Glaciation
Glaciation
Introduction
Glaciation is the accumulation of ice in a given area when more snow falls in winter than melts in summer. The resulting body of ice is termed a glacier. When ice accumulates over many years, a glacier’s own weight causes it to flow like an extremely viscous (thick) liquid. As a glacier flows, it tends to cover an ever-larger land area. If a glacier covers more than 12 million acres (50,000 km2), it is termed an ice sheet. Antarctica and Greenland are each mostly covered by ice sheets. The Antarctic ice sheet is about 10 times more massive than that of Greenland, and contains about 90% of the world’s ice.
When the climate changes and less snow falls in winter than melts in summer in a given location, glaciers retreat. Left behind by retreating glaciers are tell-tale signs, especially mineral sediments, that paleoclimatologists use to trace the history of the fairly regular advances and retreats of glaciation over large portions of Earth’s land area. The geological period from about 1.8 million to 10,000 years ago, the Pleistocene, has featured a number of such advances and retreats. These are often called ice ages.
Historical Background and Scientific Foundations
Earth’s history of repeating global glaciation is determined from deposits left behind on land, sea floor sediments, and actual ice core samples that are collected by drilling deep into an ice sheet. In 2003, a team of European scientists completed drilling of an ice core 9,800 ft (3,000 m) into the North Greenland ice sheet. The effort took seven years. The core reveals the history of the ice sheet, and of climate in the Northern Hemisphere, from the present to about 120,000 years ago.
Swiss born geologist Louis Agassiz (1807–1873) first suggested the concept of glaciation over large areas of land when he came to North America and saw evidence of glacial changes in the landscapes of vast areas. He saw boulders of rock types from distant locations that were scarred from being pushed by advancing glacial ice that reminded him of the glacier-scoured rocks in valleys in Switzerland but on a much larger scale.
Repeated advances and retreats of ice are also visible in channels carved in the landscape that are now lakes and in glacial deposits of collected sand and rock that form long regular hills called eskers. In some elevated locations, ancient fossil shorelines are evidence of periods when glaciation had retreated and sea levels were much higher than they are now.
Serbian engineer Milutin Milankovitch (1879–1958) developed a mathematical explanation for the evidence of past periods of glaciation based on cyclical variations of Earth’s orbit that would change the amount of sunlight reaching Earth and therefore change the climate. At least for the Pleistocene period Milankovitch’s theory is in close agreement with glaciation cycles of 20,000, 40,000, and 100,000 years. The variations Milankovitch considered are variations in the shape of Earth’s orbit around the sun; changes in the tilt of Earth’s axis to the orbit; and the precession effect that changes the direction of Earth’s axis of rotation much as a spinning top wobbles.
Periods of glaciation earlier than the Pleistocene Epoch are attributed partly by scientists to shifting landmasses that were associated with Earth’s plate movements. The plate movements created polar wandering, mountain ranges where plates converged, and opened and closed ocean basins. Pre-Pleistocene plate movements resulted in changes in the atmosphere and ocean circulation, driving changes in climate.
Impacts and Issues
The most recent period of widespread global glaciation began about 70,000 years ago. It reached its peak about 20,000 years ago and ended about 10,000 years ago. During that period about 32% of the total land area was covered in ice. Presently about 10% is covered. The glaciation that remains stores about 70% of Earth’s freshwater.
Climate is affected by many factors. Change in the chemical composition of Earth’s atmosphere is one of these. In particular, the human-caused rise of several greenhouse gases, especially carbon dioxide (CO2) and methane (CH4), is presently changing global climate. These gases warm Earth’s climate, affecting oceanic dynamics as glacial ice melts rapidly in most mountain ranges in parts of the Greenland and Antarctic ice sheets. This melting—especially around Greenland—adds freshwater to the oceans at an unusually high rate, locally diluting the sea’s saltiness and so, potentially, altering the large-scale flows of ocean waters. This, in turn, may affect climate.
