Ramsey's theorem: Difference between revisions

Revision as of 04:47, 29 January 2014

Isostasy (Greek ísos "equal", stásis "standstill") is a term used in geology to refer to the state of gravitational equilibrium between the earth's lithosphere and asthenosphere such that the tectonic plates "float" at an elevation which depends on their thickness and density.

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 ^[1] 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).

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 geological time flows laterally such that the load of the lithosphere is accommodated by height adjustments.

The general term 'isostasy' was coined in 1889 by the American geologist Clarence Dutton.

Isostatic models

Three principal models of isostasy are used:

The Airy-Heiskanen Model - where different topographic heights are accommodated by changes in crustal thickness, in which the crust has a constant density
The Pratt-Hayford Model - where different topographic heights are accommodated by lateral changes in rock density.
The Vening Meinesz, or flexural isostasy model - where the lithosphere acts as an elastic plate and its inherent rigidity distributes local topographic loads over a broad region by bending.

Airy and Pratt isostasy are statements of buoyancy, while flexural isostasy is a statement of buoyancy while deflecting a sheet of finite elastic strength.

Airy

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:

h₁⋅ρ₁ = h₂⋅ρ₂ = h₃⋅ρ₃ = ... h_n⋅ρ_n

For the simplified picture shown the depth of the mountain belt roots (b₁) are:

$(h_{1}+c+b_{1})\rho _{c}=(c\rho _{c})+(b_{1}\rho _{m})$
${b_{1}(\rho _{m}-\rho _{c})}=h_{1}\rho _{c}$
$b_{1}={\frac {h_{1}\rho _{c}}{\rho _{m}-\rho _{c}}}$

where $\rho _{m}$ is the density of the mantle (ca. 3,300 kg m^-3) and $\rho _{c}$ is the density of the crust (ca. 2,750 kg m^-3). Thus, we may generally consider:

b₁ ≅ 5⋅h₁

In the case of negative topography (i.e., a marine basin), the balancing of lithospheric columns gives:

$c\rho _{c}=(h_{w}\rho _{w})+(b_{2}\rho _{m})+[(c-h_{w}-b_{2})\rho _{c}]$
${b_{2}(\rho _{m}-\rho _{c})}={h_{w}(\rho _{c}-\rho _{w})}$
$b_{2}=({\frac {\rho _{c}-\rho _{w}}{\rho _{m}-\rho _{c}}}){h_{w}}$

where $\rho _{m}$ is the density of the mantle (ca. 3,300 kg m^-3), $\rho _{c}$ is the density of the crust (ca. 2,750 kg m^-3) and $\rho _{w}$ is the density of the water (ca. 1,000 kg m^-3). Thus, we may generally consider:

b₂ ≅ 3.2⋅h_w

Pratt

For the simplified model shown the new density is given by: $\rho _{1}=\rho _{c}{\frac {c}{h_{1}+c}}$ , where $h_{1}$ is the height of the mountain and c the thickness of the crust.

Vening Meinesz / flexural

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.

Isostatic effects of deposition and erosion

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.

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.

Isostatic effects of plate tectonics

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.Template:Fact

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.Template:Fact

Isostatic effects of ice sheets

Mining Engineer (Excluding Oil ) Truman from Alma, loves to spend time knotting, largest property developers in singapore developers in singapore and stamp collecting. Recently had a family visit to Urnes Stave Church. 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 cliffs and associated wave-cut platforms 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.

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 gravitational field and rotation rate of the Earth, polar wander, and earthquakes.

Eustasy and relative sea level change

Mining Engineer (Excluding Oil ) Truman from Alma, loves to spend time knotting, largest property developers in singapore developers in singapore and stamp collecting. Recently had a family visit to Urnes Stave Church. 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.

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 margins and changes in the spreading rate of the ocean floor.

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.

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.

References

43 year old Petroleum Engineer Harry from Deep River, usually spends time with hobbies and interests like renting movies, property developers in singapore new condominium and vehicle racing. Constantly enjoys going to destinations like Camino Real de Tierra Adentro.

