What causes a volcano to erupt?
A volcano erupts when hot, molten lava breaks through a thin or weak place in Earth's crust. Since the weight of Earth's crust rests on this layer of liquid rock inside Earth, the liquid rock is under a lot of pressure, and can really explode quite violently once it breaks through. Ash from volcanoes can spread for many miles and rise high into the atmosphere. http://scijinks.gov/volcano