VOLCANO GEODESY - THE SEARCH FOR MAGMA RESERVOIRS AND THE FORMATION OF ERUPTIVE VENTS

Routine geodetic measurements are made at only a few dozen of the worl d's 600 or so active volcanoes, even though these measurements have pr oven to be a reliable precursor of eruptions. The pattern and rate of surface displacement reveal the depth and rate of pressure increase wi thin shallow magma reservoirs. This process has been demonstrated clea rly at Kilauea and Mauna Loa, Hawaii; Long Valley caldera, California; Campi Flegrei caldera, Italy; Rabaul caldera, Papua New Guinea; and A ira caldera and nearby Sakurajima, Japan. Slower and lesser amounts of surface displacement at Yellowstone caldera, Wyoming, are attributed to changes in a hydrothermal system that overlies a crustal magma body . The vertical and horizontal dimensions of eruptive fissures, as well as the amount of widening, have been determined at Kilauea, Hawaii; E tna, Italy; Tolbachik, Kamchatka; Krafla, Iceland; and Asal-Ghoubbet, Djibouti, the last a segment of the East Africa Rift Zone. Continuousl y recording instruments, such as tiltmeters, extensometers, and dilato meters, have recorded horizontal and upward growth of eruptive fissure s, which grew at rates of hundreds of meters per hour, at Kilauea; Izu -Oshima, Japan; Teishi Knell seamount, Japan; and Piton de la Fournais e, Reunion Island. In addition, such instruments have recorded the hou r or less of slight ground movement that preceded small explosive erup tions at Sakurajima and presumed sudden gas emissions at Galeras, Colo mbia. The use of satellite geodesy, in particular the Global Positioni ng System, offers the possibility of revealing changes in surface stra in both local to a volcano and over a broad region that includes the v olcano.