The Great Desert



The Valles Caldera is the "type specimen" of a caldera complex volcano - a relatively unfamiliar type of volcano but very common through the world and through geologic history. Yellowstone National Park is on three overlapping caldera complex volcanos. Long Valley in eastern California is one also; the San Juan mountains in southwestern Colorado are built on a mass of Tertiary-age caldera complexes, and much of the Mesozoic and Tertiary rock of Nevada, Idaho, and eastern California are caldera complexes and their erupted ash-flow tuffs. Metamorphosed ash-flow tuffs are also common in the Precambrian rocks of northern New Mexico, so caldera complex volcanoes have been important through much of the Earth's history!

Oblique of Whole Valles Volcano
Before the Storm

Viewed from the Space Shuttle (looking southeast), the Valles caldera volcano is the dark oval area nearly filling the left side of the image. The volcano is about 80 km across and 1.5 km tall, even flatter than a basaltic shield volcano. Most of the volcano landform is lava and ash - the center or caldera is the source of all the action.
Prelude Another Space Shuttle view of the Valles caldera (the rough oval in the left 2/3 of the scene). At the top right are the town of Los Alamos and its National Laboratories. The story begins about 1.2 million years ago, as a blob of water-rich granite magma (~a million cubic kilometers) rose towards the surface.
Topographic Rim

Ring fracture and Eruption. As the magma neared the surface, it broke the overlying rock along a ring fracture (black line and erupted at about 1.14 million years ago. As the magma erupted, water dissolved in it flashed to steam, spraying the magma as huge clouds of ash and steam. This fiery cloud surged downhill like water (arrows), and deposited its ash in layers of ash-flow tuff, up to hundreds of meters thick. This ash-flow tuff, named the Bandelier tuff, has a volume of ~300 cubic kilometers!
Caldera and Lake. During the eruption, the plug of rock bounded by the ring fracture settled down into the rapidly diminishing body of magma. Steep walls surrounded where the plug had been, and they collapsed as massive landslides into the hole left by the sinking plug. The heads of those landslides are now the topographic rim of the caldera (black line), which is 20-24 km in diameter and about 300 m deep. Rain water and snow melt then filled the caldera, although one can imagine that lake to be steaming hot and sulfurous from the eruptions just finished and ongoing. But eruption of the Bandelier tuff was not the final act of the Valles caldera.

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Copyright Allan Treiman, LPI.
Updated 09/06/03.
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