adapted to HTML from lecture notes of Prof. Stephen A. Nelson Tulane
Glaciers constitute much of the Earth that makes up the cryosphere, the
part of the Earth that remains below the freezing point of water. Most
glacial ice today is found in the polar regions, above the Arctic and
Antarctic Circles. While glaciers are of relatively minor importance
today, evidence exists that the Earth's climate has undergone fluctuations
in the past, and that the amount of the Earth's surface covered by
glaciers has been much larger in the past than in the present. In fact,
much of the topography in the northern part of North America, as well as
in the high mountain regions of the west, owe their form to erosional and
depositional processes of glaciers. The latest glaciation ended only
10,000 years ago.
Definition of a glacier
A glacier is a permanent (on a human time scale, because nothing on the
Earth is really permanent) body of ice, consisting largely of
recrystallized snow, that shows evidence of downslope or outward movement
due to the pull of gravity.
Types of Glaciers
- Relatively small glaciers which occur at higher elevations in
Smallest of these occupy hollows or bowl-shaped depressions on sides
of mountains (cirque glaciers).
As cirque glaciers grow larger they may spread into valleys and flow
down the valleys as valley glaciers. Paths these valley glaciers take
are controlled by existing topography.
If a valley glacier extends down to sea level, it may carve a narrow
valley into the coastline. These are called fjord glaciers, and the
narrow valleys they carve and later become filled with seawater after
the ice has melted are fjords.
If a valley glacier extends down a valley and then covers a gentle
slope beyond the mountain range, it is called a piedmont glacier.
If all of the valleys in a mountain range become filled with
glaciers, and the glaciers cover then entire mountain range, they are
called ice caps
Ice Sheets: (Continental glaciers):
are the largest types of glaciers on Earth. They cover large areas of the
land surface, including mountain areas. Modern ice sheets cover Greenland
and Antarctica. These two ice sheets comprise about 95% of all glacial ice
currently on Earth. They have an estimated volume of about 24 million km3.
If melted, they contain enough water to raise sea level about 66m (216
ft.). This would cause serious problems for coastal cities (L.A., NY,
Washington DC, New Orleans, Miami, SF etc). The Greenland ice sheet is in
some places over 3000 m (9800 ft) thick and the weight of ice has
depressed much of the crust of Greenland below sea level. Antarctica is
covered by two large ice sheets that meet in the central part along the
Transantarctic Mountains. These are the only truly polar ice sheet on
earth (North Pole lies in an ocean covered by thin layer of ice.
Ice Shelves: Ice shelves
are sheets of ice floating on water and attached to land. They usually
occupy coastal embayments, may extend hundreds of km from land and reach
thicknesses of 1000 m.
Glaciers can also be classified by their internal temperature.
Temperate glaciers - Ice in a temperate glacier is at a
temperature near its melting point.
Polar glaciers - Ice in a polar glacier always maintains a
temperature well below its melting point
The Formation of Glacial Ice
Glaciers can only form at latitudes or elevations above the snowline,
which is the elevation above which snow can form and remain present year
round. The snowline, at present, lies at sea level in polar latitudes and
rises up to 6000 m in tropical areas. Glaciers form in these areas if the
snow becomes compacted, forcing out the air between the snowflakes. As
compaction occurs, the weight of the overlying snow causes the snow to
recrystallize and increase its grain-size, until it increases its density
and becomes a solid block of ice.
Changes in Glacier Size
A glacier can change its size by Accumulation, which occurs by addition of
snowfall, compaction and recrystallization, and Ablation, the loss of mass
resulting from melting, usually at lower altitude, where temperatures may
rise above freezing point in summer. Thus, depending on the balance
between accumulation and ablation during a full season, the glacier can
grow or shrink
Movement of Glaciers
Glaciers move to lower elevations under the force of gravity by two
Internal Flow -
called creep, results from deformation of the ice crystal structure - the
crystals slide over each other like deck of cards. This type of movement
is the only type that occurs in polar glaciers, but it also occurs in
temperate glaciers. The upper portions of glaciers are brittle, when the
lower portion deforms by internal flow, the upper portions may fracture to
form large cracks called crevasses. Crevasses occur where the lower
portion of a glacier flows over sudden change in topography
Basal sliding -
meltwater at base of glacier reduces friction by lubricating the surface
and allowing the glacier to slide across its bed. Polar glaciers are
usually frozen to their bed and are thus too cold for this mechanism to
The velocity of glacial ice changes throughout the glacier. The velocity
is low next to the base of the glacier and where it is contact with valley
walls. The velocity increases toward the center and upper parts of the
is the modification of the land surface by the action of glaciers.
