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3.2.2.2. Metamorphic rocks

Metamorphic rocks

Metamorphic rocks are formed from other previously formed rocks (sedimentarymagmatic or metamorphic) that have been subjected to great pressures and temperatures but without melting.

If fusion were to occur, the resulting rock would not be metamorphic, but magmatic.

Under these conditions of pressure and temperature, the minerals that are part of the original rock are transformed into different ones, which originates a new rock, the metamorphic rock. Only a rearrangement of the atoms that form the minerals occurs, so the chemical composition of the metamorphic rock is the same as that of the rock from which it comes.

Metamorphism is the set of changes that occur in the rock, without reaching fusion, when the rock is subjected to different temperature or pressure conditions than those that originated it. The resulting rock is called metamorphic rock.

For example, from the sedimentary rock clay the metamorphic rock slate is obtained; from igneous rock granite is obtained from the metamorphic rock gneiss, and marble is obtained sedimentary rock limestone.

classification of metamorphic rocks can be made according to their external appearance:

Lamellar metamorphic rocks

When great pressure acts on the rocks, the particles that make up the rock are oriented perpendicular to the direction of the forces acting on them, forming a characteristic sheet structure called slaty or schistosity (depending on the rock in question).

Some clay minerals, during metamorphism, change their structure to form mica crystals, which are sheet-shaped and cause this type of rock to separate into sheets when broken. They are said to have foliation. This can be seen on the blackboard, for example.

Depending on the degree of metamorphism, the transformation of the rock will be less or greater. That is, metamorphic rocks can increase their degree of metamorphism and will give rise to metamorphic series.

For example, from clay, a sedimentary rock, different metamorphic rocks can be formed depending on their degree of metamorphism:

Clay - Slate - Schist - Gneiss

  • Slates: They are formed when the metamorphism is low grade. They show slatyness, separating easily into flat and thin sheets. They are usually black. They have a slight shine because they contain mica.
  • Schists: They are formed when metamorphism is medium grade. Schistosity is characterized by slightly deformed laminae. Its mica content  gives it a strong shine.
  • Gneises: They are formed when metamorphism is high grade. Its sheets are very deformed and irregular. It is made up of mica and large crystals of quartz  and feldspar.

Crystalline metamorphic rocks

In this case, the increase in pressure and temperature does not cause the minerals to organize into sheets, but rather to adopt a crystalline structure. Its appearance is homogeneous, without foliation. When they break, they do so with an uneven surface.

Some examples of crystalline metamorphic rocks are:

Causes of metamorphism

In more advanced Geology courses the different types of metamorphism will be seen. Here we are going to limit ourselves to naming what may be the causes that cause the metamorphism of a rock:

  • The burial produced by the accumulation of sedimentary rocks, one on top of the other, can cause an increase in pressure and temperature that causes the rock to metamorphose.
  • The rocks located next to a rising magma can also be affected by this increase in temperature.
  • In areas of tectonic stress, rocks subjected to great pressure can be metamorphosed.

Do not confuse metamorphism with metamorphosis

Although the two words have the same origin (from the Greek meta , change, and morpho , form), metamorphism refers to the transformation, without change of state, of the mineral structure of a rock that has been subjected to conditions of temperature or pressure. Instead, we will use metamorphoses to talk about the shape changes that many insects, amphibians, etc. go through. from birth to adulthood.