Biogenic and precipitate rocks form from the deposition of minerals from chemicals dissolved from all other rock types.įorces that drive the rock cycle Plate tectonics
Clastic rocks can be formed from fragments broken apart from larger rocks of any type, due to processes such as erosion or from organic material, like plant remains. Sedimentary rocks can be formed from the lithification of these buried smaller fragments ( clastic sedimentary rock), the accumulation and lithification of material generated by living organisms ( biogenic sedimentary rock - fossils), or lithification of chemically precipitated material from a mineral bearing solution due to evaporation ( precipitate sedimentary rock). While an individual grain of sand is still a member of the class of rock it was formed from, a rock made up of such grains fused together is sedimentary. This fragmented material accumulates and is buried by additional material. Weathering and erosion break the original rock down into smaller fragments and carry away dissolved material. Rocks exposed to the atmosphere are variably unstable and subject to the processes of weathering and erosion. Any pre-existing type of rock can be modified by the processes of metamorphism. This contact metamorphism results in a rock that is altered and re-crystallized by the extreme heat of the magma and/or by the addition of fluids from the magma that add chemicals to the surrounding rock ( metasomatism). Another main type of metamorphism is caused when a body of rock comes into contact with an igneous intrusion that heats up this surrounding country rock. These rocks commonly exhibit distinct bands of differing mineralogy and colors, called foliation. Regional metamorphism refers to the effects on large masses of rocks over a wide area, typically associated with mountain building events within orogenic belts.
Rocks exposed to high temperatures and pressures can be changed physically or chemically to form a different rock, called metamorphic.
Epidotization occurs also in rocks of this group, and consists in the development of epidote from biotite, hornblende, augite or plagioclase feldspar. In uralitization, secondary hornblende replaces augite chloritization is the alteration of augite (biotite or hornblende) to chlorite, and is seen in many diabases, diorites and greenstones. Serpentinization is the alteration of olivine to serpentine (with magnetite) it is typical of peridotites, but occurs in most of the mafic rocks. Kaolinization is the decomposition of the feldspars, which are the most common minerals in igneous rocks, into kaolin (along with quartz and other clay minerals) it is best shown by granites and syenites. Silicification, the replacement of the minerals by crystalline or crypto-crystalline silica, is most common in felsic rocks, such as rhyolite, but is also found in serpentine, etc. Secondary changes Įpigenetic change (secondary processes occurring at low temperatures and low pressures) may be arranged under a number of headings, each of which is typical of a group of rocks or rock-forming minerals, though usually more than one of these alterations is in progress in the same rock. Any of the three main types of rocks (igneous, sedimentary, and metamorphic rocks) can melt into magma and cool into igneous rocks. These rocks are fine-grained and sometimes cool so rapidly that no crystals can form and result in a natural glass, such as obsidian, however the most common fine-grained rock would be known as basalt. As a result of volcanic activity, magma (which is called lava when it reaches Earth's surface) may cool very rapidly on the Earth's surface exposed to the atmosphere and are called extrusive or volcanic rocks. A rock that cools within the Earth is called intrusive or plutonic and cools very slowly, producing a coarse-grained texture such as the rock granite. If the conditions no longer exist for the magma to stay in its liquid state, it cools and solidifies into an igneous rock. When rocks are pushed deep under the Earth's surface, they may melt into magma. Legend: A = magma chamber (batholith) B = dyke/dike C = laccolith D = pegmatite E = sill F = stratovolcano processes: 1 = newer intrusion cutting through older one 2 = xenolith or roof pendant 3 = contact metamorphism 4 = uplift due to laccolith emplacement.
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