Glaciers
A glacier is a persistent body of dense ice exceeding a surface area of 0.1 km² constantly moving under its own gravity; it forms where the accumulation of snow exceeds its ablation (melting and sublimation) over many years, often centuries. Glaciers slowly deform and flow due to stresses induced by their weight, creating crevasses, seracs, and other distinguishing features. They also abrade rock and debris from their substrate to create landforms such as cirques and moraines. Glaciers form only on land and are distinct from the much thinner sea ice and lake ice that form on the surface of bodies of water.
On Earth, 99% of glacial ice is contained within vast ice sheets in the polar regions, but glaciers may be found in mountain ranges on every continent, and on a few high-latitude oceanic islands. Between 35°N and 35°S, glaciers occur only in the Himalayas, Andes, a few high mountains in East Africa, Mexico, New Guinea and on Zard Kuh in Iran.
Glacial ice is the largest reservoir of freshwater on Earth. Many glaciers from temperate, alpine and seasonal polar climates store
water as ice during the colder seasons and release it later in the form
of meltwater as warmer summer temperatures cause the glacier to melt, creating a water source
that is especially important for plants, animals and human uses when
other sources may be scant. Within high altitude and Antarctic
environments, the seasonal temperature difference is often not
sufficient to release meltwater.
Because glacial mass is affected by long-term climate changes, e.g., precipitation, mean temperature, and cloud cover, glacial mass changes are considered among the most sensitive indicators of climate change and are a major source of variations in sea level.
Found only in Antarctica and Greenland, ice sheets are enormous
continental masses of glacial ice and snow expanding over 50,000 square
kilometers. The ice sheet on Antarctica is over 4200 meters thick in
some areas, covering nearly all of the land features except the
Transantarctic Mountains, which protrude above the ice. Another example
is the Greenland ice sheet.
Ice shelves occur when ice sheets extend over the sea, and float on
the water. In thickness they range from a few hundred meters to over
1000 meters. Ice shelves surround most of the Antarctic continent.
Retreating ice shelves may provide indications of climate change. For
example, the Larsen Ice Shelf has been retreating since the spring of
1998. Check out some of the remotely sensed satellite images scientists used to monitor the ice shelf breakup.
Ice caps are miniature ice sheets, covering less than 50,000 square
kilometers. They form primarily in polar and sub-polar regions that are
relatively flat and high in elevation. To really see the difference
between an ice cap and an ice sheet, compare Iceland and Greenland on a
globe or world map. The much smaller mass of ice on Iceland is an ice
cap.
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| glacier |
Glaciers begin to form when snow remains in the same area year-round,
where enough snow accumulates to transform into ice. Each year, new
layers of snow bury and compress the previous layers. This compression
forces the snow to re-crystallize, forming grains similar in size and
shape to grains of sugar. Gradually the grains grow larger and the air
pockets between the grains get smaller, causing the snow to slowly
compact and increase in density. After about two winters, the snow turns
into firn—an
intermediate state between snow and glacier ice. At this point, it is
about half as dense as water. Over time, larger ice crystals become so
compressed that any air pockets between them are very tiny. In very old
glacier ice, crystals can reach several inches in length. For most
glaciers, this process takes over a hundred years.
Glaciers Motion
Glaciers move, or flow, downhill due to gravity and the internal deformation of ice. Ice behaves like a brittle solid until its thickness exceeds about 50 m (160 ft). The pressure on ice deeper than 50 m causes plastic flow.
At the molecular level, ice consists of stacked layers of molecules
with relatively weak bonds between layers. When the stress on the layer
above exceeds the inter-layer binding strength, it moves faster than the
layer below.
Glaciers also move through basal sliding. In this process, a glacier slides over the terrain on which it sits, lubricated
by the presence of liquid water. The water is created from ice that
melts under high pressure from frictional heating. Basal sliding is
dominant in temperate, or warm-based glaciers.
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Types of Glaciers
Ice Sheets
Found only in Antarctica and Greenland, ice sheets are enormous
continental masses of glacial ice and snow expanding over 50,000 square
kilometers. The ice sheet on Antarctica is over 4200 meters thick in
some areas, covering nearly all of the land features except the
Transantarctic Mountains, which protrude above the ice. Another example
is the Greenland ice sheet.
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Ice Sheets |
Ice Shelves
Ice shelves occur when ice sheets extend over the sea, and float on
the water. In thickness they range from a few hundred meters to over
1000 meters. Ice shelves surround most of the Antarctic continent.
Retreating ice shelves may provide indications of climate change. For
example, the Larsen Ice Shelf has been retreating since the spring of
1998. Check out some of the remotely sensed satellite images scientists used to monitor the ice shelf breakup.
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Ice Shelves |
Ice Caps
Ice caps are miniature ice sheets, covering less than 50,000 square kilometers. They form primarily in polar and sub-polar regions that are relatively flat and high in elevation. To really see the difference between an ice cap and an ice sheet, compare Iceland and Greenland on a globe or world map. The much smaller mass of ice on Iceland is an ice cap.
