When magma moves into a volcanic system, and closer to the surface of the earth, the area surrounding the volcano may move upward and outward. This swelling is typically measured using the Global Positioning System (GPS).
Continuously recording GPS instruments are the most used type of volcanic deformation-monitoring equipment in the world. 24 satellites, orbiting the Earth twice each day, transmit their position in orbit to receivers on the surface of the Earth. The receivers record vertical and horizontal position, and by comparing these measurements through time, it is possible to determine the amount of movement for a specific location on the surface of the earth.
The GPS receivers near the Mono Lake volcanic field are part of the 46 instruments that make up the Long Valley Caldera deformation monitoring network. The first instrument was installed in 1995 and the network was updated and modernized between 2006 and 2008. Significant caldera uplift at Long Valley was measured in the years 1978-1983, 1990-1995, 1996, and 1997-1998.