What is a metamorphic rock?
It's a rock that "changes form", or it alters the mineralogy, texture, and chemical composition of a rock. This doesn't include melting, as this would enter the realm of igneous rocks.
Types of metamorphosis
There are two types of metamorphosis. The first is contact metamorphosis, where there is a large temperature increase, due to an intrusive igneous body (body of magma).
The second is called regional metamorphosis, which occurs during mountain building, where there is a large scale deformation due to high heat and pressure.
Agents of metamorphosis
This provides the energy for the chemical reactions that cause most metamorphism. The place that the energy could come from are:
- Intrusive igneous body
- Increasing depth
This increases with depth. This is similar to water pressure, also known as confining pressure, where the distribution is uniform in all directions. This causes the spaces between minerals to compact, and rocks are created. If the forces aren't even distributed, the rocks are shortened in some directions.
If recrystallization occurs simultaneously with compaction, this creates foliations. Foliation only occurs if the crystals in rocks are realigned such that they appear in parallel bands. They occur perpendicular to the differential stress.
Chemically Active Fluids
There could be chemically-active fluids in the tight spaces within rocks. They are usually a combination of water, and another volatile liquid (prone to sudden change).They move ions from one place to another causing larger or longer crystals to form.