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Tectonic Plate

Tectonic plate

A Tectonic plate is a piece of the Earth's crust (or lithosphere). The surface of the Earth consists of seven major tectonic plates and many more minor ones. The plates are around 100 km (60 miles) thick and consist of two principal types of material: oceanic crust (also called sima from silicone and magnesium) and continental crust (sial from silicone and aluminium). Under both lies a relatively plastic layer of the Earth's mantle called the asthenosphere, which is in constant motion. This is in turn underlaid by a solid layer of mantle. The composition of the two types of crust differs markedly. Oceanic crust consists largely of basaltic rocks, while the continental crust consists principally of lower density granitic rocks rich in aluminium and silica. The two types of crust also differ in thickness, with continental crusts considerably thicker than oceanic. The churning of the asthenosphere carries the plates along in a process known as continental drift, which is explained by the theory of plate tectonics. Interaction between the plates creates mountains and volcanoes, as well as giving rise to earthquakes and other geological phenomena. The boundaries of the plates do not coincide with those of the continents. For instance, the North American Plate covers not only North America but also Greenland, far eastern Siberia and northern Japan. As far as is known, the Earth is the only planet in the Solar System to possess tectonic plates, although there have been suggestions that Mars may also have possessed plates in the past before the planet's crust froze in place.

See also


- List of tectonic plates

External links


- [http://element.ess.ucla.edu/publications/2003_PB2002/2003_PB2002.htm Bird, P. (2003) An updated digital model of plate boundaries] also available as a large (13 mb) PDF file [http://element.ess.ucla.edu/publications/2003_PB2002/2001GC000252.pdf]
- [http://snobear.colorado.edu/Markw/Mountains/03/week3.html Map of tectonic plates] Category:Plate tectonics ja:プレート (地学)

Crust (geology)

to exosphere. Partially to scale.]] In geology, a crust is the outer layer of a planet, part of its lithosphere. Planetary crusts are generally composed of a less dense material than that of its deeper layers. The crust of the Earth is composed mainly of basalt and granite. It is cooler and more rigid than the deeper layers of the mantle and core. On partially-molten planets, such as Earth, the lithosphere is floating on fluid interior layers. Because of the partially-fluid upper mantle, or asthenosphere, underneath, floating lithospheres can be broken into tectonic plates that move. Sea floor crust is different from that of the continents. The oceanic crust (sima) is 5 to 10 km thick and is composed primarily of a dark, dense rock called basalt. The continental crust (sial) is 20-70 km deep and is composed of a variety of less dense rocks.

See also


- Continental drift
- Plate tectonics

External links


- [http://quake.wr.usgs.gov/research/structure/CrustalStructure/ USGS Crust Thickness Map] Category:Geology Category:Plate tectonics ms:Kerak bumi ja:地殻 simple:Crust th:เปลือกโลก

Mantle (geology)

to exosphere. Partially to scale.]] exosphere waves.]] The Earth's mantle is the layer in the structure of the Earth that lies directly under the Earth's crust and above the Earth's outer core. The term is also applied to the structure of other planets. Earth's mantle lies roughly between 30 and 2,900 km below the surface. The boundary between the crust and the mantle is the Mohorovičić discontinuity, named for its discoverer, and is usually called the Moho. The Moho is a boundary at which there is a sudden change in the speed of seismic waves. At one time some thought that the Moho was the structure at which the earth's rigid crust moved relative to the mantle. Current research places this zone of movement within the mantle, from 70 km (43 mi) below the ocean crust to 150 km (93 mi) below the continental crust. The mantle just below the crust is composed of cold and therefore rigid mantle fused to the crust but at the same time separated from it by the Moho. This rigid layer of crust and the upper mantle forms the lithosphere. The mantle differs substantially from the crust in its mechanical characteristics and its chemical composition. It is chiefly the difference of chemistry on which the distinction between crust and mantle is based. Mantle rock consists of olivines, different pyroxenes and other mafic minerals. Typified by peridotite, dunite, and eclogite, mantle rocks also possesses a higher portion of iron and magnesium and a smaller portion of silicon and aluminium than the crust. In the mantle, temperatures range between 100°C at the upper boundary to over 3,500°C at the boundary with the core. Although these temperatures far exceed the melting points of the mantle rocks at the surface, particularly in deeper ranges, they are almost exclusively solid. The enormous lithostatic pressure exerted on the mantle prevents them from melting. The subregion of the mantle extending about 250 km (155 mi) below the lithosphere is called the asthenosphere. It some regions of the earth, this subregion of the mantle is associated with a region of the mantle that passes seismic waves more slowly. This region is called the low-velocity zone. The cause of this low velocity zone is still debated. Currently theories include the influence of temperature and pressure or the existence of a small amount of partial melt. Due to the temperature difference between the Earth's crust and outer core there is a convective material circulation in the mantle. Hot material ascends as a plutonic diapir from the border with the outer core, while cooler (and heavier) material sinks downward. This is often in the form of large-scale lithospheric downwellings at plate boundaries called subduction zones. During the ascent the material of the mantle cools down adiabatically. The temperature of the material falls with the pressure relief connected with the ascent, and its heat distributes itself over a larger volume. Near the lithosphere the pressure relief can lead to partial melting of the diapir, leading to volcanism and plutonism. The convection of the Earth's mantle is a chaotic process (in the sense of fluid dynamics), which is thought to drive the motion of plates. Plate motion should not be confused with the older term continental drift which applies purely to the movement of the crustal components of the continents. The movements of the lithosphere and the underlying mantle are thereby partially decoupled, since due to the rigidity of the lithosphere, a tectonic plate can only move as a whole. Continental drift is therefore only a diffuse image of the movements at the upper limit of the Earth's mantle. The convection of the mantle is not yet clarified in detail. There are different theories, according to which the Earth's mantle is divided into different floors of separate convection. Although there is a tendency to larger viscosity at greater depth, this relation is far from linear, and shows layers with dramatically decreased viscosity, in particular in the upper mantle and at the boundary with the core [http://www2.uni-jena.de/chemie/geowiss/geodyn/poster2.html]. Due to the low viscosity in the upper mantle one could reason that there should be no earthquakes below approximately 300 km depth. However, in subduction zones, the geothermal gradient can be lowered, increasing the strength of the surrounding mantle, and allowing earthquakes to occur down to a depth of 400 km and 670 km. The pressure at the bottom of the mantle is ~140 GPa (1.4 Matm). There exists increasing pressure as one travels deeper into the mantle, the entire mantle, however, is still thought to deform like a fluid on long timescales. The viscosity of the upper mantle ranges between 1021 and 1024 Pa·s, depending on depth [http://www2.uni-jena.de/chemie/geowiss/geodyn/poster2.html]. Thus, the upper mantle can only flow very slowly. Why is the inner core thought solid, the outer core thought liquid, and the mantle solid/plastic? The melting points of iron rich substances are higher than pure iron. The core is composed almost entirely of pure iron, while iron rich substances are more common outside the core. So, surface iron-substances are solid, upper mantle iron-substances are semi-molten (as it is hot and they are under relatively little pressure), lower mantle iron-substances are solid (as they are under tremendous pressure), outer core pure iron is liquid as it has a very low melting point (despite enormous pressure), and the inner core is solid due to the overwhelming pressure found at the centre of the planet. Category:Geology Category:Geophysics Category:Planetary science ja:マントル th:เนื้อโลก

Asthenosphere

The asthenosphere (from an invented Greek a + 'sthenos "without strength") is the region of the Earth between 100-200 km below the surface—but perhaps extending as deep as 400 km—that is the weak or "soft" zone in the upper mantle. It lies just below the lithosphere, which is involved in plate movements and isostatic adjustments. In spite of its heat, pressures keep it plastic, and it has a relatively low density. Seismic waves, the speed of which decrease with the softness of a medium, pass relatively slowly though the asthenosphere, the cue that originally alerted seismologists to its presence; thus it has been given the name low-velocity zone. Under the thin oceanic plates the asthenosphere is usually much nearer the seafloor surface, and at mid-ocean ridges it rises to within a few kilometres of the ocean floor. The upper part of the asthenosphere is believed to be the zone upon which the great rigid and brittle lithospheric plates of the Earth's crust move about. Due to the temperature and pressure conditions in the asthenosphere, rock becomes ductile, moving at rates of deformation measured in cm/yr over lineal distances eventually measuring thousands of kilometers. In this way, it flows like a convection current, radiating heat outward from the Earth's interior. Above the asthenosphere, at the same rate of deformation, rock behaves elastically and, being brittle, can break, causing faults. The rigid lithosphere is thought to "float" or move about on the slowly flowing asthenosphere, creating the movement of crustal plates described by Plate tectonics theory. Although its presence was suspected as early as 1926, the worldwide occurrence of the asthenosphere was confirmed by analyses of earthquake waves from the Chilean Great earthquake of May 22, 1960.

