Custom Processing

ALOS PALSAR    The L1.1 (SLC) custom processing service has been discontinued. ASF is in the process of becoming a mirror to the JAXA archive and will soon provide worldwide coverage. If you are currently not able to find L1.1 products using Vertex or the ASF API, keep checking as new products are added to our archive daily.

Sentinel-1     ASF HyP3 (pronounced “hype”) is a user service for processing Synthetic Aperture Radar (SAR) imagery that addresses many common issues for users of SAR data:

  • Most SAR data require at least some processing to remove distortions before they are analysis-ready
  • SAR processing requires a lot of computing resources
  • Software for SAR processing is complicated to use and can be prohibitively expensive
  • Producing analysis-ready SAR data is hard to learn

HyP3 On-Demand Radiometric Terrain Correction (RTC) and Interferometric SAR (InSAR) processing using GAMMA software are available through the ASF Vertex data search application. This allows you to use the powerful search tools of Vertex and then directly order data for processing without leaving the application. Processing is done in the cloud and results are available to download in less than an hour. 

Step-by-step instructions for using the Vertex On-Demand processing services are available in the RTC On Demand! and InSAR On Demand! user guides.

GAMMA RTC and InSAR processing are also available programmatically using the HyP3 API and HyP3 SDK. Detailed information about what these services provide and how to use them can be found in the ASF HyP3 User Guide.

Glacier Power – How do Glaciers Move?

Glaciers Are Solid Rivers

  • A glacier is a large accumulation of many years of snow, transformed into ice. This solid crystalline material deforms (changes) and moves.
  • Glaciers, also known as “rivers of ice,” actually flow. Gravity is the cause of glacier motion; the ice slowly flows and deforms (changes) in response to gravity. 
  • A glacier molds itself to the land and also molds the land as it creeps down the valley. Many glaciers slide on their beds, which enables them to move faster.
  • Rock that falls onto the glacier’s surface is incorporated into the glacier and erodes the bed, forming sediment. The glacier and its load of rock debris flow down-valley.
  • A glacier discharges snow from its accumulation area in the same way a stream discharges water from its watershed.
  • Sometimes, in cold climates with a lot of snow, like Alaska, glaciers flow all the way down to sea level. These glaciers carve fjords and make icebergs.
  • At the glacier’s face, ice which has been melting, fracturing, and has been battered by the sea breaks off as icebergs – a process, called calving, that balances the flow of ice from behind.
Muir Glacier, Alaska. Photo by Austin Post.

Glacier Advance and Retreat

Glaciers advance and retreat. If more snow and ice are added than are lost through melting, calving, or evaporation, glaciers will advance. If less snow and ice are added than are lost, glaciers will retreat.

Accumulation Zone: Where snow is added to the glacier and begins to turn to ice – Input Zone
In this zone, the glacier gains snow and ice.

  1. This is the upper region of the glacier.
  2. Water seeps through accumulated snow and gradually forms horizontal “ice lenses” and vertical “glands.”
  3. Eventually, the whole mass compresses into a deep bed of dense ice.
  4. The ice flows like a conveyor belt driven by gravity and ever mounting snows.

Ablation Zone: Where the glacier loses ice through melting, calving, and evaporation – Output Zone
In this zone, the glacier loses ice.

  1. This is the lower region of the glacier.
  2. Meltwater flows out to the terminus through hidden channels and tunnels.
  3. Oldest ice is the deepest.

Equilibrium Line: An equilibrium line divides the two areas. This spot is like an old-fashioned pair of scales used to weigh gold dust.

  1. If the glacier’s scale, or budget, is balanced with enough new ice added to replace the loss, the glacier is stable, with little advance or retreat.
  2. If the balance is tipped, the glacier shifts and either advances or retreats.

Motion and Movement

Mass Balance: The difference between the amount of material that a glacier accumulates and the amount lost during ablation is called its mass balance. The equilibrium line moves down (1) or up (2) a glacier as the mass balance changes.