Increases in greenhouse gases may both stimulate and be stimulated by warming climate. As ice ages in the past have ended, rising temperatures stimulated changes that increased carbon dioxide and methane levels in the atmosphere. These changes sent temperatures even higher. Current levels of carbon dioxide and methane in the atmosphere are higher than the highest levels ever detected through studies of ice cores from Antarctica. Present CO2 and methane concentrations are higher than any time in at least the last 800,000 years, and atmospheric CO2 is growing rapidly as a result of fossil-fuel burning and deforestation by human beings.
In 2008, the United Nations Environment Programme (UNEP) announced that human-induced global warming is causing mountain glaciers around the world to melt rapidly. From 2004–2005 to 2005–2006, the rate of glacial melting doubled. The United Nations scientists forecast that all tropical glaciers, such as those in the Andes Mountains in South America, would probably disappear within 15 years. Accelerated melting of the Greenland ice sheet and of the ice on the West Antarctic Peninsula has also been observed and attributed by scientists to human-caused global climate change.
Primary Source Connection
In the following excerpt from John Muir’s first published work, “Yosemite Glaciers,” the author compares various aspects of the Yosemite Valley to the pages of a book. By examining those aspects, he is able to read—that is, to reconstruct—the shape and activity of the glacier itself.
WORDS TO KNOW
ICE AGE: Period of glacial advance.
ICE CORE: A cylindrical section of ice removed from a glacier or ice sheet in order to study climate patterns of the past.
MILANKOVITCH CYCLES: Regularly-repeating variations in Earth’s climate caused by shifts in its orbit around the sun and its orientation (i.e., tilt) with respect to the sun.
PALEOCLIMATOLOGY: The study of past climates throughout geological history, and the causes of variations among those climates.
PLEISTOCENE EPOCH: The geologic period characterized by ice ages in the Northern Hemisphere, from 1.8 million to 10,000 years ago.
PRECESSION: The comparatively slow torquing of the orbital planes of all satellites with respect to Earth’s axis, due to the bulge of Earth at the equator which distorts Earth’s gravitational field. Precession is manifest by the slow rotation of the line of nodes of the orbit (westward for inclinations less than 90 degrees and eastward for inclinations greater than 90 degrees).
Unlike the geologists of his time who argued that the Yosemite Valley was formed by the activity of an earthquake, Muir asserted (and convincingly demonstrated) that it was carved out by the work of glaciers, mighty rivers of ice which rushed over the surface of the Earth, shaping it and leaving sediments that became its soil, beds for its rivers, and ranges of mountains. More than a first-rate geologist, in his descriptions of the valley and the glacier that formed it, Muir showed his love for nature and the processes of nature that excited his admiration and his reverence.
YOSEMITE GLACIERS
YOSEMITE VALLEY September 28th, 1871. Two years ago, when picking flowers in the mountains back of Yosemite Valley, I found a book. It was blotted and storm-beaten; all of its outer pages were mealy and crumbly, the paper seemed to dissolve like the snow beneath which it had been buried; but many of the inner pages were well preserved, and though all were more or less stained and torn, whole chapters were easily readable. In this condition is the great open book of Yosemite glaciers today; its granite pages have been torn and blurred by the same storms that wasted the castaway book. The grand central chapters of the Hoffman, and Tenaya, and Nevada glaciers are stained and corroded by the frosts and rains, yet, nevertheless, they contain scarce
one unreadable page; but the outer chapters of the Pohono, and the Illilouette, and the Yosemite Creek, and Ribbon, and Cascade glaciers, are all dimmed and eaten away on the bottom, though the tops of their pages have not been so long exposed, and still proclaim in splendid characters the glorious actions of their departed ice. The glacier which filled the basin of the Yosemite Creek was the fourth ice-stream that flowed to Yosemite Valley. It was about fifteen miles in length by five in breadth at the middle of the main stream, and in many places was not less than 1,000 feet in depth. It united with the central glaciers in the valley by a mouth reaching from the east side of El Capitan to Yosemite Point, east of the falls. Its western rim was rayed with short tributaries, and on the north its divide from the Tuolumne glacier was deeply grooved; but few if any of its ridges were here high enough to separate the descending ice into distinct tributaries. The main central trunk flowed nearly south, and, at a distance of about 10 miles, separated into three nearly equal branches, which were turned abruptly to the east.