External link


- [http://www.geology.sdsu.edu/how_volcanoes_work/Heat.html San Diego State University, "The Earth's internal heat energy and interior structure"] Category:Geology Category:Plate tectonics ja:アセノスフェア

Oceanic crust

Oceanic crust is the part of Earth's lithosphere which underlies the ocean basins. It is thinner (generally less than 10 km thick) but more dense than continental crust, about 3.3 g/cc (grams per cubic centimeter). Oceanic crust is composed of mafic basaltic rocks. Most of the present day oceanic crust is less than 200 million years old because it is continuously being created at oceanic ridges and destroyed by being pulled back into the mantle in subduction zones by the processes of plate tectonics. Category:Plate tectonics

Continental crust

The continental crust is the layer of granitic and sedimentary rock which forms the continents and the areas of shallow seabed close to their shores, known as continental shelves. It is less dense and more rigid than the material of the Earth's mantle and thus "floats" on top of it. Continental crust is also less dense than oceanic crust, though it is considerably thicker, averaging 20 to 80 km versus the average oceanic thickness of around 5-10 km. As a consequence, when active margins of continental crust meet oceanic crust in subduction zones, the oceanic crust is subducted. Its relative low density keeps the continental crust from being subducted or re-cycled back into the mantle. For this reason the oldest rocks on Earth are within the "cratons" or cores of the continents, rather than in repeatedly recycled oceanic crust. Continental crust is thickest beneath mountain ranges with a deep root. This fact results from the isostatic uplift associated with orogeny (mountain formation). As it is still being formed today, the amount of continental crust has been increasing over geological time. About 40% of the Earth's surface is now underlain by continental crust.

External link


- [http://www.geo.cornell.edu/geology/classes/geochemdata/CrustalAbundances.html Average composition of Continental Crust]
- [http://earth.leeds.ac.uk/assyntgeology/extra_info/ehistory.htm Making new continents] Category:Plate tectonics ko:대륙 지각

Density

: For other senses of "density", see density (disambiguation). Density (symbol: ρ - Greek: rho) is a measure of mass per unit of volume. The higher an object's density, the higher its mass per volume. The average density of an object equals its total mass divided by its total volume. A denser object (such as iron) will have less volume than an equal mass of some less dense substance (such as water). The SI unit of density is the kilogram per cubic metre (kg/m3) :\rho = \frac where :ρ is the object's density (measured in kilograms per cubic metre) :m is the object's total mass (measured in kilograms) :V is the object's total volume (measured in cubic metres) Under specified conditions of temperature and pressure, density of a fluid is defined as described above. However, the density of a solid material can be different, depending on exactly how it is defined. Take sand for example. If you gently fill a container with sand, and divide the mass of sand by the container volume you get a value termed loose bulk density. If you took this same container and tapped on it repeatedly, allowing the sand to settle and pack together, and then calculate the results, you get a value termed tapped or packed bulk density. Tapped bulk density is always greater than or equal to loose bulk density. In both types of bulk density, some of the volume is taken up by the spaces between the grains of sand. Also, in terms of candy making, density is affected by the melting and cooling processes. Loose granular sugar, like sand, contains a lot of air and is not tightly packed, but when it has melted and starts to boil, the sugar loses its granularity and entrained air and becomes a fluid. When you mold it to make a smaller, compacted shape, the syrup tightens up and loses more air. As it cools, it contracts and gains moisture, making the already heavy candy even more dense.

Other units

Density in terms of the SI base units is expressed in terms of kilograms per cubic metre (kg/m³). Other units fully within the SI include grams per cubic centimetre (g/cm³) and megagrams per cubic metre (Mg/m³). Since both the litre and the tonne or metric ton are also acceptable for use with the SI, a wide variety of units such as kilograms per litre (kg/L) are also used. Imperial units or U.S. customary units, the units of density include pounds per cubic foot (lb/ft³), pounds per cubic yard (lb/yd³), pounds per cubic inch (lb/in³), ounces per cubic inch (oz/in³), pounds per gallon (for U.S. or imperial gallons) (lb/gal), pounds per U.S. bushel (lb/bu), in some engineering calculations slugs per cubic foot, and other less common units. The maximum density of pure water at a pressure of one standard atmosphere is 999.972 kg/m³; this occurs at a temperature of about 3.98 °C (277.13 K). From 1901 to 1964, a litre was defined as exactly the volume of 1 kg of water at maximum density, and the maximum density of pure water was 1.000 000 kg/L (now 0.999 972 kg/L). However, while that definition of the litre was in effect, just as it is now, the maximum density of pure water was 0.999 972 kg/dm3. During that period students had to learn the esoteric fact that a cubic centimetre and a millilitre were slightly different volumes, with 1 mL = 1.000 028 cm3. (often stated as 1.000 027 cm3 in earlier literature).

Measurement of density

A common device for measuring fluid density is a pycnometer. A device for measuring absolute density of a solid is a gas pycnometer.

Density of substances

Perhaps the highest density known is reached in neutron star matter (see neutronium). The singularity at the centre of a black hole, according to general relativity, does not have any volume, so its density is undefined. The most dense naturally occurring substance on Earth is iridium, at about 22650 kg/m3. A table of densities of various substances:
SubstanceDensity in kg/m3
Iridium22650
Osmium22610
Platinum21450
Gold19300
Tungsten19250
Uranium19050
Mercury13580
Palladium12023
Lead11340
Silver10490
Copper   8960
Iron   7870
Tin   7310
Titanium   4507
Diamond   3500
Basalt   3000
Granite   2700
Aluminium   2700
Graphite   2200
Magnesium   1740
PVC   1300
Seawater   1025
Water   1000
Ice   917
Polyethylene   910
Ethyl alcohol   790
Gasoline   730
Aerogel   .003
any gas   0.0446 times the average molecular mass, hence between 0.09 and ca. 10.0 (at standard temperature and pressure)
For example air   1.2
Note the low density of aluminium compared to most other metals. For this reason, aircraft are made of aluminium. Also note that air has a nonzero, albeit small, density. Aerogel is the world's lightest solid.

