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Conrad Discontinuity
The Conrad discontinuity corresponds to the sub-horizontal boundary in continental crust at which the seismic wave velocity increases in a discontinuous way. This boundary is observed in various continental regions at a depth of 15 to 20 km, however it is not found in oceanic regions. The Conrad discontinuity (named after the seismologist Victor Conrad) is considered to be the border between the upper continental crust and the lower one. It is not as pronounced as the Mohorovičić discontinuity, and absent in some continental regions. Up to the middle 20th Century the upper crust in continental regions was seen to consist of felsic rocks such as granite (sial, for silica-aluminium), and the lower one to consist of more magnesium-rich mafic rocks like basalt (sima, for silica-magnesium). Therefore, the seismologists of that time considered that the Conrad discontinuity should correspond to a sharply defined contact between the chemically distinct two layers, sial and sima. How ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Continental Crust
Continental crust is the layer of igneous, sedimentary, and metamorphic rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called ''sial'' because its bulk composition is richer in aluminium silicates (Al-Si) and has a lower density compared to the oceanic crust, called ''sima'' which is richer in magnesium silicate (Mg-Si) minerals. Changes in seismic wave velocities have shown that at a certain depth (the Conrad discontinuity), there is a reasonably sharp contrast between the more felsic upper continental crust and the lower continental crust, which is more mafic in character. The continental crust consists of various layers, with a bulk composition that is intermediate (SiO2 wt% = 60.6). The average density of continental crust is about , less dense than the ultramafic material that makes up the mantle, which has a density of around . Continental crust is also less dense ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Basalt
Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial planet, rocky planet or natural satellite, moon. More than 90% of all volcanic rock on Earth is basalt. Rapid-cooling, fine-grained basalt is chemically equivalent to slow-cooling, coarse-grained gabbro. The eruption of basalt lava is observed by geologists at about 20 volcanoes per year. Basalt is also an important rock type on other planetary bodies in the Solar System. For example, the bulk of the plains of volcanism on Venus, Venus, which cover ~80% of the surface, are basaltic; the lunar mare, lunar maria are plains of flood-basaltic lava flows; and basalt is a common rock on the surface of Mars. Molten basalt lava has a low viscosity due to its relatively low silica content (between 45% and 52%), resulting in rapidly moving lava flo ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Plate Tectonics
Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large tectonic plates which have been slowly moving since about 3.4 billion years ago. The model builds on the concept of ''continental drift'', an idea developed during the first decades of the 20th century. Plate tectonics came to be generally accepted by geoscientists after seafloor spreading was validated in the mid to late 1960s. Earth's lithosphere, which is the rigid outermost shell of the planet (the crust and upper mantle), is broken into seven or eight major plates (depending on how they are defined) and many minor plates or "platelets". Where the plates meet, their relative motion determines the type of plate boundary: '' convergent'', '' divergent'', or ''transform''. Earthquakes, volcanic activity, mountain-building, and oceanic tr ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Kaapvaal Craton
The Kaapvaal Craton (centred on Limpopo Province in South Africa), along with the Pilbara Craton of Western Australia, are the only remaining areas of pristine 3.6–2.5 Ga (billion years ago) crust on Earth. Similarities of rock records from both these cratons, especially of the overlying late Archean sequences, suggest that they were once part of the Vaalbara supercontinent. Description The Kaapvaal Craton covers an area of approximately and is joined to the Zimbabwe Craton to the north by the Limpopo Belt. To the south and west, the Kaapvaal Craton is flanked by Proterozoic orogens, and to the east by the Lebombo monocline that contains Jurassic igneous rocks associated with the break-up of Gondwana. The Kaapvaal Craton formed and stabilised between 3.7 and 2.6 Ga by the emplacement of major granitoid batholiths that thickened and stabilised the continental crust during the early stages of an arc-related magmatism and sedimentation cycle. The craton is a mixture o ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Vredefort Impact Structure
The Vredefort impact structure is the largest verified impact structure on Earth. The crater, which has since been eroded away, was around across when it was formed. The remaining structure, comprising the deformed underlying bedrock, is located in present-day Free State province of South Africa. It is named after the town of Vredefort, which is near its centre. The structure's central uplift is known as the Vredefort Dome. The impact structure was formed during the Paleoproterozoic Era, 2.023 billion years (± 4 million years) ago. It is the second-oldest known impact structure on Earth, after Yarrabubba. In 2005, the Vredefort Dome was added to the list of UNESCO World Heritage Sites for its geologic interest. Formation and structure The asteroid that hit Vredefort is estimated to have been one of the largest ever to strike Earth since the Hadean Eon some four billion years ago, originally thought to have been approximately in diameter. The bolide that created the ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Metamorphism
Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of chemically active fluids, but the rock remains mostly solid during the transformation. Metamorphism is distinct from weathering or diagenesis, which are changes that take place at or just beneath Earth's surface. Various forms of metamorphism exist, including regional, contact, hydrothermal, shock, and dynamic metamorphism. These differ in the characteristic temperatures, pressures, and rate at which they take place and in the extent to which reactive fluids are involved. Metamorphism occurring at increasing pressure and temperature conditions is known as ''prograde metamorphism'', while decreasing temperature and pressure characterize ''retrograde metamorphism''. Metamorphic petrology is the study of metamorphism. Metamorphic petrologists re ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Geologist
A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes Earth and other terrestrial planets, as well as the processes that shape them. Geologists usually study geology, earth science, or geophysics, although backgrounds in physics, chemistry, biology, and other sciences are also useful. Field research (field work) is an important component of geology, although many subdisciplines incorporate laboratory and digitalized work. Geologists can be classified in a larger group of scientists, called geoscientists. Geologists work in the energy and mining sectors searching for natural resources such as petroleum, natural gas, precious and base metals. They are also in the forefront of preventing and mitigating damage from natural hazards and disasters such as earthquakes, volcanoes, tsunamis and landslides. Their studies are used to warn the general public of the occurrence of these events. Geologists are also important contributors to climate ch ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
John Wiley & Sons
John Wiley & Sons, Inc., commonly known as Wiley (), is an American multinational publishing company founded in 1807 that focuses on academic publishing and instructional materials. The company produces books, journals, and encyclopedias, in print and electronically, as well as online products and services, training materials, and educational materials for undergraduate, graduate, and continuing education students. History The company was established in 1807 when Charles Wiley opened a print shop in Manhattan. The company was the publisher of 19th century American literary figures like James Fenimore Cooper, Washington Irving, Herman Melville, and Edgar Allan Poe, as well as of legal, religious, and other non-fiction titles. The firm took its current name in 1865. Wiley later shifted its focus to scientific, technical, and engineering subject areas, abandoning its literary interests. Wiley's son John (born in Flatbush, New York, October 4, 1808; died in East Orange, New Je ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Sima (geology)
In geology, sima () is an antiquated blended term for the lower layer of Earth's crust. This layer is made of rocks rich in magnesium silicate minerals. Typically, when the sima comes to the surface, it is basalt, so sometimes this layer is called the 'ocean layer' of the crust. The sima layer is also called the 'basal crust' or 'basal layer' because it is the lowest layer of the crust. Because the ocean floors are mainly sima, it is also sometimes called the 'oceanic crust'. The name 'sima' was taken from the first two letters of silica and of magnesia. Comparable is the name 'sial', which is the name for the upper layer of Earth's crust (continental crust). Petrology The sima has a higher density (2800 to 3300 kg/m3) than the sial, which is due to larger amounts of iron and magnesium, and smaller amounts of aluminium. When the denser sima comes to the surface it forms mafic rocks, or rocks with mafic minerals. The most dense sima has less silica and forms ultramafic rocks ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Mafic Rock
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, :de:Louis V Pirsson, Louis Valentine 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 apat ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Seismic Wave
A seismic wave is a wave of acoustic energy that travels through the Earth. It can result from an earthquake, volcanic eruption, magma movement, a large landslide, and a large man-made explosion that produces low-frequency acoustic energy. Seismic waves are studied by seismologists, who record the waves using seismometers, hydrophones (in water), or accelerometers. Seismic waves are distinguished from seismic noise (ambient vibration), which is persistent low-amplitude vibration arising from a variety of natural and anthropogenic sources. The propagation velocity of a seismic wave depends on density and elasticity of the medium as well as the type of wave. Velocity tends to increase with depth through Earth's crust and mantle, but drops sharply going from the mantle to Earth's outer core. Earthquakes create distinct types of waves with different velocities. When recorded by a seismic observatory, their different travel times help scientists locate the quake's hypocen ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Sial
In geology, the term sial refers to the composition of the upper layer of Earth's crust, namely rocks rich in aluminium silicate minerals. It is sometimes equated with the continental crust because it is absent in the wide oceanic basins, but 'sial' is a geochemical term rather than a plate tectonic term. As these elements are less dense than the majority of Earth's elements, they tend to be concentrated in the upper layer of the crust. The uppermost layer of the crust is called the sial, consisting of silicate and aluminium (Si = silicate, Al = aluminium). On average, the thickness of the sial is till 25 km from the surface. The continents are composed mainly of lighter rock material formed from silicon and aluminium, so the sial is thick over the continents and very thin or absent on the ocean floor, especially the Pacific Ocean. Average density of the sial is 2.7 gm/cc. Geologists often refer to the rocks in this layer as felsic, because they contain high levels of feldspar ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
