of basalt in the opening 

that often resembles whale baleen plates (Rogers and 

Mosch 1997a).

Occasionally, gas pressure may build up behind 

patches of wall lining. When the pressure exceeds the 

strength of the still-plastic lining, a patch may literally 

explode from the wall. These ruptures have out-turned 

edges and highly vesicular interiors. 

When the level of lava in a tube drops quickly, highly 

plastic linings may curl or roll down toward the floor. 

In some cases, they fold back and collapse on 

themselves. Big Skylight Cave has excellent examples 

of these features. 

• Shelves—horizontal banks of basalt along the walls of a 

lava tube. These interior forms may also be referred to 

as “benches” when particularly wide or as “levees” if 

they confined the flowing lava to a portion of the floor. 

Shelves are left by lava that coursed through the tubes, 

and mark a period of time when lava discharge 

remained relatively constant. Generally, wider shelves

indicate a longer period of lava flow at that level. 

• Stalactites—basalt deposits hanging down from the 

ceiling of a lava tube. Lava stalactites are commonly 

called “lavacicles” because of their resemblance to 

icicles. Lava stalactites can be conical, bulbous, 

cylindrical, or tear-drop shaped (Hill and Forti 1997). 

Some lava stalactites at El Malpais National 

Monument resemble shark teeth, and others resemble 

soda straws. Rogers and Mosch (1997b) noted globular 

lavacicles that look like columns of small grapes, and 

others that resemble golf tees. 

• Stalagmites—deposits of basalt that build upward from 

the cave floor. Lava stalagmites form where spots on 

the ceiling melt and dribble blobs of lava below. They 

are much less numerous than lavacicles because 

dripping lava is often incorporated into the fluid floor 

or swept away from the point of origin by floor 

movement. Most stalagmites in El Malpais National 

Monument look like small-diameter columns of grapes 

(Rogers and Mosch 1997b). These stalagmites are 

commonly found along the edges of passages below 

cracks in linings. At other places, however, lava 

dripped through a central crack to spawn a series of 

stalagmites in the center of the passage. When a single 

lavacicle supplied lava to a crusted, slow-moving floor, 

a trail of small stalagmites may have formed along the 

floor (Rogers and Mosch 1997b).

Xenoliths

Some lava tubes in El Malpais National Monument, such 

as Junction Cave, Lava Crater, and of course Xenolith 

Cave, contain xenoliths. Xenoliths are inclusions in 

volcanic rocks that are commonly longer than 0.3 m (1.0 

ft) and differ distinctly from their host rocks (Mabery et 

al. 1999). Xenoliths may have come from the sides of a 

magma chamber or the walls of an erupting lava conduit, 

and, thus provide clues about volcanic processes and

information about rocks in the local crust or mantle. 

According to Mabery et al. (1999), lava flows and latestage cinders from Bandera Crater contain xenoliths that 

originated in Earth’s mantle. They are often found in the 

cinder pits at this location. Mantle-derived xenoliths 

were brought to the surface by fast-rising magma and 

encased as the lava cooled (National Park Service 2003a). 

These xenoliths provide rare opportunities for first-hand 

study of Earth’s mantle, which is generally inaccessible 

because it lies 100–200 km (60–120 mi) below Earth’s 

surface (Robertson 2011).

By contrast, some xenoliths may have originated at the 

surface as blocks of rock from bluffs and narrows that 

fell onto lava flows (National Park Service 2006b). 

According to Laughlin et al. (1971), xenoliths of 

sandstone, limestone, granite, gabbro, and granodiorite 

are present in some lava flows at El Malpais National 

Monument. These xenoliths provide clues about the 

bedrock under the lava flows and help to determine the 

erosive history of the valleys (National Park Service 

2006b). 

Cave Minerals

Using x-ray diffraction, Rogers and Mosch (1997b) 

identified numerous minerals with various sources in the 

lava tubes at El Malpais National Monument (table 2). 

Gypsum, epsomite, mirabilite, thenardite, calcite, trona, 

and burkeite appear to have drawn carbonate and sulfate 

from windblown dust derived from the weathering of 

sedimentary rocks in the area. The silica of cristobalite 

appears to have been leached from unstable pumice and 

glassy ash of the volcanic terrain (Rogers 1991b). 

Xenoliths within the lava flows provided raw materials 

for glass, opal, and malachite. These minerals form a 

variety of speleothems, including hairs, moonmilk, and 

coralloids (Marinakis 1997). Hairs are composed of 

single crystal fibers and resemble thin strands of hair 

(Hill and Forti 1997). Coralloids are nodular, globular, or 

coral-like in shape and form from films of water (Hill and 

Forti 1997). Moonmilk consists of white, finely 

crystalline material that feels like powder when dry and 

cream cheese when moist (Hill and Forti 1997).
ELMA Geologic Resources Inventory Report 29

Table 2. Cave minerals

Name Chemical Formula Speleothem Type Occurrence

Amorphous silica glass SiO2 Bubbly, green, glass coatings on 

xenoliths

Rare (known only in 

Xenolith Cave)

Burkeite Na6(CO3)(SO4)2 Crusts Rare (known only in 

Outlaw Cave)

Calcite CaCO3 Flowstone, draperies, coralloids, 

and moonmilk

Common

α-Cristobalite SiO2 Coralloids Uncommon

Epsomite MgSO4 • 7H2O Hairs and crystals Rare 

Glaserite (K, Na)Na(SO4)2 Constituent of mirabilite and 

thenardite “snowballs”

Rare (known only in 

Braided Cave)

Gypsum CaSO4 • 2H2O Sugary crusts, stalactites, 

stalagmites, columns, 

flowstone, hairs, and powder

Common

Malachite Cu2(CO3)(OH)2 Green stains in xenoliths Rare 

Mirabilite Na2SO4 • 10H2O Hairs Rare 

Opal-A SiO2 • nH2O Coralloids (e.g., “cave coral”) Common

Opal-CT SiO2 • nH2O Crystals on xenoliths Rare (known only in 

Xenolith Cave)

Thenardite Na2SO Powder and coralloids (e.g., 

“snowballs”)

Rare 

Trona Na3(CO3)(HCO3) • 2H2O Crusts Rare (known only in 

Outlaw Cave)

Sources: Rogers (1991b) and Rogers and Mosch (1997b).

Ice Caves

Lava tubes that contain ice accumulations are called “ice 

caves.” These features are abundant in El Malpais 

National Monument (Marinakis 1997; National Park 

Service 2006b). Ice forms when percolating rainwater 

and melting snow enter the freezing zone of a cave. 

Although the amount of ice varies from year to year, 

large amounts of ice accumulate seasonally, beginning in 

spring and persisting into early–middle summer (Rogers 

and Mosch 1997b). Much of the ice melts by later 

summer. Minimal amounts of ice form during the early 

winter because lower temperatures freeze the 

groundwater and the winter air contains very little 

moisture (Rogers and Mosch 1997b). Although most ice 

in caves at El Malpais is clear to white, part of the ice 

floor in Ice Cave (also called Candelaria Ice Cave)—the 

large, privately owned cave in the Bandera lava flow—is 

pale green. Algae apparently flourish in this part of the 

ice floor, which receives direct sunlight (Rogers and 

Mosch 1997b).

Marinakis (1997) surveyed four caves in El Malpais 

National Monument with massive perennial ice deposits, 

three caves with significant perennial ice accumulations, 

and dozens of other caves with smaller amounts of 

seasonal and perennial ice. Seasonal ice formations in 

caves consist of icicles and ice stalagmites, which usually 

reach their maximum size during March or April. 

Perennial accumulations of ice may contain ice 

flowstone, frozen ponds, ice needles, and ice crystals

(figs. 30 and 31). 

Ice accumulations in the ice caves are culturally and 

scientifically significant. For example, in the 1860s, 

soldiers from Fort Wingate—established on the site of 

what is now the town of San Rafael—used to visit Ice 

Cave and haul ice back to the fort (Laughlin et al. 1993a). 

The ice accumulations have scientific value for the study 

of paleoclimate. Thompson et al. (1991) extracted three 

ice cores from Ice and La Marchinita caves, which they 

dated and analyzed for pollen, microparticle 

concentrations, oxygen isotope ratios, and nitrate

(NO3-), chlorine (Cl-

), sulfate (SO4

2

), and phosphate 

(PO4) contents. Results indicated that the ice laminations 

are not annual, but represent occasional layers between 

periods of melt. Thompson et al. (1991) obtained 

radiocarbon dates for the older ice deposits using 

accelerator mass spectrometry. At Ice Cave, a twig 

enclosed near the bottom of a 4-m- (13-ft-) thick bank of 

laminated ice was 3,166 ± 77 years old (Thompson et al. 

1991).

Figure 30. Ice crystals. Ice accumulates seasonally in the lava tubes at 

El Malpais National Monument. Seasonal ice features include icicles, 

ice stalagmites, and ice crystals (shown in this photograph). National 

Park Service photograph.
30 NPS Geologic Resources Division

Figure 31. Perennial ice. Perennial ice accumulations produce ice 

flowstone, frozen ponds, and frozen floors. Some ice accumulations 

at El Malpais have persisted for more than 3,000 years. This 

photograph was taken in the Red Room of Crystal Ice Cave at Lava 

Beds National Monument, where similar ice accumulations occur. 

National Park Service photograph.

Eolian Features and Processes

Maxwell (1986) included eolian deposits (Qe) within El 

Malpais National Monument, and described these 

deposits as recent accumulations of windblown silt and 

sand. Maxwell (1986) only mapped the most prominent 

accumulations, which occur in the form of sand sheets, 

dune fields, longitudinal dunes, and smaller deposits. 

These landforms appear on aerial images (National Park 

Service 2006b). 

At present, the friable Zuni Sandstone (Jz) supplies 

sediment for eolian transport. The North Pasture 

Ridges—a significant Cretaceous outcrop of Zuni 

Sandstone south of the Narrows Picnic Area—is a source 

area. Although usually made of basalt, some kipukas are 

made of sandstone (fig. 21), which serves as another 

source of eolian sediment (National Park Service 2006b). 

During the Middle Jurassic Period (175 million–161 

million years ago), the Zuni and Entrada sandstones were 

part of a vast desert where sand was piled into dunes and 

spread into sheets. The crossbeds and flat-lying beds of 

the sandstone are indicative of eolian deposition (Lucas 

2010). At nearby El Morro National Monument, the 

Zuni Sandstone is stratigraphically equivalent to the 

Entrada Sandstone and the overlying Bluff (Cow Springs) 

Sandstone to the northeast (Lucas et al. 2003). Wellknown Delicate Arch and the other arches in Arches

National Park in Utah are made of Entrada Sandstone

(Graham 2004). Notably, La Ventana Natural Arch 

formed in Zuni Sandstone (see “Sandstone Features”

section).

Eolian processes are significant for landscape evolution 

at El Malpais National Monument. They provide 

windblown dust, called loess, which fills in voids 

(vesicles and cracks) in the lava flows. Older flows 

generally exhibit more eolian “infilling,” which smooths

out a lava flow’s surface over time. As a result, many 

older flows, such as the El Calderon flow in the Cave 

Junction area, are virtually flat (Dunbar 2010). By 

contrast, a clean, silt-free surface like that of the 

McCartys flow indicates that a flow is younger. In a flow 

of moderate age, some larger depressions are filled with 

silt. 

Infilling of voids with loess is the first stage of soil 

development on the lava flows. As a lava flow fills in and 

attains a smooth surface, vegetation augments 

sedimentation by influencing moisture content and 

trapping sediment. Bauman (1999) studied the influences 

of climate, sediment source, and surface cover types on 

soil formation within the Carrizozo lava flow in central 

New Mexico. The study found that soils on the flows 

were of eolian origin and not the products of basalt 

weathering. These findings may be applicable to the lava 

flows at El Malpais National Monument.

Biologic and Geologic Connections

With a name meaning “The Badlands,” El Malpais may 

seem an unlikely spot for biological diversity. Yet, the 

national monument hosts a variety of habitats that 

support a large number of plant and animal species, 

including some of the oldest trees in the American 

Southwest. Geologic features create these special 

habitats (Lightfoot 1997).

Lava-Flow Habitat

The McCartys (Qbm), Bandera (Qbb), Hoya de Cibola 

(Qbw), Twin Craters (Qbt), and El Calderon (Qbc) flows 

are the principal geologic features that define the lavaflow habitat at El Malpais National Monument 

(Lightfoot 1997). These flows constitute a “mesic island”

(an area that is moister than the land around it) 

surrounded on all but the northwest side by drier 

habitats (Bleakly 1997). Investigators have hypothesized 

that the porous nature of the lava acts as a reservoir that 

traps and holds moisture from winter snowmelt and 

summer rainfall (Lindsey 1951; Grissino-Mayer and 

Swetnam 1997). This water-holding capacity allows the 

lava to support tree species such as aspen (Populus 

tremuloides), Douglas-fir (Pseudotsuga menziesii 

menziesii), and ponderosa pine (Pinus ponderosa) in a 

region that would otherwise be inhospitable. 

The lava-flow habitat varies across an elevation gradient. 

Temperature and moisture conditions vary across this 

gradient from cooler and wetter on the upper-elevation 

lava flows to warmer and drier on the lower-elevation 

flows (Lightfoot 1997). The northwestern part of the 

national monument is highest, with elevations of up to 

2,438 m (8,000 ft), and is contiguous with the Zuni 

Mountains, which are densely covered with conifers 

(Bleakly 1997). Mixed-conifer woodlands—composed 

primarily of ponderosa pine (Pinus ponderosa) with 

lesser numbers of Douglas-fir (Pseudotsuga menziesii), 

Rocky Mountain juniper (Juniperus scopulorum), and 

piñon pine (Pinus edulis)—blanket most of the four older 

flows (El Calderon, Twin Craters, Hoya de Cibola, and 

Bandera) and adjacent areas to the northwest. The fifth 

and youngest flow, the McCartys flow, is barren or 

sparsely vegetated with shrubs, grasses, and stunted 

conifers. The stunted trees, referred to as the “Pygmy 

Forest,” are of interest to visitors and are highlighted in 
ELMA Geologic Resources Inventory Report 31

the Lava Falls area trail guide (National Park Service 

2009). These trees occur on the eastern part of the 

McCartys flow, where apparently they are able to find 

enough water to survive, but not enough to reach the 

loftier heights of trees on the flows to the northwest. 

Other species are also affected by the elevation gradient: 

lichen cover is much greater on lava at higher elevations 

in the Bandera Crater area than at lower elevations on 

the McCartys flow. 

The lava flows at El Malpais National Monument host 

some incredibly old, still-living specimens of Douglas-fir, 

including a tree referred to as the “Yoda Tree” by 

national monument staff (Dana Sullivan, chief ranger, 

and Dave Hays, resource manager, El Malpais NM and 

El Morro NM, conference call, 9 December 2011). 

According to the New Mexico Bureau of Mines and 

Mineral Resources (1993), this tree’s first year of life was 

719 CE (Common Era; preferred to “AD”), making it 

1,293 years old in 2012. The lava also preserves some 

very old dead trees and pieces of wood that would have 

decayed under less-arid conditions (New Mexico Bureau 

of Mines and Mineral Resources 1993). The “Bannister 

Tree,” named in honor of eminent dendrochronologist 

Dr. Bryant Bannister, has an inner tree ring dated at 200 

BCE (Before Common Era; preferred to “BC”) and an 

outer ring dated at 550 CE (Lightfoot 1997). Piecing 

together samples from ancient, living trees to tree-ring 

patterns in dead trees enabled the development of a 

chronology used to help decipher past climate and fire 

histories (New Mexico Bureau of Mines and Mineral 

Resources 1993). 

The lava flows also affect animal species. Melantropic 

lizards that live on the lava are darker in color than are

those that live off the flows (National Park Service 

2006b). However, melanism apparently has not 

produced any distinct subspecies at El Malpais (Hooper 

1941; Lightfoot 1997). Furthermore, certain endemic 

species of invertebrates are found only on lava flows or 

in the entrances to lava-tube caves at El Malpais 

(Lightfoot 1997). In the early 1990s, an extensive 

invertebrate inventory resulted in the discovery of 17 

new species and one new genus of arthropods. Most of 

these species appear to be endemic (Lightfoot et al. 

1994). 

