Montserrat is situated in the northern part of the Lesser Antilles, a volcanic island arc formed along the junction where the Atlantic tectonic plate subducts beneath the Caribbean plate. Nearly all the islands along the arc are the result of subduction related volcanism. Most have andesitic stratovolcanoes, which have been produced by explosive eruptions, coupled with the growth of lava domes and associated pyroclastic flows, and the occasional extrusion of lava flows.
Montserrat is only 16 km long (north - south) and 10 km wide (east - west), and is built almost exclusively of volcanic rocks. The island comprises three volcanic centres or massifs of differing age. These are, from oldest to youngest: the Silver Hills in the north; the Centre Hills in the centre; and the active volcano of the Soufriere Hills and South Soufriere Hills in the south. In addition, Garibaldi Hill and St. George's Hill form two smaller, isolated topographic highs. The island's interior is densely vegetated, with the exception of areas affected by the recent eruption. Rock exposures are thus largely limited to coastal cliffs, road cuttings, and inland cliffs.
The island is mainly composed of andesitic lavas and volcaniclastic rocks produced by dome-forming eruptions; although the South Soufriere Hills are of basaltic to basaltic-andesite composition. The main products consist of the remnants of andesite lava domes; andesitic breccias representing the talus of previous lava domes; pyroclastic flow deposits formed by the collapse of lava domes; lahar and debris avalanche deposits; and subordinate tephra fall deposits. There are zones of hydrothermally altered rocks, and active fumarole fields (known locally as soufrieres) occur on the Soufriere Hills Volcano.