Crater lakes are often part of the life cycle of a volcano. To begin with, a magma chamber becomes active deep underground and causes an eruption. The power of the pressure in the magma chamber also cracks the rocks above.

When the magma has mostly been blown out, the weight of rock on the surface is what was there before plus what the volcano has just erupted. Sometimes this causes the whole cone to collapse back down the magma chamber, leaving only a rim to mark where the top of the cone once was.

Over many centuries, rain falling in this crater begins to form a lake because, like any other bowl, there is nowhere for the water to run away to. This is the stage you see at Crater Lake Oregon. But you can also see the volcanic cone rebuilding itself. This is Wizard Island. It may not look much now (although it is actually hundreds of metres tall as it rises from the lake bed), but it will build up a massive cone in the next few thousand years.