The geological setting

It is only since the 1960s that Yellowstone's geology and volcanic history have been fully appreciated. In addition to mapping various solidified lava flows, geologists from the USGS (United States Geological Survey) have identified three distinct welded tuff deposits that extend over thousands of square kilometres both within and outside the park. This discovery is significant because tuff is formed only from highly explosive volcanic eruptions. Furthermore, the enormous volume of tuff that has been deposited provides evidence for past eruptions on a truly colossal scale.

The discovery of these deposits (collectively known as the Yellowstone group of tuffs) created a new question. How could such a huge amount of ash and lava bave been produced when there was no obvious volcano to be seen in the park? The answer eluded (ускользал, не приходил на ум) geologists until it was realised that the central area of Yellowstone National Park actually sits inside an enormous caldera, 72 km long by 48 km wide. From the ground there isn't a volcano or caldera in sight, but this is because it is so big that you are inside it! The full scale and outline of this caldera can only be appreciated from the air. Calderas are formed by the eruption of large bodies of magma through the crust.

Measurement of seismic waves travelling beneath the caldera has shown that they slow down significantly around 3 km beneath the ground. This indicates that there is a lot of molten magma, rather than just solid rock, at relatively shallow depths within the crust. Detailed seismic surveys have made it possible to estimate the dimensions of this body of magma. It is approximately 48 km long, 19 km wide and 10 km thick. The relatively low density of this molten material compared with solid rock also accounts for the lower than normal gravity values that are measured across the Yellowstone Plateau.

Like Hawaii, Yellowstone is far from any plate margin and therefore its volcanism must be explained by a hot spot. As at Hawaii (perhaps the best known hot spot), geologists believe that a plume of mantle is rising upwards from the asthenosphere, continually injecting magma into the lithosphere beneath Yellowstone.

No one knows exactly why mantle plumes occur where they do, but their existence in some places rather than others must relate to irregularities in the output of heat from around the Earth's core. Although both Hawaii and Yellowstone are located over hot spots, Hawaii is on oceanic crust while Yellowstone is on continental crust. This is the reason why the type of volcanism experienced in the two areas is so different.

Those who have studied volcanoes in Hawaii, such as Mauna Loa and Kilauea, will know that these are classic examples of shield volcanoes that frequently emit hot, runny basaltic lava in predominantly nonviolent eruptions. Yellowstone eruptions, on the other hand, are extremely rare but incredibly explosive. Yellowstone provides an excellent example of a hot spot under continental crust.