Volcanoes have similar causes to earthquakes.Sheets of rock beneath the earth's surface push against each other with amazing power.The rock melts and becomes lava.The lava is forced up the earth's surface,where it erupts from volcanoes.Scientists can often predict when volcanoes will erupt,and give people plenty of time to escape.But when a volcano called Nevado del Ruiz erupted in 1985,the people in the nearby town of Armero were caught by surprise.