Ultra-high-pressure metamorphism

Metamorphic rock is one of three major rock types and is formed when pressure, temperature, or environmental chemicals cause either an igneous or sedimentary rock to change in density and mineralogical structure. Metamorphosis can be caused by extraterrestrial impact, exposure to heat from the earth’s core, exposure to chemical environments, and tectonic forces. Ultra-high-pressure metamorphism is a type of metamorphism that occurs at extremely high pressures and usually accompanies the collision of continental plates.

Metamorphic Petrology

Metamorphic petrology is a branch of geology that studies transformations in mineral composition, texture, color, and other qualities that occur within rocks. These transformations occur when the rocks are subjected to pressures, chemical environments, and temperatures different from those in which the rock originally formed.

Metamorphism can be distinguished from diagenesis, which is another transformative process that affects certain types of rock after solidification. Diagenesis occurs at low temperatures and pressures, whereas the process affecting metamorphic rock typically occurs at temperatures higher than 200 degrees Celsius (392 degrees Fahrenheit) and at pressures greater than 300 megapascals. Metamorphic rocks are sufficiently unique in structure and chemistry that they are considered one of the three basic rock types.

Sedimentary rocks are formed from sediment, a layer of debris that forms at the top layer of the earth’s crust and is composed of loose minerals, organic waste, and bits of other rocks that have eroded into smaller fragments. Through burial and compression, this surface material can become lithified—that is, cemented and compressed to form rocks.

Sedimentary rocks come in three types: clastic, chemical, and organic. Clastic rocks are composed almost entirely of bits of other rocks that have become compacted into solid structures. Chemical rocks are sedimentary rocks that form through some chemical process. Many types of chemical rocks result from situations in which a riverbed or shallow sea gradually dries, after which underwater sediment may become solidified through dehydration and other chemical processes to become sedimentary rock. Organic rocks are formed from the accumulation of organic debris in the sediment that becomes lithified through compaction. Coal is an example of an organic sedimentary rock that forms from the remains of wood and other organic material that has become buried and compacted through millions of years.

Igneous rocks form when molten earth originating within the earth’s mantle rises close to the surface and then cools and solidifies to form solid bodies. The formation of igneous rock involves a change of state from liquid to solid, which is accompanied by a reduction in heat.

Igneous rocks come in two main types: plutonic and volcanic. Plutonic rock forms when magma slowly cools in pockets beneath the surface, sometimes taking millions of years to form. Plutonic rock is one of the most common types of rock and commonly underlies mountains and other major geologic features. Volcanic rock forms when magma breaks through the surface as lava and then cools and hardens into solid structures. Magma and volcanism have played major roles in the formation of the lithosphere. Geologists estimate that the majority of the earth’s crust is composed of igneous rock.

Metamorphic rocks may start their existence as either igneous or sedimentary rocks but are subjected to an environment such that they undergo a change in chemical and physical properties. Most metamorphic rocks form deep in the earth’s crust or the upper layers of the mantle, where temperatures and pressures are sufficient to induce significant chemical and physical changes. Though metamorphosis generally begins at 200 degrees Celsius (degrees 392 Fahrenheit) and at 300 megapascals, it can occur at much higher temperatures and pressures. The limit of the metamorphic process occurs when the rock becomes hot enough to melt. At this point, the rock becomes magma. Any further structural transformations fall into the category of igneous rock formation.

Metamorphism always involves the loss of water and an overall reduction in density. Some of the most familiar metamorphic rocks are gemstones such as diamonds and rubies. Rubies and sapphires form from the metamorphosis of certain types of clay, which give rise to corundum. Further pressure and temperature increases will cause the corundum to develop glass-like mineral inclusions including rubies and sapphires. Diamonds also develop from the metamorphic process acting on coal that has been compressed at depths for millions of years.

There are two basic stages to the metamorphic process. First, the volume of the rock is reduced as the rock is compacted, which reduces empty air space and water from the sample. After this, further pressure will cause recrystallization, which is a process by which the relationships and position of the individual grains, molecules, or atoms of a rock change to form a new crystal structure with a reduced volume. Many common stones, including marble, result from recrystallization; marble is a recrystallized form of limestone that has been subjected to intense pressures.

Types and Grades of Metamorphism

Low-grade metamorphism occurs at temperatures between 200 and 320 degrees Celsius (608 degrees Fahrenheit) and at relatively low pressures, where high-grade metamorphosis occurs at temperatures above 320 degrees Celsius and at higher pressures. Rocks formed from low-grade metamorphism tend to be hydrous minerals, meaning that the rocks have water incorporated into their crystal matrix.

As pressure and temperature increase, water molecules are destabilized and removed from the rock. High-grade metamorphic rocks are characterized by having low levels of water molecules in their structure.

Many different physical conditions can lead to metamorphosis. One of the most basic types is contact metamorphism, which occurs when magma rises from the mantle into the crust. Only some of the surrounding rock becomes hot enough to melt. Just outside of this melting zone is a layer of rock that is subjected to intense heat, which may be sufficient in some cases to bring about metamorphic changes in the rock.

In rare cases, metamorphic rocks result from extraterrestrial impact events, such as when a meteor hits the surface of the earth. An impact of this type can generate sufficient pressure over a small area to bring about metamorphosis in the impacted rock. Evidence of an extraterrestrial impact also may include the presence of shock lamellae, which are thin strips of glass that form in the opposite direction from the rock’s original crystalline structure. Shock lamellae form only under conditions of rapid, intense pressure, such as those that accompany a meteor impact.

