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Isostasy

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Stacy Dohm
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« on: May 02, 2007, 02:04:01 am »

Simply put, "isostasy" is the scientific process that excludes the possibility of a continent ever sinking. Let's find out more about this important process!

Isostasy 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. It 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. 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 Himalaya, 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 observed by Archimedes in his bath, where he saw that when an object was immersed, an amount of water equal in volume to that of the object was displaced. 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.

« Last Edit: May 02, 2007, 02:06:13 am by Stacy Dohm » Report Spam   Logged

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Stacy Dohm
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« Reply #1 on: May 02, 2007, 02:06:59 am »

Isostatic models

Three principal models of isostasy are used:
   The Airy-Heiskanen Model
- where different topographic heights are accommodated by changes in crustal thickness.
   The Pratt-Hayford Model
- where different topographic heights are accommodated by lateral changes in rock density.
   The Vening Meinesz Model
- where the crust acts as an elastic plate and its inherent rigidity spreads topographic loads over a broader region. This hypothesis was put forward to explain how feautures of grand magnitude like the himalayas could be explained using a regional isostatic compensation rather than a localised one which is the case for the first two models.
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Stacy Dohm
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« Reply #2 on: May 02, 2007, 02:08:48 am »

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.

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.


Isostatic effects of ice-sheets

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 which 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 Ice Age is still continuing.


Eustasy and relative sea level change

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 climatic changes. When the Earth's climate cools, a greater proportion of the earths water is stored on land masses in the form of Glaciers ,snow, etc. This results in a relative fall in global sea levels (relative to a stable land mass). The refilling of ocean basins by glacier 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 vary from as little as zero to +2 mm/a, its determination being difficult due to the need to separate out local vertical crustal movements.

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.


Further reading

Lisitzin E. (1974) "Sea level changes". Elsevier Oceanography Series, 8
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Mark of Australia
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« Reply #3 on: May 03, 2007, 11:29:47 pm »

Great topic ,Great posts Stacy

I have always been puzzled by how isostasy relates to Atlantis .Those common statements about how we have mapped the Atlantic ocean floor and there is no missing continent so there is no Atlantis.. Firstly ,any Atlantologist worth her salt knows Atlantis is NOT a continent.Secondly ,the mid-Atlantic ridge looks like a sunken continent if a portion of it were to be raised in the North Atlantic,so to me there does appear to be a sunken continent in the Atlantic! I guess they say there isn't one because of there initial bias against Atlantis.

I guess they mean that there is no neatly outlined sunken landmass at a uniform depth and in high-relief from the surrounding sea floor. Oh how convenient if it were that easy .

But then ,there is one ,the Great Meteor Seamount which is in a location matching Plato's description ,it appears like a sunken land even though it is not the size of a continent.

I think isostasy is the key ,if Atlantis was a 'large island'  and it sank ,why should it conveniently stick out like a sore thumb just for us ?
I reckon that it would sink to a depth where it would be in equilibrium like the surrounding ocean floor, so it would be flush with the surrounding crust.,,But since Atlantis is described as having a mountain range ,those mountains should be expected to exist now at depth .so there may be some dispacement of the 'Atlantis plate',a few hundred meters perhaps?

So I think we are looking for a region of Seamounts in the North Atlantic that appear to rise from the abyssal plain not some convenient terrace of crust in the outline of a large separate slab.

I guess the best candidates are the mid-Atlantic ridge and the Azores. My personal choice is the Horseshoe Seamount chain.

As Stacy quotes, they think that all isostatic rebound is gradual ,I think that is just their dogma at work . Plato's account of Atlantis describes a geological movement that would involve isostasy at a catastrophic pace. 

Ok so what causes an isostatic imbalance that behaves so violently ?  Well I don't know !!  Lips sealed   But maybe like how Stacy mentions the Himalayas being out of equilibrium so that they are raised up by another force,they could then be suddenly brought back to equilibrium if that pressure was to cease. The entire Himalaya sinking several kilometers ?

Maybe Plato did make it all up and we are wasting our time. None of us would be here if we really believed that .

I think we are in a Catch-22 situation -  'They' wont consider large-scale,sudden,recent movements of the crust unless there is evidence (ruined buildings on the ocean floor at great depth)  ,and they wont consider Atlantis because there is no evidence of the required fast geological movement.

I think only man-made ruins at great depth will do the trick because of the nature of geological evidence ,rocks and sediment can be interpreted to say whatever they want it to say. We need a discovery so astounding that it forces an immediate reassessment of geological theory.

