Heat flow and the mid-Atlantic rift volcanism of San Miguel Island, Azores

Abstract

The volcanoes of San Miguel Island appear to be fed by a single elongated magma chamber at a mean depth of about 5 km. Magma probably rises from the upper mantle to that chamber through the fractures of a branch of the mid-Atlantic rift which is assumed to cross the central part of the island. The magma then flows to either side of the rift along the chamber and, as solidification proceeds, the temperature is supposed to be kept close to 1200°C (about 1500 K), which gives a theoretical gradient of 100–400°C km −1 (or mK m −1), depending on position and on thickness of the magma chamber roof. At the flank of one of the active volcanoes of San Miguel, a gradient up to 300°C km −1 was recently measured in a borehole. This corresponds to a heat flow close to 15 μcal cm −2 sec −1 (equivalent to 0.63 W m −2) through the magma chamber roof which must be about 4 km thick.