GLACIERS
TEACHER BACKGROUND
Mountain glaciers, the most common form of glaciers, are flowing ice masses
formed in regions where average annual snowfall exceeds average annual
snow melt. Glaciers can be as big as a continent, such as Antarctica, or a s
small as a mountain valley such as most of the glaciers in Alaska. Some
states, such as California, have glaciers so small Alaskans would probably call
them “glacierettes”.
At various times in the distant past, glaciers covered more than half the land in
Alaska creating or influencing many of the state’s hills, valleys, rivers, and
lakes. Today Alaska has more glaciers than any other state covering more than
26,000 square miles. Most of them are in Southeastern Alaska, Southcentral
Alaska, and the Alaskan Peninsula.
All glaciers are huge masses of ice that began as compacted snow. If the
climate is cold enough, some of the snow that falls each year does not melt.
Year after year, snow builds layer upon layer to become a permanent snow
field. Through evaporation, partial melting, and refreezing, the snowflakes
become tiny grains of ice. This process is aided by pressure from the mass of
overlying snow layers. As the process continues, air is squeezed out and the
lower layers of ice grains become solid ice. When the ice reaches a certain
thickness, its great mass causes the bottom layers to melt and begins the
slow flow of ice downhill.
Glaciers can both grow and recede. If more snow falls than melts, then the
glacier is said to be growing. If the glacier melts faster than the snow falls,
then the glacier is said to be receding. Most glaciers in the world are now
receding, although Alaska has a few that are advancing such as the Hubbard
glacier near Yakutat, the Taku glacier near Juneau, and the Harvard and Meir
glaciers in Prince William Sound. Some glaciers flow down to the sea such a s
the Taku glacier near Juneau. These are called tidewater glaciers and are
common in Southeast. Alaska is reported to have sixteen of the remaining
thirty tidewater glaciers in the world. Pieces of these glaciers break off and float
away as ice bergs. When this happens, the glacier is said to be calving.
Since the ice of a glacier is somewhat flexible, it tends to flow within the
confines of the valley. As the glacier moves in one direction or another, the ice
may crack and groan loudly as it travels. These large cracks are called
crevasses. Tremendous force is exerted by the mass of a glacier so that the
rocks and soil are ground beneath it as it slowly moves along a path of least
resistance.
Rocks trapped in the base and sides of the glacier scrape
bedrock, valley floors, and canyon walls. Where ice can force its way into
cracks, it plucks out large pieces of rocks. The erosion feature that forms at the
head of a glacier is called the cirque. This basin-like feature is developed by a
combination of abrasion and plucking.
Once a critical mass of ice
71
accumulates in the cirque, it moves downhill under the influence of gravity and
frequently follows a former stream course. The glacier gradually widens and
deepens the area until the valley becomes more U than V-shaped. A “river of
ice”, moving slowly down a valley, carries enormous amounts of rock and
debris along with it. When the debris is deposited and the glacier begins to
recede, the mound left behind is called the moraine.
Ice and surface snow melt can form streams which carry away many rock
particles gathered by the glacier, depositing the debris in other locations on the
out wash plain valley floor. Some glaciers have formed large green colored
rivers and lakes as seen on the Kenai Peninsula.
With our enormous amounts of glaciers, some of which are tidal, the state
draws many scientists to its ice fields to study geologic processes in action.
72
73
74
75
76
GLACIERS
TEACHER BACKGROUND
QUESTIONS AND ANSWERS
What is a glacier?
A glacier is a large mass of ice, snow, water, debris and rock. Glaciers form
from an accumulation of unmelted snow that has turned to ice over a period of
many years.
How do glaciers grow?
Glaciers grow when more snow builds up than melts or evaporates over many
years. A growing glacier is said to be advancing.
How does a glacier get smaller:
When less snow falls than melts or evaporates over a long period of time, the
glacier gets smaller (recedes).
How does a glacier move?
Glaciers move in two ways:
1. They slide on thick layers of water from melted ice within and beneath the
glacier.
2. Glaciers move downhill because of the ice mass pressing down and
forward with the force of gravity.
What happens when a glacier moves?
As a glacier moves forward, it overrides rocks which become frozen into the
base and sides. Rocks also are plucked from the valley and walls. These
sides and base act like giant files grinding, scratching and gouging bedrock.
These actions result in a more U-shaped valley.
Avalanche and rockfall debris on top, below, and within glaciers is carried
along, often traveling within the ice. As the glaciers melt, the debris i s
deposited in what is called a moraine.
What happens to water from a melted glacier?
Melting glaciers often feed lakes and rivers. Many lakes in Alaska and other
parts of North America were originally formed by glaciers.
There are glacial rivers and streams throughout Alaska. They carry large
amounts of clay and silt particles (glacial flour). There are such large amounts
of glacial flour coming from some glaciers, that the stream may look milky or it
may have a green, turquoise, or blue color. Sometime, depending on the
glacier and the time of year, the stream may look very muddy. If you can get a
container of stream water from a glacier and let sediments settle, you may be
surprised by the amount gathered.
