Root Glacier. Photo by the Alaska Geographic Society.

Glacier Power – What is a Glacier?

A Glacier Begins with Small Snowflakes

Glaciers are massive and incredibly powerful but they begin with small snowflakes. 

Each lacy, delicate crystal flake is unlike any other; imagine how many it takes to make a glacier as snow gradually changes into glacier ice.

What is a Glacier? A glacier is a huge mass of many years of snow, ice, rock, sediment, and water. It originates on land and moves down slope under the influence of its own weight and gravity.

Each glacier is different in its own special way and each glacier has a different surrounding environment.

Types of Glaciers (By Where They Are)

Ice Sheets, Shelves, and Caps

Ice Sheets

  •  Found in Antarctica and Greenland.
  •  Large masses of ice which cover more than 50,000 square kilometers!
  •  Some ice sheets that flow into the sea have ice streams.

Ice Shelves are slabs of glacier ice which float on the sea. Glaciers that discharge into the sea from higher ground become detached from the bed and float, spreading out to cover large bays.

Ice Caps

  • They are smaller versions of Ice Sheets which cover less than 50,000 square kilometers. A large ice cap could cover an area as large as the entire Seward Peninsula on Alaska’s west coast!
  • Ice accumulates on a high area in the middle of an ice cap and spills down on all sides.
The Ross Ice Shelf in Antarctica. Illustration by A. Stubsjoen.

Mountain Glaciers

Cirques

  • Are bowls or basins carved out of a mountain.
  • Are very short in relation to their width.

Valley Glaciers

  • Are found in high altitudes.
  • May flow downwards from cirques, ice caps, highland ice fields, or ice sheets.
  • Are shaped like long tongues of ice.
  • Bering and Hubbard Glaciers are valley glaciers and are the longest in the Americas (200 and 150 kilometers long).

Tidewater Glaciers

  • Are valley glaciers that enter the sea. At the water, they either remain grounded or float.

Piedmont Glaciers

  • Formed where mountain valleys open into larger valleys or onto plains.
  • Valley glaciers that spread out into wide lobes.

Ice Aprons or Hanging Glaciers

  • Form where mountain slopes seem almost too steep to hold any snow
  • Often make avalanches.

Types of Glaciers (By Temperature)

Warm (or Temperate) Glaciers

  • Ice is at the melting point throughout.
  • Meltwater is abundant in summer and continues to flow throughout the winter.
  • A well-developed drainage network exists, including moulins, ice lenses, and water pockets.
  • Form in most mountain regions outside the Arctic and Antarctic.
  • The glacier is eroded.

Cold Glaciers

  • Most of the ice is at temperatures below freezing.
  • Are frozen to the bed and erode very little.
  • Are found in the polar and sub-polar regions (sub-polar glaciers melt at the surface in summer).
Moulin Worn Down into the Glacier. Photo by Hambrey.

Beginnings

Remember, even the largest glacier begins with a small snowflake! Here is an example of a snowflake crystal. It has been greatly magnified and is a scanning electron micrograph of the snow crystal. Next to it is what happens to the snow crystals when they begin the process of compression and are on their way to becoming glacier ice. These other crystals are also scanning electron micrographs of firn crystals.

Scanning Electron Micrograph of a Snow Crystal. W.P. Wergin & E.F. Erbe, ARS, U.S.D.A.
Scanning Electron Micrograph of Firn Crystals. W.P. Wergin & E.F. Erbe, ARS, U.S.D.A.

What is a Glacier?

Vocabulary Plus!

glacier
ice sheet
ice cap
cirque
valley glacier
tidewater glacier
piedmont glacier
hanging glacier
warm (temperate) glacier 
cold glacier
Antarctica
Greenland
Highland Ice Field 
Synthetic Aperture Radar

Review Questions
(some of the answers may come from the vocabulary list)

  1. How does the glacier move down slope?
  2. Are all glaciers the same?
  3. Where are the world’s largest ice sheets found?
  4. Do ice shelves float?
  5. Which are bigger, ice shelves or ice caps?
  6. From what sources might you see a valley glacier flowing?
  7. What does SAR stand for?
  8. In what season is meltwater abundant, summer or winter?
  9. Do cold glaciers erode a lot?

Brain Challenge!
When rivers freeze, do they turn into glaciers?

Project: Snowball Melt and Freeze
Let’s see how a glacier is made! 
You’ll need:

  • A small mound of snow from outside (if available)
  • OR 10 to 12 ice cubes, finely crushed
  • OR scraped ice from the sides of your freezer at home
  • A small handful of dirt
  • A few pebbles

Now, mix the snow with the dirt and pebbles, then form into a tight ball. Put in the freezer. Once the ball’s frozen hard, bring it back out into room temperature. Let it rest until it just starts to melt. Pack tightly again with your hands. Return to freezer, and remove again once it’s hard.

Repeat the steps two or three times. Notice that each time you pack the snowball, it gets tighter and more compact. That means there are fewer air bubbles in the ice because of the pressure applied by your hands. This is similar to how a glacier is made. The weight of snow and ice on a glacier presses the snow tight over time, removing many air bubbles.

(Courtesy of Glaciers of North America, by S. Ferguson)

Glacier Power – Where Have Glaciers Been?

Find your clues!

There are clues which tell us if a glacier has been over certain landscapes.

Think about it!

Imagine a landscape of mountains, trees, and wildflowers. Up in the mountains a glacier has been growing for some time and now begins to creep and flow over the land. What do you think happens?

Here’s a hint!

Imagine a bulldozer going over mountains, trees, and flowers. The bulldozer would definitely leave a mark and probably tear out the trees. Think of a glacier as a natural bulldozer.

Glaciers are Sculptors!

Glaciers sculpt and carve away the land, transport material, and create glacial landforms. A landscape can be dramatically re-shaped from a glacier’s passing. When glaciers carve and sculpt they are eroding the landscape. Eroding means to move dirt, rock, or other material from the ground. Boulders, broken rocks, and debris can be carried in and on the glacier ice and deposited far from their original locations. Sometimes the debris is even pushed ahead of a glacier and then left behind in mounds, or, rocks found at the end of a glacier may have come from the beginning.

Glacial landforms are clues to let us know where glaciers have been.

Glacier Landforms

Glaciated Valleys

Valleys with a U-shape, often with steep vertical cliffs. Sometimes entire mountains have been removed to create a U-valley.

Fjords

Long, narrow coastal valleys with steep sides and rounded bottoms. They were originally carved below sea level by their glaciers. After the glaciers left, sea water entered and covered the valley floors.

Cirques

Steep-sided basin-shaped depressions on a mountainside, carved out by a glacier.

Aretes

Sharp, narrow ridges formed by a glacier on a mountain.

Drumlins

Smooth rounded mounds of glacial till (rock, dirt, and debris) deposited under a glacier.

A Cirque Glacier. Photo by Hambrey.

