|
Guiana Shield
The Guiana Shield (; ; ; ) is one of the three cratons of the South American Plate. It is a 1.7 billion-year-old Precambrian geological formation in northeast South America that forms a portion of the northern coast. The higher elevations on the shield are called the Guiana Highlands, which is where the table-like mountains called tepuis are found. The Guiana Highlands are also the source of some of the world's most well-known waterfalls such as Angel Falls, Kaieteur Falls and Cuquenan Falls. The Guiana Shield underlies Guyana (previously ''British Guiana''), Suriname (previously ''Dutch Guiana''), and French Guiana (or ''Guyane''), much of southern Venezuela, as well as parts of Colombia and Brazil. The rocks of the Guiana Shield consist of metasediments and metavolcanics ( greenstones) overlain by sub-horizontal layers of sandstones, quartzites, shales and conglomerates intruded by sills of younger mafic intrusives such as gabbros. Geology The oldest rocks in the ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] [Amazon] |
South America
South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a considerably smaller portion in the Northern Hemisphere. It can also be described as the southern Subregion#Americas, subregion of the Americas. South America is bordered on the west by the Pacific Ocean, on the north and east by the Atlantic Ocean, and to the south by the Drake Passage; North America and the Caribbean Sea lie to the northwest. The continent includes twelve sovereign states: Argentina, Bolivia, Brazil, Chile, Colombia, Ecuador, Guyana, Paraguay, Peru, Suriname, Uruguay, and Venezuela; two dependent territory, dependent territories: the Falkland Islands and South Georgia and the South Sandwich Islands; and one administrative division, internal territory: French Guiana. The Dutch Caribbean ABC islands (Leeward Antilles), ABC islands (Aruba, Bonaire, and Curaçao) and Trinidad and Tobago are geologically located on the South-American continental shel ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] [Amazon] |
Kaieteur Falls
Kaieteur Falls is one of the most powerful single-drop waterfalls in the world. It is located on the Potaro River in Kaieteur National Park, central Essequibo Territory, Guyana. It is 226 metres (741 ft) high when measured from its plunge over a sandstone and conglomerate cliff to the first break. It then flows over a series of steep cascades that, when included in the measurements, bring the total height to 251 metres (822 ft). While many falls have greater height or greater flow, few have the combination of height and water volume, and Kaieteur is among the most powerful waterfalls in the world with an average flow rate of 663 cubic meters per second (23,400 cubic feet per second). Kaieteur Falls is about four and a half times the height of Niagara Falls, on the border between Canada and the United States, and about twice the height of Victoria Falls, on the border of Zambia and Zimbabwe in Africa. Upriver from the falls, the Potaro Plateau stretches out to the ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] [Amazon] |
Fault (geology)
In geology, a fault is a Fracture (geology), planar fracture or discontinuity in a volume of Rock (geology), rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust (geology), crust result from the action of Plate tectonics, plate tectonic forces, with the largest forming the boundaries between the plates, such as the megathrust faults of subduction, subduction zones or transform faults. Energy release associated with rapid movement on active faults is the cause of most earthquakes. Faults may also displace slowly, by aseismic creep. A ''fault plane'' is the Plane (geometry), plane that represents the fracture surface of a fault. A ''fault trace'' or ''fault line'' is a place where the fault can be seen or mapped on the surface. A fault trace is also the line commonly plotted on geological maps to represent a fault. A ''fault zone'' is a cluster of parallel faults. However, the term is also used for the zone ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] [Amazon] |
Mafic
A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include basalt, diabase and gabbro. Mafic rocks often also contain calcium-rich varieties of plagioclase feldspar. Mafic materials can also be described as ferromagnesian. History The term ''mafic'' is a portmanteau of "magnesium" and "ferric" and was coined by Charles Whitman Cross, Joseph P. Iddings, Louis V. Pirsson, and Henry Stephens Washington in 1912. Cross' group had previously divided the major rock-forming minerals found in igneous rocks into ''salic'' minerals, such as quartz, feldspars, or feldspathoids, and ''femic'' minerals, such as olivine and pyroxene. However, micas and aluminium-rich amphiboles were excluded, while some calcium minerals containing little iron or magnesium, such as wollastonite or apatite, were included in t ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] [Amazon] |
Gneiss
Gneiss (pronounced ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. This rock is formed under pressures ranging from 2 to 15 kbar, sometimes even more, and temperatures over 300 °C (572 °F). Gneiss nearly always shows a banded texture characterized by alternating darker and lighter colored bands and without a distinct Cleavage (geology), cleavage. Gneisses are common in the ancient crust of Continental Shield, continental shields. Some of the oldest rocks on Earth are gneisses, such as the Acasta Gneiss. Description image:Orthogneiss Geopark.jpg, Orthogneiss from the Czech Republic In traditional English and North American usage, a gneiss is a coarse-grained metamorphic rock showing compositional banding (gneissic banding) but poorly developed schistosity and indistinct Cleavage (geology), cleavage. In other words, it i ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] [Amazon] |
Feldspar
