How Are Metamorphic Rocks Formed |❓Question-and-Answer FAQ

How Are Metamorphic Rocks Formed?

Metamorphic rocks are one of the three main types of rocks, along with igneous and sedimentary rocks. But how exactly are metamorphic rocks formed? Understanding the process of metamorphism helps explain why these rocks have such unique properties.

The Process of Metamorphism

Metamorphism is the process by which existing rocks are changed by heat, pressure, and/or chemical reactions. The conditions for metamorphism typically exist deep below the Earth’s surface. Rocks become buried miles underground and are subjected to enormous amounts of heat and pressure from the surrounding rock layers. They may also interact with hot, chemically active fluids. Under these intense conditions, the minerals, textures, and overall composition of the rocks begin to change and transform into metamorphic rocks.

There are three main types of metamorphism:

• Contact Metamorphism - when a hot igneous intrusion contacts and bakes surrounding rocks
• Regional Metamorphism - when rocks are subjected to high temps and pressures as they are buried deep in the Earth's crust
• Hydrothermal Metamorphism - when hot, reactive fluids flow through and alter rocks

The degree of change rocks undergo depends on the intensity of the metamorphic environment. Higher temperatures and pressures lead to more dramatic metamorphism. The changes that take place transform the rocks into completely new types without melting them. This is what distinguishes metamorphism from melting processes like igneous activity.

Characteristics of Metamorphic Rocks

Metamorphic rocks have distinct characteristics that reflect the intense process of metamorphism they undergo. Some key features include:

• Foliated texture - Many metamorphic rocks have a banded or foliated appearance. This is caused by the realignment of minerals in response to pressure.
• Crystalline structure - Increased temperatures allow the component minerals to recrystallize into larger, well-formed crystals.
• New minerals - Existing minerals transform into different minerals that are stable under metamorphic conditions, like garnet, mica, and staurolite.
• No fossils - The intense metamorphic environment destroys any fossils that might have been present in the original rocks.

These traits allow geologists to easily identify rocks that have undergone metamorphism.

Parent Rocks and Metamorphic Rock Types

Almost any type of pre-existing rock can become a metamorphic rock. The original rocks that undergo metamorphism are referred to as parent rocks. Common parent rocks include:

• Shale - metamorphoses into slate, phyllite, or schist
• Sandstone - metamorphoses into quartzite
• Limestone - metamorphoses into marble
• Basalt - metamorphoses into greenstone

The type of metamorphic rock that forms depends on the parent rock composition and the degree of metamorphism. Some of the main types of metamorphic rocks include:

• Slate - fine grained, foliated rock derived from shale
• Schist - medium to coarse grained rock with aligned mica flakes
• Gneiss - banded rock with light and dark mineral bands
• Quartzite - hard metamorphic rock made of interlocking quartz crystals
• Marble - recrystallized limestone composed of calcite or dolomite

Metamorphic Facies and Zones

Geologists use the concept of metamorphic facies and metamorphic zones to categorize metamorphic rocks based on the specific conditions under which they formed. A metamorphic facies refers to a group of metamorphic minerals that are indicative of certain temperatures and pressures. Some examples include:

• Greenschist facies - lower temperature, produced by regional metamorphism
• Amphibolite facies - higher temperature, associated with mountain building
• Granulite facies - very high temperature metamorphism

Metamorphic zones describe the location where certain facies of metamorphic rocks are found exposed at the surface. The boundaries between metamorphic zones are called isograds. Some major metamorphic zones include:

• Zeolite zone - lowest grade zone near the surface
• Greenschist zone - characterized by the greenschist metamorphic facies
• Amphibolite zone - dominated by amphibolite facies gneiss and schist

Examining the metamorphic minerals and zones helps geologists reconstruct the pressure and temperature conditions that produced specific metamorphic rocks.

Metamorphism and Plate Tectonics

The movement and collision of tectonic plates generates ideal conditions for metamorphism deep in the crust. Subduction zones, where oceanic crust descends beneath continental crust, produce high pressure and temperature zones that metamorphose rocks. Continental collisions that form mountain belts bury and heat up rocks to amphibolite or even granulite metamorphic conditions.

Areas with high rates of metamorphism include:

• Subduction zones like Japan and the Pacific Northwest
• Continental collision belts like the Himalayas
• Zones of crustal thickening like the Appalachian Mountains

Understanding how plate tectonics generates metamorphic settings provides important context for studying metamorphic rocks.

Key Steps in Metamorphic Rock Formation

To summarize, here are the key steps in how metamorphic rocks are formed:

1. Existing igneous, sedimentary or metamorphic rocks get buried deep in the crust.
2. The rocks are subjected to high heat and pressure from being buried.
3. Minerals transform into new metamorphic minerals stable under the new conditions.
4. Rock textures recrystallize and realign due to pressure and heat.
5. With increasing metamorphism, rocks transform into higher metamorphic grades.
6. Uplift and erosion eventually expose the metamorphic rocks at the surface.

Understanding this process helps geologists recognize how rocks exposed at the surface once experienced the intense conditions deep within the Earth.

Metamorphism and the Rock Cycle

Metamorphism is a key process in the rock cycle. It takes existing rocks and transforms them into new types without melting. Metamorphism typically follows periods of uplift and erosion that expose deep crustal rocks at the surface. In turn, metamorphic rocks can undergo uplift and erosion to become the source materials for new sedimentary rocks. The cyclical nature of metamorphic processes demonstrates the interconnectedness of all rock types in the rock cycle.

Conclusion

Metamorphism occurs when rocks undergo recrystallization and chemical changes due to exposure to intense heat, pressure, and chemically active fluids. The process creates distinctive metamorphic rocks like gneiss, schist, slate, and marble that form under specific metamorphic conditions related to plate tectonics processes. Understanding how metamorphism transforms existing rocks into new types is key for interpreting the geological history of metamorphic terrains.

Key Points

• Metamorphism involves the chemical and textural changes in rocks from increased heat and pressure.
• Foliation, new mineral growth, and lack of fossils are key traits of metamorphic rocks.
• Metamorphic facies and zones characterize metamorphic conditions.
• Plate collisions produce ideal environments for regional metamorphism.
• Metamorphism is an important part of the rock cycle.