El Malpais Geology: New Mexico's Youngest Lava and Its Lava Tubes

The badlands

El Malpais National Monument protects the volcanic wilderness of the Zuni-Bandera Volcanic Field in New Mexico. Its name — "the badlands" — goes back to the Spanish exploration era and reflects the brutally rough terrain of its young lava. A national monument was first proposed in the 1930s; the monument and the neighboring El Malpais National Conservation Area, managed with the Bureau of Land Management, were established in 1987.

• Youngest lava: the McCartys flow, erupted 3,900 years ago, is the youngest lava flow in New Mexico — and one of its longest basaltic flows, studied to understand how lava flows inflate and as an analog for lava on Mars.
• Lava tubes: one of the longest lava tube systems in the world, with rare minerals and unique habitats at openings and skylights — some holding perennial ice.
• In the park system: one of 19 units set aside mainly for their volcanoes, one of 33 with at least one volcano, and one of 17 with a volcano that erupted in historic or prehistoric times.

Learn more in the El Malpais Volcanic Resources Summary.

The setting

• The field: the monument covers the five young lava flows of the Zuni-Bandera field's latest phase, in the last 60,000 years; the field first erupted about 700,000 years ago.
• Buried below: the flows mostly buried older volcanic rocks and Pennsylvanian to Cretaceous sedimentary rocks. The monument lies at the southern edge of the Zuni Uplift, whose Precambrian core is exposed along the northern boundary.
• Between two provinces: the field sits at the southern margin of the Colorado Plateau, with its thick crust, and west of the Rio Grande Rift, where the crust has thinned — both areas of recent basaltic volcanism.
• The Jemez lineament: the field lies on this line of ten young volcanic fields and centers from southeastern Arizona to northeastern New Mexico and southeastern Colorado, which includes Mount Taylor, the Valles Caldera and Capulin Volcano.
• Monogenetic: like the fields at Sunset Crater, Capulin and Craters of the Moon, Zuni-Bandera is a monogenetic field — each vent erupts through one period of activity, typically where magma supply is low.

The rocks

• Precambrian: igneous and metamorphic rocks about 1,400 million years old, just outside the monument in the Zuni uplift, from when crustal fragments were added to the growing continent.
• Permian: in the northwest, in kīpukas — patches of older ground surrounded by lava — the red beds of the Abo Formation (the oldest rock in the monument), the coastal Yeso Formation, the shoreline and dune Glorieta Sandstone, and the shallow-sea San Andres Limestone.
• Jurassic: near the eastern boundary, the wind-blown Entrada Sandstone at the Narrows, and the Zuni Sandstone, which forms a prominent bluff east of the McCartys flow, La Ventana Natural Arch nearby, and Inscription Rock at El Morro.
• Cretaceous: rocks of the Western Interior Seaway — the Dakota Sandstone, the Mancos Shale and the Tres Hermanos Sandstone.

Three phases of volcanoes

The five El Malpais flows, oldest to youngest:

• El Calderon (under 60,000 years): from a cinder cone in the north; about 22 miles (35 km) long, partly buried by younger flows.
• Twin Craters (18,000 years): from four cinder cones; the shortest of the five, mostly tube-fed pāhoehoe with some channeled ʻaʻā.
• Hoya de Cibola (over 11,000 years): pāhoehoe from the monument's only shield volcano.
• Bandera (11,000 years): mostly ʻaʻā, unlike the others; still showing its surface, with tree molds and a network of lava tubes.
• McCartys (3,900 years): from a small vent in the southeast, its glassy rind well preserved.

Basalt magma is runny, so its lava travels far. Eruptions are effusive to mildly explosive — 0 to 2 on the Volcanic Explosivity Index — from low Hawaiian fire fountains to Strombolian bursts like fireworks.

Features of the lava

• Pāhoehoe and ʻaʻā: both occur, pāhoehoe more commonly — hummocky, billowy, ropy or wrinkled, against ʻaʻā's rough clinkers. Viscosity decides which forms; pāhoehoe can turn to ʻaʻā far from the vent.
• Surface features: squeeze-ups, tumuli and pāhoehoe toes, especially on the McCartys flow.
• Inflation: when lava arrives faster than a flow can spread, the flow thickens from within, lifting its hardened crust. The McCartys flow has lava-rise plateaus and steep-sided inflation pits.
• Lava tubes: more than 60 miles (95 km) in total, in every El Malpais flow except the inflated McCartys; with long collapsed sections, fragile speleothems and rare cave minerals.
• Kīpukas: islands of older volcanic rock, sedimentary rock or wind-blown sand ringed by younger lava.
• Volcanoes: at least 11 cinder cones — steep piles of cinders at about 30 degrees, built in eruptions of months to a few years; Bandera Crater is breached — and Hoya de Cibola, a small shield about 1.5 miles (2.4 km) across and 300 feet (90 m) high.

The McCartys flow, New Mexico's youngest, still shows its glassy rind. NPS photo by Katie KellerLynn, Colorado State University.

Fossils and caves

• Marine fossils: invertebrates in the San Andres Limestone, the Oak Canyon Member of the Dakota and the Tres Hermanos Sandstone.
• Tree molds: where lava surrounded trees in the El Calderon, Bandera and McCartys flows — El Malpais is one of five park units known to have them.
• Cave remains: bones and packrat middens in lava tubes, not yet dated; significant Holocene fossils are likely.
• Caves: more than 400 lava tube caves, including the southernmost ice caves in North America, with ice, fossils, archeological sites and bat habitat. 93% of the monument is volcanic pseudokarst — caves formed without rock dissolving — and 4% true karst.

Fossils are protected by the Paleontological Resources Preservation Act of 2009, caves by the Federal Cave Resources Protection Act of 1988.

Plants, a trail, and hazards

• Plants: the geology shapes the plant life, including stands of exceptionally old trees. Kīpukas on sedimentary ground grow different communities than volcanic rock. Cinder phacelia (Phacelia serrata) grows on volcanic cinders only here and around Sunset Crater in Arizona.
• The Zuni-Acoma Trail: a rough 7.5-mile (12-km) route across the McCartys flow, marked by cairns — part of an ancient path between the pueblos of Zuni and Acoma, used for at least hundreds of years.
• Volcanic hazard: individual vents won't erupt again, but the field may. The USGS's 2018 national assessment rates it very low threat, with a score of 5.4.
• Earthquakes: a moderate hazard — a 2% chance in 50 years of shaking of 12–14% of gravity, roughly VI on the Modified Mercalli scale, and between 4 and 10 damaging shakings expected in 10,000 years.
• Old quarries: six abandoned mineral land sites with 28 features — five cinder ("lava rock") quarries and one sandstone quarry off New Mexico 117 — plus four more sites found by the inventory.

Overview of the monument's digital geologic data. NPS Geologic Resources Inventory.

The Geologic Resources Inventory

The Park Service's Geologic Resources Inventory produces digital geologic maps and reports for more than 270 natural resource parks:

• scoping summaries — records of the meetings where Park Service staff and local geologists planned the mapping and the report;
• digital geologic maps, for GIS software, with a user guide and ancillary information, and on newer products Google Earth and online map services;
• reports that use the maps to discuss a park's setting, significant features, management issues and geologic history;
• posters, a static view of the map data.

A Soil Resources Inventory has not yet been completed for El Malpais. The monument belongs to the Colorado Plateaus Physiographic Province.

Sources

Based on "NPS Geodiversity Atlas—El Malpais National Monument, New Mexico," National Park Service; a work of the United States government in the public domain. A thumbnail of the inventory report's cover is not reproduced.