A huge amount of loose ash and mud debris has been deposited on the flanks of the volcano. Much of the remaining vegetation is killed by the effects of ash. When there is heavy rainfall, this debris gets washed down the main valleys giving rise to mudflows (sometimes known as lahars). This hazard will continue until vegetation becomes re-established.
Ash is made of fine particles of volcanic rock and can be produced by three main processes (illustrated below): phreatic eruptions (caused by magma heating groundwater); explosions and pyroclastic flows.
Most of the ash on Montserrat has been blown to the west over Plymouth. Ash kills vegetation, can be a hazard to human and animal health and causes roofs to collapse.