I think it's fair to say that everyone here is struck by how coherent and detailed Platos account of Atlantis is ,and that is why we consider it to be real history of a real place. We only have his description to work with ,but I think it points the way to a discovery whose implications will ultimately destroy the paradigm of geology. Not to mention starting in earnest the study of an amazing civilisation.

Oh ,I got a bit carried away there.  Smiley
« Last Edit: May 04, 2007, 02:58:59 pm by Mark Ponta » Report Spam   Logged
Bianca
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« Reply #4 on: May 04, 2007, 07:23:53 pm »



Really nice post, Mark!  Get carried away like that any time, as far as I am
concerned.

Unfortunately, as we have discussed on another thread, there ARE discoveries
that are being kept from the public at large, and for all we know there is plenty
of proof of the existence of Atlantis.......

Love and Peace,
B
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Mark of Australia
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« Reply #5 on: May 05, 2007, 01:57:03 am »

Hi Bianca  ,thanks

Yeah ,the thought that knowledge is being kept from us is worrying ,but it is a fine line between concern and paranoia ,delusions,conspiracy.

This whole subject of Atlantis is curious in that it is on the outer of 'serious' science and academics would consider most of what is on this forum as nonsense. I admit that I think there are some deluded individuals here ( I mean that in an affectionate way Grin) ...but then all of us would be considered deluded by the average garden variety of university professor. It's a disorienting state of affairs .

It makes me be very cautious in considering and dismissing ideas. I don't really dismiss any idea ,It's just that I have to judge whether an idea is going to bear fruit any time soon and so is it worth the consideration at this moment .I am on the look out for ideas and theories that can actually be tested for an answer .Hopefully that test for empirical evidence of Atlantis is not too far away ,so that we Atlantis researchers don't have to be on the fringe anymore.

It's not made any easier when there is the suggestion that evidence is being suppressed or hoarded. ...err and I have to admit I have some ideas that I am keeping to myself for now ,but I intend to release them when they are ready.
 
 Nevertheless I am optimistic that we shall have an answer soon . Smiley

Now that I think of it , it would be so depressing if Atlantis really was a fable  ,  Embarrassed Tongue
« Last Edit: May 05, 2007, 02:01:46 am by Mark Ponta » Report Spam   Logged
Bianca
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« Reply #6 on: May 05, 2007, 06:36:40 am »



Well, Mark, as you know, the only degree I have is in COMMON SENSE.  So, here's
what I think.   

All daydreaming and fantasizing put aside (and Plato too), unless one is truly con-
vinced that the Earth is only 5,000 or so years old (as the Bible says), one has to
really take a look at how old this planet is and its ever-changing condition.  Then
one has to consider the archaeological discoveries up to date, at least the ones
that we have been "allowed" to know of.  Man has been around a LONG time also.

Considering all that and the cataclysms we have witnessed ourselves, we really
have no idea how many "Atlantises" there may have been.  It wouldn't surprise me at
all if all the theories  on this board may eventually prove true.  The problem is what
were these civilizations called.  I don't think any of them will be plainly labeled
ATLANTIS.

To think that we (within recent history) are the only civilizations on the Planet
EVER  is as arrogant as thinking we are the only inhabited world in this universe, or that
"The Sun revolves around the Earth".

Love and Peace,
B
                           
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Mark of Australia
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« Reply #7 on: May 05, 2007, 06:59:13 am »

I fully agree with your sentiment Bianca ,I'm also self taught ,no 'official' degree.

 Common sense tempered with good logic and scientific method ,yes that is the way.  Arrogance is a big problem ,it is so hard to guard against.

That was a very eloquent post Bianca and I also think that there may be many lost chapters in the history of civilisation .Atlantis is just one of them.

I think the way to discover those lost chapters is to have a better understanding of the geological history of Earth.I have said it before that I think geological history is wrong and until we get it right we will not know where to look for those lost chapters.

Finding Atlantis will be the first step in rewriting the geological history books I believe.This one of the reasons why Atlantis is so important ,it'll pave the way for lots of other theories and expeditions.
« Last Edit: May 05, 2007, 07:11:12 am by Mark Ponta » Report Spam   Logged
Stacy Dohm
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« Reply #8 on: May 07, 2007, 01:09:09 am »

Mark,

I'm glad that someone has head of Isostasy here besides me!

As you know, Antarctica is my preference for Atlantis, however, some very strange things have been found in the Atlantic, geologically speakng! 

