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Can a Continent Sink?

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Author Topic: Can a Continent Sink?  (Read 4215 times)
Adam Hawthorne
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« on: April 08, 2007, 11:19:40 pm »

It is said that there is no sunken landmass (of the size Plato suggests) in the Atlantic Ocean.

However, I thought it would be worthwhile to investigate ways where a landmass can be submerged, which we shall do now.
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Adam Hawthorne
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« Reply #1 on: April 08, 2007, 11:21:17 pm »



The Juan de Fuca plate sinks below the North America plate at the Cascadia subduction zone.

Subduction


The Juan de Fuca plate sinks below the North America plate at the Cascadia subduction zone.In geology, a subduction zone is an area on Earth where two tectonic plates meet and move towards one another, with one sliding underneath the other and moving down into the mantle, at rates typically measured in centimeters per year. An oceanic plate ordinarily slides underneath a continental plate; this often creates an orogenic zone with many volcanoes and earthquakes. In a sense, subduction zones are the opposite of divergent boundaries, areas where material rises up from the mantle and plates are moving apart
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Adam Hawthorne
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« Reply #2 on: April 08, 2007, 11:23:12 pm »


Oceanic plates are subducted creating oceanic trenches
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Adam Hawthorne
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« Reply #3 on: April 08, 2007, 11:24:01 pm »

General description

Subduction zones mark sites of convective downwelling of the Earth's lithosphere (the crust plus the strong portion of the upper mantle). Subduction zones exist at convergent plate boundaries where one plate of oceanic lithosphere converges with another plate and sinks below it to depth of approximately 100 km. At that depth the peridotite of the oceanic slab is converted to eclogite, the density of the edge of the oceanic lithosphere increases and it sinks into the mantle. It is at subduction zones that the Earth's lithosphere, oceanic crust, sedimentary layers, and trapped water are recycled into the deep mantle. Earth is the only planet where subduction is known to occur; neither Venus nor Mars have subduction zones. Without subduction, plate tectonics could not exist and Earth would be a very different planet: Earth's crust would not have differentiated into continents and oceans and all of the solid Earth would lie beneath a global ocean.

Subduction results from the difference in density between lithosphere and underlying asthenosphere. Where, very rarely, lithosphere is denser than asthenospheric mantle, it can easily sink back into the mantle at a subduction zone; however, subduction is resisted where lithosphere is less dense than underlying asthenosphere. Whether or not lithosphere is denser than underlying asthenosphere depends on the nature of the associated crust. Crust is always less dense than asthenosphere or lithospheric mantle, but because continental crust is always thicker and less dense than oceanic crust, continental lithosphere is always less dense than oceanic lithosphere. Oceanic lithosphere is generally not denser than asthenosphere but continental lithosphere is lighter. Exceptionally, the presence of the large areas of flood basalt that are called large igneous provinces (LIPs), which result in extreme thickening of the oceanic crust, can cause some sections of older oceanic lithosphere to be too buoyant to subduct. Where lithosphere on the downgoing plate is too buoyant to subduct, a collision occurs, hence the adage "Subduction leads to orogeny". Most subduction zones are arcuate, where the concave side is directed towards the continent. This is especially so where a back-arc basin develops between the subduction zone and the continent. The arcuate configuration probably results from differential friction between the tectonic plates, and the likely agent that would reduct the interplate friction is serpentinite, but a large batch of unconsolidated sediment could cause similar effects as well.

Subduction zones are associated with the deepest earthquakes on the planet. Earthquakes are generally restricted to the shallow, brittle parts of the crust, generally at depths of less than 20 km. However, in subduction zones, earthquakes occur at depths as great as 700 km. These earthquakes define inclined zones of seismicity known as Wadati-Benioff zones (after the scientists who discovered them), which outline the descending lithosphere. Seismic tomography has helped outline subducted lithosphere in regions where there are no earthquakes. Some subducted slabs seem not to be able to penetrate the major discontinuity in the mantle that lies at a depth of about 670 km, whereas other subducted oceanic plates can penetrate all the way to the core-mantle boundary. The great seismic discontinuities in the mantle - at 410 and 670 km depth - are disrupted by the descent of cold slabs in deep subduction zones.

