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Isostasy
« on: May 04, 2021, 04:20:17 PM »
The wiki page on isostasy  includes several problems.  Given it’s (perceived) purpose to validate the arguments against RE gravity it’s worth exploring some of these.

First, the opening sentence is a mischaracterization:
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Isostasy is a concept in the(sic) Geology invoked to explain why the Earth’s structures do not behave in accordance to Gravity(sic), which states that greater mass should have greater attraction.
This is portrayed as a statement of fact, when the reality is that Isostacy is a concept that was developed to explain the equal (iso) balance (stasis) of earth’s surface and upper crust resting above the more ductile mantle.

Mainstream definitions and descriptions of isostasy provide a better sense of the concept:

From Joseph A. DiPietro in “Geology and Landscape Evolution 2nd edition:
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Isostasy is the rising and settling of a portion of the Earth’s lithosphere when weight is removed or added in order to maintain equilibrium between buoyancy forces that push the lithosphere upward and gravity forces that pull the lithosphere downward
From Wikipedia:
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Isostasy or isostatic equilibrium is the state of gravitational equilibrium between Earth’s crust and mantle such that the crust “floats” at an elevation that depends on its thickness and density.

The introduction continues with further misconceptions, stating:
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It is expected that there should be a greater gravitational attraction from mountains than from hills, plateaus, and oceans, since mountains are more massive, yet ‘gravity’ readings do not reflect this.
Here it’s entirely unclear who expects that, or why it would be expected. The reason regional gravity measurements do not reflect that is explained by isostacy (and is illustrated by the figure of ‘inverse mountains’ lower on the wiki page.  Essentially, mountains are commonly composed of thick accumulations (tens of km locally) of low density rocks. The weight of these low-density rocks depresses the boundary with the underlying high-density mantle rocks, which then extend for ~2900 km down into Earth’s interior. The depression of the contact is what creates the overall mass-deficit in mountainous regions which produces the reduced gravity signal.

From Karner and Watts (1983): “gravity anomalies and flexure of the lithosphere at mountain ranges”:
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The Bouguer gravity anomaly over the Himalayan, Alpine, and Appalachian mountains is characterized by a generally asymmetric gravity “low”, which spans the mountains and associated foreland basins. The minimum of the gravity “low” is generally systematically displaced from the region of greatest topographic relief and shows no obvious relationship to surface geology. In addition, the Alps and Appalachians are associated with a generally symmetric gravity “high” that is unrelated to the topographic relief. Together the gravity low and high form a characteristic positive‐negative anomaly “couple”.

The arguments against Isostasy in the Wiki revolve around the comments by two geologists. The first is an exploration geologist from Australia, Louis Hissink, provides a discussion against gravity and isostasy, stemming from some anomalous downhole geophysical data readings. It’s unclear what downhole tools were used, or how the results were anomalous, so it’s impossible (I would argue not relevant) to discuss the merits of the origins for his skepticism, but the central points he brings up demonstrate a lack of understanding of the concept of isostacy as a whole. After claiming parts of plate tectonic theory do not support the theory of gravity, he continues:
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Because of this manner of thinking, which leads to the illogical scenario of low density rocks floating in a more dense substrate, ice caps are believed to depress the crust underneath them, and when the ice melts, the crust re-adjusts by expanding upwards … but just how a rock of density 1 kg/M^3(sic) can sink into crust of density 2.7 kg/M^3(sic) is explained by the principle of isostasy. This assertion is simply crazy – logical but crazy and came about from misinterpreting the earlier surveying data where the plumbline did not deflect as expected from calculations compensating for the mass of the adjacent mountain.
It’s unclear exactly what is ‘crazy’ about isostasy. There is nothing illogical about the scenario of low density material floating in more dense substrate. Take a bucket of water (1 g/cm3), filled to the brim, and place it on a scale. Now place large block of ice (0.9g/cm3) into the bucket. A large volume of water will spill out, the ice will partially float in the bucket, with the highest point of ice rising above the rim of the bucket, and the resulting mass of the bucket-water-ice block will be lower than with just water – isostasy in a bucket!

Hissink mentions crustal emergence around the Baltic Sea and in Canada as proof of glacial isostasy. And it’s worth expanding on glacial isostatic rebound for two reasons: first, regions in Canada and the Baltic experiencing high rates of glacial isostatic rebound are tectonically stable, limiting the potential influence of modern plate tectonic forces on the data; and second glaciers are great, and the deposits they left behind give a great opportunity to explore some fundamental concepts.

