Re: erosion, ice sheets, glaciers, glaciation and u-shaped valleys


Daryl Krupa wrote:
> don findlay wrote:
> <snip>
> > I would think compressibility is a very real property
> > to take into account when it comes to crustal loads,
> > but I don't think it figures in questions of
> > isostatic equilibrium, ...does it?
>
> It does not, usually.

Well, surely it should be. But compressibility is strength. And
what's the strength of the mantle (with a crust on top) when isostacy
has to be considered? I mean, we consider the mantle to be a viscous
fluid (on a very long time scale), but it is also a shell (with a
crustal skin) to the Earth. And it's pretty solid too (going by
seismic response) So, if this property is "not usually taken into
account", surely it should be? What value is there in talking isostacy
if it is not?


> > Isostacy just deals with 'floatation' does it not?
>
> That depends on what definition you are implying by
> " 'flotation ' ".

Well I put it like that because I don't fully understand how people can
talk about the crust floating on the mantle in view of the above
comment about strength. We happily talk about the crust undergoing
viscous deformation on "geologically long time scales", but it can fail
in an instant (Tsunamis). How long does it take for an ice *** to
build up? A few thousand years? Pretty dry down there in the
Antarctic. Gets a lot less snow than most other places get rain.


> > and the lateral balance when you move loads
> > about on the surface. Shifting weights.
>
> The key word there is "lateral":
> Depression of the surface results from lateral movement of
> plastic mantle material away from the load.
> After removal of the load,
> Uplift of the surface results from lateral movement of
> mantle material toward the formerly loaded area.
>
> > Which leads to the question:-
> > does a ship float *on* the water, or *in* the water (crust on/in
> > mantle), and the equivalent one, is the ice floating *on* the crust or
> > *in* the crust (and the attendant questions of 'forebulge' that you
> > mention) (but no, I wasn't talking about that.)
>
> This is more than one question.
> The crust does not float on the mantle, or in it.
> The crust rests on the mantle.
> Ice on land is not floating.
> I am having trouble determining your meaning of "floating".
>
> > > Therefore, I do not know follow your meaning.
> <snip>
> > > Please elucidate your references to 'heave' and 'swell'
> > > in relation to the effect of an accumulation of ice on
> > > the movement of the Earth's surface.
> >
> > (Heave/ swell - depress/ raise) ...
>
> Heave is an upward movement, not a depressing movement.
> Thus my confusion.
>
> > So, correct me if I'm wrong, but
> > I don't think notions of crustal strength and compressibility are much
> > taken into account when considering 'isostacy', except loosely to
> > estimate slow rates of 'rebound',
>
> Rates of rebound can be rather closely measured by
> dating marine material on raised beaches and measuring
> their elevation.
> No discussion of crustal properties is required.
> One need only observe the effects of the isostatic
> movement, whatever its cause or complicating geophysical
> factors.
> I have not seen
> notions of crustal strength and compressibility
> mentioned in discussions of estimates or measurements of
> isostatic rebound.
> I have seen
> notions of crustal strength
> mentioned in discussions of tectonic movement and faulting,
> but isostatic movement is not usually a part of those discussions,
> IMO.
> I do not recall seeing
> notions of crustal compressibility
> mentioned in discussions of vertical movements of
> the Earth's crust; rather, the matters of mantle viscosity
> and load are mentioned.
>
> > and yet it would seem to be very important in itself,
> > as a factor in whether isostacy works *at all* or not.
>
> Isostatic happens. There is no question of that.
> It is easily observed in areas that have been deglaciated
> since the Last Glacial Maximum (LGM). E.g., by me.


