Craters Of The Moon National Monument & PreservePage 19 sur 35
Volcanic Craters: How Cinder Cones, Maars, Domes and Big Volcanoes Get Their Bowls
A volcanic crater is a bowl- or funnel-shaped hollow, usually directly above the vent that erupts. Craters often crown a volcano's summit but can also sit over side vents of composite and shield volcanoes. They form during eruptions, blasted out by explosions, including steam-driven (phreatic) and magma–water (phreatomagmatic) ones. Some outlast the eruption for a long time but eventually erode away; others are buried by later eruptions or filled by growing lava domes. During eruptions a crater can grow, from bigger blasts or from its walls slumping into the vent, or be breached by a sideways blast, as at Mount St. Helens in 1980.
Mount Martin’s summit crater, Katmai National Park and Preserve. U.S. Geological Survey.
Cinder cone craters
Usually a simple round hollow at the top, kept open by explosions during the eruption, and partly built, since cinders pile up around the vent. Loose cinders slumping in is the main way they widen. Some are armored by spatter: cinders still molten when they land, welding together into solid rock, and the best-preserved cinder cone craters are often spatter-rimmed.
| Crater | Park | Size | Age |
|---|---|---|---|
| Sunset Crater | Sunset Crater Volcano NM, Arizona | ~300 ft deep, up to ~1,900 ft across | erupted 1085 CE |
| Capulin | Capulin Volcano NM, New Mexico | ~1,400 ft across, nearly 300 ft deep; spatter rim | 54,200 years |
| Cinder Cone | Lassen Volcanic NP, California | outer crater ~900 ft across, ~200 ft deep; double rim | about 1650 CE |
| Wizard Island | Crater Lake NP, Oregon | under 500 ft wide, ~70 ft deep | — |
Cinder Cone's double rim probably reflects an eruption that waxed and waned near its end.
Sunset Crater. National Park Service.
Into Capulin’s crater; spatter in the foreground. National Park Service.
Cinder Cone, Lassen Volcanic National Park. National Park Service.
Maar craters
Maars are cut down into the ground surface rather than built up inside a volcano, so older "country rock" may show in their walls under only a thin coat of tephra. At Ubehebe Crater in Death Valley, orange Miocene sedimentary rock shows in the walls, and the deposits of the phreatomagmatic blast that made it are about 160 ft thick at the rim.
Summit craters of composite volcanoes
These are complex and changeable. New craters are blasted out, or old ones enlarged, when a new eruption begins; domes can partly or wholly fill them. Gases escape even between eruptions, so fumaroles are common, and crater lakes may be acidic; together they alter the rock throughout the crater. Some hold glaciers.
- Mount Griggs (Katmai): a flat top made of three nested craters; the outer one, 0.9 mile wide, probably formed by collapse, and is mostly filled by an inner cone.
- Mount Martin (Katmai): intense fumaroles and a lake that comes and goes, about 1,000 ft across.
- Redoubt (Lake Clark): reshaped in the 1989–1990 and 2009 eruptions; 1.1 miles wide and breached on the north by the Drift glacier. In May 2009 a lava dome grew in it, after explosions destroyed at least two earlier ones.
- Mount Rainier: the East Crater is filled by a glacier holding 2.2 miles of surveyed ice caves melted by fumaroles, which also keep its rim mostly bare; about 1,400 ft across.
The summit of Mount Griggs. U.S. Geological Survey.
Redoubt’s summit crater on 16 May 2009. U.S. Geological Survey.
Domes with craters
Most lava domes have no crater, since they ooze out quietly, but some have explosive ones. Lassen Peak, a large dome of 27,000-year-old dacite, opened a new crater at the start of its 1914–1917 eruption, which went on to produce another dome, a lava flow, pyroclastic and pumice deposits, and lahars that swept many miles down the northeast valley.
The 1915 crater at Lassen Peak’s summit. U.S. Geological Survey.
The same view, labeled. U.S. Geological Survey.
Not really craters: collapse features
Some hollows on volcanoes form not by explosion but by collapse.
- Pit craters are steep-sided pits that form from the bottom up as ground collapses into a void. Craters of the Moon has at least 174, from 16 ft to 1,300 ft across, mostly in chains above fissures. In Hawaiʻi Volcanoes, they line Kīlauea's East and Southwest Rift Zones, several visible from Chain of Craters Road; Mauna Loa has ten named ones. Halemaʻumaʻu, in Kīlauea's summit caldera, is a pit crater that has often held a lava lake; oral tradition calls it the home of Pele.
- Calderas are collapses more than 0.6 miles (1 km) across, over a magma chamber emptied by an eruption, and are both parts of volcanoes and a volcano type of their own. Crater Lake fills one formed when Mount Mazama erupted 7,700 years ago; rain and snow took several hundred years to fill it.
Pit Craters leading to Big Craters, Craters of the Moon, Idaho. National Park Service.
Pauahi Crater on Chain of Craters Road, Hawaiʻi Volcanoes. National Park Service.
Crater Lake, in a caldera. National Park Service.
Parks with craters include Aniakchak, Bandelier, Bering Land Bridge, Capulin Volcano, Crater Lake, Craters of the Moon, Death Valley, El Malpais, Grand Canyon, Grand Canyon-Parashant, Haleakalā and Hawaiʻi Volcanoes.
Sources
- National Park Service, "Volcanic Craters"; rewritten in hubnx's own words. Photographs: National Park Service and U.S. Geological Survey. Two openly licensed photographs by private photographers are not reproduced here.