Glaciers Alaskas Rivers of Ice_1of2
Transcript
00:00Alaska exhibits many contrasts. Majestic mountains, long convoluted shorelines, and thousands
00:12of glaciers. Less than 200 square miles of glaciers exist in other states, including
00:19Washington, Wyoming, Montana, Oregon, California, Colorado, Idaho, and Nevada. The Glacier
00:29Ice in the rest of the United States combined totals less than the area of a single large
00:35Alaska glacier. Alaska's glaciers range in size from tiny cirque glacier remnants, covering
00:42fractions of a square mile, to massive piedmont glaciers, such as Bering and Malaspina, each
00:49covering more than 2,000 square miles, and each larger than the state of Rhode Island.
00:55With a glacier cover of about 29,000 square miles, Alaska has about one-half the glacier
01:01cover of Asia, the same glacier cover as Russia, two-and-a-half times the glacier cover of
01:08China and Tibet, three times the glacier cover of South America, six times the glacier cover
01:14of Iceland, 12 times the glacier cover of Europe, 75 times the glacier cover of New
01:21Zealand, about 150 times the glacier cover of the rest of the United States, and about
01:28a thousand times the glacier cover of Africa. Glaciers are an important part of Alaska's
01:34landscape. Most visitors to Alaska rarely see the largest of Alaska's glaciers, Bering
01:40and Malaspina, except perhaps from the window of an airplane. Rather, the majority view
01:46a variety of medium-to-small valley glaciers, Mendenhall, Matanuska, Exit, Portage or Worthington,
01:55accessible by automobile or bus. Or they encounter the glaciers of Tracey Arm, Glacier Bay, Yakutat
02:03Bay, Prince William Sound or Kenai Fjords from the deck of a tour boat, cruise ship
02:09or ferry. Few Alaska visitors walk on a glacier or even touch glacier ice, save for those
02:16who collect the small ice chunks that drift to the shore of Portage Lake, near Anchorage.
02:22The few who are fortunate enough to explore the surface of a glacier or to linger in front
02:28of a tidewater glacier and watch it calve, form memories and take home stories that last
02:33a lifetime. This is the story of Alaska's glaciers. For those who have not had the opportunity
02:40to examine and explore firsthand the majesty, the beauty and the power of a living Alaskan
02:49river of ice. No one knows the exact number of glaciers in Alaska, but it may exceed 50,000.
02:58Only about 650 of them have names. Since the late 19th century, more than 95% of Alaskan
03:05glaciers that extend below elevations of 5,000 feet have been retreating, thinning or stagnating.
03:13During the 20th century, as many glaciers retreated and shrank in size, they separated
03:20into individual retreating tributary glaciers. Ironically, even though the area and volume
03:26of glacier ice in Alaska decreased, the number of glaciers actually increased. We are lucky
03:34to live in a period of time, both climatic and geologic, when glaciers exist and when
03:40we have the technical capability to travel to them, fly over them and photograph them.
03:47Less than 300 years ago, much of the world remained ignorant of the existence of glaciers.
03:54Less than 200 years ago, the majority of the geological community failed to recognize the
03:59importance of glacial erosion. And slightly more than a century ago, more than 99% of
04:06all Alaskan glaciers lay secluded, completely unknown. Today, scientists are answering many
04:13questions about why glaciers form, how they move, and what they do.
04:21Water is a naturally occurring chemical compound made up of hydrogen and oxygen, H2O.
04:29The most abundant chemical compound found on the surface of the Earth. Water can exist
04:35in three physical states, as water vapor, a gas, as liquid water, and as ice, a solid.
04:44At different times, all three occur in the glacier environment. The three can and do
04:50frequently coexist. Not surprisingly, most of a glacier is composed of water, generally
04:58more than 95%. Glacier ice constitutes the largest reservoir of fresh water on Earth,
05:06and aside from the oceans, the second largest reservoir of water. Most water resides in
05:11the oceans, 97.2%. The remaining 2.8% occurs in the atmosphere, lakes, and rivers, groundwater,
05:19soil, and glaciers. To look at it in a different way, if there were 1,000 drops of water on
05:30Earth, 972 would be in the oceans. Less than one would be in the atmosphere. Less than
05:38one would be in lakes and rivers. About six would be in groundwater and soil moisture.
