Metamorphic rocks constitute the deep roots of the Earth’s crust and occur exposed at surface, covering about 12% of the planet’s exterior. Metamorphic rocks result from modifications of the structure and mineral content of original igneous and sedimentary rocks (called protoliths) due to processes that occur predominantly at the solid-state, collectively known as metamorphism. Studying metamorphic rocks is the key to understand the processes that happens deep within the crust, like deformation or earthquakes, at conditions that we cannot access directly. The deepest ‘hole’ in the world, indeed, only drilled to about 11 km depth, a small fraction of the 30-70 km of thickness of the crust, or the nearly 200 km of the lithosphere, the outer ‘layer’ of our planet. Thankfully, we have entire mountain chains that expose deep metamorphic rocks, smashed and squeezed by collisions between continents, and volcanoes that kindly brings up fragments of the rocks magma encounters when it rises!
Metamorphism encompasses a wide range of processes that transforming rocks at depth. The IUGS Subcommission on the Systematics of Metamorphic Rocks defines metamorphism as:
“…a subsolidus process leading to changes in mineralogy, composition and/or texture of a rock. These changes are due to physical and/or chemical conditions that differ from those normally occurring at the surface of planets and in zones of cementation and diagenesis below this surface. The metamorphic processes may coexist with partial melting and may also involve changes in the bulk chemical composition of the rock”
This very complex definition highlights that metamorphism is a large container that includes all those processes occurring at the solid state, at depth (to distinguish metamorphism from near surface processes like diagenesis or weathering), and lead to changes in the mineral content and structure/texture of a rock. Therefore, the definition encompasses deformation (changes in shape) and mineralogic changes. Metamorphism ‘may coexist with partial melting’ and includes, therefore, migmatites, rocks that lie on the borderline between the metamorphic and the igneous process. The last sentence includes also a reference to metasomatism, which is the modification of the chemistry of a rock through the interaction with aqueous fluid.
The usage of ‘metamorphism’ as a container is highlighted by the existence of terms like ‘lightning metamorphism’ (i.e. local modification of a rock/sediment due to a lightning strike) or ‘combustion metamorphism’ (i.e. modification of a rock/sediment exposed to natural fires). These last two terms are also examples of metamorphism ‘at conditions that differ from those normally occurring at the surface’.
In this section, I will focus mainly on the ‘classic’ metamorphic rocks that form at depth and result from tectonic processes, such as subduction, mountain building, intrusion of magma, and deformation. E-mail me if you would like to see some difficult metamorphic concept explained or images of specific rock structures. If you like what I am doing, follow me on my socials or support me at the price of a coffee!