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Isostatic Rebound

A cache by geo-tek Send Message to Owner Message this owner
Hidden : 1/16/2014
Difficulty:
1.5 out of 5
Terrain:
1 out of 5

Size: Size: other (other)

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Geocache Description:




Isostatic Rebound is the rise of land masses that were depressed by the huge weight of ice sheets during the last glacial period that occurred about 10,000-20,000 years ago, through a process known as isostasy. It affects northern Europe and almost the whole of Canada and to a lesser extent northern U.S.A.  Imagine the weight of a kilometer high ice sheet pushing down on the rock below. The ice depressed the rock and sucked water out of the ocean that tended to lower sea levels, and the ground level below the ice. (Isostasy in geology is gravitational equilibrium between the earth's inner and outer layers. Isostatic sea level was also lower than it is now because a lot of water was sucked up by the ice.)
When the ice melted the uplift or rebound took place in two distinct stages. The initial uplift following deglaciation was almost immediate due to the elastic response of the crust as the ice load melted. After this elastic phase and sea level returned to its normal state, uplift proceeded by slow viscous flow so the rate of uplift decreased exponentially after that. Today, typical uplift rates are of the order of 1-5 mm/year
Under glacial/ isostatic rebound,  an area that had been under a kilometer of glacial ice like Vancouver is now very slowly rising as the rock rebounds.
This occurs very slowly because what we're talking about is the earth's solid crust having been pushed down. As B.C. rises, the earth has to slowly readjust. But eventually, if the ice doesn't come back, the earth's crust will reach isostatic equilibrium.
In the early 1900's, the sea level was at GZ on lower Lonsdale Ave at the plaque that says sea level used to be here. But about 8,000 years ago the sea level was higher than it was now when ground level was first depressed by ice, but the ice had melted. Then the ground rose to where it is today and sea level dropped. The rise was quick at first and slower and diminishing now. (refer to History and isostatic effects of the last ice sheet in southern British Columbia by John J Clague and Thomas S. James.)

Throw our isostatic rising of the land in with global warming and sea level rise and the situation becomes more complicated.

If you are interested in this same process going on in Europe, you can visit this European cache (visit link) and read the English translation if your German is not very good. The gist of the cache is "To this day, Scandinavia rises annually by about 9 mm per year. (From Isostatic Rebound)". There is a Norwegian cache as well. It rises annually about 10mm/year. (visit link)

Use e-mail to answer when you log the cache if possible.
If your answer is wrong, I will let you know.  

1. How far above sea level do you estimate this point is?
2. What do you think the two factors are which are involved in raising the sea level shoreline to this point and how much did each contribute?
3. What is the year on the plaque on the sidewalk?

Additional Hints (Decrypt)

Decryption Key

A|B|C|D|E|F|G|H|I|J|K|L|M
-------------------------
N|O|P|Q|R|S|T|U|V|W|X|Y|Z

(letter above equals below, and vice versa)



 

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