Edited by John P. Rafff.rty, Associate Editor, Eartii Sciences
- Educational Publishing -
IN ASSOCIAIION WITH
- Educational Publishing -
IN ASSOCIAIION WITH
Published in 2011 by Britannica Educational Publishing (a trademark of Encyclopedia Britannica, Inc.) in association with Rosen Educational Services, LLC 29 East 21st Street, New York, NY 10010.
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Library of Congress Cataloging-in-Publication Data
Glaciers, sea ice, and ice formation / edited by John P. Rafferty—ist ed.
p. cm. — (Dynamic earth) "In association with Britannica Educational Publishing, Rosen Educational Services." Includes bibliographical references and index. ISBN 978-1-61530-189-8 (eBook)
i. Glaciers. 2. Sea ice. 3. Ice. I. Rafferty, John P. II. Series: Dynamic earth.
On the cover: The Vatnajokull glacier in Iceland's Jokulsarlon Glacier Lake. Michele Falzone/ Photographer's Choice/Getty Images
On page 12: The San Rafael glacier in Chile, c. 1950. Three Lions/Hulton Archive/Getty Images
On pages 20,226, 232, 235, 246: Ice melting in Lake Baikal, located in southeast Siberia. Shutterstock.com
Pages 21, 29, 53, 74, 116, 145, 182 © www.istockphoto.com/Pablo Caridad
Chapter 1: Ice on Planet Earth 21
The Water Molecule 21
The Ice Crystal 23
Hoarfrost and Rime 24
Mechanical Properties 25
Thermal Properties 26
Optical Properties 27
Electromagnetic Properties 27
Ice Ages 28
Chapter 2: Permafrost 29 The Origin and Stability of Permafrost 30 Air Temperature and Ground Temperature 31
Climatic Change 32 Permafrost Distribution In the Northern Hemisphere 33 The Local Thickness of Permafrost 34
The Effects of Climate 35 The Effects of Water Bodies 35 The Effects of Solar Radiation, Vegetation, and Snow Cover 36
Types of Ground Ice 37
Ice Wedges 39
Active Wedges, Inactive Wedges, and
Ice-Wedge Casts 40 Surface Manifestations of Permafrost and Seasonally
Frozen Ground 41
Polygonal Ground 42
Thermokarst Formations 43
Patterned Ground 45
Soil Flow 46
The Study of Permafrost 47 Problems Posed by
Permafrost Engineering 48 Development in
Permafrost Areas 50 Chapter 3:
Ice in Lakes and Rivers 53
The Seasonal Cycle 53
Ice Formation in Lakes 54
Nucleation of Ice Crystals 56 The Effects of Wind Mixing 57
The Rates of Growth 58
Variations in Ice Structure 60
Ice Decay 61
Thinning and Rotting 61
Melting 62 The Geographic Distribution of Lake Ice 63
Ice Formation in Rivers 64
Accumulating Ice Cover 65
Growth of Fixed Ice Cover 67
Ice Buildups 67
Decay and Ice Jams 69
River Ice Modification 70 The Geographic Distribution of River Ice 72
Chapter 4: Glaciers and
Ice Sheets 74 The Formation and
Characteristics of Glacier Ice 76
Mass Balance 78
Heat or Energy Balance 80
Glacier Flow 81 Mountain Glaciers and Other
Smaller Ice Masses 84 Classification of
Mountain Glaciers 84
Surface Features 86 Mass Balance of
Mountain Glaciers 88 The Flow of
Mountain Glaciers 90
Glacier Hydrology 92
Glacier Floods 93
Glacier Surges 94
Tidewater Glaciers 95
Rock Glacier 96
The Great Ice Sheets 97
The Antarctic Ice Sheet 97
The Greenland Ice Sheet 99 Accumulation and Ablation of the Ice Sheets 102
Net Mass Balance 105
The Flow of the Ice Sheets 107
Ross Ice Shelf 108
The Information from
Deep Cores 110 The Response of Glaciers to Climatic Change 113
Glaciers and Sea Level 114
Chapter 5: Glacial
General Considerations 116
Glaciers and Topography 116
Glacial Erosion 118
Glacial Deposition 121
Erosional Landforms 124 Small-Scale Features of Glacial Erosion 124
Rock Polish 124
Striations 125 P-Forms and Glacial
Grooves 126 The Erosional Landforms of Valley Glaciers 126 Cirques, Tarns, U-shaped Valleys, Arêtes, and Horns 127
Hanging Valleys 130
Paternoster Lakes 130
Roches Moutonnées 131
