Metamorphic and Sedimentary Rocks: Formation and Classification
Facies and Metamorphic Rocks
A metamorphic facies comprises a set of different metamorphic rocks that have withstood similar pressures and temperatures. Two different initial rocks subjected to similar metamorphic conditions will result in two different metamorphic rocks with mostly different minerals, but with some identical. Although the textures of metamorphic rocks usually distinguish them from magmatic or sedimentary rocks, the following types are common:
Granoblastic Texture
Equivalent to the grainy texture of magmatic rocks, where minerals are present in similar large sizes.
Porphyroblastic Texture
Corresponds to the texture of porphyritic rocks where large crystals are surrounded by smaller ones.
Crystalloblastic Texture
Recrystallized ore is very at ease.
Types of Metamorphism
Although temperature, pressure, and fluids are present simultaneously in most metamorphic processes, they are not involved with the same intensity in all cases. Depending on the values of these agents, we can speak of three types of metamorphism:
- Pressure Metamorphism: Pressure is involved in greater proportion than temperature.
- Thermal Metamorphism: Temperature plays a much higher role than pressure.
- Regional Metamorphism: Both factors act with great intensity.
Pressure Metamorphism
Occurs usually in shallow areas of the crust where faults occur and where there are very high pressures. This type of metamorphism is also called dynamometamorphism. The rocks formed due to pressure are mainly the effects of brecciation and schistosity. Two zones are usually distinguished: the breccia zone with relatively small pressures and the mylonites zone with high pressures.
Thermal Metamorphism
Occurs when the temperature increases under a rock, provided that this increase is not accompanied by a considerable rise in pressure. The most well-known type occurs when magma comes into contact with another rock generated at low temperature, which can occur by intrusion of magma, magmatic contact between lava and substrate, or in the vicinity of a vein. In these cases, thermal metamorphism is called contact metamorphism and produces processes such as dehydration, recrystallization, mineralogical adjustments, and metasomatism.
Regional or Dynamothermal Metamorphism
Affects large continental regions and creates rocks with typically metamorphic schistose mineralogy and texture, such as slates, schists, and gneisses. Regional metamorphism occurs when temperature, pressure, and fluids act simultaneously. If pressures and temperatures are very high, the rock can partially melt and cause features of rocks intermediate between magmatic and metamorphic rocks, called gneiss. When the rock melts completely, magma is formed.
Associated Minerals and Metamorphism Deposits
There are natural deposits of minerals formed by metamorphism, which are exploited for their economic interest. These include abundant deposits of irregular size and shape with sulfur oxides, graphite, and other minerals.
Metamorphism and Plate Tectonics
Plate tectonics can distinguish four regions of the earth’s crust where metamorphism occurs:
- Continental Crust Metamorphism: Due to the variable thickness of the continental crust, different types of metamorphism can occur. In the deepest regions, pressures and temperatures can be very high, resulting in high-grade metamorphism.
- Ocean Floor Metamorphism: The heat output of magma generates essentially thermal metamorphism.
- Compressive Margins Metamorphism: Subduction of oceanic crust beneath another crust fosters conditions for intense regional metamorphism.
- Transform Fault Metamorphism: Horizontal movement of blocks creates pressure metamorphism (dynamometamorphism).
Sedimentation and Sedimentary Rocks
Igneous and metamorphic rocks are more abundant in the Earth’s crust due to high pressures and temperatures. These conditions do not normally exist on the Earth’s surface, so sedimentary rocks are particularly numerous. Sedimentary rocks are layered in the place where they were formed.
Origin of Sedimentary Rocks
External geological agents like the atmosphere, wind, water, and ice continually destroy rocks near the surface, a phenomenon known as denudation. Erosion leads to the formation of particles of variable size that can be transferred to a sedimentation basin. The materials deposited in a sedimentation basin are known as sediments, and these form sedimentary rocks that exist only in shallow areas on Earth. Sediments include rock fragments, single minerals, crystals of dissolved substances, and remains of animals and plants.
Diagenesis
Over time, sediments in sedimentary basins undergo transformations that turn them into sedimentary rocks. This process is called diagenesis or lithification. Diagenesis comprises compaction and cementation.
Compaction
Is the volume loss of sediment by reducing the size of pores between fragments. Compaction is generally due to the weight exerted on the upper layers of sediments.
Cementation
Is the union of fragments embedded in the sediments. It occurs when aqueous solutions in the pores bring materials that crystallize in the gaps. The main cements are CaCO3 (limestone) or SiO2 (silicon). Sometimes, sediments are weakly linked by a clay matrix.
As a result of diagenesis, sedimentary rocks can be differentiated into three types of constituent materials:
- Plot: The set of particles that comprise the most abundant and essential part of the rock.
- Array: Set of finer particles, generally clay, deposited in the pores of the plot.
- Cement: The material crystallized in the pores of the plot.
Sedimentary Rocks and Their Classes
Sedimentary rocks can be divided into detrital (mechanical origin) and chemical or organic (biochemical origin). They can also be divided into detrital rocks, chemical rocks, and organic rocks, although most consist of both types of sediments.
Detrital Rocks
Are formed by fragments of other rocks or minerals that have been eroded and transported to a sedimentation basin. They are classified by the average size of their fragments:
- Rudites: Fragments larger than 2 millimeters.
- Arenites: Fragments between 2 and 0.06mm.
- Lutites: Microscopic minerals.
Chemical Rocks
Include chemical sediments like crystals of water-soluble substances and remains of living beings. Main groups include:
- Carbonate Rocks: Formed by crystallization of carbonates or accumulation of animal remains.
- Siliceous Rocks: Consist of silica and are not from organisms with siliceous skeletons.
- Evaporites: Formed by evaporation of water containing soluble salts.
- Carbon Rocks: Light-colored rocks formed from plant remains.
- Oil: A complex mixture of solid, liquid, and gaseous substances that burns easily and produces heat energy. It is believed to come from the transformation of sediment formed by plankton, bacteria, and plants in enclosed seas.