Kames

Steep-sided hills of sand and gravel deposited by glacial streams or in crevasses.

Horns

Steep-sided peaks, shaped like pyramids, formed when cirque glaciers erode on three or more sides of a mountain.

Tarn

A small lake filling a hollow which was eroded out by ice or dammed by a moraine. Frequently found with cirques.

Two Processes that Create Glacier Landforms

Erosion

Process by which material is worn away from the Earth’s surface.
fjords
glaciated valleys
horns
aretes
cirques
tarns

Deposition

The laying down of matter by a natural process.
till
moraines
kettles
boulders
drumlins
kames

A Kettle is the result of a very large block of ice being left behind as a glacier recedes. The melting forms potholes which are sometimes filled with water in a glacier, till, or outwash plain. Vegetation may grow up around kettles.

A Kettle. Photo by James Roush.
Vegetation growing around a kettle filled with water. Photo by McMillan.

What about Valley Shapes?

If a glacier makes a U-shaped valley when it flows over the landscape, then what process makes a V-shaped valley?

Rivers! A river transports dirt and material (sediment). If a river has more energy than it needs to transport its load of sediment, it will use that energy to cut downward, eventually creating a V-shaped valley.

A river will follow the path of least resistance. A glacier will force its way through almost anything. Rivers and glaciers each transform the landscape, but in different ways. Look at the three pictures. What differences do you see?

As you might guess from its name, a V-shaped valley has different slopes than a U-shaped valley. A U-shaped valley has gentle, over-steepened slopes. Also, stream or river-cut V-shaped valleys may meander and curve, while glacier U-shaped valleys are straighter in their course.

Something to think about:

Glaciers are made of ice and rivers are made of water. Think of a solid then a liquid. At home, pour some water over an old rock, then rub a piece of ice over the rock with some pressure. What differences do you think there will be between the two processes?

Why do we want clues to find out where glaciers have been?

What we see today gives us clues of what happened in the past. For two million years, North America, Europe, and Asia were covered in ice sheets. Glaciers were everywhere. As the temperatures got slightly warmer, these glaciers began to melt.

World Detectives

Now, we only see glaciers where the temperature and precipitation is just right. As we learn more and more about glaciers, we realize that glaciers leave clues about where they’ve been. Geologists and glaciologists are world detectives who put pieces of the big Ice Age puzzle together to find out what happened in the past. The puzzle pieces also help determine what may happen in the future. Cool!

Glaciologists learn to spot clues glaciers leave behind. They can tell if the glacier has been in the area.

Has a glacier been where you live?

CLUES:

Scraped Rock
Silt
Valley with a U-Shape
Huge Boulders
Cirques
Fjords

An Erratic Boulder. Illustration by Amy Stubsjoen.

The image above is an example of a glacier clue! It’s a 60-ton boulder left behind by a glacier. Many such large surface boulders are scattered across the upper Midwestern United States and in other areas where glaciers have been. These boulders are called erratics which is derived from a Latin word which means, “to wander.” The boulders have “wandered” from their original place.

Glacier Detectives – Remember:

Check to see if a valley is U-shaped or V-shaped because that will be a special clue as to “where a glacier has been.” Be a detective with us and discover the clues glaciers leave behind! Here’s a warm-up for detective work you can do later on!

A Glacier Power Warm-up!

There is a U-shaped valley and huge boulders are lying all over the landscape. How did those huge boulders get there? Well, I bet a bulldozer could plow them down the landscape. Remember, a glacier is similar to a bulldozer so a glacier must have transported those boulders.

Where have Glaciers Been?

Vocabulary Plus!
U-shaped valleys
V-shaped valleys
erosion
deposition
horns
kames
drumlins
aretes
cirques
fjords
glaciated valleys

Review Questions

(some of the answers may come from the vocabulary list)

  1. What three continents were covered in ice sheets for 2 million years?
  2. What made the glaciers melt two million years ago?
  3. Why do geologists and glaciologists study the Ice Age?
  4. Do glaciers leave clues behind? Yes or No?
  5. Name two clues that glaciers leave.
  6. What two things have to be just right for a glacier to exist?
  7. What does deposition mean?
  8. What does erosion mean?
  9. What is the difference between a U-shaped valley and a V-shaped valley?

Brain Challenge

Have glaciers been around where you live? How can you tell?

Exercise: Connect the Related Words

U-Shaped Valley

V-Shaped Valley

river

glacier

silt

fjords

Grand Canyon

Project: See How Types of Valleys are Formed

You will need:

  • A milk jug full of water
  • Several ice cubes

Find a slope or side of a hill outside. (Or make your own by mounding dirt, gravel, and thick mud into a “hill” in the bottom of a plastic-lined box. Let dry). First, pour a thin stream of water from the jug down the slope. Empty the jug in this way. Can you see how the water meanders in a thin stream? Think about how rocks and sediment would move freely if caught in a stream of water. Water causes a V-shaped valley to form by wearing land down over time.

Second, take an ice cube and scrape it down the slope along another path. See how it pushes dirt and other material out of its way? A glacier is like a chisel; it creeps and carves its way along the ground, leaving a U-shape in its wake.

(Courtesy Glaciers of North America, By S. Ferguson)

Glacier Power- Why Do Scientists Study Glaciers?

Think of a glacier as a huge ice box with the answers about how our world was a long time ago locked inside. All we have to do is open up the ice box and find the answers. We study glaciers for many reasons. We can find out how the atmosphere was and what kind of mammals lived thousands of years ago. Scientists also teach all of us about the wonders of glaciers.

Geologic Processes
Climate
Global Warming Versus the Ice Ages
Satellite Imagery Aids Scientists’ Glacier Study

Geologic Processes

In the segment, Where Have Glaciers Been?, we learned that glaciers are sculptors whose tools are the geologic processes of erosion and deposition. They sculpt and carve away the land, transport material, and create glacier landforms. These landforms are clues to let scientists know where glaciers have been and what processes contribute to making the Earth look like it does today. Here are two photos where a few noticeable glacier landforms appear together.

Brooks Range, Alaska. Photo by McMillan.
A U-Shaped Valley. Photo by McMillan.

The Brooks Range photo above to the left shows several geologic features caused by glaciers. Glaciers can gradually scour or carve back into a mountain face. If this happens on three sides of a mountain, a sharp horn is left in the middle. A cirque is typically left at the base of the mountain, like someone used a giant ice cream scoop to scoop out a part. You can see a cirque in this photo from the Brooks Range. You can also see a tarn in this photo. It is the little lake at the base of the cirque. Tarns commonly occur with cirques. The fuzzy images are only mosquitoes flying in front of the photographer’s camera lens.

The photo above to the right shows a valley with a distinct u-shape. Any time scientists see a valley with a u-shape to it, they can comfortably guess that it was carved by a glacier. Rivers tend to give v-shapes to valleys, but glaciers give u-shapes to valleys.

