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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.
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.
- This is the upper region of the glacier.
- Water seeps through accumulated snow and gradually forms horizontal “ice lenses” and vertical “glands.”
- Eventually, the whole mass compresses into a deep bed of dense ice.
- 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.
- This is the lower region of the glacier.
- Meltwater flows out to the terminus through hidden channels and tunnels.
- 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.
- 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.
- 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.
- Gains more than it loses = positive mass balance
- 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.
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.
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.
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.
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?
(some of the answers may come from the vocabulary list)
- What causes the glacier to be in motion?
- True or False: Glaciers slide on their beds and this enables them to move faster.
- True or False: Glaciers can’t flow down to sea level or carve fjords.
- What is the zone where a glacier gains snow and ice?
- What is the zone where a glacier loses ice through melting and calving?
- What is the difference between the amount of material that a glacier accumulates and the amount it loses during ablation?
- If the glacier gains more than it loses, will the glacier have a positive or negative mass balance?
- True or False: The snout is another name for the terminus on a glacier.
- Name one type of moraine.
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
lowest end of a glacier
soil and rock debris
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)
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.
Recipe for a Glacier
- Snowfall on a glacier is the first step in the formation of glacier ice.
- As snow builds up, snowflakes are packed into grains.
- The weight of the overlying snow causes the grains below to become coarser and larger. (Fresh snow is about 90 percent air.)
- 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.
How do Glaciers Form?
(Some of the answers may come from the vocabulary list.)
- The formation of a huge glacier begins with a single, small _____________?
- What types of summer temperatures need to occur for a glacier to form?
- How does over-lying weight affect the snow?
- What is wetted snow that has survived one summer without being transformed into glacier ice?
- How long does it take for firn to form?
- When does firn become glacial ice?
- What is the line that separates bare ice from snow at the end of the ablation season?
- What is the difference between a perennial snow patch and a glacier?
- What causes a glacier to move downhill?
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
- 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
- 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.
- After 24 hours, remove the sealed container and drain all the water. Use some tissue to pat dry the snow.
- Reseal the container and put it back in the ice bath for a few hours.
- 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)
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.
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.
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.
Strange Glacier Phenomena
(some of the answers may come from the
- Can glaciers make sounds?
- What are the small black wingless springtail bugs that live in firn on glaciers?
- What do ice worms eat?
- There is an image of a fossil in glacial till in this section. What is the fossil?
- What are sudden glacial outburst floods of water that can be catastrophic?
- Do ice quakes sound like earthquakes (a rumbling sound) or do they make a hissing and crackling sound?
- Are moulins holes in a glacier or the steel spikes you put on your boots to hike on a glacier?
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.
- outburst flood
- sounds like crispy rice cereal
- holes in a glacier allowing water to flow
- things that can be trapped in a glacier for
thousands of years
- 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)
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.
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.
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
- Ice Axes
- Crevasse Probes
- 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
(some of the answers may come from the vocabulary list)
- What is a tower of ice surrounded on all sides by crevasses?
- What causes a block of ice to break off and fall?
- What do snow bridges cover on a glacier?
- Does wind drifting cause mechanical hardening in the snow?
- Should you walk over a snow bridge?
- What do glacier travelers wear on their boots so they don’t slide on the ice?
- Name two correct ways to travel on a glacier.
- Name a piece of proper clothing to wear when traveling on glaciers.
- If they go fast enough, snow machines can cross a snow bridge safely. True or False?
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?
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.