http://news.nationalgeographic.com/news/2011/04/pictures/110407-volcano-first-descent-magma-chamber-geographic-lava-iceland/#/06-into-icelands-volcano_34292_600x450.jpg
Magma chambers are usually located a few kilometers under the surface. When magma rises through cracks in the earth's crust it is less dense than surrounding rock. The magma chamber creates by a upwelling of heat fomr the earth's mantle. It distributes heat and molten rock up to the base of the crust. Magma is much denser than solid rock, and it will come up through cracks and fractures in the earth. If some of the magma cannot find its way out of the path upwards, it will gather into a reservoir and that where the magma chamber will form. Constant movement of more magma into the chamber will start to cause pressure in the chamber to rise. The magma chamber may not be able to contain the pressure from the magma, and it may rupture. The magma then rises upward through the fissure, resulting in the eruption of the volcano.
If a magma chamber experiences a stop of the flow the magma before it releases into a volcanic eruption. If this happens the magma may start to cool and crystalize where it is. When it does this, mineral crystals will form and settle to the bottom while the rest of the magma, drained of the elements pools to the top. Some of the specific minerals will form first and the crystallization will become more silica rich. When this happens the process is know as the fractional crystallization and results in a stratified igneous body, known as layered intrusion.
If a magma chamber experiences a stop of the flow the magma before it releases into a volcanic eruption. If this happens the magma may start to cool and crystalize where it is. When it does this, mineral crystals will form and settle to the bottom while the rest of the magma, drained of the elements pools to the top. Some of the specific minerals will form first and the crystallization will become more silica rich. When this happens the process is know as the fractional crystallization and results in a stratified igneous body, known as layered intrusion.