Igneous rock is hardened magma that can occur above or below ground. [1] It can melt into magma, erode into sediment, or be tightly compressed to become metamorphic. Since the rock cycle is a continuous process, the cycle does not stop after quartzite is formed. Eventually, quartzite rock could turn into sedimentary or igneous rock to continue the cycle. We see rocks in our environment, of different shapes and sizes. Rocks never stay the same and constantly change over time. The rock cycle is a basic term in geology that describes time-consuming transitions through geological time. Rocks can be: (1) from minerals, each of which has a specific crystal structure and chemical composition; (2) from pieces of other rocks; (3) vitreous (such as obsidian); or (4) contain material produced by living organisms (e.g. Charcoal containing carbon from plants).
Different types of rocks form in different environments from Earth to or below the Earth`s surface. For example, igneous rocks form when molten rocks in the mantle or crust (see plate tectonics) slowly cool and harden underground (e.g., granite) or harden rapidly when they erupt from a volcano (e.g., basalt). Rocks that undergo enough heat and pressure in the earth without melting turn into metamorphic rocks. Rocks exposed by orogeny or even modest uplift and erode, and the resulting sediments can form sedimentary rocks. The formation and transformation of different rock types can take many paths through the rock cycle depending on environmental conditions, as shown in the diagram below. Sedimentary rocks are compacted sediments that can come from any of the other rocks,[1] plus the remains of living things. It can erode into sediments or be pressurized into metamorphic rocks and melted into magma, which forms igneous rocks. Transformation into metamorphic rocks requires high temperature and/or pressure conditions, transformation into sedimentary rocks takes place via the intermediate sediment step, and transformation into igneous rocks takes place via the intermediate magma step: these processes can take place in different orders, and the cycle lasts forever. The Earth has several processes to alter rocks. Wind and water can form sediments from rocks, and the movement of one tectonic plate against another creates enormous heat and pressure that greatly affects rocks. [4] Subduction converts all species into magma, which eventually enters the cycle as igneous rock. High temperature and pressure occur in subduction zones and in areas where two plates of the continental lithosphere collide to create a mountain range, while increased pressure without high temperature is created when sedimentary rocks are buried deep under more sediment.
Sediments form when rocks are lifted, altered and eroded, and the resulting dirty material is deposited in marine or terrestrial basins. If the sediments are buried under other sedimentary layers, they can be lithified into sedimentary rock. Magma forms when rock is melted. This melting can occur when a lithospheric plate sinks into the Earth`s crust in a subduction zone, or when a mid-ocean ridge opens and creates decompression melts in the athenosphere below the back. When magma solidifies, it becomes an igneous rock. The concept of the rock cycle was first proposed by James Hutton, the founder of modern geology in the 18th century. The rock cycle is the process that describes the gradual transformation between the three main types of rocks: sedimentary, metamorphic and magmatic. It occurs continuously in nature through geological time. The rock cycle describes the processes by which the three main types of rocks (magmatic, metamorphic and sedimentary) transform from one type to another. The formation, movement and transformation of rocks result from the Earth`s internal heat, the pressures of tectonic processes and the effects of water, wind, gravity and biological (including human) activities.
The texture, structure and composition of a rock show the conditions under which it formed and tell us something about the history of the earth. Molten lava cools to form igneous rocks that form in Hawaii National Park (left) metamorphic rocks in Death Valley National Park (right). Source: NPS Igneous Rocks and NPS Metamorphic Rocks There are three main types of rocks: sedimentary, igneous and metamorphic. Each of these rocks is formed by physical changes such as melting, cooling, erosion, compaction or deformation, which are part of the rock cycle. Sedimentary rocks are formed from pieces of other existing rocks or organic matter. There are three different types of sedimentary rocks: clastic, organic (biological) and chemical. Clastic sedimentary rocks, such as sandstone, are formed from clasts or pieces of other rocks. Organic sedimentary rocks such as coal are formed from hard biological materials such as plants, shells and bones, which are compressed into rock. The formation of clastic and organic rocks begins with the alteration or decomposition of exposed rock into small fragments. Through the process of erosion, these fragments are removed from their source and transported to a new location by wind, water, ice or biological activity.
As soon as the sediment has settled somewhere and accumulates sufficiently, the lowest layers are compacted so densely that they form solid rock. Chemical sedimentary rocks such as limestone, halite and flint are formed from chemical precipitation. A chemical precipitate is a chemical compound — for example, calcium carbonate, salt, and silica — that forms when the solution in which it is dissolved, usually water, evaporates, leaving the compound behind. This happens when water moves through the earth`s crust, altering the rock and dissolving some of its minerals to transport it to another location. These dissolved minerals are precipitated when the water evaporates. Metamorphic rocks are rocks that have been modified from their original shape by immense heat or pressure. Metamorphic rocks have two classes: deciduous and non-deciduous. When a rock with flat or elongated minerals is subjected to immense pressure, the minerals line up in layers and create foliation. Foliation is the alignment of elongated or plated minerals, such as hornblende or mica, perpendicular to the direction of pressure exerted. An example of this transformation is granite, an igneous rock. Granite contains long, flat minerals that are not initially aligned, but when enough pressure is added, these minerals move in the same direction when pressed into flat sheets.