Metamorphism and crustal thickening
Metamorphism refers to the geological process that alters the mineral composition and structure of rocks due to high temperature and pressure, resulting in the formation of metamorphic rocks. This process plays a significant role in shaping Earth's crust, particularly through crustal thickening associated with tectonic activity, such as continental collisions and the uplift of faults. Metamorphic rocks, which can originate from any type of rock (igneous, sedimentary, or even older metamorphic rocks), are classified into distinct categories based on their mineral composition, temperature, and pressure conditions.
The formation processes of metamorphic rocks include regional, contact, dynamic, hydrothermal, shock, and burial metamorphism, each characterized by different environmental conditions and physical changes. Key factors influencing metamorphism include heat from Earth's interior, pressure from overlying rock layers, and tectonic processes that can either facilitate or halt the transformative processes. This interplay creates a dynamic rock cycle where rocks continuously change forms through weathering, sedimentation, and metamorphism, reflecting the interconnected nature of geological processes and the evolution of Earth's crust over time. Understanding metamorphism and crustal thickening provides insights into the history and structure of our planet, informing geological studies and resource exploration.
Metamorphism and crustal thickening
Metamorphism produces a wide array of Earth's rocks that form under circumstances related to high pressure and heat. Tectonic activity, particularly when it results in crustal thickening, is a driving factor in producing metamorphic rocks. Uplifting at faults has brought many metamorphic rocks to the surface, providing scientists the opportunity to make inferences about Earth's interior.
Metamorphic Rocks
Petrology, the scientific study of rocks, recognizes three main types of rocks: sedimentary, igneous, and metamorphic. Sedimentary rocks are formed from the deposition of organic material and minerals at Earth's surface and within bodies of water. Igneous rocks are the products of magma and lava that have cooled and solidified. Metamorphic rocks are rocks (of any type) that have transformed physically, chemically, or both due to the influence of high temperature and pressure. Metamorphic rocks make up a large part of Earth's crust; common types include gneiss, marble, quartzite, schist, and slate.
The first form of a metamorphic rock is called the protolith, and it can be igneous, sedimentary, or even an older metamorphic rock. When a protolith is subjected to a temperature greater than about 200 degrees Celsius (C), or 392 degrees Fahrenheit (F), and a pressure above about 300 megapascals (MPa)—which equals 43,500 pounds per square inch—that protolith can transform in many ways. Metamorphism is a distinct process from diagenesis, which describes changes that can occur in sedimentary rocks under 200 degrees C and 300 MPa. Likewise, at temperatures and pressures high enough that a metamorphic rock begins to melt, it is no longer considered a metamorphic process; it is igneous.
Metamorphic rocks contain index minerals that are found only at the high temperatures and pressures of metamorphism; these minerals include andalusite, kyanite, sillimanite, and staurolite. Some other minerals can also be found in metamorphic rocks, including amphiboles, feldspars, micas, olivines, pyroxenes, and quartz. These minerals are stable at relatively high temperatures and pressures, and the points at which they become unstable can be used to deduce information about the conditions under which the metamorphic rock formed.
Three general types of classification can be applied to metamorphic rocks: facies, grades, and textures. Facies are groupings of metamorphic rocks by chemical and mineral compositions that are typically found within specific pairings of temperature and pressure conditions. For example, the lowest-grade facies is the zeolite facies; it describes metamorphic rocks at the lowest temperature and pressure conditions that can occur without entering diagenesis. This generally applies to certain sediments that undergo burial metamorphism. The highest grade is called the eclogite facies, which occurs at pressures around 1.2 gigapascals (174,000 pounds per square inch) and temperatures exceeding 600 degrees Celsius (1112 degrees Fahrenheit).
