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CHAPTER 17
Class
Date
Glaciers
1 Glaciers: Moving Ice
SECTION
KEY IDEAS
As you read this section, keep these questions in mind:
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•
•
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How do glaciers form?
How are the two glacier types alike and different?
How do glaciers move?
What are the features of glaciers?
How Do Glaciers Form?
A glacier is a large, compact mass of moving ice.
Natural forces compact snow into glaciers in a similar
way to how you make a snowball by squeezing snow.
Glaciers are found at high elevations and in polar
regions. In these areas, snow stays on the ground all year
long. Ice and snow build up and form a permanent mass
called a snowfield.
At high elevations,
snowfields can
be found above a
certain elevation
known as the
snowline.
Temperatures in these areas remain near or below
0 °C. The low temperatures generally cause snow to build
up every year instead of melting. Parts of the snowfield
do occasionally melt and then refreeze. Over time, this
process changes the snow into grainy ice called firn.
In deep layers of snow and firn, the top layers press
down and flatten the ice beneath. The pressure of the
top layers also squeezes air from between the ice grains.
The snow and firn continue to build up, forming a glacier.
As the glacier becomes bigger and heavier, it eventually
begins to move downhill under its own weight.
A glacier’s size depends on how much snow builds
up and how much ice is lost through melting. When new
snow piles up faster than it melts, the glacier gets bigger.
When snow and ice melt more quickly than new snow is
added, the glacier shrinks.
READING TOOLBOX
Summarize As you read this
section, highlight sentences
that relate to the Key Ideas
questions. When you finish
reading, write a short answer
to each Key Ideas question
using the underlined
information.
LOOKING CLOSER
1. Explain How can you tell
that the snow and ice in the
picture is not simply due to
cold weather?
READING CHECK
2. Describe How does firn
form?
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Name
SECTION 1
Class
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Glaciers: Moving Ice continued
ALPINE AND CONTINENTAL GLACIERS
READING CHECK
3. Infer Why do you think
today’s continental glaciers
exist only in Greenland and
Antarctica?
There are two types of glaciers: alpine and continental.
An alpine glacier is a narrow, wedge-shaped mass of
ice. Alpine glaciers form in mountainous areas. The
mountains around them limit them to a small area.
In contrast, a continental glacier is a huge sheet
of ice that is not confined by surrounding landforms.
Continental glaciers, or ice sheets, can cover millions
of square kilometers and can be thousands of meters
thick. Today, continental glaciers can be found only in
Greenland and Antarctica. If these glaciers melted, they
would raise the worldwide sea level by more than 50
meters.
A Comparison of Alpine and Continental Glaciers
Alpine Glaciers
Continental Glaciers
• are narrow and wedge shaped
• are found in mountainous regions
• are confined to a small area by
surrounding topography
• are broad and may cover millions of
square kilometers
• are found only in Greenland and
Antarctica
• are not limited by surrounding
topography
How Do Glaciers Move?
Both rivers and glaciers flow downward due to the
force of gravity. In fact, glaciers are sometimes called
“rivers of ice.” However, rivers and glaciers move in
different ways. Unlike the water in a river, the ice in a
glacier moves very slowly. In a single year, some glaciers
may travel only a few centimeters. The ice in glaciers
also cannot flow easily around barriers. Glaciers move
through two basic processes: basal slip and internal
plastic flow.
READING CHECK
4. Explain In the process of
basal slip, why does the ice
beneath the glacier melt?
BASAL SLIP
Many glaciers move through the process of basal slip.
Basal slip happens because the weight of a glacier exerts
pressure that lowers the melting point of the ice. The ice
melts where the glacier touches the ground.
The water mixes with sediment beneath the glacier.
This mixture acts as a lubricant between the ice and
the ground, making the glacier slide forward. Glaciers
can move by sliding over this thin layer of water and
sediment.
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Glaciers: Moving Ice continued
INTERNAL PLASTIC FLOW
Glaciers can also move by a process called internal
plastic flow. In this process, pressure deforms the ice
grains under a glacier. The grains then slide over each
other, causing the glacier to flow slowly.
The rate of flow varies in different parts of the
glacier. The slope of the ground and the thickness and
temperature of the ice determine the flow rate. The edges
of a glacier move more slowly than the center because
of friction with the rock beneath. For the same reason,
the top layers of the glacier flow more quickly than the
bottom layers.
READING CHECK
5. Identify In the process of
internal plastic flow, which
parts of the glacier will move
most quickly?
Starting position
End position
LOOKING CLOSER
Measurement stakes driven into an alpine glacier move as the glacier flows. Their
start and end positions show how the central part of the glacier moves more
quickly than the edges.
6. Explain Based on the
figure, how can you tell that
the center of the glacier has
moved more quickly than the
edges?
What Are the Main Features of Glaciers?
Although a glacier flows, low pressure on the glacier’s
surface ice causes the ice to remain brittle. The glacier
flows unevenly beneath the surface. As tension builds,
large cracks, called crevasses, can form on the surface.
A continental glacier moves outward in all directions
from its center. Certain parts of the glacier may
eventually stretch out over the ocean and form ice
shelves. Sometimes, large blocks of ice called icebergs
break off from the ice shelves. The icebergs then drift
into the ocean. Most of an iceberg is below the surface of
the water and is therefore not visible from the surface.
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Section 1 Review
SECTION VOCABULARY
alpine glacier a narrow, wedge-shaped mass
of ice that forms in a mountainous region and
that is confined to a small area by surrounding
topography
basal slip the process that causes the ice at the
base of a glacier to melt and the glacier to
slide
continental glacier a massive sheet of ice that
may cover millions of square kilometers, that
may be thousands of meters thick, and that is
not confined by surrounding topography
crevasse in a glacier, a large crack or fissure that
results from ice movement
glacier a large mass of moving ice
internal plastic flow the process by which
glaciers flow slowly as grains of ice deform
under pressure and slide over each other
1. Describe In your own words, describe how crevasses form.
2. Infer Glaciers shrink and grow depending on how much snow and ice they lose
or gain. Scientists study glaciers for evidence of climate change. Why might
glaciers be good indicators of climate change?
3. Explain Why are glaciers called “rivers of ice”?
4. Compare What is one significant difference between basal slip and internal plastic
flow?
5. Explain What is the relationship between an ice sheet, an ice shelf, and an
iceberg?
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