Climate

How can glaciers be used to obtain information on long-term climate change?

Snow is porous with lots of air pockets. Everything that was in the atmosphere when it snowed will be trapped and buried under more snow instead of getting washed away by rain.

So What’s in the Atmosphere?

  • Trace Gasses
  • Volcanic Ash
  • Pollen
  • Dust

Air bubbles are trapped as porous snow becomes firn. As snow changes to glacier ice, there is more and more ice and less and less pore space.

When glacier ice finally forms, the pore spaces are closed off. All of the things listed above are trapped in the air bubbles. Each pore contains a trapped sample of the atmosphere.

The trapped air is under pressure. That is why a popping sound can be heard on many glaciers; the sound is made as pressurized air escapes from the ice. If you put a piece of glacier ice into a glass of water, the glass might explode as the pressurized air blasts out of the melting ice!

Global Warming Versus Ice Ages

In the past million years there have been nine full glacial periods, separated by much shorter interglacials, or warm spells.

  • Each glacial period lasted about 100,000 years.
  • Each interglacial period lasted about 10,000 years.
  • Slow variations in Earth’s orbit, called Milankovitch cycles, effect the vast, slow changes in ice sheets, glaciers, and sea level.
  • The most recent great ice age existed some 18,000 years ago and buried over 30% of the world’s land surface under thick ice and snow.

Today, humans burn hydrocarbon fuels releasing excessive carbon dioxide in the atmosphere and creating the greenhouse effect which can cause more melting of snow. The Earth’s atmosphere traps solar radiation because of gases present such as carbon dioxide, water vapor, and methane. Those gases allow sunlight to pass through but absorb the heat radiated back from the Earth’s surface. Scientists are examining possible relationships between glacial melting and rise in sea levels in order to determine how strong or long-lasting the greenhouse effect may become. If the greenhouse effect increases, Earth may undergo serious global warming.

An ice sizzle. Computer Graphic by Donna Sandberg.

Our world’s climate is warmer now than at any other time since 6,000 B.C., the “Thermal Maximum,” but geologists say firmly that the Ice Age is still with us; we are only living in a slightly warmer spell of it. To support this, they point out that:

  • The entire Arctic Ocean is covered with ice.
  • Huge domes of ice lie atop Antarctica and Greenland.
  • Glacial rivers of ice in Canada, Alaska, and even on mountains at the Equator help regulate Earth’s weather.

We consider our time in history to be “normal” in terms of temperature but, if we consider the vast cycles which effected the past and will effect the future, and also add humankind’s influence into that equation, our time in history may not be “normal.”

Ice Age: Will the great continental ice sheets begin growing again under the unavoidable Milankovitch cycles?

Global Warming: Will human influence cause the greenhouse effect to delay the natural planetary cycle and bring an age of “global warming?”

Satellite Imagery Aids Scientists in Glacier Study

Black Rapids Glacier: Dr. Craig Lingle, a glaciologist of the Geophysical Institute, University of Alaska Fairbanks, has performed extensive research on Black Rapids Glacier, Alaska. His research tools include use of satellite imagery.

SAR Image © ESA 1992.

Bering Glacier: the largest glacier in North America descends 190 km from high in the Chugach-St. Elias Mountains to a lake filled with icebergs on the south-central coast of Alaska. Dr. Lingle and others used satellite imagery to study a surge of Bering Glacier.

Bering Glacier Surge: Over 200 surge-type glaciers identified in North America are located in the high, heavily ice-covered mountains of southern Alaska and the Yukon Territory.

Near its terminus, Bering Glacier spreads out 47 km. This glacier is known to have been surging in cycles this century, approximately every 20 years. A major surge began during spring 1993, after a 26 year quiet period.

Crevasses on Bering Glacier Near the Grindle Hills
Extensive fresh crevassing and bulging of the glacier surface were discovered by scientists while flying over the glacier to reach remote camps for field work.

Glaciers are seen through clouds and darkness.
Usually the coastal mountains and glaciers of Alaska are obscured by cloudy weather. For the first time, regular repeated measurements of a surging glacier have been made with satellite imagery through clouds and winter darkness. James Roush, a geology graduate of the University of Alaska Fairbanks, observed the progress of the Bering Glacier surge with satellite synthetic aperture radar (SAR) images received at the Alaska Satellite Facility (ASF). These images were from the European Remote Sensing satellite (ERS-1).

A sequence of images was terrain-corrected.
Terrain-correction includes geocoding and co-registration which means the pixels in the images are referenced to absolute geographic coordinates such as latitude-longitude or universal transverse mercator, i.e. UTM. A pixel is the smallest image-forming unit of a video display.

Why do Scientists Study Glaciers?

Vocabulary Plus!

glacial period
interglacial
Milankovitch cycles
SAR
ERS-1
ASF
glaciologist
Bering Glacier
Black Rapids Glacier

Review Questions
(some of the answers may come from the vocabulary list)

  1. Name one thing you can find out by studying glaciers. 
  2. As the ice compacts, is there more/less (circle one) pore space?
  3. Glaciers sometimes make popping sounds. Why?
  4. What is another big word for “warm spells”?  
  5. How long was each interglacial period? 
  6. 18,000 years ago, there was an ice age. How much (%) of the world’s land surface was covered under thick ice?   
  7. Dr. Craig Lingle likes to study glaciers. What kind of scientist is he?
  8. What is the largest glacier in North America?
  9. What did James Roush and Dr. Craig Lingle use in order to look at glaciers from space?

Brain Challenge!
Do you think there will be an Ice Age or Global Warming in the next 100 years? Why? (Don’t worry, there is no wrong answer!)

Exercise: Why Study?
Circle the items below that are in the Earth’s atmosphere:

A. Pollen
B. Dust
C. Floating marbles
D. Ash from volcanoes
E. Lost homework
F. Gasses

Why or when might the Earth undergo global warming?

Glacier Study: True or False
Decide whether the following statements are true or false. 
If false, correct the statement.

Ice rivers in different parts of the world help regulate the Earth’s weather.
An interglacial is the exact center of a glacier.
Very slow changes or variations in the earth’s orbit are known as Milankovitch cycles.
Our climate is colder now than it has ever been before.
The greenhouse effect is caused by the growing demand for fresh garden vegetables all year round.

Why, for the first time, were scientists able to make regular repeated measurements of the surge of Bering Glacier by using SAR satellite imagery?

Project: World Glaciers
Look at a map of the world. See if you can find and point to the areas that currently have much of the Earth’s ice. 
Ask your teacher or parent for help, if you need to.

What do these areas have in common with each other? 
Would you expect to find large ice masses in central Africa or South America? 
Why or why not?