Grades describe relative pressure and temperature conditions under which a metamorphic rock formed. For example, metamorphic rocks are considered low grade between about 200 and 320 degrees Celsius (392 and 608 degrees Fahrenheit) and under about 600 MPa (87,000 pounds per square inch), while high grade refers to rocks formed at hotter, higher-pressure conditions. Low-grade metamorphic rocks tend to have an abundance of hydrous minerals, whereas high-grade rocks do not, because hydrous volatiles such as water and carbon dioxide evaporate as the temperature rises.
Texture provides even more details. In general, metamorphic rocks can be foliated or nonfoliated. Foliated rocks have distinct layers that develop through the rotations of minerals (such as mica or chlorite) as a rock undergoes stress or strain on one side. Nonfoliated rocks, which are less common, undergo stress uniformly on all sides, so layering does not occur. Foliated rocks are further divided into classes of slates, phyllites, schists, and gneisses, while nonfoliated rocks can be granoblastic or hornfelsic. Metamorphic rock textures also can be described as idioblastic or xenoblastic. Idioblastic rocks are bounded by their crystal faces, while xenoblastic rocks do not show their crystal faces.
Metamorphism occurs because the application of heat and pressure causes the ions and atoms in rocks to reorganize themselves, which alters the crystal structure. A variety of conditions provide the necessary heat and pressure for a metamorphic rock to form. Being located deep in the earth is enough to cause change. Tectonic processes also contribute, as pieces of crust are pushed under one another, causing increased friction, heat, and pressure. The intrusion of magma into a rock also causes metamorphism.
Tectonic activity has caused uplifting and erosion to reveal many metamorphic rocks at Earth's surface. This process gives scientists easy access to study the rocks and to make inferences about Earth's interior.
Types of Metamorphism
Metamorphism varies widely in its process and its results, and it can be classified through a variety of types. These types are discussed here.
Regional metamorphism refers to changes through a wide region, such as the lower continental crust, brought on by an orogenic event (tectonic activity leading to severe deformation of Earth's crust). The lower crust is affected by its depth and by tectonic processes such as continental collisions, which result in uplifting and subduction of crust (processes full of friction, heat, and pressure). Most rock formed by regional metamorphism is highly foliated; slate, schist, and gneiss are common occurrences.
Contact metamorphism is caused by the intrusion of magma. Hot magma cools into an igneous rock, forming a region called the contact metamorphism aureole; metamorphism occurs locally due to heat, which lessens farther from the aureole. Contact metamorphism typically produces nonfoliated hornfels, and it is also common to find ore minerals around the contact zone. This process, called metasomatism, adds chemicals from surrounding rocks, often carried by water, and causes a drastic chemical or physical change in the contacted metamorphic rock.
Dynamic metamorphism, also called cataclasis, occurs at areas of moderate to high strain, such as fault zones. The rock undergoes metamorphosis due to mechanical deformation, which itself is caused by shearing and sliding along a fault. In simpler terms, the rock is crushed or shattered. This process does not involve much temperature change.
Hydrothermal metamorphism occurs under conditions of high temperature and moderate pressure, and in the presence of a hydrothermal fluid such as magma and hot groundwater or hot ocean water. Hydrothermal metamorphism generally occurs at the surface, often resulting in rich ore deposits. One area that exhibits hydrothermal metamorphism is Yellowstone National Park in the northwestern United States.
Shock or impact metamorphism is the result of shock waves—ultra-high-pressure conditions caused by a comet or meteorite impact, or a huge volcanic eruption. This type of metamorphism forms minerals like silicon dioxide polymorphs (such as coesite and stishovite), which are stable at high pressures. Shock metamorphism leaves tell-tale textural signs such as planar fractures, shock lamellae, and shatter cones. The effects of shock metamorphism have been found at every identified impact site on Earth.
Burial metamorphism crosses somewhat with diagenesis, but the process “grades up” to metamorphosis as pressure and temperature rise. Sedimentary rocks buried several hundred meters below the surface came from this type of metamorphism.
Prograde metamorphism occurs as temperature and pressure increase, and water and carbon dioxide are lost. Retrograde metamorphism is the opposite process. It is less common because the volatiles (water and carbon dioxide) must be present.