  1. Gains more than it loses = positive mass balance
  2. Loses more than it gains = negative mass balance

Ice Flow: Glaciers move by internal deformation (changing due to pressure or stress) and sliding at the base. Also, the ice in the middle of a glacier actually flows faster than the ice along the sides of a glacier as shown by the rocks in this illustration (right).

Glacier Bed: Glaciers move by sliding over bedrock or underlying gravel and rock debris. With the increased pressure in the glacier because of the weight, the individual ice grains slide past one another and the ice moves slowly downhill. The sliding of the glacier over its bed is called the basal slip. Water lubrication is crucial to either process.

The ice in the middle of a glacier flows faster than the ice along the sides of the glacier. Illustration by Erica Herbert.

Revealed by Satellite Radar

These images allow glaciologists to study in very fine detail the way in which glacier ice flows downhill. An “interferogram” is an image made from the comparison of two radar satellite scenes of a glacier. The cycle, or repeating, color patterns represent an overlaying of information about surface elevation (like topographic maps) with information about how fast the surface of the glacier is moving.

The glaciers in the images are part of the Bagley Icefield in Southcentral Alaska. On the mountains (which are stationary), the color bands represent increasing elevation. On the glacier surface the color bands primarily represent surface speed.

In these images the color bands are like a series of parallel moving sidewalks, each moving slightly faster than its neighbor as one traverses from the edge of the glacier towards the center, so that the ice in the middle is moving the fastest.

Bagley Ice Field interferogram from synthetic aperture radar (SAR).
Interferogram of Bagley Ice Field.

Moraine: Moraines are mounds, ridges, or other distinct accumulations of unsorted, unlayered mixtures of clay, silt, sand, gravel, and boulders. There are many types of moraines:

  • Terminal or toehold – The advancing ice scrapes and grinds the bedrock boulders and gravel beneath it and pushes ahead of itself a ridge or terminal moraine of rock and earth. A terminal moraine helps to anchor the glacier’s ice.
  • Lateral – their rock material comes from the valley walls.
  • Medial – When two lateral moraines combine, or a tributary glacier joins the main flow, they form a single medial moraine, which extends as a long, dark stripe 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. Medial moraines can create interesting swirls and loops. 
  • Ablation – an accumulation of melted-out rocks (sometimes just sparse collections of glacial till).
  • End and Push – created near the margin of a glacier, at the terminus.
  • Ground and Dump – glaciers often dump out their supply of rocks as they retreat.
Moraines from tributaries. Barnard Glacier, Alaska, 1949.
Looping medial moraines. Photos by James Roush.

Terminus: The terminus is the lowest end of a glacier. Also called the snout, toe, or leading edge. Near the terminus, the glacier’s surface thins and stretches and breaks into a mosaic of crevasses. Below, the terminus of Hubbard Glacier in Alaska is shown as a large chunk of it is breaking off (also called, “calving”).

Meltwater flows through hidden channels and tunnels, reaching the base of the ice to lubricate its flow, and pours from under its face in a silt-laden cloud.

Nunatak is an Inuit term for an island of bedrock or mountain projecting above the surface of an ice sheet, highland icefield, or mountain glacier. The glacier flow has gone around the bedrock, leaving behind this distinct geologic feature.

Scientists use stakes to measure glacier movement. In the picture to the right below, the glacial stream velocity is being measured by a scientist.

Stakes measuring glacier movement at the Bering Glacier in Alaska. Photo by James Roush.
Scientist measures the movement of the Bering Glacier in Alaska. Photo by James Roush.

Glaciers advance and retreat in response to changes in climate. As long as a glacier accumulates more snow and ice than it melts or calves, it will advance.

How do Glaciers Move?

Vocabulary Plus!

accumulation zone




ablation zone

equilibrium line

mass balance




crevasse probe






glacier bed

basal slip





Review Questions

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

  1. What causes the glacier to be in motion?
  2. True or False: Glaciers slide on their beds and this enables them to move faster.
  3. True or False: Glaciers can’t flow down to sea level or carve fjords.
  4. What is the zone where a glacier gains snow and ice?
  5. What is the zone where a glacier loses ice through melting and calving?
  6. What is the difference between the amount of material that a glacier accumulates and the amount it loses during ablation?
  7. If the glacier gains more than it loses, will the glacier have a positive or negative mass balance?
  8. True or False: The snout is another name for the terminus on a glacier.
  9. Name one type of moraine.