Kipukas

Hole-in-the-Wall is a 2,710-ha (6,700-ac) kipuka that 

consists of ponderosa pine parklands and grassland 

surrounded by lava (fig. 16). Because of their 

inaccessibility, several kipukas within El Malpais 

National Monument were never or only slightly 

disturbed by grazing or logging. Hence, natural 

vegetative assemblages remain intact in these areas 

(National Park Service 2006b). Kipukas host remnants of 

vegetation that were more widespread prior to 

disturbance. For example, Hidden Kipuka marks the 

southernmost extent of alligator juniper (Juniperus 

deppeana). 

Because the “kipuka pines” have a fire regime similar to

those of surrounding forests from which embers can

travel and ignite, these pines have not attained the old 

ages of the Douglas-firs that grow on the Bandera flows 

(Dave Hays, resource manager, El Malpais NM and El 

Morro NM, e-mail communication, 16 December 2011). 

Nevertheless, kipukas shelter some older ponderosa 

pines because these areas were not logged. 

Cinder-Cone Habitats

Steep-sided cinder cones are common in the western 

part of El Malpais National Monument (Lightfoot 1997). 

The loose cinders provide good places for seeds to take 

root. The cinders may also retain water deep below the 

surface, providing moisture for trees, plants, and 

wildflowers. Several plants in the national monument, 

such as bracken ferns (Pteridium aquilinum pubescens), 

cinders phacelia (Phacelia serrata), and limber pine 

(Pinus flexilis), grow only on cinders (National Park 

Service 2004a).

Lava-Tube Habitats

The sunlight that pours through skylights, for instance at 

Big Skylight and Four Windows caves, provides an 

opportunity for life to thrive where it otherwise might 

not. Spiders, mites, crickets, and other small creatures 

live beneath these sunlit openings in gardens of delicate 

green moss (National Park Service 2003a), which 

Marinakis (1997, p. 162) called “luxuriant moss gardens.” 

Flora and fauna flourish in the cool cave air beneath 

most skylights. Unique assemblages of lichens and 

mosses grow at the mouths of caves. The cave 

microclimate is similar to Arctic climates, where these 

plants typically grow. Ferns similar to those that grow in 

the Pacific Northwest also grow at the mouths of caves 

and in collapse structures (Bleakly 1997).

Various types of organisms inhabit El Malpais caves, 

including those that have adapted to life in complete 

darkness, those that inhabit gypsum and calcium 

deposits, and those that inhabit ice accumulations 

(National Park Service 2003a) (see “Cave Minerals” and 

“Ice Caves” sections). Fauna include microorganisms 

that form “cave crusts” and “lava wall slime,” as 

described in Northup and Welbourn (1997). Northup 

and Welbourn (1997) provided a list of vertebrates and 

invertebrates species from six caves (Bat, Big Skylight, 

Braided, Four Windows, Junction, and Navajo caves) in 

El Malpais National Monument; this inventory identified 

62 species. Lightfoot (1997) described several new 

species of crickets from some of the national 

monument’s more remote caves. In addition, Mexican 

free-tailed bats (Tadarida brasiliensis) inhabit the lavatube caves. Although bighorn sheep (Ovis canadensis) no 

longer roam the area, their skeletal remains are 

commonly found in the caves (Marinakis 1997) (see 

“Paleontological Resources” section). 

Sandstone and Limestone Habitats

Sandstone and limestone ridges consist of rocky slopes, 

as well as cliffs and bluffs, with or without vegetation. 

Sandstone and limestone escarpments are found in the 

north-central area of El Malpais National Monument, 

such as Cerritos de Jaspe and Oak Ridge, and along the 
32 NPS Geologic Resources Division

east side of the national monument, including the 

Sandstone Bluffs area (Lightfoot 1997).

Some birds are associated specifically with rock habitats, 

regardless of surrounding vegetation (Lightfoot 1997). 

Such species include the band-tailed pigeon (Columba 

fasciata), rock dove (Columba livia), white-throated swift 

(Aeronautes saxatalis), cordilleran flycatcher (Empidonax 

occidentalis), cliff swallow (Hirundo pyrrhonota), rock 

wren (Salpinctes obsoletus), and canyon wren (Catherpes 

mexicanus). Of these, the cordilleran flycatcher is largely 

associated with lava flows and the cliff swallow with 

sandstone. The other birds are found on both lava and 

sandstone (Lightfoot 1997).

Tinajas and Playas

Natural potholes called tinajas occur in the sandstone

rock units at the national monument (fig. 32), and 

smaller ones occur on the lava flows. These features are 

common in the sandstone bluffs on the east side of the 

national monument, Hidden Kipuka, Cerritos de Jaspe, 

and Oak Ridge (Lightfoot 1997). Although ephemeral, 

tinajas are a significant source of surface water in the 

national monument (National Park Service 2006b). One 

tinaja, Laguna Juan Garcia, may be fed by a natural 

(groundwater) seep (National Park Service 2006b). 

Tinajas provide habitats for some aquatic invertebrates 

and breeding pools for amphibians. Humans are known 

to have enhanced the natural tinajas for use in watering 

livestock (National Park Service 2006b). Some of the 

“stock tanks” are cultural features dating back to the 

1200s and 1300s (Dana Sullivan, chief ranger, and Dave 

Hays, resource manager, El Malpais NM and El Morro 

NM, conference call, 9 December 2011).

Figure 32. Tinaja. Potholes called tinajas formed at Sandstone Bluffs 

Overlook. The pools are ephemeral and fill with rainwater during 

monsoonal thunderstorms. U.S. Geological Survey photograph, 

available at http://3dparks.wr.usgs.gov/elma/html2/elma0074.htm

(accessed 4 May 2012).

Other water-related features at El Malpais National 

Monument are small playas that develop along the edges 

of lava flows. These small ephemeral lakes and associated 

livestock ponds are present in the southern part of El 

Malpais (Lightfoot 1997). When water is present, the 

playas provide important habitat to aquatic insects, 

crustaceans, amphibians, and water birds (Lightfoot 

1997). When the ponds dry out partially or completely, 

the mud flats provide important habitats for many insects 

and songbirds (Lightfoot 1997). The temporary water 

and mud flats also provide habitat to several species of 

aquatic birds and shorebirds that otherwise would not 

occur at El Malpais, such as great blue (Ardea herodias) 

and green-backed (Butorides striatus) herons, whitefaced ibis (Plegadis chihi), green-winged teal (Anas 

crecca) and northern pintail (A. acuta) ducks, and 

killdeer (Charadrius vociferus) and mountain (C. 

montanus) plovers. Most of these species are migrants 

that stop to forage on their way to or from breeding 

grounds (Lightfoot 1997).

Eolian Habitats

Sand dunes and sandy areas are common on the east side 

of El Malpais. Although a few eolian deposits (Qe) are 

mapped within the national monument, the majority are 

mapped just east of its boundary. Sand habitats are best 

developed between the McCartys flow and the bases of 

sandstone bluffs from Sandstone Bluffs Overlook to 

North Pasture (Lightfoot 1997). The soil mounds of 

pocket gophers (Thomomys bottae and T. talpoides) are 

evidence that these species inhabit these deposits 

(Lightfoot 1997).

Soil Crusts

Interpretive materials for El Malpais National 

Monument warn visitors about accidentally stepping on 

soil crusts, which can wipe out hundreds of years of 

growth and promote erosion (National Park Service 

2004b). Soil crusts appear as knobby, black, ground 

coverings. A distinctive feature related to both biologic 

and geologic resources, they are composed of loess and 

weathered rocks, as well as cyanobacteria, lichens, and 

mosses. They are an important and widespread 

component of the ecosystem, benefiting soil quality in 

the following ways: increasing water infiltration, 

stabilizing soils, fixing atmospheric nitrogen for vascular 

plants, providing carbon to the interspaces between 

vegetation, secreting metals that stimulate plant growth, 

capturing nutrient-carrying dust, and increasing soil 

temperatures by decreasing surface albedo (KellerLynn 

2005).

Paleontological Resources

Tweet et al. (2009) completed a paleontological resource 

inventory and monitoring report for the Southern 

Colorado Plateau Network, including El Malpais 

National Monument. No field-based inventory of these 

units has been conducted to date (Dana Sullivan, chief 

ranger, and Dave Hays, resource manager, El Malpais 

NM and El Morro NM, conference call, 9 December 

2011).

The most noteworthy type of paleontological resource at 

El Malpais National Monument is tree molds, such as

those on the surface of the Bandera flows. Tree molds 

form when a lava flow engulfs a forest or individual tree 

in its path. The tree begins to burn, but releases water 

and other vapors that sufficiently cool the surrounding 
ELMA Geologic Resources Inventory Report 33

lava, leaving an imprint of the tree in the hardening 

basalt. A vertical tree mold develops when lava 

surrounds a standing tree; a horizontal mold develops 

when an erect tree falls onto the surface of a flow. Both 

types of mold are preserved at El Malpais National 

Monument (fig. 33). 

Investigators have begun to document these features 

within the national monument, and have identified four 

localities on the Bandera flows where they are present

(Dana Sullivan, chief ranger, and Dave Hays, resource 

manager, El Malpais NM and El Morro NM, conference 

call, 9 December 2011). Tree molds are known from 

other National Park System units, including Craters of 

the Moon National Monument and Preserve in Idaho, 

Lava Beds National Monument in California, Puuhonua 

o Honaunau National Historical Park in Hawaii 

(Thornberry-Ehrlich 2011b), and Hawaii Volcanoes 

National Park in Hawaii (Thornberry-Ehrlich 2009).

Quaternary plant debris, packrat middens, and bones 

occur in lava tubes (Tweet et al. 2009). Bighorn sheep 

(Ovis canadensis) remains occur within Braided Cave 

(Ken Mabery, superintendent, Scotts Bluff NM, 

conference call, 16 March 2006); all that is known about 

the age of these remains is that they predate the 1950s. 

Native peoples may have used the lava tubes as traps, 

similar to the use of “buffalo jumps,” in this case to 

corner bighorn sheep during hunts (Dana Sullivan, chief 

ranger, and Dave Hays, resource manager, El Malpais 

NM and El Morro NM, conference call, 9 December 

2011).

The Quaternary sedimentary rocks and deposits at El 

Malpais National Monument include eolian deposits 

(Qe), alluvium (Qal), terrace gravels (Qtg), and colluvium 

(Qac). To date, only ancient wood has been reported 

from these deposits (Tweet et al. 2009). The national 

monument has a lengthy tree-ring record dating back to 

136 BCE. (Poore et al. 2005; Stahle 2008), making the 

monument’s resources useful for climate studies (see 

“Lava-Flow Habitat” section). Quaternary vertebrate 

fossils have not been reported from these deposits within 

the national monument, but fossils of proboscideans, 

equids, ground sloths, camels, llamas, beaver, musk oxen, 

and bison are known from nearby locations (Morgan and 

Lucas 2003, 2005). Invertebrate fossils from these nearby 

deposits include terrestrial gastropods, freshwater 

bivalves, gastropods, and ostracodes (Drakos et al. 2003). 

Several references list New Mexico’s Quaternary fossils, 

including Harris (1993) and Morgan and Lucas (2000, 

2003, 2005).

Paleontological resources are also known from “middleaged” rocks at El Malpais National Monument, including 

the San Andres Limestone (Psa), Oak Canyon Member 

of the Dakota Sandstone (part of Kdo), Twowells 

Tongue of the Dakota Sandstone (Kdt), and Tres 

Hermanos Sandstone (Kth). Fossils in these units are 

primarily marine invertebrates. These units crop out as 

sandstone bluffs and limestone kipukas within the 

national monument. 

Although the following units, which Maxwell (1986) 

mapped within El Malpais National Monument, have not 

yet yielded fossils within the national monument’s 

boundaries, exposures of these formations outside of the 

national monument are known to contain fossils. Hence, 

future field investigations within the national monument 

could lead to the recovery of fossils from one or more of 

the following units: Abo Formation (Pa), Yeso Formation 

(Py), Glorieta Sandstone (Pg), Entrada Sandstone (Je), 

Zuni Sandstone (Jz), Clay Mesa Tongue of Mancos Shale 

and lower part of Dakota Sandstone (Kbo) (in addition 

to the Oak Canyon Member of the Dakota Sandstone, 

which is known to be fossiliferous within the national 

monument), and Paguate Tongue of Dakota Sandstone 

(Kdp). 

 

Figure 33. Tree molds. Horizontal (left) and vertical (right) tree molds are notable fossils at El Malpais National Monument. Horizontal molds 

form when a tree falls onto the surface of a lava flow, and vertical molds form when a tree remains standing. In both cases, the tree began to 

burn, but released water and other vapors that sufficiently cooled the surrounding lava, leaving an imprint of the tree in the hardening 

basalt. Photographs by Larry Crumpler (New Mexico Museum of Natural History and Science). 
34 NPS Geologic Resources Division

Sandstone Features

The official map and guide for El Malpais National 

Monument describes the landscape as an area “where 

sharp lava meets smooth sandstone” (National Park 

Service 2003b). This juxtaposition of rock types is 

particularly notable and picturesque at Sandstone Bluffs 

Overlook (see cover photograph and figs. 34 and 35). 

Within the national monument, two sandstone units 

stand out—the Jurassic Zuni Sandstone (Jz) and the 

Cretaceous Dakota Sandstone (Kdo). These units were 

deposited under very different environmental 

conditions. The sand grains that compose the Zuni 

Sandstone were transported across a vast desert and 

deposited as sand dunes about 160 million years ago. The 

Dakota Sandstone represents the initial advance of the 

Western Interior Seaway into New Mexico about 96 

million years ago. The ridgetop at Sandstone Bluffs 

Overlook is composed of Dakota Sandstone. From this 

perch, visitors can gaze across the valley, taking in a 

sweeping panoramic view of the cinder cones of the 

Zuni–Bandera volcanic field and Mount Taylor, which 

rises 3,445 m (11,302 ft) above sea level, in the distance 

(Crumpler et al. 2003). Mount Taylor is a truncated 

conical composite volcano with an estimated original 

height of 3,960–4,270 m (13,000–14,000 ft) above sea 

level (Crumpler 2010).

Several lava flows are encompassed within the field of 

view at Sandstone Bluffs Overlook, including flows with 

sources at Hoya de Cibola (Qvw), Bandera Crater (Qcb), 

Twin Craters (Qct), and El Calderon (Qcc) (Crumpler et 

al. 2003). The darker flow nearest to the base of the 

sandstone cliffs is the McCartys flow (Qbm). 

Features of the Zuni Sandstone include La Vieja (“the 

Old Woman”) (fig. 36) and hoodoos (fig. 37). Both occur 

along Highway 117. La Ventana Natural Arch, located in 

the El Malpais National Conservation Area adjacent to 

the national monument and identified on the park map 

(fig. 1), developed in Zuni Sandstone (fig. 38). The arch is 

visible from the road and accessible via a short trail from 

the parking area on Highway 117.

Figure 34. Sandstone and lava. The contrast between light-colored sandstone and dark-colored basalt figures prominently in the El Malpais 

landscape. At Sandstone Bluffs Overlook, the dark lava of the McCartys flow (Qbm) and the light-colored Dakota Sandstone (Kdo) bluff are 

juxtaposed. Photograph by Katie KellerLynn (Colorado State University).
ELMA Geologic Resources Inventory Report 35

Figure 35. Sandstone Bluffs Overlook. Looking north from the Sandstone Bluffs Overlook, Mount Taylor looms in the distance. Part of the 

Mount Taylor volcanic field, the peak has an elevation of 3,445 m (11,301 ft) and formed during the same eruptive period that generated the 

Cebollita Mesa east of El Malpais National Monument, between 3.73 million and 1.57 million years ago. The Sandstone Bluffs consist of 

gently dipping Jurassic and Cretaceous sandstone that crops out along the eastern flank of the Cebollita Mesa on the eastern margin of the 

national monument. Forests partly mask the dark Quaternary lava flows in the valley. U.S. Geological Survey photograph, available at:

http://3dparks.wr.usgs.gov/elma/html2/elma0012.htm (accessed 4 May 2012).

Figure 36. La Vieja. Along the eastern side of El Malpais National Monument, the Highway-17 corridor passes many picturesque sandstone 

features, including La Vieja—“the Old Woman”—eroded from Zuni Sandstone. National Park Service photograph.
36 NPS Geologic Resources Division

Figure 37. Sandstone spires. Erosion of Zuni Sandstone created spires near Sandstone Bluffs Overlook. Some develop into pedestals with 

overhanging rock called “hoodoos.” National Park Service photograph.

