Answer: - Magma chamber.
Explanation:-
1. A magma chamber is a cavity which acts a reservoir of magma and it feeds a volcano.
2.A magma is basically a mixture of molten and semi molten rocks.
3. The magma is less dense than the layers of rocks and as a result of this it tends to move upwards towards the surface of the earth but when it encounters a layer of rocks in this upward journey that hinders its movement then it gets collected in a large pool which is known as the magma chamber.
4. These magma chambers are under a great pressure which results in the breaking of the rock which surrounds the magma chamber with time. Once, these rocks break the magma can then travel upwards and this results in volcanic eruption.
Thus, these magma chambers are the cavities containing the magma that feeds the volcano.
