Chapter 8: Metamorphism and Metamorphic Rocks

 

8.1 

 

Metamorphic rock comes from igneous, sedimentary, or other metamorphic rocks. The rock that is transformed into a metamorphic rock is called a parent rock. Metamorphism is the process that transforms the mineralogy, texture, and often the chemical composition of the rock. 

 

Metamorphic grade is the intensity of heat and pressure a rock has undergone. Low‑grade rocks show only slight changes, medium‑grade rocks develop new minerals like garnet, and high‑grade rocks form under extreme conditions that nearly erase the rock’s original features.

 

8.2

 

There are four agents of metamorphism:

 

1. Heat

 

Heat is the most important of the metamorphic agents as it drives the recrystallization of the existing minerals in the parent rock. An increase in temperature causes the atoms of a mineral to vibrate more rapidly. New or enlarged mineral grains at the expense of old mineral grains is called recrystallization. 

 

2. Pressure

 

Pressure increases with depth because the thickness of the rock above increases. Rocks that are buried are subject to confining pressure which means there is pressure equally on all sides. As pressure increases, the tiny spaces between mineral grains are squeezed shut. The rock becomes denser and more compact. If the pressure becomes extremely high, the atoms inside a mineral can rearrange into a tighter, more stable structure. That transformation from one mineral form to another is called a phase change (or a change between polymorphs).

 

3. Differential Stress

 

Differential stress is the kind of pressure that pushes harder in one direction than in the others. Instead of squeezing a rock equally from all sides (which is confining pressure), differential stress applies a stronger force from a particular direction. This usually happens where tectonic plates collide. Because the stress is uneven, minerals and rock layers respond by bending, flattening, or aligning in the direction of least resistance.

 

4. Chemically Active Fluids

 

Chemically active fluids are hot, ion‑rich solutions that move through the tiny spaces and fractures in rock during metamorphism, allowing atoms to dissolve, migrate, and re‑attach in new arrangements. Because these fluids speed up mineral reactions, they help minerals recrystallize more easily, grow larger, or change into entirely new minerals without the rock needing extreme temperatures or pressures.

 

8.5

 

Contact metamorphism happens when rock is “baked” by the intense heat of nearby magma. Because the heat is localized around the intrusion, only the rocks touching or close to the magma chamber are altered, and the changes are driven mostly by temperature rather than pressure. This produces a narrow zone of metamorphosed rock, called a metamorphic aureole, where minerals recrystallize into new, heat‑stable forms without significant deformation.

 

Hydrothermal metamorphism occurs when hot, chemically active fluids circulate through rock, usually near mid‑ocean ridges or other volcanic settings. These fluids dissolve ions, transport them, and trigger rapid mineral reactions that replace original minerals with new ones. Instead of high pressure or extreme heat, the key driver is fluid‑rock interaction, which can dramatically change a rock’s chemistry and create mineral‑rich deposits.

 

Subduction zone metamorphism develops where an oceanic plate is pushed deep beneath another plate. As the slab descends, it experiences very high pressure but relatively low temperature compared to other metamorphic settings. This unusual combination produces distinctive high‑pressure minerals, like blueschist, that form only under the cold, compressed conditions of a subduction zone. The rock is squeezed intensely but not heated enough to melt or fully recrystallize.

 

Regional metamorphism affects vast areas of crust during large‑scale mountain‑building events. When continents collide, rocks are buried, compressed, and heated over millions of years, creating broad belts of metamorphic rock. Because both temperature and differential stress increase, minerals grow, align, and recrystallize, producing foliated textures such as schist and gneiss. This is the most widespread form of metamorphism and shapes the cores of major mountain ranges.