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| '''Isostasy''' (Greek [[wikt:ἴσος|''ísos'']] "equal", [[wikt:στάσις|''stásis'']] "standstill") is a term used in [[geology]] to refer to the state of [[gravity|gravitational]] equilibrium between the [[earth]]'s [[lithosphere]] and [[asthenosphere]] such that the [[tectonic plate]]s "float" at an elevation which depends on their thickness and density.
| | "Why does my computer keep freezing up?" I was asked by a lot of folks the cause of their pc freeze issues. And I am fed up with spending much time in answering the query time and time again. This article is to tell we the real cause of your PC Freezes.<br><br>Your PC registry starts to get mistakes plus fragmented the more we employ the computer because we enter more information each time, and also create changes inside our systems plus setup. When the registry starts to receive overloaded and full of mistakes, your computer may eventually crash. It can be done to fix it on your however quite dangerous, specifically should you have no extensive experience inside doing this. Therefore, do NOT even attempt to do this yourself.<br><br>Needless to say, the upcoming logical step is to receive these false entries cleaned out. Fortunately, this might be not a difficult task. It is the second thing you should do when you observed the computer has lost speed. The first is to make certain there are no viruses or severe spyware present.<br><br>There are tricks to create the slow computer work effective and quickly. In this short article, I may tell you just 3 best strategies or techniques to avoid a computer of being slow and instead of which create it quicker plus function even much better than before.<br><br>In a word, to speed up windows XP, Vista startup, it's very important to disable certain startup goods plus clean and optimize the registry. You are able to follow the steps above to disable unwanted programs. To optimize the registry, I suggest you use a [http://bestregistrycleanerfix.com/tune-up-utilities tuneup utilities] software. Because it is really risky for we to edit the registry by oneself.<br><br>Turn It Off: Chances are in the event you are like me; then you spend a lot of time on your computer on a daily basis. Try providing a computer some time to do completely nothing; this might sound funny yet should you have an older computer you may be asking it to do too much.<br><br>Whenever the registry is corrupt or full of mistakes, the signs is felt by the computer owner. The slow performance, the frequent system crashes and the nightmare of all computer owners, the blue screen of death.<br><br>All of these difficulties will be easily solved by the clean registry. Installing the registry cleaner allows you to utilize the PC without worries behind. You usually capable to use we system without being scared which it's going to crash inside the middle. Our registry cleaner usually fix a host of mistakes on the PC, identifying missing, invalid or corrupt settings in the registry. |
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| This concept is invoked to explain how different topographic heights can exist at the Earth's surface. When a certain area of lithosphere reaches the state of isostasy, it is said to be in ''isostatic equilibrium''. Isostasy is '''not''' a process that upsets equilibrium, but rather one which restores it (a negative feedback). It is generally accepted <ref>A.B. Watts, Isostasy and flexure of the lithosphere,Cambridge Univ. Press., 2001</ref> that the Earth is a dynamic system that responds to loads in many different ways. However, isostasy provides an important 'view' of the processes that are happening in areas that are experiencing vertical movement. Certain areas (such as the [[Himalayas]]) are ''not'' in isostatic equilibrium, which has forced researchers to identify other reasons to explain their topographic heights (in the case of the Himalayas, which are still rising, by proposing that their elevation is being "propped-up" by the force of the impacting [[Indian plate]]). | |
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| In the simplest example, isostasy is the principle of [[buoyancy]] where an object immersed in a [[liquid]] is buoyed with a force equal to the weight of the displaced liquid. On a geological scale, isostasy can be observed where the Earth's strong lithosphere exerts stress on the weaker asthenosphere which, over [[geologic timescale|geological time]] flows laterally such that the load of the lithosphere is accommodated by height adjustments.
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| The general term 'isostasy' was coined in 1889 by the American geologist [[Clarence Dutton]].
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| == Isostatic models ==
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| Three principal models of isostasy are used:
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| # The [[George Airy|Airy]]-[[Veikko Aleksanteri Heiskanen|Heiskanen]] Model - where different topographic heights are accommodated by changes in [[Crust (geology)|crustal]] thickness, in which the crust has a constant density
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| # The [[John Henry Pratt|Pratt]]-[[John Fillmore Hayford|Hayford]] Model - where different topographic heights are accommodated by lateral changes in [[Rock (geology)|rock]] [[density]].
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| # The [[Vening Meinesz]], or [[flexural isostasy]] model - where the [[lithosphere]] acts as an [[Elasticity (physics)|elastic]] plate and its inherent rigidity distributes local topographic loads over a broad region by bending.
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| Airy and Pratt isostasy are statements of buoyancy, while [[flexural isostasy]] is a statement of buoyancy while deflecting a sheet of finite elastic strength.
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| === Airy ===
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| [[File:Airy Isostasy.jpg|thumb|right|Airy isostasy, in which a constant-density crust floats on a higher-density mantle, and topography is determined by the thickness of the crust.]]
