Yellowstone is over a Hot Spot.
On the surface Yellowstone appears as a flat plateau marked by geothermal features but underneath the surface is where the excitement originates. Yellowstone’s geodynamic setting is designated a “hot spot” (Breining, 2007; Rosi et al.,1999). According to the 1999 Firefly Guide to Volcanoes “a hot spot is an area in the middle of a plate where magma rises from the mantle; the magma erupts through the plate, creating a series of volcanoes whose alignment indicates the movement of the plate over the hot spot” (p. 99). A hot spot can drift slowly across the plate (Van Rose, 1992). Yellowstone is also known as an intraplate volcano (Rosi et al., 1999).
Yellowstone is a Caldera.
Yellowstone’s structure is a caldera which is defined in the Firefly guidebook (1999) as a “large circular depression shaped like a cauldron or basin produced by the collapse of a volcanic structure, usually the roof of a magma chamber” (p.100). The collapse of a magma chamber usually results from the loss of internal pressure following a large eruption. The sequence of the phenomena is recreated in stages. First, is a plinian eruption column from a single vent. Magma is ejected from the chamber at a great speed, producing pyroclastic flows. Next, the roof of the magma chamber begins to collapse. More vents are forced open which collapse and finally the caldera is formed. After their formation, caldera depressions can be sites of further volcanic activity. After the caldera is formed, the eruptive materials from the explosion can accumulate, leading to the partial or even total refilling of the caldera. In some cases the depression is no longer visible (USGS, 2013).
It has Erupted Three Times Already.
The explanation of why Yellowstone is a caldera goes back millions of years. As noted by Rosi et al.(1999), "The volcanic activity at Yellowstone took place in three cycles over the course of the last two million years. Each cycle involved the emission of rhyolitic lava and was an explosive event of enormous size. These large explosive eruptions took place 2 million, 1.3 million and 600,000 thousand years ago. Following these large eruptive explosions a caldera depression formed in the Yellowstone area with an extension of 28 by 47 miles and a depth of several hundred yards. Later volcanic activity, together with the erosive action of glaciers, partially covered and reshaped the walls of the caldera so much that now they are barely visible” (p.238-239).
The Magma is Rising Again.
The caldera floor can raise up when new magma is forced up as the magma chamber refills. This is called resurgence. (Rosi et al., 1999). In the past ten years, scientists at Yellowstone have observed that two vents known as "resurgent domes" are rising again. On of them, the South Creek dome, is causing the Yellowstone Lake to "tilt" southward, affecting the shore lines. Fascinating discoveries, such as more geysers, rock spires, and natural chimney pipes have been discovered under the lake ("Yellowstone lakes," 2012). According to recent research, Oskin (2013), has reported that the 37 mile long, 18 miles wide, and 3-7 miles deep magma chamber under Yellowstone is fifty percent larger than originally thought. Underneath this magma reservoir is a plume of hot rock rising from the Earth's hot mantle that is almost 14 thousand cubic miles in size (Sealy, 2013).