Unit 4: Glaciers Slides

Chapter 15
Topic 15.1
What is a Glacier?
Much of Earth’s fresh water is frozen in
valley glaciers or continental glaciers.
Glaciers form above the snowline from
compressed snow called firn. The snow line
elevation is high in areas close to the
equator and gets lower in areas farther from
the equator. Formation of glaciers requires
adequate snowfall and low temperatures.
The two main types of glaciers are valley
glaciers and continental ice sheets.
Topic 15.2
Glacial Movement and Erosion
Gravity causes glaciers to move. The ice front
of a glacier may advance or retreat
depending on conditions. If a glacier’s ice
front is near the sea, icebergs may break off.
As glaciers move, they erode the underlying
surface, leaving behind characteristic
erosional features. Glaciers move by basal
slip and internal plastic flow. Glaciers erode
the valleys through which they flow,
producing characteristic landforms.
Topic 15.3
Glacial Deposits
When glaciers retreat, the leave
behind deposits. Till is unsorted,
unstratified material deposited directly
by ice. Outwash is sorted, stratified
material deposited by meltwater from
the ice. When glaciers melt, they
deposit their sediments, creating
features that can be readily
identifiable. Glaciers may form lake
basins by eroding the land or by
depositing sediments.
Topic 15.4
Ice Ages
Earth has experienced a number of
periods of widespread glaciation.
Evidence for such ice ages comes from
erosional and depositional features.
Scientists have identified a number of
possible causes for glaciation. During an
ice age, glacial advances alternate with
glacial retreats. The Milankovitch theory is
the most commonly accepted theory of
the cause of the ice ages.
Key Terms
continental glacier
valley glacier
glacial valley
ice age
internal plastic flow
Milankovitch theory
outwash plain
Topic 15.1
What is a Glacier?
About 75% of Earth’s
freshwater is frozen
as glaciers. Natural
forces compact
snow to make
enormous masses of
moving ice.
A large mass of ice and
snow that exists yearround and moves under
the influence of gravity.
They have carved some of the most
spectacular features on the surface of
the earth. They form in areas that are
always covered by snow. Layer after
Snow line
The lowest elevation
that permanent snow
reaches in summer.
 The elevation of the snowline varies from
place to place. At the equator, it is 5,500
m above sea level. Near the poles, it is at
sea level. Snowfields are formed by the
accumulation of snow and ice above the
snowline. They cover most of the land in
the polar regions and the tops of some
mountains at lower latitudes.
Partially compacted
and refrozen snow
which has yet to
become a glacier.
In the deepest layers of
accumulating snow, the pressure
of the overlying ice becomes so
great that the ice grains flatten
and the air between the firn is
squeezed out. It loses its white
color and becomes a bight steelblue color. When the mass
becomes large enough, it begins
to move downslope due to
gravity. Where are temperatures
are low and snowfall is high,
glacial formation is greatest.
There are two main
types of glaciers:
1. Valley
2. Continental
Valley glacier
A long, slow moving,
wedge shaped
glacier that moves
within valley walls.
They develop in high mountain
regions and flow down slope due
to the force of gravity. They are
also known as alpine glaciers,
after the Alps in south-central
Europe. They vary in size.
continental glacier
A large sheet of ice
that covers a large
part of a continent.
Today, they are found only in Greenland
and Antarctica. The Antarctic ice sheet is
1 ½ times the size of the United States. In
some places it is more than 4,000 m thick.
90% of Greenland is covered by its ice
sheet. It is about the size of Mexico and is
3,000 m thick. Scientists estimate that if
melted, the ice would release enough
water to raise sea level by 60 m if melted.
Read Textbook pages 318 - 320.
Complete Regents Questions
Topic 15.2
Glacial Movement and Erosion
Glaciers are sometimes called
rivers of ice. Continental
glaciers may move lass than a
centimeter a day, while valley
glaciers can move several
meters in a day. As they move,
they erode and deposit earth
materials. However, they move
very differently than water does.
Examine the
seasonal migration
of snow cover.
They move in two ways:
1. Basal slip
2. Internal plastic
Basal slip
Movement at the
base of the glacier.
