Glacial Processes and Landforms
(summarized principally from Easterbrook, 1993)
Snow -> Firn (névé) -> Glacial Ice
Fluffy snow -> ice granules -> interlocking ice crystals
Snow is compacted and broken into smaller shards. Snow crystals also melt along points of contact. The meltwater may trickle down to the center of the snow crystal or down into unerlying void spaces and refreeze. This process gradually transforms fluffy snow into granular ice into an interlocking network of ice crystals that we know as glacial ice. The process takes only a few years for temperate glaciers to hundreds or thousands of years for low- accumulation polar glaciers.
Dust falling on ice in summer may yield a visible annual layering (laminations).
internal flow (plastic deformation)
if > ~30 meters thick
depends on thickness + slope
e.g. will flow at 10 degrees 60 meters thick
if glacier is wet-based (ice not frozen to bedrock)
aided by deformation of water-saturated basal sediments
especially in thin temperate glaciers on steep slopes aided by meltwater at base
may account for 0 -> 75% of total glacial movement
"Stick- slip" (seismic) movement yields percussion marks: crescent-shape fractures
sliding also yields groove, striations, and glacial polish
Crevasses form from tensional stresses in upper 30 - 50 meters of a glacier
transverse crevasses form across a glacier where slope steepens
chevron crevasses form along margin (pointing up valley); lateral friction
longitudinal crevasses: where ice spreads laterally
radial crevasses: where ice spreads radially.
ogives: seasonal flow bands where ice flows over ice falls.
Types of Glaciers
Thermal Classification - (ideal classification)
temperate or wet-based glaciers:
ice near pressure-melting temperature except near upper surface in winter
water found throughout thickness
fast snow -> firn -> ice transition
hi flow rates: m/yr to m/day
much basal sliding
large outwash plains from voluminous meltwater
polar or cold-base glaciers
below pressure-melting temperature
no meltwater-basal ice frozen to bedrock
no melting-ablation by calving, sublimation, wind erosion
no basal sliding
slow flow: cm/yr to m/yr
no erosion of subglacial bedrock
no outwash plain
frozen at base
surface melting in summer
But really many polar glaciers are wet-based
geothermal heat + volcanoes
many below sea level
valley glaciers that terminate as broad radial lobes where they are not confined by valley walls or adjacent ice streams
Ice Sheets (continent size) & Ice Caps (< 50,000 km2)
not confined by topography
flow in direction of slope of glacier surface outward from zone of greatest accumulation
Greenland ice sheet covers 1.7 million km2
Antartic ice sheet covers ~ 13 million km2 (actually, East and West Antarctic ice sheets)
These ice sheets bury 5000 meter high mountain ranges
Dynamic Classification - Glacier Mass Balance
Accumulation vs. Ablation
(precipitation) vs. (melting, evaporation, sublimation, calving, wind erosion)
Zone of Accumulation - Equilibrium Line - Zone Of Ablation
10 - 100x faster than normal
some glaciers surge periodically ~ every 15 - 100 yrs
speeds of 10s of meters per day
Beneath wet-based glaciers with basal sliding, the bedrock is abraded by boulders, gravel, sand, and rock flour embedded in the base of the ice. Rock flour is the silt-size remains of rock ground up by glacial abrasion. Boulders excavate deep, broad grooves, sand and gravel make smaller striations, and rock flour polishes the bedrock surface.
Abrasion increases with increasing ice thickness up to a point. It then decreases due to increasing friction and a resulting decrease in basal sliding. At greatest thickness, till lodgement occurs.
Plucking (quarrying): If basal ice freezes to the surface locally, joint blocks can be plucked out as ice moves. Also, blocks on the downstream side of bumps may be broken out.
Running water at the base of glaciers may also erode, creating familiar fluvial features.
Cirques are bowl shaped features at the head of a glacial valley. They have steep headwalls and a low threshold of rock or moraine. A tarn (lake) may fill a cirque after the glacier is gone.
Cirques may begin in nivation basins where freeze thaw beneath a snow bank breaks up rock which is then removed by creep, solifluction, and rill wash, thereby hollowing out a depression.
Bergschrund are large, very deep glacial crevasses near the headwall of a cirque- between the ice and headwall.
Cirques are more common on north and east facing slopes in northern hemisphere because these sides remain cooler with less afternoon sunlight to melt the snow.
Horns are steep faceted mountain peaks, sculpted and surrounded by three or more cirques and their steep headwalls.
Arêtes are high, pinnacled ridges formed by cirques eroding each side of the ridge.
Cols are low areas between higher stretches of a ridge (arête) where opposing cirques have started to cut through the ridge.
Glacial Valleys have a characteristic parabolic shape, which vary from the prototypical deep, steep-sided, flat-bottomed, U-shape valley, which forms in resistant bedrock, to wide shallow troughs, which form in less resistant bedrock.
Tributary glaciers dont erode as deeply as main glaciers. While the top surface of tributary and main glaciers meet at the same level, their bottoms dont. After the glaciers melt, the bottoms of tributary valleys may lie far above the main valley floor. These are called hanging valleys. They are commonly the source of high waterfalls.
