Magma chamber

A magma chamber is a large pool of liquid rock under the surface of the Earth. 

The molten rock, or magma, in such a chamber is less dense than the surrounding  rock. This produces buoyant forces on the magma that tend to drive it upwards. 

If the magma finds a path to the surface, then the result may be a volcanic eruption. Many volcanoes sit over magma chambers. 
These chambers are hard to detect if they are deep within the Earth. Most of those known are close to the surface.

 References 

Category:Volcanoes