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Monday, 16 January 2012

Ice streams

Above:  two distinct ice streams in the ice sheet edge, NW Greenland.
Lower pic:  two ice streams separated by a zone of more sluggish ice in Wilkes Land, Antarctica.  The ice embayment coincides with the zone of sluggish ice -- on either side of it the ice is moving much faster, and has pushed out across the coast in distinct tongues.


A number of people have asked recently what an ice stream is.  I have put in various posts before on this topic -- I suggest you do a search via the search box.  An ice stream is simply a stream of ice within an ice cap or ice sheet which is moving more rapidly than the surrounding ice.  That may be because there is a rock trough below and a large input of ice from further up-glacier;  there may then be ice melting on the bed and a subsequent acceleration in ice velocity.

"Streamlines" are simply the recreated ice flow directions, conventionally shown by arrows.   Note that you can also recreate streamlines for ice flow in areas affected by an ice cover, even though you are not dealing with an area covered by an ice stream. 

An ice stream may be highly crevassed -- and there is often another crevassed zone on the flanks of the ice stream where sluggish flanking ice is literally ripped apart.

Within the ice stream ice movement directions (and hence directions of erratic transport) will be rather consistent.  In the intervening areas (which are sometimes "ice shed" areas) ice movement directions may be highly eccentric and variable.  High rates of ice evacuation in an ice stream will lower the ice surface there, and cause ice from the intervening areas to flow towards the ice stream.  If the ice stream slows down, or if accumulation patterns vary, ice movement directions may literally be reversed in these zones.  Erratic movements in these areas will be very erratic indeed.......

The Irish Sea Glacier must have been an ice stream, flanked by ice-covered terrain on either side.  There must also have been big ice streams in the Moray Firth region and in the Firth of Forth. This is the BRITICE recreation for 23,000 years ago -- in the Devensian Glaciation:

One of the areas we have been talking about recently -- Aberdeenshire -- was in effect squeezed between these ice streams.  No wonder that geomorphologists have difficulty in determining exactly how and when erratics have been moved.......

See also:
http://brian-mountainman.blogspot.com/2009/10/on-bristol-channel-ice-stream.html

9 comments:

Robert John Langdon said...

Its OK Brian I will not mention boats in this post... oops too late!

It seems that lots of your ideas are based on this BRITICE MODEL of the UK - sadly I have a problem with its accuracy and therefore its credibility.

Its well accepted by 'most' Geologists ( http://geology.geoscienceworld.org/content/7/12/584.abstract) that a condition known as a 'Geomagnetic Excursion' occurs either (this is when geologists get confused, again) after the 'excursion' as the additional ice 'wobbles' the earth moving the magnetic field or more likely (in my humble view) the sun's radiation is deflected by the alteration in the magnetic field and THEN the ice age happens.

Either way the Gothenburg Excursion occurred 12 - 13.5kya, but that is completely missing from these BRITICE animations.

Any ideas how this is justified?


RJL

BRIAN JOHN said...

Very few of my ideas are based on the BRITICE model, as you call it. The BRITICE maps are a synthesis of evidence from hundreds of scientific publications -- the causes of glaciations and glacial oscillations are many and varied. The article you cite does nothing to question the overall sequence of glacial episodes already well established, and does nothing to question what we already know about ice expansion and contraction during glacial phases. What it does is offer another refinement in the "trigger mechanisms" -- all of which work in a mutually reinforcing way when glaciations happen. Sometimes it's difficult to separate out cause from effect. Then you get positive feedback mechanisms kicking in.

I don't actually see what you are on about.

For an overview, see my book called "The Winters of the World." Out of print long since, but you might find a copy somewhere....

Robert John Langdon said...

Brian

This model is showing rapid ice shrinkage in the period in question to such a degree in the period 12 - 13.5 kya, which clearly is WRONG as the article suggest there should be ice INCREASE due to this effect.

To suggest "I don't actually see what you are on about" shows that you do not question if these models are accurate - although you place them on your blog as evidence to support SOME of your ideas.

Does 'the winters of the world' identify how the 'Gothenburg', 'Mono Lake', 'Lake Mungo' and 'Laschamp' Magnetic Reversals affected the growth in glaciation and consequently world temperatures during the last ice age?

Moreover, does it explain if the 'event' PROCEEDS or FOLLOWS the ice increase - or are you one of the ice just happens camp, lets look at the pretty patterns?

RJL

BRIAN JOHN said...

As in any scientific investigation, you establish a sequence of events as best you can -- and continue to test it across a wide area -- and having done that, you seek explanations.

You do not find your explanations first, and then try to arrange the evidence to fir the explanation.

I disagree with a lot of the material on the BRITICE map -- as I have made clear in previous posts. i prefer to trust my own field knowledge and my understanding of the published literature.

BRIAN JOHN said...

Oops -- should be "to fit the explanation."

Anonymous said...

Robert

I too found the BRITICE maps questionable and commented so on a previous blog.

Annie O.

BRIAN JOHN said...

On what bases, Annie, do you find the maps questionable? Share your doubts with us -- you are among friends.....

Anonymous said...

Brian

Doggerland disappeared under water far too early on the BritIce animations and did not correlate to trawler evidence of Mesolithic peoples living in the Northern part near Orkney and Shetland.

If Robert is right and the Ice Age lingered further to 10,000BC - then the flooding waters could not existed as they were tied up in the ice cap, which would have covered northern Europe.

Annie O.

BRIAN JOHN said...

Annie -- I am afraid you misunderstand the situation. Some of the BRITICE maps are attempts to record glacial features through the British Isles -- based on collected field evidence. Other BRITICE maps are MODELS -- the ice sheet animations come in many shapes and sizes, some extreme and some conservative. They are currently testing these models against reality! Models are almost always imperfect -- but they are improved over time.

We are -- or should be, if it were not for greenhouse gases -- still in the Ice Age. The ice of the last glaciation melted away gradually -- there was certainly some left in the Younger Dryas, but most of the big ice sheet melting was complete by then -- and sea level had already risen substantially from below -100m to near present sea-level. Please type in "Doggerland" into the search box, and also look up "eustasy" -- I have a lot of info on the site.

There is still discussion going on with respect to the interactions between eustatic and isostatic effects in the North Sea.