See also


- ISO 31: volumic mass
- Dord
- Standard temperature and pressure
- Relative density (specific gravity)
- Charge density
- Energy density
- Population density Category:Continuum mechanics Category:Introductory physics Category:Fundamental physics concepts Category:Physical quantity Category:Physical chemistry ms:Ketumpatan ja:密度 th:ความหนาแน่น

Granite

Granite is a common and widely-occurring group of intrusive felsic igneous rocks that form at great depths and pressures under continents. Granite consists of orthoclase and plagioclase feldspars, quartz, hornblende, biotite, muscovite and minor accessory minerals such as magnetite, garnet, zircon and apatite. Rarely, a pyroxene is present. Ordinary granite always carries a small amount of plagioclase, but when this is absent the rock is referred to as alkali granite. An interesting proportion of plagioclase feldspar causes granite to pass into granodiorite. A rock consisting of equal proportions of orthoclase and plagioclase plus quartz may be considered a quartz monzonite. A granite containing both muscovite and biotite micas is called a binary granite. Depending upon the proportions of feldspar and quartz, the Mohs hardness of granite ranges between 5.5 and 7 [http://www.findstone.com/daniel1.htm]. The average density is 2.75 g/cm<sup>3</sup> with a range of 1.74 to 2.80. The extrusive equivalent of plutonic granite rock is called Rhyolite. The word granite comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a crystalline rock.

Occurrence

Granite occurs as relatively small, less than 100 km<sup>2</sup> stock-like masses and as large batholiths often associated with orogenic mountain ranges and is frequently of great extent. Small dikes of granitic composition called aplites are associated with granite margins. In some locations very coarse-grained pegmatite masses occur with granite. Granite has been intruded into the crust of the Earth during all geologic periods; much of it is of Precambrian age. Granite is widely distributed throughout the continental crust of the Earth and is the most abundant basement rock that underlies the relatively thin sedimentary rock veneer of the continents.

Origin

There are two theories for the origin of granite. The magmatic theory states that granite is derived by the crystal fractionation of magma. Thus granite bodies are the result of intrusion of liquid magma into the existing rocks. The granitization theory states that granite is formed in place by extreme metamorphism. There is evidence to support both theories, and both are useful to explain different observed features. The two may actually merge: as metamorphic conditions increase to the melting point of the metamorphosed granite, it will melt and become a liquid magma, and then harden into igneous granite.

Uses

Antiquity

The Red Pyramid of Ancient Egypt (c.26th century BC), named for the light crimson hue of its exposed granite surfaces, is the third largest of Egyptian pyramids. Menkaure's Pyramid, likely dating to the same era, was constructed of limestone and granite blocks. The Great Pyramid of Giza (c.2580 BC) contains a huge granite sarcophagus fashioned of "Red Aswan Granite." The mostly ruined Black Pyramid dating from the reign of Amenemhat III once had a polished granite pyramidion or capstone, now on display in the main hall of the Egyptian Museum in Cairo (see Dahshur). Other uses in Ancient Egypt, [http://www.eeescience.utoledo.edu/Faculty/Harrell/Egypt/Mosques/CAIRO_Rocks_1.htm] include columns, door lintels, sills, jambs, and wall and floor veneer. How the Egyptians worked the solid granite is still a matter of debate. Dr. Patrick Hunt [http://hebsed.home.comcast.net/hunt.htm] has postulated that the Egyptians used emery shown to have higher hardness on the Mohs scale.

Modern

Granite has been extensively used as a dimension stone and as flooring tiles in public and commercial buildings and monuments. Polished granite has been a popular choice for kitchen countertops due to its high durability and aesthetic qualities. In the world of sports, curling rocks are traditionally fashioned of granite. <center> <gallery> Image:Granite azul noce.jpg|<center>Azul Noce (Spain)</center> Image:Granite giallo.jpg|<center>Giallo Veneziano (Brazil)</center> Image:Granite_gran_violet.jpg|<center>Gran Violet (Brazil)</center> Image:Granite lavanda blue.jpg|<center>Lavanda Blue (Brazil)</center> </gallery> </center>

See also


- List of minerals
- List of rocks

External link


- [http://www.geologynet.com/granite1.htm The Emplacement and Origin of Granite] Category:Igneous rocks Category:Granite domes ko:화강암 ja:花崗岩

Silica

The chemical compound silicon dioxide, also known as silica, is the oxide of silicon, chemical formula SiO2. It is found in nature in several forms, including quartz and opal. In fact, silica has 17 crystalline forms (see [http://www.minsocam.org/MSA/collectors_corner/arc/silicanom.htm Nomenclature of Silica]). Also, many forms of life include silica structures, including microorganisms such as diatoms, plants such as horsetail, and animals such as hexactinellid sponges. It is manufactured in several forms including glass (in colorless high purity form called fused silica), synthetic amorphous silica and silica gel (used e.g. as desiccants in brand new clothes and leather goods). Silica, with alumina, is a crucial ingredient in clay and allows for the development of a interlocking crystal matrix after firing in earthenware, stoneware and porcelain ceramic processes. Silica is a major ingredient of Portland cement. The ceramic re-entry heat protection tiles mounted on the bottom side of the Space Shuttles are made mostly of silica, as are the firebricks used in steel processing. The most common constituent of sand in inland continental settings and non-tropical coastal settings is silica, usually in the form of quartz because the considerable hardness of this mineral resists erosion. However, the composition of sand varies according to local rock sources and conditions. Inhaling crystalline silica dust can lead to silicosis. Variants found in high-pressure impacts are coesite and stishovite. Silica is also used as a food additive, primarily as a flow agent in powdered foods, or to absorb water. See the ingredients list for [http://www.bk.com/Food/Nutrition/ingredients.aspx Burger King]. The chemical stability of silicon dioxide and its electrical insulation properties are a major reason why silicon is the dominant material for semiconductor devices. It is used to separate the active regions of devices and to form insulating surfaces.

Chemistry

Silicon dioxide can be formed when silicon is exposed to oxygen (or air) at extremely high temperatures. This can occasionally happen naturally in fires, or in lightning strikes onto sand. Silicon dioxide is attacked by strong acids particularly hydrofluoric acid (HF). HF is used to remove or pattern silicon in the semiconductor industry.

Reference


- R. K. Iler, The Chemistry of Silica (ISBN 047102404X)

External links


- (Tridymite)
- (Quartz)
- (Cristobalite)
- [http://www.cdc.gov/niosh/npg/npgd0552.html NIOSH Pocket Guide to Chemical Hazards] (amorphous)
- [http://www.cdc.gov/niosh/npg/npgd0553.html NIOSH Pocket Guide to Chemical Hazards] (crystalline, as respirable dust) Category:Silicon compounds Category:Oxides Category:Ceramics ja:二酸化ケイ素

Continental drift

The concept of continental drift was first proposed by Alfred Wegener. In 1912 he noticed that the shapes of continents on either side of the Atlantic Ocean seem to fit together (for example, Africa and South America). Francis Bacon, Antonio Snider-Pellegrini, Benjamin Franklin, and others had noted much the same thing earlier. The similarity of southern continent fossil faunae and some geological formations had led a relatively small number of Southern hemisphere geologists to conjecture as early as 1900 that all the continents had once been joined into a supercontinent known as Pangaea. The concept was initially ridiculed by most geologists, who felt that an explanation of how a continent drifted was a prerequisite and that the lack of one made the idea of drifting continents wholly unreasonable. The theory received support through the controversial years from South African geologist Alexander Du Toit as well as from Arthur Holmes. The idea of continental drift did not become widely accepted as theory until the 1950s in Europe. By the 1960s, geological research conducted by Robert Dietz, Bruce Heezen, and Harry Hess along with a rekindling of the theory including a mechanism by J. Tuzo Wilson led to acceptance among North American geologists. The hypothesis of continental drift became part of the larger theory of plate tectonics. This article deals mainly with the historical development of the continental drift hypothesis before 1950. See: plate tectonics for information on current ideas underlying concepts of continental drift.

Various data

South America and Africa are moving apart at 3 cm per year, due to the seafloor spreading along the Mid-Atlantic Ridge. This is comparable to the growth speed of a fingernail. The fastest recorded seafloor spreading takes place along the East Pacific Rise at 15 cm per year.

