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Saturday, 3 August 2013

Glaciers, protection and erosion in the uplands


 It's not very often that you get a straight piece of glacial geomorphology making it onto the prime news pages on the BBC web site -- but here is something from today.  The report is quite interesting, and the paper presents some neat observations which back up glaciological theory quite nicely -- showing that cold ice protects the landscape, that temperate or warm ice erodes, and that even more erosion and downcutting occurs on the mountain slopes above and at the edges of the ice -- with the glacier acting as a conveyor belt for transporting away shattered rock debris.  We see this very often in ice fronts, where ridges of terminal and lateral moraine are shown to be very thin indeed -- we might even call them superficial, with dark rock material extending to a depth of just a few metres, and with clean ice lower down.

Not sure I would agree with the suggestion that cold ice in Alpine snowfields protects the landscape so much that this helps to explain the height of the Alps (which are still being lifted tectonically)...... but this is nonetheless a very welcome contribution to the literature.

http://www.bbc.co.uk/news/science-environment-23553094



Alpine glaciers 'protect mountain peaks from erosion' 

By Simon Redfern Reporter, BBC News

Instead of wearing mountains down, evidence from Europe's high Alps shows that glaciers shield summits from erosion, acting as a protective lid. French scientists studied erosion on Mont Blanc, western Europe's highest peak, below and around its glaciers. Cold ice at the highest points froze to the mountain rock and played little part in erosion, the team said.  In contrast, water and rain eroded glacier-free areas 10 times faster than areas protected by the glacier.

"Mountains don't grow to infinity, so there must be another mechanism which has lowered the summit of Europe” --  Prof Fritz Schlunegger Bern University

The research was part of CĂ©cile Godon's doctoral research at UniversitĂ© de Savoie, located on the edge of the French Alps, and appeared in the journal Earth and Planetary Sciences Letters.  The study focused on the Bossons glacier, which flows down the northern face of Mont Blanc towards the French town of Chamonix.  Rock debris, carved from the mountain at the toe of the Bossons glacier and sediments washed out in high mountain streams were compared with erosion in nearby glacier-free areas.  The researchers found that the cold glacial ice protected the mountain from erosion, rather than promoting it.

Rising ice

These results may explain the high altitude of the Alps. Driven by the tectonic collision of Europe with Africa, the high alpine bedrock is rising about one millimetre each year.  Glacier-free areas of the Alps erode at a similar rate but where the mountains are protected by ice, the peaks wear away at one tenth that rate.

Fritz Schlunegger, from Bern University, Switzerland, was not involved in the work and commented: "This group has used sediments at the end of Bossons glacier to determine where erosion is happening beneath the ice.  "Most material has been derived from the non-frozen part the glacier, while higher up towards (the summit of) Mont Blanc - where the glacier is frozen to the ground - erosion is much less," he told BBC News.  "This is really the first time, according to my knowledge, where this has been convincingly shown in a quantitative way and using a natural example.

"However, mountains don't grow to infinity, so there must be another mechanism which has lowered the summit of Europe. According to (Dr) Godon's findings, this erosion is not related to glaciers, so we still have to think about other possibilities."

Around the globe, mountain glaciers - especially those at low latitudes - are retreating in response to climate change, scientists say.  Reports earlier this year indicated that glaciers around Mount Everest had lost more than one eighth of their area in the past 50 years, and the snowline had retreated 180 metres up the mountain sides.    Dr Godon's results suggest that changes like these could change the shapes of the world's highest mountains, and that climate and mountain landscape are intimately linked.
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The Bossons glacier protects Europe's summit from erosion

by C. Godon, J.L. Mugnier, R. Fallourd, J.L. Paquette, A. Pohl, J.F. Buoncristiani
Available online 28 June 2013

Abstract

The contrasting efficiency of erosion beneath cold glacier ice, beneath temperate glacier ice, and on ice-free mountain slopes is one of the key parameters in the development of relief during glacial periods. Detrital geochronology has been applied to the subglacial streams of the north face of the Mont-Blanc massif in order to estimate the efficiency of erosional processes there. Lithologically this area is composed of granite intruded at ~303 Ma within an older polymetamorphic complex. We use macroscopic features (on ~10,000 clasts) and U–Pb dating of zircon (~500 grains) to establish the provenance of the sediment transported by the glacier and its subglacial streams. The lithology of sediment collected from the surface and the base of the glacier is compared with the distribution of bedrock sources. The analysis of this distribution takes into account the glacier's surface flow lines, the surface areas beneath temperate and cold ice above and below the Equilibrium Line Altitude (ELA), and the extent of the watersheds of the three subglacial meltwater stream outlets located at altitudes of 2300 m, 1760 m and 1450 m.
Comparison of the proportions of granite and metamorphics in these samples indicates that (1) glacial transport does not mix the clasts derived from subglacial erosion with the clasts derived from supraglacial deposition, except in the lower part of the ice tongue where supraglacial streams and moulins transfer the supraglacial load to the base of the glacier; (2) the glacial erosion rate beneath the tongue is lower than the erosion rate in adjacent non-glaciated areas; and (3) glacial erosion beneath cold ice is at least 16 times less efficient than erosion beneath temperate ice. The low rates of subglacial erosion on the north face of the Mont-Blanc massif mean that its glaciers are protecting “the roof of Europe” from erosion. A long-term effect of this might be a rise in the maximum altitude of the Alps.

1 comment:

Myris of Alexandria said...

A most prestigious publication!
It is known, of course, as Earth and Sanitary Appliances.
They take but the best.
M