(Courtesy of Glaciers of North America, By S. Ferguson)

Alaska Satellite Facility DAAC Research Agreement

I understand that the data received from the Alaska Satellite Facility can be used only under the following terms and conditions:

  1. The data are for my use for bona fide research purposes only. No commercial use is allowed of the data or any products derived there from.
  2. The data will not be reproduced or distributed to any other parties, except that they may be shared among named members of my research team (co-investigators) and with other researchers who have signed a similar research agreement. I will be responsible for compliance with this condition for the data I obtain from ASF. Furthermore, I am responsible for compliance to these agreement terms by members of my research team with whom I share these data.
  3. I will submit for publication in the open scientific literature results of research accomplished with the requested data, including derived data sets, and the algorithms and models used. Application demonstrations are not required to supply algorithms or models.
  4. I agree to provide, if requested, a copy of such results including derived data sets, algorithms, models, and documentation, to the ASF for archive and distribution. Application demonstrations are not required to supply algorithms or models.
  5. I agree to pay the marginal cost to ASF of filling my specific requests including reproducing and delivering the data.
  6. I also understand that a product which involved ASF data in its production can only be freely distributed by me if it is in such a form that the original backscatter values cannot be derived from it.

I understand that if these conditions are violated NASA may take appropriate action, including the following:

NASA Investigators : Termination of the research agreement, and potential loss of funding support by NASA. Subsequent access would be under terms for commercial use, as outlined by the respective flight agency.

Other Users : Termination of the research agreement, and notification by NASA to the investigator’s sponsoring agency and to the relevant space agency of the violation. Subsequent access would be under terms for commercial use, as outlined by the respective flight agency.

Special Conditions for ERS Data :

I acknowledge and agree to respect the full title and ownership by the European Space Agency of all ERS-1 and -2 data. I agree to clearly mark all ERS-1 and -2 data, irrespective of the form in which it is reproduced, in such a way that the European Space Agency’s copyright is plain to all, as follows:

“© ESA (year of reception).”

Special Conditions for RADARSAT-1 Data :

I understand that the intellectual property rights of RADARSAT-1 data are reserved solely for the Canadian Space Agency (CSA), and I am entitled only to the right to utilize the data. I agree to clearly mark all RADARSAT-1 data, irrespective of the form in which it is reproduced, in such a way that the CSA copyright is plain to all, as follows:

“© Canadian Space Agency (year of reception).”

Reports or publications describing RADARSAT-1 experiments which are copyrighted must provide royalty free rights for NASA, NOAA, CSA and RADARSAT International (RSI) to reproduce or use such work for their own purposes.

Dipole eddies

How do I interpret SAR images?

Interpretation of synthetic aperture radar (SAR) images is not always straightforward, in part because of the non-intuitive, side-looking geometry.

Here are some general rules of thumb:

Water

Regions of calm water and other smooth surfaces appear black (the radar reflects away from the spacecraft). In the ESA image to the right of eddies around islands in the Bering Sea (© ESA 1992), the shades of grey indicate both rough water and ice in various stages of formation.

Wind-roughened water can backscatter brightly in the presence of capillary waves, which occur when the resulting waves are close in size to the incident radar’s wavelength.

Surfaces

Rough surfaces appear brighter, as they reflect the radar in all directions, and more of the energy is scattered back to the antenna. A rough surface backscatters even more brightly when it is wet.

  • Surface variations near the size of the radar’s wavelength cause strong backscattering.
  • If the wavelength is a few centimeters long, dirt clods and leaves might backscatter brightly.
  • A longer wavelength would be more likely to scatter off boulders than dirt clods, or tree trunks rather than leaves.

Slopes

Any slope leads to geometric distortion. Radar signals that return to the spacecraft from a mountaintop arrive earlier or at the same time as the signal from the foot of the mountain, seeming to indicate that the mountaintop and the foot of the mountain are in nearly the same place — or the mountaintop may also appear before the foot. In a SAR image with layover, the mountains look as if they have fallen over toward the sensor. Steeper angles lead to more extreme layover, where mountain tops appear to lay over their base. Layover appears bright.

Geometric distortions are corrected by doing terrain correction. ASF has terrain correction tutorials for both Sentinel-1 and ERS-1 and -2, JERS-1, and RADARSAT-1. PALSAR RTC products are available already radiometrically terrain-corrected.

Hills and other large-scale surface variations tend to appear bright on one side and dim on the other. (The side that appears bright was facing the SAR.) Where slopes are very steep, the dim side may be completely dark because no radar signal is returned at all. This is called shadow. Slope-influenced brightness is corrected by doing radiometric correction. ASF has tutorials which combine radiometric and terrain correction instructions.

Various combinations of polarizations for transmitted and received signals have a large impact on the backscattering of the signal. The right choice of polarization can help emphasize particular topographic features.

Dipole eddies
Dipole eddies swirl in the vicinity of the Bering Sea's Sarichef Strait, between Hall and St. Matthew Islands, in this ERS-1 image acquired on 15 February 1992. The eddies are tidal generated and were observed only when frazil (slush-like ice) and grease ice acted as tracers. ©ESA, 1992
Radar shadow occurs behind vertical features or steep slopes where the radar beam can't reach. Diagram courtesy of Natural Resources Canada.
Radar shadow behind steep slopes. Image courtesy of Natural Resources Canada.

Man-Made Structures
In urban areas it is at times challenging to determine the orbit direction. All buildings that are perfectly perpendicularly aligned to the flight direction show very bright returns.

Due to the reflectivity and angular structure of buildings, bridges, and other human-made objects, these targets tend to behave as corner reflectors and show up as bright spots in a synthetic aperture radar (SAR) image. A particularly strong response — for example, from a corner reflector or ASF’s receiving antenna — can look like a bright dot or a cross in a processed synthetic aperture radar (SAR) image.

Brooklyn neighborhoods such as Bedford -Stuyvesant with north-south running streets show strong radar return, as the buildings are oriented perpendicular to the imaging radar beam.
Closeup of Brooklyn street grid.
ASF's DJR 9 corner reflector shows a bright return amidst agricultural fields. Delta Junction, Alaska.
Icebergs float from the calving Mendenhall glacier, which originates in Alaska's Coast Range. The glacier velocity dataset reveals that about 40 percent (approximately 20 cubic km) of ice lost annually in Alaska is due to calving alone, mostly from a few coastal glaciers. © UAF

Glacier Power

Glacier Power started as a 1997 middle-school curriculum supplement produced by the Alaska Satellite Facility (ASF) in collaboration with glaciologists, local scientists, teachers, students, and artists. Although parts of the supplement have become outdated, several components of Glacier Power are still user favorites and rank among ASF’s most-visited pages.

ASF has updated Glacier Power content in the form of Q&A pages and lesson plans for teachers. Many of the Q&A pages contain vocabulary lists, review questions, or exercises.

Data Formats in Depth

1. CEOS (Committee on Earth Observation Satellites)

A standard format published in 1988, used for radar data and originally expected to be used with tape media. The format does not specify a naming convention.