External links

Template:Cite EB1922

↑ A.B. Watts, Isostasy and flexure of the lithosphere,Cambridge Univ. Press., 2001

[1] A.B. Watts, Isostasy and flexure of the lithosphere,Cambridge Univ. Press., 2001

[1]

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-I am 28 years old and my name is Romaine Fisher. I life in Fort-De-France (France).<br><br>Feel free to surf to my web page: [http://hemorrhoidtreatmentfix.com/internal-hemorrhoids-treatment internal hemorrhoids treatment]
+'''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.
+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]]).
+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.
+The general term 'isostasy' was coined in 1889 by the American geologist [[Clarence Dutton]].
+== Isostatic models ==
+Three principal models of isostasy are used:
+# 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
+# The [[John Henry Pratt|Pratt]]-[[John Fillmore Hayford|Hayford]] Model - where different topographic heights are accommodated by lateral changes in [[Rock (geology)|rock]] [[density]].
+# 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.
+Airy and Pratt isostasy are statements of buoyancy, while [[flexural isostasy]] is a statement of buoyancy while deflecting a sheet of finite elastic strength.
+=== Airy ===
+[[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.]]
+[[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]]
+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:
+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>
+<br />
+For the simplified picture shown the depth of the mountain belt roots (b<sub>1</sub>) are:
+<br />
+<big><math> (h_1+c+b_1)\rho_c = (c\rho_c)+(b_1\rho_m) </math></big>
+<br />
+<big><math> {b_1(\rho_m-\rho_c)} = h_1\rho_c </math></big>
+<br />
+<big><math> b_1 = \frac{h_1\rho_c}{\rho_m-\rho_c} </math></big>
+<br />
+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:
+<br />
+<big>b<sub>1</sub> ≅ 5⋅h<sub>1</sub></big>
+In the case of negative topography (i.e., a marine basin), the balancing of lithospheric columns gives:
+<br />
+<big><big><math> c\rho_c = (h_w\rho_w)+(b_2\rho_m)+[(c-h_w-b_2)\rho_c] </math></big>
+<br />
+<big><math> {b_2(\rho_m-\rho_c)} = {h_w(\rho_c-\rho_w)} </math></big>
+<br />
+<big><math> b_2 = (\frac{\rho_c-\rho_w}{\rho_m-\rho_c}){h_w} </math></big>
+<br />
+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:
+<br />
+<big>b<sub>2</sub> ≅ 3.2⋅h<sub>w</sub></big>
+<br />
+=== Pratt ===
+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.
+=== Vening Meinesz / flexural ===
+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.
+==Isostatic effects of deposition and erosion==
+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.
+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.
+==Isostatic effects of plate tectonics==
+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}}
+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}}
+==Isostatic effects of ice sheets==
+{{Main|post-glacial rebound}}
+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.
+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.
+== Eustasy and relative sea level change ==
+{{Main|Eustasy}}
+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]].
+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]].
+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.
+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.
+==References==
+{{Reflist}}
+==Further reading==
+* Lisitzin, E. (1974) "Sea level changes". Elsevier Oceanography Series, 8
+*{{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.
+==See also==
+* [[Clarence Dutton]], who coined the term ''isostasy'' in 1889
+* [[John Fillmore Hayford]]
+* [[William Bowie]]
+* [[Marine terrace]]
+==External links==
+* {{Cite EB1922|Isostasy|author=[[Richard Dixon Oldham]]}}
+[[Category:Geodynamics]]
+[[Category:Geology]]
+[[Category:Geomorphology]]
+[[Category:Buoyancy]]

Ramsey's theorem: Difference between revisions

Revision as of 04:47, 29 January 2014

Contents

Isostatic models

Airy

Pratt

Vening Meinesz / flexural

Isostatic effects of deposition and erosion

Isostatic effects of plate tectonics

Isostatic effects of ice sheets

Eustasy and relative sea level change

References

Further reading

See also

External links

Navigation menu

Ramsey's theorem: Difference between revisions

Revision as of 04:47, 29 January 2014

Isostatic models

Airy

Pratt

Vening Meinesz / flexural

Isostatic effects of deposition and erosion

Isostatic effects of plate tectonics

Isostatic effects of ice sheets

Eustasy and relative sea level change

References

Further reading

See also

External links

Navigation menu

Search