A region, postulated to exist below the Earth’s surface, in which magma is received from a source region in the deep crust or upper mantle, stored, and from which it moves to the Earth’s surface at the site of a volcano. The existence of ‘magma chambers’ is often invoked by geochemists to provide a location for processes such as fractional crystallization, because the chemistry of lavas is explicable only in terms of such processes. When magma moves rapidly from the chamber, as in a pyroclastic flow eruption, the unsupported chamber roof may collapse to produce a caldera at the surface. The diameter of the caldera can be used to estimate the diameter of the underlying magma chamber; diameters of up to 40 km are not uncommon for terrestrial subvolcanic magma chambers.