Field briefing: why volcanoes erupt

Rock deep inside the Earth can melt because of intense heat, forming magma. Magma is often less dense than the solid rock around it, so it can rise through cracks. It may collect in a magma chamber beneath a volcano. As more magma and gas gather, pressure can increase. If the pressure forces its way through a vent, an eruption occurs. Once magma reaches the Earth's surface, it is called lava. An eruption may also release ash, gas and rock fragments. These materials can be dangerous, and ash may travel far from the volcano. Volcanoes are common where tectonic plates meet: at some boundaries plates move apart, while at others one plate sinks beneath another. Not every volcano erupts in the same way, and scientists monitor changes such as earthquakes and gas emissions to help assess hazards.

Volcano Watch questions