05:45And 21 would be in glaciers. Of the 2.8% that is fresh water, three-fourths is contained
05:54in glaciers. Three times more water exists frozen in glacier ice than all of the liquid
06:01fresh water on and in Earth. Most glacier ice is located in Earth's polar regions,
06:07with 91.4% in Antarctic glaciers and ice shelves, and 7.9% in Greenland glaciers.
06:15The remaining 0.7% is located in ice caps, ice fields, and glaciers of North America,
06:24Asia, South America, Europe, Africa, and the islands of New Zealand and Irian Jaya. Using
06:33the 1,000 drop analogy, 914 would be in Antarctica, 79 would be in Greenland, about four in North
06:41America, about two in Asia, and less than one in South America, Europe, Africa, and
06:50the islands of New Zealand and Irian Jaya combined. Of the four North American drops,
06:57one would be in Alaska. Glaciers cover about 3.1% of Earth's surface, about 10.7% of the
07:05land, and about 5% of Alaska. Less than 20,000 years ago, during the peak of the most recent
07:16phase of the Ice Age, a period called the Pleistocene, glaciers covered about 25% of
07:24Earth's land surface. If all of the glacier ice were to melt, sea level would rise about
07:30265 feet, flooding every coastal city on the planet. If Alaska's glaciers melted, sea level
07:38would rise less than one foot. In simplest terms, a glacier is a mixture
07:47of ice and rock that moves downhill over a bed of solid rock or sediment under the influence
07:54of gravity. But a glacier contains more than just moving ice and rock. At different times
08:01of the year, liquid water may be on, in, or under the glacier. As this water flows, it
08:09transports sediment. As the ice moves, it modifies the bed over which it flows, sometimes
08:17eroding, and sometimes depositing sediment. In reality, an Alaskan glacier is a complex
08:24and dynamic system that is composed of the glacier, its bed, and surrounding environment,
08:32and the sediment it erodes and deposits. All are interrelated. All are continuously changing
08:40in response to short-term fluctuations in temperature and precipitation, and to longer-term
08:46fluctuations in climate. To understand how an Alaskan glacier system
08:52behaves, let's examine the components of a glacier and its environment. First, the glacier
08:58ice. Second, the complex array of deposits a glacier produces as it advances, melts in
09:06place, or retreats. And third, the bed, valley, fjord, or channel in which and over which
09:16the glacier flows. Glacier ice is different from all other ice
09:24on Earth. Unlike lake ice, or sea ice, or even refrigerator ice, which forms by the
09:33freezing of liquid water, glacier ice evolves through the metamorphism of snow. Six-sided
09:40elongated crystals form through compaction, compression, and recrystallization of individual
09:48snowflakes to eventually produce glacier ice. This metamorphism happens not overnight,
09:56but rather over a number of years. As each successive year's snowfall creates a new layer
10:03on top of the older layers, weight and pressure increase, and transformation takes place.
10:11The more new snow that accumulates, the greater the overburden weight and pressure, and the
10:18quicker the change occurs. Each new layer alters the density, volume, and crystal structure
10:25of the snow below. In simple terms, density is the comparison of the weight of any object
10:33to an equal volume of liquid water. By definition, a cube of liquid water that measures one centimeter
10:41by one centimeter by one centimeter in size, or one cubic centimeter, has a weight of one
10:48gram, and a density of one. A centimeter is a length of a little less than one-half inch.
10:56Newly fallen snow consists of individual six-sided crystals that have a density of .1 to .3. Glacier
11:06ice has a density of about .9. Thus, in the metamorphic process that changes snow to ice,
11:14a volume decrease of up to nine times occurs. Snow, generally after surviving one summer
11:22melt season, converts to a material called fern. In each successive year, the fern becomes
11:29denser as air is forced out. The density of fern ranges from about .4 to .8. As fernification,
11:39the process of changing snow to fern to glacier ice occurs, original layering disappears,
11:46and individual snowflakes merge to form granules of solid ice. These original layers frequently
11:54undergo deformation and contortion during fernification. The change from snow to glacier
12:01ice may take 5 to 20 years in areas of high accumulation, or many decades in areas of
12:08lower accumulation. Warming above the freezing point may completely destroy crystal formation
12:14and establish ice layers instead. Finally, when an interconnected network of ice crystals
12:21forms that will not permit the movement of air and liquid water, glacier ice is created.
