Three extraordinarily large explosive eruptions in the past 2.1 million years each created a giant caldera within or west of Yellowstone National Park with the spread of enormous volumes of hot, fragmented volcanic rocks as pyroclastic flows over vast areas within times as short as a few days or weeks. The accumulated hot ash, pumice, and other rock fragments welded together from their heat and the weight of overlying material to form extensive sheets of hard lava-like rock. In some sections, these welded ash-flow tuffs are more than 400 m thick! These ash-flow sheets—from oldest to youngest, the Huckleberry Ridge, Mesa Falls, and Lava Creek Tuffs—account for more than half the material erupted from Yellowstone. The enormous outpouring of magma, 280 to 2,450 km3 during each explosive event, led to the collapse of magma-chamber roofs, causing the ground above to subside by many hundreds of meters to form the calderas.

Before and after these caldera-forming events, eruptions in the Yellowstone area produced rhyolitic and basaltic rocks—large rhyolite lava flows and some smaller pyroclastic flows in and near where the calderas collapsed and basalt lava flows around the margins of the calderas.

Photograph of the Huckleberry Ridge Tuff on top of Mount Everts, Yellowstone National Park, Wyoming
Welded tuffs form capping layer on top of Mount Everts
Huckleberry Ridge and Lava Creek ash-flow tuffs

At the top of Mount Everts east of Mammoth Hot Springs, a brownish-colored cliff held an important clue about the number of caldera-forming eruptions at Yellowstone. Click on image for the clue.

A general sequence of events was repeated in the evolution of each of Yellowstone's three volcanic cycles:

  1. A broad area, larger than that which will become the caldera is slowly uplifted. This uplift reflects the development and rise of large volumes of rhyolite to form a magma chamber at shallow depths in the Earth's crust. Stretching of the crust above the inflating magma chamber leads to concentric and radial fracturing and faulting at the surface, typically accompanied by the extrusion of lava flows from these fractures.

  2. At a critical stage in the evolution of the magma chamber, enormous volumes of the overpressurized rhyolite magma erupt explosively through the ring-fracture zone created above the magma chamber during inflation and uplift, producing extensive ash-flow sheets. As the eruptions partly empty the chamber of its magma, the roof of the magma chamber collapses along the same ring fractures to produce a large caldera.

  3. Postcollapse volcanism includes the extrusion of rhyolite lavas and smaller explosive eruptions of pyroclastic flows within or adjacent to the the caldera. In the present-day Yellowstone caldera, lakes formed where streams draining into or along the margin of the caldera were dammed by these thick intracaldera rhyolite flows, including Shoshone, Lewis, Heart, and Yellowstone Lakes. Shortly following collapse, the caldera floor may be uplifted by hundreds of meters in a process known as resurgent doming; this uplift reflects renewed pressure as magma rises again into the magma chamber. Hydrothermal activity (such as hot springs and geysers) occurs during all three stages but, in the third stage, it becomes the dominant or only visible sign at the surface of magmatic activity below.

Aerial photo of enormous rhyolite flows of the Madison Plateau, Yellowstone National Park, Wyoming
Thick rhyolite lava flows of the Madison Plateau
Scientists infer that rhyolite lava flows (left photo) as well as the caldera-forming ash-flow tuffs were fed from shallow magma chambers filled by the melting of rocks of the lower continental crust below Yellowstone. The heat needed to facilitate the melting process was supplied by the repeated injections of basalt magma from the mantle into the shallow crust. Click on images for a larger-sized image and description.
Photograph of Undine Falls and basalt flow, Yellowstone National Park, Wyoming
Undine Falls spills over basalt lava flow
Basalt lava flows (left photo), though subordinate in volume to rhyolites, have erupted throughout the 2.3 million-year volcanic history of the Yellowstone area. The magma feeding these eruptions originated in the upper part of Earth's mantle and resided only briefly in the crust before erupting at the surface. Click on images for a larger-sized image and description.

The long-term nature of volcanism in this part of North America suggests that more eruptions will occur as the Yellowstone National Park continues to evolve. The most recent series of eruptions, 160,000 to 70,000 years ago, extruded more than 20 thick rhyolite lava flows and domes, most of them within the youngest caldera. Other postcaldera lavas are basalts, erupted around the margins of the rhyolitic calderas. Based on Yellowstone's history, the next eruptions are likely to expel lavas, which might be either rhyolites or basalts, possibly accompanied by moderate explosive activity. Far less likely would be another enormous outpouring of material that could lead to a fourth caldera.
Copyright © 2006-21 Claud "Sonny" Rouch, all rights reserved. Website by OACYS Technology. Cover photo by Roberts Engineering.