Geologic Research Reveals Typical Eruption Sequence
Volcanic rock deposits from eruptions in the Yellowstone Plateau Volcanic Field reveal that a typical sequence occurs as the magma storage region evolves. Caldera forming events are the most known and talked about, but lava flows both precede and follow the massive explosive eruptions.
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 the image to learn more.
A general sequence of events was repeated in the evolution of each of Yellowstone's three volcanic cycles:
- Slow uplift occurs over a broad area (larger than that of the future caldera). This uplift reflects the development of a magma storage region within the Earth's crust several kilometers below the surface. As the North American tectonic plate moves to the southwest, lesser evolved (basalt) magma within the Yellowstone hotspot rises, interacts with, and stalls within the dense crust. The stalled magma interacts with surrounding rock, cools, and evolves into a rhyolite; all the while, the hotspot continues to feed magma from deep within the Earth. Stretching of the crust above the inflating magma chamber leads to concentric and radial ("ring") fracturing and faulting at the surface, typically accompanied by the extrusion of basalt and rhyolite lava flows from these fractures.
- During the evolution of the magma chamber, excess pressure builds up within the system. At a critical stage in enormous volumes of rhyolite magma erupt explosively through the ring-fracture zone that was created during inflation and uplift. Massive quantities of tephra and pyroclastic density currents produce extensive ash-flow sheets (ignimbrite deposits). As the eruptions progressively empties the storage region of its magma, the roof of the magma chamber collapses along the same ring fractures to produce a large caldera.
- After collapse, rhyolite lava flows and smaller explosive eruptions occur within or adjacent to the caldera. Shortly following collapse, the caldera floor may be uplifted by hundreds of meters (feet) in a process known as resurgent doming. This uplift reflects renewed pressure as magma rises again into the magma chamber. Magma rises from the hotspot and stalls beneath the larger rhyolite magma storage region, but some finds its way to the surface along its edges and erupts as plateau-margin basalt lava flows. 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.
In the present-day Yellowstone caldera, lakes formed where streams draining into or along the margin of the caldera were dammed by these thick intra-caldera rhyolite flows, including Shoshone, Lewis, Heart, and Yellowstone Lakes.
Scientists infer that rhyolite lava flows 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.
Basalt lava flows, 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 post-caldera 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.