The Earths metallic core is cloaked by a mantle of molten rock, which is in a constant state of agitation as it rises, cools, and sinks. Tremendous pressures build up beneath the Earths relatively thin crustthe ground on which we walk. This endless churning has split the surface into 20 or so rocky slabs, or plates, that slowly drift about on the mantle. Every so often, like a shaken bottle of carbonated soda, hot, liquefied, gas-infused rock called magma squirts out the top in the form of lava.
Sometimes the eruption is sudden and violent, as was the case at Mount St. Helens in southwestern Washington State in 1980, and at the Philippines Mount Pinatubo in 1991. At other timesdepending on the nature of the magmaeruptions are relatively slow and quiet. People had plenty of time to get out of the way of Hawaiis Mauna Loa volcano in 1984. Its lava crept down the slope at about the speed of thick honey.