Figure 38. La Ventana Natural Arch. Adjacent to El Malpais National Monument, El Malpais National Conservation Area (administered by the 

Bureau of Land Management) contains La Ventana Natural Arch, which spans 41 m (135 ft). The arch is composed of Jurassic Zuni Sandstone

that formed as part of a vast desert 160 million years ago. Aerial photograph by Larry Crumpler (New Mexico Museum of Natural History and 

Science). Inset is U.S. Geological Survey photograph, available at http://3dparks.wr.usgs.gov/elma/html2/elma0030.htm (accessed 4 May 

2012). 
ELMA Geologic Resources Inventory Report 37

Geologic History

This section describes the rocks and unconsolidated deposits that appear on the digital 

geologic maps of El Malpais National Monument, the environment in which those units 

were deposited, and the timing of geologic events that formed the present landscape. 

For simplicity, and to gain a general understanding of the 

geologic history of El Malpais National Monument, the 

rocks in the area can be divided (fig. 39) into three main 

types: (1) very old metamorphic and igneous rocks of 

Precambrian age (approximately 1.4 billion years old); 

(2) sedimentary rocks of Permian, Triassic, Jurassic, and 

Cretaceous ages (299 million–65.5 million years old); and 

(3) young volcanic rocks, most of which erupted in the 

past 700,000 years. A fourth type includes 

unconsolidated deposits of Pleistocene or Holocene age 

(<2.6 million years old), including alluvium, colluvium, 

landslide deposits, and windblown sand and silt. 

The Precambrian rocks record a complex geologic 

history that involved the assembly and stabilization of 

fragments of oceanic and continental crust, called 

terranes, onto the North American continent (Strickland 

et al. 2003; Karlstrom et al. 2004). The process of adding 

fragments of crust to a preexisting continent is called 

accretion. These ancient rocks can be seen in the El 

Calderon area north of Highway 53. 

The “middle-aged” sedimentary rocks make up the 

distinctive gray, buff, and yellow ridges and cliffs that 

flank the national monument. These sediments 

accumulated over millions of years as shallow seas 

invaded and withdrew from the area. Some of these 

rocks formed in deep marine waters, some along 

shorelines and beaches, and some in rivers or other 

distributary channels leading to the sea. During 

terrestrial periods, when the ocean retreated, sediments 

were deposited in deserts and river valleys. These rocks 

are well displayed at the Sandstone Bluffs Overlook.

Young rocks representing Pleistocene and Holocene 

volcanism dominate the landscape at El Malpais 

National Monument. The Zuni-Acoma Trail provides a 

good opportunity to view and compare lava flows of 

different ages (Dunbar 2010). From east to west, hikers 

first encounter the McCartys flow (map unit symbol 

Qbm), which is about 3,900 years old, then the 11,000-

year-old Bandera flows (Qbb), followed by the Twin 

Craters flows (Qbt), which are about 18,000 years old. 

The trail ends on the El Calderon flows (Qbc), which are

less than 60,000 years old. 

Precambrian Crystalline Rocks

Immediately north of El Malpais National Monument, 

Maxwell (1986) mapped a variety of Precambrian rocks, 

including schist (PCsc), gneisses (PCig, PChg, and 

PCmg), quartzite (PCqt), granites (PCgg, PCag, and 

PCpg), aplites (PCap and PCpa), and hornblende, 

gabbro, and intrusive basalt (PChb). Precambrian rocks 

in the southwestern United States, such as those exposed 

in the Zuni Mountains, record the accretion of 

comparatively young terranes onto an older (Archean

and Proterozoic) craton (Strickland et al. 2003) (fig. 40). 

Shear zones in the rocks record periodic deformation, 

and dikes record subsequent magmatism (Strickland et 

al. 2003). 

Strickland et al. (2003) proposed that the Jemez 

lineament is a tectonic boundary between two accreted 

Proterozoic terranes. The long-lived tectonic and 

magmatic history recorded within the Zuni Mountains is 

evidence that the Jemez lineament is a crustal-scale zone 

of weakness consistent with episodic reactivation of an 

accretionary boundary (Strickland et al. 2003). The Zuni 

Mountains on the southern edge of the Jemez lineament 

show evidence of repeated deformation and magmatism 

from Proterozoic through Cenozoic times (Strickland et 

al. 2003).

Sedimentary Rocks

The sedimentary rocks of El Malpais National 

Monument were deposited during the Permian, Triassic, 

Jurassic, and Cretaceous periods. These time periods 

span about 233 million years, from 299 million to 65.5 

million years ago. Maxwell (1986) mapped rocks dating 

to all of these periods in the vicinity of the national 

monument.

Permian Period

Most of the Permian Period (299 million–251 million 

years ago) was characterized by marine conditions near 

El Malpais. However, uplift in northern New Mexico, 

southern Colorado, and southeastern Utah during the 

earliest part of this period produced a huge apron of 

continental sand, gravel, silt, and mud. Streams 

distributed these sediments southward across central 

New Mexico. The terrestrial Abo Formation (Pa) records 

this deposition of clastic material.

The Yeso Formation (Py), which overlies and thus is 

younger than the Abo Formation (Pa), is made up of 

several distinct beds of rocks that suggest a north–south 

transition from continental to marine conditions. The 

deposits include cross-stratified and horizontally 

stratified sandstones of eolian dune and sand-sheet 

origin, wavy-bedded sandstones formed in sanddominated sabkhas, gypsiferous rocks formed in coastal 

sabkhas and hypersaline lagoons, ripple-laminated 

sandstones originating in coastal-plain and tidal settings, 

and carbonate rocks formed in coastal sabkha, 

restricted-marine, and marine-shelf environments 

(Stanesco 1991). On a smaller scale, vertical repetition of 

sediments at intervals of 1–8 m (3.3–26 ft) within the 

Yeso Formation (Py) indicates cyclic deposition 
38 NPS Geologic Resources Division

Figure 39. El Malpais rock types. The rocks in the El Malpais area can be divided into three main types: (1) very old (Precambrian) metamorphic 

and igneous rocks, (2) sedimentary rocks of Permian, Triassic, Jurassic, and Cretaceous ages, and (3) young volcanic rocks of mostly 

Quaternary age. A fourth set of geologic materials includes unconsolidated alluvium, colluvium, landslide deposits, and windblown sand and 

silt of Quaternary age. Map by Jason Kenworthy (NPS Geologic Resources Division) using compiled GRI digital geologic map data.
ELMA Geologic Resources Inventory Report 39

Figure 40. Paleogeographic maps for El Malpais National Monument. The images of land that became North America represent all major 

geologic eras—Archean and Proterozoic (combined and commonly called Precambrian), Paleozoic (Permian), Mesozoic (Triassic, Jurassic, and 

Cretaceous), and Cenozoic (Quaternary). The geologic history of El Malpais National Monument encompasses the formation of an ancient 

mountain or rift system during the Precambrian, repeated inundation by seas during the Paleozoic and Mesozoic eras, uplifting of mountains 

and shedding of sediments into basins during the Mesozoic and Cenozoic eras, and crustal extension (pulling apart of Earth’s crust) and 

volcanism during the Cenozoic Era. Red stars show the approximate location of land that is now El Malpais National Monument during 

various points in geologic time. White lines represent the approximate location of the equator. “mya” = million years ago. Map units refer to 

those on the geologic map by Maxwell (1977, 1986). Graphic compiled by Jason Kenworthy (NPS Geologic Resources Division) based on 

paleogeographic maps created by Ron Blakey (Colorado Plateau Geosystems, Inc.), available at http://cpgeosystems.com/index.html (accessed 

25 January 2012).
40 NPS Geologic Resources Division

(Stanesco 1991). The cycles are initiated by a layer of 

limestone with a sharp lower contact, which probably 

indicates rapid marine transgression when the ocean 

spread across the land surface (Blakey and Middleton 

1983). The cycles coarsen upward, indicating regression 

(fall in sea level/advance in shoreline) of a retreating sea, 

as well as representing the lateral migration of eolian 

sand-sheet and dune strata over supratidal mud-flat and 

coastal sabkha deposits (Stanesco 1991). 

The contact between the Yeso Formation (Py) and the 

overlying Glorieta Sandstone (Pg) represents the slow 

migration of a shoreline, transitioning from the coastal 

sabkha of the Yeso Formation to a marginal marine 

setting. The Glorieta Sandstone preserves widespread 

beach deposits of clean quartz sand, crossed by 

meandering streams flowing southward toward the 

ocean (Rawling 2010). The Glorieta Sandstone was 

deposited in streams and windblown dunes on the 

margin of a Permian sea between 268 million and 245 

million years ago (Rawling 2010).

The San Andres Limestone (Psa) overlies the Glorieta 

Sandstone (Pg). The fossil content and dolomitization of 

the limestone indicate that the depositional environment 

was a warm, shallow, saline sea with abundant marine 

life. This unit was deposited along the bottom of a 

shallow epicontinental seaway that covered much of 

New Mexico (McLemore 2010). At the end of the middle 

Permian Period (approximately 260 million years ago), 

the seas retreated and erosion occurred. During this 

period of exposure, karst features such as sinkholes, 

caves, and underground drainages formed in the 

limestone. However, Maxwell (1986) did not describe 

any such features in the San Andres Limestone near El 

Malpais National Monument.

Triassic and Jurassic Periods

The ocean retreated from northwestern New Mexico

near the end of the Permian Period, and the area was 

above sea level during Triassic and Jurassic times (251 

million–145 million years ago) (Kelley 2010). The 

Triassic fluvial Moenkopi and Chinle formations (TRcm) 

rest unconformably atop the San Andres Formation (Psa)

(fig. 4). Jurassic rocks include the eolian Entrada 

Sandstone (Je); the Todilto Limestone Member of the 

Wanakah Formation (Jt), which was deposited in saline 

waters; the eolian Zuni Sandstone and Wanakah 

Formation (Jz); and the fluvial Morrison Formation (Jm). 

Some of the most famous rock formations in the western 

United States were deposited during the Triassic and 

Jurassic periods. These include the Triassic Chinle 

Formation (part of Maxwell’s TRcm), which is known 

for its petrified wood and is particularly well exposed at 

Petrified Forest National Park in Arizona (KellerLynn 

2010). The Jurassic Morrison Formation (Jm) is known 

for its dinosaur bones and is the source of the spectacular 

fossil discoveries within Dinosaur National Monument 

in Colorado and Utah (Graham 2006).

Cretaceous Period

An extensive seaway called the Western Interior Seaway 

characterizes the Cretaceous Period (145 million–65.5 

million years ago) in the western United States. The 

Western Interior Seaway extended from the Arctic to the 

tropics, covering the entire west-central part of the 

North American continent. This sea left thick deposits of 

shale, siltstone, and sandstone. In the vicinity of El 

Malpais National Monument, Maxwell (1986) mapped 

the following Cretaceous rocks: Dakota Sandstone (Kdo, 

Kdp, and Kdt), Mancos Shale (Kdo, Kmd, Kmr, and 

Kmw), Tres Hermanos Sandstone (Kth), Gallup 

Sandstone (Kg), and Crevasse Canyon Formation (Kcc). 

North of El Malpais National Monument in the 

McCartys quadrangle, Maxwell (1977) mapped other 

members and tongues of the Dakota Sandstone (Kdc and 

Kds), Mancos Shale (Km, Kmm, Kmwc, and Kmc), 

Gallup Sandstone (Kgm, Kgu, and Kgl), and Crevasse 

Canyon Formation (Kcg, Kcd, Kcs, and Kcdi) (see Map 

Unit Properties Table). These strata record the 

transgression (advance) and regression (retreat) of the 

sea in a variety of nearshore and marine settings. 

In general, marine waters spread across the land surface 

from the north/northwest in response to global changes 

in sea level (Hunt and Kelley 2010). Subsidence of basins 

facilitated the deposition of an extremely thick sequence 

of sediments. At least 1,555 m (5,100 ft) of relative sealevel rise or subsidence was necessary to accommodate 

the total Upper Cretaceous section in the southern 

Colorado Plateau and adjacent areas (Molenaar 1983). 

Molenaar (1983) documented five major transgressiveregressive cycles that took place during the Upper 

Cretaceous Period in the southern Colorado Plateau and 

adjacent areas. Three of these are recorded in the rocks 

around El Malpais National Monument.

The Dakota Sandstone and lower part of the Mancos 

Shale were deposited during the first advance of the 

Western Interior Seaway (Hunt and Kelley 2010).

Maxwell (1986) mapped various tongues of these units. 

Tongues extend outward beyond the main body of a 

formation, but are of limited extent and ultimately 

disappear laterally. Intertonguing of sediments is an 

indication of the migration of marine waters back and 

forth across an area. The Paguate (Kdp) and Twowells 

(Kdt) tongues of the Dakota Sandstone record a minor 

regression within an overall transgression, or perhaps a 

stillstand in shoreline when sand was spread over the 

shallow continental shelf (Molenaar 1983). Separating 

these tongues of the Dakota Sandstone are the Clay Mesa 

(part of unit Kdo) and Whitewater Arroyo (Kmw) 

tongues of the Mancos Shale, which also record 

transgression. 

Notably, Maxwell (1986) described the Dakota 

Sandstone as being locally disturbed by trails and 

burrows. These sediments were intensely bioturbated by 

marine organisms living in shallow waters along the 

shores of the Western Interior Seaway (Hunt and Kelley 

2010). In addition, Maxwell (1986) documented 

carbonaceous material and conglomerate within the 
ELMA Geologic Resources Inventory Report 41

Dakota Sandstone, which indicate deposition in swamps 

and rivers. Organic-rich carbonaceous (or “coaly”) 

material accumulated in swamps, whereas gravel 

consolidated into conglomerate in stream channels. 

These nonmarine deposits were associated with lakes, 

rivers, and levees within coastal areas. Regression of the 

initial advance of the Western Interior Seaway is 

recorded by the upper part of the Rio Salado Tongue of 

the Mancos Shale (Kmr) and the Tres Hermanos 

Sandstone (Kth) (Molenaar 1983). 

The second major advance of the Western Interior 

Seaway is represented by the upper part of the Tres 

Hermanos Sandstone (Kth) and the D-Cross Tongues of 

the Mancos Shale (Kmd). The associated regression of 

this cycle is preserved only in rocks in New Mexico and 

adjacent northeastern Arizona. Widespread beach 

deposits of the Gallup Sandstone (Kg) document this 

retreat. In addition, the upper part of the D-Cross 

Tongue (Kmd) and lower part of the Crevasse Canyon 

Formation (Kcc) were deposited during this regression.

The last transgression recorded in the El Malpais area is 

the third advance of the Western Interior Seaway. The 

Gibson Coal and Dalton Sandstone members of the 

Crevasse Canyon Formation (Kcc) record this advance.

By the end of the Cretaceous Period, the western margin 

of North America was tectonically active. Oceanic crust 

was being subducted under North America, and active 

tectonism provided the compressional forces that caused

the uplift of the Rocky Mountains. This mountainbuilding event is known as the Laramide Orogeny. 

Mountain building and uplift brought Precambrian 

rocks, such as those in the Zuni Mountains north of El 

Malpais National Monument, to the surface and 

ultimately displaced the Western Interior Seaway (Smith 

and Siegel 2000). Waters withdrew to the northeast.

Volcanic Rocks

Mount Taylor, which is a distinctive landmark in the El 

Malpais viewshed, marks the beginning of volcanic 

activity in the El Malpais region (fig. 41). Basalt flows 

from this period of volcanism cap the mesas to the east of 

the national monument (fig. 1). Maxwell (1986) mapped 

basalt flows on Cebollita Mesa, Mesa Negra, and Horace 

Mesa (Tb; fig. 14). This volcanic activity is not related to 

the Zuni–Bandera volcanic field. The Mount Taylor 

eruptions predate Zuni–Bandera eruptions, including the 

El Malpais event.

Figure 41. Mount Taylor and the McCartys lava flow. Mount Taylor, in the background, represents the beginning of volcanic activity in the El 

Malpais region. The McCartys lava flow, in the foreground, represents the most recent activity in the region and the youngest lava flow 

within the state of New Mexico. Photograph by Larry Crumpler (New Mexico Museum of Natural History and Science).
42 NPS Geologic Resources Division

Three episodes of volcanism have occurred within the 

Zuni–Bandera volcanic field (Laughlin et al. 1993b), 

creating the landscape of El Malpais. The first episode of 

volcanic activity occurred about 700,000 years ago. 