Rock Drumlins 131 The Erosional Landforms of Continental Glaciers 131
Depositional Landforms 135 The Depositional Landforms of
Valley Glaciers 136
The Depositional Landforms of
Continental Glaciers 137 Meltwater Deposits 138
Glaciofluvial Deposits 139 Glaciolacustrine Deposits 141
Periglacial Landforms 143
Felsenmeers, Talus, and Rock Glaciers 143
Chapter 6: Icebergs and Sea Ice 145
The Origin of Antarctic Icebergs 145
The Origin of Arctic Icebergs 148
Iceberg Structure 150
Iceberg Size and Shape 153 Erosion and Melting 155
The Distribution of Icebergs and Their Drift Trajectories 157
Iceberg Scour and Sediment Transport 162
The Climatic Impacts of Icebergs 165
The Impacts on Ice Sheets and Sea Level 165 The Impact on Ocean Structure 166
Iceberg Detection, Tracking, and
tabular tabular drvduck drvduck
Sea Ice 170
Ice Salinity, Temperature, and Ecological
Sea Ice Formation and Features 172
Pack Ice Drift and Thickness 176
Sea Ice and its
Interactions with the
Oceans, Atmosphere, and Climate 177
The Emerging Impacts of Recent Changes to
Sea Ice 179
Chapter 7: The Arctic and Antarctic 182
The Arctic 182 Continental Ice Sheets of the Past 183
Drainage and Soils 189 Present-Day Glaciation 191
Glacier Groups 193
The Arctic Ocean 201
Oceanography 202 Sea Ice in the
Arctic Ocean 206
Physical Features 211
Antarctic Glaciation 213
The Surrounding Seas 217
The Power of Ice 224
Appendix: Significant Glaciers, Ice Sheets, and Ice Shelves 226
Amery Ice Shelf 226
Beardmore Glacier 226
Filchner Ice Shelf 227
Larsen Ice Shelf 227
Laurentide Ice Sheet 228
Ronne Ice Shelf 229
Shackleton Ice Shelf 229
Skelton Glacier 230
Wilkins Ice Shelf 230
ater is at once simple and complex. Its study begins with a single molecule. A water molecule consists of two hydrogen atoms and one oxygen atom, as noted by its chemical formula H2O. In water's gaseous state, thermal energy—heat—enables single water molecules to float freely, mostly independently of one another. In water's liquid state, hydrogen atoms constantly form and break bonds with other hydrogen atoms, as exhibited by water's characteristic fluidity. In the solid phase, water's molecular bonds are dictated by the oxygen atoms, which form crystalline shapes, or ice.
On Earth, ice appears in many forms, from the ground-imbedded nature of permafrost to the towering majesty of mountainous glaciers. Each of these varied states is examined thoroughly in Glaciers, Sea Ice, and Ice Formation. Readers will virtually traverse pingos, rappel down cirques, and explore the frozen tundra of the Arctic and Antarctic as the complexities of this deceptively simple entity are revealed.
Unlike most materials, water's solid state is less dense than its liquid state, a factor that explains why ice floats above liquid water. People witness natural ice formations seasonally on lakes, ponds, and other small bodies of water. As average daily temperatures fall, thermal energy is released from liquid water until the water's surface temperature cools to the freezing point, and then below the freezing point—a process called supercooling. When this occurs, ice particles begin to form above the water's surface.
Though wind and currents typically interfere with the freezing process, ice can form on fast-flowing rivers. Ice crystals, called frazil, can appear as a thin layer of ice in a slow-moving river or as slush in faster currents. As ice forms, it slows down the velocity of the water. As long as air temperatures remain below freezing 32° F (0°C) near the surfaces of lakes and rivers, ice will thicken from the bottom layers up. In other words, the water is at its coldest when the freezing ofits surface begins. As temperatures increase, however, ice on rivers and lakes decays due to the penetration of solar radiation.