Brain Challenge!

When climbing a glacier, if you could only bring one other thing with you besides warm clothes, boots, and a camera, what would you bring?

Exercise: Connect the Words with Definitions
Draw lines to connect the words to their definitions

Ablation zone

Accumulation zone



Equilibrium line


lowest end of a glacier

soil and rock debris

equal melting/adding

losing snow

adding snow

ice breaking off

Project: More Silly Putty Cigars

Roll some Silly Putty into a cigar shape to make it look like a glacier. Then grab the ends and pull it slowly apart. See it sag and still stay as one piece. This is like ice. When ice moves slowly, it flows and deforms.

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

Firn line marked on Vedretta di Fellario Orientale Glacier in the Italian Alps. The firn line is the zone that separates bare ice from snow at the end of the ablation season. Photo by Alean.

Glacier Power – How do Glaciers Form?

From Snowflakes to Rivers of Ice


Glaciers are massive and incredibly powerful, but they begin with small snowflakes. Imagine how many snowflakes make a glacier as snow gradually changes into glacier ice.

The firn line on a glacier is the zone that separates bare ice from snow at the end of the ablation season.

Recipe for a Glacier

  1. Snowfall on a glacier is the first step in the formation of glacier ice.
  2. As snow builds up, snowflakes are packed into grains.
  3. The weight of the overlying snow causes the grains below to become coarser and larger. (Fresh snow is about 90 percent air.)
  4. Melted snow quickly refreezes forming ice. How the snow changes and how much time it takes to develop into glacier ice depends on the temperature.

In an area where there is more snowfall than summer snow melt, perennial snow patches appear in the mountains and remain at the end of summer. Glaciers can form in areas where summer temperatures are too low for all of the snow to melt.

When the weight of the ice and snow (thickening snowfield) becomes great enough, they begin to move (flow down-slope). When signs of flow appear in a perennial snow patch, a glacier has begun! No longer only a mass of ice and snow, it is a glacier!

All About Firn

  • Firn is wetted snow that has survived one summer without being transformed to ice. It is in the metamorphic process of snow-becoming-ice. Eventually, firn changes into solid glacier ice.
  • Firn takes about a year to form. (In colder parts of the world, this could take as long as 100 years.)
  • Firn becomes glacier ice when the interconnecting air passages between the grains are sealed off. In glacier ice, air is present only as bubbles. Ice may become denser by more compression of the bubbles.

Remember, the scanning electron micrographs of the firn crystals and the snowflake shown in What is a Glacier?  Here again, you can see the great difference between snow and firn. There is also a great difference between firn crystals and glacier ice crystals.

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

How do Glaciers Form?


firn line

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

  1. The formation of a huge glacier begins with a single, small _____________?
  2. What types of summer temperatures need to occur for a glacier to form?
  3. How does over-lying weight affect the snow?
  4. What is wetted snow that has survived one summer without being transformed into glacier ice?
  5. How long does it take for firn to form?
  6. When does firn become glacial ice?
  7. What is the line that separates bare ice from snow at the end of the ablation season?
  8. What is the difference between a perennial snow patch and a glacier?
  9. What causes a glacier to move downhill?

Brain Challenge!
What would Alaska look like if all of the glaciers melted?

Exercise: Circle the Facts
Circle all the statements that are true about each word is given (more than one statement may be true).


