Capulin Volcano National MonumentPage 16 sur 19
Monogenetic Volcanic Fields: Hundreds of One-Time Volcanoes, from Alaska to Arizona
A volcanic field is a cluster of volcanoes, or an area covered by volcanic rock. In a monogenetic field, almost all the volcanoes erupt only once: cinder cones, maars, tuff rings and eruptive fissures, each with a single period of activity. Most such fields are basaltic lava flows dotted with clusters of cinder cones or maars, sometimes with a composite or shield volcano near the middle.
They form where magma supply is low. The volcanoes do not share plumbing: each new batch of magma finds its own way up, so a field can grow to hundreds of separate volcanoes, with long gaps between eruptions, over periods as long as several million years. Each eruption is small, but together a field can outweigh a big composite volcano, or even some shield volcanoes.
The Craters of the Moon volcanic field from Landsat. NASA Earth Observatory.
In the national parks
At least 13 National Park System units hold all or part of a monogenetic field.
| Park | Field | Notes |
|---|---|---|
| Bering Land Bridge NPres, Alaska | Espenberg | North America's northernmost volcanic field, above the Arctic Circle: five maars and five small shield volcanoes |
| Bering Land Bridge NPres | Imuruk Lake | about 75 vents with small shields and cinder cones; the latest, the Lost Jim flow, 1,000–2,000 years ago |
| Capulin Volcano NM, New Mexico | Raton-Clayton | the easternmost young field in the U.S., about 8,000 sq mi, active on and off for 9 million years; Capulin, one of its youngest, erupted 54,200 years ago |
| Craters of the Moon NM, Idaho | Craters of the Moon, Wapi, Kings Bowl | on the Great Rift of the Snake River Plain; about 620 sq mi, eight eruptive episodes since about 15,000 years ago |
| El Malpais NM, New Mexico | Zuni-Bandera | active since about 700,000 years ago; unusually long, voluminous and varied basalt flows, with lava tubes |
| Mojave NPres, California | Cima | about 25,600 acres of cinder cones and basalt flows, a National Natural Landmark; active from about 7 million to 15,000 years ago |
| Sunset Crater Volcano NM, Arizona | San Francisco | about 1,800 sq mi and some 600 volcanoes since about 6 million years ago; youngest, Sunset Crater, erupted in 1085 CE |
The San Francisco field, mostly basaltic cinder cones, also has lava domes and composite volcanoes: the San Francisco Peaks, with Humphreys Peak, at 12,633 ft Arizona's highest point.
A cinder cone in the Cima field, Mojave National Preserve. National Park Service.
The San Francisco Volcanic Field. U.S. Geological Survey.
Magma and eruptions
Basalt and other mafic lavas are the most common, though compositions vary. El Malpais's field erupts only basalt; Raton-Clayton may be the most varied field in the country, from ultramafic rock to rhyolite (36–70 percent silica).
Eruptions range from gentle to violent:
- Basalt flows, the most common product, pour from fissures or from vents at the base of cinder cones, usually effusive (VEI 0), in Hawaiian-style fire fountains, sometimes building spatter cones along fissures.
- Cinder cone eruptions are usually VEI 1–2, rarely more; Sunset Crater's was sub-Plinian, VEI 4.
- Phreatomagmatic eruptions, where magma meets groundwater, surface water or permafrost, blast out maars and tuff rings and can be violently explosive.
Fields usually lie in continental settings, often where the crust is stretching or beside rifts. Several ring the edge of the Colorado Plateau: Kolob (Zion), Uinkaret (Grand Canyon and Grand Canyon-Parashant), San Francisco (Sunset Crater), Hopi Buttes (Petrified Forest) and Zuni-Bandera (El Malpais). Vents may line up along rifts and faults.
Features
- Basalt flows are the hallmark: fluid lava that can cover large areas.
- Cinder cones are the most common volcanoes; most of the San Francisco field's are far smaller than Sunset Crater, about 1,000 ft tall.
- Maars and tuff rings form where there is water or ice in the ground. The Hopi Buttes field, mostly north of Petrified Forest, is one of the most extensive maar-diatreme fields on Earth, and the Espenberg field holds the largest known maars on Earth, formed where magma met permafrost.
- Lava tubes are common in basalt flows, at Bering Land Bridge, Craters of the Moon and El Malpais among others.
- Spatter cones are built of lava bombs still molten when they landed, welded into spatter (agglutinate).
The edge of a basalt flow at Craters of the Moon. National Park Service.
Sunset Crater, about 1,000 ft tall. National Park Service.
The Devil Mountain maars, Espenberg field, Bering Land Bridge. U.S. Geological Survey.
These huge maars formed when magma met permafrost. U.S. Geological Survey.
A spatter cone at Craters of the Moon, with Big Cinder Butte behind. National Park Service.
Hazards
Low magma supply means long quiet spells. Individual cones, maars and fissures are one-time vents and should not erupt again, but a new vent may open somewhere in the field, usually heralded by earthquakes and underground rumbling. Day to day, the hazards are rockfall and debris flows on cone slopes, and fissures, rough ground and lava tubes on old flows (geohazard guidance).
Sources
- National Park Service, "Monogenetic Volcanic Fields"; rewritten in hubnx's own words. Photographs and images: National Park Service, U.S. Geological Survey and NASA.