How to Open

CEOS data may be viewed using ASF’s MapReady. ESA’s Sentinel-1 Toolbox (S1TBX) is also available.

For users who do not wish to use MapReady or S1TBX, some useful information:

  • CEOS is a wrapper — image data are wrapped in an image file descriptor and record headers which must be discarded in order to work with the data.
  • Each file starts with a file descriptor record, which provides details on the format used to store the data. In addition, there is one record header for each line of data. Each line has a 12-byte record header (which contains the record count in the file, record type identification, and record length in bytes); data values, and possibly fill values
  • Note that the units for state vectors may be either meters or kilometers (not a worry if using MapReady).

CEOS File Format and Content Information

Data format information for ASF’s CEOS L1 and L1.5 (image) products:

File Type Description ERS-1, ERS-2, JERS-1 and RSAT-1 File Extension (L1) ALOS PALSAR File Prefix (L1.5)
SAR VOLUME file Stores the volume-management and file-management information. N/A VOL-
SAR LEADER file Contains detailed metadata. Useful records include the Dataset Summary, Platform Position, and the Facility-Related records. Contents can be read in plain text if the .L.txt file is available. .L LED-
SAR DATA file Contains image data .D IMG-
SAR TRAILER file .P contains basic metadata including orbit, beam mode, start/stop times and lat/long corners and center. Can be read with a text editor.
TRL contains a file descriptor, and for L1.1 and L1.5 data, low resolution image data.
.P TRL-

Product CEOS Naming Conventions

The CEOS format does not specify a naming convention, so naming is facility or agency-specific.

2. GeoTIFF (Tagged Information File Format)

Format for handling images and data within a single raster file, by including header tags such as size, definition, image-data arrangement, and applied image compression.

The GIS-friendly GeoTIFF format is an extension of TIFF that includes georeferencing or geocoding information embedded within a TIFF file (such as latitude, longitude, map projection, coordinate systems, ellipsoids, and datums) so an image can be positioned correctly on maps of Earth. It is a public domain metadata standard.

A georeferenced image is oriented in parallel with orbit direction:
Descending: The scene start is at the top of the image, and scene end is at the bottom.
Ascending: The scene end is at the top of the image, and the scene beginning is at the bottom
A geocoded image is projected on a map oriented in a north-south direction:
Descending
Ascending

Product GeoTIFF Naming Conventions

ALOS PALSAR RTC GeoTIFF File Extension and Description

File Extension Description Example
_HH.tif _HV.tif _VH.tif _VV.tif Terrain-corrected product stored in separate files for each available polarization in GeoTIFF format. AP_26939_PLR_F3170_RT1_HH.tif AP_26939_PLR_F3170_RT1_HV.tif AP_26939_PLR_F3170_RT1_VH.tif AP_26939_PLR_F3170_RT1_VV.tif
.iso.xml ISO-compliant metadata in XML format AP_26939_PLR_F3170_RT1.iso.xml
.inc_map.tif Incidence angle map in GeoTIFF format AP_26939_PLR_F3170_RT1.inc_map.tif
.ls_map.tif Layover/shadow mask in GeoTIFF format AP_26939_PLR_F3170_RT1.ls_map.tif
.dem.tif Digital elevation model used for terrain correction in GeoTIFF format AP_26939_PLR_F3170_RT1.dem.tif
.geo.jpg Browse image of the amplitude (including world and auxiliary file) in JPEG format AP_26939_PLR_F3170_RT1.geo.jpg
.kmz Browse image in Google Earth format AP_26939_PLR_F3170_RT1.kmz

3. SAFE (Standard Archive Format for Europe)

SAFE Product Folder Structure​

Sentinel data products use a Sentinel-specific variation of the SAFE format, an ESA folder structure containing data and information as follows:

The manifest.safe file is an XML file containing the mandatory product metadata common to all Sentinel-1 products.

Annotation datasets contain metadata describing the properties and characteristics of the measurement data or how they were generated. For each band of data there is a product annotation data set that contains metadata describing the main characteristics corresponding to that band such as the state of the platform during acquisition, image properties, polarization, Doppler information, swath merging and geographic location. Calibration annotations contain calibration information and the beta naught, sigma naught, gamma and digital number look-up tables that can be used for absolute product calibration. Noise data annotations contain the estimated thermal noise look-up tables. Annotated data sets are provided in XML format.

Measurement datasets contain the binary information of the actual acquired or processed data. For Level-0 this is the instrument data, for Level-1 it is processed data. Measurement datasets are provided in GeoTIFF format (georeferenced) for Level-1 products. There is one measurement data set per polarization and per swath. TOPSAR SLC products contain one complex measurement data set in GeoTIFF format (georeferenced) per swath per polarization. Level-1 GRD products contain one detected measurement data set in GeoTIFF format (georeferenced) per polarization.

In the Preview folder, quick-look datasets are power detected, averaged and decimated to produce a lower resolution version of the image. Single polarization products are represented with a grey scale image. Dual polarization products are represented by a single composite color image in RGB with the red channel (R) representing the first polarization, the green channel (G) represents the second polarization and the blue channel (B) represents an average of the absolute values of the two polarizations.

Representation datasets found in the Support folder contain information about the format or syntax of the measurement and annotated data sets and can be used to validate and exploit these data. Representation data sets are provided as XML schemas.

Sentinel-1 SAFE Naming Convention

The top-level Sentinel-1 product folder name is composed of upper-case alphanumeric characters separated by an underscore (_).

Product Name
Example:
S1A_IW_SLC__1SDV_20170413T131907_20170413T131934_016126_01AA29_8754.SAFE

Naming Notes:

  • In Resolution (R), underscore (_) is a valid input to mean “not applicable.”
  • Product Class — A (Annotation) is internal only and not distributed.
  • In Start/Stop Date-Time, date and time are separated by the character ‘T’
  • Absolute orbit (OOOOO) ranges from 000001 to 999999.
  • Mission data-take ID (DDDDDD) ranges from 000001 to FFFFFF.
  • The folder extension is always “SAFE”

4. HDF5

Hierarchical Data Format (HDF) is a set of file formats designed to store and organize large amounts of data.

HDF5 simplifies the file structure to include only two major types of object:

  • Datasets, which are multidimensional arrays of a homogeneous type
  • Groups, which are container structures which can hold datasets and other groups

HDF5 is a general purpose file format and programming library for storing scientific data. Use of the HDF library enables users to read HDF files on multiple platforms regardless of the architecture the platforms use to represent integer and floating point numbers.

ASF DAAC SAR datasets available in HDF5 format and tools

  • Seasat products are offered in either HDF5 or GeoTIFF formats. HDF5 products may be viewed with MapReady.
  • All SMAP standard products are in the Hierarchical Data Format version 5 (HDF5). SMAP products can be viewed with Panoply.