Maxwell (1986) mapped the deposits of this event as old 

basalt flows (Qb), which are deeply weathered and 

largely covered by soil and alluvium (Qac). The old basalt 

flows underlie the area west of the national monument 

and Hole-in-the-Wall kipuka (fig. 16). 

The second pulse of lava erupted about 150,000 years ago 

and is represented by the Chain of Craters (fig. 16). 

Maxwell (1986) mapped these deposits as basalt flows 

(Qbu), volcanic debris (Qv), and cinder cones (Qc). 

Although the first and second pulses of volcanic activity 

within the Zuni–Bandera volcanic field are part of the El 

Malpais landscape, generally they are not considered 

part of “the Malpais flows” (Maxwell 1986). The main El 

Malpais event occurred less than 60,000 years ago (table 

1). It is the youngest pulse of volcanic activity within the 

Zuni–Bandera volcanic field and is represented by most 

of the vents within El Malpais National Monument

(Laughlin et al. 1993b). This event is represented by the 

following units mapped by Maxwell (1986): El Calderon 

(Qbc, Qcc, and Qvc), Twin Craters (Qbt, Qct, and Qvt), 

Oso Ridge (Qbo and Qvo), Zuni (Qbz), Paxton Springs 

(Qbp, Qcp, and Qvp), Hoya de Cibola (Qbw and Qvw), 

Bandera (Qbb, Qcb, and Qvb), and McCartys (Qbm and 

Qcm). Maxwell (1977) mapped the McCartys flow as 

unit Qbd and the Zuni flows as unit Qba in the McCartys 

quadrangle north of El Malpais National Monument. 

Interestingly, each of these eruptions was monogenetic 

(representing a single event), except for Paxton Springs, 

which was polygenetic. Since mapping by Maxwell 

(1986), investigators have determined that the Paxton 

Springs vents (Qcp) produced two separate eruptions, 

5,000 years apart (Laughlin et al. 1994a) (see “Paxton 

Springs Flows” section and table 1). This is a very 

unusual characteristic for a cinder cone (Dunbar 2010)

Unconsolidated Deposits

The post-volcanic geologic narrative of El Malpais 

National Monument includes the deposition of alluvium 

(Qal); alluvium, colluvium, and soil (Qac); and landslide 

deposits (Ql) (Maxwell 1986). Also, Maxwell (1977) 

mapped terrace alluvium and gravel (Qtg) and colluvium 

of the McCartys Mesa (Qc). Many of these deposits 

formed during a previous wet period in the Pleistocene 

Epoch (2.6 million–11,700 years ago), which was 

followed by progressive changes in climate to presentday arid conditions (Maxwell 1986). During periods 

characterized by wetter climatic conditions, large 

amounts of water percolated through porous basalt and 

sandstones and soaked and weakened underlying shales, 

causing large blocks of basalt and sandstone to break 

away from mesa edges and slide down shale slopes. As 

precipitation lessened, landslides occurred less 

frequently and less alluvial material was carried away. 

Eventually, slides stabilized and the valleys were choked 

with alluvium that covered many of the lower slides and 

filled valleys to form broad, flat floors (Maxwell 1986). 

Flash flooding, characteristic of present-day arid 

conditions, produced deep gullies in the alluvium, 

exhuming some landslide blocks (Maxwell 1986). Eolian 

deposits (Qe) are characteristic of drier conditions and 

represent the Holocene Epoch (the last 11,700 years). 

Windblown dust is a primary agent in landscape 

evolution today, filling in the vesicles and cracks of lava 

flows and smoothing their surfaces. 
ELMA Geologic Resources Inventory Report 43

Geologic Map Data 

This section summarizes the geologic map data available for El Malpais National 

Monument. The Geologic Map Overview Graphic displays the geologic map data draped 

over a shaded relief image of the park and surrounding area. The foldout Map Unit 

Properties Table summarizes this report’s content for each geologic map unit. Complete GIS 

data are included on the accompanying CD and are also available at the GRI publications 

website (http://www.nature.nps.gov/geology/inventory/gre_publications.cfm).

Geologic Maps

Geologic maps facilitate an understanding of an area’s 

geologic framework and the evolution of its present 

landscape. Using designated colors and symbols, 

geologic maps portray the spatial distribution and 

relationships of rocks and unconsolidated deposits. 

There are two primary types of geologic maps: surficial 

and bedrock. Surficial geologic maps encompass deposits 

that are frequently unconsolidated and formed during 

the past 2.6 million years (the Quaternary Period). 

Surficial map units are differentiated by geologic process 

or depositional environment. Bedrock geologic maps 

encompass older, generally more consolidated 

sedimentary, metamorphic, and/or igneous rocks. 

Bedrock map units are differentiated based on age 

and/or rock type. For reference, a geologic time scale is 

included as figure 3. Bedrock and surficial geologic map 

data are provided for El Malpais National Monument. 

Geologic maps also may show geomorphic features, 

structural interpretations, and locations of past geologic 

hazards that may be prone to future activity. 

Additionally, anthropogenic features such as mines and 

quarries may be indicated on geologic maps. 

Source Maps

The Geologic Resources Inventory (GRI) team converts 

digital and/or paper source maps into GIS formats that 

conform to the GRI GIS data model. The GRI digital 

geologic map product also includes essential elements of 

the source maps, including unit descriptions, map 

legend, map notes, references, and figures. The GRI team 

used the following source maps to produce the digital 

geologic data for El Malpais National Monument. These 

source maps provided information for the “Geologic 

Issues,” “Geologic Features and Processes,” and 

“Geologic History” sections of this report.

Maxwell, C. H. 1977. Preliminary geologic map of the 

McCartys quadrangle, Valencia County, New Mexico 

(scale 1:24,000). Open-File Report OF-77-380. U.S. 

Geological Survey, Washington, D.C., USA.

Maxwell, C. H. 1986. Geologic map of El Malpais lava 

field and surrounding area, Cibola County, New 

Mexico (scale 1:62,500). Miscellaneous Investigations 

Series Map I-1595. U.S. Geological Survey, 

Washington, D.C., USA.

Geologic GIS Data

The GRI team implements a GIS data model that 

standardizes map deliverables. The data model is 

included on the enclosed CD and is also available online 

(http://science.nature.nps.gov/im/inventory/geology/Ge

ologyGISDataModel.cfm). This data model dictates GIS 

data structure, including layer architecture, feature 

attribution, and relationships within ESRI ArcGIS 

software. The GRI team digitized the data for El Malpais 

National Monument using data model version 2.1. 

GRI digital geologic data are included on the attached 

CD and are available through the NPS Integrated 

Resource Management Applications (IRMA) portal 

(https://irma.nps.gov/App/Reference/Search?SearchTyp

e=Q). Enter “GRI” as the search text and select El 

Malpais National Monument from the unit list.

The following components and geology data layers are 

part of the data set:

• Data in ESRI geodatabase and shapefile GIS formats

• Layer files with feature symbology (see table 3) 

• Federal Geographic Data Committee (FGDC)–

compliant metadata

• A help file (.pdf) document that contains all of the 

ancillary map information and graphics, including 

geologic unit correlation tables, and map unit 

descriptions, legends, and other information captured 

from source maps.

• An ESRI map document file (.mxd) that displays the 

digital geologic data

• A KML/KMZ version of the data viewable in Google 

Earth. Not all data layers may be represented in the 

Google Earth data.
44 NPS Geologic Resources Division

Table 3. Geology data layers in the El Malpais National Monument GIS data

Data Layer Maxwell 

source map

Data Layer 

Code

On Overview 

Graphic?

Google Earth 

Layer?

Geologic Attitude and Observation Points 1977, 1986 ATD No No

Mine Point Features 1986 MIN No No 

Geologic Point Features 1986 GPF No No 

Volcanic Point Features 1986 VPF Yes No 

Volcanic Line Features 1986 VLF Yes Yes

Faults 1977, 1986 FLT Yes Yes

Linear Geologic Units (fluorite vein) 1986 GLN No Yes

Deformation Area Boundaries 1986 DEFA No Yes

Deformation Areas 1986 DEF No Yes

Geologic Contacts 1977, 1986 GLGA Yes Yes

Geologic Units 1977, 1986 GLG Yes Yes

Map Symbology 1977 SYM No No 

Geologic Map Overview Graphic

The Geologic Map Overview Graphic (in pocket) 

displays the GRI digital geologic data draped over a 

shaded relief image of the park and surrounding area. 

For graphic clarity and legibility, not all GIS feature 

classes may be visible on the overview, as indicated in 

table 3. Cartographic elements and basic geographic 

information have been added to the overview. Digital 

elevation data and geographic information, which are 

part of the overview graphic, are not included with the 

GRI digital geologic GIS data for the park, but are 

available online from a variety of sources.

Map Unit Properties Table

The geologic units listed in the Map Unit Properties 

Table (in pocket) correspond to the accompanying 

digital geologic data. Following the structure of the 

report, the table summarizes the geologic issues, features, 

and processes, and geologic history associated with each 

map unit. The table also lists the geologic time period, 

map unit symbol, and a simplified geologic description of 

the unit. Connections between geologic units and park 

stories are also summarized.

Use Constraints

Graphic and written information provided in this section 

is not a substitute for site-specific investigations. 

Ground-disturbing activities should neither be permitted 

nor denied based upon the information provided here. 

Minor inaccuracies may exist regarding the location of 

geologic features relative to other geologic or geographic 

features on the overview graphic. Based on the source 

map scales—1:62,500 for Maxwell (1986) and 1:24,000 

for Maxwell (1977)—and U.S. National Map Accuracy 

Standards, geologic features represented here are within 

32 m (105 ft) at map scale 1:62,500 and 12 m (40 ft) at 

map scale 1:24,000 (horizontally) of their true location.

Please contact GRI with any questions.
xxxx Geologic Resources Inventory Report 45

Glossary 

This glossary contains brief definitions of geologic terms used in this report. Not all geologic 

terms used are listed. Definitions are based on those in the American Geological Institute 

Glossary of Geology (fifth edition; 2005). Additional definitions and terms are available at: 

http://geomaps.wr.usgs.gov/parks/misc/glossarya.html. 

absolute age. The geologic age of a fossil, rock, feature, 

or event in years; commonly refers to radiometrically 

determined ages.

accretion. The gradual addition of new land to old by the 

deposition of sediment or emplacement of landmasses 

onto the edge of a continent at a convergent margin.

alluvial fan. A fan-shaped deposit of sediment that 

accumulates where a hydraulically confined stream 

flows to a hydraulically unconfined area. Commonly 

out of a mountainous area into an area such as a valley 

or plain.

alluvium. Stream-deposited sediment.

aphanitic. Describes the texture of fine-grained igneous 

rocks where the different components are not 

distinguishable with the unaided eye.

aplite. A light-colored intrusive igneous rock 

characterized by a fine-grained texture. Emplaced at a 

relatively shallow depth beneath Earth’s surface.

arc. See “volcanic arc” and “magmatic arc.”

arkose. A sandstone with abundant feldspar minerals, 

commonly coarse-grained and pink or reddish.

arroyo. A small, deep, flat-floored channel or gully of an 

ephemeral or intermittent stream in the arid and 

semiarid regions of the southwestern United States.

ash (volcanic). Fine material ejected from a volcano (also 

see “tuff”).

backwasting. Wasting (gradual erosion) that causes a

slope to retreat without changing its declivity 

(gradient). 

barrier island. A long, low, narrow island formed by a 

ridge of sand that parallels the coast.

basalt. A dark-colored, often low-viscosity, extrusive 

igneous rock.

beach. A gently sloping shoreline covered with sediment, 

commonly formed by the action of waves and tides.

bed. The smallest sedimentary strata unit, commonly 

ranging in thickness from one centimeter to a meter or 

two and distinguishable from beds above and below.

bedding. Depositional layering or stratification of 

sediments.

bedrock. A general term for the rock that underlies soil 

or other unconsolidated, surficial material.

bedset. A relatively continuous succession of beds 

bounded by surfaces (called bedset surfaces, or bedset 

boundaries) of erosion or non-deposition.

bentonite. A soft clay or greasy claystone composed 

largely of smectite. Formed by the chemical alteration 

of glassy volcanic ash in contact with water.

bioturbation. The reworking of sediment by organisms.

bomb. A pyroclast ejected while still viscous and shaped 

while in flight. Commonly greater than 64 mm (2.5 in.) 

in diameter and often hollow or vesicular inside.

breakdown (cave). The collapse of the ceiling or walls of 

a cave; also, the accumulation of debris thus formed.

breccia. A coarse-grained, generally unsorted 

sedimentary rock consisting of cemented angular 

clasts greater than 2 mm (0.08 in).

breccia (volcanic). A coarse-grained, generally unsorted 

volcanic rock consisting of partially welded angular 

fragments of ejected material such as tuff or ash.

calcareous. Describes rock or sediment that contains the 

mineral calcium carbonate (CaCO3).

caldera. A large bowl- or cone-shaped depression at the 

summit of a volcano. Formed by explosion or collapse.

carbonaceous. Describes a rock or sediment with 

considerable carbon content, especially organics, 

hydrocarbons, or coal.

carbonate. A mineral that has CO3

-2 as its essential 

component (e.g., calcite and aragonite).

carbonate rock. A rock consisting chiefly of carbonate 

minerals (e.g., limestone, dolomite, or carbonatite).

cinder. A glassy pyroclastic fragment that falls to the 

ground in an essentially solid condition.

cinder cone. A conical volcanic feature formed by the 

accumulation of cinders and other pyroclasts, 

normally of basaltic or andesitic composition.

clast. An individual grain or rock fragment in a 

sedimentary rock, produced by the physical 

disintegration of a larger rock mass.

clastic. Describes rock or sediment made of fragments of 

pre-existing rocks (clasts).

clay. Can be used to refer to clay minerals or as a 

sedimentary fragment size classification (less than 

1/256 mm [0.00015 in]).

claystone. Lithified clay having the texture and 

composition of shale but lacking shale’s fine layering 

and fissility (characteristic splitting into thin layers).

coastal plain. Any lowland area bordering a sea or ocean, 

extending inland to the nearest elevated land, and 

sloping very gently seaward; it may result from the 

accumulation of material along a coast.

colluvium. A general term for any loose, heterogeneous, 

and incoherent mass of soil material and/or rock 

fragments deposited through the action of surface 

runoff (rainwash, sheetwash) or slow continuous 

downslope creep.

concretion. A hard, compact aggregate of mineral matter, 

subspherical to irregular in shape; formed by 

precipitation from water solution around a nucleus 

such as shell or bone in a sedimentary or pyroclastic 

rock. Concretions are generally different in 

composition from the rocks in which they occur.
46 NPS Geologic Resources Division

conglomerate. A coarse-grained, generally unsorted, 

sedimentary rock consisting of cemented, rounded 

clasts larger than 2 mm (0.08 in).

contact. The surface between two types or ages of rock. 

continental crust. Crustal rocks rich in silica and alumina 

that underlie the continents; ranging in thickness from 

35 km (22 mi) to 60 km (37 mi) under mountain ranges.

continental shelf. The shallowly submerged part of a 

continental margin extending from the shoreline to the 

continental slope with water depths less than 200 m 

(660 ft).

cordillera. A Spanish term for an extensive mountain 

range; used in North America to refer to all of the 

western mountain ranges of the continent.

craton. The relatively old and geologically stable interior 

of a continent (also see “continental shield”).

cross-stratification. Arrangement of strata inclined at an 

angle to the main stratification. 

crust. Earth’s outermost compositional shell, 10 to 40 km 

(6 to 25 mi) thick, consisting predominantly of 

relatively low-density silicate minerals (also see 

“oceanic crust” and “continental crust”).

crystalline. Describes a regular, orderly, repeating 

geometric structural arrangement of atoms.

debris flow. A moving mass of rock fragments, soil, and 

mud, in which more than half the particles of which 

are larger than sand size. 

décollement. A large-displacement (kilometers to tens of 

kilometers), shallowly-dipping to sub-horizontal fault 

or shear zone.

deformation. A general term for the processes of faulting, 

folding, and shearing of rocks as a result of various 

Earth forces such as compression (pushing together) 

and extension (pulling apart).

delta. A sediment wedge deposited where a stream flows 

into a lake or sea.

delta plain. The level or nearly level surface composing 

the landward part of a large or compound delta; 

strictly, an alluvial plain characterized by repeated 

channel bifurcation and divergence, multiple 

distributary channels, and interdistributary flood 

basins.

