Chapter 15-1 Glaciers
Pages 318-337
What is a Glacier?
• ¾ of earth’s fresh water is frozen in
glaciers.
• A Glacier is –
• A large mass of compacted snow and ice
that moves under the force of gravity
Where glaciers form
• Form in areas always covered
by snow
• Snow builds up from year to
year
• Glaciers may occur in any part
of the world
– Closer to the equator, the
higher the elevation
• The lowest elevation at which
a layer of permanent snow
occurs in summer is called the
SNOW LINE
How Glaciers form
• Mountain basins above the snow line are
filled with snow year round.
• In these snowfields, buried snow becomes
compressed and recrystallizes into a
rough, granular material called FIRN or
NEVE.
Firn
• Resembles the ice of a packed snowball
• Weight of the firn may turn the lower
layers into solid ice. This ice may begin to
move outward or downward due to the
overlying weight of the firn above – this
now is a glacier.
Types of glaciers
• Valley glacier
– Moves within valley
walls
• Continental glacier
– Covers a large portion
of a continent
Valley glaciers
• Forms in the upper valleys of the world’s
mountain ranges where snowfall exceeds
snowmelt
• Under the snow, the firn changes into ice
• The ice stays in the valley and slowly
begins to move downhill due to gravity
• Valley glaciers are
also known as
alpine glaciers
• Valley glaciers
occur on all
continents except
for Australia
• Valley glaciers may very
small (2km) to very
large (100km) and
hundreds of meters
thick
– Large valley glaciers
are found in Alaska
and Himalayas
– Glacier National Park
has many remnant
valley glaciers
Continental Glaciers
• In polar regions, large areas are covered
in thick masses of ice
– Only mountain peaks are visible called
nunataks
• Are circular or oval in shape
– Greenland = 1.7 million km2 and 3 km thick
– Antartica = 13.7 million km2 and 4 km thick
• Largest C. G. is found in Antartica
• A glacier less than
50,000 km2 is
sometimes called an
icecap
– Iceland, Baffin Island,
Spitsbergen
Ice cap in 1979 and 2003
Glacial Movement and
Erosion
Ch 15-2
Pages 321-325
How do glaciers move?
• Some glaciers move a few centimeters a
day, while some move several meters in a
week or month?
• Move faster after snowy winter, in summer
and on steep slopes
• Friction differences exist between valley
floor and valley walls – why?
• A typical glacier will move more rapidly at
its surface and center than at its sides.
• The heavy weight of the ice above can
melt the ice at the base.
– This water or slushy layer reduces the friction
and may increase the movement of the
glacier. This is called basal slip.
• Basal slip occurs at the base of the
glacier, but does not account for the rest of
the glacier’s movement.
• Basal slip does not occur in extreme cold
Plastic flow
• In the interior of a glacier, the grains of ice
may deform, or change shape, due to the
pressure of the snow and ice above.
• Near the bottom of the glacier, the grains
are almost flat and can allow grains to slip
past one another to create forward
movement.
crevasses
• When glaciers move
over a steep
downward change of
slope, the ice is rigid
and crevasses or
deep surface cracks
form.
• Are rarely larger than
50 meters deep
How do we study glacial
movement?
• Scientists will study glacial movement by
placing a row of stakes across a glacier to
study its movement.
• Also use satellite data to track glacial
movement and positioning over time.
Ice front
• The ice front occurs where the glacier’s ice
melts as fast as it moves.
• A glacier continually is moving forward due
to gravity
Icebergs
• Where the snow line is close to sea level,
glaciers meet the sea.
• As the glacier moves into the sea, large
blocks of ice break off - this is called
calving
• Occurs in Antartica, northern Alaska,
Siberia
How Glaciers Cause Erosion
• Glaciers are very powerful agents of
erosion.
• Remove loose rock from valley walls (from
fine powder to house-size boulders) and
transport this material until it is deposited.
• Some material may fall on top of glacier,
become apart of the glacier, or be dragged
beneath the glacier.
• When this unsorted
and unstratified rock
material is deposited
by a retreating, or
melting glacier, it is
called TILL
• The accumulation of
till is called a
MORAINE
• With Valley glaciers,
material may be
deposited on the sides of
the glacier – forming a
lateral moraine
• If two valley glaciers meet
to form a single glacier,
the lateral moraines may
combine to form a medial
(middle) moraine.
• When clay and silt particles are mixed by a
glacier by the crushing of rock below the
glacier, it forms ROCK FLOUR
• When this rock flour mixes with water, it
takes on a milky white appearance and
this meltwater is called GLACIAL MILK
• When rocks pieces
frozen into the glacier
cut away at existing
bedrock on the valley
floor and walls, it may
leave behind long
parallel scratches
called STRIATIONS
– Striations show
the general
direction of the
glacier
• Glaciers may also
remove large sections of
softer bedrock material
from the valley walls
through process called
PLUCKING
– This is caused by
melting and refreezing
if the glacier along this
material, and it tends
to pluck the softer
bedrock from the
valley wall.
Effects of Erosion
•
Glaciers may leave
behind
characteristic
formations:
1. Roches Moutonnees
– is an outcropping
of bedrock that looks
like a flock of sheep
from a distance
2. Valley glaciers often
erode a river valley
from a typical Vshaped valley into
the classic Ushaped glacial
valley.
• 3. Main valley
glaciers are larger
than their tributary
glaciers, and erode
the valley more as
well.
– Forms a deeper ushaped main glacial
valley than the
tributary valleys, this is
called HANGING
VALLEYS
• When a stream is
present in the
tributary valley, and it
now falls into the
deeper main valley,
this is called a
HANGING
VALLEY
WATERFALL
• At the head of a former valley
glacier is a semi-circular
basin called a CIRQUE
• THIS CIRQUE MAY FILL UP WITH
WATER AND FORM A SMALL
LAKE CALLED A CIRQUE
LAKE
• When two cirques are
formed next to each
other, the divide
between the two may
be narrow and sharp.
This divide is called
an ARETE
• If three or more
cirques meet, they will
form a pyramidshaped peak called a
HORN
• Valley glaciers tend
to make mountain
peaks more jagged
and pronounced,
while continental
glaciers tend to
smooth off mountain
peaks and leave them
polished and
rounded.