Resurgent Calderas: Earth's Largest Volcanoes at Valles Caldera and Yellowstone

The largest volcanic features on Earth are not soaring cones or massive shields. They are resurgent calderas: broad basins, often tens of miles across, left when an enormous eruption empties part of the magma reservoir beneath and its roof collapses. Their eruptions send columns of ash and pumice into the stratosphere, where winds carry the ash around the globe, and pour pyroclastic flows across huge areas inside and outside the caldera walls.

Valles Caldera National Preserve: the flat grassland lies on sediments of a drained caldera lake, and the low hills are lava domes erupted after the collapse. NPS.

What makes a caldera "resurgent"

The defining feature is a resurgent dome: after the collapse, magma moving in the chamber below pushes the caldera floor back up. A resurgent dome is an uplift, not an eruption, which sets it apart from a lava dome. A resurgent caldera is a broad depression ringed by low, steep cliffs or hills, the caldera walls. Its floor may be fairly flat where lakes once filled the basin, and inside are the resurgent dome and the lava flows and domes of later eruptions.

The biggest, such as Yellowstone, are popularly called supervolcanoes. Some geologists use the word for volcanoes that have produced eruptions of the highest magnitude, 8 on the Volcanic Explosivity Index (VEI); others prefer to speak of super eruptions, since a caldera system erupts at many sizes over its life and VEI 8 eruptions are rare in the geologic record. Caldera-forming eruptions are usually ultra-Plinian, VEI 7 to 8.

Five stages

Before the collapse, a caldera system's eruptions are usually smaller and varied, and may build lava domes, cinder cones or even composite volcanoes. The caldera-forming cycle then runs through five stages:

1. Swelling. Large volumes of silica-rich magma collect near the surface, lifting and faulting the ground.
2. Eruption and collapse. Magma bursts out along ring fractures in a vast eruption column, raining pumice and ash far and wide and sending thick pyroclastic flows across the area. As parts of the chamber empty, the roof collapses.
3. Widening. Unstable slopes fail and widen the caldera; lakes may fill the depression; small eruptions of lava domes or flows follow.
4. Resurgence. Magma moving in the shallow reservoir pushes the floor up.
5. Ring eruptions. Lava flows and domes erupt along the ring fracture.

Resurgent calderas form on continents. Yellowstone sits above a mantle plume, a continental hot spot; the Valles Caldera lies beside the Rio Grande Rift.

The magma system beneath Yellowstone: a deep reservoir of basaltic magma and a smaller, shallower one of rhyolitic magma. Modified from a USGS graphic.

Valles Caldera and Bandelier

Valles Caldera, in the Jemez Mountains of north-central New Mexico, is the type example of a resurgent caldera, the one against which others are compared; studies since the 1960s have shaped how geologists understand them. It is nearly 14 miles (22 km) across. Its resurgent dome, Redondo Peak, rises about 3,300 feet (1,000 m) above the floor to 11,258 feet (3,431 m), and the caldera walls in the Jemez stand about 1,650 feet (500 m) high. Several later lava domes line up along its ring fracture.

Two calderas lie in the Jemez Mountains: the Toledo Caldera, formed 1.6 million years ago and mostly outside the preserve, and the Valles Caldera, formed 1.24 million years ago, which made most of the preserve's landforms.

Next door, Bandelier National Monument is built of the Bandelier Tuff, ash-fall and pyroclastic-flow deposits from both caldera eruptions and from an earlier eruption 1.85 million years ago. Its upper part, the Tshirege Member, erupted 1.2 million years ago as the Valles Caldera formed and makes up most of the walls of Frijoles Canyon.

Redondo Peak, the resurgent dome of the Valles Caldera; the other hills inside the walls are lava domes erupted after the collapse. NPS.

Bandelier Tuff in the walls of Frijoles Canyon, Bandelier National Monument. NPS.

Yellowstone's three calderas

The Yellowstone Plateau, with its steaming geyser basins and cliffs of obsidian, has had some of the largest eruptions in geologic history: three caldera-forming eruptions during the Pleistocene.

The first is the largest; the Huckleberry Ridge Tuff alone covers about 6,000 square miles (15,500 sq km) and erupted in three main pulses that cooled at least partly before the next. The second, about 10 miles (16 km) across, is the smallest. After the third, great rhyolite lava flows spread over the plateau, most of them between 160,000 and 70,000 years ago. Yellowstone has two resurgent domes, Sour Creek and Mallard Lakes, each with gentle flanks cut by faults from the uplift.

The caldera wall along the Gibbon River, Yellowstone National Park. NPS.

Huckleberry Ridge Tuff about 20 km north of the caldera rim, near Mammoth Hot Springs. USGS.

Rhyolite lava flows erupted after the Yellowstone Caldera formed. USGS.

Ash and hot water

The VEI 7 and 8 eruptions that make these calderas spread ash across huge areas, in layered deposits sorted by grain size. The pyroclastic flows, ground-hugging currents of superheated gas, ash, pumice and rock, can cross ridges, travel many tens of miles and fill whole valleys; an eruption may come in phases years or decades apart.

The minimum area covered by ash from Valles Caldera and Yellowstone's Lava Creek eruption, with Long Valley Caldera and Mount St. Helens in 1980 for comparison. USGS.

The magma beneath a resurgent caldera also heats hydrothermal systems, far shallower than the magma itself. Yellowstone has more than 10,000 hydrothermal features, including more than 500 geysers.

The Yellowstone area. Modified from a USGS graphic.

Yellowstone's hydrothermal features; different fluids make different kinds of features. Modified from a USGS graphic.

Hazards

Calderas bring hazards both near and far. Nearby: ash clouds and falling ash and tephra, pyroclastic flows and surges, landslides, fumaroles and earthquakes. Farther away: ash clouds and ash fall. Later eruptions may be quiet lava flows or explosive, driven by magma or by magma meeting water. Both Yellowstone and Valles have active geothermal areas with their own dangers to visitors.

Sources

• National Park Service, "Resurgent Calderas"; rewritten in hubnx's own words. Photographs and figures: NPS and USGS. Two figures on the original page, by Colorado State University and from a commercial book, are not reproduced here.