Those branch basins are laid among the highest spurs of the Hoffman range and abound in small, bright lakes, set in the solid granite without the usual terminal moraine dam. The structure of those dividing spurs is exactly similar, all three appearing as if ruins of one mountain, or rather as perfect units hewn from one mountain rock during long ages of glacial activity. As their north sides are precipitous, and as they extend east and west, they were enabled to shelter and keep alive their hiding glaciers long after the death of the main trunk. Their basins are still dazzling bright, and their lakes have as yet accumulated but narrow rings of border meadow, because their feeding streams have had but little time to carry the sand of which they are made. The east bank of the main stream, all the way from the three forks to the mouth, is a continuous, regular wall, which also forms the west bank of the Indian Canon glacier-basin. The tributaries of the west side of the main basin touched the east tributaries of the cascade, and the great Tuolumne glacier from Mount Dana, the mightiest ice-river of this whole region, flowed past on the north. The declivity of the tributaries was great, especially those which flowed from the spurs of the Hoffman on the Tuolumne divide, but the main stream was rather level, and in approaching Yosemite was compelled to make a considerable ascent back of Eagle Cliff. To the concentrated currents of the central glaciers, and to the levelness and width of mouth of this one, we in a great measure owe the present height of the Yosemite Falls. Yosemite Creek lives the most tranquil life of all the large streams that leap into the valley, the others occupying the canons of narrower and, consequently, of deeper glaciers, while yet far from the valley, abound in loud falls and snowy cascades, but Yosemite Creek flows straight on through smooth meadows and hollows, with only two or three gentle cascades, and now and then a row of soothing, rumbling rapids, biding its time, and hoarding up the best music and poetry of its life for the one anthem at Yosemite, as planned by the ice.
When a bird’s-eye view of Yosemite Basin is obtained from any of its upper domes, it is seen to possess a great number of dense patches of black forest, planted in abrupt contact with bare gray rocks. Those forest plots mark the number and the size of all the entire and fragmentary moraines of the basin, as the latter eroding agents have not yet had sufficient time to form a soil fit for the vigorous life of large trees.
Wherever a deep-wombed tributary laid against a narrow ridge, and was also shielded from the sun by compassing rock-shadows, there we invariably find more small terminal moraines, because when such tributaries were melted off from the trunk they retired to those upper strongholds of shade, and lived and worked in full independence, and the moraines which they built are left entire because the water-collecting basins behind are too small to make streams large enough to wash them away; but in the basins of exposed tributaries there are no terminal moraines, because their glaciers died with the trunk. Medial and lateral moraines are common upon all the outside slopes, some of them nearly perfect in form, but down in the main basin there is not left one unaltered moraine of any kind, immense floods having washed down and leveled them into harder meadows for the present stream, and into sandy flower beds and fields for forests.
Such was Yosemite glacier, and such is its basin, the magnificent work of its hands. There is sublimity in the life of a glacier. Water rivers work openly, and so the rains and the gentle dews, and the great sea also grasping all the world: and even the universal ocean of breath, though invisible, yet speaks aloud in a thousand voices, and proclaims its modes of working and its power: but glaciers work apart from men, exerting their tremendous energies in silence and darkness, outspread, spirit-like, brooding above predestined rocks unknown to light, unborn, working on unwearied through unmeasured times, unhalting as the stars, until at length, their creations complete, their mountains brought forth, homes made for the meadows and the lakes, and fields for waiting forests, earnest, calm as when they came as crystals from the sky, they depart.
The great valley itself, together with all its domes and walls, was brought forth and fashioned by a grand combination of glaciers, acting in certain directions against granite of peculiar physical structure. All of the rocks and mountains and lakes and meadows of the whole upper Merced basin received their specific forms and carvings almost entirely from this same agency of ice. I have been drifting about among the rocks of this region for several years, anxious to spell out some of the mountain truths which are written here; and since the number, and magnitude, and significance of these ice-rivers began to appear, I have become anxious for more exact knowledge regarding them; with this object, supplying myself with blankets and bread, I climbed out of the Yosemite by Indian Cañon, and am now searching the upper rocks and moraines for readable glacier manuscript.