dendrochronology. The study of annual growth rings of 

trees for dating of the recent past. 

diabase. An intrusive igneous rock consisting primarily 

of the minerals labradorite and pyroxene.

dike. A narrow igneous intrusion that cuts across 

bedding planes or other geologic structures.

dolomite. A carbonate sedimentary rock of which more 

than 50% by weight or by areal percentages under the 

microscope consists of the mineral dolomite (calciummagnesium carbonate).

dolomitic. Describes a dolomite-bearing rock, or a rock 

containing dolomite.

drainage. The manner in which the waters of an area 

pass of flow off by surface streams or subsurface 

conduits. Also, the processes of surface drainage of 

water from an area by streamflow and sheet flow, and 

the removal of excess water from soil by downward 

flow. 

dune. A low mound or ridge of sediment, usually sand, 

deposited by wind. Common dune types include 

“barchan,” “longitudinal,” “parabolic,” and 

“transverse” (see respective listings).

dune field. Extensive deposits on sand in an area where 

the supply is abundant. As a characteristic, individual 

dunes somewhat resemble barchans but are highly 

irregular in shape and crowded.

eolian. Describes materials formed, eroded, or deposited 

by or related to the action of the wind. Also spelled 

“Aeolian.” 

epicontinental. Describes a geologic feature situated on 

the continental shelf or on the continental interior. An 

“epicontinental sea” is one example.

escarpment. A steep cliff or topographic step resulting 

from vertical displacement on a fault or by mass 

movement. Also called a “scarp.”

fault. A break in rock along which relative movement has 

occurred between the two sides.

fissile. Capable of being easily split along closely spaced 

planes. 

foliation. A preferred arrangement of crystal planes in 

minerals. In metamorphic rocks, the term commonly 

refers to a parallel orientation of planar minerals such 

as micas.

formation. Fundamental rock-stratigraphic unit that is 

mappable, lithologically distinct from adjoining strata, 

and has definable upper and lower contacts.

fracture. Irregular breakage of a mineral. Any break in a 

rock (e.g., crack, joint, fault).

friable. Said of a rock or mineral that is easily crumbled, 

e.g., a poorly cemented sandstone. 

gabbro. A group of dark-colored, intrusive igneous 

rocks. The intrusive equivalent of basalt. 

geology. The study of Earth including its origin, history, 

physical processes, components, and morphology.

gneiss. A foliated rock formed by regional 

metamorphism with alternating bands of dark and 

light minerals.

granite. An intrusive igneous (plutonic) rock composed 

primarily of quartz and feldspar. Mica and amphibole 

minerals are also common. Intrusive equivalent of 

rhyolite. 

granodiorite. A group of intrusive igneous (plutonic) 

rocks containing quartz, plagioclase, and potassium 

feldspar minerals with biotite, hornblende, or, more 

rarely, pyroxene, as the mafic components.

groundmass. The material between the large crystals in a 

porphyritic igneous rock. Can also refer to the matrix 

of a sedimentary rock.

gully. A small channel produced by running water in 

earth or unconsolidated material (e.g., soil or a bare 

slope). 

gypsiferous. Gypsum-bearing. 

gypsum. The most common sulfate mineral (calcium 

sulfate). Frequently associated with halite and 

anhydrite in evaporites.

hoodoo. A pillar of rock developed by erosion of 

horizontal strata of varying hardness. Typically found 

in climatic zones where most rainfall is concentrated 

during a short period of the year.

hornito. A small mound of spatter built on the back of a 

lava flow, formed by the gradual accumulation of clots 
ELMA Geologic Resources Inventory Report 47

of lava ejected through an opening in the roof of an 

underlying lava tube.

hypersaline. Excessively saline; with salinity substantially 

greater than that of average seawater. 

igneous. Refers to a rock or mineral that originated from 

molten material; one of the three main classes of 

rocks—igneous, metamorphic, and sedimentary.

inflation. Process by which a local area of pahoehoe lava 

swells as a result of injection of lava beneath its surface 

crust.

isotopic age. An age expressed in years and calculated 

from the quantitative determination of radioactive 

elements and their decay products; “absolute age” and 

“radiometric age” are often used in place of isotopic 

age but are less precise terms.

karst topography. Topography characterized by 

abundant sinkholes and caverns formed by the 

dissolution of calcareous rocks.

lacustrine. Pertaining to, produced by, or inhabiting a 

lake or lakes.

lagoon. Typically, a narrow water body that is parallel to 

the shore and is between the mainland and a barrier 

and parallel to the shore. Little or no freshwater influx 

and limited tidal flux cause elevated salinities.

lamination. Very thin, parallel layers.

landslide. Any process or landform resulting from rapid, 

gravity-driven mass movement.

lava. Still-molten or solidified magma that has been 

extruded onto Earth’s surface though a volcano or 

fissure.

lavacicle. A general term applied to nearly anything that 

protrudes into a lava tube.

levee. Raised ridge lining the banks of a stream. May be 

natural or artificial.

levee (speleology). A retaining wall of hardened lave 

along the side of a lava channel, built up incrementally 

by overflow, overthrusting of lava crusts of blocks, or 

spatter; also, the freestanding cooled edge of a lava 

tongue of flow left after evacuation of the molten lava. 

lignite. A brownish-black coal that is intermediate in 

coalification between peat and subbituminous coal.

limestone. A sedimentary rock consisting chiefly of 

calcium carbonate, primarily in the form of the mineral 

calcite. 

lineament. Any relatively straight surface feature that can 

be identified via observation, mapping, or remote 

sensing, often reflects crustal structure.

loess. Windblown silt-sized sediment, generally of glacial 

origin.

longitudinal dune. Dune elongated parallel to the 

direction of wind flow.

magma. Molten rock beneath Earth’s surface capable of 

intrusion and extrusion. 

magmatic arc. Zone of plutons or volcanic rocks formed 

at a convergent boundary.

mantle. The zone of Earth’s interior between the crust 

and core.

mass wasting. A general term for the downslope 

movement of soil and rock material under the direct 

influence of gravity.

mechanical weathering. The physical breakup of rocks 

without change in composition. Synonymous with 

“physical weathering.”

member. A lithostratigraphic unit with definable 

contacts; a member subdivides a formation.

mesa. A broad, flat-topped erosional hill or mountain 

bounded by steeply sloping sides or cliffs.

metamorphic. Describes the process of metamorphism 

or its results. One of the three main classes of rocks—

igneous, metamorphic, and sedimentary.

mud flat. A relatively level area of fine silt along a shore 

(as in a sheltered estuary) or around an island, 

alternately covered and uncovered by the tide, or 

covered by shallow water; a muddy tidal flat barren of 

vegetation. 

mudflow (mass movement). A general term for a massmovement landform and a process characterized by a 

flowing mass of predominantly fine-grained earth 

material possessing a high degree of fluidity during 

movement. 

oceanic crust. Earth’s crust formed at spreading ridges 

that underlies the ocean basins. Oceanic crust is 6 to 7 

km (3 to 4 miles) thick and generally of basaltic 

composition.

orogeny. A mountain-building event.

outcrop. Any part of a rock mass or formation that is 

exposed or “crops out” at Earth’s surface.

parting. A plane or surface along which a rock readily 

separates.

plastic. Describes a material capable of being deformed 

permanently without rupture.

porosity. The proportion of void space (e.g., pores or 

voids) in a volume of rock or sediment deposit.

principle of superposition. The concept that sediments 

are deposited in layers, one atop another, i.e., the rocks 

on the bottom are oldest with the overlying rocks 

progressively younger toward the top.

pyroclast. An individual particle ejected during a volcanic 

eruption.

pyroclastic. Describes clastic rock material formed by 

volcanic explosion or aerial expulsion from a volcanic 

vent; also pertaining to rock texture of explosive 

origin. In the plural, the term is used as a noun.

radiometric age. An age expressed in years and 

calculated from the quantitative determination of 

radioactive elements and their decay products. 

regression. A long-term seaward retreat of the shoreline 

or relative fall of sea level.

relative dating. Determining the chronological 

placement of rocks, events, or fossils with respect to 

the geologic time scale and without reference to their 

numerical age.

rift. A region of crust where extension results in 

formation of many related normal faults, often 

associated with volcanic activity.

rock. A solid, cohesive aggregate of one or more minerals.

rockfall. Mass wasting process where rocks are dislodged 

and move downslope rapidly; it is the fastest mass 

wasting process.

sabkha. A coastal environment in an arid climate just 

above high tide. Characterized by evaporate minerals, 
48 NPS Geologic Resources Division

tidal-flood, and eolian deposits. Common in the 

Persian Gulf. 

sand. A clastic particle smaller than a granule and larger 

than a silt grain, having a diameter in the range of 1/16 

mm (0.0025 in) to 2 mm (0.08 in).

sand sheet. A large irregularly shaped plain of eolian 

sand, lacking the discernible slip faces that are 

common on dunes.

sandstone. Clastic sedimentary rock of predominantly 

sand-sized grains.

scarp. A steep cliff or topographic step resulting from 

displacement on a fault, or by mass movement, or 

erosion. Also called an “escarpment.”

schist. A strongly foliated metamorphic rock that can be 

readily be split into thick flakes or slabs. Micas are 

arranged in parallel, imparting a distinctive sheen, or 

“schistosity” to the rock.

scoria. A bomb-size pyroclast that is irregular in form and 

generally very vesicular.

sediment. An eroded and deposited, unconsolidated 

accumulation of rock and mineral fragments.

sedimentary rock. A consolidated and lithified rock 

consisting of clastic and/or chemical sediment(s). One 

of the three main classes of rocks—igneous, 

metamorphic, and sedimentary.

sequence. A major informal rock-stratigraphic unit that 

is traceable over large areas and defined by a sediments 

associated with a major sea level transgressionregression.

shale. A clastic sedimentary rock made of clay-sized 

particles that exhibit parallel splitting properties.

shear zone. A zone of rock that has been crushed and 

brecciated by many parallel fractures due to shear 

strain.

shield volcano. A volcano in the shape of a flattened 

dome, broad and low, built by flows of very fluid 

basaltic lava. The Hawaiian Mauna Loa volcano is one 

example. 

silt. Clastic sedimentary material intermediate in size 

between fine-grained sand and coarse clay (1/256 to 

1/16 mm [0.00015 to 0.002 in]).

siltstone. A variably lithified sedimentary rock composed 

of silt-sized grains. 

sinkhole. A circular depression in a karst area with 

subterranean drainage and is commonly funnelshaped.

slope. The inclined surface of any geomorphic feature or 

measurement thereof. Synonymous with “gradient.”

slump. A generally large, coherent mass movement with a 

concave-up failure surface and subsequent backward 

rotation relative to the slope.

soil. Surface accumulation of weathered rock and 

organic matter capable of supporting plant growth and 

often overlying the parent material from which it 

formed. 

spatter. An accumulation of initially very fluid 

pyroclasts, usually stuck together, coating the surface 

around a volcanic vent.

spatter cone. A low, steep-sided cone of spatter built up 

on a fissure of vent, usually composed of basaltic 

material.

speleothem. Any secondary mineral deposit that forms in 

a cave.

spring. A site where water issues from the surface due to 

the intersection of the water table with the ground 

surface.

squeeze-ups. A small extrusion of viscous lava, from a 

fracture or opening on the solidified surface of a flow, 

caused by pressure. It may be marked by vertical 

grooves.

stalactites. Calcite deposits that form as water drips from 

the roof of a cave.

stalagmites. Mounds of calcite that commonly form 

beneath stalactites from dripping water in a cave.

strata. Tabular or sheet-like masses or distinct layers of 

rock.

stratification. The accumulation, or layering of 

sedimentary rocks in strata. Tabular, or planar, 

stratification refers to essentially parallel surfaces. 

Cross-stratification refers to strata inclined at an angle 

to the main stratification.

stratigraphy. The geologic study of the origin, 

occurrence, distribution, classification, correlation, 

and age of rock layers, especially sedimentary rocks.

stratovolcano. A volcano that is constructed of 

alternating layers of lava and pyroclastic deposits, 

along with abundant dikes and sills.

stream. Any body of water moving under gravity flow in 

a clearly confined channel.

stream channel. A long, narrow depression shaped by the 

concentrated flow of a stream and covered 

continuously or periodically by water.

subduction zone. A convergent plate boundary where 

oceanic lithosphere descends beneath a continental or 

oceanic plate and is carried down into the mantle.

subsidence. The gradual sinking or depression of part of 

Earth’s surface.

talus. Rock fragments, usually coarse and angular, lying 

at the base of a cliff or steep slope from which they 

have been derived.

tectonic. Relating to large-scale movement and 

deformation of Earth’s crust.

tephra. A collective term used for all pyroclastic material, 

regardless of size, shape, or origin, ejected during an 

explosive volcanic eruption.

terrane. A large region or group of rocks with similar 

geology, age, or structural style.

terrestrial. Relating to land, Earth, or its inhabitants.

tongue (stratigraphy). A member of a formation that 

extends and wedges out away from the main body of a 

formation.

topography. The general morphology of Earth’s surface, 

including relief and locations of natural and 

anthropogenic features.

trace fossil. Tracks, trails, burrows, coprolites (dung), 

etc., that preserve evidence of organisms’ life activities, 

rather than the organisms themselves.

transgression. Landward migration of the sea as a result 

of a relative rise in sea level.

ultramafic. Describes rock composed chiefly of mafic 

(dark-colored, iron and magnesium rich) minerals. 

unconformable. Said of strata that do not succeed the 

underlying rocks in immediate order of age of in 
ELMA Geologic Resources Inventory Report 49

parallel position, especially younger strata that do not 

have the same dip and strike as the underlying rocks. 

Also, said of the contact between unconformable 

rocks.

unconformity. An erosional or non-depositional surface 

bounded on one or both sides by sedimentary strata. 

An unconformity marks a period of missing time.

uplift. A structurally high area in the crust, produced by 

movement that raises the rocks.

vent. An opening at Earth’s surface where volcanic 

materials emerge.

vesicle. A void in an igneous rock formed by a gas bubble 

trapped when the lava solidified.

vesicular. Describes a volcanic rock with abundant holes 

that formed from the expansion of gases while the lava 

was still molten.

vitric. Describes pyroclastic material that is 

characteristically glassy.

volcanic. Describes anything related to volcanoes. Can 

refer to igneous rock crystallized at or near Earth’s 

surface (e.g., lava).

volcanic arc. A commonly curved, linear, zone of 

volcanoes above a subduction zone. 

wash. A term used especially in the southwestern United 

States for the broad, gravelly dry bed of an intermittent 

stream, generally in the bottom of a canyon; it is 

occasionally swept by a torrent of water.

weathering. The physical, chemical, and biological 

processes by which rock is broken down.

xenolith. A rock particle, formed elsewhere, entrained in 

magma as an inclusion.

ELMA Geologic Resources Inventory Report 51

Literature Cited 

This section lists references cited in this report. Contact the Geologic Resources Division for 

assistance in obtaining these documents. 

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http://earthquake.usgs.gov/earthquakes/states/events/

1906_11_15.php (accessed 4 February 2011).

U.S. Geological Survey. 2009b. New Mexico earthquake 

information. Online information. USGS Earthquake 

Hazards Program, Golden, Colorado, USA. 

http://earthquake.usgs.gov/earthquakes/states/index.p

hp?regionID=31 (accessed 4 February 2011).

Volcano Video Productions. 2004. Lava flows and lava 

tubes: what they are, how they form. DVD. Volcano 

Video Productions, Hilo, Hawaii, USA. 

http://www.volcanovideo.com (accessed 9 January 

2012). 

Walkup, L. 2012. NPS volcanic resources/hazard 

inventory: El Malpais National Monument. Draft 

report by GSA GeoCorps America guest scientist 

(3/12/12). National Park Service, Geologic Resources 

Division, Lakewood, Colorado, USA.

Young, R., and L. Norby. 2009. Geological monitoring. 

Geological Society of America, Boulder, Colorado, 

USA. 

http://www.nature.nps.gov/geology/monitoring/index

.cfm (accessed 27 February 2012).

ELMA Geologic Resources Inventory Report 59

Additional References

This section lists additional references, resources, and websites that may be of use to 

resource managers. Web addresses are current as of May 2012. 

Geology of National Park Service Areas

National Park Service Geologic Resources Division 

(Lakewood, Colorado): http://nature.nps.gov/geology/

NPS Geologic Resources Inventory: 

http://www.nature.nps.gov/geology/inventory/gre_pu

blications.cfm

Harris, A. G., E. Tuttle, and S. D. Tuttle. 2003. Geology of 

national parks. 6th edition. Kendall/Hunt Publishing 

Co., Dubuque, Iowa, USA.