12:28The thickness of Alaskan glacier ice accumulations varies from a minimum of a few hundred feet
12:34to more than 3,000 feet. For ice to flow, it must be more than 50 feet thick. Because
12:41of differences in glacier motion and flow, ice crystals of different size and type are
12:47segregated into bands or layers, called folia, with the layering or banding termed foliation.
12:56Often sequences of folia consist of alternating layers of clear and bubbly ice, or alternating
13:03layers of fine-grained and coarse-grained ice. Rarely does the foliation correspond
13:10to the original snow and fern accumulation layers.
13:18Glacier ice has a density range of 0.88 to 0.92, depending on the amount of air that
13:25is trapped between and within crystals, and often appears blue. It is impermeable to air
13:32and water. Individual ice crystals may grow to more than a foot long. By comparison, refrigerator
13:40ice, sea ice, and lake ice are minimally crystalline, are much less dense, and contain much more
13:47air. All form quickly, without any overburden pressure.
13:53Glacier ice looks blue because the physical characteristics of water molecules absorb
13:58all colors except blue, which is reflected. At one time, glaciologists used the French
14:05term neve interchangeably with fern, but in today's vocabulary, neve means an area
14:12covered with perennial snow, or the area of accumulation at the upper end of a glacier.
14:19Air trapped within glacier ice is frequently subject to pressures that may exceed 1,000
14:25pounds per square inch. When glacier ice calves at near sea-level pressures, producing icebergs,
14:33bubbles contained in the icebergs are in equilibrium with the depths and pressures under which they
14:39formed. As melting progresses and the surrounding glacier ice barrier thins, the pressurized
14:46air bubbles escape, frequently producing a resounding pop. With a sound known as ice
14:53sizzle or bergy seltzer, hundreds of these bubbles break almost simultaneously, creating
14:59a continuous crackling sound reminiscent of Rice Krispies.
15:05The energy released through ice sizzle is intense. Sound generated by ice sizzle has
15:11been detected up to 100 miles away by underwater hydrophones, sound detection devices used
15:18in tracking distant ship movements. Glaciologists laughingly tell of celebrating the completion
15:25of a deep ice core drilled to more than 2,000 feet. The crew chilled the party's liquid
15:30refreshments with ice from the bottom of the core. The celebration quickly turned disastrous
15:37as glass after glass cracked under the impact of the bursting high-pressure bubbles. To
15:43experience ice sizzle on a greatly reduced scale, a person needs only a glass of water
15:49and a refrigerator ice cube. Although there is no pressure difference between the water
15:54and the bubbles in the ice cube, small audible pops can be heard as the cube melts.
16:02As melting continues, cracks may develop along bubble planes and the ice may split or fracture.
16:09In a similar fashion, glacier icebergs may fracture or roll over following intense episodes
16:15of ice sizzle. Even beached icebergs can be heard popping away as they sit on the sand.
16:23If you ever have the opportunity to be near an iceberg, listen closely and you will hear
16:27the symphony of bubbles as they play the themes of ice sizzle and Burgi Seltzer.
16:34Liquid water constitutes an integral part of an Alaskan glacier. Streams exist in, on
16:41and under most of them. During summer months, much of a glacier's surface becomes wet with
16:48streams flowing into crevasses or narrow tubular chutes called moulons or glacier mills. Running
16:56water frequently melts channels 15 feet wide or more into the surface of a glacier.
17:10Glacier sediment of any size, whether microscopic silt particles or boulders larger than a house,
17:17is called drift. Drift can be deposited on the surface of a glacier in several ways,
17:23by running water, by avalanches, or through the air as falls of rock, dust or volcanic
17:31ash. Drift can also accumulate around the entire perimeter of the bed of a glacier and
17:38can build up distinct layers within the glacier. The term stems from the early impression that
17:45all these sedimentary deposits attributed to glaciers were the result of the great biblical
17:50flood described in Genesis.