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| [[File:Backstripping and eustasy correction.jpg|thumb|Airy isostasy applied to a real-case basin scenario, where the total load on the mantle is composed by a crustal basement, lower-density sediments and overlying marine water]]
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| The basis of the model is the [[Pascal's law]], and particularly its consequence that, within a fluid in static equilibrium, the hydrostatic pressure is the same on every point at the same elevation (surface of hydrostatic compensation). In other words: | |
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| h<sub>1</sub>⋅ρ<sub>1</sub> = h<sub>2</sub>⋅ρ<sub>2</sub> = h<sub>3</sub>⋅ρ<sub>3</sub> = ... h<sub>n</sub>⋅ρ<sub>n</sub>
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| <br /> | |
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| For the simplified picture shown the depth of the mountain belt roots (b<sub>1</sub>) are:
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| <br />
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| <big><math> (h_1+c+b_1)\rho_c = (c\rho_c)+(b_1\rho_m) </math></big>
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| <br />
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| <big><math> {b_1(\rho_m-\rho_c)} = h_1\rho_c </math></big>
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| <br />
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| <big><math> b_1 = \frac{h_1\rho_c}{\rho_m-\rho_c} </math></big>
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| <br />
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| where <math> \rho_m </math> is the density of the mantle (ca. 3,300 kg m<sup>-3</sup>) and <math> \rho_c </math> is the density of the crust (ca. 2,750 kg m<sup>-3</sup>). Thus, we may generally consider:
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| <br /> | |
| <big>b<sub>1</sub> ≅ 5⋅h<sub>1</sub></big>
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| In the case of negative topography (i.e., a marine basin), the balancing of lithospheric columns gives: | |
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| <br />
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| <big><big><math> c\rho_c = (h_w\rho_w)+(b_2\rho_m)+[(c-h_w-b_2)\rho_c] </math></big>
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| <br />
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| <big><math> {b_2(\rho_m-\rho_c)} = {h_w(\rho_c-\rho_w)} </math></big>
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| <br />
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| <big><math> b_2 = (\frac{\rho_c-\rho_w}{\rho_m-\rho_c}){h_w} </math></big>
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| <br />
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| where <math> \rho_m </math> is the density of the mantle (ca. 3,300 kg m<sup>-3</sup>), <math> \rho_c </math> is the density of the crust (ca. 2,750 kg m<sup>-3</sup>) and <math> \rho_w </math> is the density of the water (ca. 1,000 kg m<sup>-3</sup>). Thus, we may generally consider:
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| <br /> | |
| <big>b<sub>2</sub> ≅ 3.2⋅h<sub>w</sub></big>
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| <br /> | |
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| === Pratt ===
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| For the simplified model shown the new density is given by: <math> \rho_1 = \rho_c \frac{c}{h_1+c} </math>, where <math>h_1</math> is the height of the mountain and c the thickness of the crust.
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| === Vening Meinesz / flexural ===
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| This hypothesis was suggested to explain how large topographic loads such as [[seamounts]] (e.g. [[Hawaiian Islands]]) could be compensated by regional rather than local displacement of the lithosphere. This is the more general solution for [[lithospheric flexure]], as it approaches the locally-compensated models above as the load becomes much larger than a flexural wavelength or the flexural rigidity of the lithosphere approaches 0.
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| ==Isostatic effects of deposition and erosion==
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| When large amounts of sediment are deposited on a particular region, the immense weight of the new sediment may cause the crust below to sink. Similarly, when large amounts of material are eroded away from a region, the land may rise to compensate. Therefore, as a mountain range is eroded down, the (reduced) range rebounds upwards (to a certain extent) to be eroded further. Some of the rock strata now visible at the ground surface may have spent much of their history at great depths below the surface buried under other strata, to be eventually exposed as those other strata are eroded away and the lower layers rebound upwards again.
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| An analogy may be made with an [[iceberg]] - it always floats with a certain proportion of its mass below the surface of the water. If more ice is added to the top of the iceberg, the iceberg will sink lower in the water. If a layer of ice is somehow sliced off the top of the iceberg, the remaining iceberg will rise. Similarly, the Earth's lithosphere "floats" in the asthenosphere.
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| ==Isostatic effects of plate tectonics==
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| When continents collide, the continental crust may thicken at their edges in the collision. If this happens, much of the thickened crust may move ''downwards'' rather than up as with the iceberg analogy. The idea of continental collisions building mountains "up" is therefore rather a simplification. Instead, the crust ''thickens'' and ''the upper part of the thickened crust'' may become a mountain range.{{fact|date=July 2012}}
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| However, some continental collisions are far more complex than this, and the region may not be in isostatic equilibrium, so this subject has to be treated with caution.{{fact|date=July 2012}}
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| ==Isostatic effects of ice sheets==
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| {{Main|post-glacial rebound}}
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| The formation of [[ice sheets]] can cause the Earth's surface to sink. Conversely, isostatic post-glacial rebound is observed in areas once covered by ice sheets that have now melted, such as around the [[Baltic Sea]] and [[Hudson Bay]]. As the ice retreats, the load on the [[lithosphere]] and [[asthenosphere]] is reduced and they ''rebound'' back towards their equilibrium levels. In this way, it is possible to find former [[sea cliff]]s and associated [[wave-cut platform]]s hundreds of metres above present-day [[sea level]]. The rebound movements are so slow that the uplift caused by the ending of the last [[glacial period]] is still continuing.
