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Tuesday, 7 December 2010

Anglian isostatic rebound

These diagrams, widely used on the web, illustrate how isostasy works.  On the top diagram, the point at which the depression of the lithosphere ends and the forebulge effect begins is the position of the "zero isobase" -- sometimes referred to as the hinge line.  Note that this might be more than 100 km beyond the ice edge.

Note that the curve of isostatic uplift follows an exponential law, just as the eustatic sea-level rise curve does.  In some circumstances, the initial uplift when a glaciation comes to an end may be as high as 10m per century, but you seldom see any signs of that in the landscape (for example, in the form of strandlines) because the recovering landmass is still heavily covered by ice.  By the time the sea starts to make an impact on the coastline, the initial pulse of uplift is already over, and uplift rates may have reduced to c 3m per century.  Tilted strandlines then become the key indicators of what has been going on.

If the land is rising at about 1m per century, and the sea is rising at the same rate, then you get a "stillstand" which will allow vast deltas to form at the mouths of torrential glacial rivers.  I studied this sort of thing in East Greenland in 1962.  In the area we were looking at, there were stillstands at 101m, 67m, and 35m above present sea level.  If there is a pulse in the sea-level rise, then there is a coastal transgression.  If there is a jerk or increase in isostatic recovery, then there may be a coastal regression of relative sea-level fall.

One other feature which I didn't deal with in earlier posts is the nature of the mantle beneath an area affected by glacier ice.  Many of the studies of glacial isostasy and shorelines have been undertaken in Scandinavia and Canada -- in areas underlain by PreCambrian Shield rocks which are very stable and rigid.  In contrast, the rocks of Southern England are soft and flexible.  In such areas, isostatic responses might be faster than in rigid shield areas, and the mechanisms might also be different.  It has been suggested that internal deformation and even faulting might occur in the earth's crust, whereas beneath a shield area the rigid block might be pushed down to displace parts of the mantle, which literally flow sideways to accommodate the extra load.  When the load is removed, the mantle adjusts by allowing material to flow back inwards again -- very, very slowly.

At the moment, there is a lot still to be done both on the modelling front and on the observation / evidence front.


Robert Langdon said...

Interesting model!

Probably explains Doggerland if the 'forebulge' was exaggerated due to 'technoplate tilt' which would have been a major factor with over a giga tonne of weight on the edge of the European plate, just off the west coats of Britain.

Even so how do Geologists know how far the landscape has dropped?

The Great Lakes model by James Clark 1990 showed that it dipped by 700m - Unless you have data from Antarctica showing existing land levels under the ice (wiki says 2500m) - but then again what would the natural height of Antarctica be without the ice?

Yes we know how that can be measured by satellite today quite accurately (actually, i'm taking that for granted in this country US no problem - but cant find any real evidence or the mathematical formula to date!!)

Moreover, to get an accurate mathematical model you will need accurate data over the last 50 to 100 years to eliminate fluctuations and Geology satellites have been around only 20 years?

Are you Geologists running out of fingers?


BRIAN JOHN said...

The isostatic depression associated with the big ice sheets is pretty well known now. I'm not sure what point you are trying to make re the Great Lakes...... if the ice was to disappear from Antarctica some parts would be a lot higher and other parts would be a little higher .. as with Greenland.

I'm not sure what point you are trying to make about geologists. (By the way, I'm not one myself...) But much of the actual hard evidence in the field for past shorelines, rates of uplift and depression etc has been collected by geomorphologists and palaeobotanists. There are, as I say, hundreds of papers and thousands of radiocarbon dates -- and none of them supports your idea of a deep Mesolithic submergence in Southern England.