The salinity of the world's oceans keeps them from freezing at the same temperature as bodies of freshwater. Salt ions disrupt the formation of ice crystals at the normal freezing temperature. However, in extreme polar areas, sea ice can form. There are three kinds of sea ice: landfast ice, which is attached to another surface or caught between icebergs; marine ice, which forms on the bottom of Antarctica's ice shelves; and pack ice, which drifts with currents and wind. As with ice on lakes and rivers, sea ice experiences changes throughout the seasons.
Sea ice serves several purposes, such as providing a habitat to ocean life. Arctic and Antarctic sea ice supports bacteria, algae, and fungi that live off the nutritive brine and, in turn, become a food source for organisms such as krill. Sea ice also provides a measure of climate control. Snow that comes to rest on the ice insulates the ocean water from colder air above, retaining heat that influences weather and helps to sustain ocean life.
Ice also forms on land as well as within bodies of water. Permafrost, which is moisture-laden ground that is frozen for two years or longer, covers about 20 percent of Earth's surface. It exists in climates where the average annual air temperature is 32° F (0°C) or colder. For permafrost to form, consistently low air temperatures draw the thermal energy from deep inside the earth, freezing water within the soil. Each winter, the layers of permafrost grow deeper until halted by the heat of Earth's core. It takes thousands of years for permafrost to embed itself hundreds of feet below the surface.
In summer, warmer air temperatures heat the top "active" layer of the ground, a region that is just a few meters thick. As permafrost freezes and thaws year after year, it produces large-scale geomorphic polygon patterns on the surface. Thawing permafrost can create small mounds or depressions in the earth, as well as larger, more substantial tunnels and caverns.
There are five types of ground ice found in permafrost: pore, segregated (Taber), foliated (wedge), pingo, and buried ice. Each type is the result of the way in which water is distributed in the ground. For instance, pingos are hills of soil and rock with an ice core; some have been measured at almost 200 feet (30 metres). Pingos form as groundwater freezes, causing the water and sediment to "heave" upward.
Permafrost dictates how hundreds of thousands of people live. Parts of Canada, Alaska, China, and Russia lie on permafrost. Crop and plant growth are difficult in these areas, at best. Construction and transportation companies must factor in the changes in the active permafrost layer when planning infrastructure. Engineers have managed to work around some problems. For example, parts of the Trans-Alaska Pipeline System run above ground, raised above the surface on lines of individual supports. However, permafrost remains a daunting obstacle; some buildings, roads, railroads, and pipes need annual replacement.
Large bodies of ice that form on land aboveground, due to the recrystallization of solid water such as snow, are called glaciers. Glaciers start with ice crystals. As snow falls, the individual flakes break down under the forces of wind, evaporation, and pressure from overlying layers of precipitation to forge small, hard grains of ice. As more snow falls, the layers become more densely packed, and ice grains become larger, rounder, and less likely to allow gas and liquid to penetrate the mass. For the glacier to grow, it needs to accumulate more mass through precipitation, condensation, and other processes than will be lost through erosion, via melting or runoff. The process of erosion is known as ablation.
Glaciers are among the largest moving objects on Earth, though typically their motion is so slow as to be imperceptible. They have the properties of ice, and therefore can suffer the same consequences caused by stress. Top layers can become brittle and crack, creating deep crevasses. Lower layers are more plastic, meaning they spread out to relieve the pressure of weight. Stress from the top layers may cause the glacier's base to slip over the bedrock beneath. Water at the base—sometimes supplied by leaking crevasses or friction at the base—facilitates glacial movement.
While a majority of the world's ice can be found in regions near the North and South poles, glaciers can grow elsewhere--including in the cold, mountainous terrain near the Equator. Mountain glaciers are especially susceptible to bed slips as gravity acts on their mass, pulling them down mountain slopes. Types of mountain glaciers include valley glaciers and ice fields. Piedmont glaciers are found in flat areas, fed by valley glaciers. Hanging glaciers cling precariously to mountains, sometimes resulting in avalanches.
