Author: geosciencejournal

Back-arc basin, submarine basin that forms behind an island arc. Such basins are typically found along the western margin of the Pacific Ocean near the convergence of two tectonic plates. Back-arc basins are sites of significant hydrothermal activity, and the deep-sea vents that occur in these regions often harbour diverse biological communities. A back-arc basin is formed by the process of back-arc spreading, which begins when one tectonic plate subducts under another. Subduction creates a trench between the two plates and melts the mantle in the overlying plate, which causes magma to rise toward the surface. Rising magma increases the pressure at the top of the overlying plate that creates rifts in the crust above and causes the volcanoes on the island arc to erupt. As additional magma breaks through the cracks in the crust, one or more spreading centres develop, which widen the seafloor and expand the section of the overlying plate behind the trench. As the basin expands, the leading edge of the overlying plate may be forced oceanward, causing the trench to roll back over the subducting plate, or it may serve as a sea anchor by remaining fixed in place relative to the top of the subducting plate. In the latter case, the enlargement of the basin forces the trailing part of the overlying plate to move in the opposite direction.

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Seamount, large submarine volcanic mountain rising at least 1,000 m above the surrounding deep-sea floor; smaller submarine volcanoes are called sea knolls, and flat-topped seamounts are called guyots. Great Meteor Table mount in the northeast Atlantic, standing more than 4,000 m above the surrounding terrain, with a basal diameter of up to 110 km, illustrates the size that such features can attain. The sides of larger seamounts generally are concave upward and rarely slope more than 14°; smaller seamounts lack this concavity and can have sides as steep as 35°. In plan, seamounts tend to be elliptical or elongate, possibly because the lavas are extruded from linear rifts in the seafloor. Most material dredged from seamounts is microcrystalline, or glassy, oceanic basalt that probably formed as submarine lava flows. The summits and flanks of seamounts are generally covered with a thin layer of marine sediment. Seamounts are exceedingly abundant and occur in all major ocean basins. A linear cluster of seamounts may result when several are fed by lava extruded from a single linear rift. Most Pacific seamounts occur in linear clusters or elongate groups of 10 to 100. The individual seamounts in a chain may share a common ridge connecting their bases, as in the Mid-Pacific Mountains. Seamount chains in the Pacific basin tend to be aligned northwesterly, and several chains are intimately associated with fracture zones; the Eltanin Fracture Zone in the southwestern Pacific is an example. At least one seamount chain, the New England Seamounts, lies in the northwestern Atlantic. No seamount chains have been reported from the Indian Ocean, possibly because that basin has been less extensively surveyed.

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Oceanic ridge, continuous submarine mountain chain extending approximately 80,000 km  through all the world’s oceans. Individually, ocean ridges are the largest features in ocean basins. Collectively, the oceanic ridge system is the most prominent feature on Earth’s surface after the continents and the ocean basins themselves. In the past these features were referred to as mid-ocean ridges, but, as will be seen, the largest oceanic ridge, the East Pacific Rise, is far from a mid-ocean location, and the nomenclature is thus inaccurate. Oceanic ridges are not to be confused with aseismic ridges, which have an entirely different origin. Oceanic ridges are found in every ocean basin and appear to girdle Earth. The ridges rise from depths near 5 km to an essentially uniform depth of about 2.6 km and are roughly symmetrical in cross section. They can be thousands of kilometres wide. In places, the crests of the ridges are offset across transform faults within fracture zones, and these faults can be followed down the flanks of the ridges. The flanks are marked by sets of mountains and hills that are elongate and parallel to the ridge trend. New oceanic crust is formed at seafloor spreading centres at these crests of the oceanic ridges. Because of this, certain unique geologic features are found there. Fresh basaltic lavas are exposed on the seafloor at the ridge crests. These lavas are progressively buried by sediments as the seafloor spreads away from the site. The flow of heat out of the crust is many times greater at the crests than elsewhere in the world. Earthquakes are common along the crests and in the transform faults that join the offset ridge segments. Analysis of earthquakes occurring at the ridge crests indicates that the oceanic crust is under tension there. A high-amplitude magnetic anomaly is centred over the crests because fresh lavas at the crests are being magnetized in the direction of the present geomagnetic field.

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Pelagic sediment or pelagite is a fine-grained sediment that accumulates as the result of the settling of particles to the floor of the open ocean, far from land. These particles consist primarily of either the microscopic, calcareous or siliceous shells of phytoplankton or zooplankton; clay-size siliciclastic sediment; or some mixture of these. Trace amounts of meteoric dust and variable amounts of volcanic ash also occur within pelagic sediments. Based upon the composition of the ooze, there are three main types of pelagic sediments: siliceous oozes, calcareous oozes, and red clays. The composition of pelagic sediments is controlled by three main factors. The first factor is the distance from major landmasses, which affects their dilution by terrigenous, or land-derived, sediment. The second factor is water depth, which affects the preservation of both siliceous and calcareous biogenic particles as they settle to the ocean bottom. The final factor is ocean fertility, which controls the amount of biogenic particles produced in surface waters.

