Magma chamber

The residing magma starts to cool, with the higher melting point components such as olivine crystallizing out of the solution, particularly near to the cooler walls of the chamber, and forming a denser conglomerate of minerals which sinks (cumulative rock).

If magma resides in a chamber for a long period, then it can become stratified with lower density components rising to the top and denser materials sinking.

Additionally, the removal of the lower melting point components will tend to make the magma more viscous (by increasing the concentration of silicates).

[12] If the magma is not vented to the surface in a volcanic eruption, it will slowly cool and crystallize at depth to form an intrusive igneous body, one, for example, composed of granite or gabbro (see also pluton).

[14] In Iceland, Thrihnukagigur, discovered in 1974 by cave explorer Árni B. Stefánsson and opened for tourism in 2012, is the only volcano in the world where visitors can take an elevator and safely descend into the magma chamber.