A. is wet snow that has survived one summer without being completely turned into ice

B. are plants your cat likes to eat

C. takes a year to form

D. becomes glacier ice when it is more compressed

E. is what you call snow when it’s freshly fallen on the glacier


A. change to firn

B. is the name of your pet rattlesnake

C. is the first step in the formation of glacier ice

D. is a six-sided crystal

E. can’t form glacier ice

Snow on a glacier:

A. is only used for snowball fights

B. builds up on a glacier in the accumulation zone

C. causes the grains of snow beneath it to enlarge

D. will melt instantly because the glacier is so hot

E. will feed all the ice worms

Project: Firn Structure

You’ll need:

  • snow or ice shavings from ice cubes or freezer frost
  • very cold water (close to freezing)
  • a small container that you can seal
  • a larger container
  1. Mix the ice in your small container with enough water to make a slushy snow. Seal the container. Next, make an ice bath with a mixture of half water and half ice and sink your sealed container into the bath. If you’re able, put the whole experiment into a refrigerator.
  2. After 24 hours, remove the sealed container and drain all the water. Use some tissue to pat dry the snow.
  3. Reseal the container and put it back in the ice bath for a few hours.
  4. When this is done, pull the sealed container out and look at the remaining ice with a magnifying lens of at least 5x magnification. You should see clusters of rounded ice particles, very similar to the structure of firn.

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

The Start of a Moulin. Photo by James Roush.

Glacier Power – How are Glaciers Strange?

Glacier Voices

Glaciers are giants that seem to come to life with strange voices, mysterious powers and unusual life forms. These voices can be of a substantial volume. The sounds that they produce can be as comforting as your breakfast cereal or as terrifying as a creature from Jurassic Park.

  • Ice Sizzles can sound like Rice KrispiesTM or Pepsi ColaTM
  • Ice Quakes are the first indication that a crevasse is forming but they don’t sound like the low rumbling of earthquakes. Fractures that cause ice quakes make a hissing or traveling cracking sound which sometimes comes from within the glacier, even though no crack is visible on the surface.
  • Moulins, which are holes in the glacier, allow for waterflow and make loud roaring sounds.

Glacier Life

As glaciers shift and change the face of the earth with their giant hands, they delicately support some of the tiniest creatures alive. Glaciers create unusual environments sensitive to the animal kingdom’s need for existence.

  • Glacier fleas are small black wingless springtail bugs that live in firn on glaciers.
  • Ice worms feed on algae and pollen, as they thrive in the cold temperatures of glaciers.


Fossils may be trapped in glaciers for thousands of years.

A cut of the fossilized log. Photo by James Roush.
This fossilized log has been exposed after many years. Photo by James Roush.
This tree stump on Lesser Island was buried under a glacier for about 3,000 years. Photo by Kristina Alhnas.

Glacier Force

When the Hubbard Glacier surged in 1986, a tongue of ice blocked the mouth of Russel Fjord creating a very large lake. The first signs of a surge are thickening of ice in the upper part of a glacier and then the appearance of lots of crevasses. During a surge, a glacier can flow more than 100 times faster than it normally flows.

Jokulhlaups (or “outburst floods”) can bring a sudden end to the surge of a glacier by releasing stored subglacial water. This water, on which the glacier was “walking,” enables the glacier to slide rapidly on its bed. Jokulhlaups are sudden glacial outburst floods of water that can be catastrophic. During the summer of 1994 the surge of Bering Glacier was ended by a Jokulhlaup or outburst flood with a sudden release of stored water from within the glacier. The force of the Jokulhlaup caused large segments of ice to calve. The enormous splashes and force represented were awesome.

The force of the pent-up water bursting forth is amazing. Huge boulders of ice are rolled and swallowed easily.

Hubbard Glacier, Alaska, Prior to a Surge. 1983 Photo by Hambrey.
Hubbard Glacier, Alaska, During a Surge. 1986 Photo by Hambrey.

Strange Glacier Phenomena

Vocabulary Plus!

ice sizzles 
ice quakes
glacier fleas
ice worms

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

  1. Can glaciers make sounds?
  2. What are the small black wingless springtail bugs that live in firn on glaciers?
  3. What do ice worms eat?
  4. There is an image of a fossil in glacial till in this section. What is the fossil?
  5. What are sudden glacial outburst floods of water that can be catastrophic?
  6. Do ice quakes sound like earthquakes (a rumbling sound) or do they make a hissing and crackling sound?
  7. Are moulins holes in a glacier or the steel spikes you put on your boots to hike on a glacier?

Brain Challenge!
Would you ever want to be an ice worm?
Why or why not?