Evidence for continental drift


- Sir Francis Bacon first noticed this peculiarity in the 17th century. Note: This section contains evidence available to Wegener's contemporaries and predecessors Sir Francis Bacon Evidence for continental drift is now extensive, in the form of plant and animal fossils of the same age found around different continent shores, suggesting that these shores were once joined. For example the fossils of the freshwater crocodile found in Brazil and South Africa. Another illustrative example is the discovery of fossils of the aquatic reptile Lystrosaurus from rocks of the same age from locations in South America, Africa, and Antarctica. There is also living evidence - the same animals being found on two continents. An example of this is a particular earthworm found in South America and South Africa. The complementary shapes of the facing sides of South America and Africa is obvious, but is a temporary coincidence. In millions of years, seafloor spreading, continental drift, and other forces of tectonophysics will further separate and rotate those two continents. It was this temporary feature which inspired Alfred Wegener to study what he defined as continental drift.

Permo-Carboniferous

Permo-Carboniferous was a period of great glaciation that occurred about 250 million years ago. It is one of the many ice ages that has occurred on this Earth. This is also an era that has been used to submit proof that the continents were once a large land mass called Gondwana. Permo-Carboniferous rocks are widely distributed in Gondwana. The widespread distribution of Permo-Carboniferous glacial sediments in South America, Africa, Madagascar, Arabia, India, Antarctica and Australia was one of the major pieces of evidence for the theory of continental drift. Glacial activity spanned virtually the whole of Carboniferous and Early Permian time (A.G. Smith 1997). Toward the end of the Carboniferous, and around 290 million years ago, Gondwanda hovered over the south polar regions, where glacial centres expanded across the continents, as evidenced by glacial deposits of tillites along with striations in ancient rocks. Those heavily grooved by the advancing glaciers showed lines of ice flow away from the equator and toward the poles, which is the opposite direction if the continents were situated where they are today. Overall, the southern continents drifted together over the South Pole, and massive ice sheets radiating outward from a central point crossed the present continentall boundaries. The Permo-Carboniferous ice sheet is so extensive that it can fit within a latitude circle of 50 degrees (A.G.Smith 1997) (Rahul Megharaj 1985).

The debate over continental drift

Before geophysical evidence started accumulating after World War II, the idea of continental drift caused sharp disagreement among geologists. Wegener had introduced his theory in 1912 at a meeting of the German Geological Association. His paper was published that year and expanded into a book in 1915. In 1921 the Berlin Geological Society held symposium on the theory. In 1922 Wegener's book was translated into English and received a wider audience. In 1923 the theory was discussed at conferences by Geological Society of France, the Geological Section of the British Association for the Advancement of Science, and the Royal Geological Society. The theory was carefully but critically reviewed in the journal Nature by Philip Lake. On November 15, 1926, the American Association of Petroleum Geologists (AAPG) held a symposium at which the continental drift hypothesis was vigorously debated. The resulting papers were published in 1928 under the title Theory of continental drift. Wegener himself contributed a paper to this volume. One of the main problems with Wegener's theory was that he believed that the continents "plowed" through the rocks of the ocean basins. Most geologists did not believe that this could be possible. Plate tectonics, a modern update of the old ideas of Wegener about "plowing" continents, accommodates continental motion through the mechanism of seafloor spreading. New rock is created by volcanism at mid-ocean ridges and returned to the Earth's mantle at ocean trenches. Remarkably, in the 1928 AAPG volume, G. A. F. Molengraaf of the Delft Institute (now University) of Technology proposed a recognisable form of seafloor spreading in order to account for the opening of the Atlantic Ocean as well as the East Africa Rift. Arthur Holmes (an early supporter of Wegener) suggested that the movement of continents was the result of convection currents driven by the heat of the interior of the Earth, rather than the continents floating on the mantle. The ideas of these two people led to the theory of plate tectonics, which replaced the theory of continental drift, and became the accepted theory in the 1960s (based on data that started to accumulate in the late 1950s).

Further reading


- Friedlander, Michael W. At the Fringes of Science, pages 21-26, Westview, 1995, ISBN 0-8133-2200-6. 1998 edition with new epilog: ISBN 0-8133-9060-5.
- Le Grand, H. E. Drifting Continents and Shifting Theories, Cambridge University, 1988, ISBN 0521311055 (paperback) and ISBN 0521322103 (hardback).

External links


- [http://www.ux1.eiu.edu/~cfjps/1300/cont_drift.html A brief introduction to Plate Tectonics, based on the work of Alfred Wegener.]
- [http://www.scotese.com/earth.htm Maps of continental drift, from the Precambrian to the future] Category:Plate tectonics ms:Teori hanyutan benua ko:대륙이동설 ja:大陸移動説 th:การเลื่อนไหลของทวีป

Mountain

has one of the largest visible base-to-summit elevation differences anywhere]] A mountain is a landform that extends above the surrounding terrain in a limited area. A mountain is generally much higher and steeper than a hill, but there is considerable overlap, and usage often depends on local custom. Some authorities define a mountain as a peak with a topographic prominence over an arbitrary value: for example, the Encyclopædia Britannica requires a prominence of 2,000 feet (610 m). 24% of the Earth's land mass is mountainous; 10% of the world's 6 billion people live in mountainous regions. All the world's major rivers are fed from mountain sources, and more than half of humanity depends on mountains for water [http://www.animana.org/tab2/22troubleattop.shtml]. The adjective montane is used to describe mountainous areas and the things associated with them.

Heights

Heights of mountains are generally given as heights above mean sea level. The Himalayas average 5km above sea level, whilst the Andes average 4km. Most other mountain ranges average 2-2.5km. The highest mountain on Earth is Everest, 8850 m, set in the world's most significant mountain range, the Himalaya. Other definitions of height are possible. The peak that is farthest from the centre of the Earth is Chimborazo in Ecuador. At 6,272 m above sea level it is not even the tallest peak in the Andes, but because the Earth bulges at the equator and Chimborazo is very close to the equator, it is 2,150 m further away from the Earth's centre than Everest. The peak that rises farthest from its base is Mauna Kea on Hawaii, whose peak is over 9,000 m above its base on the floor of the Pacific Ocean. The tallest known mountain in the solar system is Olympus Mons, located on Mars.

Characteristics

The altitude of mountains means that the tops exist in higher cold layers of the atmosphere. They are consequently often subject to glaciation and erosion through frost action. This produces the classic mountain peak shape. Some mountains have glacial lakes, created by melting glaciers; for example, there are an estimated 3000 in Bhutan. Sufficiently tall mountains have very different climatic conditions at the top than at the base, and will thus have different life zones at different altitudes on their slopes. The plants and animals of a zone are somewhat isolated when the zones above and below are inhospitable, and many unique species occur on mountainsides as a result. Extreme cases are known as sky islands. Cloud forests are forests on mountain sides which attract moisture from the air, creating a unique ecosystem. Mountains are not generally favored for human habitation; the weather is harsher, less food is available, and there is little level ground suitable for farming. At very high altitudes, there is less oxygen in the air, and less protection against solar radiation (UV). Acute mountain sickness (caused by hypoxia - a lack of oxygen in the blood) affects over half of lowlanders who spend more than a few hours above 3500 metres. Despite some biological adaptation by peoples who have lived on mountains for hundreds or thousands of years, babies' average birthweight is reduced by 100 grams for every 1000-metre gain in altitude. Most mountains of the world have been left in their natural state, and are today primarily used for recreation. Some mountains are very difficult to climb, and offer spectacular views. Some people therefore enjoy the sport of mountaineering. Mountains are also the site for the sport of downhill skiing. People engaging in these activities often stay at mountain resorts built for the purpose.