OK Daryl, but the link between raised beaches and glaciation is still
an assumption . What do you do with places where there are raised
beaches, but no ice?
<http://www.ncdc.noaa.gov/paleo/pubs/tudhope2001/tudhope.html>


> > My facetious remark was a crude way of introducing the question,
> > in view of the implication that follows from isostatic rebound, that
> > the 'lifting of the land' to the extent of (say) creating the himalayas
> > should be a measure of something like melting ice.
>
> I see no question, above.
> I do not know what you refer to with the term
> "something like melting ice".
> I can only guess at your meaning.
> Please write for comprehension.

The question:- What (globally) lifts layered stratigraphic sequence
from below the base level of erosion (sea-level) to the highest parts
of the Earth's crust?


> The Himalaya is an erosional remnant of
> a previously uplifted area.
> Isostacy did not create the uplifted area.

So what did? We're not just talking Himalayas, we're talking plateaus
globally where there is no 'subduction' of convenient crust (when we
know crust doesn't subduct - unless empirical field evidence
specifically requires it to do so, ..and often no erosion to speak of:-
http://users.indigo.net.au/don/to/null.html


> Isostacy is said to maintain it as an uplifted area,
> as erosion reduces the load on the mantle.
> Melting of Himalayan ice would result in uplift, just as
> erosion of Himalayan rocky material would result in uplift.

The question about the supportive strength of the crust not being taken
into account again. And relative rates of erosion/ melting versus
isostatic adjustment.


> > Why haven't the Deccan Traps, and the Ethiopian Highlands 'sunk',
>
> I imagine that addition of lava to the Deccan surface
> might create a depressing load, but the matter is
> complicated by the uplifting and depressing effects of
> volcanic movement of lava and magma beneath the surface;
> this is not a useful example or analogy in this discussion.
> The Ethiopian Highlands have presumably risen as a result of
> erosive removal of load there; I do not follow your meaning.

I mean that there are great tracts of dense material that sit on top of
the sialic crust apparently without depressing it. How much is the
antarctic ice *** (mere water) estimated to depress the crust? -
7km?? Where are the seven km throws on faults around the Deccan Traps
or the Ethiopian Highlands?


> > if it's a question of buoyancy?
> > They are more dense after all, than regular crust.
>
> The average density of the earth's crustal material is
> greater than that of the Deccan Traps or the Ethiopian
> Highlands; I do not follow your meaning.
> You must define your meaning of
> "regular crust".

**SIALIC/ GRANITIC/** crust. = less dense than mantle material/ basic
lavas/ dolerite/ gabbroic intrusions. (Are you including the mantle
crust in *YOUR* 'average regular crust'? If so we have a problem.
And why don't all the vertically oriented mafic units in the Archaean,
all the vertically oriented greenstone belts, ..creep back down into
the mantle? they should, shouldn't they, if it's a question of
density.


> > Or where are the 'forebulges' for them?
>
> Because the Ethiopian Highlands are not a site of
> accumulation of sedimentary load, they have no forebulge;
> I do not follow your meaning.

I mean they are a great slab of basaltic lavas sitting on sialic crust
and should push up a so-called 'forebulge'


> The Deccan Traps are many millions of years old;
> presuambly, any forebulge that might have been detected
> around them (not that that area is a site of accumulation
> of sedimentary load, either) would have become part of
> the general tectonic movement of the area long ago, and
> would no longer be detectable.

*Tectonic* movement? I don't think so. You mean it would have crept
further way, so that you don't see it any more? Can you point to any
geological examples of 'forebulge' in the real world? Is the Ganges
Plain a forebulge to the Himalayas?. Or would the craton to the south
be 'forebulge'? Or maybe you mean that the forebulge would have evened
itself out by creep, and added itself to the fluid medium in which the
Himalyas are floating, thereby raising them imperceptibly. In other
words, with time there is no forebulge. (Creep again, and why don't
mafic units creep back down into the mantle)

>
> > The question goes to the root of whether isostacy is
> > a geological (or just a mathematical) concept.
>
> Isostatic movement of the Earth's crust is
> an observed and measurable phenomenon.
> It is not just a concept.