HDF5 Data Recipe

How to View Seasat HDF5 Files in ASF MapReady

Product HDF5 Naming Conventions

5. UAVSAR

From JPL’s Polarimetric (PolSAR) Data Format and Interferometric (InSAR) Pair Data Format.

Polarimetric and RPI Product File Formats

SLC files (.slc): calibrated single look complex files; floating point format, little endian, 8 bytes per pixel, corresponding to the scattering matrix.

  • Polarimetric product: one SLC file for each polarization (HH, HV, VH, and VV)
  • Repeat-pass interferometric product: one SLC file for each flight track (track 1, T1; and track 2, T2). Files are available by request, but are not normally included in the data distribution.

Ground projected files (.grd): calibrated complex cross products (Polarimetric product) or interferometric products (RPI product) projected to the ground in simple geographic coordinates (latitude, longitude). There is a fixed number of looks for each pixel. Floating point or complex floating point, little endian, 8 or 4 bytes per pixel.

HGT file: the DEM that the imagery was projected to, in the same geographic coordinates as the ground projected files. Floating point (4 bytes per pixel), little endian, ground elevation in meters.

Annotation file (.ann): a text file with metadata.

Additional Polarimetric Product File Formats

MLC files (.mlc): calibrated multi-looked cross products, floating point format, three files 8 bytes per pixel, three files 4 bytes per pixel, little endian.

Compressed Stokes Matrix product (.dat): AIRSAR compressed stokes matrix format for software compatibility (AIRSAR Integrated Processor Documentation). 10 bytes per pixel.

Terrain slope file (.slope): The terrain slope file (.slope) contains the derivatives of the digital elevation model (DEM) in the East and North direction. The file is an array of two interleaved floating point numbers (2 x 4 bytes per pixel) with geometry identical to the other ground-projected data layers (.grd, .hgt, .inc). For each interleaved pixel, the first 4 byte value is the terrain slope in the east direction, and the second 4 byte value is the slope in the north direction. Floating point, little endian.

Incidence angle file (.inc): the local incidence angle, the angle between the surface normal and the radar line of site. The file consists of 4-byte floating point values, co-registered with the slope file. The floating point, little endian file contains values reported in radians, as shown in the image.

Terrain slope file (.slope): The terrain slope file (.slope) contains the derivatives of the digital elevation model (DEM) in the East and North direction. The file is an array of two interleaved floating point numbers (2 x 4 bytes per pixel) with geometry identical to the other ground-projected data layers (.grd, .hgt, .inc). For each interleaved pixel, the first 4 byte value is the terrain slope in the east direction, and the second 4 byte value is the slope in the north direction. Floating point, little endian.

Incidence angle file (.inc): the local incidence angle, the angle between the surface normal and the radar line of site. The file consists of 4-byte floating point values, co-registered with the slope file. The floating point, little endian file contains values reported in radians, as shown in the image.

Additional RPI Product File Formats

AMP files (.amp1 and .amp2): calibrated multi-looked amplitude products, one file per repeat track, floating point format 4 bytes per pixel, little endian.

INT files (.int): interferogram product, one file per pair of repeat tracks, complex floating point format 8 bytes per pixel, little endian.

UNW files (.unw): unwrapped interferometric phase product, one file per pair of repeat tracks, floating point format 4 bytes per pixel, little endian.

COR files (.cor): interferometric correlation product, one file per pair of repeat tracks, floating point format 4 bytes per pixel, little endian.

KML and KMZ files (.kml or .kmz): these files allow you to view a representation of their corresponding file type in Google Earth or similar software.

PNG files (.png): these are representations of the corresponding products in standard PNG Format.

Formation of Interferometric Products:

Prior to creating interferometric products, the SLC images from both tracks are co-registered to each other using GPS data and the data itself to estimate and compensate for the variable motion between the tracks.

The single look complex (SLC) data for each track is summed in range and azimuth by the number of looks specified in the annotation file (“Number of Looks in Range” and “Number of Looks in Azimuthz”) (typically 3 looks in range and 12 looks in azimuth), divided by the product of the number of looks in range and azimuth, and output as the amp1 and amp2 files.

The interferogram in the .int file is formed by multiplying the single look complex image from track 1 times the complex conjugate of the single look complex image from track 2. The resulting complex values are then summed in range and azimuth according to the desired number of looks in the range and azimuth direction, with each pixel then divided by the product of the the number of azimuth looks and the number of range looks.

The correlation file .cor is formed by dividing the interferogram values (the .int file) by the product of the multilooked amplitude values for track 1 and track 2 (the .amp1 and .amp2 files).

The unwrapped interferometric phase file UNW (the .unw file) is obtained by applying the Goldstein/Werner method on the interferogram: Goldstein, R. M. and Werner, C. L., 1998. Radar interferogram filtering for geophysical applications. Geophysical Research Letters, 25(21):4035-4038.

Calibration of the data:

Please see JPL’s calibration page for documentation on calibration of the data.

UAVSAR Product Naming Conventions

6. AIRSAR (data format)

From JPL’s AirSAR website.

File Headers

All AIRSAR data files start with 3-4 header records.

  • First header: general information about data file including # of lines and samples and offset to the first data record
  • Parameter header: information specific to the scene
  • Calibration header: information on data calibration
  • DEM header: only present for TOPSAR data; contains the elevation offset and elevation increment needed to translate the integer*2 values to elevations in meters

Data Modes

POLSAR Data Mode

The POLSAR operating mode collects twelve channels of data, four in each of the three frequencies of AIRSAR: P-, L-, and C-band.  The four data channels are:

HH horizontally polarized transmit wave, horizontally polarized receive wave
HV horizontally polarized transmit wave, vertically polarized receive wave
VH vertically polarized receive wave, horizontally polarized receive wave
VV vertically polarized receive wave, vertically polarized receive wave

TOPSAR Data Modes

XTI1 – Generates a C-band DEM along with L- and P-band polarimetry
XTI2 – Generates a C-band and an L-band DEM, along with P-band polarimetry
(see exception below for when P-band data will not be present)

TOPSAR data are processed on the AIRSAR Integrated Processor (ver. 5.1 or ver. 6.1).

Data Files

POLSAR Data Files

Data Format — Compressed Stokes matrix data in slant range. File Size (10 km) is 15 Mbytes. CM data are oriented so that each pixel sample is decreasing azimuth (along track) and each pixel line is of increasing range (cross-track).

Each POLSAR set typically contains:

cm4212_c.dat
cm4212_l.dat
cm4212_p.dat
cm4212.gif
cm4212_meta.airsar
hdo4212.log

TOPSAR Data Files

Each TOPSAR scene typically contains:

  • 4 or 8 DEM (C-band or C-band and L-band) and related data files
  • 1 or 2 Polarimetric data files (see note below for exception)

They have the following files:

DEM Data (C-band, maybe L-band)

  • c.vvi2 – C-band VV polarization only
  • integer2 (signed 16 bit)
  • .demi2 – C-band DEM
  • byte file
  • .corgr – Correlation coefficient map
  • .incgr – Local incidence angle map

Polarimetric Data

  • l.datgr  – L-band polarimetry compressed Stokes matrix
  • p.datgr – P-band polarimetry compressed Stokes matrix

Note: Due to FCC restrictions, since 1994, P-band data are not included for TOPSAR datasets collected at 40 MHz bandwidth over sites in the United States.