Geology

mountain resort.]] A mountain is usually produced by the movement of lithospheric plates, either orogenic movement or epeirogenic movement. The compressional forces, isostatic uplift and intrusion of igneous matter forces surface rock upwards, creating a landform higher than the surrounding features. The height of the feature makes it either a hill or, if higher and steeper, a mountain. The absolute heights of features termed mountains and hills vary greatly according to an area's topography. The major mountains tend to occur in long linear arcs, indicating tectonic plate boundaries and activity. Mountain creation tends to occur in discrete periods, each referred to as an orogeny. The orogeny may last millions of years, and the uplifted region is being eroded away, producing valley-and-peak topography, even while the uplift is taking place. Two types of mountain are formed depending on how the rock reacts to the tectonic forces – block mountains or fold mountains. The compressional forces in continental collisions may cause the compressed region to thicken, so the upper surface is forced upwards. In order to balance the weight, much of the compressed rock is forced downwards, producing deep "mountain roots". Mountains therefore form downwards as well as upwards (see isostasy). However, in some continental collisions part of one continent may simply override part of the other, crumpling in the process. Some isolated mountains were produced by volcanoes, including many apparently small islands that reach a great height above the ocean floor. Block mountains are created when large areas are widely broken up by faults creating large vertical displacements. The uplifted blocks are block mountains or horsts. The intervening dropped blocks are termed graben: these can be small or form extensive rift valley systems. This form of landscape can be seen in East Africa, the Vosges, the Basin and Range province of Western North America and the Rhine valley. Where rock does not fault it folds, either symmetrically or asymmetrically. The upfolds are anticlines and the downfolds are synclines; in asymmetric folding there may also be recumbent and overturned folds. The Jura mountains are an example of folding. Over time, erosion can bring about an inversion of relief: the soft upthrust rock is worn away so the anticlines are actually lower than the tougher, more compressed rock of the synclines.

See also


- List of mountains
- List of highest mountains
- Latin names of mountains
- Mountain range and list of mountain ranges
- List of mountains on Venus
- List of mountains on the Moon
- Gallery of mountains
- Peak

External links


- [http://www.ga.com.pl/tatry21.htm Pics from the Tatra Mountains - Poland]
- [http://bivouac.com Canadian Mountain Encyclopedia] - an exhaustive index of North American peaks, including thousands of unnamed ones. Includes the United States and Mexico as well as Canada. Category:Landforms Category:Mountains Category:Mountaineering Category:Geomorphology ko:산 ms:Gunung ja:山 simple:Mountain

Earthquake

: An earthquake is a sudden and sometimes catastrophic movement of a part of the Earth's surface. Earthquakes result from the dynamic release of elastic strain energy that radiates seismic waves. Earthquakes typically result from the movement of faults, planar zones of deformation within the Earth's upper crust. The word earthquake is also widely used to indicate the source region itself. The Earth's lithosphere is a patch work of plates in slow but constant motion (see plate tectonics). Earthquakes occur where the stress resulting from the differential motion of these plates exceeds the strength of the crust. The highest stress (and possible weakest zones) are most often found at the boundaries of the tectonic plates and hence these locations are where the majority of earthquakes occur. Events located at plate boundaries are called interplate earthquakes; the less frequent events that occur in the interior of the lithospheric plates are called intraplate earthquakes (see New Madrid Seismic Zone). Earthquakes also occur in volcanic regions and as the result of a number of anthropogenic sources, such as reservoir induced seismicity, mining and the removal or injection of fluids into the crust. Seismic waves including some strong enough to be felt by humans can also be caused by explosions (chemical or nuclear), landslides, and collapse of old mine shafts, though these sources are not strictly earthquakes.

Characteristics

Large numbers of earthquakes occur on a daily basis on Earth, but the majority of them are detected only by seismometers and cause no damage ([http://neic.usgs.gov/neis/general/magnitude_intensity.html magnitude] 5). Most earthquakes occur in narrow regions around plate boundaries down to depths of a few tens of kilometres where the crust is rigid enough to support the elastic strain. Where the crust is thicker and colder they will occur at greater depths and the opposite in areas that are hot. At subduction zones where plates descend into the mantle earthquakes have been recorded to a depth of 600 km. Large earthquakes can cause serious destruction and massive loss of life through a variety of agents of damage, including fault rupture, vibratory ground motion (i.e., shaking), inundation (e.g., tsunami, seiche, dam failure), various kinds of permanent ground failure (e.g. liquefaction, landslide), and fire or a release of hazardous materials. In a particular earthquake, any of these agents of damage can dominate, and historically each has caused major damage and great loss of life, but for most of the earthquakes shaking is the dominant and most widespread cause of damage. There are four types of seismic waves that are all generated simultaneously and can be felt on the ground. S-waves (secondary or shear waves) and the two types of surfaces waves (Love waves and Rayleigh waves) are responsible for the shaking hazard. Rayleigh waves Rayleigh waves Most large earthquakes are accompanied by other, smaller ones, that can occur either before or after the principal quake — these are known as foreshocks or aftershocks, respectively. While almost all earthquakes have aftershocks, foreshocks are far less common occurring in only about 10% of events. The power of an earthquake is distributed over a significant area, but in the case of large earthquakes, it can spread over the entire planet. Ground motions caused by very distant earthquakes are called teleseisms. The Rayleigh waves from the Sumatra-Andaman Earthquake of 2004 caused ground motion of over 1 cm even at the seismometers that were located the greatest distance from it. Using such ground motion records from around the world it is possible to identify a point from which the earthquake's seismic waves appear to originate. That point is called its "focus" or "hypocenter" and usually proves to be the point at which the fault slip was initiated. The location on the surface directly above the hypocenter is known as the "epicenter". The total size of the fault that slips, the rupture zone, can be as large as 1000 km, for the biggest earthquakes. Just as a large loudspeaker can produce a greater volume of sound than a smaller one, large faults are capable of higher magnitude earthquakes than smaller faults are. Earthquakes, especially those that occur beneath oceans or seas (also called seaquake) and have large vertical displacements, can give rise to tsunamis, either as a direct result of the deformation of the sea bed due to the earthquake, or as a result of submarine landslips or "slides" indirectly triggered by it.

Earthquake Size

The first method of quantifying earthquakes was intensity scales. In the United States the Mercalli (or Modified Mercalli, MM) scale, is commonly used while Japan (shindo) and the EU (European Macroseismic Scale) each have their own scales. These assign a numeric value (different for each scale) to a location based on the size of the shaking experienced there. The values 6 (normally denoted ‘’VI’’) in the MM scale for example is: Everyone feels movement. People have trouble walking. Objects fall from shelves. Pictures fall off walls. Furniture moves. Plaster in walls might crack. Trees and bushes shake. Damage is slight in poorly built buildings. No structural damage. The problem with these scales is the measurement is subjective, often based on the worst damage in an area and influenced by local effects like site conditions that make it a poor measure for the relative size of different events in different places. For some tasks related to engineering and local planning it is still useful for the very same reasons and thus still collected. If you feel an earthquake in the US you can report the effects to the USGS here: [http://pasadena.wr.usgs.gov/shake/ Did you feel it?] The first attempt to qualitatively define one value to describe the size of earthquakes was the magnitude scale (the name being taking from similar formed scales used on the brightness of stars). In the 1930s, a California seismologist named Charles F. Richter devised a simple numerical scale (which he called the magnitude) to describe the relative sizes of earthquakes in Southern California. This is known as the “Richter scale”, “Richter Magnitude” or “Local Magnitude” (ML). It is obtained by measuring the maximum amplitude of a recording on a Wood-Anderson torsion seismometer (or one calibrated to it) at a distance of 600km from the earthquake. Other more recent Magnitude measurements include: body wave magnitude (mb), surface wave magnitude (Ms) and duration magnitude (MD). Each of these is scaled to gives values similar to the values given by the Richter scale. However as each is also based on the measurement of one part of the seismogram they do not measure the overall power of the source and can suffer from saturation at higher magnitude values (larger events fail to produce higher magnitude values).These scales are also empirical and as such there is no physical meaning to the values. They are still useful however as they can be rapidly calculated, there are catalogues of them dating back many years and are they are familiar to the public. Seismologists now favor a measure called the seismic moment, related to the concept of moment in physics, to measure the size of a seismic source. The seismic moment is calculated from seismograms but can also by obtained from geologic estimates of the size of the fault rupture and the displacement. The values of moments for different earthquakes ranges over several order of magnitude. As a result the moment magnitude (MW) scale was introduced by Hiroo Kanamori, which is comparable to the other magnitude scales but will not saturate at higher values. seismogram on February 28 2001.]] 2001 of the shaking of the Nisqually earthquake on February 28 2001.]]

