Elvey building on the UAF campus is home to the Geophysical Institute

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The Alaska Satellite Facility is located on the Troth Yeddha’ campus of the University of Alaska Fairbanks and is a part of the Geophysical Institute . ASF offices and ground station operations are located in the CT Elvey and West Ridge Research Buildings.

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Elvey building on the UAF campus is home to the Geophysical Institute


Alaska Satellite Facility
Geophysical Institute
University of Alaska Fairbanks
2156 Koyukuk Drive
Fairbanks, AK 99775 USA

Interferogram of Okmok Island.


Common questions and answers about Synthetic Aperture Radar

Unlike the aperture in a camera, which opens to let in light, radar aperture is another term for the antenna on the spacecraft or aircraft. The radar antenna first transmits electromagnetic energy toward Earth and then receives the returning energy after it reflects off of objects on the planet. In the NASA image below, the radar antenna is the rectangle at the Earth end of the 1978 Seasat satellite. The data collected by the radar antenna are then transmitted to another kind of antenna on Earth — such as the antennas of the ASF Satellite Tracking Ground Station — so they can be stored and processed.

In general, the larger the antenna, the more unique information scientists can obtain about an object — and the more information, the better the image resolution. However, antennas in space are not large. So scientists use the spacecraft’s motion, along with advanced signal-processing techniques, to simulate a larger antenna.

Synthetic aperture radar (SAR) interferometry (InSAR) detects motion or elevation by comparing radar signals from two or more images of the same scene. The images are taken at different times from the same vantage point in space. SAR interferometry is often used to detect surface changes (for use in seismology, for example) or to generate digital elevation maps. The InSAR image below shows deformation on Okmok, a volcano in the Aleutian Islands. 

Interferogram of Okmok Island.
Image courtesy of Zhong Lu, © ESA 2008

Because the radar wavelength is longer than particles in a cloud, such as droplets, the signal traveling through a cloud is mostly unaffected by any refraction at the boundaries of the different media.

In microwave remote sensing, scientists measure the time and magnitude of the signal backscattered from the ground to the radar antenna. The magnitude of the signal defines the brightness of a given pixel in the image. The resulting image has a grayscale. Scientists sometimes colorize SAR images to highlight certain data or features.

The interpretation of synthetic aperture radar (SAR) images is not straightforward. The reasons include the non-intuitive, side-looking geometry. Here are some general rules of thumb:

  • Regions of calm water and other smooth surfaces appear black (the incident radar reflects away from the spacecraft).
  • Rough surfaces appear brighter, as they reflect the radar in all directions, and more of the energy is scattered back to the antenna. Rough surface backscatter even more brightly when it is wet.
  • Any slopes lead to geometric distortions. Steeper angles lead to more extreme layover, in which the signals from the tops of mountains or other tall objects “layover” on top of other signals, effectively creating foreshortening. Mountaintops always appear to tip towards the sensor.
  • Layover is highlighted by bright pixel values. The various combinations of the polarization for the 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.
  • 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.
  • 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.
  • Wind-roughened water can backscatter brightly when the resulting waves are close in size to the incident radar’s wavelength.
  • 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.)
  • 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 SAR image. A particularly strong response — for example, from a corner reflector or ASF’s receiving antenna — can look like a bright cross in a processed SAR image.

In ASF’s full-resolution synthetic aperture radar (SAR) images, objects can be distinguished as small as about 30 meters wide. Some of the smaller items scientists have spotted have been ships and their wakes. When the synthetic aperture radar (SAR) happens to be aligned at a certain angle, long thin objects such as roads or even the Alaskan oil pipeline can also be seen. Objects can be much smaller than the resolution and still be observable such as bright point objects. They only need to be perfectly aligned with the look direction of the synthetic aperture radar (SAR) sensor.

As the spacecraft moves along in its orbit, the radar antenna transmits pulses very rapidly in order to obtain many backscattered radar responses from a particular object. The synthetic aperture radar (SAR) processor could use all of these responses to obtain the object’s radar cross-section (how brightly the object backscattered the incoming radar), but the result often contains quite a bit of speckle. Generally considered to be noise, speckle can be caused by an object that is a very strong reflector at a particular alignment between itself and the spacecraft, or by the combined effect of various responses all within one grid cell. To reduce speckle, the data are sometimes processed in sections that are later combined — called looks. The more looks used to process an image, the less speckle. However, resolution is reduced, and information is lost in this process. Several research groups are developing/improving algorithms to reduce speckle while saving as much accurate information as possible.

Noise is defined as random or regular interfering effects that degrade the data’s information-bearing quality. Speckle is a scattering phenomenon that arises because the resolution of the sensor is not sufficient to resolve individual scatterers. Physically speaking, speckle is not noise, as the same imaging configuration leads to the identical speckle pattern. Speckle is removed by multi-looking. See “What is a ‘look'” above.