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Zeolite facies, one of the major divisions of the mineral facies classification of metamorphic rocks, the rocks of which formed at the lowest temperatures and pressures associated with regional metamorphism. It represents the transition between the sedimentary processes of diagenesis and the distinct regional metamorphism exhibited by the greenschist facies. This facies was first proposed for rocks subject to a load pressure of about one-fifth of a kilobar and temperatures of 200 to 300 °C. Minerals typically found under these conditions include the zeolites, albite, quartz, and prehnite. Volatile components are very important in the chemistry of this facies because, under zeolite facies, the temperature and pressure can change the chemical potentials of water and carbon dioxide so as to produce the mineralogy of the greenschist facies.

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Troglobites are small creatures that have adapted to a permanent life in a cave. They are so well-adapted to life in a cave that they would be unable to survive in the surface environment. To survive in the darkness, troglobites have highly-developed senses of hearing, touch and smell. The darkness of the cave eliminates their need for sight. As a result, they are usually blind with undeveloped eyes that might be covered by a layer of skin. The darkness eliminates the advantage of camouflage coloring, and many troglobites are albino. Many types of animals have evolved into troglobites. Some of the most familiar types of troglobites are spiders, beetles, gastropods, fish, millipedes, and salamanders. Turbellarians, pseudoscorpions, harvestmen, isopods, amphipods, decapods, collembolans, and diplurans are also represented in Earth’s troglobite collection.

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Pyroclastic falls, also known as volcanic fallout, occur when tephra – fragmented rock ranging in size from mm to tens of cm is ejected from a volcanic vent during an eruption and falls to the ground some distance away from the vent. Falls are usually associated with Plinian eruptive columns, ash clouds or volcanic plumes. Tephra in pyroclastic fall deposits may have been transported only a short distance from the vent or if it is injected into the upper atmosphere, may circle the globe. Any kind of pyroclastic fall deposit will mantle or drape itself over the landscape, and will decrease in both size and thickness the farther away it is from its source. Tephra falls are usually not directly dangerous unless a person is close enough to an eruption to be struck by larger fragments. The effects of falls can be, however. Ash can smother vegetation, destroy moving parts in motors and engines, and scratch surfaces. Scoria and small bombs can break delicate objects, dent metals and become embedded in wood. Some pyroclastic falls contain toxic chemicals that can be absorbed into plants and local water supplies, which can be dangerous for both people and livestock. The main danger of pyroclastic falls is their weight: tephra of any size is made up of pulverized rock, and can be extremely heavy, especially if it gets wet. Most of the damage caused by falls occurs when wet ash and scoria on the roofs of buildings causes them to collapse. Pyroclastic material injected into the atmosphere may have global as well as local consequences. When the volume of an eruption cloud is large enough, and the cloud is spread far enough by wind, pyroclastic material may actually block sunlight and cause temporary cooling of the Earth’s surface.

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Ecosystem, the complex of living organisms, their physical environment, and all their interrelationships in a particular unit of space. An ecosystem can be categorized into its abiotic constituents, including minerals, climate, soil, water, sunlight, and all other nonliving elements, and its biotic constituents, consisting of all its living members. Linking these constituents together are two major forces: the flow of energy through the ecosystem, and the cycling of nutrients within the ecosystem. The fundamental source of energy in almost all ecosystems is radiant energy from the Sun. The energy of sunlight is used by the ecosystem’s autotrophic, or self-sustaining, organisms. Consisting largely of green vegetation, these organisms are capable of photosynthesis i.e., they can use the energy of sunlight to convert carbon dioxide and water into simple, energy-rich carbohydrates. The autotrophs use the energy stored within the simple carbohydrates to produce the more complex organic compounds, such as proteins, lipids, and starches that maintain the organisms’ life processes. The autotrophic segment of the ecosystem is commonly referred to as the producer level.

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Hypsometry, the science of measuring the elevation and depth of features on Earth’s surface with respect to sea level. Data collected using hypsometers, wire sounders, echo sounders, and satellite-based altimeters is used to quantify the distribution of land at different elevations across a given area and the surface-area distribution of the oceans and their marginal seas with depth. Scientists can show how the areas of oceans, marginal seas, and terrestrial basins change with elevation and depth using a special curve known as a hypsometric, or hypsographic, curve.

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Biodiversity, also called biological diversity, the variety of life found in a place on Earth or, often, the total variety of life on Earth. A common measure of this variety, called species richness, is the count of species in an area. some of the differences between places the tropics, for example, have more biodiversity than temperate regions but raw species count is not the only measure of diversity. Furthermore, biodiversity encompasses the genetic variety within each species and the variety of ecosystems that species create. Although examining counts of species is perhaps the most common method used to compare the biodiversity of various places, in practice biodiversity is weighted differently for different species, the reason being that some species are deemed more valuable or more interesting than others.

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