When mountain glaciers advance over bedrock, they greatly alter the landscape. Their enormous mass can "absorb" terrain as the melting base refreezes around rocks, prying them out of the surface. As a glacier retreats, or melts, it may leave behind what is known as till, comprised of huge boulders, hills of smaller rocks, or tiny bits of "rock flour." Moraines, fjords, and arêtes are land formations molded by glacial movement.
The largest glaciers, called ice sheets, extend over a large area and flow in many directions. The Antarctic and Greenland ice sheets hold 99 percent of the world's glacier ice. The Antarctic ice sheet covers an area of roughly 5.3 million square miles (13.8 million square kilometres). Though much smaller than the Antarctic sheet, the Greenland ice sheet is still much larger than any mountain glacier.
Glacial flow on ice sheets generally drains from high elevations to basins that feed ice streams. Flow is very slow in the interior of ice sheets, perhaps inches each year, speeding up towards the outer edges, hundreds of feet per year. The flow may create ice shelves, which are thick slabs of an ice sheet that extend into the ocean. Ice shelves may be hammered by waves, tides, ocean currents, sea ice, and other icebergs. These stresses cause crevasses that eventually sever and calve, or break apart from the ice shelf, unleashing icebergs.
Antarctica produces the largest icebergs. Called tabular icebergs, they may have a freeboard (height above water) of over 150 feet (45 metres) and be up to 1,300 feet (395 metres) thick. Occasionally, some of these bergs measure over 150 miles (240 km) long. In contrast, most Arctic icebergs are calved from fast-flowing glaciers; the mountainous terrain stresses the glacial flow, creating more crevasses. Arctic icebergs generally are smaller than their Antarctic counterparts. Even ice islands, the largest of the Arctic icebergs, are only about 200 feet (60 metres) thick. All icebergs, regardless of their region of origin, are much larger than they appear above water.
All icebergs warm as soon as they break away from their parent glacier. Meltwater eats away at the glacier from the top, although it may refreeze as it seeps into the berg's cold core. Ocean waves also cause erosion. Icebergs may travel great distances, losing mass as they enter warmer waters. Some icebergs carry sediment and plant life, which help scientists identify the iceberg's origin.
As evidenced by the HMS Titanic tragedy of 1912, icebergs have historically been a danger to ships. Currently; ice patrols in the North Atlantic track icebergs and transmit reports to ships in the area. No such system is in place in the Southern Ocean. Ships, aircraft, and, most recently, satellites are used to keep track of icebergs that may invade shipping lanes. If icebergs need to be removed, they are usually towed. Special explosives that could fragment icebergs have not yet been perfected. Difficulties are compounded by the danger of affixing devices to capsize-prone icebergs.
Glaciologists believe that massive ice formations such as the Antarctic and Greenland ice sheets, offer a glimpse of Earth's past—and possibly its future. Snow that has fallen for millions of years has accumulated in layers of ice that, when extracted as cores and tested, reveal climatic data from prehistoric times. Air bubbles in glacial ice have confirmed growing amounts of carbon dioxide in the air. The increases in atmospheric carbon dioxide concentration is a contributing factor to global warming, which is an increase in Earth's near-surface air temperatures believed to be caused by human activities. Global warming has been cited as a reason for the decrease in Arctic sea ice; the near disappearance of the Ward Hunt Ice Shelf in Canada's Ellesmere Island, along with parts Antarctica's Larsen and Wilkins ice shelves; and the continued retreat of the world's mountain glaciers. Global warming also has been named a culprit in rising sea levels, although the amount of that increase due to glacial ablation is uncertain. Recent core samplings offer evidence that climate change can occur quite rapidly, perhaps as much as 9° to i3°F (5° to 7°C) within a single decade. In addition, it has been estimated that if all Earth's glaciers melted, sea levels would likely rise about 300 feet (90 metres) worldwide, and humans would lose three-fourths of Earth's freshwater to the ocean. Facts and statistics such as these only serve to underscore the tremendous power of ice.
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