AIRSAR Naming Conventions

TOPSAR Data Conversions

Data collected since 1993 are processed using Version 5.1 and Version 6.1.

Note that the integer*2 data will need to be converted using the following equations:

Convert demi*2 data to elevations in meters:
hs = (elevation increment) * DN + (elevation offset)

  • Elevation increment and offset are found in the DEM header record
  • DN is the integer*2 (signed) number from the .demi2 data file

Convert vvi*2 data to radar cross sections
sigma naught =(DN**2)/(General Scale Factor GFS)

  • DN is the integer*2 (signed) number as the amplitude (linear value) from the .vvi2 data file
  • GSF is in the second field of the Calibration Header.
  • Note that the GSF = 10**6

Polarimetric data collected in the TOPSAR mode are read the same way as POLSAR data.

Glacier Power – What is Glacier Anatomy?

Anatomy of a Glacier

The anatomy of a glacier as it slowly slides down the valley. Illustration by Donna Redhead.

Anatomy of a Glacier Definitions

Key terms are highlighted

  1. The accumulation (input) zone is where a glacier gains snow and ice through snowfall and compression. Ice begins to flow like a conveyor belt, driven by gravity and ever mounting snows.
  2. In the lower region or ablation (output) zone, the glacier loses ice through melting and evaporation. Older ice is carried down to greater and greater depth.
  3. An equilibrium line divides the two areas. This spot is like an old fashioned pair of scales used to weigh gold dust.
  4. Advancing ice scrapes and grinds the bedrock, boulders, and gravel beneath it and pushes a ridge of terminal moraine in front.
  5. Another, or tributary, glacier sometimes joins the main flow, adding another strip of lateral moraine debris. The two lateral moraines combine to form a single medial moraine, which now extends down the middle of the glacier towards the snout.
  6. When two lateral moraines combine, they form a single medial moraine, which extends down the middle of the glacier towards the snout. When medial moraines come close to one another near the terminus, a glacier may look multicolored or striped.
  7. Glacier ice melts and fractures, and the sea often batters it. Finally, chunks of ice break off as icebergs in a process called calving, which balances the flow of ice from behind.
  8. Near the terminus (end) of a glacier, its surface thins and stretches and breaks into a mosaic of crevasses.
  9. Meltwater flows through hidden channels and tunnels.
  10. Snow to Ice: Water seeps through accumulated snow and gradually forms horizontal “ice lenses” and vertical “glands.” Eventually, the whole mass compresses into a deep bed of dense ice.
  11. Ice Flow: Bending of a vertical bore hole (left) shows how a glacier moves by internal deformation and sliding at the base (red arrow).
  12. Glacier Bed: Glaciers move by sliding over bedrock or underlying gravel and rock debris. With the increased pressure because of the weight, the individual ice grains slide past one another and the ice moves slowly downhill. Water lubrication is crucial to either process. The sliding of the glacier over its bed is called basal slip.

Anatomy of a Surge

Surge

When a glacier surge occurs, rapid downstream motion and stretching of the ice can advance the terminus by many kilometers. The glacier may swallow its own valley. A surge is a short period when a glacier can go as much as 100 times faster than it normally goes. Traveling waves during the surge and the advancing surge front can actually move much more rapidly than the rest of the glacier. The rest of the glacier is also traveling faster than usual.

Surge Definitions

A. When not surging, the glacier contains cracks and tunnels that drain off meltwater. See: Moulin

B. If the meltwater system gets blocked, water pressure can lift and lubricate part of the ice stream, letting the glacier slide rapidly downhill. See: Surge

C. While the surge happens, the glacier heaves and undulates like a moving caterpillar. See: Galloping Glacier

D. Surges can end suddenly as the blocked water gushes out at the face. These downward gallops occur in only a small percentage of Alaska’s glaciers. See: Jokulhlaup

A surge is a short period when a glacier can go as much as 100 times faster than it normally goes. Illustration by Erica Herbert.

Types of Crevasses

A diagram showing the many types of crevasses. Figure by Hambrey, modified by Lingle & Sandberg.

Also see crevasses.

Anatomy

Vocabulary Plus

accumulation zone
ablation zone
equilibrium line
moraine
tributary
terminal moraine
calving
terminus
meltwater
moulin
surge
jokulhlaup

Review Questions

Look at the first diagram

  1. Is the accumulation zone near the upper or lower region of a glacier?
  2. Is the ablation zone near the upper or lower region of a glacier?
  3. The process of calving stops, balances, or increases (pick one) the flow of ice from behind.
  4. True or False: Meltwater flows along the top of glaciers.
  5. Explain what process is occurring in #3 on the glacier anatomy image. What does it tell you?

Look at “Anatomy of a Surge,” which shows the stages of a surging glacier

  1. What is the fastest a glacier can surge: 5 times faster than it normally goes or 100 times faster than it normally goes?
  2. In a surge, can a glacier swallow its own valley?
  3. In the diagram in this section that shows what happens when a glacier surges, look at letter B and find out what it’s about.

The next figure is a diagram of kinds of crevasses created by stresses within glaciers. Look closely.

9. Draw a diagram of shearing at the side of a glacier. (Hint: it’s in the diagram of “types of crevasses”)

Brain Challenge!
Do you think a surging glacier could knock down Denali?

Exercise: Matching

___accumulation zone

___ablation zone

___tributary

___moraine

___terminus

A. A glacier that joins the main flow.

B. Where a glacier gains snow and ice through snowfall and compression

C. Helps anchor the glacier’s ice.

D. In the lower region of a glacier; this loses ice through melting and evaporation

E. The end of a glacier; the surface thins and stretches

Project: Drawing Glacier Anatomy
See if you can draw these parts of a glacier from memory. If you need help, look in the glossary for definitions.

  • Terminus
  • Tributary
  • Accumulation Zone

Check yourself! Go back to the anatomy section of Glacier Power and look. Were you right?

Glacier Power – What are Crevasses?

Crevasses

A crevasse is a crack in the surface of a glacier. Photo by Kristina Ahlnas.
A scientist climbing into a crevasse. Photo by Kristina Ahlnas.
A ladder inside a crevasse. Photo by Kristina Ahlnas.

A crevasse is a crack in the surface of a glacier caused by extensive stress within the ice. For example, extensive stress can be caused by stretching if the glacier is speeding up as it flows down the valley. Crevasses can also be caused by the ice flowing over bumps or steps in the bedrock.

