Montserrat's Soufriere Hills volcano erupted because the underlying pressures beneath the rock grew too high and exploded. A pyroclastic surge was observed, which is a mixture of hot gas and rock that flows at high speeds down the sides of the blown volcano. The presence of this flow means Montserrat experienced a thick lava flow that was impeded, built up and collapsed into a fast-moving pyroclastic surge.
Volcanic eruptions occur when magma from deep beneath the Earth's surface escapes to the top and emerges as lava. If the magma contains dissolved gases like hydrogen, carbon dioxide and sulfur dioxide, the sudden release of pressure at the less dense surface results in a massive explosion. When the magma is deep within the Earth, the pressure keeps the magma semi-solid and the contents very hot.
The Earth's mantle is beneath the crust and on top of the radioactive iron core. This radioactivity partially melts the mantle, and heat transfer by convection occurs because the lower mantle is warmer than the upper mantle. Warm substances rise and cooler substances fall, so convection cells arise and the mantle becomes a constantly-roiling mass. This motion causes plate tectonics and magma breaks through weak layers of rock above, forming a volcano.