“water always finds its level” is a line that is read throughout this site, because it’s true! The fun part is that in basins affected by isostatic adjustment, ‘level’ is not static. We know the Great Lakes are ‘level’ currently, but the shorelines of lakes that occupied the Great Lakes basins in the past rise towards the North and northeast, on exponential curves! The elevations of the shoreline of Glacial Lake Algonquin rises from Below modern water surface elevations of Lake Michigan-Huron(176m asl) in the south, to greater than 375 m asl north and northeast of Georgian Bay.

Here’s a paper that looks and past and recent uplift across the Great Lakes region: https://www.erudit.org/fr/revues/gpq/2005-v59-n2-3-gpq1624/014754ar/  - note that the rate of uplift decreases over time and the total magnitude increases toward the N-NE, such that older lakes are more tilted (because ‘level’ has been changed so much from subsequent isostatic adjustment). Critically, modern GPS data and water level gauges show ongoing adjustments, in line with the warping of paleo-lake shorelines:

modern water level gauge data and uplift modelling in Great lakes
http://www.greatlakescc.org/wp36/wp-content/uploads/2017/07/PresentDayTiltinggrlakes_gsab2005.pdf

GPS data showing vertical and horizontal movement across US, Canada, and parts of Greenland
https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2006GL027081

Demonstrating the case for glacioisostasy places the discussion of isostasy relative to mountains and ocean trenches into a clearer space by demonstrating the physical, measurable effects of loading huge volumes of low-density material (ice) onto more dense material (crustal rocks and sediments). It is an excellent analog for the mountains (low density rocks in earth’s upper crust) overlying and depressing denser mantle rock – just like the block of ice in a bucket of water.
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In the wiki article on isostasy, David Pratt provides additional comments on perceived ‘discrepancies’. These are again based on poor understanding of what is ‘expected’  for gravitational attraction above mountains and continents compared to low-lying areas. A quote from Physicist Maurice Allais is provided to characterize isostasy as a ‘pseudoexplanation’, but a link to ref [15] is not provided in the page’s footnotes. Pratt’s section is concluded with an argument from incredulity asking why positive gravity anomalies can occur in some areas with vertical tectonic movements.

The wiki page then discusses an excerpt from an article in the Journal of the Geological Society of India (vol. 58, Nov 2001). I don’t have access to the full article, but it does appear as though the article’s authors are suggesting the issue arises from conflicting datums, and they begin to propose an answer to the perceived conflict between negative and positive gravity anomalies, which
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may be overcome by applying free air correction factor to all the anomalies for a constant height, in the free air, as in the case of airborne surveys.
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The alternative explanation that concludes the Wiki page on isostasy leads the reader to oft-discussed gravimeter-seismometer arguments, which I will not repeat here.

It’s worth noting that other parts of the FES wiki – see the Ice wall page – do support the idea that ice in Antarctic outlet glaciers and ice streams does depress the underlying crust there.

TL;DR:
Who cares? Isostasy is a minor detail and doesn’t actually say anything about the shape of the earth.

It IS, though, an important part of our understanding of geological processes and of regional variations in the strength of gravitational attraction by the Earth. Dismissal of gravity as a viable theory, partially based on the perceived flaws between observed gravity signals and what is ‘expected’ along major geological features at surface is a common folly here. The Wiki article does not provide any real evidence refuting the theory of isostasy. Therefore, the observation that the strength of earth’s gravity vary by location remains valid. This is an observation that creates significant issues for FET’s equivalency-based arguments against gravity, i.e. that what we ‘feel’ as Earth’s gravity accelerating us downward could equally be described as the Earth accelerating us upward. Until UA can account for local and regional variations in observed gravitational attraction, it can’t be viewed as a viable alternative to RE gravity.

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Offline Tom Bishop

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Re: Isostasy
« Reply #1 on: May 05, 2021, 11:22:46 AM »
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but the central points he brings up demonstrate a lack of understanding of the concept of isostacy as a whole

It is interesting that you think that you know better than geologists, but I don't see a source that directly contradicts any of his statements. You have posted some definitions of isostasy, but this doesn't contradict what he says about it. You are basically telling us that you, a random person on the internet, disagrees with the assessment of a geologist. My response is "Okay.  ::)"

Post a quote from him and then a quote from another geologist which directly contradicts that statement. You are avoiding quoting him at all, or directly contradicting those statements. Since you have failed to do this, and are only posting definitions of isostasy, which that author doesn't even appear to disagree with in his comments, I don't see what you are contradicting here.
« Last Edit: May 05, 2021, 11:34:27 AM by Tom Bishop »

