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Mineral Genesis |
How do minerals form? |
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The Theory of Plate Tectonics There is a natural order and beauty to the universe, and scientists spend most of their working lives trying to discovery the fundamental rules that describe that order. Mineralogists and geologists are no exception to this rule. By the mid-sixties, most geologists accepted the hypothesis or theory of plate tectonics. This theory explained so much about the distribution of the different kinds of rocks that make up the Earth that it literally rocked the foundations of geology. The idea that the Earth's crust is broken up into tectonic plates that skate over the surface of the Earth, was difficult for many people, geologists included, to accept. The major argument against plate tectonics (or continental drift, an earlier theory), was the difficulty of identifying a mechanism that cold move entire continents large distances. It wasn't until early in the 1950's that the evidence for plate tectonics fell into place.
The Mechanism driving Plate Tectonics Upwelling of hot partially molten asthenospheric material at spreading centers and the sinking of cold, dense oceanic plates at subduction zones are the surficial expression of the convective forces that drive the movement of the tectonic plates on the Earth's surface. Although, geophysicists are still debating the existence of one or two layers of convection cells in the mantle, it is agreed that the gravitational forces responsible for mantle convection and subduction drive plate tectonics. The outer 100 kilometers of the Earth that deforms rigidly is called the lithosphere. It is this portion of the Earth that is fragmented into the tectonic plates that flow over the hotter ductile (plastic) convecting asthenosphere.  The hot, ductile, peridotite (an example from the Basin and Range is shown to the left) that makes up the asthenosphere rises close to the Earth's surface at the mid-oceanic ridges. The upwelling of the hot low density asthenosphere results in decompression melting of the peridotite (photograph to the left) to generate basalt lavas. These basalt lavas separate, rise, and are erupted to become new oceanic crust. The plates move away from each other over time, as younger basaltic crust continues to plaster on to the edges of the plates. The other example of divergent plate boundary occurs when a continental plate is broken up by upwelling asthenosphere and subsequent magma injection. The rift zone in East Africa and parts of the Arabian peninsula, and the Rio Grande rift in New Mexico appear to be in the early stages of this process.
Subduction is the result of two tectonic plates converging, as one plate, typically the oceanic plate, is forced down or subducted under the less dense continental plate. Continental plates are composed of a irregularly stratified base of stratified mafic and intermediate igneous rocks, metamorphic gneisses and schists, intruded and/or interstratified intermediate through felsic plutonic and volcanic rocks, covered by an volumetrically insignificant layer of sedimentary rocks and volcanic rocks. The continental rocks are less dense than the sea-water altered basalts, gabbros and peridotites that make up the oceanic plates. The gravitational pull of the denser oceanic plate drives the subduction. 
Occasionally, the oceanic plate is thrust up over the less dense continental plate in a convergent process called obduction. The peridotite, gabbros, and basalts exposed on Cyprus and Oman ophiolite complexes are believed to be part of an obducted oceanic plate. Convergence can also take place between two dominantly oceanic plates. The collision of two continental plates or two oceanic plates are also examples of convergent plate movement. When two continents collide, the two continental masses buckle and are pushed upward and sideways. The collision of India into Asia 50 million years ago caused the Eurasian plate to override the Indian Plate. The continued slow convergence of the Indian and Eurasian plates over millions of years produced the folded and deformed Himalayas, the highest continental mountains in the world. 
Transform boundaries occur when two plates slide horizontally past each another. The large faults or fracture zones that make up a transform boundary typically connect two spreading centers. Most transform faults occur in oceanic plates. One well-known continental transform fault is the San Andreas fault. Faulting will not cause California to "fall" into the ocean, but in time, the part of California located west of the San Andreas fault will move north with respect to the rest of North American. The average rate of plate movement is slow. According to the United States Geological Survey, the Arctic Ridge has the slowest rate (less than 2.5 cm/yr), and the East Pacific Rise in the South Pacific about 3,400 km west of Chile, has the fastest rate (more than 15 cm/yr). These rates may seem insignificant but they are not considering the huge expanse of geologic time. 2.5 cm/year is approximately the rate at which human fingernails grow. The Wonder of Plate Tectonics The theory of plate tectonics is considered to be the greatest geologic discovery of this century because it can explain so many different geologic phenomena. Plate tectonics explains the reason for the global distribution of volcanoes and earthquakes, the shape of continents, the global distribution of plant and animal fossils, the location of marine fossils and sediments at the top of mountains, and the distribution of minerals. The approximately 3,000 minerals on Earth exist because of the distribution of specific chemical elements and the wide range in pressure and temperature conditions. Both the distribution and range of geologic environments and elements are controlled, in large part, by plate tectonics. Minerals crystallize and are stable at a range of specific temperature and pressure conditions. Crystallization also requires certain elements to be present. If the temperature and pressure conditions change beyond the stability field of a mineral, the mineral becomes unstable. Given enough time and enough energy, an unstable mineral will breakdown and change to another mineral or minerals by reacting with other unstable minerals present to form new minerals or phases (a phase is a homogeneous, physically distinct portion of matter occurring in the same physical state in a heterogeneous environment or system). Phases can be crystalline solids, liquids, gases, or fluids (fluids are materials that have some of the properties of both liquids and gases). An example may illustrate these points. Picture a basalt erupting, cooling and then crystallizing along the Juan de Fuca ridge. After crystallization, sea-water will infiltrate and circulate through the cooling basalt. In time, some of the fine-grained pyroxene and plagioclase minerals that make up the basalt, and that are no longer within their stability field, will start to break down at the lower temperature. The presence of water will speed this reaction. The elements in the decomposing pyroxene and plagioclase will recombine to produce clay minerals, chlorite, and other low temperature minerals that are stable in the presence of water at the current temperature and pressure conditions. After some much longer period of time (in the order of millions of years), the Juan de Fuca plate that has been moving east away from the spreading center, is subducted eastward beneath North America. The temperature and pressure conditions change as the slab slowly descends into the Earth. At some point, the pressure and temperature conditions change enough (pressure increases, temperatures will decrease), and blueschist facies metamorphism of the altered basalts occurs. High pressure minerals such as lawsonite, albite feldspar, jadeite (a pyroxene group mineral), glaucophane (an amphibole group mineral), and pyrope (Mg-rich garnet) form at the expense of the minerals in the hydrated altered basalt. If subduction continues, temperature and pressures continue to increase, until the the T-P conditions of the eclogite facies are reached. At this point, the hydrous minerals in the blueschist will breakdown and recrystallize to form an eclogite which consists of anhydrous pyrope and a Na-rich pyroxene called omphacite. Pyrope and omphacite are stable at the higher temperature and pressure conditions. The water that was released with the breakdown of the hydrous minerals, will move upwards through the rock column. The presence of water at some higher level in the upper mantle peridotite or lower crustal rocks may instigate partial melting as some of the minerals become unstable at those pressure and temperature conditions, triggering another series of mineral breakdown and crystallization reactions. The Rock Cycle A dynamic balance exists between constructive and destructive forces operating on Earth. Subduction and continent-continent collision build mountains; weathering and erosion tear them down. Rocks and minerals are constantly being crystallized, decomposed, and mechanically broken down into smaller fragments or altered into other minerals. The products of weathering and erosion are carried by water, wind, ice, and air, as sediments to the ocean where they are deposited. With continued accumulation, the sediments are compacted under the pressure of overlying, younger sediments, and are transformed through the processes of lithification into sedimentary rocks. 
The sedimentary minerals and rocks recrystallize under heat and pressure to produce metamorphic minerals and rocks. Continued heating may result in melting to form magmas which in term crystallize under various temperature and pressure conditions to form igneous rocks of a wide variety of compositions. Magmas that crystallize slowly at depth in magma chambers produce coarse-grained intrusive or plutonic rocks. Magmas that erupt at the Earth's surface cool much more rapidly and may form volcanic glasses (obsidian), fine-grained lavas, or pyroclastic rocks depending on magma composition and water and gas content. Pyroclastic rocks (also called tuffs) consist of consolidated accumulations of material explosively erupted from volcanoes. This material can range in size from tiny fragments of glass shards or minerals to huge bombs larger than a car. Volcanic bombs the size of small houses were erupted from Mayan Volcano, Philippines on March 1, 2000. Minerals are produced by three major categories of processes: igneous, sedimentary, and metamorphic processes. Water plays an important role in each of these processes. Water changes the temperature and pressures at which igneous and metamorphic reactions occur. Water is capable of dissolving, transporting, recombining, and precipitating minerals. Water may be released during volcanism, plutonism, and metamorphism to form heated hydrothermal solutions that precipitate ore minerals or alter and breakdown preexisting minerals. Hot seawater circulating through oceanic ridge basalts dissolve metals and ultimately erupt and deposit a fine precipitate of sulfide mineral from fumaroles located on the ocean bottom called black smokers. Meteoric waters concentrate ore minerals by the process of supergene enrichment. 
 Water is by far the the most important agent in the deposition of the detrital sediments that are compacted, consolidated and cemented into clastic sedimentary rocks. The majority of limestones are the accumulated remains of shell and coral fragments precipitated by invertebrate marine animals in shallow seas. Evaporite deposits consist of layers of gypsum, halite and halite that were chemically precipitated from evaporating salt-rich waters in confined marine basins or playas (seasonal) lakes in arid or semi arid climates. Brines (salty aqueous solutions) derived from the waters in sedimentary basins dissolve metals and later precipitate them in Mississippi Valley-type sulfide deposits in carbonate rocks. Low temperature aqueous solutions precipitate carbonate and sulfates in solutions cavities in carbonate and other sedimentary rocks.
Click on the following links to learn more about igneous processes and volcanism, metamorphism and sedimentary processes, and hydrothermal and aqueous solutions.
The plate tectonic diagrams on these pages are taken from the online edition of This Dynamic Earth: The Story of Plate Tectonics, by permission of the authors, Jacquelyne Kious and Robert I. Tilling of the United States Geological Survey. All diagrams are courtesy of the United States Geological Survey. We recommend their site as an excellent source of more information on plate tectonics.
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