MUIR, JOHN. “YOSEMITE GLACIERS.” NEW YORK TRIBUNE (DECEMBER 5, 1871).
See Also Climate Change; Global Warming; Ice Ages; Snow and Ice Cover
BIBLIOGRAPHY
Books
Lurie, Edward. Louis Agassiz: A Life in Science. John Hopkins University Press, Baltimore, MD: 1996.
Weart, Spencer R. The Discovery of Global Warming. Harvard University Press, Cambridge, MA: 2004.
Web Sites
National Snow and Ice Data Center. “All About Glaciers.” 2007. http://www.nsidc.org/glaciers (accessed March 25, 2008).
U.S. Environmental Protection Agency. “Atmospheric Changes.” 2007. http://www.epa.gov/climatechange/science/recentac.html (accessed March 26, 2008).
Miriam C. Nagel
Glaciation
Glaciation
Introduction
Glaciation is the ice and snow cover that results when more snow falls in winter than melts in summer. When the snow accumulates over the years, the weight of it turns the mass into ice that flows outward and downward to cover an ever-larger land mass. If the advancing glaciation covers more than 12 million acres (50,000 sq km), it is called an ice sheet. Antarctica and Greenland are each largely covered by an ice sheet.
When the climate changes and less snow falls in winter than melts in summer, glaciation retreats. Left behind from glaciation are tell-tale signs that paleoclimatologists use to trace the history of the fairly regular advances and retreats of glaciation over very large portions of Earth's land mass, especially over a period of 1.8 million years ago to 10,000 years ago called the Pleistocene, an epoch in geologic history that is often referred to as the Ice Ages.
Historical Background and Scientific Foundations
The history of repeating global glaciation is determined from deposits left behind on land, sea floor sediments, and actual ice core samples that are collected by drilling deep into an ice sheet. In 2003, a team of European scientists
completed drilling of an ice core 9,800 ft (3,000 m) into the North Greenland ice sheet. The effort took seven years. When the core is completely analyzed it will trace the history of the ice sheet back about 120,000 years.
Swiss born geologist Louis Agassiz (1807–1873) first suggested the concept of glaciation over large areas of land when he came to North America and saw evidence of glacial changes in the landscapes of vast areas. He saw boulders of rock types from distant locations that were scarred from being pushed by advancing glacial ice that reminded him of the glacier scoured rocks in valleys in Switzerland but on a much larger scale.
Repeated advances and retreats of ice are also visible in channels carved in the landscape that are now lakes and in glacial deposits of collected sand and rock that form long regular hills called eskers. In some elevated locations, ancient fossil shorelines are evidence of periods when glaciation had retreated and sea levels were much higher than they are now.
Serbian engineer Milutin Milankovitch (1879–1958) developed a mathematical explanation for the evidence of past periods of glaciation based on cyclical variations of Earth's orbit that would change the amount of sunlight reaching Earth and therefore change the climate. At least for the Pleistocene period Milankovitch's theory is in close agreement with glaciation cycles of 20,000, 40,000, and 100,000 years. The variations Milankovitch considered are variations in the shape of Earth's orbit around the sun; changes in the tilt of Earth's axis to the orbit; and the precession effect that changes the direction of Earth's axis of rotation much as a spinning top wobbles.
Periods of glaciation that are more ancient than the Pleistocene Epoch are attributed by scientists to shifting land masses that were associated with Earth's plate movements. The plate movements created polar wandering, mountain ranges where plates converged, and opened and closed ocean basins. Pre-Pleistocene plate movements resulted in changes in the atmosphere and ocean circulation, driving changes in climate.
Impacts and Issues
The most recent period of widespread global glaciation began about 70,000 years ago. It reached its peak about 20,000 years ago and ended about 10,000 years ago. During that period about 32% of the total land area was covered in ice. Presently about 10% is covered. The glaciation that remains stores about 70% of Earth's freshwater.
Historically the warm periods have lasted about 10,000 years. Climate is affected by small changes. Some scientists believe in a Chaos Theory that refers to small indeterminate events as the trigger of severe climate changes. There is speculation that adding pollution to the atmosphere is one of those events. The rise of two key greenhouse gases, carbon dioxide and methane, is changing the climate. The combination of adding atmospheric changes to changing oceanic dynamics as glacier ice melts and adds freshwater to dilute salty ocean water at a record pace, particularly around Greenland, could trigger severe climate changes.