Causes

Most earthquakes are powered by the release of the elastic strain that accumulate over time, typically, at the boundaries of the plates that make up the Earth's lithosphere via a process called Elastic-rebound theory. The Earth is made up of tectonic plates driven by the heat in the Earth's core. these plates collide against each other all the time but sometimes the gaps between them are stressed. Eventually, the plates make way and all that energy is sent out in the form of seismic waves. Deep focus earthquakes, at depths of 100's km, are possibly generated as subducted lithospheric material catastrophically undergoes a phase transition since at the pressures and temperatures present at such depth elastic strain cannot be supported. Some earthquakes are also caused by the movement of magma in volcanoes, and such quakes can be an early warning of volcanic eruptions. A rare few earthquakes have been associated with the build-up of large masses of water behind dams, such as the Kariba Dam in Zambia, Africa, and with the injection or extraction of fluids into the Earth's crust (e.g. at certain geothermal power plants and at the Rocky Mountain Arsenal). Such earthquakes occur because the strength of the Earth's crust can be modified by fluid pressure. Earthquakes have also been known to be caused by the removal of natural gas from subsurface deposits, for instance in the northern Netherlands. Finally, ground shaking can also result from the detonation of explosives. Thus scientists have been able to monitor, using the tools of seismology, nuclear weapons tests performed by governments that were not disclosing information about these tests along normal channels. Earthquakes such as these, that are caused by human activity, are referred to by the term induced seismicity. Another type of movement of the Earth is observed by terrestrial spectroscopy. These oscillations of the earth are either due to the deformation of the Earth by tide caused by the Moon or the Sun, or other phenomena.

Preparation for earthquakes


- Emergency preparedness
- Household seismic safety
- Seismic retrofit
- Earthquake prediction

Specific fault articles


- Alpine Fault
- Calaveras Fault
- Hayward Fault Zone
- North Anatolian Fault Zone
- New Madrid Fault Zone
- San Andreas Fault

Specific earthquake articles


- Shaanxi Earthquake (1556). Deadliest known earthquake in history, estimated to have killed 830,000 in China.
- Cascadia Earthquake (1700).
- Kamchatka earthquakes (1737 and 1952).
- Lisbon earthquake (1755).
- New Madrid Earthquake (1811).
- Fort Tejon Earthquake (1857).
- Charleston earthquake (1886). Largest earthquake in the Southeast and killed 100.
- San Francisco Earthquake (1906).
- Great Kantō earthquake (1923). On the Japanese island of Honshu, killing over 140,000 in Tokyo and environs.
- Kamchatka earthquakes (1952 and 1737).
- Great Chilean Earthquake (1960). Biggest earthquake ever recorded, 9.5 on Moment magnitude scale.
- Good Friday Earthquake (1964) Alaskan earthquake.
- Ancash earthquake (1970). Caused a landslide that buried the town of Yungay, Peru; killed over 40,000 people.
- Sylmar earthquake (1971). Caused great and unexpected destruction of freeway bridges and flyways in the San Fernando Valley, leading to the first major seismic retrofits of these types of structures, but not at a sufficient pace to avoid the next California freeway collapse in 1989.
- Tangshan earthquake (1976). The most destructive earthquake of modern times. The official death toll was 255,000, but many experts believe that two or three times that number died.
- Great Mexican Earthquake (1985). 8.1 on the Ritcher Scale, killed over 6,500 people (though it is believed as many as 30,000 may have died, due to missing people never reappearing.)
- Whittier Narrows earthquake (1987).
- Armenian earthquake (1988). Killed over 25,000.
- Loma Prieta earthquake (1989). Severely affecting Santa Cruz, San Francisco and Oakland in California. Revealed necessity of accelerated seismic retrofit of road and bridge structures.
- Northridge, California earthquake (1994). Damage showed seismic resistance deficiencies in modern low-rise apartment construction.
- Great Hanshin earthquake (1995). Killed over 6,400 people in and around Kobe, Japan.
- İzmit earthquake (1999) Killed over 17,000 in northwestern Turkey.
- Düzce earthquake (1999)
- Chi-Chi earthquake (1999).
- Nisqually Earthquake (2001).
- Gujarat Earthquake (2001).
- Dudley Earthquake (2002).
- Bam Earthquake (2003).
- Parkfield, California earthquake (2004). Not large (6.0), but the most anticipated and intensely instrumented earthquake ever recorded and likely to offer insights into predicting future earthquakes elsewhere on similar slip-strike fault structures.
- Chuetsu Earthquake (2004).
- Indian Ocean Earthquake (2004). One of the largest earthquakes ever recorded at 9.0. Epicenter off the coast of the Indonesian island Sumatra. Triggered a tsunami which caused nearly 300,000 deaths spanning several countries.
- Sumatran Earthquake (2005).
- Fukuoka earthquake (2005).
- Kashmir earthquake (2005). Killed over 79,000 people. Many more at risk from the Kashmiri winter.
- Lake Tanganyika earthquake (2005). See also List of earthquakes

See also


- Earthquake insurance
- Earthquake lights
- Elastic-rebound theory
- Catastrophe modeling
- Geophysics
- Interplate earthquake
- Intraplate earthquake
- Megathrust earthquake
- List of earthquakes
- Plate tectonics
- List of tectonic plates
- Seismic wave
- Seismology
- Tsunami
- The VAN method to predict earthquakes

External links


- [http://www.eqnet.org/ EQNET: Earthquake Information Network]
- [http://neic.usgs.gov/ The U.S. National Earthquake Information Center]
- [http://earthquake.usgs.gov/faq/ USGS Earthquake FAQs]
- [http://www.ssn.unam.mx/ Mexican Sismological Service] Reports earthquakes in Mexico. Updated regularly.
- [http://wapi.isu.edu/envgeo/EG5_earthqks/eg_mod5.htm Environmental Geology - GEOL 406/506 (Earthquakes)]
- [http://www.quakes.bgs.ac.uk/hazard/ems1.htm The European Macroseismic Scale]
- [http://simscience.org/crackling/Advanced/Earthquakes/GutenbergRichter.html Gutenberg-Richter] power law of earthquake frequency against magnitude
- [http://www.guardian.co.uk/flash/0,5860,1121610,00.html Interactive guide: Earthquakes] an educational presentation on why earthquakes happen by Guardian Unlimited
- [http://www.geowall.org Geowall]- an educational 3d presentation system for looking at and understanding earthquake data
- [http://www.sciencecourseware.com/VirtualEarthquake/ Virtual Earthquake] educational site explaining how epicenters are located and magnitude is determined
- [http://www.pbs.org/newshour/science/earthquake/ PBS NewsHour - Predicting Earthquakes]
- [http://www.lamit.ro/earthquake-early-warning-system.htm Earthquake Warning System] Personal Earthquake warning system. Highly advanced detector, featuring sos signals and carrying strip.
- [http://www.data.scec.org/ Southern California Earthquake Data Center]
- [http://www.emsc-csem.org/ European-Mediterranean Seismological Centre (EMSC)]
- [http://www.gfz-potsdam.de/geofon/seismon/globmon.html Global Seismic Monitor at GFZ Potsdam]
- [http://earthquake.usgs.gov/bytopic/eqmonitoring/history/part09.php USGS Earthquake Monitoring History]
- [http://tsunami.geo.ed.ac.uk/local-bin/quakes/mapscript/demo_run.pl Global Earthquake Report – chart updated with each new earthquake or aftershock]
- [http://hraun.vedur.is/ja/englishweb/index.html Earthquakes in Iceland during the last 48 hours], updated automatically once every 2 minutes.
- [http://www.data.scec.org/recenteqs/Quakes/quakes0.html Recent earthquakes in California and Nevada ]
- [http://neic.usgs.gov/neis/eqlists/10maps_world.html USGS – Largest earthquakes in the world since 1900]
- [http://www.armageddononline.org/earthquake.php The Destruction of Earthquakes - and a List of the Worst ever recorded]
- [http://www.losangelesearthquakes.com Los Angeles Earthquakes plotted on a Google map]
- [http://rev.seis.sc.edu Seismograms for recent earthquakes via REV, the Rapid Earthquake Viewer]
- [http://www.iris.edu Incorporated Research Institutions for Seismology (IRIS)], earthquake database and software
- [http://www.iris.edu/seismon/ IRIS Seismic Monitor], world map of recent earthquakes
- [http://www.iris.edu/seismo/ SeismoArchives], Seismogram Archives of Significant Earthquakes of the World Category:Seismology Category:Geological hazards ko:지진 ms:Gempa bumi ja:地震 simple:Earthquake th:แผ่นดินไหว

