The recycling of the Earth's outer layers. Rocks are continually being formed, destroyed, and re-formed in an endless cycle of change that takes millions of years. The processes involved include the formation of igneous rock from magma (molten rock); surface weathering and erosion; the compaction and cementation of sediments into sedimentary rock; and metamorphism, chemical and physical changes brought about by heat and pressure, producing metamorphic rock.
Formation of igneous rock
The newest rocks found on the Earth's surface are those caused by active volcanoes. When magma cools and solidifies, the minerals within it crystallize, and the magma becomes igneous rock. When a volcano erupts it may throw out molten magma that has built up below the surface in an igneous intrusion. Magma that flows out of the volcano is known as lava.
Extrusive rocks are those that form from the molten magma above ground. As the magma cools rapidly on the surface, the minerals in the rock crystallize quickly to form small interlocking crystals. A common type of extrusive igneous rock is basalt. Sometimes magma cools and solidifies before it reaches the surface, forming intrusive rocks. Intrusive igneous rock has a much larger crystal structure because the molten magma cools more slowly underground, allowing the crystals more time to grow. An example of intrusive igneous rock is granite.
Surface weathering and erosion
Rocks exposed on the surface of the Earth are very slowly broken down into fragments through the action of sun, wind, rain, and ice, in a combination of physical weathering and chemical weathering. The broken pieces of rock are transported by gravity, water (rivers and streams), wind, or ice (glaciers), and are further broken up and smoothed during the process. The movement of the fragments causes further erosion of the rock surfaces. In a river, the river bed is broken down and the smallest particles (clay and sand) are deposited at the river mouth forming an estuary, or in lakes. The sea is also an important agent of erosion, causing coastal erosion.
Rock particles to sedimentary rock
Rock particles are deposited as sediments, often at the bottom of a lake or sea. Layers of sediment build up, sometimes trapping animals and plants that will later become fossils. At this point the sediments are described as unconsolidated because the mixture is loosely packed together, for example as gravel beds. Gradually the layers get thicker as more material is deposited. The transformation of sediments into sedimentary rocks is called lithification. The layers of sediments are compressed (compacted) by the weight of the material above. Water is squeezed out and the grains are fused together. In certain cases the material is bound together by new minerals, such as calcium carbonate, which crystallize around the rock particles. This process is known as cementation (not to be confused with the cement used in the building industry). Sedimentary rocks are composed of grains, often in layers, and may contain fossils.
Formation of metamorphic rock
Plate tectonics (movements of the plates that form the Earth's outermost layer) cause further change. Large areas of sedimentary rock may be folded and uplifted to form mountain ranges, such as the Himalayas, while in the active subduction zones of the tectonic plates, where one plate dips under another, the temperature and pressure will rise causing the rock structure to change or metamorphose, forming metamorphic rock. The huge pressures and high temperatures cause chemical changes in the minerals of the rock, and the new minerals to be compressed and aligned in a parallel pattern. Rocks buried deep in the crust will be heated by about 30°C/90°F per kilometre depth. Metamorphic rock will also form next to an intrusion of magma (molten rock), often found near a volcano where the magma heats the surrounding rock. This type of change is called thermal or contact metamorphism. Examples of metamorphic rock are slate and marble.
Completion of the rock cycle
If the rock is heated until it melts then magma is formed. The material that once formed a mountain peak may now become the molten lava forced out of an active volcano, and the rock cycle is complete. The timespan for these changes is enormous. Some of the oldest known rocks come from the Precambrian era, and are at least 570 million years old.
The energy needed to power the rock cycle comes from the heat generated by the radioactive decay of elements in the material that makes up the mantle (the zone between the Earth's crust and core). The convection currents generated cause the semi-solid material of the mantle to rise, moving the tectonic plates, which causes fractures allowing volcanoes to form at the plate edges (see plate tectonics).
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