Phil's Hiking Resources

Glaciers


Glaciers currently cover about 10% of the Earth’s surface and 80% of the fresh water on Earth is frozen in the form of Glacier ice.  There are two main types of glaciers: alpine glaciers (about 1%) which are found in mountain terrain like the Alps, and continental glaciers (about 99%) which are associated with the polar extremes of Greenland & Antarctica and previously in the ice ages much more

The largest glacier in the Alps is the Aletsch Glacier which covers more than more than 45 square miles in southern Switzerland.  It descends round the south of the Jungfrau into the Upper Rhône valley.

Glaciers are the result of snow accumulating faster than it melts away over many years.  Eventually a glacier becomes massive enough flow under its own weight.

Some features associated with glaciers include: the distinctive U-shaped valley often with a corrie at its head; formations such as drumlins and moraines; as well as ice features such as Seracs, Bergschrunds and Crevasses.

The upper part of a glacier that receives most of the snowfall and least thawing is called the accumulation zone.  The snow here is subjected to repeated freezing and thawing, changing it into “névé” a young granular ice.  Over the following year, under the weight of the accumulating snow and ice above it, this névé then fuses into a denser “firn” ice.  Over a further period of more years, layers of fern ice become more compacted to eventually become solid blue glacial ice.  The distinctive blue tint of the glacial ice is due to the same scattering effect of short wave light that gives the sky its blue colour.

Once the depth of ice becomes sufficient (about 50 meters), the lower layers of ice start to flow plastically under the pressure, allowing the glacier as a whole to move slowly like a thick fluid.  The upper layers of ice however remain brittle and fractures and cracks form as the ice travels at different speeds over irregular terrain.  These fractures widen to become “crevasses” as a glacier flows over a convex surface or round the outside of a bend.  “Seracs” (named after a crumbly white Swiss French cheese) are large and often unstable blocks of ice formed when crevasses intersect.  Glaciers also have a “Bergschrund” which is a crevasse at the head and sides formed when the glacier moves downslope leaving its borders firmly frozen to the rock

The weight of snow & ice in the accumulation zone along with the movement of the glacier is responsible for causing deep erosion of the underlining rock.

At the other end of the glacier is the “ablation zone”, where more ice is lost through melting than gained from new snowfall and the eroded sediments are deposited.  The place where the glacier thins to nothing is called the ice front.

Between these extremes is an equilibrium line where the loss of ice by melting is balanced by accumulation of new snow.  Here the downward erosive forces of the accumulation zone and the tendency of the ablation zone to deposit sediments tend to balance out.  However lateral erosive forces are not balanced; and this strong sideways erosion is how glaciers widen river-carved “v-shaped” valleys into classic “u-shaped” glacial valleys.

As mentioned the action of the ice in accumulation zone of a glacier is responsible for causing deep erosion of the underlining rock.  After the glacier has melted there is often an amphitheatre-shaped depression left, called a corrie or cirque (which often contains a small lake).  If more than one glacier is eroding a mountain, then other unique features may form, for example the Matterhorn was formed by 3 glaciers eroding back to leave a pyramidal peak or “horn”.  Arêtes are often formed by 2 glaciers eroding back towards each other.

Glaciers erode the underlining ground by “plucking” and “scouring”.
As a glacier moves over rock, it loosens and picks up chunks of rock. This process is known as plucking, and it is occurs because pressurised sub-glacial water penetrates small fractures in the rock and subsequent freezing expansion separates them from the bedrock.

Scourging or glacial abrasion is the effect of the weight of ice and embedded rocks moving over the ground.  This can form glacial striations (linear rock scratches) and Chatter marks on the underling rock.  The action of fine-grained material within a glacier on hard bedrock can lead to a fine glacial polish.

Roche Moutonnee are elongated rounded outcrops of resistant bedrock.  These have a gentle “stoss” slope on the up-glacier side smoothed by abrasion & scouring, and a rough fractured steep face on the “lee” side due to plucking.  A “Crag and Tail” is a similar but larger feature where a resistant rock obstacle protects the rock on the lee side from erosion by an advancing glacier (Edinburgh Castle makes a fine example).

Glacial moraines are linear mounds of “till” formed by glacial deposition and are exposed after the glacier has retreated.  Till is mixture of rock, gravel and boulders in a fine powder or clay matrix.  Moraines usually appear as;

  • terminal moraine is deposited at the foot of the glacier, (recessional moraines are a series of terminal moraines left as a glacier retreats)
  • lateral moraines are left along the sides, and
  • medial moraines are formed at the surface in the middle of two coalescing tributary glaciers from their combining lateral moraines
  • terminal and lateral moraines often trap a lake behind them

Drumlins are elongated whale-shaped hills made mainly of till. There is some controversy amongst geologists over exactly how they are formed. Their heights vary from 15 to 50 meters and they can reach a kilometre in length.  They are often in drumlin fields of similarly shaped, sized and oriented hills. The steeper side of the hills looks toward the direction from which the ice advanced, while the longer slope follows the ice’s direction of movement.

A Glacial erratic is a boulder transported and deposited by a glacier and having a composition different than the bedrock upon which it is sitting.  Erratics can range in size from pebbles to massive boulders (weighing thousands of tons) and the distances involved can be considerable.

Glacial retreat

Since the “Little Ice Age” ended around 1850, glaciers have retreated substantially.  However this glacial retreat did slow, and even reversed, between 1950 and 1980.  Since about 1980, glacier retreat has accelerated again and this appears to strongly correlated with global warming and the increase of atmospheric greenhouse gases.

This has led to glacier retreat becoming increasingly rapid and ubiquitous, so much so that many glaciers have disappeared and the existence of a great number of the remaining glaciers of the world is threatened.

The consequences of this may be far reaching. 

  • Many countries rely on glaciers for water supplies, hydro-electric power production, fresh silt to rejuvenate fields, and for tourism.
  • The loss of glaciers releases fresh water to the sea, rather than in a sustainable and useful way.
  • The consequence is that sea levels will rise, which may be disastrous for communities that live near costal regions and other low altitudes.
  • Additionally Fresh water less dense than sea water and a large influx of freshwater may upset the delicate balance of ocean currents and so change global weather patterns.
  • There is a real danger that this could fuel a vicious cycle leading to further warming & melt.

References


Author: Phil Newby

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Author: Phil Newby