The acceleration of gravity varies between points in space and depends a.o. on the distance to the center of the Earth, geographic latitudr, height above sea level (or geoid), and the distribution of mass around the measuring point. Gravity measurements can be made with better relative accuracy than most other measurements. If they are applied with precise geodetic measurements they can be used to determine the mass change beneath the measuring point. They have been used e.g. to estimate the density of the mass responsible for the inflation and deflation of volcanoes.
Gravity measurements at Kleifarvatn
Recent publications of the Institute of Earth Sciences on this topic:
Rymer, H., C. Locke, B. G. Ófeigsson, P. Einarsson, and E. Sturkell (2010). New mass increase beneath Askja volcano, Iceland – a precursor to renewed activity?Terra Nova, 22, 309-313.doi: 10.1111/j.1365-3121.2010.00948.x
Ágústsdóttir, Þ., Magnús Tumi Gudmundsson and Páll Einarsson (2010). A gravity study of silicic domes in the Krafla area, North Iceland. Jökull 60, 135-148.