After the radar sends its microwave signal toward a target, the target reflects part of the signal back to the radar antenna. That reflection is called backscatter. Various properties of the target affect how much it backscatters the signal.

  • Sentinel-1
  • PALSAR (Faraday rotation can be a factor.)
  • RADARSAT-1 (The most suitable RADARSAT-1 data for InSAR were acquired during and after the Modified Antarctic Mapping Mission in the fall of 2000.)
  • ERS-1
  • ERS-2
  • JERS-1

IfSAR is another term for InSAR. InSAR is the more common term, particularly for satellite-borne sensors. IfSAR has been used more by the military and/or for airborne sensors.

Layover is a type of distortion in a synthetic aperture radar (SAR) image. The radar is pointed to the side (side-looking) for imaging. 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 synthetic aperture radar (SAR) image with layover, the mountains look as if they have “fallen over” towards the sensor.

Where features are shifted from their actual location, the resulting geolocations are incorrect. This effect can be removed by the technique of terrain correction (also see “What is terrain correction?” below).

As with shadows from sunlight, shadows in synthetic aperture radar (SAR) images appear behind vertical objects. Mountains may appear to have black shadows behind them, depending on the steepness of the slope. The shadows appear black because no radar signals return from there.

Radiometric correction involves removing the misleading influence of topography on backscatter values. For example, the correction eliminates bright backscatter from a steep slope, leaving only the backscatter that reveals surface characteristics such as vegetation and soil moisture.

Animation showing the effect of radiometric correction.
ASF DAAC 2014; © JAXA/METI 2008

Terrain correction is the process of correcting geometric distortions that lead to geolocation errors. The distortions are induced by side-looking (rather than straight-down looking or nadir) imaging, and compounded by rugged terrain. Terrain correction moves image pixels into the proper spatial relationship with each other. Mountains that look as if they have “fallen over” towards the sensor are corrected in their shape and geolocation.

Comparison showing the effect of terrain correctio.
ASF DAAC 2014; © JAXA/METI 2008.

Most digital elevation models (DEM) are geoid-based and require a correction before they can be used for terrain correction. The DEM included in an ASF radiometrically terrain corrected (RTC) product file was converted from source DEM orthometric height to ellipsoid height using the ASF MapReady geoid_adjust tool. This tool applies a geoid correction so that the resulting DEM relates to the ellipsoid.

An online tool is available that computes the height of the geoid above the WGS84 ellipsoid, and will show the amount of correction that was applied to the source DEM used in creating an RTC product.
Comparison of DEM heights.

Orthorectification corrects geometric distortions in imagery, just as terrain correction does (see “What is terrain correction?” above). The term ‘orthorectification’ is used more often in association with aerial and optical imagery. Terrain correction generally refers to synthetic aperture radar (SAR) imagery.

A georeferenced image has the location of the four corners of the image and the information needed to put the data into a projection. Geocoded data is already projected. Each point in the image is associated with a geographic coordinate.

  • C-band (~5.3 GHz)
    Applies to ERS-1, ERS-2, RADARSAT-1, Sentinel-1
    – Variety of applications, but particularly sea ice, ocean winds, glaciers
  • L-band (~1.2 GHz)
    Applies to PALSAR, UAVSAR, AIRSAR, JERS-1, Seasat
    – Provides vegetation penetration
    – Applications included sea ice, tropical forest mapping, soil moisture
    – Subject to ionospheric effects
  • P-band (~0.4 GHz)
    Applies to some products of UAVSAR
    – Greatest penetration depth through vegetation and into soil
    – Ideal for soil moisture and biomass
    – Difficult to operate from space due to ionospheric effects

SAR backscatter

Fundamentals of SAR – Media Resources

Elipsoid, Orthometric, and Geoid heghts relationship

PALSAR RTC DEM Information

Most DEMs are geoid-based and require a correction before they can be used for terrain correction. The DEM included with an ASF RTC product was converted from the orthometric height of the source DEM to ellipsoid height using the ASF MapReady geoid_adjust tool. This tool applies a geoid correction so that the resulting DEM relates to the ellipsoid.

The GeoidEval Utilty is a free online tool that computes the height of the geoid above the WGS84 ellipsoid at a specific location, and will show the amount of correction that was applied to the source DEM used in creating an RTC product.

Elipsoid, Orthometric, and Geoid heghts relationship

About the source DEMs used for ALOS PALSAR RTC processing

The quality of an ALOS PALSAR RTC product is directly related to the quality of the digital elevation model (DEM) used in the radiometric terrain correction (RTC) process. The table below summarizes the various DEM sources and the map indicates which DEM was used in a given location.