By descending into a crevasse, scientists can observe the layers of snow from past years or, deeper down, the ice crystals of the glacier.

Crevasses can be small, but many are quite large. Most are squeezed shut by the pressure of the ice below about 30 m (100 ft.). Crevasses are like windows into a glacier.

Types of Crevasses

Crevasse Type Description
Bergschrunds Form at the beginning of the glacier where the glacier pulls away from the rock wall at its head.
Longitudinal or splay crevasses Form in the direction of a glacier flow, and where ice slowly spreads out sideways to cover a larger area. Commonly found near the terminus of glaciers (glacier termini).
Marginal or shear crevasses Form near the side. Marginal crevasses are caused by shear between the valley wall and the glacier. They form a herring- bone pattern, pointing about 45° up-glacier from the valley wall.
Transverse crevasses Form across a glacier in a region where the speed is increasing, which causes stretching (tensile stress) in the direction of glacier flow. They fan across the glacier. They are common in the accumulation zone and near steepening slopes, such as an ice fall.
Radial crevasses Form where a glacier turns a corner. The ice on the outside of the bend has to travel faster than the ice on the inside corner. This tension pulls the ice farther out from the corner, creating crevasses which radiate out from the inside wall.

Crevasse Diagram

A diagram showing the many types of crevasses. Figure by Hambrey, modified by Lingle & Sandberg.
A Bergschrund Crevasse. Photo by Hambrey.
Crevasses, Upper Seward Glacier, Alaska, 1948. Photo by Sharp.

Crevasses

Vocabulary Plus!

crevasse
bergschrund
longitudinal
marginal
transverse
radial
terminus
shear
accumulation zone

Review Questions
(some of the answers may come from the vocabulary list)

  1. What can cause a crevasse?
  2. Why don’t crevasses reach to the very bottom of the glacier?
  3. Where is the most common place on a glacier to find longitudinal or splay crevasses?
  4. What causes marginal crevasses?
  5. In a deep crevasse, can scientists see the ice crystals of a glacier? Yes or No?
  6. Why do scientists go down into crevasses: to observe the layers of snow or to test their bravery?
  7. Transverse crevasses form across a glacier where the speed is 
    (pick one) increasing or decreasing.
  8. Radial crevasses form where a glacier turns a corner. True or False?
  9. In which zone of a glacier are transverse crevasses most common?

Brain Challenge!
What would you do if you ever fell into a crevasse while climbing on a glacier?

Exercise: Word Scramble – Types of Crevasses

1. DSUNRCHGBER
Forms at the beginning of the glacier where the glacier pulls away from the rock wall at its head.
2. LRDAIA
They form where a glacier turns a corner. The ice on the outside of the bend has to travel faster than the ice on the inside corner.
3. VSERESNRAT
Forms across a glacier in a region where the speed is increasing, which causes stretching in the direction of glacier flow.
4. LLNGOUTIDINA
Forms in the direction of glacier flow; where ice slowly spreads out sideways to cover a larger area.

Project: Silly Putty® Cigars

Silly Putty® can be a lot of fun. Roll some into a cigar shape to make it look like a glacier. Pull it apart quickly and watch how it might fracture or break apart. This is like ice. When ice moves quickly, it fractures and breaks.

Ball it up and roll another cigar. This time, put it in a freezer for an hour or two. Now try to pull it apart. It breaks easily! This is like ice in polar or subpolar glaciers where crevasses can be deeper than those in the ice of temperate glaciers.

(Courtesy of Glaciers of North America, By S. Ferguson)

Glacier Power – Why is Glacier Ice Blue?

Glacial ice is a different color from regular ice. It is so blue because the dense ice of the glacier absorbs every other color of the spectrum except blue — so blue is what we see!

It’s Not Just Frozen Water!

Sometimes the glacial ice appears almost turquoise. Its crystalline structure strongly scatters blue light. The ice on a glacier has been there for a really long time and has been compacted down so that its structure is pretty different from the ice you normally see. Glacial ice is a lot different from the frozen water you get out of the freezer.

It’s Not Just Frozen Snow!

Glacial ice is not just frozen compacted snow. There are other things in the ice that make it much different from the ice in your home. Glaciers move through rock and soil as they carve their way down a slope. This means the ice is going to have a lot more ingredients than just water.

Wow! Just Imagine…

What would happen if you broke off a big chunk of ice from a glacier and put it in your glass of water? Would it be any different from the ice in your freezer at home? What would happen to all those air bubbles that have been trapped under pressure?

  • If your chunk of glacial ice melted in your glass of water, you would have dirt, gravel, and even organic matter (living stuff) in your water.
  • All those pressurized air bubbles would rush out so fast that they might explode your glass!
Glacial ice is a different color than regular ice. It is so blue because the dense ice of the glacier absorbs every other color of the spectrum except blue, so blue is what we see. Photo by Hambrey.

Why is Glacier Ice Blue?

Vocabulary Words
absorb
reflect
spectrum
organic matter

Review Questions
(some of the answers may come from the vocabulary list)

  1. Glacier ice is so blue because the dense ice of a glacier absorbs/reflects (circle one) every other color of the spectrum except blue/yellow (circle one).
  2. Glacial ice is different than regular ice. True or False?
  3. Are there rocks in glacial ice? Why or Why not?
  4. What could happen to your glass of water if you dumped glacial ice in it?
  5. What is the stuff called that is either alive now or was alive in the past (it may be trapped in glacier ice)?
  6. True or False: Glacial ice is just like the water in your freezer.
  7. What would be in your glass of water if the glacial ice melted?
  8. Glaciers are just frozen compacted snow. True or False?
  9. The ice on a glacier has been there for a long time and has been compacted down. True or False?

Brain Challenge!

If all the glaciers in the world melted, what would happen? (Use your imagination!)

Exercise: Blue Ice
1. Glacier ice is blue because:

A. Its structure strongly scatters or reflects blue light

B. It is lonely

C. It absorbs every other color in the spectrum except blue

D. Yellow and green make blue

2. Think about it:
How is glacier ice different from the ice in your freezer?

Project: Bubbles in an Ice Cube

Glacier ice is highly pressurized. Bubbles in glacier ice get squeezed and pushed around. Sometimes you can see round bubbles that have been squeezed into long rods or flat plains. Coarse-bubbly ice looks whiter than most other ice because it is filled with small bubbles. This kind is usually found near ablation areas of a glacier. Coarse-clear ice is free of bubbles and is the bluest ice of all. This kind is usually found near the margins and terminus of a glacier.

Look at ice cubes formed in your freezer. These ice cubes first froze on their outsides and trapped air bubbles toward the center. As a result, the exterior is bubble-free while the interior has bubbles. Bubbles between the outside and the inside of the cube are probably longer and more extended. Can you see the differences?

(Courtesy of Glaciers of North America, by S. Ferguson)