Introduction

Volcanoes are a natural way that the Earth and other planets have of cooling off. Planets are hot inside because it is about 16-40km deep under the crust (hard surface made of rocks and soil) and the rock around is a good insulator. Heat inside planets escapes towards their surfaces. Heat sometimes melts rocks, which geologists called magma, then rise buoyantly toward the planet's surface, because it is less dense than the surrounding rock (like a hot air balloon rising through the cooler air). When the magma and included gases break through the crust, an eruption occurs. When it reaches the surface, it is called lava. The build up of ash and lava flows around the eruption hole (or vent) and accumulates to makes a volcano. Some volcanoes erupt for only a short time and never erupt again. Large volcanoes such as strato volcanoes and shields erupt many thousands times throughout their lifetimes of hundreds of thousands to a few million years.