WORDS TO KNOW
ICE AGE: Period of glacial advance.
ICE CORE: A cylindrical section of ice removed from a glacier or an ice sheet in order to study climate patterns of the past. By performing chemical analyses on the air trapped in the ice, scientists can estimate the percentage of carbon dioxide and other trace gases in the atmosphere at that time.
MILANKOVITCH CYCLES: Regularly repeating variations in Earth's climate caused by shifts in its orbit around the sun and its orientation (i.e., tilt) with respect to the sun. Named after Serbian scientist Milutin Milankovitch (1879–1958), though he was not the first to propose such cycles.
PALEOCLIMATOLOGY: The study of past climates, throughout geological history, and the causes of the variations among.
PLEISTOCENE EPOCH: Geologic period characterized by ice ages in the Northern Hemisphere, from 1.8 million to 10,000 years ago.
PRECESSION: The comparatively slow torquing of the orbital planes of all satellites with respect to Earth's axis, due to the bulge of Earth at the equator, which distorts Earth's gravitational field. Precession is manifest by the slow rotation of the line of nodes of the orbit (westward for inclinations less than 90 degrees and eastward for inclinations greater than 90 degrees).
As ice ages in the past have ended, the rising temperatures stimulated changes that increased carbon dioxide and methane levels in the atmosphere. These changes sent temperatures even higher. Current levels of carbon dioxide and methane in the atmosphere are higher than the highest levels ever detected through studies of ice cores from Antarctica. Present CO2 and methane concentrations are higher than any time in at least the last 650,000 years, and growing. Questions remain among climatologists on whether Earth is approaching another ice age as past cycles suggest or whether the human factor will change the cycles of glaciation.
See Also Glacier; Glacier Retreat; Ice Ages.
BIBLIOGRAPHY
Books
Lurie, Edward. Louis Agassiz:A Life in Science. John Hopkins University Press, Baltimore, MD: 1996.
Weart, Spencer R. The Discovery of Global Warming. Harvard University Press, Cambridge, MA: 2004.
Web Sites
“All About Glaciers.” National Snow and Ice Data Center, 2007. < http://www.nsidc.org/glaciers> (accessed August 16, 2007).
“Atmospheric Changes.” U.S. Environmental Protection Agency, 2007. < http://www.epa.gov/climatechange/science/recentac.html> (accessed August 16, 2007).
“Past Climate Cycles: Ice Age Speculations.” American Institute of Physics, 2007. < http://www.aip.org/climate/cycles.htm> (accessed August 16, 2007).
Glaciation
Glaciation
Glaciation is an extended period of time during which glaciers are present and active. It also refers to all the processes that form glaciers and that are at work within a glacier. A glacier is a land-based mass of highly compacted ice that moves downward and outward under its own weight due to gravity . Glaciers may be large enough to cover a continent or small enough to fill a mountain valley and periods of glaciation can last hundreds, thousands, or millions of years.
A glacier is formed by a series of processes that begins with accumulation. Accumulation occurs when the buildup of snow and ice through snowfall, avalanching, or wind transport during cold months greatly exceeds the loss through melting or sublimation (the direct conversion of a solid to a gas) in warmer months. As snow accumulates and deepens, its weight causes increased pressure that converts snowflakes first into granular snow and eventually into dense ice granules. Continual and sustained accumulation, compaction, melting, and refreezing eventually create a very dense mass of inter-locking ice with about 10% void space. As a comparison, freshly fallen snow contains about 80% void space. The formation of new glacial ice takes several decades.
A mass of accumulated snow and ice is not strictly considered a glacier until it begins to move downhill. Glacial ice will begin to move when it becomes too thick and heavy to hold its position against gravity. The instant that glacial ice will begin to move depends on the steepness of the ground, the ice thickness, and the ice temperature . In mountainous regions a glacier will start to creep when it reaches a thickness of 65.5–131 ft (20–40 m). Glaciers usually move very slowly, less than one meter a day, but can move up to 164 ft (50 m) a day.