17:56Sediment deposited directly by ice is called till and is generally poorly sorted and not
18:02layered. Glacial geologists recognize two types of till, lodgment till, material plastered
18:09into place by ice as it moves forward, and ablation till, material dropped to the ground
18:16as stagnant ice melts in place. After a glacier has melted, a blanket of till, called ground
18:23moraine, usually covers all surfaces over which a glacier flowed.
18:31If the terminus of a glacier remains essentially in the same place for a period of time, a
18:36ridge or mound of till develops adjacent to the ice. If this moraine is the farthest
18:43down valley, marking the maximum extent of the glacier, it is called the terminal or
18:49end moraine. Up-valley are successive moraines, each representing a position where the ice
18:56margin temporarily stood still during a period of retreat.
19:02These are termed recessional moraines. Both terminal and recessional moraines often block
19:09meltwater streams, creating moraine-dammed glacial lakes.
19:15Yolkalups are outburst floods that form through the failure of glacier ice-dammed or moraine-dammed
19:22lakes or through subglacial volcanic eruptions. The term is adopted from Iceland, where a
19:30detailed recorded history of periodic glacier floods extends back to the 14th century. Sedimentary
19:38deposits document many Alaskan Glacier Yolkalup events.
19:44As a glacier slides over bedrock, it carries and drags a load of sediment and rock known
19:51as the basal till layer. This layer, frozen into the basal ice, polishes and scrapes away
19:59small rock particles known as rock flour through abrasion.
20:06In addition to abrasion, which produces material on the order of fractions of an inch to inches,
20:13glaciers quarry large blocks of bedrock prepared for transport by the freezing and thawing
20:19of water in cracks, joints and fractures. This transportation of larger blocks is known
20:26as glacial plucking. Plucked particles may reach dimensions in tens of feet or more.
20:35Two other types of moraines occur on the surface of a glacier, lateral moraines and medial
20:42moraines. A lateral moraine develops on each side of a valley glacier and consists of abraded
20:50sediment and plucked rock material from the side valley walls, or rock and sediment that
20:57avalanche onto the surface of the ice. Where two valley glaciers coalesce, lateral
21:04moraines merge to form a medial moraine, one that is now in the middle of the combined
21:11glacier. Lateral moraines are frequently preserved after a glacier melts away, generally as a
21:18veneer of sediment plastered on the valley wall. Medial moraines rarely survive, as melt-water
21:26streams rework them and transport away much, if not all, of the sediment. Large valley
21:33glaciers or piedmont glaciers may show 20 or more distinct medial moraines, indicating
21:40the enormous number of tributary glaciers that have joined to form the large glacier
21:46system. Running glacial melt-water transports much
21:52glacially eroded sediment, depositing it in layers in front of the glacier on a broad,
21:58low-angle surface known as an outwash plain. Occasionally a block of ice is left behind
22:04during retreat or carried by a melt-water stream onto the outwash plain, then buried
22:11in the sediment. As the ice melts, a depression called a kettle forms and continues to enlarge
22:18until all the ice melts. A pitted outwash plain is one with many kettles.
22:26On the outwash plain, the volume of sediment deposited is often so great that it cannot
22:32be transported at one time by the quantity of water available. Braided streams dominated
22:39by meandering channels and ever-changing bars continuously rework the sediment.
22:46Examples are the Copper River, north of Cordova, the Alsek River, south of Yakutat, and the
22:54Sissitna River, south of the Alaska Range. These braided streams transport rock flour
23:01and deposit it in large plumes in lakes, bays, Cook Inlet, and the Gulf of Alaska.
