Coronado National MemorialСтраница 18 из 25

Ancient Calderas of the Southwest: Big Bend, Chiricahua, Gila and Saguaro

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.

Rock spires of welded volcanic tuff

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.

Tilted bands of rock on a mountainside

Rocks on the southeast flank of the Pine Canyon caldera, Big Bend. National Park Service.

Rock towers filling a valley under evening sky

The stone spires of Chiricahua, carved from the Rhyolite Canyon Tuff. National Park Service.

Mountains and rolling hills seen from a pass

Looking east from Montezuma Pass into the Montezuma caldera. National Park Service.

Rugged desert peaks with saguaro cacti in front

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.