The notorious 1980 "bulge" at Mount St. Helens, extraordinary ups and downs of Yellowstone and Long Valley calderas, sliding south flank of Kīlauea volcano, and subtle swelling of Augustine volcano in Alaska prior to its 2005 eruption: these are just a few examples of surface deformation, and they provide important information about the inner structure and workings of volcanoes.
Changes to a volcano's ground surface (volcano deformation) appear as swelling, sinking, or cracking, which can be caused by magma, gas, or other fluids (typically water) moving underground or by movements in the earth's crust due to motion along faults. Often, this deformation is very small in magnitude—a few centimeters (inches) or less—and so can only be detected and monitored with very sensitive instruments.
The Volcano Hazards Program installs and maintains networks of instruments around volcanoes to monitor changes to the ground surface over time. Ground-based measurements are made of specific points on the earth's surface with Global Positioning System (GPS) receivers, tiltmeters, and strainmeters. Space-based observations of large areas are accomplished using satellite technologies such as InSAR. By combining these with data from other monitoring techniques (geologic observations plus earthquake and gas detection), VHP scientists can better understand what is happening beneath the ground, know when a volcano's behavior shifts away from "background," and provide appropriate warning when a volcano is showing signs that it may erupt.