23:10Valleys leading away from the front of a glacier also may receive a substantial amount of outwash
23:16fill. These are called valley train deposits. Some sedimentary deposits accumulate where
23:24running water comes into direct contact with ice. The most common type of deposit is a
23:30poorly sorted sand and gravel mass called caim, which forms in direct contact with stagnant
23:37ice. Caims, which generally have some stratification, develop within cracks, holes, or crevasses
23:45in the ice, or between the ice and the land surface. A well-stratified deposit known as
23:51a caim terrace often builds up between the glacier and its valley wall. When the ice
23:58melts, the terrace remains along the valley side and is often confused with a lateral
24:04moraine. Crevasse fills and eskers are other common
24:09kinds of caim deposits. Running water on the surface of a glacier often washes sediment
24:16into crevasses, where the sediment remains. When the ice melts, the crevasse fills persist
24:23as long, steep, narrow, stratified ridges. Running water under a glacier may erode a
24:30meandering channel in the ice that can fill with sediment. As the glacier melts, a meandering
24:36stratified sediment ridge called an esker may emerge from underneath the retreating
24:42terminus. Eskers vary in height from several feet to more than 100 feet, and in length
24:49from a few hundred feet to tens of miles. Where glacier meltwater streams empty into
24:56lakes or bays, deltas form. When the glacier terminus ends in a body of water, blocks
25:03of ice break off and float away. The process of the blocks separating from the main glacier
25:10is known as calving. If any sediment is embedded in the icebergs that drift away, that sediment
25:17is referred to as being ice-rafted away from the glacier. As the iceberg melts or turns
25:24over, the sediment it carries falls to the bottom of the lake, bay, fjord or ocean. During
25:31the Pleistocene, ice-rafted sediment eroded by Alaskan glaciers was deposited in the Pacific
25:38Ocean. Rocks which originated in the St. Elias and Chugach Mountains have been recovered
25:44in cores collected from the seafloor more than 1,000 miles south of their Alaskan source.
25:55Glaciers generally flow along the path of least resistance. They commonly occupy a stream
26:01valley or a fault trench and widen and modify it by abrasion and plucking. Streams flow
26:09in V-shaped valleys. Glaciers modify the shape of these stream valleys and change them to
26:15a characteristic U-shaped glacial valley. In the process of widening and deepening coastal
26:23valleys, glaciers often erode the valley floor to below sea level and extend the valley onto
26:29the continental shelf or into deep bays where valleys become fjords. Alaska has hundreds
26:37of fjords, the best known located along the Gulf of Alaska coast, in the Inside Passage,
26:44Glacier Bay and Prince William Sound. Large fjords may be up to five miles wide and have
26:52nearly two miles of vertical relief. Glacier valleys are not only rounded or U-shaped in
27:02cross-section but have rounded amphitheater-like basins cut in their upper ends or sides. These
27:09half-bowl shaped depressions called cirques have lips at their lower ends called thresholds.
27:17Small glaciers with in cirques may have tongues of ice cascading or even avalanching down
27:24the valley walls until they reach the main glacier. Glaciers change topography. They
27:31tend to round, deepen, over-steepen and streamline the surfaces they contact. Glaciers may be
27:39the most efficient mechanism on Earth for erosion. Glaciers may be the most efficient
27:46in the world for erosion. Valley glaciers often develop in parallel, closely spaced
27:50valleys. As individual glaciers erode and widen their valleys, the bedrock ridge that
27:57separates two adjacent glaciers narrows and becomes over-steepened. A series of narrow,
28:04jagged spires, much like the edge of the blade of a serrated knife, develop along the ridge
28:10crest. These features are called aretes, after the French word for fish bones. Eventually,
28:18with continued erosion, the arete disappears and the glaciers merge. As glaciers erode
28:28headward or up-valley into the side of a mountain, they modify the configuration of the mountain's
28:35summit to form a steep-sided, sharp-pointed, pyramidal mountain peak called a horn. When
28:42four glaciers erode a symmetrically shaped horn, it's termed a matter horn, after the
28:48famous peak in the Swiss Alps. Horns, aretes and cirques, the most common bedrock features
28:55that can be observed in recently deglaciarized parts of Alaska, are often seen poking through
29:01the edges of Alaska's many valley glaciers. As a glacially eroded area emerges from melting
29:12ice, many large, rounded, asymmetrical bedrock knobs, called ruches moutonnées, after the
29:18French term for a grazing sheep, begin to emerge. These knobs, which form sub-glacially,
29:25have a gentle slope on their up-glacier side and a steep to almost vertical face on their
29:31down-glacier side. This geometry is due to the glacier gliding and gently overriding
29:37the near side and then plucking blocks of rock from the knob's far side as it flows
29:43past. Glacial geologists use the shape and orientation of ruches moutonnées to help
29:50interpret glacier flow directions. When a glacier thins, as melting increases or as
29:58a recession begins, a sharp line that marks the maximum extent of the glacier's margins
30:04appear above the ice on the side of the glacier. This line, which may either be a change in
30:10type or presence of vegetation, or a change from weathered to unweathered bedrock, is
30:16called a trim line. Comparison of the height of the most recent trim line above the glacier
30:23with the height of the ice surface is useful in determining a glacier's health.
