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.
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.
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.