Galciations have occurred so recently in N. America and Europe, that
weathering, mass wasting, and stream erosion have not had time to alter
the landscape. Thus, evidence of glacial erosion and deposition are still
present. Since glaciers move, they can pick up and transport rocks and
thus erode. Since they transport material and can melt, they can also
deposit material. Glaciated landscapes are the result of both glacial
erosion and glacial deposition.
Small scale erosional features
Glacial striations - long parallel scratches and grooves that are
produced at the bottom of temperate glaciers by rocks embedded in the
ice scraping against the rock underlying the glacier (see figure 11.18
in your text).
Glacial polish - rock that has a smooth surface produced as a result
of fined grained material embedded in the glacier acting like
sandpaper on the underlying surface.
Landforms produced by mountain glaciers
Cirques - bowl shaped depressions that occur at the heads of
mountain glaciers that result form a combination of frost wedging,
glacial plucking, and abrasion. Sometimes small lakes, called tarns
occur in the bottom of cirque
Glacial Valleys - Valleys that once contained glacial ice
become eroded into a "U" shape in cross section. Stream erosion, on
the other hand, produces valleys that are "V" shaped in cross section
Hanging Valleys - When a glacier occupying a smaller
tributary valley meets the larger valley, the tributary glacier
usually does not have the ability to erode its base to the floor of
the main valley. Thus, when the glacial ice melts the floor of the
tributary valley hangs above the floor of the main valley and is
called a hanging valley. Waterfalls generally occur where the hanging
valley meets the main valley
Fjords - Fjords are narrow inlets along the seacoast that
were once occupied by a valley glacier, called a fjord glacier.
Landforms produced by Ice Caps and Ice Sheets
Abrasional features - The same small-scale abrasional features such
as striations and glacial polish can occur beneath ice caps and ice
sheets, particularly in temperate environments.
Streamlined forms - The land surface beneath a moving continental
ice sheet can be molded into smooth elongated forms called drumlins
Since glaciers are solid they can transport all sizes of sediment, from
huge house-sized boulders to fine-grained clay sized material. The glacier
can carry this material on its surface or embedded within it. Thus,
sediment transportation in a glacier is very much different than that in a
stream. Thus, sediments deposited directly from melting of a glacial can
range from very poorly sorted to better sorted, depending on how much
water transport takes place after the ice melts. All sediment deposited as
a result of glacial erosion is called Glacial Drift.
Ice Laid Deposits
Till - nonsorted glacial drift deposited directly from ice. Till
consists of a random mixture of different sized fragments of angular
rocks in a matrix of fine grained, sand- to clay-sized fragments that
were produced by abrasion within the glacier. This fine-grained
material is often called rock flour because it is really ground up
rock. A till that has undergone diagenesis and has turned into a rock
is called a tillite.
Erratics - a glacially deposited rock or fragment that now rests on
a surface made of different rock. Erratics are often found many
kilometers from their source, and by mapping the distribution pattern
of erratics geologists can often determine the flow directions of the
ice that carried them to their present locations.
Moraines - are deposits of till that have a form different from the
underlying bedrock. Depending on where it formed in relation to the
glacier moraines can be:
Ground Moraines - these are deposited beneath the glacier and result
in a hummocky topography with lots of enclosed small basins.
End Moraines and Terminal Moraines are deposited at the low
elevation end of a glacier as the ice retreats due to ablation
Lateral Moraines are deposits of till that were deposited along the
sides of mountain glaciers.
Medial Moraines - When two valley glaciers meet to form a larger
glacier, the rock debris along the sides of both glaciers merge to
form a medial moraine. These black streaks in an active glacier, as
well as the deposits left behind after the ice melts are called medial
Glacial Marine drift - Glaciers that reach the oceans or even lakes,
may calve off into large icebergs which then float on the water
surface until they melt. Upon melting, the rock debris that they
contain becomes immediately deposited on the sea floor or lakebed as
an unsorted chaotic deposit. Sometimes single large rock fragments
fall out on the floor of the water body, and these are called
Stratified Drift - Glacial drift can be picked up and moved by
meltwater streams which can then deposit that material as stratified
Outwash Plains - Streams running off the end of a melting glacier
are usually choked with sediment and form braided streams, which
deposit poorly sorted stratified sediment in an outwash plain. These
deposits are often referred to as outwash.
Outwash Terraces - If the outwash streams cut down into their
outwash deposits, the banks from river terraces called outwash
Kettle Lakes - If depressions form underneath a glacier and remain
after the glacier is melted then water filling these depressions
become small lakes where fine-grained sediment is deposited. The state
of Minnesota is called the land of a thousand lakes, most of which are
Kames and Kame Terraces. Streams and lakes forming on top of
stagnant ice may deposit stratified sediment on top of the glacier.
When the glacier melts these deposits are set down on the ground
surface. The former lake deposits become kames, and the former stream
deposits become kame terraces.