30:33As glaciers slide over their beds, they often polish, fracture, groove, or striate the surface.
30:41When ice motion is irregular, large particles may grind along the bottom and carve individual
30:47or multiple crescent-shaped fractures, gouges, or chatter marks, generally perpendicular
30:53to the direction of glacier flow. Striations and grooves are long, straight, parallel furrows
31:00oriented in the direction of glacier motion.
31:07Glaciers are classified either by their size, from tiny cirque glaciers to continental-covering
31:14ice sheets, by their thermal characteristics, polar versus temperate, or by other traits,
31:20such as ending in tidewater. The classification used here is based on size, shape, and geographic
31:27location. Glaciers in Alaska range in size from smaller than a football field to larger
31:33than the state of Rhode Island. A cirque glacier is a small glacier that forms within a cirque
31:40basin. Today, many cirque glaciers are not thick enough to reach the lip and remain in
31:45the basin. Some overtop the threshold and flow down slope. Cirque glaciers can be found
31:51on the summits of many of the highest mountains in the state, including Mounts McKinley, St.
31:56Elias, and Fairweather. Small cirque glaciers are sometimes called glacierettes.
32:04Valley glaciers originate from ice accumulating in one or more basins or cirques, or from
32:10overflows from an ice field or ice cap on top of a plateau. Many of the larger valley glaciers
32:18in Alaska exceed 20 miles in length. Hubbard Glacier, more than 70 miles, is the longest.
32:26The most spectacular examples of Piedmont glaciers in Alaska are Malaspina and Bering
32:32glaciers, each with Piedmont lobes covering areas greater than 800 square miles. A Piedmont
32:39glacier forms when one or more valley glaciers flows from a confined valley into a plain
32:45where it can expand into a broad, fan-shaped ice mask at the base of the mountain. An ice
32:53field covers a mountainous area where large, interconnecting valley glaciers are separated
32:58by mountain peaks and ridges, which project through the ice as noon attacks. The lower
33:05parts of the valley glaciers serve as outlet glaciers and drain ice from the ice field.
33:11Alaskan ice fields include the Stikine, Juneau, Hardin, and Sargent. Each has an area of more
33:20than 500 square miles. An ice cap is a dome-shaped or plate-like cover of perennial snow and
33:28glacier ice that completely covers the summits of a mountain mass so that no peaks emerge
33:34through it. The term also applies to a continuous cover of snow and ice on an Arctic or Antarctic
33:41landmass that spreads outward in all directions because of its mass. When thickness becomes
33:47great enough, tongues of ice overflow the basins or plateaus as valley glaciers. Ice
33:54caps cloak several Canadian Arctic islands. During the Pleistocene, parts of southern
34:00and southeastern Alaska were covered by large, ice-cap-like subcontinental glaciers. Ice
34:08sheets, or continental glaciers, are vast accumulations of glacier ice and snow that
34:15completely blanket a large landmass. The Antarctic and Greenland ice sheets are the
34:21largest on Earth. The Antarctic ice sheet covers more than 5 million square miles and
34:27in places exceeds 14,000 feet in thickness. An ice shelf is a floating glacier that forms
34:35when a land-based glacier extends into the ocean. Antarctica has many of these floating
34:42glaciers, with the largest, the Ross Ice Shelf, covering six times the area of Alaska's
34:49glaciers. Thermal characteristics of glaciers fall into two categories, temperate and polar.
