Gravity effects of volcanic islands

Abstract

Gravity surveys of six volcanic islands in the Southern Cook Group (South-West Pacific Ocean) show that the radial anomaly gradient near the reef is much larger than the calculated effect for an island of uniform density. The gravity surveys also show that the difference between the modified Bouguer anomalies at the centre of each island and at the reef edge are much larger than the calculated effects for an island of uniform density. The observed values, however, are consistent with the calculated effects of an uncompensated island platform of density 2·35 g/cm³ which contains a core of density 2·87 g/cm³ and radius equal to the radius of the island at sea level. The gravity observations are incompatible with the theory of local isostatic compensation and therefore some form of regional compensation must apply. Gunn's isobaric hypothesis (lithospheric thickness 50 km) would give rise to a maximum depression below each island of less than 0·1 km and hence would have negligible gravitational effect. However, if the lithospheric thickness was only 10 km, the maximum depression would be 0·8 km and the densities of the island platform and core would be 0·14 g/cm³ larger than those computed on the basis of an uncompensated island.