Eskers - Eskers are long sinuous ridges of sediment deposited by
streams than ran under or within a glacier. The sediment deposited by
these streams becomes an esker after the ice has melted.
The last glaciation ended about 10,000 years ago. But the period between
10,000 years ago and 3 my ago (Pleistocene epoch) was a time of many
glacial and interglacial ages.
During this period sea level fluctuated because:
during glaciations the continental land masses were depressed by
weight of ice.
during glacial periods much sea water was tied up in glaciers so sea
level was lower.
during interglacial periods sea level was higher due to melting of
during interglacial periods land that were covered with ice during a
glaciation are uplifted due to removal of the weight of the ice.
Based on evidence from glacial deposits and glacial erosion features
geologists have been able to document at least 4 glaciations during the
Pleistocene. But recent studies of deep-sea sediments and dating of these
deposits suggest that there were at least 30 glaciations that occurred
during the Pleistocene. This evidence comes from studies of fossils found
in deep-sea sediment cores, and what they tell us about ocean surface
temperatures in the past. The results come from studies of the isotopes of
Oxygen has two major isotopes, 18O, which is considered heavy, and
16O, which is considered light. Both of these isotopes are stable and
non-radiogenic, so their ratio is constant through time.
Because 16O is lighter, it is preferentially evaporated with sea
water from the oceans, and thus gets concentrated in the water that
eventually falls on the continents as rain or snow. Because of this,
18O gets concentrated in ocean water.
During constant climatic conditions the 16O lost to evaporation
returns to the oceans by rain and streams, so that the ratio of 18O to
16O (18O / 16O) is constant.
But, during a glaciation, some of the 16O gets tied up in glacial
ice and does not return to the oceans. Thus during glaciations the 18O
/ 16O ratio of sea water increases.
During an interglaciation, on the other hand, the 16O that was tied up in
glacial ice returns to the oceans causing a decrease in the 18O / 16O
ratio of seawater.
Thus, we expect that during glaciations the 18O / 16O ratio in seawater
will be high, and during interglaciations the 18O / 16O ratio in seawater
will be low.
Since organisms that live in the oceans extract Oxygen from seawater to
form their carbonate (CO3-2) shells, measuring the 18O / 16O ratio in the
shells of dead organisms gives a record of past ocean temperatures. The
record for the past two million years is shown below and in figure 11.30
on page 332 of your text. This suggests about 30 glaciations separated by
interglaciations during the past 2 million years.
During the last 1 million years it appears that each glacial -
interglacial cycle has lasted about 100,000 years, but earlier cycles were
about 40,000 years long.
Other periods of glaciation are known from the geologic record, mainly
from preserved glacial striations and tillites (consolidated till). The
earliest recognized glaciation occurred about 2.3 billion years ago, but
at least 50 other glaciations are recognized to have occurred during the
Causes of Glacial Ages
In order to understand what causes these cycles of glacial - interglacial
episodes we need a much better understanding of what causes global climate
changes. Because human history is so short compared to the time scales on
which global climate change occurs, we do not completely understand the
causes. However, we can suggest a few reasons why climates fluctuate.
Long term variations in climate (tens of millions of years) on a
single continent are likely caused by drifting continents. If a
continent drifts toward the equator, the climate will become warmer.
If the continent drifts toward the poles, glaciations can occur on
Short-term variations in climate are likely controlled by the amount
of solar radiation reaching the Earth. Among these are astronomical
factors and atmospheric factors.
Astronomical Factors -
Variation in the eccentricity of the Earth's orbit around the sun
has periods of about 400,0000 years and 100,000 years.
Variation in the tilt of the Earth's axis has a period of about
Variation in the way the Earth wobbles on its axis, called
precession, has a period of about 23,000 years.
The combined effects of these astronomical variations results in
periodicities similar to those observed for glacial - interglacial
- the composition of the Earth's atmosphere can be gleaned from air
bubbles trapped in ice in the polar ice sheets. Studying drill core
samples of such glacial ice and their contained air bubbles reveals the
During past glaciations, the amount of CO2 and methane, both greenhouse
gasses that tend to cause global warming, were lower than during
During past glaciations, the amount of dust in the atmosphere was higher
than during interglacial periods, thus more heat was likely reflected from
the Earth's atmosphere back into space.
The problem in unraveling what this means comes from not being able to
understand if low greenhouse gas concentration and high dust content in
the atmosphere caused the ice ages or if these conditions were caused by
the ice ages.
Changes in Oceanic Circulation
- small changes in ocean circulation can amplify small changes in
temperature variation produced by astronomical factors. Other factors
The energy output from the sun may fluctuate.
Large explosive volcanic eruptions can add significant quantities of
dust to the atmosphere reflecting solar radiation and resulting in