Although a glacier is constantly creeping downhill, the front edge of it may appear to remain in the same place, or even retreat uphill. This is because at the same time a glacier is moving, it may be growing or shrinking. A glacier fluctuates depending on the rate of the accumulation of new ice versus the rate of ablation, or the loss of ice due to melting, sublimation, and wind erosion . A glacier will appear to advance if accumulation exceeds ablation and it will recede if ablation is greater than accumulation. If the two are equal, the glacier will appear to remain stationary. When a glacier advances far enough to reach a body of water , large chunks may break off and fall in into the water. This is called calving and is the source of icebergs .
Throughout geologic time , Earth has experienced major periods of glaciation when glaciers covered large portions of Earth's surface for up to many millions of years. In the last 500,000 years, four major periods of glaciation have occurred. During these times, ice sheets several kilometers thick covered as much as 30% of the global land surface. This type of glaciation, which extends over vast areas of lowlands and mountains, is known as continental glaciation. The most recent major glaciation ended about 10,000 years ago.
Today, continental glaciation occurs only at the polar regions, mostly in Greenland and Antarctica . Active glaciation in other parts of the world exists in mountainous regions at high altitudes and is called alpine glaciation. The causes of glaciation are not completely understood, but major periods of glaciation are attributed to decreases in the amount of sunlight the earth receives due to very long-term cyclic variations in Earth's rotation and angle of orbit.
See also Glacial Landforms
Glaciation
Glaciation
The covering of the earth's surface with glacial ice. The term also includes the alteration of the surface of the earth by glacial erosion or deposition. Due to the passage of time, ice erosion can be almost unidentifiable; the weathering of hard rock surfaces often eliminates minor scratches and other evidence of such glacial activities as the carving of deep valleys. The evidence of deposition, known as depositional imprints, can vary. It may consist of specialized features a few meters above the surrounding terrain, or it may consist of ground materials several meters in thickness covering wide areas of the landscape.
Only 10% of the earth's surface is currently covered with glacial ice, but it is estimated that 30% had been covered with glacial ice at some time. During the last major glacial period, most of Europe and more than half of the North American continent were covered with ice. The glacial ice of modern day is much thinner than it was in the ice age , and the majority of it (85%) is found in Antarctica . About 11% of the remaining glacial ice is in Greenland, and the rest is scattered in high altitudes throughout the world.
Moisture and cold temperatures are the two main factors for the formation of glacial ice. Glacial ice in Antarctica is the result of relatively small quantities of snow deposition and low loss of ice because of the cold climate . In the middle and low latitudes where the loss of ice, known as ablation, is higher, snowfall tends to be much higher and the glaciers are able overcome ablation by generating large amounts of ice. These types of systems tend to be more active than the glaciers in Antarctica, and the most active of these are often located at high altitudes and in the path of prevailing winds carrying marine moisture.
The topography of the earth has been shaped by glaciation. Hills have been reduced in height and valleys created or filled in the movement of glacial ice. Moraine is a French term used to describe the ridges and earthen dikes formed near the edges of regional glaciers. Ground moraine is material that accumulates beneath a glacier and has low-relief characteristics, and end moraine is material that builds up along the extremities of a glacier in a ridge-like appearance.
In England, early researchers found stones that were not common to the local ground rock and decided they must have "drifted" there, carried by icebergs on water. Though geology has changed since that time, the term remains and all deposits made by glacial ice are usually identified as drift. These glacial deposits, also known as till, are highly varied in composition. They can be a fine grained deposit, or very coarse with rather large stones present, or a combination of both.
Rock and other soil-like debris are often crushed and ground into very small particles, and they are commonly found as sediment in waters flowing from a glacial mass. This material is called glacial flour, and it is carried downstream to form another kind of glacial deposit. During certain cold, dry periods of the year winds can pick up portions of this deposit and scatter it for miles. Many of the different soils in the American "Corn Belt" originated in this way, and they have become some of the more important agricultural soils in the world.
[Royce Lambert ]
RESOURCES
BOOKS
Flint, R. F. Glacial and Pleistocene Geology. New York: Wiley, 1972.