Ice Streams and Outlet Glaciers
Ice streams are channelized glaciers that flow more rapidly than the
surrounding body of ice. For instance, the Antarctic ice sheet has many
ice streams flowing outward.
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Mountain Glaciers
These glaciers develop in high mountainous regions, often flowing out
of icefields that span several peaks or even a mountain range. The
largest mountain glaciers are found in Arctic Canada, Alaska, the Andes
in South America, the Himalayas in Asia, and on Antarctica.
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Mountain Glaciers |
Valley Glaciers
Commonly originating from mountain glaciers or ice fields, these
glaciers spill down valleys, looking much like giant tongues. Valley
glaciers may be very long, often flowing down beyond the snow line,
sometimes reaching sea level.
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Valley Glaciers |
Piedmont Glaciers
Piedmont glaciers occur when steep valley glaciers spill into
relatively flat plains, where they spread out into bulb-like lobes. The
Malaspina Glacier in Alaska is one of the most famous examples of this
type of glacier, and is the largest piedmont glacier in the world.
Spilling out of the Seward Ice Field, Malaspina Glacier covers over
5,000 square kilometers as it spreads across the coastal plain.
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Piedmont Glaciers |
Cirque Glaciers
Cirque Glaciers are named for the bowl-like hollows they occupy,
which are called cirques. Typically, they are found high on
mountainsides and tend to be wide rather than long.
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Cirque Glaciers |
Hanging Glaciers
Also called ice aprons, these glaciers cling to steep mountainsides.
Like cirque glaciers, they are wider than they are long. Hanging
glaciers are common in the Alps, where they often cause avalanches due
to the steep inclines they occupy.
Geologic work of glaciers
Glacial erosion
Glaciers themselves do relatively little significant erosion because ice is so soft. Under the weight of an ice sheet thousands of feet thick continental glaciers detach material from the surface by crushing the underlying bedrock. Once the material is loosened from the surface, ice can quarry (also known as plucking) the rock by freezing around and into fractures, then lifting it from the surface. The rock embedded in the ice gouges and smoothes bedrock surfaces by abrasion. Striations are fine scratches left in bedrock by abrasion. At a larger scale, linear grooves are ground into the bedrock in the direction of ice movement. Episodic movement leaves crescent-shaped marks called chatter marks gouged into the bedrock. The constant abrasion of exposed rock also creates polished bedrock
Glacier Transport and Deposition
Glacial drift is the general term applied to materials eroded from the surface and deposited by glaciers. Glaciers transport the embedded material towards the front of the glacier as if they were on a conveyor belt, or is deposit directly beneath the ice. Most material is embedded in the lowest few meters of the glacier and along its sides. Little drift material is lodged in the interior as flow through most of the glacier is laminar, except at the nose where thrust faulting of the ice occurs. When the ice becomes so burdened by its load of soil and rock fragments, it deposits the mixture of fine and coarse textured material in place as glacial till. Till is distinguished by its lack of sorting.
Avalanch
An avalanche (also called a snowslide or snowslip) is a rapid flow of snow down a slope. Avalanches are typically triggered in a starting zone from a mechanical failure in the snowpack (slab avalanche) when the forces on the snow exceed its strength but sometimes only with gradually widening (loose snow avalanche). After initiation, avalanches usually accelerate rapidly and grow in mass and volume as they entrain more snow. If the avalanche moves fast enough some of the snow may mix with the air forming a powder snow avalanche, which is a type of gravity current.
Slides of rocks or debris, behaving in a similar way to snow, are also referred to as avalanches. The remainder of this article refers to snow avalanches.
The load on the snowpack may be only due to gravity, in which case failure may result either from weakening in the snowpack or increased load due to precipitation. Avalanches that occur in this way are known as spontaneous avalanches. Avalanches can also be triggered by other loads such as skiers, snowmobilers, animals or explosives. Seismic activity may also trigger failure in the snowpack and avalanches.
Although primarily composed of flowing snow and air, large avalanches have the capability to entrain ice, rocks, trees, and other material on the slope, and are distinct from mudslides, rock slides, and serac collapses on an icefall. Avalanches are not rare or random events and are endemic to any mountain range that accumulates a standing snowpack. Avalanches are most common during winter or spring but glacier movements may cause ice and snow avalanches at any time of year. In mountainous terrain, avalanches are among the most serious objective natural hazards to life and property, with their destructive capability resulting from their potential to carry enormous masses of snow at high speeds.
There is no universally accepted classification of avalanches—different classifications are useful for different purposes. Avalanches can be described by their size, their destructive potential, their initiation mechanism, their composition and their dynamics.
Most avalanches occur spontaneously during storms under increased load due to snowfall. The second largest cause of natural avalanches is metamorphic changes in the snowpack such as melting due to solar radiation. Other natural causes include rain, earthquakes, rockfall and icefall. Artificial triggers of avalanches include skiers, snowmobiles, and controlled explosive work.
Avalanche initiation can start at a point with only a small amount of snow moving initially; this is typical of wet snow avalanches or avalanches in dry unconsolidated snow. However, if the snow has sintered into a stiff slab overlying a weak layer then fractures can propagate very rapidly, so that a large volume of snow, that may be thousands of cubic meters, can start moving almost simultaneously.
