THE plates of Earth, the giant slabs that cover the planet, are always moving. Mount Pinatubo erupted because the Pacific plate went under the Continental plate. When a plate moves under another plate, it causes friction. The friction creates heat, which makes the rocks melt. When the rocks become liquid, they need room to expand. To find a place, they usually push upward in a volcano. When the top of the volcano can't hold back the lava anymore, it explodes.
