metamorphic rocks are rocks formed by physical or chemical alteration of other pre-existing rocks. The original rock is called protolith and it can be an igneous rock, a sedimentary rock, or even another previous metamorphic rock.
They tend to be denser, more compact and harder than igneous and sedimentary rocks, and therefore more resistant to erosion. Metamorphic rocks are very abundant in the earth’s crust and occupy approximately 12% of its surface.
The process of transforming a rock into a metamorphic rock is known as metamorphism, which means “change of form”, and is produced mainly by the action of high temperatures, high pressures, presence of hot fluids or a combination of these factors, but without the rock melting or changing state.
The tectonic activity of the lithosphere causes movements and phenomena that lead rocks of all kinds to meet new conditions, especially pressure and temperature, which can cause metamorphic changes in their structure and composition. Among the tectonic processes responsible, subduction and continental collision stand out.
The representation of the pressure and temperature conditions as a function of depth generates a graph in which you can see the areas delimited between sedimentation, diagenesis, metamorphism and magmatism:
The intensity of the metamorphic conditions during the metamorphosis process can be variable. This intensity is often referred to as metamorphic grade and is classified into:
- very low: 100 to 200-250ºC)
- bass: 200-250 to 400-450ºC)
- medium: 400-450 to 600-650ºC)
- tall: above 600ºC
Metamorphic rocks contain minerals that only form at high temperatures and pressures and are associated with metamorphism. These minerals are known as metamorphic minerals and its quantity serves as an indication of the degree of metamorphism that the rock has undergone, as well as the temperature and pressure reached during the metamorphic process. Examples of metamorphic minerals are sillimanite, kyanite, staurolite, andalusite or garnet.
Each of these minerals is formed under specific metamorphic conditions and their representation in the graph above gives rise to those known as metamorphic facies that represent the groups of rocks associated with a certain range of pressure and temperature.
Along with metamorphic minerals, metamorphic rocks can contain other minerals, such as olivine, mica, feldspar or quartz, but they have not necessarily formed by metamorphism, but are usually minerals formed by crystallization of igneous rocks that resist high temperatures and pressures. being able to remain unchanged during the metamorphic process.
One of the main chemical mechanisms of metamorphism is the recrystallization. Under high pressure and temperature, the atoms in mineral crystals can rearrange by diffusion or dislocation to form a different mineral, even though the composition of the mineral remains constant. For example, limestone, a rock of sedimentary origin, can recrystallize to form marble, a metamorphic rock; Similarly, clay, another sedimentary rock, can recrystallize to form mica, a rock that can be igneous or metamorphic in origin.
types of metamorphism
Metamorphism, from a geological point of view, is usually classified into two large groups:
- regional metamorphism: affects large orogenic zones that can cover hundreds of kilometers. Some subtypes are orogenic, dynamic, and burial metamorphism. The rocks undergo great directional stresses, which is why they usually present typical foliar structure together with important deformations.
- local metamorphism: It is produced by direct contact between rocks and increases in temperature in adjacent formations, for example by contact with lava flows or with igneous intrusions. Some types are contact, hydrothermal, or impact metamorphism.
Regional metamorphism is associated with geological phenomena that affect large orogenic zones and large rock masses for long periods of time. It can often be seen in great mountain ranges and ancient continental shields. Regional metamorphism is usually the result of a combined action of high pressures and temperatures that depend fundamentally on the depth reached.
Most of the lower continental crust is metamorphic rock formed by regional metamorphism. Among the most common phenomena that generate regional metamorphism, the subduction, continental collision and horizontal tectonic movements.
Regional metamorphism usually results in dense and hard metamorphic rocks with foliated texture. The crystallization of metamorphic minerals occurs in the direction of the pressures suffered by the rocks, resulting in the characteristic layers of the foliated texture.
In fault zones, a type of regional metamorphism known as dynamic metamorphism. The main agent of metamorphism is the pressure generated by the movement between the blocks of the fault. The resulting rocks do not usually present foliation, since they are frequently crushed by shears.
Another type of regional metamorphism is burial metamorphism, which is due to the increase in pressure and temperature produced by the accumulation of large strata in sedimentary basins. Burial metamorphism does not usually generate foliation.
Local metamorphism is produced by changes in pressure and temperature in localized regions and is not, in general, directly related to tectonic movements. One of the main types of local metamorphism is contact metamorphismwhich occurs in rocks that enter contact with igneous intrusions (plutons, dikes) or that remain covered by lava flows.
Mineral recrystallization in contact metamorphism occurs in a disorderly manner without a marked alignment, so foliation does not appear. Marble and granite are common examples of rocks formed by contact metamorphism.
The hydrothermal metamorphism It is a type of local metamorphism that involves chemical reactions caused by the circulation of fluids at high temperature. It is one of the types of metamorphism most frequently associated with changes in the chemical composition of the original rock (metasomatism). It is common on spreading mid-ocean ridges (oceanic metamorphism).
A very striking type of local metamorphism is the impact metamorphism. This type of metamorphism is produced by large increases in pressure and temperature associated with the impact of meteorites. It can be seen around impact craters on the Earth’s surface and is very common on the lunar surface. By this mechanism, metamorphic minerals can form on the Earth’s surface that would otherwise only form deep in the mantle.
Most of the metamorphic rocks on planet Earth have a layered structure which is called foliation or foliate texture. This structure is due to the alignment of the mineral crystals according to the direction of the compression forces suffered by the rock during metamorphosis.
Layers can appear as different colored bandsattending to the different minerals that form each band, or as fissile bladesfor example in the mica and the board. Other examples are the shale and the gneiss (same minerals as granite but foliated).
The non-foliated texture is typical of metamorphic phenomena without large directional deformation forces, with large but similar deformation forces from various axes, or in rocks that do not present minerals with different crystallization habits (planar minerals). One of the most common examples of non-foliated metamorphic rock is marble, formed by recrystallization of limestone and dolomite; other examples are quartzite, serpentinite or cancagua.