Capulin Volcano: How a Near-Perfect Cinder Cone Formed 54,200 Years Ago

In 1890, a letter to the Commissioner of the General Land Office called Capulin one of the "most perfect specimens of extinct volcanoes in North America." That letter led to the land being withdrawn from settlement, and in 1916 it became a national monument. Capulin Volcano, in northeastern New Mexico, stands 2,494 m (8,182 ft) above sea level and nearly 400 m (1,300 ft) above the surrounding plain — one of the tallest and most perfectly formed cinder cones in North America.

A textbook cinder cone

Cinder cones — also called scoria cones — are the most common kind of volcano on land. They are simple piles of scoria that falls around a vent during moderately explosive eruptions of low-silica, basaltic magma. Their slopes are set by the angle of repose, 25 to 32 degrees for ash and cinders: when cinders pile up more steeply, they avalanche down the sides and into the crater, and those avalanches build the cone's layers. Few cinder cones are as large or as symmetrical as Capulin, which makes it the archetype of the form — and the reason it is in the National Park System.

Richman (2010) mapped 2 boca ramparts, 16 lava cascades, 19 lava lakes, 15 lava levees, 18 lava ridges, 1 pooled lava flow, 1 push-up, 2 rafted cinder cones, 24 spatter deposits, 1 spatter flow, 23 squeeze-ups, 18 tumuli and 7 vents.

How it erupted

1. A lava flow first, from a fissure or small vents — cinders lying on top of it show it came before the cone.
2. Cone building. Intermittent explosive bursts threw cinders a few hundred feet into the air in glowing, firework-like "rooster tails" — a Strombolian eruption. The cone grew fast at first, then more slowly, since each foot of height needed more material as the base widened.
3. Spatter. As the eruption weakened and the column shrank, cinders and volcanic bombs were still molten when they landed on the rim, and welded together into spatter. That spatter has slowed erosion and helped the cone keep its shape.
4. The boca. After the cone-building phase, vents at the base took over in a gentler, Hawaiian style: low fire fountains, lava lakes, lava tubes and ropy pahoehoe lava, flowing first south and then north.

The rock is Capulin Basalt — technically a trachybasalt, richer in sodium and potassium than true basalt. Fragments and crystals of Dakota Sandstone caught up in it are its hallmark, and quartz from that sandstone helps explain its fairly high silica content (50–55 percent).

How old it is

For decades Capulin was thought to be less than 10,000 years old, because alluvium beneath one of its lava flows had been correlated with the Folsom archeological site. Accurate dating arrived only in the 1990s: a new technique in the mid-1990s put the eruption at 56,000 to 62,000 years ago. Refined argon-argon dating now gives 54,200 ± 1,800 years. The eruption itself probably lasted a few years or less.

Geologic map of Capulin Volcano National Monument. Credit: National Park Service, Geologic Resources Inventory.

The Raton-Clayton volcanic field

Capulin sits near the middle of the Raton-Clayton volcanic field, about 20,000 square kilometers stretching more than 130 km (80 mi) between the towns of Raton and Clayton in northeastern New Mexico and southeastern Colorado. It is the easternmost young volcanic field in North America, with an estimated 125 vents: cinder cones, volcanic domes, and the large andesite volcano Sierra Grande.

The field's largest volcano, Sierra Grande, formed 3.8 to 2.8 million years ago and rises about 2,200 ft above the plain southeast of Capulin. It is shaped like a shield volcano but built of andesite, a thicker, "pastier" lava than the basalt of typical shield volcanoes such as Kilauea. To the northwest, mesas such as Johnson Mesa are capped with some of the field's oldest lava.

There have been no eruptions for 36,000 years, but the field's history of eruptions and pauses suggests it is not finished. A future eruption would probably open a new vent: cinder cones are monogenetic, built in one episode, after which their plumbing is plugged with solidified magma.

The field lies on the Jemez lineament, a southwest-to-northeast line of young volcanic centers across northern New Mexico. Unlike the chain that built the Hawaiian Islands, it is not a hot-spot track — eruption ages do not progress along it. It is probably a long-lived weakness in the lithosphere dating to the Precambrian, when North America grew as island chains collided with it. El Malpais National Monument (the Zuni-Bandera field) and Bandelier National Monument (tuffs from the Valles Caldera) lie on it too.

The view, and what else to see

• The summit road. The view from the rim probably led Homer Farr, custodian of what was then Capulin Mountain National Monument from 1923 to 1955, to build a road to the top. The "dramatic view" is one of the monument's four statements of significance, with the classic cinder cone, the diverse volcanic field and the cone's accessibility.
• Playa lakes to the south and east, which fill in the wet season and dry out, are almost the only nonvolcanic features in view.
• Loess, windblown dust, fills cracks and holes in the lava flows, helps form soil, and serves as a rough way to date flows.
• The K-T boundary, marking the extinction 66 million years ago of about half of all species, dinosaurs included, is not in the monument but is exposed nearby at Goat Hill.
• No fossils have been found in the monument, and none are likely.

The monument lies in the Raton section of the Great Plains physiographic province, which has far more relief than most of the Great Plains, with lava-capped mesas and volcanoes rising above Cretaceous and Cenozoic sedimentary rock.

Sources

Based on "NPS Geodiversity Atlas—Capulin Volcano National Monument, New Mexico," National Park Service, a work of the United States government in the public domain, drawing on the park's Geologic Resources Inventory report.