Final answer:
The underground pocket where magma collects is called a magma chamber. They are crucial in the formation of plutonic or intrusive igneous rocks and are found in volcanic regions where the caprock can trap the steam from heated water.
Step-by-step explanation:
The underground pocket where magma collects is known as a magma chamber. These chambers are usually found in volcanic regions, under the Earth's surface. The magma in these chambers can reach water temperatures in excess of 180°C due to the intense pressure that exists at such depths. A caprock often lies above the fractured rock within which magma chambers sit, able to trap the pressurized steam generated by the heated water.
Magma chambers are key to the formation of igneous rocks. If the magma remains underground and cools slowly, it forms coarse-grained igneous rocks like granite, which is plutonic or intrusive. Conversely, when magma erupts onto the Earth's surface, it cools quickly and forms fine-grained igneous rocks like basalt.
These formations can be associated with different geological activities such as tectonic plate boundaries and mantle hot spots. Mantle plumes, for instance, can create hot spots by heating the lithosphere from below, resulting in the formation of magma chambers and potentially leading to volcanic activity far from plate boundaries.