Kiver, E. P., and D. V. Harris. 1999. Geology of U.S. 

parklands. John Wiley and Sons, Inc., New York, New 

York, USA.

Lillie, R. J. 2005. Parks and plates: the geology of our 

national parks, monuments, and seashores. W.W. 

Norton and Co., New York, New York, USA.

NPS Geoscientist-in-the-Parks (GIP) internship and 

guest scientist program: 

http://www.nature.nps.gov/geology/gip/index.cfm

NPS Views Program (geology-themed modules are 

available for geologic time, paleontology, glaciers, caves 

and karst, coastal geology, volcanoes, and a wide variety 

of geologic parks): 

http://www.nature.nps.gov/views/layouts/Main.html#/V

iews/

NPS Resource Management Guidance and Documents

1998 National Parks Omnibus Management Act: 

http://www.gpo.gov/fdsys/pkg/PLAW105publ391/pdf/PLAW-105publ391.pdf

NPS 2006 Management Policies (Chapter 4; Natural 

Resource Management): 

http://www.nps.gov/policy/mp/policies.html#_Toc157

232681

NPS-75: Natural Resource Inventory and Monitoring 

Guideline: 

http://www.nature.nps.gov/nps75/nps75.pdf

NPS Natural Resource Management Reference Manual 

#77: http://www.nature.nps.gov/Rm77/

Geologic Monitoring Manual: 

Young, R., and L. Norby, editors. 2009. Geological 

monitoring. Geological Society of America, Boulder, 

Colorado, USA.

http://nature.nps.gov/geology/monitoring/index.cfm

NPS Technical Information Center (TIC; Denver, 

Colorado; repository for technical documents): 

http://etic.nps.gov/

Geological Surveys and Societies

New Mexico Bureau of Geology and Mineral Resources:

http://geoinfo.nmt.edu/ 

U.S. Geological Survey: http://www.usgs.gov/

Geological Society of America: 

http://www.geosociety.org/

American Geological Institute: http://www.agiweb.org/

Association of American State Geologists: 

http://www.stategeologists.org/

U.S. Geological Survey Reference Tools

U.S. Geological Survey National Geologic Map Database 

(NGMDB): http://ngmdb.usgs.gov/

U.S. Geological Survey Geologic Names Lexicon 

(GEOLEX; geologic unit nomenclature and summary): 

http://ngmdb.usgs.gov/Geolex/geolex_home.html

U.S. Geological Survey Geographic Names Information 

System (GNIS; official listing of place names and 

geographic features): http://gnis.usgs.gov/

U.S. Geological Survey GeoPDFs (download searchable 

PDFs of any topographic map in the United States): 

http://store.usgs.gov (click on “Map Locator”)

U.S. Geological Survey Publications Warehouse (USGS 

publications, many available online): 

http://pubs.er.usgs.gov

U.S. Geological Survey Tapestry of Time and Terrain 

(descriptions of physiographic provinces): 

http://tapestry.usgs.gov/Default.html

ELMA Geologic Resources Inventory Report 61

Appendix: Scoping Meeting Participants 

The following people attended the GRI scoping meeting for El Malpais National Monument, 

held on 30 March 2006, or participated in the post-scoping conference call, held on 9 

December 2011. Discussions during these meetings supplied a foundation for this GRI 

report. The scoping summary document is available on the GRI publications website 

(http://www.nature.nps.gov/geology/inventory/gre_publications.cfm).

2006 Scoping Meeting Participants

Name Affiliation Position

Tim Connors NPS Geologic Resources Division Geologist/GRI Maps

Kayci Cook Collins El Malpais and El Morro national monuments Superintendent

Nelia Dunbar New Mexico Bureau of Geology and Mineral Resources Geologist

Bruce Heise NPS Geologic Resources Division Geologist

Katie KellerLynn Colorado State University Geologist/Research Associate

Ron Kerbo NPS Geologic Resources Division Cave Specialist

Ken Mabery* Fort Necessity National Battlefield and Friendship Hill 

National Historic Site (at time of scoping) Superintendent

Greer Price New Mexico Bureau of Geology and Mineral Resources Geologist/Chief Editor

Peter Scholle New Mexico Bureau of Geology and Mineral Resources State Geologist

Herschel Schulz El Malpais and El Morro national monuments Chief Ranger

*Participated in 16 March 2006 conference call only. Formerly with El Morro and El Malpais national monuments; with Scotts Bluff National 

Monument in 2012.

2011 Conference Call Participants

Name Affiliation Position

Tim Connors NPS Geologic Resources Division Geologist/GRI Maps

David Hays El Malpais and El Morro national monuments Resource Manager

Katie KellerLynn Colorado State University Geologist/Research Associate

Jason Kenworthy NPS Geologic Resources Division Geologist/GRI Reports

Dana Sullivan El Malpais and El Morro national monuments Chief Ranger

The Department of the Interior protects and manages the nation’s natural resources and cultural heritage; provides 

scientific and other information about those resources; and honors its special responsibilities to American Indians, 

Alaska Natives, and affiliated Island Communities.

NPS 103/117008, September 2012
National Park Service

U.S. Department of the Interior

Natural Resource Stewardship and Science

1201 Oakridge Drive, Suite 150

Fort Collins, CO 80525

www.nature.nps.gov

EXPERIENCE YOUR AMERICATM
El Malpais National Monument

New Mexico

National Park Service

U.S. Department of the Interior

Produced by the Geologic Resources Inventory September 2012

 Overview of Digital Geologic Data 

for El Malpais National Monument

This figure is an overview of compiled digital geologic 

data. It is not a substitute for site-specific investigations. 

Minor inaccuracies may exist regarding the location of 

geologic features relative to other geologic or 

geographic features on the figure. Based on the source 

map scale (1:24,000 and 1:62,500) and U.S. National 

Map Accuracy Standards, geologic features 

represented here are within 13 (24k) and 32 (62.5k) 

meters / 40 (24k) and 104 (62.5k) feet (horizontally) of 

their true location.

This figure was prepared as part of the NPS Geologic 

Resources Division’s Geologic Resources Inventory. 

The source maps used in creation of the digital 

geologic data product were: 

Maxwell, Charles H. 1986. Geologic Map of El Malpais 

Lava Field and Surrounding Area, Cibola County, New 

Mexico (1:62,500 scale). Miscellaneous Investigations 

Series Map I-1595. U.S. Geological Survey

Digital geologic data and cross sections for El Malpais 

National Monument, and all other digital geologic data 

prepared as part of the Geologic Resources Inventory, 

are available online at the NPS Integrated Resource 

Management Applications Portal (IRMA): 

https://irma.nps.gov/App/Reference/Search. (Enter 

“GRI” as the search text and select El Malpais National 

Monument from the unit list.)

117

117

53

53

40 San Rafael

To Albuquerque

El Malpais (BLM)

Ranger Station

El Malpais (NPS)

Information Center

Northwest New Mexico

Visitor Center

El Morro

National Monument

highways

structures

 ̧ ice caves

* lava cone

NPS Boundary

Infrastructure

Geologic Point Features

Volcanic Point Features

Volcanic Line Features

Faults

@

Geologic Contacts

@

Geologic Units

 Tb

Qbz Zuni flows

crater, known or certain

crest line of crater, known or certain

lava tube, approximate

unknown offset/displacement, known or certain

unknown offset/displacement, approximate

unknown offset/displacement, concealed

unknown offset/displacement, queried

unknown offset/displacement, inferred

known or certain

approximate

concealed

queried

map boundary

Qe Eolian deposits

Qal Alluvium

Ql Landslide deposits

Qac Alluvium, colluvium, and soil

Qbm McCartys flow

Qcm McCartys cinder cone

Qbb Bandera flows

Qcb Bandera crater

Qvb Bandera cinder field

Qbw Hoya de Cibola flows

Qvw Hoya de Cibola shield volcano

Qbp Paxton Springs flows

Qcp Paxton Springs crater

Qvp Paxton Springs cinder field

Qbo Oso Ridge flows

Qvo Oso Ridge lava cone

Qbt Twin Crater flows

Qct Twin Crater cinder cones

Qvt Twin Crater mixed pyroclastics

 and flows

Qbc El Calderon flows

Qcc El Calderon crater

Qvc El Calderon cinder field

 Qc Cinder cones

 Qv Volcanic debris

Qbu Basalt flows

Qb Old basalt flows

 Basalt flows on Cebollita Mesa, Mesa Negra, and Horace Mesa

 Kcc Crevasse Canyon Formation

 Kg Gallup Sandstone

Kmd Mancos Shale, D-Cross Tongue

Kmr Mancos Shale, Rio Salado Tongue

Kmw Mancos Shale, Whitewater Arroyo Tongue

 Kth Tres Hermanos Sandstone

 Kdt Twowells Tongue of Dakota Sandstone

Kdp Paguate Tongue of Dakota Sandstone

 Clay Mesa Tongue of Mancos Shale and Lower Part of Dakota Sandstone

 Jm Morrison Formation

 Jz Zuni Sandstone and Wanakah Formation

 Jt Todilto Limestone Member of Wanakah Formation

 Je Entrada Sandstone

TRcm Chinle and Moenkopi (?) Formations

 Psa San Andres Limestone

 Pg Glorieta Sandstone

 Py Yeso Formation

 Pa Abo Formation

PChb Hornblendite, gabbro, and intrusive basalt

PCpa Porphyritic aplite

PCap Gneissic aplite

PCpg Porphyritic granite

PCag Aplitic granite

PCgg Gneissic granite

PCmg Quartz monzonite gneiss

PCqt Quartzite

PChg Hornblende gneiss

PCig Injection gneiss

PCsc Biotite schist

sbz Silicified breccia zone

 f Fluorite vein

 Kdo

Sheet 1: Full Extent of Digital Data

EL MALPAIS NATIONAL MONUMENT

Area of Detail (Sheet 2) 

0 2 4 6 8 North

Kilometers

0 2 4 6 8

Miles
EL MALPAIS

NATIONAL CONSERVATION AREA

EL MALPAIS

NATIONAL CONSERVATION AREA

EL MALPAIS

NATIONAL

CONSERVATION

AREA

C

H

A I N

O F C

R

A

T E

R

S

 ̧

***

* *

*

*

* *

*

* *

* *

*

*

53

53

117

40

117

Ql

Qbb

Qvo

Qal

Qc

Qb

Qbm

Tb

Psa

Qbw

Ql

Qal

Qbc

Qb

Qal

Qb

Qbb

Qal

Py

PCgg

Qbt

Ql

Qbt

Psa

Qv

Py

Qal

Qv

Qbw

Pg

Pa

Pg

PCpa

Qal

PCgg

Kth

Py

PCgg

Py

Ql

Qbc

Pa

Ql

Ql

Ql

Qv Qe

Qvt

Pa

Pa

Qbp

Qe

Kdo

Qvw

Qal

Kdp

Qe

Qvb

Pg

Psa

Pa

Qal

Qac

PCag

Qbb

Pg

Pa

Kth

Qbo

Pa

Pg

Kcc

PCgg

Pa

Qbw

Kcc

Pa

Psa

Psa

Kdo

PChg

Ql

Qal

Jz

Qe

PCap

Kdt

PCgg

Qal

Pg

Qvt

Pa

Py

Pa

Py

Qbw

Qb

Psa

Ql

Kdo

Qv

PCgg

TRcm

Kth

Qal

Qal

Kdp

Kdo

Qal

Pg

Psa

Pa

Qbw

Ql

Ql

Qbp

Qbb

TRcm

Qvt

Qac

Kg

Ql

Qbp

Py

Kth

Qal

Ql

Ql

Qe

Qcb

Qc

Kg

PCap

Kmw

Qc

Qal

Pa

Qvc

Kdo

Pg

Qal

Kth

Kdo

Qal

TRcm

Pa

Psa

Qct

TRcm

Qal

Qe

Qal

Qal

Pa

Qal

Tb

TRcm

TRcm

PChg

Qbw

Psa

PCap

Pa

Qe

Qb

Qcp

Qcc

Qal

Psa

Jz

Pg

PCap

Py

Kg

Qal

Kmr

Psa

Jz

Qbp

Kcc

PCig

Qbu

Qe

Qal

Qal

Qal

Ql

Qal

PCgg

Qal

Psa

PCqt

Kg

PCap

Kth

PCpg

Qbz

PCap

Qal

PCmg

Qal

Kmr

Kdt

Qe

Qal

Qbb

PCap

Kdt

Qbw

PChb

Qbw

Pg

Pg Py

Py

Qal

Kdp

Qc

Qal

Qal

Qv

Qc

Qb

Qb

Qbw

Qbm

Qbm

Sandstone Bluffs Overlook

North

Northwest 

New Mexico

Visitor Center

Cerro Bandera

8372ft

2552m

Junction 

Cave

El Calderon

Lava

Crater

Bandera Crater

Ice Caves

Lost Woman 

Crater

Twin Craters

Braided Hoya Cave de Cobiloa 

Cerro

Hoya

Cerro

Encierro

McCartys

Crater

La Ventana

Natural Arch

 Overview of Digital Geologic Data 

for El Malpais National Monument

National Park Service

U.S. Department of the Interior

0 0.5 1 1.5

Miles

0 1 2 3

Kilometers

UTM NAD83 ZONE 12N

Produced by the Geologic Resources Inventory September 2012

El Malpais National Monument

New Mexico

El Malpais (BLM)

Ranger Station

El Malpais (NPS)

Information Center

Sheet 2: Area of Detail

HOLE-IN-THE-WALL

EL MALPAIS NATIONAL MONUMENT
ELMA Map Unit Properties Table, Sheet 1 

Map Unit Properties Table: El Malpais National Monument 

Gray-shaded rows indicate units not mapped within El Malpais National Monument. Italicized text corresponds to report sections.

Age Map Unit

(Symbol) Geologic Description Geologic Issues Geologic Features and Processes Geologic History

QUATERNARY 

(Holocene)

Eolian deposits

(Qe)

Recent accumulations of windblown silt and sand. Forms sand sheets, 

dune fields, longitudinal dunes, and smaller deposits.

Erosion—Qe is highly susceptible to erosion. Issue in 

the Sandstone Bluffs Overlook area.

Disturbed Lands—Qe is easily disturbed by grazing.

Channel diversions cut through Qe. 

Eolian Features and Processes—significant for

landscape evolution. Amount of “dust infilling” in lava 

flows provides a means of determining relative ages of 

lava flows.

Biologic and Geologic Connections—eolian habitat 

(e.g., sand dunes). Inhabited by pocket gophers 

(Thomomys bottae and T. talpoides).

Paleontological Resources—Qe may yield Quaternary 

fossils.

Maxwell (1986) mapped only the most prominent 

accumulations. Occur mostly to the east of El Malpais

National Monument, but also on lava flows within the 

national monument. Characteristic of drier Holocene 

climatic conditions.

QUATERNARY 

(Holocene and Pleistocene)

Alluvium

(Qal)

Mostly fine-grained, stream-deposited silt and sand. A few local lenses 

contain coarse sand or pebbles. As much as 15 m (50 ft) exposed in 

recent gullies. Includes some eolian (Qe) and colluvial (Qac) deposits.

Mass Wasting—landslides (Ql) are usually older than 

Qal that covers valley floors.

Flooding—flash flooding produces deep gullies in Qal, 

exhuming some landslide blocks (Ql).

Lava Flows—depending on age, lava flows may cover 

or be covered by Qal. For example, McCartys flow 

(Qbm) overlies older alluvium. Qal covers El 

Calderon flows (Qbc) near vent.

Paleontological Resources—Qal may yield Quaternary 

fossils.

Much of the alluvium on the valley floor today spread 

across during the Pleistocene wet period, but alluvium 

also includes material deposited during present-day 

flooding.

Terrace 

alluvium and 

gravel

(Qtg)

Composed largely of pebbles, cobbles, and boulders of volcanic and 

Precambrian rocks in a matrix of silt and sand. Unknown. Paleontological Resources—Qtg may yield Quaternary fossils.

Terraces represent previous floodplains that were 

active during the Pleistocene Epoch. Indicates erosion 

and transport of exposed Precambrian rocks in the 

Zuni Mountains.

Landslide 

deposits

(Ql)

Composed mostly of rotational-block slides of Tertiary basalt and 

Cretaceous sandstone and shale that have slid over soft shale units. 

