Crater - The pit or depression at the top of the volcano. Caldera - A destructive feature that marks the site of collapse of a volcano's summit. Form when magma chamber beneath the volcano is emptied. Anatomy of a Volcanic Eruption Magma is generated at depth. Because magma is less dense it begins to rise.
Lithostatic pressure drops as magma rises and it begins to boil.
This releases gas which exerts outward pressure. At depth of 3-5 km magma reaches gravitational equilibrium. If outward pressure of gas exceeds lithostatic pressure an eruption occurs. Viscosity of the magma also an important consideration. Can remain suspended for weeks or years.