Wiktionary
n. (context geology English) A large underground pool of molten rock lying under the surface of the earth's crust.
Wikipedia
A magma chamber is a large underground pool of liquid rock found beneath the surface of the Earth. The molten rock in such a chamber is under great pressure, and given enough time, that pressure can gradually fracture the rock around it creating outlets for the magma. If it finds a way to the surface, then the result will be a volcanic eruption; consequently many volcanoes are situated over magma chambers. Magma chambers are hard to detect, and most of the known ones are therefore close to the surface of the Earth, commonly between 1 km and 10 km under the surface. In geological terms, this is extremely close to the surface, although in human terms, it is considerably deep underground.
Usage examples of "magma chamber".
Beneath the surface is a magma chamber that is about forty-five miles across—.
Hesitating only to displace the cooler, enclosing rocks, the flow formed an underground reservoir of molten material known as a magma chamber, where it built up immense pressures.
For it to be trapped in this basalt implies a metamorphic slush - lava partially melting rock in the throat above the magma chamber, and then throwing it all up.
He'd been successful at Fort Weyr in drilling into the old, still-hot magma chamber that provided delightful quantities of heat, especially for the hardening of dragon eggs on the sandy floor of the Hatching Ground.