Rock Types: Igneous, Sedimentary and Metamorphic Rocks Explained
Rock Types: Igneous, Sedimentary and Metamorphic Rocks
Rocks are classified according to their origin. The main criterion used to classify rocks is their origin, which gives three main groups: igneous (magmatic) rocks, sedimentary rocks and metamorphic rocks.
Igneous (Magmatic) Rocks
Igneous rocks form from the solidification of magma. When magma cools and solidifies inside the Earth or on its surface, it generates igneous rocks. Rocks formed on the surface (for example, basalt) are called volcanic, while those formed slowly inside the Earth are called plutonic.
Texture of Igneous Rocks
The texture depends on the degree of crystallization and the size of the crystals:
- Holocrystalline: entirely constituted by crystals.
- Hypocrystalline: crystals within a glassy (vitreous) matrix.
- Vitreous: the rock presents an amorphous mass with the appearance of glass.
- Homometric: crystals of similar size.
- Heterometric: crystals of different sizes, such as porphyritic textures.
- Extreme heterometry: some crystals have very different sizes.
Sedimentary Rocks
Sedimentary rocks originate from the erosion of any type of rock. The process follows these steps:
- Weathering: the action of air, water and living beings breaks rocks into fragments and makes them easier to alter.
- Erosion, transport and sedimentation: weathered materials are removed by rivers, wind, glaciers and the sea, and deposited in other areas. Transport can occur as solid-state materials pushed by wind and glaciers, as suspension of fine-medium particles, as saltation or rolling of coarse particles, or as dissolutions carried by water.
- Sediment: loose material that has been eroded, transported and deposited.
- Diagenesis: dissolved materials circulate through the pores and gaps between sediments, precipitating as a cement. Under pressure and compaction, the sediment is transformed into rock.
Cementation and compaction, located below the surface, are part of diagenesis. Sedimentary rocks have two main characteristics:
- Stratification: sediments are deposited in horizontal layers (strata).
- Fossils: organisms buried along with sediment can be preserved. A fossil is any remnant of an organism preserved in rocks.
Classification of Sedimentary Rocks
They are classified by the size of their constituent materials:
- Detrital (clastic) rocks: formed from solid materials transported by suspension, saltation or dragging.
- Conglomerate: fragments larger than 2 mm (clasts) embedded in a fine matrix.
- Sandstone: fragments of medium size between 0.06 and 2 mm, visible to the naked eye.
- Claystone (argillite): very fine fragments not visible to the naked eye, resulting from diagenesis.
- Non-detrital rocks: all the rest, formed from dissolved materials in water or from the remains of organisms.
Metamorphic Rocks
Metamorphism is the transformation of rocks in the solid state as a consequence of an increase in pressure and/or temperature. Rocks are not melted; instead, changes occur such as loss of water, modification of texture, cohesion and color, and transformations of the minerals they contain. The depth at which rocks are located influences the temperature they reach.
Foliation
Metamorphic rocks are characterized by foliation, the preferential orientation of laminar or elongated crystals. There are three types:
- Slaty: flat crystals, foliation not very visible.
- Schistose: undulating foliation with observable crystals.
- Gneissic: large crystals divided into alternating light and dark bands.
Granoblastic rocks have no foliation. They lack elongated or layered crystals; all their crystals have similar dimensions in every direction.