Includes some rock slides and mudflows, and talus and fan 

accumulations, some of which are derived from outcrops and others

from landslide debris.

Mass Wasting—extensive landslide deposits east of El 

Malpais National Monument.

Flooding—Ql blocks become exposed during flash 

floods.

None reported.

Landslide deposits are generally older than alluvium 

(Qal) and formed during Pleistocene wet period, 

which was followed by progressive changes in climate 

to present-day arid conditions.

Alluvium, 

colluvium, and 

soil

(Qac)

Soil and alluvium. Locally covered by recent alluvium (Qal) and eolian 

deposits (Qe).

Seismic Activity—disturbed by recent faulting in 

southern part of the map area.

Lava Flows—Qac covers old basalt flows (Qb).

Paleontological Resources—Qac may yield Quaternary 

fossils.

Started accumulating during Pleistocene wet period, 

but still being deposited. May be as young as 

Holocene eolian deposits (Qe).

Colluvium of 

McCartys Mesa

(Qc)

Mantling deposits composed largely of colluvial soil, caliche, terrace 

sand and gravel, alluvium, and windblown sand. Includes local small 

outcrops of Mancos Shale and Twowells Tongue. 

Note: “Qc” is used as the map symbol for both colluvium of McCartys 

Mesa (Maxwell 1977) and cinder cones (Maxwell 1986).

Unknown. None reported.

Started accumulating during Pleistocene wet period, 

but still being deposited. Initial deposition of this 

colluvium predates deposition of landslide deposits 

(Ql).

McCartys flow 

and cinder cone

(Qbm and Qcm)

Flows (Qbm) and cinder cone (Qcm) are composed of basalt that is 

generally unweathered, uneroded, and relatively barren of vegetation. 

Contains plagioclase and olivine phenocrysts. Plagioclase phenocrysts 

predominant within 4 km (2.5 mi) of the crater. Olivine phenocrysts 

predominant in remainder of the flow.

Note: Maxwell (1977) mapped the McCartys flow using map symbol 

Qbd in the McCartys quadrangle.

Cave Management—contains lava-tube caves.

Flooding—flooding causes erosion between the 

sandstone bluffs and the McCartys flow. 

Disturbed Lands—military used Qbm as a bombing 

range and Qcm as a target.

Seismic Activity—surface faults mapped by Maxwell 

(1986) have not been field verified.

Geochronology, Research, and Interpretation—dated 

using 14C, 3

He, and 36Cl methods. Currently under 

study. Ideal field-trip location.

Volcanic Features—flows (notable pahoehoe) exhibit 

inflation pits and mounds, grooved lava, squeeze-ups

and squeeze-outs, aa lava, ropes, and spatter cones.

Eolian Features and Processes—mostly clean, silt-free 

surface.

Biologic and Geologic Connections—Qbm and Qcm

are part of lava-flow habitat/“mesic island.” Qbm

hosts “Pygmy Forest” and provides lava-tube habitat.

Sand habitats are best developed between Qbm and 

bases of sandstone bluffs. Qbm forms one side of 

Hole-in-the-Wall kipuka.

Sandstone Features—black basalt of Qbm contrasts 

with light-colored sandstone (Jz and Kdo) at 

Sandstone Bluffs Overlook.

Part of El Malpais event. Approximately 3,900 years 

old. 

Flow originated at a small cinder cone (Qcm). Some 

lava flowed to the south. More flowed to the north, 

extending to Rio San Jose, and eastward along 

Interstate 40. 
Gray-shaded rows indicate units not mapped within El Malpais National Monument. Italicized text corresponds to report sections.

ELMA Map Unit Properties Table, Sheet 2 

Age Map Unit

(Symbol) Geologic Description Geologic Issues Geologic Features and Processes Geologic History

QUATERNARY 

(Holocene and Pleistocene)

Bandera flows, 

crater, and 

cinder field

(Qbb, Qcb, and 

Qvb)

Rock of flows (Qbb) and crater (Qcb) is similar to that of the McCartys 

flow (Qbm). Ultramafic inclusions in final tephra eruptions.

Fine cinders of cinder field (Qvb) surround north, east, and west sides 

of crater (Qcb), and cover sedimentary rocks and older basalt flows.

Cave Management—contains extensive lava-tube

system and ice caves.

Flooding—near agricultural source area of pollutants 

that enter lava-tube system during flash floods. 

Disturbed Lands—past mining produced highly visible 

cinder quarry.

Geochronology, Research, and Interpretation—dated 

using 14C, 3He, 36Cl, and Ar-Ar methods. U-series 

dating of gypsum crusts in lava tubes. Currently under 

study. Ideal field-trip location.

Volcanic Features—lava flows (notable aa), crater, and 

cinder field. Ideal example of breached cinder cone; 

symmetrical and distinctive. Flows host spatter cones.

Lava Tubes—hosts longest lava-tube system in El 

Malpais National Monument, in the continental 

United States. Contains xenoliths.

Ice Caves—ice caves in flows, including privately 

owned/commercially operated Ice Caves.

Biologic and Geologic Connections—part of lava-flow 

habitat/”mesic island.” Hosts mixed-conifer 

woodlands. Provides lava-tube cave habitat. Notable 

lichen cover. Qbb forms one side of Hole-in-the-Wall 

kipuka.

Paleontological Resources—tree molds.

Part of El Malpais event. Approximately 11,000 years 

old. 

Qbb originated at Bandera Crater (Qcb) adjacent to 

Highway 53.

Hoya de Cibola 

flows and shield 

volcano

(Qbw and Qvw)

Basalt of Qbw and Qvw is similar to that of the McCartys flow (Qbm) 

but slightly higher in Al2O3 and alkalis, and lower in MgO.

Cave Management—lava-tube caves (e.g., Braided 

Cave).

Geochronology, Research, and Interpretation—dated 

using Ar-Ar method, but results seem “too old.” Exact 

(absolute) age still under consideration.

Volcanic Features—lava flows and shield volcano.

Biologic and Geologic Connections—part of lava-flow 

habitat/“mesic island.” Hosts mixed-conifer 

woodlands. Provides lava-tube cave habitat. Qbw

forms one side of Hole-in-the-Wall kipuka.

Part of El Malpais event.

Dunbar and Phillips (2004) used map symbol “Qh” 

for Hoya de Cibola flows, and “Qbw” for Bluewater 

flows west of Grants, which were not mapped by 

Maxwell (1986).

Western flows originated from Hoya de Cibola crater 

and shield volcano (Qvw) in western part of El 

Malpais National Monument. Lava flowed south, 

east, and probably northeast. 

Paxton Springs 

flows, crater, 

and cinder field

(Qbp, Qcp, and 

Qvp)

Flows (Qbp), crater (Qcp), and cinder field (Qvp) composed of 

vesicular basalt. Contains olivine phenocrysts.

Consists of volcanic material from two separate eruptions; the only 

polygenetic cinder cone vent known from the Zuni–Bandera volcanic 

field. 

Geochronology, Research, and Interpretation—dated 

using 36Cl method. Separated into northern flow 

(20,700 years old) and southern flow (15,000 years 

old) by Dunbar and Phillips (2004).

Volcanic Features—lava flows, crater, and cinder field. 

Cinder field (Qvp) includes three small cinder cones.

Originated from a volcanic center in the Precambrian 

core of the Zuni Mountains and flowed 3 km (2 mi) 

down Agua Fria Creek, and 27 km (17 mi) northeast 

down Zuni Canyon to the area between Grants and 

San Rafael. 

Zuni flows

(Qbz)

Basalt flows.

Note: Maxwell (1977) mapped the oldest Zuni basalt flow (Qba) in the 

McCartys quadrangle.

Geochronology—exact age unknown. Volcanic Features—lava flows.

Lava flows from Zuni Mountains, north of El Malpais 

National Monument. Originated from volcanic 

centers about 1.7 km (1 mi) north of boundary.

Oso Ridge flows 

and lava cone

(Qbo and Qvo)

Basalt flows (Qbo) from Oso Ridge (northwest of Bandera Crater). 

Lava cone (Qvo) composed largely of breccia, welded pyroclastic 

debris, and glassy flows.

Geochronology—exact age unknown. Volcanic Features—lava flows and cone.

Lava flows in the Zuni Mountains, north of El 

Malpais National Monument. Originated from a lava 

cone (Qvo) on Oso Ridge. Basalt flows (Qbo) 

extended eastward to Agua Fria Creek and southward 

almost to Malpais flows. Overlain by the Paxton 

Springs flows (Qcb). 

Twin Craters 

flows, cones, 

and mixed 

pyroclastics 

(Qbt, Qct, and 

Qvt)

Basalt flows (Qbt) from Twin Craters cinder cones (Qct). Mixed 

pyroclastics (Qvt) surround the cinder cones (Qct) and partially bury 

sedimentary rocks in the area. 

Flows (Qbt) originated from complex of cinder cones (Qct), which 

extends in northeast-trending line with Twin Craters at the

approximate center of the line, Lost Woman center to southwest, and 

La Tetra center to northeast.

Geochronology, Research, and Interpretation—consists 

of several overlapping flows from different vents, 

which made them difficult to delineate for early 

investigators, including Maxwell (1986). Recently 

dated using 14C, 3He, and 36Cl methods. 

Volcanic Features—lava flows, cones, and mixed 

pyroclastics.

Biologic and Geologic Connections—part of lava-flow 

habitat/“mesic island.” Hosts mixed-conifer 

woodlands. Provides lava-tube cave habitat.

Part of El Malpais event. Approximately 18,000 years 

old. 

Includes many prominent El Malpais features such as 

Twin Craters, Lost Woman, La Tetra, Lava Crater, 

and Cerro Candelaria.

Includes most flows under or adjacent to Highway 53.
Gray-shaded rows indicate units not mapped within El Malpais National Monument. Italicized text corresponds to report sections.

ELMA Map Unit Properties Table, Sheet 3 

Age Map Unit

(Symbol) Geologic Description Geologic Issues Geologic Features and Processes Geologic History

QUATERNARY 

(Holocene and Pleistocene)

El Calderon 

flows, crater, 

and cinder field

(Qbc, Qcc, and 

Qvc)

El Calderon crater (Qcc), cinder field (Qvc), and flows (Qbc) are 

composed of coarse-grained, porous basalt. Qbc largely covered by 

alluvium (Qal) or younger flows near source. Some rocks included with 

the old basalt flows (Qb), notably Cerro Bandera and Cerro Rendija, are 

similar and may correlate with El Calderon. 

Cave Management—trail to El Calderon Bat Cave is 

prime location for observing bat flights.

Disturbed Lands—abandoned cinder quarry on the 

side of El Calderon crater (Qcc).

Geochronology, Research, and Interpretation—flows 

dated using 36Cl and K-Ar methods. 

Volcanic Features—lava flows, crater, and cinder field.

Eolian Features and Processes—El Calderon flows 

noticeably infilled with loess.

Biologic and Geologic Connections—part of lava-flow 

habitat/“mesic island.” Hosts mixed-conifer 

woodlands. Provides lava-tube cave habitat.

Represents first eruption of El Malpais episode of 

Zuni–Bandera volcanic field. Less than 60,000 years 

old. 

Flows (Qbc) originated from El Calderon crater (Qcc) 

below and northeast of Cerritos de Jaspe. 

Basalt flows 

(Qbu), cinder 

cones (Qc), and 

volcanic debris 

(Qv)

Basalt flows, cinder cones, and volcanic debris west of El Malpais 

National Monument. Maxwell (1986) found these flows, cones, and 

volcanic debris to be stratigraphically related. Qv composed largely of 

weathered lava covered with coarse lava blocks and scoria.

Note: “Qc” is used as the map symbol for both colluvium of McCartys 

Mesa (Maxwell 1977) and cinder cones (Maxwell 1986).

Disturbed Lands—cinders difficult to restore. 

Geochronology, Research, and Interpretation—

currently under study.

Volcanic Features—lava flows, cinder cones, and 

volcanic debris (lava, lava blocks, and scoria).

Represent second episode of volcanism in Zuni–

Bandera volcanic field. Approximately 150,000 years 

old. 

West of El Malpais flows. Represented by Chain of 

Craters area. Volcanic debris associated with cinder 

cones and with a few older flows.

Old basalt flows

(Qb) Deeply weathered. Largely covered with soil and alluvium (Qac). None reported.

Biologic and Geologic Connections—underlie many 

kipukas (e.g., Hole-in-the-Wall). Deeply weathered 

and covered by Qac. Produce open grasslands with

small outcrops of basalt. 

Represent first episode of volcanism in Zuni–Bandera 

volcanic field. Approximately 700,000 years old. 

Cover much of western half of map area. Same as 

flows at nearby El Morro National Monument.

TERTIARY

(Pliocene)

Basalt flows on 

Cebollita Mesa, 

Mesa Negra, 

and Horace 

Mesa 

(Tb)

Generally aphanitic and vesicular olivine basalt and associated scoria. 

Flows generally 20–30 m (65–100 ft) thick, 100 m (300 ft) or more near 

smaller centers of eruption, and 300 m (1,000 ft) or more at Cebollita 

Peak.

Note: Maxwell (1977) mapped basalt on Horace Mesa as Tb in the 

McCartys quadrangle.

Mass Wasting—part of rotational slide (Ql).

Volcanic Features—lava flows cap mesas seen from El 

Malpais National Monument. 

Biologic and Geologic Connections—largely covered 

with soil, alluvium, and pine trees.

Part of Mount Taylor volcanic field, and not related to 

Zuni–Bandera activity. About 3.73 million to 1.57 

million years old. 

East of El Malpais National Monument. 

Diabase dikes

(Td)

Very fine-grained diabase with grains of plagioclase, augite, magnetite, 

and apatite. Contacts with the enclosing rock are generally marked by a 

thin aphanitic chill border in the dike and a hardened or silicified zone 

in the enclosing sandstone or shale. Range in thickness from a few 

decimeters to slightly more than 6 m (20 ft). 

Unknown.

Volcanic Features—igneous rock intruded “middleaged rocks” such as Gallup Sandstone and Mancos 

Shale. 

Intruded at the time of Mount Taylor volcanism.

UPPER CRETACEOUS

Crevasse 

Canyon 

Formation

(Kcc)

Comprises an 80–100-m- (260–330-ft-) thick sequence of nonmarine

sandstone, siltstone, carbonaceous shale, and coal beds. Members are 

(from youngest to oldest) Gibson Coal Member, Dalton Sandstone 

Member, and Dilco Coal Member.

Note: Maxwell (1977) mapped the Gibson Coal Member (Kcg), Dalton 

Sandstone Member (Kcd), Stray Sandstone Member (Kcs), and Dilco 

Coal Member (Kcdi) in the McCartys quadrangle.

Disturbed Lands—coal present, but not an abandoned 

mineral land (AML) issue. None reported. Marine setting. Records retreat and advance of Western Interior Seaway. 

Gallup 

Sandstone

(Kg)

Composed of an upper sandstone member, a middle silty shale unit, and 

a lower silty unit. Total thickness 80–100 m (260–330 ft).

Note: Maxwell (1977) mapped the upper tongue (Kgu) and lower 

tongue (Kgl), as well as a main body (Kgm), in the McCartys 

quadrangle.

Disturbed Lands—lower silty unit contains coaly 

lenses, but not an AML issue. None reported. 

Marine and sandy beach settings. Records an entire 

transgression–regression cycle of Western Interior 

Seaway. Recorded regression is unique to New 

Mexico and adjacent northeastern Arizona. 

Mancos Shale, 

D-Cross 

Tongue 

(Kmd)

Light- to dark-gray shale containing local lenses and concretionary 

zones of sandy fossiliferous limestone, and thin-bedded sandstone in 

southeastern part of map area. Outcrops are sparse and generally 

confined to small areas under cliffs with overlying sandstones. 

Mass Wasting—weathers to steep, soft slopes almost 

entirely covered by landslides, talus, or colluvium. Paleontological Resources—marine invertebrates. Marine setting. Records advance and subsequent retreat of Western Interior Seaway. 
Gray-shaded rows indicate units not mapped within El Malpais National Monument. Italicized text corresponds to report sections.

ELMA Map Unit Properties Table, Sheet 4 

Age Map Unit

(Symbol) Geologic Description Geologic Issues Geologic Features and Processes Geologic History

UPPER CRETACEOUS

Tres Hermanos 

Sandstone

(Kth)

Composed of three units—sandstone at top, highly variable middle unit 

(see description below), and light-yellowish-gray sandstone at base.