Exercise: Crossword Puzzle
Choose 5 out of the 7 words given for the crossword puzzle.

Possible words:
ice worms
ice sizzle


  1. outburst flood
  2. sounds like crispy rice cereal


  1. holes in a glacier allowing water to flow
  2. things that can be trapped in a glacier for 
    thousands of years
  3. living in a glacier

Project: Hair Spray the Snow
Hey kids! If there’s snow outside, here’s a cool project to try! Get a clear piece of plastic that has been chilled outside. Grab a bottle of hairspray. Go outside and catch a few snowflakes. Spray the hairspray to preserve the snowflakes. Look at the snowflakes with a hand lens. Draw a picture of what you see! NEAT!

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

Seracs. Photo by James Roush.

Glacier Power – How Dangerous are Glaciers?

Glaciers Have Their Own Warning Signs

Glaciers can be dangerous in many ways. However, as long as you keep safety in mind, visiting a glacier can be a wonderful experience.

Walking too close to a glacier can be hazardous! Often the ice will form cliffs at the terminus (the end of the glacier) or at the margins (the sides). Sometimes the ice makes towers called seracs.

Be Careful!

These cliffs and ice towers are unstable and can fall. Glaciers are always moving slowly, even though you usually can’t see them move. The movement causes stress. The stress causes cracking, which causes blocks of ice to break off and fall. Sometimes an entire serac or section of the ice front can collapse. People standing too close could be killed by falling ice.

Crevasses are dangerous

Sometimes crevasses are not visible because they are covered by surface snow. This can happen during winter snowstorms when wind causes the drifting snow to build out from the upwind side of the crevasse. Mechanical hardening of the snow, caused by wind drifting, enables the snowflakes or grains to stick together as the snow bridges out toward the downwind side of the crevasse. Finally, the crevasse is completely covered. In this way, large crevasses can be entirely hidden beneath a thin layer of snow.

Sometimes a crevasse stretching for a long way across a glacier will have a single snow bridge, which may sag into the crevasse under its own weight. Snow bridges can be strong enough to support the weight of a person, but crossing them is risky. People, snow machines (no matter how fast they are going) and, in Antarctica, even large pieces of machinery have been known to fall into covered or bridged crevasses.

Living Near Glaciers Can Be Dangerous!

In the village of Randa in Switzerland, parts of the hanging glacier below the summit have broken and fallen. Ice avalanches in the winter can cause enormous masses of snow to move and the subsequent avalanches have reached as far as the tiny village in the foreground.


A person should never walk on a glacier alone. The risk of slipping on the ice and sliding into an open crevasse, or of breaking through and falling into a hidden crevasse is too great. It would be very hard, or impossible, for a single person to get out of a crevasse without companions who have a rope and other equipment. This is especially true if the person is injured in the fall.

Glaciologists and mountaineers or glacier travelers are all extremely wary of crevasses. Before making camp on a glacier, they will use crevasse probes (a 10 meter long metal rod) to detect hidden crevasses. They also practice methods of rescuing a companion who has fallen into a crevasse, and of getting themselves out. For safety, they tie themselves together in groups of two or three using a rope about 45 meters (150 ft) long. They carry ice axes to stop themselves from sliding. If they are pulled down by one person falling into a crevasse, the ice axes help stop the fall. To keep from slipping on ice, they wear crampons, which are steel spikes attached to the bottoms of their boots. 

Climbing Group. Photo by Hambrey.

The correct way to travel on a glacier:

  • Travel in a team
  • Team members may be roped together
  • Have an experienced glacier traveler with your team
  • Use proper equipment

Special Equipment:

  • Ice Axes
  • Ropes
  • Crampons
  • Crevasse Probes

Proper Clothing:

  • Boots, waterproof and warm
  • High tech materials for warm clothing, or dress in layers of clothing
  • Don’t forget your handy duct tape!