34:57Temperate glaciers, including most in Alaska, remain warm enough during a part of the year
35:03for liquid water to form through melting. Polar glaciers evolve where the annual temperature
35:09stays below the freezing point so that liquid water is never present. In Alaska, polar glaciers
35:16are confined to high elevations. Several other terms describe unusual occurrences of glacier
35:23ice, such as rock glaciers, tidewater glaciers, and reconstituted glaciers. Rock glaciers
35:31frequently head in a circ and consist of a valley-filling accumulation of angular rock
35:37blocks. They resemble a glacier in shape, but have little or no visible ice at the surface.
35:44Investigations have shown that ice fills the spaces between rock blocks, and that rock
35:50glaciers move, although very slowly. A reconstituted glacier, also known as a reconstructed glacier
36:00or a glacier reminier, forms when pressure-melting regulation joins ice blocks that accumulate
36:07below the terminus of a hanging or circ glacier. Much of the ice in a reconstituted glacier
36:14accumulates through avalanching, or ice falls. Once reconstituted, these glaciers behave
36:21like normal valley glaciers.
36:26Flow within glaciers is a function of ice thickness, geometry, depth of the channel
36:32or valley, and temperature. In simplistic terms, the upper 100 to 150 feet of a glacier
36:39deforms and flows in a brittle fashion, often developing elongated cracks that may extend
36:46hundreds or thousands of feet across the glacier's surface. The cracks, which often change in
36:52size and shape as a glacier flows down valley, are called crevasses. Below that depth, pressure
37:00on the ice increases. Flow is more plastic, and the glacier moves fastest. It also slides
37:07the fastest in its center. Toward the bottom of the glacier, friction with a bed decreases
37:13the flow rate. At the head of a valley glacier, a single large crevasse or series of small
37:20crevasses develops, where moving ice pulls away from the rock or cirque wall. This crevasse
37:27system is called bird shrund. Typically, glaciers flow at rates of inches to 2 to 3 feet per
37:35day. Some glaciers, however, occasionally experience sudden large-scale, short-lived
37:40increases in their rates of movement, 10 to 100 or more times faster than normal. These
37:46rapid rates of movement are called surges. Austin Post, a retired U.S. Geological Survey
37:52glaciologist who has studied surges for nearly 50 years, attributes these rapid movements
37:58to a remarkable instability that occurs at periodic intervals in certain glaciers. Others
38:04have suggested that certain surges were the result of earthquakes, avalanches, and local
38:10increases in snow accumulation. However, recent studies of variegated and bearing glaciers
38:16suggest that blockage of subglacial drainage channels causes an increase in water pressure
38:23and volume below and within the glacier. This, in turn, results in a thin water layer at
38:29the base of the glacier that lubricates the bed and causes the glacier to move like a
38:35hydroplaning car. Based on aerial photographs, Post has identified more than 200 North American
38:42glaciers that either were surging or that had surged in the recent past. Alaska hosts
38:49at least two-thirds of these surging glaciers. Post identified the glaciers on the basis
38:54of intense crevassing and folded surface moraines that were produced by rapid ice displacements,
39:01characteristic of surge movements, or distinctive surface features that resulted from previous
39:08surges. He found that surging glaciers have specific geographic occurrences. In Alaska,
39:14they are restricted to the Alaska Range, eastern Wrangell Mountains, eastern Chugach Mountains,
39:21and the St. Elias Mountains near Yakutat and Glacier Bay. No surging glaciers have been
39:27identified in the Coast Mountains, west and central Wrangell Mountains, west and central
39:33Chugach Mountains, Kenai Mountains, or the Brooks Range. Post's analysis of surging glaciers
39:40showed that they exist in maritime to continental climates and in temperate to subpolar environments.
39:47Surging is independent of elevation, bedrock type, valley configuration, glacier orientation,
39:55or size. Malaspina and Bering glaciers show evidence of multiple surges. During a short
40:02interval of its 1956-1957 surge, Muldrow glaciers sped forward as much as 1,150 feet per day.