I think it is (just a concept, principally), and that the evidence for
it (rise of Scandinavia etc) is due to something else, and I repeat
what I say just below here, that ideas of isostatic movement begin with
the geological observation that the land has risen relative to
sea-level in areas where ice has melted, which we can know from the
presence of raised beaches, rejuvenation of drainage, and the stuff
dumped by glaciers. But that so-called rise could just as easily be
due to sea-level fall, and nothing to do with rebound.


> > It begins with the geological observation that
> > the land has risen in areas where ice has melted
> > (glacial dump)
>
> Again, it is not clear what you mean by "glacial dump";
> it could be the land, those areas, the ice,
> melting of ice, rising of land, or something else.
> When you introduce new terms, you must define them.

"Glacial dump" = stuff dumped by glaciers (moraines, eskers, large
boulders, till etc.)


> > but maybe it's that the sea level has fallen in a big way,
>
> No.

Rather peremptory. Care to enlarge? How do we know it is the land that
has risen, and not sea-level that has fallen? Convince me.


> > and the small eustatic rise that we see at the present day could
> > be due to a much smaller effect - like *THAT* being the melting of ice.
> > In other words, if we're going to melt ice then the net result should
> > be eustatic rise in sea level, not the 'lifting' of land, which would
> > be much slower.****
>
> This is correct, but I do not see why the modern
> eustatic rise of sea level implies a previous
> large eustatic fall in sea level.
> Quite the opposite has occurred since the LGM;
> I do not follow your meaning.

I didn't say it very well. And I guess it's more complicated than I do
say. What I was saying there was just that if you melt the ice, then
the *SEA*- level rise is dissipated all around the globe, but the land
that the ice melts off (being a restricted area) eventually pops up
more than the slight rise in sea-level. ( The key word there is
eventually). So the net result (eventually) is isostatic rise of the
land.. In the case of the antarctic the question is academic, since
just about the only ice that melts (apart from the dry valleys) is the
glaciers floating in the sea, and of course when they melt, then
nothing happens to sea level.


> > But the land *has* lifted, ..to the extent of exhuming stratigraphic
> > sequence world-wide. How come (in an 'isostatic readjustment' model)?
> > ICE? I don't think so.
>
> Exhumation results from erosion.

Well, that's the question:- Does it? How much erosion does there have
to be to lift the sea-floor up to the height of the Colorado Plateau?
Or the Jurassic sea-floor up to the height of the Himalayas? How does
erosion happen, when the sequence is at base level? Lower than base
level in fact. You can talk about underthrusting of the Indian
contintinent, but exhumed flat-lying sequences (plateaus) are
distributed globally , and at various heights. So, ..whilst some
exhumation *might* result from erosion (though I question that on
grounds of 'strength' of the crust), that's not what we're looking at
when we talk of the stratigraphic sequence sitting on the continental
crust, which was once under the sea and is now above it. You can't
argue 'erosion' for that, now can you?


> Erosive removal of material during exhumation
> decreases load and results in movement of mantle material
> toward the area of decreased load.
> That lateral movement of mantle material lifts the surface,
> or maintains its altitude, allowing continued erosion and
> exhumation.
> The areas that get eroded have been uplifted by
> whatever forces
> before exhumation begins.
> Exhumation does not initiate uplift.
> Exhumation is the result of erosion is the result of uplift.
>
> I do not know why you dismiss "ICE" with regard to
> matters of exhumation and uplift.
> Ice can erode, and therefore ice can exhume.
> Addition of ice depresses a surface, and removal of ice
> raises a surface, just as does the addition or removal of
> any other mass resting on a surface.
>
> Again, I do not follow your meaning.

It's a question of scale, .. and again the fact that the strength of
the crust is not taken into account (the time it takes for things to
adjust), as it should be if you are saying that isostacy is controlled
by lateral mantle movement.