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| In addition to the vertical movement of the land and sea, isostatic adjustment of the Earth also involves horizontal movements. It can cause changes in the [[Earth's gravity|gravitational field]] and [[Rotation of Earth|rotation rate of the Earth]], [[polar wander]], and [[earthquake]]s.
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| == Eustasy and relative sea level change ==
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| {{Main|Eustasy}}
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| Eustasy is another cause of relative [[sea level change]] quite different from isostatic causes. The term ''eustasy'' or ''eustatic'' refers to changes in the amount of water in the oceans, usually due to [[global climate change]]. When the Earth's climate cools, a greater proportion of water is stored on land masses in the form of glaciers, snow, etc. This results in falling global sea levels (relative to a stable land mass). The refilling of ocean basins by [[glacial meltwater]] at the end of ice ages is an example of eustatic [[sea level rise]].
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| A second significant cause of eustatic sea level rise is thermal expansion of sea water when the Earth's mean temperature increases. Current estimates of global eustatic rise from tide gauge records and [[satellite altimetry]] is about +3 mm/a (see 2007 IPCC report). Global sea level is also affected by vertical crustal movements, changes in the rotational rate of the Earth, large scale changes in [[continental margin]]s and changes in the spreading rate of the [[ocean floor]].
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| When the term ''relative'' is used in context with ''sea level change'', the implication is that both eustasy and isostasy are at work, or that the author does not know which cause to invoke.
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| Post-glacial rebound can also be a cause of rising sea levels. When the sea floor rises, which it continues to do in parts of the northern hemisphere, water is displaced and has to go elsewhere.
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| ==References==
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| {{Reflist}}
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| ==Further reading==
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| * Lisitzin, E. (1974) "Sea level changes". Elsevier Oceanography Series, 8
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| *{{cite book |author=AB Watts |year=2001 |title=Isostasy and Flexure of the Lithosphere |publisher= Cambridge University Press |isbn=0-521-00600-7 |url=http://books.google.com/books?id=CNkiZU7enWUC&printsec=frontcover#v=onepage&q=&f=false}} A very complete overview with much of the historical development.
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| ==See also==
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| * [[Clarence Dutton]], who coined the term ''isostasy'' in 1889
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| * [[John Fillmore Hayford]]
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| * [[William Bowie]]
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| * [[Marine terrace]]
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| ==External links==
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| * {{Cite EB1922|Isostasy|author=[[Richard Dixon Oldham]]}}
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| [[Category:Geodynamics]]
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| [[Category:Geology]]
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| [[Category:Geomorphology]]
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| [[Category:Buoyancy]]
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"Why does my computer keep freezing up?" I was asked by a lot of folks the cause of their pc freeze issues. And I am fed up with spending much time in answering the query time and time again. This article is to tell we the real cause of your PC Freezes.
Your PC registry starts to get mistakes plus fragmented the more we employ the computer because we enter more information each time, and also create changes inside our systems plus setup. When the registry starts to receive overloaded and full of mistakes, your computer may eventually crash. It can be done to fix it on your however quite dangerous, specifically should you have no extensive experience inside doing this. Therefore, do NOT even attempt to do this yourself.
Needless to say, the upcoming logical step is to receive these false entries cleaned out. Fortunately, this might be not a difficult task. It is the second thing you should do when you observed the computer has lost speed. The first is to make certain there are no viruses or severe spyware present.
There are tricks to create the slow computer work effective and quickly. In this short article, I may tell you just 3 best strategies or techniques to avoid a computer of being slow and instead of which create it quicker plus function even much better than before.
In a word, to speed up windows XP, Vista startup, it's very important to disable certain startup goods plus clean and optimize the registry. You are able to follow the steps above to disable unwanted programs. To optimize the registry, I suggest you use a tuneup utilities software. Because it is really risky for we to edit the registry by oneself.
Turn It Off: Chances are in the event you are like me; then you spend a lot of time on your computer on a daily basis. Try providing a computer some time to do completely nothing; this might sound funny yet should you have an older computer you may be asking it to do too much.
Whenever the registry is corrupt or full of mistakes, the signs is felt by the computer owner. The slow performance, the frequent system crashes and the nightmare of all computer owners, the blue screen of death.
All of these difficulties will be easily solved by the clean registry. Installing the registry cleaner allows you to utilize the PC without worries behind. You usually capable to use we system without being scared which it's going to crash inside the middle. Our registry cleaner usually fix a host of mistakes on the PC, identifying missing, invalid or corrupt settings in the registry.