North American Plate

The North American Plate is a continental tectonic plate covering the continent of North America, extending eastward to the Mid-Atlantic Ridge and westward to the Cherskiy Range in East Siberia. The easterly side is a divergent boundary with the Eurasian Plate to the north and the African Plate to the south forming the northern part of the Mid-Atlantic Ridge. The southerly side is a boundary with the Cocos Plate to the west and the Caribbean Plate to the east. The westerly side is a convergent boundary with the subducting Juan de Fuca Plate to the north and a transform boundary with the Pacific Plate to the south along the San Andreas Fault. On its western edge the Farallon Plate has been subducting under the North American Plate since the Jurassic period. The Farallon Plate has completely subducted beneath the southern portion of the North American Plate leaving that part of the North American Plate in contact with the Pacific Plate and creating the San Andreas Fault. The Juan de Fuca, Cocos, and Nazca Plates are the remains of the Farallon Plate. Category:Plate tectonics

Greenland

:For the town in New Hampshire, see Greenland, New Hampshire. Greenland (Greenlandic: Kalaallit Nunaat, meaning "Land of the Greenlanders"; Danish: Grønland, meaning "Greenland") is a self-governed Danish territory. An Arctic island nation located in the continent of North America, both geographically and ethnically; politically and historically, however, Greenland is closely associated with Europe. The Atlantic Ocean and Iceland lie to Greenland's Southeast; the Greenland Sea to the East; the Arctic Ocean to the North; Baffin Bay and Canada to the West. Greenland is the world's largest island, and is the largest dependent territory by area in the world. It also contains the world's largest national park. About 81 percent of its surface is covered by ice, known as the Greenlandic ice cap. Nearly all Greenlanders live along the fjords in the south-west of the island, which has a milder climate. Most Greenlanders have both Kalaallit (Inuit) and Scandinavian ancestry, and speak Greenlandic (Kalaallisut) as their first language. Greenlandic is spoken by about 50,000 people, which is more than all the other Eskimo-Aleut languages combined. A minority of Danish migrants with no Inuit ancestry speak Danish as their first language. Both languages are official, with the West Greenlandic dialect forming the basis of the official form of Greenlandic. There is an on-going diplomatic sovereignty dispute between Canada and Greenland (represented internationally by Denmark) over the tiny Hans Island. Greenland was one of the Norwegian Crown colonies until 1815, when it formally became a Danish colony, although Norway and Denmark had been in a personal union for centuries (see Denmark-Norway). Greenland became an integral part of the Kingdom of Denmark in 1953. It was granted home rule (hjemmestyre) by the Folketing (Danish parliament) on May 1 1979. The law went into effect the following year. The Queen of Denmark, Margrethe II, remains Greenland's Head of state.

History

Greenland was home to a number of Paleo-Eskimo cultures in prehistory, the latest of which - the Early Dorset culture - disappeared around the year 200. Hereafter, the island seems to have been without humans for some eight centuries. Icelandic settlers found the land uninhabited when they arrived ca. 982. They established three settlements near the very Southwestern tip of the island, where they thrived for the next few centuries, disappearing after over 450 years of habitation. The name Greenland comes from those Scandinavian settlers. In the Norse sagas, it is said that Eiríkur Rauði (Erik the Red) was exiled from Iceland for murder. He, along with his extended family and slaves, set out in ships to find the land that was rumored to be to the northwest. After settling there, he named the land Greenland in order to attract more people to settle there. The fjords of the Southern part of the island were lush and had a warmer climate at that time, possibly due to what was called the Medieval Warm Period. These remote communities thrived and lived off farming, hunting and trading with the motherland, and when the Scandinavian monarchs converted their domains to Christianity, a bishop was installed in Greenland as well. The settlements seem to have coexisted relatively peacefully with the Inuit, who had migrated southwards from the Arctic islands of North America around 1200. In 1261, Greenland became part of the Kingdom of Norway, which was part of the Kalmar Union and later of the dual monarchy of Denmark-Norway. After almost five hundred years, the settlements simply vanished, possibly due to famine during the 15th century in the Little Ice Age, when climatic conditions deteriorated, and contact with Europe was lost. Bones from this late period were found to be in a condition consistent with malnutrition. Some believe the settlers were wiped out by plague or exterminated by Inuits. Other historians have speculated that Basque or English pirates or slave traders from the Barbary Coast contributed to the extinction of the Greenlandic communities. Denmark-Norway reasserted its latent claim to the colony in 1721. The island's ties with Norway were severed by the Treaty of Kiel of 1815, through which Sweden gained control over mainland Norway while Denmark retained all of their common overseas possessions (which at that time included small territories in India, West Africa and the West Indies, as well as lands in northwestern Europe). Norway occupied and claimed parts of (then uninhabited) Eastern Greenland in the 1920s, claiming that it constituted Terra nullius. Norway and Denmark agreed to settle the matter at the Permanent Court of International Justice in 1933, where Norway lost. Greenland was also called Gruntland ("Ground-land") on early maps. Whether Green is an erroneous transcription of Grunt ("Ground"), which refers to shallow bays, or vice versa, is not known. During World War II, Greenland was on its own, the connection to Denmark having been cut on April 9, 1940 when Denmark was occupied by Germany. Through the cryolite from the mine in Ivigtut, Greenland was able to pay for goods bought in the United States and Canada. The manner in which Greenland had been run prior to the war was altered. The Sirius Patrol, guarding the Northeastern shores of Greenland using dog sleds, was founded in 1941 and participated in defeating the Germans, which gave Denmark a better position in the postwar turmoil. In 1953 Greenland was made an equal part of the Danish Kingdom. In 1979 Greenland took one step further when home rule was granted. During the War Eske Brun was governor and ruled the Island via a 1925-law concerning the governing of the Island where, under extreme circumstances, the governors could take control. The other governor Aksel Svane was transferred to the USA as leader of the supply to Greenland commission.

Politics

Greenland's Head of State is the Danish Monarch, currently Margrethe II. The Queen's government in Denmark appoints a Rigsombudsmand (High commissioner) representing the Danish government and monarchy. Greenland has a 31 member elected parliament. The head of government is the Prime Minister, who is usually the leader of the majority party in Parliament. It is notable that Greenland is not part of the European Union, despite Denmark itself being a member state.

