Geology And Geophysics

How Rocks are Formed

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Rocks are compositions of minerals that occur naturally. They consist of elements or compounds that are homogeneous - having the same composition and structure - having regular arrangements of atoms. The Earth’s crust is composed mostly of rocks. It can be found everywhere. It forms the mountains and the seabed.

The rock is formed by breaking down and forming a new one. There are three types of rock, in which all these are can be formed into another type depending on the pressure, temperature, composition and placement. The three types of rock are sedimentary, metamorphic and igneous rock.

Igneous rocks are formed from magma. Magma is a molten material formed also from rocks beneath the Earth’s crust that have melted. Rocks lying on the top of the magma push the magma down, but the less dense magma rises and solidifies as it moves upward. The magma that reaches the surface is called the lava. The lava cools quickly until it forms into crystals or glass. Volcanoes are formed through the continual eruption of lava. Magma beneath the Earth’s surface cools slowly until it forms into large crystals. Igneous rocks are named according to minerals they contain. The minerals that are commonly found on igneous rocks are feldspar, pyroxene, quartz, amphibole, iron, olivine, magnesium and mica. The igneous rocks that are commonly found are granite, basalt, rhyolite and gabbro. Granite and rhyolite are made up of minerals called feldspar and quartz. Gabbro and basalt are made up of magnesium and iron.

The second rock type is called sedimentary. From the word itself, the sedimentary rock forms once loose sediments harden. It can also be formed through fossilization or from mineral precipitation or solidification. The sedimentary rocks are grouped into three categories depending on their formation and composition.

The first is the clastic rock (from the word clast). Clastic rocks form when existing rocks break down through chemical or physical processes. Water, wind and glaciers carry the sand and mud produced from these rocks. The sand usually forms the seashore, riverbeds and beaches. The mud usually forms the lakebeds and ocean floor, because the mud stays longer with the wind or water compared to the sand, thus the mud accumulates in a still environment. The most common clastic rocks are sandstones and mudrocks or shales, which can be formed from the hardened accumulation and compaction of sand and mud.

The second category is the chemical rock. Chemical rocks are formed through the precipitation or solidification of minerals from a solution. Chemical rocks are formed from the evaporation of a solution. The most common are called the evaporites. These are formed from the evaporation of sea or lake water. The result from this evaporation forms minerals that are called gypsum and halite.

The third category is the organic rock. From the word organic, organic rocks are formed from the fossilization of plant and animal remains. The most common among the organic rocks are the limestone and coal. The limestone is formed from the accumulation of calcium carbonate. This calcium carbonate can be found in skeletons, shells and corals. The coal is formed from the accumulated carbon compound of plant remains in swampy areas.

Metamorphic rocks are also formed from pre-existing rocks that have undergone structural changes. This change occurs through high temperature or pressure. Unlike the igneous rock, metamorphic rocks are formed without melting the pre-existing rock. Pressure and temperature are the key factors for the formation of the metamorphic rocks. The change in these factors affects the stability range of a mineral or minerals.

Going beyond this stability range, the mineral in a rock breaks down and forms into another mineral. Same goes with the combination of minerals. Temperature and pressure affect its stability ranges, thus making the minerals react to each other through the movement of atoms in the solid state to form a new combination; this process is called metamorphism. The metamorphic rocks are named according to their texture. The higher the temperature or the pressure is, the grainier the rock will become.

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