Re: Isostasy
« Reply #2 on: May 05, 2021, 11:50:10 AM »
You are basically telling us that you, a random person on the internet, disagrees with the assessment of a geologist. My response is "Okay.  ::)"
Aren't you a random person on the internet who spends a lot of their time disagreeing with the assessment of scientists about pretty much everything. I guess our response should be the same?
"On a very clear and chilly day it is possible to see Lighthouse Beach from Lovers Point and vice versa...Upon looking into the telescope I can see children running in and out of the water, splashing and playing. I can see people sun bathing at the shore
- An excerpt from the account of the Bishop Experiment. My emphasis

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Online Iceman

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Re: Isostasy
« Reply #3 on: May 05, 2021, 12:31:38 PM »

It is interesting that you think that you know better than geologists, but I don't see a source that directly contradicts any of his statements. You have posted some definitions of isostasy, but this doesn't contradict what he says about it. You are basically telling us that you, a random person on the internet, disagrees with the assessment of a geologist. My response is "Okay.  ::)"

Post a quote from him and then a quote from another geologist which directly contradicts that statement. You are avoiding quoting him at all, or directly contradicting those statements. Since you have failed to do this, and are only posting definitions of isostasy, which that author doesn't even appear to disagree with in his comments, I don't see what you are contradicting here.

I don't claim to know more than "geologists" here, but I do think I know more about evidence for isostasy than the geologists quoted in the wiki page. But people shouldnt just believe some random guy on the internet, which is why I included actual definitions of isostasy, and excerpts from (or descriptions and  direct links to) peer-reviewed publications and real-world examples on the subject.

Obviously I cant force you to read them and if your only response is "okay", well then...OK!

I posted that for other people so that in the future when they're met with the same ol song and dance about how UA is sufficient to account for our perceived weight/acceleration on Earth because "we dont actually measure gravity, because the observations dont fit the model" (a crude paraphrasing there), people will have something to look back to.

That is, if they dont want to go to google scholar and go directly to the subject matter themselves!

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Offline Tom Bishop

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Re: Isostasy
« Reply #4 on: May 07, 2021, 12:04:28 AM »
I do think I know more about evidence for isostasy than the geologists quoted in the wiki page.

Feel free to present it then. Post a quote from the geologist cited in the Wiki and then post another quote from another geologist which directly contradicts that statement.

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Obviously I cant force you to read them and if your only response is "okay", well then...OK!

I read it. I didn't see what was being contradicted. The person you are referring to in the Wiki isn't saying that the definition of isostasy is wrong. If you think that there is something that is wrong then post the quotes side by side for us.
« Last Edit: May 07, 2021, 12:06:16 AM by Tom Bishop »

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Online Iceman

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Re: Isostasy
« Reply #5 on: May 07, 2021, 12:10:10 AM »
That's fine, anyone else reading might see the quotes I supplied from Hissink, then the quoted portions of abstracts from a journal article, followed by examples of modern GPS monitoring compared to older (postglacial) crustal rebound in formerly glaciated areas.

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Offline Tom Bishop

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Re: Isostasy
« Reply #6 on: May 08, 2021, 01:01:26 PM »
I'll ask for a third time; can you actually put up two quotes side by side, one from the geologist in the Wiki and another from another geologist and show a statement which is directly contradicted by another?

It appears that you can't do that. Since you can't do that I don't see what more there is to discuss on this.

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Online Iceman

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Re: Isostasy
« Reply #7 on: May 08, 2021, 01:32:18 PM »
I'll ask for a third time; can you actually put up two quotes side by side, one from the geologist in the Wiki and another from another geologist and show a statement which is directly contradicted by another?

It appears that you can't do that. Since you can't do that I don't see what more there is to discuss on this.

I'll make you a deal.  If anyone thinks I haven't addressed both the wiki page's author's views and the quoted Geologist's assertions, as well as any potential misrepresentations of current understanding of isostasy by providing old and modern peer-reviewed sources and simple analogies, then I will break it down further.

At present I see no need to add to what was already provided in the OP. On the other hand, if there is anything in the OP that you find incorrect, or an unfair characterization, feel free to point them out and we can proceed that way.

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Offline TomInAustin

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Re: Isostasy
« Reply #8 on: May 10, 2021, 03:41:42 PM »
I'll ask for a third time; can you actually put up two quotes side by side, one from the geologist in the Wiki and another from another geologist and show a statement which is directly contradicted by another?

It appears that you can't do that. Since you can't do that I don't see what more there is to discuss on this.

That's a great rule, any time someone makes a claim they have to put up quotes from scientists.   Let's start with UA...  Go
Do you have a citation for this sweeping generalisation?