Glacier Danger and Safety

Vocabulary Plus!

snow bridges
ice axe
crevasse probe

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

  1. What is a tower of ice surrounded on all sides by crevasses?
  2. What causes a block of ice to break off and fall?
  3. What do snow bridges cover on a glacier?
  4. Does wind drifting cause mechanical hardening in the snow?
  5. Should you walk over a snow bridge?
  6. What do glacier travelers wear on their boots so they don’t slide on the ice?
  7. Name two correct ways to travel on a glacier.
  8. Name a piece of proper clothing to wear when traveling on glaciers.
  9. If they go fast enough, snow machines can cross a snow bridge safely. True or False?

Brain Challenge!

What one thing would you like to do on a glacier?

Exercise: Dress Your Friend for His/Her Hiking Adventure

Your friend, Pat, is going to hike on Bering Glacier. Choose the right clothing and gear for his/her journey.
Please draw in the correct clothing.


Here’s a list of possible items. Which ones are right?

coloring book


video games




bathing suit

hockey stick

blow dryer


experienced traveler




ice skates


Project: Double Fisherman’s Knot

When groups of climbers climb across glaciers and up mountains, they often will connect to each other with ropes. This can save lives in the case of an avalanche. The trick is to make sure one person at the end of a rope is standing in a safe spot, out of an avalanches’ way, while the person at the other end of the rope crosses the dangerous slope. This way, it reduces the chance that the exposed climber will be swept away by the force of the avalanche.

A knot that’s often used to connect two ropes is called the double fisherman’s knot. Tying knots may seem tricky at first, but it’s fun! Let’s try this knot:

You’ll need two pieces of rope, about one to one-and-a-half yards long each.

Citation Policy for Data and Imagery Accessed Through ASF

Instructions for specific datasets

Whenever you publish data processed or archived at ASF or ASF DAAC, cite ASF or ASF DAAC as the data source, following the guidelines below. Examples include but are not limited to papers, presentations, websites, videos, and books. When you discuss with news organizations your use of data processed or archived at ASF or ASF DAAC, please cite ASF or ASF DAAC as well as the data source. In general, cite the following elements:

  1. Name of dataset
  2. Source of original data (requirements may vary depending on source; see links below)
  3. Year of data acquisition 
  4. “Processed by ASF” if applicable or “Processed by ASF DAAC” if applicable
  5. “Retrieved from ASF [day month year of data access]” or “Retrieved from ASF DAAC” if the data are ASF DAAC data
  6. DOI when available (some datasets have their own DOIs)

Crediting Imagery

Include credit with each image shown in publications such as journal papers, articles, posters, and websites. See links below for specific formats and examples for datasets with their own requirements.

The general guidelines are to credit the source organization with a copyright symbol and cite the year of data acquisition. For derivative images, cite the creator, date of image creation, source organization, and date of data acquisition. 

About ASF DAAC datasets: ASF DAAC datasets are provided through the NASA Earth Science Data and Information System (ESDIS) project. ASF DAAC is one of the Earth Observing System Data and Information System (EOSDIS) Distributed Active Archive Centers (DAACs), part of the ESDIS project. NASA data are not copyrighted; however, when you publish NASA data or results derived therefrom, NASA requests that you include an acknowledgment within the text of the publication and reference list. See the examples below. 

Please note that some datasets, particularly those from foreign space agencies, have special citation requirements.

Links to Specific Citation Formats and Examples

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Some images at this website are explicitly restricted from reuse without the author’s permission. Where images are not marked, you may reuse them but must include any copyright associated with the image. Whenever you use any materials from this website, please also acknowledge ASF as follows: “Information [and images] on [SUBJECT] obtained from, Alaska Satellite Facility, UAF, [Month, Year].” 

Please send reprints and two copies of any work citing ASF data to, with “New ASF Citation” in the subject line or to

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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


  • 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.


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!

ice sheet
ice cap
valley glacier
tidewater glacier
piedmont glacier
hanging glacier
warm (temperate) glacier 
cold glacier
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.


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.


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


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


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

A Cirque Glacier. Photo by Hambrey.


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


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


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


Process by which material is worn away from the Earth’s surface.
glaciated valleys


The laying down of matter by a natural process.

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?


Scraped Rock
Valley with a U-Shape
Huge Boulders

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
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





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
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.


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
Milankovitch cycles
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.