Metamorphic Processes
Metamorphism can occur through a variety of processes, such as recrystallization, neocrystallization, phase change, pressure solution, and plastic deformation. The aforementioned metasomatism is also an important contributor.
Recrystallization occurs at about one-half the melting point of a rock (noted by degrees on the Kelvin scale). Particles change size and shape, but their identities, as atoms and ions pack together to form new crystal structures. Limestone recrystallizes into marble, for example, and small calcite crystals in sedimentary limestone transform into larger particles in metamorphic marble. Another example is clay, which can recrystallize to muscovite mica. Neocrystallization is the formation of new crystals not found in the protolith, a very slow process that involves the diffusion of atoms through solid crystal. Phase change also refers to the formation of new minerals, but these have the same formula as the protolith.
Pressure solution occurs when a rock is subjected to high pressure on one side in the presence of hot water. The rock's minerals partly dissolve and diffuse through the water to precipitate elsewhere. This process contributes to cleavage, a planar rock foliation. Plastic deformation occurs when pressure causes a metamorphic rock to shear and bend but not break. The temperature has to be high enough to avoid brittle fractures but low enough to avoid the diffusion of crystals.
Metasomatism alters a rock by introducing new chemicals from the surrounding environment—chemicals that are often delivered by a fluid. One way fluid delivery occurs is through the breakdown of hydrous minerals under high temperature and pressure in the lower crust. This breakdown releases fluid to the upper crust, where it interacts with and alters rocks.
Tectonic Processes and Metamorphism
Tectonic activity is a major factor of metamorphism, particularly in relation to crustal thickening, which can occur through crustal shortening or when one crust rides over another at a convergent boundary (causing the formation of mountains, among other results). Thickening crust becomes warmer and thus weaker. The lower portion of the crust is even warmer than the upper portion because of depth, so the lower portion becomes more plastic; this leads, ultimately, to collapse and the creation of a rift under the growing mountain. These steps all contribute to changing temperature and pressure conditions, thus causing metamorphosis in the rocks in a given region.
Tectonic activity can also lead to the stoppage of metamorphism. Crust that gets uplifted at faults is subjected to weathering and erosion, which cools the rock and returns it to a sedimentary state. All three types of rocks—sedimentary, igneous, and metamorphic—are closely connected, as demonstrated by the rock cycle. As the equilibrium of a rock's environment changes with time, the rock transforms into one of the other types.
To understand a simplified rock cycle, one can picture an igneous rock. Under the influence of weathering and erosion, that rock can break down into sediments, which undergo compaction and cementation into a sedimentary rock. This sedimentary rock then transforms under heat and pressure to become a metamorphic rock, which melts into magma. This new form then cools into an igneous rock, completing the cycle. The cycle is continuous and interacts with tectonic activity and the water cycle to affect nearly all aspects of life on Earth.
Principal Terms
diagenesis: change that can occur in sedimentary rocks under 200 degrees Celsius and 300 megapascals; a distinct process from metamorphism
facies: groupings of metamorphic rocks by chemical and mineral compositions that are typically found within specific pairings of temperature and pressure conditions
foliated: having distinct layers that occur following the rotation of minerals (such as mica or chlorite) as a rock undergoes stress or strain on one side
grade: an indicator of the relative pressure and temperature conditions under which a metamorphic rock forms; described as low-grade or high-grade
igneous rock: a product of lava or magma that has cooled and solidified
magma intrusion: the entrance of hot magma into a rock
metamorphosis: a physical and chemical transformation of a rock into a different rock under the influence of high temperature and pressure
metasomatism: a process that adds chemicals from surrounding rocks, often carried by water, causing a drastic chemical or physical change in the contacted metamorphic rock
petrology: the scientific study of rocks
protolith: the originating rock of a metamorphic rock; can be igneous, sedimentary, or metamorphic
sedimentary rock: a product of the deposition of organic material and minerals at Earth's surface and within bodies of water
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