A Magma Chamber is where magma pools beneath a volcano. As magma rises through the crust it pools as a mass of molten rock.The chamber is not an underground cave but can either be an area of fractured rock or a complete body of molten rock within which convection currents operate.
The magma will either be erupted or may cool in situ causing crystals to form and eventually solidification. Lighter Silica rich minerals will float to the top and heavier iron rich minerals to the bottom. Magma chambers can be detected up 10km below the surface. Granite,grano-diorite,dioritr and gabbro are the coarseley crystalline rocks found in solidified magma chambers.