A snowpack will fail when the load exceeds the strength. The load is straightforward; it is the weight of the snow. However, the strength of the snowpack is much more difficult to determine and is extremely heterogenous. It varies in detail with properties of the snow grains, size, density, morphology, temperature, water content; and the properties of the bonds between the grains. These properties may all metamorphose in time according to the local humidity, water vapour flux, temperature and heat flux. The top of the snowpack is also extensively influenced by incoming radiation and the local air flow. One of the aims of avalanche research is to develop and validate computer models that can describe the evolution of the seasonal snowpack over time. A complicating factor is the complex interaction of terrain and weather, which causes significant spatial and temporal variability of the depths, crystal forms, and layering of the seasonal snowpack.
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Avalanch
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Facts about glaciers
Presently, 10% of land area on Earth is covered with glacial ice, including glaciers, ice caps, and the ice sheets of Greenland and Antarctica.
Glaciers store about 75% of the world's freshwater.
Glacierized areas cover over 15 million square kilometers (5.8 million square miles).
Antarctic ice is over 4.2 kilometers (2.6 miles) thick in some areas.
In the United States, glaciers cover over 75,000 square kilometers (30,000 square miles), with most of the glaciers located in Alaska.
During the last ice age, glaciers covered 32% of the total land area.
If all land ice melted, sea level would rise approximately 70 meters (230 feet) worldwide.
Glacier ice crystals can grow to be as large as baseballs.
The land underneath parts of the West Antarctic Ice Sheet may be up to 2.5 kilometers (1.6 miles) below sea level, because of the weight of the ice.
North America's longest glacier is the Bering Glacier in Alaska, measuring 204 kilometers (127 miles) long.
Glacial ice often appears blue when it has become very dense. Years of compression gradually make the ice denser over time, forcing out the tiny air pockets between crystals. When glacier ice becomes extremely dense, the ice absorbs all other colors in the spectrum and reflects primarily blue, which is what we see. When glacier ice is white, that usually means that there are many tiny air bubbles still in the ice.
The Kutiah Glacier in Pakistan holds the record for the fastest glacial surge. In 1953, it raced more than 12 kilometers (7.5 miles) in three months, averaging about 112 meters (367 feet) per day.
In Washington state alone, glaciers provide 1.8 trillion liters (470 billion gallons) of water each summer.
Antarctic ice shelves may calve icebergs that are over 80 kilometers (50 miles) long.
Almost 90% of an iceberg is below water—only about 10% shows above water.
The Antarctic continent has has been at least partially covered by an ice sheet for the past 40 million years.
From the 17th century to the late 19th century, the world experienced a "Little Ice Age," when temperatures were consistently cool enough for significant glacier advances.
The effect of Global warming in Glaciers
Melting Glaciers, Early Ice Thaw
Rising global temperatures will speed the melting of glaciers and ice caps and cause early ice thaw on rivers and lakes.
Warning signs today:
After existing for many millennia, the northern section of the Larsen B ice shelf in Antarctica -- a section larger than the state of Rhode Island -- collapsed between January and March 2002, disintegrating at a rate that astonished scientists. Since 1995, the ice shelf's area has shrunk by 40 percent.
According to NASA, the polar ice cap is now melting at the alarming rate of nine percent per decade. Arctic ice thickness has decreased 40 percent since the 1960s.
Arctic sea ice extent set an all-time record low in September 2007, with almost half a million square miles less ice than the previous record set in September 2005, according to the National Snow and Ice Data Center. Over the past 3 decades, more than a million square miles of perennial sea ice -- an area the size of Norway, Denmark and Sweden combined -- has disappeared.
Multiple climate models indicate that sea ice will increasingly retreat as the earth warms. Scientists at the U.S. Center for Atmospheric Research predict that if the current rate of global warming continues, the Arctic could be ice-free in the summer by 2040.
At the current rate of retreat, all of the glaciers in Glacier National Park will be gone by 2070.
Sea-Level Rise
Current rates of sea-level rise are expected to increase as a result both of thermal expansion of the oceans and melting of most mountain glaciers and partial melting of the West Antarctic and Greenland ice caps. Consequences include loss of coastal wetlands and barrier islands, and a greater risk of flooding in coastal communities. Low-lying areas, such as the coastal region along the Gulf of Mexico and estuaries like the Chesapeake Bay, are especially vulnerable.
Warning signs today:
Global sea level has already risen by 4 to 8 inches in the past century, and the pace of sea level rise appears to be accelerating. The Intergovernmental Panel on Climate Change predicts that sea levels could rise 10 to 23 inches by 2100, but in recent years sea levels have been rising faster than the upper end of the range predicted.
In the 1990s, the Greenland ice mass remained stable, but the ice sheet has increasingly declined in recent years. This melting currently contributes an estimated one-hundredth of an inch per year to global sea level rise.
Greenland holds 10 percent of the total global ice mass. If it melts, sea levels could increase by up to 21 feet.
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