Middle unit around Mesa Negra is similar to the D-Cross (Kmd) or Rio 

Salado (Kmr) tongues, but grades southward to interbedded 

carbonaceous shale and sandstone containing lenticular coal beds. 

Locally grades into predominantly thin-bedded shaley sandstone and 

siltstone. In southern part of map area, grades into dark-gray, 

fossiliferous shale and light-gray and brown, thin-bedded, calcareous 

sandstone.

Disturbed Lands—coal present, but not an AML issue. Paleontological Resources—shells. 

Open-marine, shoreline, fluvial and marsh, barrierisland, coastal-plain, and delta-plain settings. Records 

mostly retreat of the Western Interior Seaway, with 

upper part showing advance along with overlying DCross Tongue (Kmd).

Mancos Shale, 

Rio Salado 

Tongue 

(Kmr)

Light- to dark-gray friable shale and silty shale containing local lenses of 

calcareous, siliceous, and sandy siltstone, and fine-grained sandstone in 

upper part. Thickness 70–90 m (230–300 ft).

Note: Maxwell (1977) mapped the Mulatto Tongue (Kmm); main body 

(Km); Whitewater Arroyo and Clay Mesa Shale Tongues, undivided 

(Kmwc); and Clay Mesa Shale (Kcdi) in the McCartys quadrangle.

Mass Wasting—friable. Weathers to steep, soft slopes 

almost entirely covered by landslides, talus, or

colluvium.

None reported. Marine setting. Records retreat of Western Interior 

Seaway. 

Twowells 

Tongue of 

Dakota 

Sandstone

(Kdt)

Fine- to very fine-grained, silty, local, thin lenses of medium- to coarsegrained sandstone. Light gray, weathers to grayish orange, light brown 

and tan. Thin to medium bedded. Resistant, slabby beds at top grade 

downward to silty sandstone containing thin interbeds of siltstone. 

Thickness 1–25 m (3–80 ft).

Mass Wasting—resistant beds at top.

Disturbed Lands—provided material in old sandstone 

quarry in Lava Falls area, which has been restored.

Paleontological Resources—uppermost part and 

overlying shale contain the extinct oyster (Pycnodonte 

newberryi Stanton). Also contains trace fossils, 

including many imprints, burrows, trails, and 

disturbed bedding.

Nearshore and marine settings. Records minor retreat 

of Western Interior Seaway within an overall advance, 

or perhaps a stillstand of shoreline. 

Mancos Shale, 

Whitewater 

Arroyo Tongue 

(Kmw)

Dark-gray shale, weathers to light gray and grayish tan. Uppermost part 

is silty shale and siltstone that is transitional into Twowells Tongue 

(Kdt). 

Mass Wasting—weathers to steep, soft slopes almost 

entirely covered by landslides, talus, or colluvium. None reported. Marine and nearshore settings. Records retreat of Western Interior Seaway. 

Paguate Tongue 

of Dakota 

Sandstone

(Kdp)

Light-brown and tan, fine- to very fine-grained sandstone. Local lenses 

medium to coarse grained, thin to medium bedded. Thickness 10–30 m 

(30–100 ft).

Mass Wasting—forms prominent cliffs along northeast 

side of El Malpais National Monument. 

Paleontological Resources—potential for fossils such as 

marine invertebrates and trace fossils (burrows and 

bioturbation). 

Nearshore and marine settings. Records minor retreat 

of Western Interior Seaway within an overall advance, 

or perhaps a stillstand of shoreline.

Clay Mesa 

Tongue of 

Mancos Shale 

and lower part 

of Dakota 

Sandstone

(Kdo)

Includes, from top to bottom, the Clay Mesa Tongue of Mancos Shale

(dark-gray, fissile shale), Cubero Tongue of Dakota Sandstone (similar 

lithology to Twowells Tongue, Kdt), the Oak Canyon Member

(calcareous or carbonaceous sandstone and siltstone), and a basal 

sandstone and conglomerate unit of Dakota Sandstone. Total thickness 

30–70 m (90–200 ft).

Note: Maxwell (1977) mapped the Cubero Sandstone Tongue (Kdc), 

Oak Canyon Member (Kdo), and basal sandstone unit of the Oak 

Canyon Member (Kds) in the McCartys quadrangle. “Kdo” is used as 

the map symbol for both the Oak Canyon Member of Dakota 

Sandstone (Maxwell 1977) and Clay Mesa Tongue of Mancos Shale and 

lower part of Dakota Sandstone (Maxwell 1986).

Mass Wasting—Clay Mesa Tongue is fissile and 

weathers to steep, soft slopes almost entirely covered 

by landslides, talus, or colluvium.

Erosion—erosion of overlying eolian sediments (Qe) 

to Kdo of Maxwell (1986) in Sandstone Bluffs 

Overlook area.

Disturbed Lands—provided material in old sandstone 

quarry in Lava Falls area, which has been restored. 

Carbonaceous material (coal) present, but not an 

AML issue.

Paleontological Resources—potential for fossils in Clay 

Mesa Tongue. Oak Canyon Member has fossils 

(marine invertebrates) in limestone lenses within El 

Malpais National Monument. Basal sandstone unit

has trace fossils (trails and burrows). 

Records first advance (transgression) of Western 

Interior Seaway into New Mexico. 

Mancos Shale—shallow marine setting.

Dakota Sandstone—nearshore and marine settings. 

Also, swamps and rivers adjacent to seaway.

UPPER 

JURASSIC 

Morrison 

Formation

(Jm)

Grayish-green mudstone and siltstone and lenticular yellowish- and 

reddish-gray fine- to medium-grained sandstone. Lenticular, palebrown to white, cross-bedded pebble conglomerate and sandstone at 

base. Pinches out about 5 km (3 mi) south of Interstate 40. Thickness up 

to 30 m (100 ft).

None reported. 

Paleontological Resources—known for dinosaur bones 

in places such as Dinosaur National Monument (Utah 

and Colorado).

Fluvial setting.

MIDDLE 

JURASSIC 

Zuni Sandstone 

and Wanakah 

Formation

(Jz)

Zuni Sandstone: variably colored, generally yellowish-gray or tan, 

locally chalk white, fine- to medium-grained sandstone. Well-rounded 

grains largely of quartz. Conspicuous bleached zone at tip of unit. 

Wanakah Formation: white to pale-brown, thin- to medium-bedded, 

very fine-grained, silty sandstone interbedded with thin, dark-brown 

mudstone and claystone. Grades into thin-bedded, fluviatile sandstone 

at base of Zuni Sandstone. Combined thickness 110–180 m (360–600 ft).

Erosion—friable sandstone.

Eolian Features and Processes—160-million-year-old 

sand dunes with very well-sorted, large-scale eolian 

cross-beds. Source of eolian sediment today. 

Eolian setting. Conspicuous bleached zone is 

remarkable at nearby El Morro National Monument.
Gray-shaded rows indicate units not mapped within El Malpais National Monument. Italicized text corresponds to report sections.

ELMA Map Unit Properties Table, Sheet 5 

Age Map Unit

(Symbol) Geologic Description Geologic Issues Geologic Features and Processes Geologic History

MIDDLE JURASSIC

Todilto 

Limestone 

Member of 

Wanakah 

Formation

(Jt)

Composed of 3–12 m (10–40 ft) of pale-olive-gray, pale-olive-brown, 

and pale-yellow limestone. Thickly bedded, coarsely crystalline in top 

part. Crinkly bedded in middle part. Platy bedded at base.

Disturbed Lands—contains uranium.

Paleontological Resources—potential for dinosaur 

tracks, algal structures, fish scales, ostracodes, and 

worm burrows. 

Deposited in a restricted marine embayment with an 

ephemeral connection to the sea.

Entrada 

Sandstone

(Je)

Reddish-orange to reddish-brown, fine- to medium-grained, eolian, 

cross-bedded, silty sandstone. None reported. 

Eolian Features and Processes—ancient sand dunes.

Paleontological Resources—potential for trace fossils.

Eolian setting. Jz and Je stratigraphically correlated at 

nearby El Morro National Monument. Makes up 

arches in Arches National Park in Utah.

TRIASSIC

Chinle (Upper 

Triassic) and 

Moenkopi(?) 

(Middle and 

Lower Triassic) 

formations

(TRcm)

Chinle Formation is 360–460 m (1,180–1,500 ft) thick in the region but 

only lowermost part, which is predominantly reddish-brown claystone 

and siltstone, is exposed in map area. Basal part of map unit is

tentatively correlated with the Moenkopi Formation, which is 

composed of pale-reddish-brown and grayish-red sandstone 

interbedded with pebble conglomerate and reddish-brown, lenticular 

siltstone and mudstone. Thickness up to 15 m (50 ft).

None reported. 

Paleontological Resources—Chinle Formation is 

known for petrified wood and other fossils outside of 

El Malpais National Monument, such as at Petrified 

Forest National Park in Arizona.

Terrestrial (fluvial) setting.

PERMIAN (Cisuralian or Early)

San Andres 

Limestone

(Psa)

Mostly gray to yellow, thick-bedded, fossiliferous dolomitic limestone 

with thin, calcareous, shale partings and thin, sandy limestone lenses. A 

yellowish-gray sandstone bed about 6 m (20 ft) thick locally present in 

lower part. Total thickness 35–44 m (115–145 ft).

Mass Wasting—forms cliffs or persistent dip slopes. Paleontological Resources—marine invertebrates in 

dolomitic limestone. Marine setting. Deposited in Permian seas.

Glorieta 

Sandstone

(Pg)

White to buff, very pure, well-sorted, medium-grained, cross-bedded 

quartz sandstone and siltstone. Weathers yellow to light brown. Well 

cemented with silica or calcite. Thickness 45–50 m (150–165 ft).

Mass Wasting—forms cliffs, ridges, or dip slopes. Paleontological Resources—potential for plant debris 

and trace fossils.

Marginal marine setting. Widespread beach deposits 

crossed by meandering streams.

Yeso Formation

(Py)

Pale-reddish-brown, medium- to fine-grained, thick- to thin-bedded, 

locally cross-bedded sandstone and siltstone. Grades upward into pink, 

white, and yellow alternating layers of friable sandstone. Three dense, 

fine-grained, light- to medium-gray limestone beds 3–4 m (10–13 ft) 

thick are present in upper part of formation. Locally overlain by 4–5 m 

(13–16 ft) of white to light-gray, poorly exposed gypsum beds. Total 

thickness about 400 m (1,300 ft).

Mass Wasting—friable sandstone.

Lava Tubes—Py may be a source of gypsum in cave 

minerals. 

Paleontological Resources—potential for marine 

invertebrates and trace fossils.

Coastal sabkha. Records transition from continental 

to marine conditions. 

Abo Formation

(Pa)

Moderate-brick-red to dusky-red, medium- to fine-grained sandstone 

and siltstone. Locally cross bedded. At base is 1–3 m (3–10 ft) of arkosic

conglomerate containing white quartz pebbles, overlain by about 10 m 

(30 ft) of dusky-red arkose containing thin, medium-gray, limestone 

beds. Thickness about 400 m (1,300 ft).

Mass Wasting—forms cliffs and steep slopes.

Paleontological Resources—potential for trace fossils, 

marine and terrestrial invertebrate fossils, and 

vertebrate fossils.

Thick wedge of terrestrial/continental material. 

Fluvial setting. Deposited on an eroded, undulating 

surface of Precambrian rocks.

PRECAMBRIAN

Hornblendite, 

gabbro, and 

intrusive basalt

(PChb)

Hornblendite, gabbro, and intrusive basalt (dikes and small irregular 

bodies). None reported. Lava Tubes—PChb gabbro is a source of xenoliths in lava tubes and flows.

Records subsequent intrusion into sutured terranes of 

volcanic arc and oceanic crust. 

Uplifted and exposed in Zuni Mountains during 

Laramide Orogeny.

Porphyritic 

aplite

(PCpa)

Moderate-orange-pink, slightly gneissic, porphyritic aplite composed 

of scattered microcline phenocrysts in medium-grained groundmass of 

quartz, orthoclase, and minor biotite. 

Occurs as stocks and dikes that intrude gneissic granite (PCgg) and 

gneissic aplite (PCap).

None reported. None reported. 

Records subsequent intrusion into sutured terranes of 

volcanic arc and oceanic crust. 

Uplifted and exposed in Zuni Mountains during 

Laramide Orogeny.

Gneissic aplite

(PCap)

Grayish- to moderate-orange-pink, sugary, well-foliated aplite. Consists 

of microcline, quartz, and some orthoclase and oligoclase. Contains 

minor amounts of muscovite and biotite. 

Occurs as irregular masses and dikes intruding or locally gradational 

into gneissic granite (PCgg).

None reported. None reported. 

Records subsequent intrusion into sutured terranes of 

volcanic arc and oceanic crust. 

Uplifted and exposed in Zuni Mountains during 

Laramide Orogeny.
Gray-shaded rows indicate units not mapped within El Malpais National Monument. Italicized text corresponds to report sections.

ELMA Map Unit Properties Table, Sheet 6 

Age Map Unit

(Symbol) Geologic Description Geologic Issues Geologic Features and Processes Geologic History

PRECAMBRIAN

Porphyritic 

granite

(PCpg)

Light-brownish-gray, porphyritic granite. Large, white orthoclase and 

small bluish quartz phenocrysts in fine-grained groundmass of quartz, 

orthoclase, hornblende, and biotite; slightly gneissic.

Occurs as dikes that intrude gneissic granite (PCgg).

Disturbed Lands—PCpg contains veins of fluorite that 

were mined during World War II.

Lava Tubes—PCpg is a source of xenoliths in lava 

tubes and flows.

Records subsequent intrusion into sutured terranes of 

volcanic arc and oceanic crust. 

Uplifted and exposed in Zuni Mountains during 

Laramide Orogeny.

Aplitic granite

(PCag)

Grayish to pink, gneissic, fine to medium grained, composed of 

potassium feldspar, quartz, and small amounts of oligoclase, biotite, and 

muscovite. 

Gradational with gneissic granite (PCgg).

Disturbed Lands—PCag contains veins of fluorite that 

were mined during World War II.

Lava Tubes—PCag is a source of xenoliths in lava 

tubes and flows.

Records accretion of terranes to older craton. 

Uplifted and exposed in Zuni Mountains during 

Laramide Orogeny.

Gneissic granite

(PCgg)

Pinkish- to medium-gray, fine- to coarse-grained, locally porphyritic, 

gneissic granite composed of orthoclase, quartz oligoclase, and 

abundant hornblende and biotite.

Disturbed Lands—PCgg contains veins of fluorite that 

were minded during World War II. Contains fluorite 

vein within El Malpais National Monument.

Lava Tubes—PCgg is a source of xenoliths in lava 

tubes and flows.

Records accretion of terranes to older craton. 

Uplifted and exposed in Zuni Mountains during 

Laramide Orogeny.

Quartz 

monzonite 

gneiss

(PCmg)

Medium to dark gray, medium grained, strongly gneissic, slightly 

banded. Composed of oligoclase, biotite, quartz, and some orthoclase. 

Occurs as xenolithic masses in gneissic granite (PCgg).

Disturbed Lands—PCmg contains veins of fluorite 

that were mined during World War II. None reported. 

Records accretion of terranes to older craton. 

Uplifted and exposed in Zuni Mountains during 

Laramide Orogeny.

Quartzite

(PCqt) Brown, fine to medium grained, locally contains phyllite beds. None reported. None reported. 

Records accretion of terranes to older craton. 

Uplifted and exposed in Zuni Mountains during 

Laramide Orogeny.

Hornblende 

gneiss

(PChg)

Dark-green gneiss. Contains lenses of gneissic gabbro and layers of 

aplite. 

Disturbed Lands—PChg contains veins of fluorite that 

were mined during World War II. None reported. 

Records accretion of terranes to older craton. 

Uplifted and exposed in Zuni Mountains during 

Laramide Orogeny.

Injection gneiss

(PCig) Quartz biotite gneiss. Disturbed Lands—PCig contains veins of fluorite that were mined during World War II. None reported. 

Records accretion of terranes to older craton. 

Uplifted and exposed in Zuni Mountains during 

Laramide Orogeny.

Biotite schist

(PCsc) Biotite schist. None reported. None reported. 

Records accretion of terranes to older craton. 

Uplifted and exposed in Zuni Mountains during 

Laramide Orogeny.