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Adam Hawthorne
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« Reply #4 on: April 08, 2007, 11:24:44 pm »

Subduction causes oceanic trenches, such as the Mariana trench. Trenches occur where one plate begins its descent beneath another. Volcanoes that occur above subduction zones, such as Mount St. Helens and Mount Fuji, often occur in arcuate chains, hence the term volcanic arc or island arc. Not all "volcanic arcs" are arced: trenches and arcs are often linear.

The magmatism associated with the volcanic arc occurs 100-300 km away from the trench. However, a relationship has been found, which relates volcanic arc location to depth of the subducted crust as defined by the Wadati-Benioff zone. Studies of many volcanic arcs around the world have revealed that volcanic arcs tend to form at a location where the subducted slab has reached a depth of about 100 km. This has interesting implications for the mechanism that causes the magmatism at these arcs. Arcs produce about 25% of the total volume of magma produced each year on Earth (~30-35 km³), much less than the volume produced at mid-ocean ridges. Nevertheless, arc volcanism has the greatest impact on humans, because many arc volcanoes lie above sealevel and erupt violently. Aerosols injected into the stratosphere during violent eruptions can cause rapid cooling of the Earth's climate.

Subduction zones are also notorious for producing devastating earthquakes because of the intense geological activity. The introduction of cold oceanic crust into the mantle depresses the local geothermal gradient and causes a larger portion of the earth to deform in a brittle fashion than it would in a normal geothermal gradient setting. Because earthquakes can only occur when a rock is deforming in a brittle fashion, subduction zones have the potential to create very large earthquakes. If this earthquake occurs under the ocean it has the potential to create tsunamis, such as the earthquake caused by subduction of the Indo-Australian Plate under the Eurasian Plate on December 26, 2004, that devastated the areas around the Indian Ocean. Small tremors that create tiny, unnoticeable tsunamis happen all the time because of the dynamics of the earth.

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Adam Hawthorne
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« Reply #5 on: April 08, 2007, 11:26:09 pm »

.

Cartoon representation of the Subduction Factory, from Y. Tatsumi JAMSTEC.
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Adam Hawthorne
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« Reply #6 on: April 08, 2007, 11:29:15 pm »

Importance
 
 
Cartoon representation of the Subduction Factory, from Y. Tatsumi JAMSTEC.
Subduction zones are important for several reasons:

1.   Subduction Zone Physics: Sinking of mantle lithosphere provides most of the force needed to drive plate motion and is the dominant mode of mantle convection.
2.   Subduction Zone Chemistry: The cold material sinking in subduction zones releases water into the overlying mantle, causing mantle melting and fractionating elements (buffering) between surface and deep mantle reservoirs, producing island arcs and continental crust.
3.   Subduction Zone Biology: Because subduction zones are the coldest parts of the Earth's interior and life cannot exist at temperatures >150°C, subduction zones are almost certainly associated with the deepest (highest pressure) biosphere.
4.   Earth's Mixmaster: Subduction zones mix subducted sediments, oceanic crust, and mantle lithosphere and mix this with mantle from the overriding plate to produce fluids, calc-alkaline series melts, ore deposits, and continental crust. For this reason, scientists increasingly refer to the "Subduction Factory", and we are intermittently and rudely reminded of its operation by earthquakes and tsunamis.
Learning more about the physics, chemistry, and biology of subduction zones requires efforts that are increasingly interdisciplinary and international. Because of the central role that subduction plays in the solid Earth system, as well as its role in maintaining equilibrium between the mantle and the hydrosphere, understanding and teaching how subduction zones operate is a scientific challenge of the first importance.
Subduction zones are also being considered as possible disposal sites for nuclear waste, where the action would carry the material into the planetary mantle, safely away from any possible influence on humanity or the surface environment.

http://en.wikipedia.org/wiki/Subduction
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Adam Hawthorne
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« Reply #7 on: April 08, 2007, 11:31:11 pm »

Orogeny
 
Orogeny (Greek for "mountain generating") is the process of mountain building, and may be studied as a tectonic structural event, as a geographical event and a chronological event, in that orogenic events cause distinctive structural phenomena and related tectonic activity, affect certain regions of rocks and crust and happen within a time frame.


Orogenic events occur solely as a result of the processes of plate tectonics; the problems which were investigated and resolved by the study of orogenesis contributed greatly to the theory of plate tectonics, coupled with study of flora and fauna, geography and mid ocean ridges in the 1950s and 1960s.

