Between 1923 – 1985, ~60 cm of uplift was measured at Yellowstone, including ~18 cm of ground deformation measured by leveling between 1976 – 1984. This was followed by a shorter period of subsidence (1985 – 1992) with displacements of ~14 cm. Up until this time it was thought that all ground deformation occurred all in the central part of the Yellowstone caldera. However, between 1997 – 2003, 12-13 cm of uplift occurred in a different region measuring ~35 x 40 km centered on the northern caldera rim. Outside of this region subsidence of up to 4 cm was recorded. Since this time, ground deformation has refturned to the central part of the caldera. Between 2004 – 2006, GPS and InSAR data show that Yellowstone uplifted by up to 7 cm/yr, with total displacements of 17 cm. Models of this deformation episode infer magma intruded ~10 km beneath the caldera. This magma input is thought to have fractured the crust, releasing hydrothermal fluids and resulting in subsidence of the region to the north, which had previously uplifted. Further analysis of GPS data between 2004 – 2010 reveal horizontal extension of up to 0.7 cm beneath Yellowstone lake. Using evidence from borehole strainmeters, it is estimated that magma is present 3 – 6 km beneath the surface of Yellowstone and that the magma is partially molten.
The first GPS measurements at Yellowstone were made in 1987. By January 2006 there were 12 GPS receivers operating at Yellowstone. Data from the Yellowstone-Snake-River-Plain GPS network can be found at the University of Utah (http://www.uusatrg.utah.edu/ts_ysrp.html).
Deformation at Yellowstone is thought to be driven by both magmatic and hydrothermal sources. Cycles of uplift and subsidence have been attributed to hydrothermal systems. As pressure within the hydrothermal system increases, the reservoir eventually fails and trapped magmatic fluids escape. This causes the hydrothermal system to depressurise, and the overburden subsides. Once the ruptured zone reseals, the hydrothermal system repressurises and the cycle begins again.
It has also been proposed that the observed patterns of uplift and subsidence at Yellowstone results from variations in the movement of molten basalt in to and out of the volcanic system. As the flux of magma increases, uplift occurs. As the flux decreases, subsidence occurs. Subsidence could also be controlled by cooling or fluid loss from the magma.