In 1954, an expedition in the Atlantic uncovered a "sunken block of continent:"


Atlantis - above the waves
traditions, theories, evidence and facts


--------------------------------------------------------------------------------


Evidence from the floor of the ocean


In a 1954 issue of Geological Society of America, Bulletin, Bruce Heezen and others reported on a seamount - an underwater mountain - that has been named Atlantis by geologists and is in the Atlantic Ocean. It has been found to have been an island about 12,000 years ago - exactly the time specified by Plato! This abstract is given:

The Atlantis, Cruiser, and Great Meteor seamounts rise from a broad ridge or plateau which extends from the Mid-Atlantic Ridge to 37N. 32W. southeast to Great Sea mount at 30N. 28W. The Atlantis Sea mount, briefly explored 1947 and 1948, was found by echo sounding and submarine photography to have a fairly flat bedrock summit area at about 180 fathoms covered in some cases by current-rippled sand. Its slopes are covered with sand or ooze symmetrically rippled at 400 fathoms and marked by slump features in 570 fathoms. A small piece of volcanic agglomerate was dredged from 400 fathoms on the north slope. About a ton of flat pteropod limestone cobbles was dredged from the summit area. One of the cobbles gave an apparent radiocarbon age of 12,000 years 900 (J.L. Kulp). The state of lithification of the limestone suggests that it may have been lithified under subaerial [i.e. above water, on land surface] conditions and that the sea mount may have been an island within the past 12,000 years. (Heezen, Bruce C., et al, "Flat-Topped Atlantis, Cruiser, And Great Meteor Sea Mounts" in Geological Society of America, Bulletin, 65:1261, 1954 (Protogonos issue 9))

In later studies, evidence was found for the remnants of a "sunken block of continent" in the middle of the Atlantic Ocean. An articlein New Scientist 1975 summarizes the result. (Anonymous, New Scientist,66:540, 1975)[/b[

Although they make no such fanciful claim from their results as to have discovered the mythical mid-Atlantic landmass, an international group of oceanographers has now convincingly confirmed preliminary findings that a sunken block of continent lies in the middle of the Atlantic Ocean. The discovery comes from analysing dredge samples taken along the line of the Vema offset fault, a long east-west fracture zone lying between Africa and South America close to latitude 11N.


The article describes the first report of "shallow-water limestone fragments" from the Vema Fracture in the Atlantic:

Four years ago two University of Miami workers, J. Honnorez and E. Bonatti, first reported the recovery of shallow-water limestone fragments from the Vema fracture zone. This limestone contained minerals indicative of a nearby granitic source unlikely to occur on the ocean floor. Neither water currents, nor more esoteric transport systems, could explain the presence of these rocks so far from the modern boundaries of the continents. The two researchers believed that, instead, the granitic grains must have been deposited close to their source.

Then the recent researchers are noted:

Now, with C. Emiliani of Miami, Paul Bronniman of the University of Geneva, M.A. Furrer of Esso Production Research, Begles, and A.A. Meyerhof, a consulting geologist from Tulsa, USA, they have carried out a more searching analysis of the dredge samples (Earth and Planetary Science Letters, vol. 26, p.Cool

Finally he notes the evidence for activity in less than 30 meters ofwater, and even some evidence for activity in soil.

The Limestones include traces of shallow-water fossils - foraminifera, green algae, bits of gastropods, and crab coprolites - implying formation in water, in one instance, less than 30 m deep. Furthermore, the limestones have been recrystallized from a high to low-magnesium form of calcite. Oxygen and carbon-isotope ratios prove conclusively that this process must have taken place subaerially (on land surface) "through the action of meteoric water enriched in light carbon while passing through a soil zone ..." A pitted limestone sample bears evidence of tidal action. Some 50 km east of the dredge site along the Vema fracture the team also recovered a thick-shelled, shallow-water, bivalve fossil from a depth of over 2000 m.

The coprolites in the sample indicate a Mesozoic age for the limestone which may well be the sedimentary capping on a residual continental block left behind as the (??) spread out into an ocean. The granitic minerals could thus have come from the bordering continents while the ocean was still in its infancy. Vertical movements made by the block appear to have raised it above sea level at some period during its history.

(from Unknown Earth: A Handbook of Geological Enigmas by William R. Corliss.)


http://www.atlantissource.com/home/forgotten_article.htm
« Last Edit: May 07, 2007, 01:12:03 am by Stacy Dohm » Report Spam   Logged

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Mark of Australia
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« Reply #9 on: May 07, 2007, 04:48:12 am »

Stacy

Yeah ,I appreciate the more scientific side of Atlantis study.Your posts are always scientific,always good .

It's interesting that you choose Antarctica for Atlantis ,I don't think there is much chance of that.What are your main reasons? and do you call it 'Atlantis' as in Plato's city or is it a generic term.
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