The Soufrière Hills volcano is composed of a lava dome complex. It is not a symmetrical cone shape, as is commonly associated with volcanoes, because the erupted lava is very viscous or sticky. As viscous lava is not very fluid, it cannot flow away from the vent easily when it is extruded. Instead it piles up on top of the vent forming a large, dome shaped mass of material.

Photo of morphology of lava dome
Graphic depiction of morphology

Characteristics of lava domes (illustrated below) include incandescence (glowing) at night; growth of spines and periodic collapses of steep sides, producing rockfalls and pyroclatic flows.

Photo showing spine growth
Collapse of the steep sides, January 1998

Incandescence, January 1997

The other large mountains of the Soufrière Hills are old domes from previous eruptions of the volcano. These include Chances Peak, Gages Mountain, Galway's Mountain, Roche's Mountain and Castle Peak. The latter is now completely engulfed by the new dome.

Map of Soufrière Hills
Photo of Castle Peak Dome