DESCRIPTION:
Gravity Measurements

Another technique used to monitor the deformation of the volcano (Web note: volcano referred to - Kilauea, Hawaii) is measurement of the force of gravity at a given point on the Earth's surface. The Earth's gravitational attraction changes in response to elevation changes (gravity is stronger the closer one is to the center of the Earth) and to subsurface mass changes (for example, gravity increases when intruding magma fills void space in the volcanic edifice). Data from a network of gravity stations on the island of Hawaii show higher gravitational attraction over the summits and rift zones of the active volcanoes. These anomalies are caused by the subsurface concentration of dense dikes and bodies of olivine, one of the first minerals to crystallize from basaltic magma.

In addition to monitoring gravity over a large area, more precise gravity measurements are made at the summits of Kilauea and Mauna Loa to record the gravitational response of the volcanoes to eruptive activity. Two gravity meters are used to make periodic readings at a stable base station and at a station near the center of inflation/deflation. Leveling or continuous tilt measurements are used to establish the station elevations at the time of gravity measurements. The gravity data, after they are corrected for elevation change, reveal the addition or loss of mass (magma) from the volcanic system.

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