The physical manifestations of orogenesis, the process of orogeny, are orogenic belts or orogens. An orogen is different from a mountain range in that an orogen may be completely eroded away, and only recognizable by studying (old) rocks that bear the traces of the orogeny. Orogens are usually long, thin, arcuate tracts of rocks which have a pronounced linear structure resulting in terranes or blocks of deformed rocks, separated generally by dipping thrust faults. These thrust faults carry relatively thin plates (which are called nappes, and differ from tectonic plates) of rock in from the margins of the compressing orogen to the core, and are intimately associated with folds and the development of metamorphism.

The topographic height of orogenic mountains is related to the principle of isostasy, where the gravitational force of the upthrust mountain range of light, continental crust material is balanced against its buoyancy relative to the dense mantle.

Erosion inevitably takes its course, removing much of the mountains, leaving the core or mountain roots, which may be exhumed by further isostatic events balancing out the loss of elevated mass. This is the final form of the majority of old orogenic belts, being a long arcuate strip of crystalline metamorphic rocks sequentially below younger sediments which are thrust atop them and dip away from the orogenic core.

http://en.wikipedia.org/wiki/Orogeny
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Adam Hawthorne
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« Reply #8 on: April 08, 2007, 11:31:57 pm »

History

Before geology, the presence of mountains was explained in Christian contexts as a result of the Biblical Deluge, for Neoplatonic thought, which influenced early Christian writers, assumed that a perfect Creation would have to have been in the form of a perfect sphere. Such thinking persisted into the eighteenth century.

Orogeny was used by Gressly (1840) and Thurmann (1854) as orogenic in terms of the creation of mountain elevations, as the term mountain building was still used to describe the processes.

Elie de Beaumont (1852) used the evocative "Jaws of a Vise" theory to explain orogeny, but was more concerned with the height rather than the implicit structures orogenic belts created and contained. His theory essentially held that mountains were created by the squeezing of certain rocks.

Suess (1875) recognised the importance of horizontal movement of rocks. The concept of a precursor geosyncline or initial downward warping of the solid earth (Hall, 1859) prompted Dana (1873) to include the concept of compression in the theories surrounding mountain-building. With hindsight, we can discount Dana's conjecture that this contraction was due to the cooling of the Earth (aka the cooling earth theory).

The cooling Earth theory was the chief paradigm for most geologists until the 1960s. It was, in the context of orogeny, contested hotly by proponents of vertical movements in the crust (similar to tephrotectonics), or convection within the asthenosphere or mantle (geology).

Steinmann (1906) recognised different classes of orogenic belts, including the Alpine type orogenic belt, typified by a flysch and molasse geometry to the sediments; ophiolite sequences, tholeiitic basalts, and a nappe style fold structure.

In terms of recognising orogeny as an event, Leopold von Buch (1855) recognised that orogenies could be placed in time by bracketing between the youngest deformed rock and the oldest undeformed rock, a principle which is still in use today, though commonly investigated by geochronology using radiometric dating.

Zwart (1967) drew attention to the metamorphic differences in orogenic belts, proposing three types, modified by Pitcher (1979);

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Adam Hawthorne
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« Reply #9 on: April 08, 2007, 11:33:33 pm »



Taconic orogeny
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« Reply #10 on: April 08, 2007, 11:35:29 pm »

Obduction

Obduction is the overthrusting of continental crust by oceanic crust or mantle rocks at a destructive plate boundary. It can occur during an orogeny.


Obduction occurs where a fragment of continental crust is caught with resulting overthrusting of oceanic mafic and ultramafic rocks from the mantle onto the continental crust. Obduction often occurs where a small tectonic plate is caught between two larger plates with the crust, both island arc and oceanic, becoming attached as a new terraine to an adjacent continent. When two continental plates collide obduction of the oceanic crust between is often a part of the resulting orogeny, or mountain building episode. New Caledonia is one example of recent obduction. The Klamath Mountains of northern California contain several obducted oceanic slabs. Obducted fragments also are found in Oman, Cyprus, Newfoundland, New Zealand, the Alps of Europe, and the Appalachians of eastern North America. The characteristic rocks of the obducted oceanic crust are the ophiolites; consisting of basalt, gabbro, peridotite, dunite, and eclogite. There are many examples of oceanic crustal rocks and deeper mantle rocks that have been obducted and exposed at the surface worldwide.