>
> > The pulsing rise of land (plateau levels) and the rise of sea-level.
> > Two entirely different-scale processes, but lumped by geophysics as
> > 'due to ice melting'. Sounds very iffy to me.
>
> I do not know where to read about
> "pulsing rise of land" (another unfamiliar term)

The 'rise of land'; the periodic rise of land; land that rises in
pulses, evidence by rejuvenation, raised beaches/ erosional platforms
(Fall in sea-level.)


> in relation to "plateau levels" as being
> the result of melting of ice.
> Perhaps you mean the webpage re: plateau levels in
> Antarctica that you had cited earlier, i.e.
> http://users.indigo.net.au/don/to/antarctica1.html
> I do not see a claim of their cause being the melting of ice.
> I see only one plateau level in the picture there,
> i.e. the "incised peneplain" of the caption.
> The incision that has occurred is the result of
> erosion, which would have been the result of uplift.

I was looking for something in the consensus that would raise
stratigraphic sequence above sea-level without meeting the problem of
erosion cited above. Isostatic rebound due to ice melting was the only
thing I could think of. As far as I know there is nothing in plate
tectonics or the wider consensus that will exhume stratigraphic
sequence from below the base-level of erosion (sea floor) other than an
ice *** melting, or underplating (which is mantle growth , which is
Earth expansion)


> Please tell me where I can see
> a a geophysical claim that
> "pulsing rise of land (plateau levels)"
> is
> "due to ice melting".

Well, I was referring to all the raised beaches, and rejuvenated
drainage in Northern Europe (and in other parts of the world as well
(eg::-
http://www.geog.uu.nl/landdegradation/images/Lossgebied_lufo.jpg
The land is rising therefore being eroded?
or the land is eroding, therefore it is rising?
Which?


> > ***And anyhow, ..I'm not sure why, when we say that there is up to half
> > a metre a day flux in the crust due to the gravitational pull of the
> > Moon, that isostatic rebound should be slow. Any ideas on that one?
>
> Lateral mantle movement controls isostatic rebound.
> That is not a tidal phenomenon; I do not follow your meaning.

Tidal is still gravitational pull, and should have its effect on the
polarity of isostatic rebound, ..an 'arrow of preference' if you like,
in other words 'tidal' should not entirely cancel.


> > There are a lot of contradictions around, when it comes to
> > 'numberologists'.
>
> I do not know what you mean by
> " 'numberologists' ".
>
> Please define this term, and the others that I have asked you about.
> After that is done, we might be able to have an intelligent
> discussion.

OK, but I think you're being deliberately obtuse; not a good harbinger
for any discussion :-)
Numerancy = what Stuart does with his mother's soup (where normal
people are into necromancy)
Numerology = the tarot of numbers - something like rolling the dice.
Numberology = the 'b' gives it weight, .slows it down, gives
respectability, .
(Then I suppose there's arithmetic)(other terms explained above)

In brief, all of this question about ice and rebound etc, is directed
towards looking for something that exhumes undeformed stratigraphic
sequence from below the base level of erosion (below sea-level) to make
up the various plateaus of the world. Epeirogeny won't do, not big
enough (and what causes that anyway?) It has to be global. The only
possible candidate ( in the consensus) is ice melting, and it runs into
the scale problem - too global, too many thousands of metres and too
much climate change. So what can it be other than outwards global
growth (underplating) and commensurate sea-level fall?

This question of flat-lying stratigraphic sequence sitting on the crust
was just beginning to be addressed properly when plate tectonics came
along. It lives on in the guiise of 'isostacy' which was all the rage
in the sixties - but isostacy is not the answer - not until the
question of strength of the crust is better addressed, and its
intrinsic ability to support weight without sinking. Cold crust sinks
on subduction zones, why not elsewhere as well? If it's a question of
more dense stuff sinking, then why aren't the ocean floors a veritable
'bog' of sinking plates with perimeters of lava fields?



> Daryl Krupa

.

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