The Relationship Between Metamorphism and Plate Tectonics
The relationship between metamorphism and plate tectonics is fundamental to understanding how the Earth's lithosphere (the rigid outer shell of the Earth) behaves under various geological processes. Metamorphism is the process by which rocks undergo physical and chemical changes in response to environmental conditions such as heat, pressure, and chemically active fluids, while plate tectonics is the theory that describes the large-scale movement and interaction of the Earth's lithospheric plates.
Metamorphism in relation to tectonic regimes:
The metamorphic facies series encountered in different tectonic regimes or settings can be summarized as follows, and are shown schematically on Figure:
- Ridges and rift valleys: characterized by high geothermal gradients contact and ocean floor metamorphism.
- Areas of magmatic activity; volcanic - plutonic complexes: greenschists amphibolites granulites.
- Areas of crustal thickening and mountain building: greenschists amphibolites granulites and type B eclogites (particularly if there are magmatic intrusions).
- Subduction zones: Characterized by low geothermal gradients: zeolite pumpellyite-actinolite facies /lawsonite albite facies blueschist facies type C eclogites.
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| What Is the Relationship Between Metamorphism and Plate Tectonics? Metamorphic facies in Plate Tectonics |
Convergent Plate Boundaries (Subduction Zones)
Metamorphism is particularly common at convergent plate boundaries, where one tectonic plate is forced beneath another in a process called subduction.
At these boundaries, rocks are subjected to intense pressures and varying temperatures, causing them to undergo high-pressure, low-temperature metamorphism. This creates metamorphic rocks such as blueschist and eclogite.
As subducting slabs descend, they carry oceanic crust and sediments into the mantle, which leads to the formation of new minerals and textures due to changes in pressure and temperature.
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| Temperature and pressure effects on metamorphism |
Why Do Metamorphic Rocks Form at Subduction Zones
Metamorphic rocks form at subduction zones due to the extreme environmental conditions—particularly high pressure and varying temperatures—that are present when one tectonic plate is forced beneath another into the Earth's mantle. The unique conditions at subduction zones create a variety of metamorphic processes that transform existing rocks into new types. Here’s why and how this happens:
High Pressure from Subduction
As the oceanic plate is subducted, it is pushed deeper into the Earth's crust and mantle, where it experiences increasing pressure. These pressures can be extremely high, often greater than at any other tectonic setting, because the descending plate sinks into the Earth’s interior.
Under these conditions, the minerals in the subducting plate become unstable and are transformed into denser minerals that are stable under high pressure. For example, basalt (oceanic crust) can be metamorphosed into blueschist or eclogite, depending on the depth and the specific pressure-temperature conditions.
Low to Moderate Temperatures in Subducting Slabs
Although the subducting plate is moving deeper, it often remains relatively cool, especially compared to the surrounding mantle. This is because the plate is descending faster than it can heat up.
The combination of high pressure and low temperature creates a distinct type of metamorphism known as high-pressure, low-temperature metamorphism. This process is responsible for forming metamorphic rocks like blueschist, which can only form under such specific conditions.
Fluid-Rich Environment
Subduction zones are often rich in fluids, especially water. This is because the subducting oceanic plate typically contains hydrated minerals (minerals that have water within their structure) and sediments from the ocean floor.
As the plate descends, the pressure causes the release of these fluids into the overlying mantle and crust, where they play a key role in metasomatism (the chemical alteration of rocks by fluid), promoting the formation of new minerals and facilitating metamorphic reactions.
This fluid interaction also aids in the partial melting of rocks, contributing to the variety of metamorphic rock types produced.
Tectonic Stress
The intense compressional stress at subduction zones also affects the rocks, deforming and transforming them. The pressure not only causes chemical changes but also physical changes, such as the alignment of minerals (foliation), which can be seen in rocks like schist and gneiss.
Tectonic stress also drives rocks to experience dynamic metamorphism along fault zones, where rocks are physically broken down and recrystallized due to intense deformation.
Depth and Temperature Variability
As the subducting plate descends deeper into the Earth, it gradually heats up, transitioning from low-temperature to moderate or even high-temperature conditions. This gradient allows different types of metamorphic rocks to form at different depths.
For instance, eclogite, a high-pressure metamorphic rock, forms at great depths where temperatures and pressures are both high. In contrast, rocks like greenschist or blueschist form at shallower depths and lower temperatures.
Regional Metamorphism
Regional metamorphism typically occurs over large areas during mountain-building events (orogeny) associated with plate collisions.
When two continental plates collide (as in the case of the Himalayas, where the Indian and Eurasian plates meet), rocks are compressed and buried deep beneath the Earth's surface. The heat and pressure from this process can transform rocks like shale into slate, schist, and eventually gneiss.
This process occurs at continental-continental collision zones, resulting in the formation of large mountain ranges.
- Moderate pressure + low temperature = Greenschist facies
- High pressure + low temperature = Blueschist facies
- Moderate pressure + high temperature = Amphibolite facies
- Highest temperature + high pressure = Granulite facies
- High temperature + highest pressure = Ecologite facies
Even though the rock material may have the same chemical composition, their mineral compositions, texture, and appearance are different, and the rocks they comprise are classed into rocks of different metamorphic grades (metamorphic facies).
Over the past century, scientists have studies the distribution and occurrences of minerals in the field as well as having manufactured them in laboratories settings, simulating the pressures and temperatures within the earth where different grades of metamorphism takes place. As a result, the temperature and pressure ranges of formation (and destruction) of many minerals, and the metamorphic rocks they form, are well known (Figure).
Divergent Plate Boundaries (Mid-Ocean Ridges)
At divergent boundaries, where tectonic plates are moving apart (such as at mid-ocean ridges), magma rises to the surface, creating new oceanic crust. As it cools, some rocks may undergo contact metamorphism due to heat from the magma.
Hydrothermal fluids also circulate through the crust at these locations, leading to hydrothermal metamorphism, where the interaction of hot fluids and rocks alters the minerals and chemistry of the rocks.
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| Generalized illustration of a convergent plate boundary showing regional metamorphism. |
Transform Boundaries and Fault Zones
At transform plate boundaries (e.g., the San Andreas Fault), where two plates slide past each other, the friction and heat generated by the movement can cause localized dynamic metamorphism.
This type of metamorphism is associated with the crushing and grinding of rocks along fault zones, leading to the formation of rocks like mylonite and cataclasite.
Mountain Building (Orogeny)
During mountain-building events (orogeny), which often occur at convergent boundaries, rocks are buried deep within the Earth’s crust, exposed to intense heat and pressure, and eventually metamorphosed. The Himalayas, for instance, are a result of the collision between the Indian and Eurasian plates, producing large-scale regional metamorphism.
Metamorphism in Oceanic Crust
In areas where oceanic plates are involved, such as in oceanic-continental subduction zones, oceanic crust undergoes metamorphism at relatively low temperatures but very high pressures. This leads to the creation of specific metamorphic rocks like serpentinite, formed through the hydration of mantle peridotite at subduction zones.
Summary
Plate tectonics provides the mechanism that drives metamorphism by supplying the heat, pressure, and fluid conditions necessary for rock transformation.
At convergent boundaries, deep burial and subduction lead to high-pressure metamorphism.
Divergent boundaries and mid-ocean ridges provide heat for contact metamorphism and hydrothermal alteration.
Transform boundaries generate localized dynamic metamorphism due to frictional forces.