Geography

European Union European Union The total area of Greenland measures 2 099 988 km², of which the ice sheet covers 1 799 992 km² (85,7%). The coastline of Greenland is 24,430 miles long (39,330 km), about the same length as the Earth's circumference at the Equator. All towns and settlements of Greenland are situated along the ice-free coast, with the population being concentrated along the Western coast. Of the 18 municipalities, 15 are in West Greenland (Aasiaat, Ilulissat, Kangaatsiaq, Qasigiannguit, Qeqertarsuaq, Upernavik, Uummannaq in the northern part, Maniitsoq, Nuuk, Paamiut, Sisimiut in the central part, and Ivittuut, Nanortalik, Narsaq, Qaqortoq in the southern part), 2 in East Greenland (Ammassalik, Illoqqortoormiut) and 1 in North Greenland (Qaanaaq). Northeastern greenland, part of North Greenland, is not part of any municipalitiy, but is the site of the world's largest national park, Northeast Greenland National Park. At least four scientific expedition stations and camps had been established in the ice-covered central part of Greenland (indicated as pale blue in the map to the right), on the ice cap: Eismitte, North Ice, North GRIP Camp and The Raven Skiway. Currently, there is a year-round station, Summit Camp, on the ice cap, established in 1989. The radio station Brondlund Fjord was, until 1950, the northernmost permanent outpost of the world. The extreme north of Greenland, Peary Land, is not covered by an ice cap, because the air there is too dry to produce snow, which is essential in the production and maintenance of an ice cap. If the Greenland ice cap were to completely melt away, Greenland would most likely become an archipelago. Between 1989 and 1993, U.S. and European climate researchers drilled into the summit of Greenland's ice sheet, obtaining a pair of two-mile (3.2 km) long ice cores. Analysis of the layering and chemical composition of the cores has provided a revolutionary new record of climate change in the Northern Hemisphere going back about 100,000 years and illustrated that the world's weather and temperature have often shifted rapidly from one seemingly stable state to another, with worldwide consequences.

Economy

Greenland suffered economic contraction in the early 1990s, but since 1993 the economy has improved. The Greenland Home Rule Government (GHRG) has pursued a tight fiscal policy since the late 1980s which has helped create surpluses in the public budget and low inflation. Since 1990, Greenland has registered a foreign trade deficit following the closure of the last remaining lead and zinc mine in 1990. Greenland today is critically dependent on fishing and fish exports; the shrimp fishing industry is by far the largest income earner. Despite resumption of several interesting hydrocarbon and mineral exploration activities, it will take several years before production can materialize. Tourism is the only sector offering any near-term potential and even this is limited due to a short season and high costs. The public sector, including publicly owned enterprises and the municipalities, plays the dominant role in Greenland's economy. About half the government revenues come from grants from the Danish Government, an important supplement to the gross domestic product.

Demographics

Culture

The Greenland National Museum and Archives[http://www.natmus.gl] is located in Nuuk.

Miscellaneous topics


- Communications in Greenland
- Transportation in Greenland
- Military of Greenland
- Foreign relations of Greenland
- University of Greenland

See also


- Danish colonization of the Americas
- History of Denmark
- List of towns in Greenland
- Danish West Indies
- Danish India
- Ellesmere Island
- Svalbard
- Cape Morris Jesup
- Oodaaq
- Kaffeklubben Island
- Ultima Thule search

References


- CIA World Factbook 2000

External links


- [http://www.nanoq.gl/english.aspx Greenland Homerule] - Official site
- [http://www.cia.gov/cia/publications/factbook/geos/gl.html Greenland] - CIA World Factbook
- [http://www.statgreen.gl/ Statistics Greenland]
- [http://www.norden.org/web/1-1-fakta/gr_kort.htm Greenland Map] - Hi-Res Map at the Nordic Ministerial Council
- [http://www.mapsofworld.com/lat_long/greenland-lat-long.html Latitude and Longitude of Important locations in Greenland] Category:Greenland Category:North Atlantic Islands Category:Islands of Denmark Category:Special territories of the EU Category:Danish dependencies Category:Former Norwegian colonies zh-min-nan:Chheⁿ-tē ko:그린란드 ja:グリーンランド simple:Greenland th:เกาะกรีนแลนด์



Solar system

The solar system comprises our Sun and the retinue of celestial objects gravitationally bound to it. Traditionally, this is said to consist of the Sun, nine planets and their 158 currently known moons; however, a large number of other objects, including asteroids, meteoroids, planetoids, comets, and interplanetary dust, orbit the Sun as well. Although the term "solar system" is frequently applied to other star systems and the planetary systems which may comprise them, it should strictly refer to our system specifically: the word "solar" is derived from the Sun's Latin name, Sol (and the term sometimes appears as Solar System). When talking about another stellar system (or planetary system), including the star(s) and bodies associated with them through gravity, it is usual to shorten it to "the system" (e.g. "the Alpha Centauri system" or "the 51 Pegasi system").

Structure and layout of the solar system

The Sun (astronomical symbol ☉) is a main sequence G2 star that contains 99.86% of the system's known mass. Its two largest orbiting bodies, Jupiter and Saturn, account for 91% of the remainder (The Oort Cloud might hold a substantial percentage, but as yet its existence is unconfirmed). In broad terms, the charted regions of our solar system consist of the Sun and its planetary system: the eight bodies in relatively unique orbits (commonly called planets or major planets) and two belts of smaller objects (which can be called minor planets, planetoids, meteoroids, planetesimals or, in the case of Pluto, planets). Objects in orbit round the Sun all lie within the same shallow plane, called the ecliptic, and all orbit in the same direction. Many are in turn orbited by moons, and the largest are encircled by planetary rings of dust and other particles. The major planets are, in order, Mercury (☿), Venus (♀), Earth (♁), Mars (♂), Jupiter (♃), Saturn (♄), Uranus (♅/10px), Neptune (♆), and Pluto (♇), though Pluto's status has been thrown into question by the discovery of (see below). Eight of the nine planets are named after or derived from gods and goddesses from Greco-Roman mythology; Earth, a Germanic word, is known in many Romance languages as Terra, the Roman goddess of the Earth. Distances within the solar system are measured most often in astronomical units, or AU. 1 AU is the distance between the Earth and the Sun, or 149 598 000 kilometers. Pluto is roughly 38 AU from the Sun, while Jupiter lies at roughly 5.2 AU. For very large distances within the solar system, such as regions beyond Pluto or the orbital circumferences of planets, the terameter (Tm, one milliard kilometers) is sometimes used. Despite the fact that many diagrams (like the image at the top of this article), for practicality's sake, represent the solar system as having each orbit the same distance apart, in actuality the orbits are largely arranged geometrically, that is, each is roughly double the distance from the Sun as the one before it. Venus’s distance from the Sun is roughly double that of Mercury, Earth’s distance is roughly double that of Venus, Mars’s double that of Earth and so on. This relationship is roughly expressed in the Titius-Bode law, a mathematical formula for predicting the semi-major axes of planets in AU. In its simplest form, it is written : a= 0.4 + 0.3\times k where k=0,1,2,4,8,16,32,64,128. By this formulation, we would expect Mercury's orbit (k=0) to be 0.4 AU, and Mars's orbit (k=4) to be at 1.6 AU. In fact their orbits are 0.38 and 1.52 AU.Ceres, the largest asteroid, lies at k=8. This law is only a rough guide, and doesn't fit all of the planets (Neptune is far closer than predicted, though Pluto lies at Neptune's predicted orbit). As of now, there is no scientific explanation for why this law "works," and many claim it is merely a coincidence. Pluto

Origin and evolution of the solar system

The current hypothesis of solar system formation is the nebular hypothesis, first proposed in 1755 by Immanuel Kant. It states the solar system was formed from a gaseous cloud called the solar nebula. It had a diameter of 100 AU and was 2-3 times the