It seems that most obductions are initiated at supra-subduction, back-arc basins. These basins are caused where the edge of the continent collapses seawards, and extension in the back-arc basin enhances volcanism and crustal accretion. Since while the continental crust collapses the upper part of the ductile lithosphere, namely the upper lithospheric mantle, is exposed, and the ophiolitic volcanism accretes on metamorphic lithologic series. As the subduction turns into mountain-building, the ophiolites and their metamorphic basement find their way to mountain tops.
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Adam Hawthorne
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« Reply #11 on: April 08, 2007, 11:38:21 pm »

Depression (geology)

Depression in geology is a landform sunken or depressed below the surrounding area. Depressions may be formed by various mechanisms, and may be referred to by a variety of technical terms.
•   A basin may be any large sediment filled depression.[1] In tectonics, it may refer specifically to a circular, syncline-like depression: a geologic basin; while in sedimentology, it may refer to an area thickly filled with sediment: sedimentary basin.[2]
•   A blowout is a depression created by wind erosion typically in either a desert sand or dry soil (such as a post-glacial loess environment).[2]
•   A graben is a down dropped and typically linear depression or basin created by rifting in a region under tensional tectonic forces.
•   An impact crater is a depression created by an impact such as a meteorite crater.
•   A kettle is left behind when a piece of ice left behind in glacial deposits melts.[3]
•   A depression may be an area of subsidence caused by the collapse of an underlying structure. Examples include sinkholes above caves[4] in karst topography, or calderas[5][6] or maars in volcanic areas.
•   A depression may be a region of tectonic downwarping typically associated with a subduction zone and island arc. Fore-arc and back-arc sedimentary basins fill with sediment from an adjacent island arc, or from continental volcanism and uplift.
•   An oceanic trench is a deep depression with steep sides located in the ocean floor. Oceanic trenches are caused by the subduction (when one tectonic plate is pushed underneath another)[7] of oceanic crust beneath either other oceanic crust or continental crust.[8]
•   A depression may result from the weight of overlying material such as an ice sheet during continental glaciation which is subsequently removed resulting in a basin which slowly rebounds. The area around the ice sheet, though not covered in ice itself, may also be pulled down by the weight of the ice sheet, which is known as peripheral depression.[9] Further from the ice, a forebulge may form, which is curved slightly upward.[10]
•   A depression may be a pothole - either a simple roadway depression or a fluvial erosional depression in a river streambed, or area affected by coastal water currents.
One of many impressive depressions is the Great Rift Valley of East Africa. Perhaps even more impressive is the Atlantic Ocean basin.


http://en.wikipedia.org/wiki/Depression_%28geology%29
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Adam Hawthorne
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« Reply #12 on: April 08, 2007, 11:42:21 pm »

Map of Sampled Seamounts


It is estimated that there are tens of thousands of seamounts in the world's oceans. They are found in every ocean basins and most latitudes. This map shows the seamounts for which SeamountsOnline currenlty has data. But note that in many cases, we only have records of one or a few species - the number of seamounts which have been well sampled is much smaller. In creating this map, a strict definition of "seamount" was not used - the map includes some features that are less than 1000m tall. If you are aware of sampling that is not represented here, please let us know.

You may use this map, and all materials on SeamountsOnline, freely for non-commercial uses (only) as long as the source is cited. Please cite this map as: K. Stocks. 2005. Map of Seamounts in SeamountsOnline. SeamountsOnline: an online information system for seamount biology. World Wide Web electronic publication. http://seamounts.sdsc.edu.

updated 2005-05-12

http://seamounts.sdsc.edu/

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Adam Hawthorne
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« Reply #13 on: April 08, 2007, 11:49:38 pm »



Space shot to our own planet:  ROV Hercules approaches a ghostly, white, carbonate spire in the Lost City Hydrothermal Field, about 2500 feet below the surface of the Atlantic Ocean. Image courtesy of IFE, URI-IAO, UW, Lost City science party, and NOAA.

http://seamounts.sdsc.edu/
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Adam Hawthorne
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« Reply #14 on: April 08, 2007, 11:53:25 pm »



A multibeam map processed into three-dimensional image of the Manning Seamount complex.
http://seamounts.sdsc.edu/
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