 The water from the melted ice acts like
a lubricant between the ice and the
ground. This allows the ice to slip on a
thin layer of water. The ice can push
against a rock causing some of the ice
to melt, letting it flow around the
barrier. In extremely cold places, basal
slip does not occur.
internal plastic flow
Grains of ice deform, or
permanently change shape
and slip past each other to
create forward movement.
This movement is usually out
from the glacier’s center.
 The speed of internal plastic flow is not the same at all parts
of the glacier. Speed is determined by the slope and
thickness of the ice. It is faster near the surface than at the
base. Friction caused by contact with the rock below
causes the speed to slow. The center also moves faster than
the sides.
Because of the pressure from
above, the inside of the
glacier moves by internal
plastic flow. The base moves
due to slip, while the surface
remains brittle. This tension
and compression causes
cracks and buckles.
A great fissure or
crack in a glacier.
They can be more than 30
m deep
Ice sheets move outward
from the center in all
directions. It can even
move out over the ocean
forming an ice shelf. The rise
and fall of tides causes large
blocks to break off. These
chunks are called icebergs.
Remember that only the top
third of an iceberg is above
the surface.
Most glaciers extend
below the snowline. As a
glacier moves downslope, ice and snow melt
away due to warmer
temperatures causing it to
thin and melt. It tends to
be the thinnest where it
melts and moves the
Ice front
The end of a glacier.
 As long as the rate of movement and
melt are equal, the ice front is
stationary. Where the snowline is close
to sea level, glaciers reach the sea. As
they extend out over the ocean, great
blocks of ice break off and fall into the
sea. This process is called calving. In
Antarctica, the snowline is at sea level.
Blocks of ice the size of Connecticut
are not uncommon.
Glaciers are powerful agents of
erosion. As they move, they
carve out the terrain over which
the move. The glacial processes
begin at the upper end of the
valley where a valley glacier
forms. As it wedges its way
through a valley, it breaks off
rock making the valley steeper.
Particles can be as small as a fine
powder or as large as a house.
The particles can ride on top of
the glacier or can be dragged
along the bottom of the glacier.
Observe how glaciers
erode bedrock
Unsorted and
unstratified rock
material deposited
dirtectly by glacial
Landforms made from
glacial till are called
A deposit of till left
behind when a
glacier retreats.
The soil of a ground moraine
is very rocky. Long Island is a
terminal moraine. They are
ridges of unsorted rock
material on the ground or on
the glacier.
Streams flowing from
melted ice are called
meltwater. Melting
occurs in a glacier during
the summer months.
Streams of meltwater
flow from the edges, the
surface, and beneath
the glacier. Because of
the presence of small
rock particles, the
meltwater often looks
like milk.
Glaciers weather
and erode
bedrock when they
drag rocks with
them, cutting and
scraping. Smaller
particles smooth
and polish.
Long, parallel
scratches left on rocks
and bedrock by
glacial movement.
They show the direction of
movement of the ice, often
creating long parallel grooves in
the landscape.
A semicircular basin
found at the head
of a glacial valley
formed by a valley
Sharp jagged ridges form
between the cirques.
A sharp divide that
separates adjoining
When several arêtes are
joined together they form a
pyramid-like peak called a
Solid rock over which a glacier
moves polishes it by scraping
the surface with fragments
picked up as the glacier moves.
The scraping action picks up
even more rock fragments.
They will eventually be rounded
smooth. The edge of the rock
facing the direction from the
which the glacier came will be
rounded and smooth. The back
side will be jagged and steep.
roche moutonnée
In French this means
“sheep rock,”
because the hill looks
like the back of a
Valley before the glacier passes
Valley after the glacier passes
Small tributary glaciers will flow into a
main valley glacier.
hanging valley
The landscape created
by a massive continental
ice sheet is very different
from the sharp rugged
features eroded by a
valley glacier.
Continental ice sheets
flatten mountains
producing relatively
smooth, rounded
 Existing valleys may be gouged out or
deepened. Rock surfaces are
smoothed and rounded. Rock
surfaces are gouged. The scratches or
grooves run parallel to the direction of
glacial movement.
Read Textbook pages
321 - 329.