Feldspar ( ; sometimes spelled felspar) is a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldspars and the ''alkali'' (potassium-sodium) feldspars. Feldspars make up about 60% of the Earth's crust and 41% of the Earth's continental crust by weight. Feldspars crystallize from magma as both intrusive and extrusive igneous rocks and are also present in many types of metamorphic rock. Rock formed almost entirely of calcic plagioclase feldspar is known as anorthosite. Feldspars are also found in many types of sedimentary rocks. Etymology The name ''feldspar'' derives from the German , a compound of the words ' ("field") and ("flake"). had long been used as the word for "a rock easily cleaved into flakes"; was introduced in the 18th century as a more specific term, referring perhaps to its comm ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] [Amazon] |
Quartz
Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical formula of Silicon dioxide, SiO2. Quartz is, therefore, classified structurally as a Silicate mineral#Tectosilicates, framework silicate mineral and compositionally as an oxide mineral. Quartz is the second most abundant mineral in Earth's continental crust, behind feldspar. Quartz exists in two forms, the normal α-quartz and the high-temperature β-quartz, both of which are chiral. The transformation from α-quartz to β-quartz takes place abruptly at . Since the transformation is accompanied by a significant change in volume, it can easily induce microfracturing of ceramics or rocks passing through this temperature threshold. There are many different varieties of quartz, several of which are classifi ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] [Amazon] |
Archean
The Archean ( , also spelled Archaean or Archæan), in older sources sometimes called the Archaeozoic, is the second of the four geologic eons of Earth's history of Earth, history, preceded by the Hadean Eon and followed by the Proterozoic and the Phanerozoic. The Archean represents the time period from (million years ago). The Late Heavy Bombardment is hypothesized to overlap with the beginning of the Archean. The Huronian glaciation occurred at the end of the eon. The Earth during the Archean was mostly a ocean world, water world: there was continental crust, but much of it was under an ocean deeper than today's oceans. Except for some rare Relict (geology), relict crystals, today's oldest continental crust dates back to the Archean. Much of the geological detail of the Archean has been destroyed by subsequent activity. The Earth's atmosphere was also vastly different in atmospheric chemistry, composition from today's: the prebiotic atmosphere was a reducing atmosphere rich in ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] [Amazon] |
Gabbro
Gabbro ( ) is a phaneritic (coarse-grained and magnesium- and iron-rich), mafic intrusive igneous rock formed from the slow cooling magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is chemically equivalent to rapid-cooling, fine-grained basalt. Much of the Earth's oceanic crust is made of gabbro, formed at mid-ocean ridges. Gabbro is also found as plutons associated with continental volcanism. Due to its variant nature, the term ''gabbro'' may be applied loosely to a wide range of intrusive rocks, many of which are merely "gabbroic". By rough analogy, gabbro is to basalt as granite is to rhyolite. Etymology The term "gabbro" was used in the 1760s to name a set of rock types that were found in the ophiolites of the Apennine Mountains in Italy. It was named after Gabbro, a hamlet near Rosignano Marittimo in Tuscany. Then, in 1809, the German geologist Christian Leopold von Buch used the term more restrictively in his d ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] [Amazon] |
Conglomerate (geology)
Conglomerate () is a sedimentary rock made up of rounded gravel-sized pieces of rock surrounded by finer-grained sediments (such as sand, silt, or clay). The larger fragments within conglomerate are called clasts, while the finer sediment surrounding the clasts is called the matrix. The clasts and matrix are typically cemented by calcium carbonate, iron oxide, silica, or hardened clay. Conglomerates form when rounded gravels deposited by water or glaciers become solidified and cemented by pressure over time. They can be found in sedimentary rock sequences of all ages but probably make up less than 1 percent by weight of all sedimentary rocks. They are closely related to sandstones in origin, and exhibit many of the same types of sedimentary structures, such as tabular and trough cross-bedding and graded bedding.Boggs, S. (2006) ''Principles of Sedimentology and Stratigraphy.'', 2nd ed. Prentice Hall, New York. 662 pp. Friedman, G.M. (2003) ''Classification of sediments and s ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] [Amazon] |
Shales
Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g., kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especially quartz and calcite.Blatt, Harvey and Robert J. Tracy (1996) ''Petrology: Igneous, Sedimentary and Metamorphic'', 2nd ed., Freeman, pp. 281–292 Shale is characterized by its tendency to split into thin layers ( laminae) less than one centimeter in thickness. This property is called '' fissility''. Shale is the most common sedimentary rock. The term ''shale'' is sometimes applied more broadly, as essentially a synonym for mudrock, rather than in the narrower sense of clay-rich fissile mudrock. Texture Shale typically exhibits varying degrees of fissility. Because of the parallel orientation of clay mineral flakes in shale, it breaks into thin layers, often splintery and usually parallel to the otherwise indistinguishable bedding pl ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] [Amazon] |
Quartzites
Quartzite is a hard, non- foliated metamorphic rock that was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tectonic compression within orogenic belts, and hence quartzite is a metasandstone. Pure quartzite is usually white to grey, though quartzites often occur in various shades of pink and red due to varying amounts of hematite. Other colors, such as yellow, green, blue and orange, are due to other minerals. The term ''quartzite'' is also sometimes used for very hard but unmetamorphosed sandstones that are composed of quartz grains thoroughly cemented with additional quartz. Such sedimentary rock has come to be described as orthoquartzite to distinguish it from metamorphic quartzite, which is sometimes called metaquartzite to emphasize its metamorphic origins. Quartzite is very resistant to chemical weathering and often forms ridges and resistant h ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] [Amazon] |
