The snow that eventually makes up ice sheet and glaciers undergoes a sequence of conversion stages:
- snow fall (density of 0.05 - 0.07) - feathery - mainly air
- settling of snow - loose granular consistency as the crystals begin to break up on collapse, and with the weight of overlying snow, plus partial melting - becomes damp and settled (0.2)
- nivation - annual and diurnal temperature changes lead to thaw-freeze alternation and the conversion of snow into ice crystals - depth hoar, this can also occur when wind packs snow together into drifts (remember that snow tends to settle on the NE slopes of upland areas as wind sweeps the snow round the top of peaks and it accumulates in the lee)
- firn or neve - increased pressure between individual grains causes pressure melting to eventually change the loose snow into a dull, white structureless mass with far less pore space (0.4 - 0.8)
- sintering - continued fusion and squeezing out of air as a result of compression by further accumulation
- glacier ice - bluish in colour and containing little air - this may take 40-50 years, depending on the location - see below - and has a density of 0.8-0.9 - water has a density of 1.
Also a process called regelation: melting and refreezing (also happens underneath glaciers) - this is similar to nivation.
ICE: Has well developed crystalline structure and displays internal pattern of thin layers, each layer representing a year’s accumulation of snow. Crystals are platy, and can be compressed more because of the shape.
Density of glacier ice is 0.9, compared with 0.1 for snow.
Takes 20-200 years for the conversion process, depending on location.
More information on this is available at TAVI MURRAY's pages at LEEDS UNIVERSITY. They are featured in some useful Powerpoint presentations which can be downloaded.
Often on the basis of size (as well as location) - Waugh's GAIA has the basic classification.
- NICHE - occupying small N facing gullies and hollows in N.Hemisphere
- CIRQUE - larger than niche glaciers and occupying semi-circular mountain hollows characterised by steep side and head-walls and a lip where the glacier spills over into the valley
- VALLEY - long, tongue-like glaciers occupying a clearly defined valley - often fed by cirque glaciers
- PIEDMONT - broad glaciers formed where constricted valley glaciers flow out onto an open lowland and are able to spread laterally
- ICE SHEET and ICE CAP - very extensive and continuous areas of ice; ice sheets are of continental dimensions; ice caps are of lesser scale
GLACIERS ARE SYSTEMS
INPUTS
Potential Energy (height of glacier / gravity)
Kinetic Energy
Precipitation
Rock debris and sediment
STORE
Snow, Ice, Sediment and Meltwater
OUTPUTS
Meltwater Streams (from summer ablation)
Fluvioglacial sediment (suspended, bed and solute load)
Deposition as Moraine
ORIENTATION AND ALTITUDE OF CIRQUES IN N.WALES
NAMES ARE FOR 4 SEPARATE MOUNTAIN RANGES
With increasing distance from the W. Coast
E= Elevation - height of lip of cirque above sea level (metres)
O= Orientation - direction with relation to the points of the compass
HEBOG SNOWDON GLYDER CARNEDD
E O E O E O E O
1 270 310 320 10 585 330 535 350
2 385 330 335 30 670 340 585 85
3 285 120 470 240 570 30 685 80
4 400 140 635 310 600 60 835 355
5 385 70 420 20 500 45 920 45
6 335 340 835 40 500 55 535 45
7 335 345 750 0 520 75 470 5
8 370 85 935 80 585 50 585 110
9 420 155 670 85 700 80 600 170
10 300 345 470 70 420 50 735 120
11 400 20 420 175 835 10
12 285 40 370 80 800 15
13 385 10 350 190 600 10
14 335 95 385 180 500 35
15 570 320 600 45
16 420 55
17 420 85
i) Draw a ROSE diagram showing the orientation of cirques.
Draw concentric circles for the scale to represent the number of corries.
Ignore the regional divisions. Count the number of cirques falling into each of the following categories: (numbers represent degrees)
N 340 - 24
NE 25 - 69
E 70 - 114
SE 115 - 159
S 160 - 204
SW 205 - 249
W 250 - 294
NW 295 - 339
ii) Describe and attempt to explain the pattern that you have found
iii) For each of the regional groups, calculate the mean altitude of the cirques
iv) Is there any relationship between cirque altitude and distance from the coast ?
If so why ?
TRANSPORTATION
Large quantities of sediment, ranging in size from boulders to fine clay are transported by glaciers on the surface, within the ice and under the ice. The release of englacial debris by ablation is important; this contains material which has fallen on to the glacier surface. Subglacial debris is derived mainly from abrasion and plucking; the material is subsequently crushed, rolled and dragged along the glacier bed, thus becoming finer and more rounded than supraglacial debris.
EROSIVE FEATURES
GLACIAL CIRQUES
Large arcuate, overdeepened hollows in the landscape, bounded by steep side and head walls. 84 cirques have been identified in Snowdonia, mainly facing NE. Similar features in the English Lake District e.g Blea Water, below High Street.
Most important features are the plucked appearance of the walls, the relatively smooth floor (a result of abrasion), the rock basin (often occupied by a small lake) and the rock lip. The ice in the cirque slopes at 26 degrees and has a thickness of around 50m.
Study of present day cirques in Norway has shed light onto their formation. At the upper limits of these glaciers there is usually a deep crevasse or bergschrund, formed as the ice pulls away from the head wall to a depth of up to 30m. It is thought that freeze thaw at the base of the bergschrund (or randkluft) breaks up the rock of the head wall. Some of the rock debris becomes trapped within the basal layers and is used to abrade the cirque floor. The glacier tends to rotate within the hollow: rotational sliding.
Where an individual peak is affected by several cirques, erosion of the back wall leads to narrow ridges: sharp crests or aretes. These may in time be lowered or destroyed creating cols, which link the cirque basins. At a late stage, isolated steep sided peaks or horns may be created.
Most cirques face between NW and NE. The altitude of the cirque lip is also related to the amount of precipitation, a feature found in Snowdonia where those furthest west are lowest, owing to the highest precipitation occurring in the west.
Cirque glaciers are usually not thick enough for plucking to occur effectively beneath the ice, but abrasion occurs: greatest at the point of rotation.
Some useful images HERE.