Complete Regents
Topic 15.3
Glacial Deposits
A valley glacier melts when
the glacier moves to a lower
elevation or when climate
changes. When the glacier
melts, all of the material
accumulated in the ice is
deposited. Glacial deposits
are easy to spot.
When a
melts, its
in nearly
place as
in the
The composition of an
erratic is different from
the bedrock they sit on.
The long axis of the drumlins are parallel to
the direction of glacial movement.
Outwash plain
plain will
by many
A bowl-like hollow found in deposits of
glacial outwash; formed when a large
block of ice was left behind when the
glacier melted.
A small, coneshaped hill of
stratified sand
and gravel
formed at a
glacial front by
sediment off
the glacier’s
Many extend hundreds of
kilometers and look like
raised winding roadways.
A long,
winding ridge
formed when
sand and
gravel fill
beneath a
Glaciers often
form lake
basins by
eroding out
surface areas,
depressions in
the bedrock
existing valleys.
Thousands of
lakes were
formed in this
Topic 15.4
Ice Ages
An ice age is also referred to as a
glacial period. The time
between glacial periods is known
as an interglacial period. Several
major glacial periods have
occurred throughout our
geologic history. The earliest was
600 million years ago. The most
recent one was 114,000 years
ago. The last advance of the last
ice sheet started to retreat only
11,000 years ago.
Ice ages begin with a longterm drop in the earth’s
average temperature. An
average temperature drop
of only 5º, and an increase in
snowfall sets the stage for an
ice age. Ice sheets advance
during periods of cold, and
retreat during warm periods.
During the last glacial period,
glaciers covered 1/3 of the
earth’s surface. Most
glaciation was located in
North America and Eurasia.
In some parts of North
America it was several
kilometers thick. So much
water was locked up in ice
that sea level was 140 m
lower than it is today.
Observe the retreat
of ice sheets from
North America.
Many theories have been
proposed to account for ice
ages. Gradual cooling and
increased precipitation
brought on the expansion of
glaciers. The theories also
explain why they retreated.
Milankovitch theory
This is the theory most
widely accepted by
scientists. According to
Milankovitch, three kinds
of periodic changes
occur in the way the
earth moves around the
1. The shape of the orbit. It
varies from nearly circular to
elongated and back to circular
every 100,000 years.
2. The earth tilts on its axis. Over
a period of 41,000 years, the tilt
ranges from 21.5 to 24.5 degrees.
3. Circular motion, or precession,
of the earth’s axis. The axis
traces a complete circle every
26,000 years.
Observe changes in
Earth's orbit that
contribute to climate
Other factors include:
1. Changes in atmospheric
carbon dioxide levels.
2. Changes in the positions
of Earth’s landmasses.
3. Changes in the amount
of solar energy reaching
Earth’s surface.
Evidence from the ocean
floor, in the form of shells,
supports this theory. The
ratio of two isotopes of
oxygen in the sea water
can show if a sediment was
laid down during an ice
There are other
theories, however,
most of them state
that the ice ages
resulted from a
change in the
amount of solar
energy reaching the
earth’s surface.
Another theory
states that tectonic
plate motion
prevents warm
water from reaching
the poles.
Continental glaciers cause
enormous changes to the Earth’s
surface. In North America, the
most recent ice age gouged out
the Great Lakes, the St. Lawrence
River valley, and the Finger Lakes
and Lake Champlain in New York
State. More than 1 million years
ago, where the lakes now lie there
were river valleys that flowed
through easily eroded sandstone
and shales. The ice widened and
deepened them. Around 14,000
years ago, the ice started to melt.
The weight of three kilometers of ice
depressed the surface of the land
about a kilometer, causing the land to
tilt to the north. Water pooled in front
of the ice and eventually drained to
the south into the Mississippi River. As
the ice sheet continued to melt, the
water levels rose forming the Mohawk,
Hudson and Susquehanna rivers.
About 7,000 years ago, the ice sheet
melted past Lakes Erie, Ontario, and
Michigan. As the ice retreated, the
land rebounded, forcing the water
flow to change direction, creating
Niagara Falls and causing the water to
drain through the St. Lawrence River.
Read Textbook pages 330
- 333.
Complete Regents
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