As magma begins to push toward the surface within a volcano, the volcano bulges outward and upward, and cracks or fissures may develop or become wider. Whatever the deformation, it is important to monitor because it could give an indication of an impending eruption.
Much of the deformation Earth’s surface consists of movements that are difficult to detect by the human eye. The displacements can be vertical or horizontal and can be measured to high precision by the use of sensitive instruments called geodetic distance meters or geodimeters.
Geodimeters measure the distance between two points. The geodimeter is situated at one particular point. A laser beam is emitted from the geodimeter, hits a retroreflector at a second point, and returns to a recording device on the geodimeter. The distance between the points is measured from the time that takes the beam to make a round trip between instrument and reflector. If the distance between two points changes with time, then the earth between the two points is deforming.
On volcanoes it has been shown that as the deformation rate increases, the probability of an eruption increases. Constant, steady displacements do not generally signify that an eruption is imminent, as they are often caused by tectonic forces rather than magma movement. In contrast, significant increases in the rate of deformation are more often related to magma movement rather than tectonic forces.