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Several national parks preserve the remains of calderas from some of the largest eruptions in North America's past. A caldera forms when an especially large eruption partly empties a shallow magma reservoir and the ground above it sinks. These eruptions probably ranged from VEI 6 to 8 ("colossal" to "apocalyptic"), with columns reaching tens of miles into the stratosphere and pyroclastic flows traveling tens of miles from the vents.
Their main surviving evidence is thick ash-flow tuff (ignimbrite) and megabreccia, jumbled blocks that fell as the caldera walls collapsed. Over time, later eruptions (including younger calderas), faulting and erosion blur or erase the rest of the structure.
The Bloodgood Canyon Tuff, a densely welded ignimbrite erupted from the Bursum caldera, fills the Gila Cliff Dwellings caldera. National Park Service.
Five parks
| Park | Caldera | Age | Notes |
|---|---|---|---|
| Big Bend NP, Texas | Pine Canyon (also called Pine Mountain), Chisos Mountains | about 32 million years | about 4 miles across; sagged down without obvious ring faults; its rhyolite tuff, from the first phase of subsidence, is found only inside it |
| Chiricahua NM, Arizona | Turkey Creek, south of the monument | about 26.5 million years | about 12 miles across; an eruption of 120 cubic miles |
| Coronado National Memorial, Arizona | Montezuma | late Jurassic | heavily eroded; probably 10–11 miles across; tuffs and collapse megabreccia |
| Gila Cliff Dwellings NM, New Mexico | Bursum and Gila Cliff Dwellings | about 28 million years | in the Mogollon–Datil field, active about 40 to 5 million years ago |
| Saguaro NP, Arizona | Tucson Mountain | 70–75 million years | 12–15 miles across; deeply eroded, but its tuff, megabreccia and later lava flows are exposed |
The famous hoodoos and spires of Chiricahua's "Wonderland of Rocks" are carved from the Rhyolite Canyon Tuff of the Turkey Creek eruption, eroded along two sets of vertical joints. Coronado National Memorial lies inside the walls of the Montezuma caldera, and Gila Cliff Dwellings sits within one caldera with ignimbrite from another around it.
Rocks on the southeast flank of the Pine Canyon caldera, Big Bend. National Park Service.
The stone spires of Chiricahua, carved from the Rhyolite Canyon Tuff. National Park Service.
Looking east from Montezuma Pass into the Montezuma caldera. National Park Service.
From Amole Peak over the Tucson Mountain District of Saguaro, which exposes the Tucson Mountain caldera. National Park Service.
Sources
- National Park Service, "Older Caldera Complexes"; rewritten in hubnx's own words. Photographs: National Park Service. A map drawn on a commercial satellite basemap is not reproduced here.
In these publicationsBig Bend National ParkSaguaro National ParkChiricahua National MonumentCoronado National MemorialGila Cliff Dwellings National Monument
Licence: CC0 1.0 (public domain) · Adapted from www.nps.gov
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