Rocks naturally occurring solid mass made up of one or several minerals. Minerals is a naturally occurring solid with a regular crystalline structure of atoms with a well-defined chemical composition. All metals counts as minerals, as do salts such as ordinary table salt (HCl) and crystals such as diamonds (pure carbon) and quarts (SiO2). Mineraloid substances can be things like volcanic glass. These follows the requirements of a mineral, but lacks an internal crystalline structure of atoms. Organic compounds such as amber and oil are not counted as minerals.
Rocks are classified in three main categories based on how they are formed: Igneous rocks (volcanic rocks), Sedimentary rocks (layered rocks) and Metamorphic rocks.
Image: Formation of igneous rocks
Igneous rocks are both on where they are formed and their mineral composition. Depending on where the magma/lava cools and solidifies. When magma solidifies underground, the resulting rock is called intrusive rocks. Lava solidifying on the surface makes extrusive rocks. So intrusive rocks intrudes on existing rock and extrusive rocks also known as volcanic rocks adds rocks on the surface. The intrusive rocks can be further subdivided into plutonic rocks and subvolcanic rocks. Plutonic solidifies deep down in the magma chamber, 1 in the figure. There the temperature is so high that it can take years for the magma to cool and solidify. That gives the minerals in the magma time to come together and grow. So in plutonic rocks you can see the minerals with the naked eye, giving them their characteristic spotted appearance. Lava that cools on the surface sets so fast that minerals do not have time to grow. Therefore, extrusive rocks, 3 in the figure, looks homogenous and monochromatic. The lava may even solidify so fast that mineral do not have time to form, forming volcanic rocks instead. Lastly, the subvolcanic rocks, 2 in the figure, is somewhere in-between the plutonic an extrusive rocks. They form when magma sets in dikes and sills before they reach the surface, usually at depths less than 2 km. How large the mineral grows is entirely dependent on the rate the magma cools.
The grouping of igneous rocks dependent on their mineral composition is mostly done on what types of silicates is present. From a high content of quartz and feldspar to low: Felsic, Intermediate, Mafic and Ultramafic. There are some rocks that falls outside of this classification. Like those with a carbonate content above 50% or high potassium, but those are comparatively rare. Felsic rocks are made from magma/lava high in feldspars and/or quartz. These minerals have a high melting point, making magmas and lavas very viscous. As a result, volcanos with felsic lavas easily grows in height as the lava do not have time to flow far before it cools. So felsic rocks are common near cone volcanos. Both feldspar and quartz is light in color and so are felsic rocks, one common example is the plutonic rock granite. Mafic rocks consist of darker mineral such as pyroxene and plagioclase. Lava made of these is more fluid, so the lava flow faster and further making low shield volcanos. A typical mafic rock is the dark extrusive rock basalt. Intermediate rocks consists of an even micks of felsic and mafic minerals. The ultramafic rocks are rocks that contains little silica, and the silica present consists of the dark and green pyroxene and olivine. Giving these rocks a greenish color.
Broadly speaking we can split sedimentary rocks in to main categories: clastic rocks and evaporites.
Clastic rocks are rocks formed from sediments. Sediments are material weathered or eroded away from a parent rock. As long as sediments are exposed to the elements, they can then be transported away by wind, water or gravity. When it reaches a final resting place it will begin to accumulate, older sediments are buried under younger sediments. With time, the pressure increases, compacting the sediments at the bottom. Under the high-pressure minerals precipitates from water trapped in between the sediments, cementing the sediments and forming a rock. The clastic rocks are generally named based the size of the sediments they contain.
Evaporites are rocks formed when minerals precipitates out of water, either because the water evaporates or become saturated with ions. Some examples of evaporites are salts. Like form when the Mediterranean Sea evaporated and made enormous deposits of rock salt or the current day salt flats of the Great Salt Lake in the USA. Other known examples are stalagmite and stalactite in grottos, which also is evaporites.
Image: Formation of sedimentary rocks.
1 – Moraine → Tillite, 2 – Riverbed → Conglomerate and Floodplain → Mudstone and Siltstone etc., 3 – Delta → Sandstone, 4 – Marine deposits near the river mouth → From Conglomerate near the river mouth to fine Sandstone further out, 5 – Light silt particles are carried further out from the river mouth → Siltstone, 6 – Small clay particles that reach the deep sea → Claystone and Shale, 7 – Turbidity flow → Turbedite and Greywacke, 8 – Coral reef → Limestone, 9 – Beach → Conglomerate and Coarse sandstone, 10 – Lagoon → Siltstone, Claystone, Shale and Evaporites, 11 – Landslide → Breccia, 12 – Desert dunes (Aeolian deposits) → Sandstone, 13 – Evaporated lake → Evaporites.
“Metamorphism” comes from Greek and meaning “after form” translating as change. The change in this case refers to a recrystallization of existing minerals, a chemical change in the rock such as formation of new minerals and/or a change in the rocks texture/appearance. Metamorphism begins with a protolith, a rock before the metamorphism that be either magmatic, sedimentary or another metamorphic rock. When the protolith is exposed to high temperatures and/or high pressure, it will change. The resulting rock can be classified based on a number factors such as what the protolith was, what minerals have formed or the texture an look of the rock.
The processes that results in metamorphism can roughly be divided in two: regional metamorphism and contact metamorphism. As the name implies, regional metamorphism takes place in a huge area. Common regions with regional metamorphism are subductions zones and areas with mountain formation. Burial metamorphism also count as a regional metamorphism, as it generally takes place over a larger area. Contact metamorphism is restricted to a smaller area as it the result of a magma intrusion into the rock and “cocking it”. There are also other types of local metamorphism such as shock metamorphism after a meteor impact. Moreover, hydrothermal metamorphism refers to hot water that permeates through a rock, many times near areas with contact metamorphism.
Image: Formation of metamorphic rocks.
1. Low-grade burial metamorphism of shale to slate. 2. At middle range temperatures and pressures mafic rocks are turned into Greenschist. 3. At low temperatures and high pressures mafic rocks are turned into Blueschist. 4. Low-grade metamorphism of shale to phyllite. 5. Intermediate metamorphism of phyllite to schist. 6. At a medium to high degree of metamorphosis, the mineral garnet is formed and we get garnet schist and eventually garnet gneiss. 7. At enough temperature and pressure, basaltic rocks become amphibolites. 8. At extremely high pressures, basaltic rocks can be transformed into eclogite. 9. Sandstone is transformed into quartzite. 10. Limestone is converted into marble. 11. Where hot magma comes into contact with limestone, skarn can form. 12. At low pressure and high temperature, hornfels is formed. 13. At high temperature, rocks can partially melt and become migmatite. 14. In shear zones and thrust zones with enough pressure, mylonite forms. 15. At high pressure and high heat, gneiss is formed.
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