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Sunday, 15 July 2012

Ice tongues -- the laws of physics still apply

These are three satellite images of ice tongues in Antarctica.  The top one shows Drygalski Ice Tongue, the middle one shows the Erebus Ice Tongue, and the third is at a location I haven't been able to identify.  These are very strange features -- essentially they are glaciers afloat, but not constrained by valley sides, so they just keep on pushing out from the coast, sometimes with the rate of forward progress faster than the rate of wastage or breakage, and sometimes being dramatically shortened if there is a major collision with a broken chunk of ice shelf or tabular berg.  Open water can have a similar effect, with tides and storm waves causing a fragmentation of the ice tongue tip, with huge tabular bergs then floating away.

These features seem to defy the laws of physics, since one would expect them simply to break up on reaching the coast, as is the case with most of the tidewater glaciers of Greenland and Alaska, for example.  The difference here is that there is a constraining medium -- sea ice or shelf ice which holds the glacier tongue in place.  So the glacier simply retains its shape, pushing out to sea in a relatively unchanged form although it is afloat and therefore vulnerable to breakup.

Such features are very rare in the northern hemisphere, although the Petermann Gletscher and other north Greenland glaciers do sometimes create tongues in situations where there is thick and persistent sea ice on the fringes of the Arctic Ocean.

The laws of physics still apply -- but don't ask me to try and explain in simple terms the strange serrated edges of the Erebus Ice Tongue...........








7 comments:

Myris of Alexandria said...

Please keep these coming!
I used to think glaciers were young and dull.
In those days of course soft rock geology was for wimps unlike today when soft rock geology is the leading edge (and where the money is) and hard rock geology is for misguided nerds.
(Geoarchaeology is for loonies!!)
M.

chris johnson said...

Still wondering how glaciers could ever have moved stones to Stonehenge. Should this help my understanding?

MPP says that such an idea defies the way glaciers are supposed to work.

I like the pics, btw.

BRIAN JOHN said...

Chris -- in answer to your question -- no, unless you are amenable to the suggestion that glaciers are rather wonderful in their infinite variety, and that they are capable of doing some extraordinary things.

But if you look back over this blog, you will find post after post explaining glacier mechanics, entrainment, transport and dumping of erratics.

Chris, re MPP, you are being rather mischievous. Not sure whether you are quoting him on or off the record, but if he really does say that the idea of a glacier moving stones to Stonehenge defies the way that glaciers are supposed to work, then he is talking through his ****. Is that clear enough?

Tony H said...

Brian, I have seen a wonderful quotable statement from Michael Parker Pearson in his new book, insofar as it is using a metaphor about icebergs...

I must find it again and let you know what it is. I find his writing style very acceptable and enjoyable on the whole, by the way.

Tony H said...

Myris

No, Geoarchaeology is for those who bravely go where no Indiana-Jones style of archaeologist has gone before.... (or has any intention of so doing, as it don't inflate their pensions oooor hteir egos...)

chris johnson said...

Brian, I should perhaps check the exact wording but I think MPP was referring to the particular characteristics of North Presceli/Pont Saeson. So a glacier picking stones from a river valley and moving them over the mountain. I looked back through your site and could not a clear parallel for this.

BRIAN JOHN said...

Chris -- glaciers move things uphill and over mountains quite frequently. The literature is full of references to stones and erratic boulders that are dumped higher up than their places of origin. Suggest you type "shear planes" into the blog search facility, and refer to the article I did with Lionel Jackson in EARTH magazine.