Most types of metamorphic rocks form when rock is buried at sufficient depths to induce metamorphic changes. At increasing depths, rocks undergo increased heating because of their proximity to the mantle and increased lithostatic pressure, which is caused by the weight of the crust overlying them. A special type of burial metamorphism, called regional metamorphism, occurs over vast areas when large volumes of rock are simultaneously buried and subjected to metamorphic influences.

One of the most common scenarios for this type of metamorphism is the collision of two continents. The force of the collision causes the crust on both sides to deform, often leading to orogenesis, the formation of mountains. When the crust buckles at the site of impact, some of the crust is thrust beneath the surface, where it is subjected to intense pressure. As this process continues, layers of crustal rock are forced deeper into the earth, leading to metamorphism as pressures and temperatures increase.

Ultra-High-Pressure Metamorphism

Ultra-high-pressure metamorphism (UHPM) is a type of metamorphism that occurs when rocks are subjected to extremely high pressures and temperatures. The conditions that result in UHPM appear to be rare, and only about twenty sites containing rocks generated by this type of metamorphism had been found between its discovery in the 1980s and 2021. Existing evidence of UHPM environments are associated with regional metamorphism related to the collision of tectonic plates and the subduction of crustal rock.

When oceanic tectonic plates are driven together, the older and therefore denser of the two plates will typically drive under the opposing plate in a process called subduction. The subducted plate is driven into the earth’s mantle, where it desolidifies and blends with the rock of the mantle. This desolidified rock may eventually become magma, some of which will eventually rise toward the surface and solidify into igneous rock.

Another type of subduction, usually called A-type subduction, involves the collision of continental plates. Because continental crust has a lower density than oceanic crust, the denser of the two plates begins to subduct, but then rebounds because the crustal rock is not dense enough to remain in the mantle. A-type subduction zones often lead to orogeny, or the formation of mountains, as the two plates buckle against each other, forcing Earth materials to rise near the impact site.

UHPM occurs where a portion of the continental crust has descended to depths greater than 100 kilometers (62 miles) before returning to the surface. Geologists detect the existence of UHPM by looking for characteristic minerals that are created only in extreme temperature and pressure regimes. Crust that is transported to sufficient depths rapidly begins to recrystallize; however, water contained within the rocks tends to stabilize the matrix, slowing the process of converting low-density rock to high-density rock. When the force causing the subduction of the rock abates, the submerged strata is far more buoyant than the surrounding rock that typically exists at this depth, causing the recrystallized rock to rise rapidly. This relatively rapid return to the surface (though still requiring thousands of years) preserves mineralogical evidence of UHPM in the rock, whereas a longer period of rising to the surface would allow metamorphosis to occur a second time, returning most of the metamorphic rock to its original form.

Evidence of UHPM

One of the most common types of rock produced by UHPM is coesite, which forms when high pressures and relatively high temperatures are applied to quartz. Quartz is one of the most abundant materials on the surface of the earth and is made from a combination of silicon and oxygen atoms arranged in a crystal lattice. Coesite is unstable at sea-level temperatures and pressures and will eventually decay in this environment, returning to quartz.

Samples of intact coesite are rarely uncovered because of coesite’s extreme instability at sea level. However, geologists can utilize indirect evidence to determine if coesite was present in a recovered sample of quartz. As coesite returns to quartz form, it forms into a type of quartz called polycrystalline quartz, which is characterized by the presence of microscopic crystal fragments called crystallites. Coesite has 10 percent less volume than quartz; therefore, when coesite metamorphoses into quartz, it expands in such a way that it causes the development of characteristic microscopic fractures in the surrounding quartz. Geologists use the presence of these pressure fractures in samples of polycrystalline quartz to determine if the quartz was subjected to UHPM environments that led to the formation of coesite.

In rare cases, geologists have discovered microdiamond inclusions in rocks subjected to UHPM environments. Microdiamonds are diamonds that are typically less than 1 micrometer in extent and are formed only in extreme high-pressure environments. Microdiamonds have been found as inclusions in zircon, a common mineral that occurs in sedimentary and igneous rocks as small inclusions. Microdiamonds also have been uncovered from samples of kyanite, a silicate mineral that forms in metamorphic environments in which the pressure rises into the thousands of pascals. Geologists have found microdiamonds formed in rocks that have undergone pressures of more than 5,000 megapascals and temperatures exceeding 1,000 degrees Celsius (1,832 degrees Fahrenheit).

Geologists are working to enhance the ability to find and study UHPM zones and to refine models that explain the formation of these extreme lithographic environments. Research into UHPM gained widespread attention in the 1990s, and since that time UHPM environments have been discovered on every continent.

The process of subduction, followed by return to the crust, takes millions of years to complete, and most evidence of UHPM has been found in sediment from ancient environments. The best-known sites to uncover UHPM minerals include the Daba Shan mountains of eastern China and the Dora-Maira massif of the Italian Alps.

Principal Terms

diagenesis: chemical and texture changes that affect sediment during and after its formation into sedimentary rock

lithification: process by which sediment is compacted or cemented into solid rock

lithostatic pressure: pressure exerted by the weight of sediment overlying the sediment sample in question

magma: superheated rock that exists in a liquid phase

megapascal: geologic unit of extreme pressure, equivalent to 1 million pascals and 145.037 pounds per square inch

petrology: branch of geology that studies rocks and the conditions that lead to the formation of rocks

polycrystalline: crystal structure consisting of crystallite fragments

recrystallization: process that occurs at high pressure, causing the atoms, molecules, or grains of a crystal to reorganize into another type of crystal

subduction zone: area in which two tectonic plates collide, forcing one plate to push beneath the other and driving the submerged plate into the earth’s mantle

Bibliography

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