The continental U.S., Hawaii, and parts of Alaska are covered with 1⁄3 arc-second National Elevation Dataset (NED) at a 10 m resolution. The rest of Alaska above 60 degrees northern latitude was only available at 60 m resolution with 2 arc-second NED data. The best resolution for Canada and Mexico at 30 m was provided by 1 arc-second NED. For the remaining globe, the Shuttle Radar Topography Mission (SRTM) GL1 data at 1 arc-second (30 m) resolution was used. Greenland and Antarctica were mostly covered by ice and glaciers and not suitable for terrain correction. For areas in Eurasia above 60 degrees northern latitude, no suitable DEMs were available.

DEMs used for ALOS PALSAR RTC processing

DEMDatumCoverage AreaDEM ResolutionProduct ResolutionResampling Approach
parts of Alaska
(1/3 arc sec)
12.5 mNo resampling
30 mDown-sampled DEM
to 30-m pixel spacing
SRTMGL1EGM96Latitudes between
60 N and 57 S degrees
~30 m
(1 arc sec)
12.5 mUp-sampled 30-m
mapping function to
12.5-m mapping function
30 mNo resampling
parts of Alaska
~30 m
(1 arc sec)
12.5 mUp-sampled 30-m
mapping function to
12.5-m mapping function
30 mNo resampling
parts of Alaska,
Canada, Mexico
~30 m
(1 arc sec)
12.5 mUp-sampled 30-m
mapping function to
12.5-m mapping function
30 mNo resampling
NED2NAVD88Alaska~60 m
(2 arc sec)
12.5 mUp-sampled 30-m
mapping function to
12.5-m mapping function
30 mUp-sampled DEM to
30-m pixel spacing

The DEMs were pre-processed by ASF to a consistent raster format (GeoTIFF) from the original source formats: height (*.hgt), ESRI ArcGrid (*.adf), etc. Many of the NASA-provided DEMs were provided as orthometric heights with EGM96 vertical datum. These were converted to ellipsoid heights using the ASF MapReady geoid_adjust tool. The pixel reference varied from the center (pixel as point) to a corner (pixel as area). The GAMMA software, used to generate the terrain-corrected products, uses pixel as area and adjusts DEM coordinates as needed. Where more than one DEM was available, the best-resolution DEM was used for processing. Complete DEM coverage from a single DEM source was required for processing to proceed.

World map showing where different source DEMs were used.
Coverage of the various DEM sources used for terrain correction.
Side-looking radar geomtry.

Basic SAR Concepts and Terminology

How Does it Work?

A synthetic aperture radar (SAR) is an active sensor that first transmits microwave signals and then receives back the signals that are returned, or backscattered, from the Earth’s surface.

Flight and Directional Terminology

The instrument measures distances between the sensor and the point on the Earth’s surface where the signal is backscattered. This distance is slant range (see illustration), which can be projected on the ground representing the ground range. The flight direction is also referred to as along-track or azimuth direction, and the direction perpendicular to the flight path is the across-track or range direction. The angle between the direction the antenna is pointing and the nadir is the look angle. The angle between the radar beam center and the normal to the local topography is the incidence angle. Both angles are sometimes used synonymously, which is only valid if the InSAR geometry is simplified neglecting the Earth’s curvature and the local topography. Because the look angle of the sensor significantly affects the behavior of backscatter, it is one of the main parameters determining the viewing geometry and the incidence angle of the backscattered signal. Depending on the characteristics of the illuminated terrain, areas of layover and shadow may occur in the imagery.

Wavelength and Effects

The wavelength of the sensor determines the penetration depth of the transmitted signal into the vegetation layer of the terrain surface. The longer the wavelength, the deeper the penetration can be, particularly in forests.

  • The energy of an X-band sensor is mainly returned at the top layer of the canopies
  • Most of the L-band signal penetrates through the upper vegetation layer and is returned at the ground surface.
  • The backscatter behavior of C-band is less predictable. Due to volume scattering effects, the layer of backscattering is less determined and does not correspond directly to a terrain surface — neither the vegetation surface nor the ground surface.
Side-looking radar geomtry.
A typical side-looking radar pointing perpendicular to the flight direction. Credit: NASA.

Resolution and Speckle

The spatial resolution of the radar sensor defines the minimum separation between the measurements the sensor is able to discriminate and determines the amount of speckle introduced into the system. Speckle is a scattering phenomenon that arises because the spatial resolution of the sensor is not sufficient to resolve individual scatterers. Speckle can be reproduced if the acquisition conditions are identical, while noise is random in nature. Speckle is removed by multi-looking. The higher the spatial resolution of the sensor, the more objects on the ground can be discriminated. The term spatial resolution is often confused with the pixel size, which is the spacing of the pixels in the azimuth and ground range direction after processing the data.

Custom Processing

ALOS-1 PALSAR    The L1.1 (SLC) custom processing service has been discontinued. ASF is in the process of becoming a mirror of the JAXA archive for PALSAR L1.1 products, so 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 On Demand User Guide or the RTC On Demand! and InSAR On Demand! StoryMaps.

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.