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Explosive Calderas: Summit, Resurgent and Ancient, in the National Parks
A caldera forms when a large eruption partly empties a magma chamber and the ground above it collapses. Explosive calderas form in especially big Plinian and ultra-Plinian eruptions, which drive ash columns high into the stratosphere and send out huge pyroclastic flows.
The climactic eruption of Mount Mazama, which formed Crater Lake’s caldera, in a historic painting by Paul Rockwood. National Park Service.
How big
Eruption size is measured on the Volcanic Explosivity Index (VEI), from 0 to 8, based on magnitude and intensity. The scale is logarithmic: each step is a tenfold increase. The 1980 eruption of Mount St. Helens was a VEI 5. Caldera-forming eruptions rank 6 to 8, so they are orders of magnitude larger. VEI 8 eruptions are super eruptions, such as those at Yellowstone 2.1 million and 0.64 million years ago.
The magma is silicic, dacite to high-silica rhyolite, often from chambers zoned in composition.
Three kinds
Summit calderas form when a VEI 6–7 eruption collapses the top of an existing composite volcano. Rain and snow may fill them to make lakes that stay at a steady level, unless the rim is breached and the lake drains. Examples: Aniakchak (Alaska), Crater Lake (Oregon), Mount Katmai (Katmai National Park, Alaska).
Resurgent calderas are far larger, many tens of miles across, and form in VEI 7–8 eruptions. They arise in areas that already had volcanism, but not on any existing volcano. Examples: Valles Caldera (New Mexico), Yellowstone (Idaho, Montana and Wyoming).
Older caldera complexes are ancient calderas whose walls later erosion or volcanism has made hard to find. Their main sign is large volumes of ash-flow tuff (ignimbrite). Most of those in or near national parks were very large and of the resurgent type. Examples: Big Bend (Texas), Chiricahua and Coronado (Arizona), Gila Cliff Dwellings (New Mexico), Saguaro (Arizona).
| Kind | Eruption size | Forms on | Parks |
|---|---|---|---|
| Summit | VEI 6–7 | an existing composite volcano | Aniakchak, Crater Lake, Katmai |
| Resurgent | VEI 7–8 | no single volcano; tens of miles across | Valles Caldera, Yellowstone |
| Older complex | mostly resurgent | eroded or buried | Big Bend, Chiricahua, Coronado, Gila Cliff Dwellings, Saguaro |
Sources
- National Park Service, "Explosive Calderas"; rewritten in hubnx's own words. Image: National Park Service.