77
Over time, glacial streams often change courses. Since running water follows
the path of least resistance, streams may split and intertwine to start new
channels. From the air or a mountain top, these streams look like they are
braided. Thus they are called “braided streams”.
78
Explore 6: The Great Glacier Race
Target Concept
• Glaciers erode rock and soil.
Targeted Standards
• SA-7
Addressed Standards
• SA-15
• SB-1
• MA-3
• MA-6
• ME-3
Assessment Task
Show how a glacier moves and how it can erode rock or soil.
Vocabulary
glacier
moraine
cirque
Background
This demonstration illustrates how glaciers move like liquid and not like a solid
block of ice. It also shows how gravity and friction cause the front and sides of
glaciers to flow downhill at different rates of speed. By participating in this
activity, students will realize that scientists use a similar method to study
glaciers.
Modeling clay and Prell brand shampoo are specified as components of “The
Great Glacier Race”. Prell has been field tested and works well. With a bit of
imagination, you can substitute some other slow-flow liquid, such as a
cornstarch mixture. Experiment for a recipe that flows slowly.
Materials per Every 4
• Metric ruler
• Measuring cup
• Metal spoon
• 1 red, 1 yellow, and 1 blue colored paper dots from a paper punch
• Timepiece to measure seconds
• Copies of Great Glacier Race, Making the Glacier, page 84
• Copies of Great Glacier Race, Getting Ready for the Race, page 85
• Copies of Great Glacier Race, The Race, page 86
• Modeling clay
• Prell Shampoo
• 16 toothpicks
• Red, yellow, blue, green markers to mark the toothpicks
Procedure
79
1. Divide the class into teams of 4. Pass out materials and distribute student
worksheets, pages 84-86.
2. Ask the students if they think glaciers move and if so, how?
3. Students follow the directions on page 84, “Making the Glacier”, to make the
rock base for the glacier out of modeling clay. The Prell shampoo is the
glacier. Students will then follow the directions found in “Getting Ready for
the Race”, page 85, to cut out hole punch dots and put them on the glacier.
These will be used to measure how the glacier moves down the valley.
4. Optional: Older students can measure the distance between each of the
toothpicks to determine where the glacier moved the fastest. During the
race place a red, yellow, and blue toothpick by its corresponding dot every
10 seconds. The toothpicks will record which part of the glacier moved the
fastest during each 10 second interval.
5. After the glacier is made and then prepared, students follow the sheet “The
Race”, page 86 to race their dots down the “valley”.
6. After everyone has had an opportunity to race their dots, compare and
discuss results.
Generalize
Ask the following questions:
1. How do you think this “glacier” moved?
2. How did the different “glaciers” around the room move? How were they the
same? How were they different?
3. How do you think this model might be like a real glacier?
4. How do you think this model may not be like a real glacier?
5. Why do you think it would be important to scientists to know how a glacier
moves?
Discuss the answers on the student pages.
Assess
Have students design, on paper, another way to model how glaciers move.
Adapted (with assistance from Jennifer Lezak) from Joe Kurtak, US Bureau of Mines
Geologist.
80
The Great Glacier Race Assessment Rubric
In Progress Towards Meets the Standard
the Standard
Science Content • Does not follow
• Follows
directions.
directions.
• Does not build or
• Builds and
demonstrate a
demonstrates a
glacial model.
glacial model.
• Does not model
• Models how a
how a glacier
glacier moves.
moves.
Communication •
of Science
Content
•
•
•
Does not tell how
a glacier erodes
rock.
Does not explain
how a glacier
moves.
Does not
distinguish how
the movement of
the model is like
and is not like a
real glacier.
Does not design
another way to
demonstrate how
a glacier moves.
•
•
•
•
Tells how a
glacier erodes
rock.
Explains how a
glacier moves.
Distinguishes
how the
movement of the
model is like and
is not like a real
glacier.
Designs another
way to
demonstrate how
a glacier moves.
Exceeds the Standard
In addition to Meets the
Standard:
• Work is neat.
• Experiments with
his/her glacier to find
out more
information.
• Discovers
something unique or
unusual while
conducting the
Glacier Race.
• Tests his/her new
idea on modeling
glacial movement.
In addition to Meets the
Standard:
• Explanations are
thorough and very
well thought out.
• Clearly understands
the difference
between a model
and a real glacier.
• Designs another
way to demonstrate
how a glacier
moves.
National Science Benchmark
Waves, wind, water, and ice shape and reshape the earth’s land surface by
eroding rock and soil in some areas and depositing them in other areas.
Change is something that happens to many things.
From Benchmarks for Literacy from the American Association for the Advancement of Science.
81
82
83
84