Yellowstone’s volcanic history spans over 2 million years and includes two of the largest known eruptions in the history of the planet. The last time Yellowstone’s caldera erupted was roughly 70,000 years ago. No eruptions have taken place since then but there have been a number of hydrothermal explosions. Yellowstone has the Earth’s largest concentration of geysers.
In its volcanic history, the Yellowstone supervolcano has gone through three major volcanic cycles making the hotspot what it is today. The climax of a volcanic cycle is an eruption, but events taking place before and after each eruption form the landscape around the caldera including lakes and ridges.
Magma build-up under the Earth’s crust causes a large area to rise, this process takes thousands of years. A magma chamber is formed at a relatively shallow depth beneath the surface. The build-up often causes cracks and fissures in the ground out of which lava-flows develop. This build-up in pressure ultimately results in eruption.
In addition to the explosive eruption, massive ash flows emanate from the site. These ash flows can be up to 400 meters thick. The Madison Plateau, pictured below, is the result of such an ash flow. Having been partially emptied of its magma, the chamber collapses producing the caldera.
Smaller eruptions continue emitting smaller amounts of ash and lava. Present day lakes in the Park such as Shoshone and Yellowstone lakes, were formed when streams or rivers drained into the caldera and were dammed by ash flows. A process called “resurgent doming” sometimes causes the floor of the supervolcano to rise by several hundred meters. Geysers and hot-springs become more visible and are signs of hydrothermal activity below the Earth’s surface.