volcano is an opening in the earth's crust through which lava, volcanic ash, and gases escape. ...
Volcanoes erupt when molten rock called magma rises to the surface. ... As the magma rises, bubbles of gas form inside it. Runny magma erupts through openings or vents in the earth's crust before flowing onto its surface as lava. If magma is thick, gas bubbles cannot easily escape and pressure builds up as the magma rises
