GLACIAL AND PERI-GLACIAL PROCESSES
More than 77% of earth’s freshwater is frozen while Ice
covers about 11% of the earth’s surface. A glacier is a mass of
ice sitting on land or floating as an ice shelf in the ocean next to land. Glaciers
form in areas of permanent snow. A snowline is the lowest
elevation where snow occurs all year-round.
ALPINE GLACIER
A glacier in a mountain range is called alpine
glacier. If the glacier is confined within a valley, it is termed a valley
glacier. A Cirque is a bowl-shaped erosional landform found at the
head of the valley. Wherever several valley glaciers pour out of their
confining valleys, they come together to form piedmont glacier at the
base of the mountain. Where alpine glaciers flow down to the sea, they break off
to form icebergs.
CONTINENTAL GLACIER
A continental glacier is a continuous mass of
ice on found on land. Its most extensive form is an ice sheet. Each
ice sheet is more than 9800feet (3,000m) deep burying all but the highest
peaks in an area; A smaller, roughly, circular form of an ice sheet is termed ice
cap, and the least extensive form of ice cover usually in mountains, is an ice
field. Ice sheets cover 80% of Greenland and 90% of Antarctica. Antarctica
alone has 91% of all glacier ice on the planet.
PROCESSES OF GLACIER
FORMATION.
A
glacier is composed of dense ice that is formed from snow and water
through a process of compaction, recrystallization and growth. Snow progresses
through transitional steps from firn (compact, granular texture) to a
denser glacial ice after many years. A glacier is an open system with
inputs and outputs that can be analyzed through observation of the growth and
wasting of the glacier itself. A firn line is the lower extent of a
fresh snow-covered area. A glacier is fed by snowfall and is wasted by ablation
(losses from its upper and lower surfaces and along its margins through
melting).
GLACIAL
MOVEMENT
As
a glacier moves downhill, vertical cracks and openings called crevasses may
develop. Crevasses result from friction with valley walls, compression, or
tension from stretching as the glacier passes over convex slopes. Sometimes a
glacier moves rapidly as in a glacier surge. As a glacier moves, it
plucks rock pieces, incorporates them into the ice and uses this debris to
scour and sandpaper underlying rocks through abrasion.
EROSIONAL AND DEPOSITIONAL
LANDFORMS CREATED BY GLACIER.
Extensive
valley glaciers profoundly reshape mountains by carving V-shaped valleys
of former streams into U-shaped glaciated valleys. As cirque walls erode
away, sharp arêtes (serrated ridges) form, dividing adjacent cirque
basins. Two eroding cirques may reduce an arête to a saddle-like feature called
col. A horn results when several cirque glaciers gouge an
individual mountain summit from all sides, forming a pyramidal peak. An
ice-carved rock basin left as a glacier retreats may fill with water to form a tarn;
A string of tarns separated by moraines is called paternoster lakes. Where
a glacial valley trough joins the ocean, and the glacier retreats, the sea
extends inland to form a fjord.
All
glacial deposits, sorted or unsorted and whether ice-borne or meltwater-borne
are termed as glacial drift. Direct deposits from ice, called till,
are unstratified and unsorted. Glacial meltwater deposits are sorted and are
called stratified drift. Highways of ice that flow from accumulation
areas high in the mountains are marked by trails of transported debris called moraines.
A lateral moraine accumulates along the sides of a glacier whilst a medial
moraine forms down the middle when two glaciers merge and their lateral
moraines combine.
CONTINENTAL
GLACIER:
Continental
glaciation leaves different features than does alpine glaciation.
A
low and rolling relief of unstratified coarse till that forms behind terminal
moraines is termed a till plain. Beyond the moraine deposits, outwash
plains of stratified drift develop stream channels that are meltwater-fed,
braided, and overloaded with debris. An esker is a winding, narrow ridge
of coarse sand and gravel that forms along the channel of a meltwater stream
beneath a glacier. When a retreating glacier leaves behind an isolated block of
ice, it becomes surrounded with debris and when the block finally melts, it
leaves a steep-sided kettle. A kame is a small hill, knob, or
mound of poorly sorted sand and gravel that is deposited directly by water or
by ice in crevasses.
Glacial
action forms two streamlined hills:
1.
Erosional roche moutonnée - an asymmetrical hill of exposed bedrock,
gently sloping upstream and abruptly sloping downstream) and …
2.
Depositional drumlin made from deposited till, streamlined in the
direction of continental ice movement with blunt end upstream and tapered end
downstream.
PERIGLACIAL LANDSCAPES
The
term periglacial describes cold-climate processes, landforms, and
topographic features that exist along the margins of glaciers, past and
present. When soil or rock temperatures remain below 0°C (32°F) for at least 2
years, permafrost (“permanent frost”) develops. An area of permafrost
that is not covered by glaciers is considered periglacial. The active
layer is the zone of seasonally frozen ground between the subsurface
permafrost layer and the ground surface. In regions of permafrost, frozen
subsurface water is termed ground ice.
An
ice age is any extended period of cold lasting several thousands of
years. The late Cenozoic Era has featured pronounced ice-age conditions
in an epoch called the Pleistocene. During the Pliestocene, alpine and
continental glaciers covered about 30% of Earth’s land area in at least 18
glacials, punctuated by interglacials of milder weather. Evidence of
ice-age conditions is gathered from ice cores drilled in Greenland and Antarctica,
from ocean sediments, from coral growth in relation to past sea levels, and
from rock. Lacustrine deposits are lake sediments that form terraces
along former shorelines. The study of past climates is termed paleoclimatology.