Glacial valley longitudinal profiles are not smooth like a graded stream, but may present a series of giant steps called rock steps or cyclopean stairs. After the glaciers melt they may be filled by a chain of paternoster lakes connected by inlet and outlet streams.
Fiords are formed where glacial valleys have eroded below present sea level.
Finger lakes formed where ice sheets flowed over an existing stream drainage network. Glacial erosion was greater in the pre-existing river valleys because of the greater thickness and weigth of ice and therefore greater abrasion.
Roches moutonées are low bedrock hills that are glacially smoothed so they are lower on the upstream side and drop off more abruptly on the downstream side as a result of plucking of large joint blocks from the downstream side.
Whalebacks have the opposite profile and therefore apparently no downstream plucking.
Glacial Sediment Transport and Deposition
by ice, en- and sub- glacial streams, wind, glacial meltwater (outwash plains), and icebergs.
till: poorly sorted, unstratified glacial sediments
erratic boulders: boulders of a type of rock not found locally that must have been transported long distances and deposited by ice.
lodgement till: deposited at the base of a glacier by basal melting. It is usually sheared and smeared by the weight of overlying ice as it continues moving. It typically develops a faric, with clasts (deposited pebbles, cobble, boulders) aligned parallel to the flow with some at 90 degrees to the flow direction.
ablation till: deposited when a glacier melts away. It is usually coarser than lodgement till
glaciomarine drift: where glaciers terminate at or float on the ocean. Glaciomarine drift is characterized by poorly-sorted and unstratified sediments with well-preserved unbroken delicate shells of marine organisms.
glacio-flvuial deposits: stratified sediments deposited on glacial outwash plain. Large seasonal changes in discharge cause sudden changes in sediment size. Sediments are deposited by networks of braided streams carrying sediment from the glacial terminus.
glacio-lacustrine drift: Where stream runoff from the end of a glacier empties into a lake near the terminus, the coarse load is deposited in prograding deltas at the lake's margin and the fine load is carried in suspension to be gradually deposited across the lake bottom or by turbidity currents that flow out into deeper water. Glacial lakes are characterized by varves which are finely laminated deposits with seasonal variation in gain size. Clay dominates in the winter when the lake is frozen over and stream discharge is low in part because glacier mostly frozen. Silt dominates in summer as melting and stream discharge increase.
glacio-aeolian deposits: windblown sand from outwash plains may form large dune fields (e.g., the Nebraska sand hills). Windblown silt and clay (rock flour) blown out of outwash plains (or deserts) settles downwind to form thick (100s of meters) loess deposits which are tan unstratified silt with some clay. The winds may either be the prevailing winds in the area or may be catabatic wind (dense cold air falling off a glacier, especially large continental ice sheets).
terminal moraines (end moraines): ridges of glacial till that form at the terminus (end) of a glacier if the terminus remains in a stable position for some time.
push moraines: moraines that have been pushed by an advancing glacier. But once the glacier overrides the moraine it may be sheared and smeared beyond recognition, so these are mostly seen as terminal moraines that have not been pushed too much. On the other hand, there is ample evidence on Long Island and elsewhere for glaciotectonic thrusting in terminal moraines.
dead-ice moraines: a broad ridge of till deposited by stagnating glaciers near the terminus. They could be thought of as low, broad terminal moraines.
recessional moraines: ridges of till deposited at the glacial terminus during a period of stillstand after the glacier has retreated from its maximum advance. They are like terminal moraines but lie behind the terminal moraine. Sometimes it is difficult to know if a moraine is a feature of recession or if it is from a later advance.
lateral moraines: from rock falls from high cliffs at the margin of a glacial valley. They wrap around to connect with the terminal moraine. They may be large compared to the terminal moraine because the side of the glacier remains stable in its valley much longer than does the terminus. Compared to a terminal moraine they lack rock flour formed by abrasion at the base of the ice.
medial moraines: form between two tributary glaciers where material from the lateral moraines are entrained in the flow. They look impressive on the surface but really do not constitute such a large mass of sediment. Medial moraine sediments on the surface of the glacier insulate the ice below from continuing ablation of the ice so the medial moraines form ridges on the glacier.
interlobate moraines: like lateral moraines grade into terminal moraine but they form in the region between two glacial lobes rather than a glacier and the valley side.
ground moraine: a sheet of till deposited behind the terminal moraine
drumlins: elongate hills of lodgement till that are typically wider and higher in the direction from which the glaciers flowed. The elongation direction is parallel to the glacial flow direction. They mostly occur as fields of drumlins behind terminal moraines. They are rarely found alone (single). They are typically 1-2 km long, 300-600 m wide, and less than 50 m high &endash; though they vary widely.
eskers: sinuous glacial ridges of sorted and stratified sand and gravel deposited by en- and sub- glacial streams. Clasts are waterworn and rounded. Tributary eskers are sometimes seen.
outwash plain: the area in front of the terminal moraine is blanketed with a sheet of sorted and stratified sediments carried by meltwater in a network of braided streams.
kettles: small depression, ponds, and lakes common in moraines. They form as a result of stranded ice blocks deposited with glacial till when a glacier melts.