One of the most spectacular phenomena that reminds us that Earth is a restless, geologically active planet are volcanic eruptions, the spectacular emission of molten lava, gas, and other materials at the surface. Volcanoes are spectacular, but are only the tip of the iceberg of what is happening underground. Lava is constantly being pumped out of a deep system that produces, stores, and transports to the surface molten material or magma from large depths.
Not all magma reaches the surface and becomes lava. Most of it remains trapped at depth, producing the ‘underground’ equivalent of a volcano: an igneous intrusion, a magmatic body that crystallized under the surface. The difference between a magma crystallizing at depth and a lava solidifying on the surface may appear insignificant, but it produces rocks with completely different texture. Volcanic or extrusive rocks, emitted on the surface, solidify very rapidly, cooled down at the contact with the atmosphere or water, and have to cope with degassing and the explosive activity that characterizes volcanic eruptions. As a consequence these rocks, tend to show very tiny crystals, a lot of volcanic glass, and eventually bubbles that formed as gas tried to escape the molten material. The explosive activity of volcanoes fragments volcanic rocks into all sort of debris, known as pyroclastic rocks or tephra.
On the contrary, magma that emplaces at depth is surrounded by rocks that are good thermal insulators and cools down over thousands to millions of years, producing plutonic or intrusive rocks. In these rocks, large crystals have time to form and develop and the resulting texture consists entirely of crystals (think of the granite countertops that are found in every kitchen). Differently from volcanoes, we cannot see magma stored at depth, but we can observe the spectacular effects of the heat it releases on the surface, in the form of geysers, hot springs or even phreatic eruptions. We alo use the energy coming from this magma, harvested as geothermal energy. Gas and other volatiles, confined in magma at such high pressure, can form hydrous minerals or exsolve from magma in the latest stages of crystallization, producing masses of huge crystals (pegmatites) that are the joy of every mineral collector, or form mineralizations within and around the intrusions, exploited by many mining activities in the world.
We, humans, like to classify things in simple (black and white) categories but, within the container of igneous rocks, there is a wide range of structures and textures, including some that are intermediate between plutonic and volcanic rocks and that document the transport of magma through the crust (i.e. subvolcanic rocks). Similarly, there are also rocks that are intermediate between igneous and metamorphic rocks (migmatites) that documented the melting of the crust the formation of magma at very high temperatures. There are several ways and viewpoints to investigate igneous rocks, from the volcanologists that study volcanoes and related hazards, to the geophysicists imaging magma at depth in the crust, and the petrologists, studying igneous processes now frozen in igneous rocks. The formation and migration of magma is linked to plate tectonics and it is only by investigating all these aspects of magmatism that we can understand what is happening beneath our feet.