Planet Earth/5i. Earth’s Ice: Glaciers, Ice Sheets, and Sea Ice
Glaciers
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In late summer there is a valley high in the Bernese alps, that is filled with the cracked and tumbled rocks that appear to have been pushed and bashed down the valley floor by giants. Large boulders, jagged and angular point upward to the surrounding steep mountain sides. These are the Aargletschers, German for Aare Glaciers, a system of two major glaciers that are the source for the Aare River in Western Switzerland. The glaciers today have retreated up the valley, a relic of their former glory, with the northern glacier Lauleraar and southern glacier Finsteraar retreating into their respective separate valleys, but two hundred years ago these great glaciers extended down the valley meeting for a combined glacier (called the Unteraar glacier) that extended for 3 kilometers, burying these rocks in thick sheets of frozen ice. The Swiss painter Caspar Wolf captured these glaciers in beautiful and dramatic paintings of large boulders tossed by giants, and massive ice sheets tumbling down the valleys of mountains, piles of ice and snow. Of blue green ice that jaggedly pointed skyward, from a landscape that bears little resemblance to the warmer Earth today. Near where the two glaciers meet, was constructed a small shelter of rocks, named ironically the Hôtel des Neuchátelois. It was here in this shelter surrounded by ice and snow, that one of the important geological studies was carried out by a Swiss scientist named Louis Agassiz, a scientist who would have a major influence on how we see the world today.
In 1832, Louis Agassiz was hired by the University of Neuchâtel in Switzerland to teach natural history. He was at the time engrossed in studying tropical fish brought back from early expeditions from the Amazon Basin of Brazil. The mountainous cold landscape of the Swiss Alps, unfortunately, was not the most ideal place to study tropical fish, but Agassiz discovered that by hiking around the mountainous region he could find the petrified remains of fish that lived along ago, buried in the rock layers. Embarking on a study of these fossil fish, Agassiz would spend most days in the mountains splitting rocks with his rock hammer and finding new species of long dead fish from millions of years ago. As Agassiz explored the mountains for new fossil sites, he began to see the valleys in a different light, observing how they appeared to be carved by the motion of ice. This captured his interests, and he began to lead field trips with his students to study the motion and movement of the glaciers that filled these valleys. Using the rock shelter as his base of operations, Agassiz drilled down through the ice to measure its thickness, and mapped the glaciers extent, as well as the boulders that were carried on its surface. His most famous experiment was sinking poles in a straight line across the glacier, and observing over the course of several years the poles in the center of the glacier moved downslope, as if the ice flowed like a slow-moving river.
How Ice Carves the Earth
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Agassiz’s study of glaciers, demonstrated the powerful force of ice in the shaping of Earth’s surface. Glaciers form either at high latitudes near the poles or at high altitudes on mountain peaks. When snow falling each winter is greater than the amount that melts during the summer, the remaining dense ice that survived the summer melt advances down slope. A glacier is a persistent body of ice that moves down slope under the influence of gravity. First forming on the tops of mountain peaks, these glaciers carve a bowl-shaped depression called a cirque, it is from here that the glacier will flow down the mountainside, carving the steep rocky slopes into a bowl shape flanked by steep peaked mountains, called an arête. The famous Matterhorn between the border of Switzerland and Italy is an example of an arête, formed by carving glaciers on each side of the mountain pinnacle. Glaciers flow and expand down the valleys, carving U shaped valley floors. This flowing ice acts like a conveyer belt for boulders and large rocks that fall from the over-steepened valley sides. These boulders and rocks are carried down the valley on top of the thick sheets of ice. Near the valley ends, the glacier will melt with the lower altitude and warmer temperatures, and the large rocks and boulders will tumble out forming a moraine. Moraines are thick piles of jumbled rocks and debris that are carried by glaciers, and they can form on the edges of glaciers, where they are called lateral moraines and at the end of the glacier they are called terminal moraines. These piles of rock are characterized by being poorly sorted (large and small rocks), with an appearance that they were formed by a large bulldozer. This poorly sorted, jagged and angular pile of rocks is called till, and when lithified into rock, tillite. Till differs from sediments transported by water in rivers, as those sediments will be polished into cobbles and rounded pebbles. Till is just a jumble of rocks of all sorts and sizes.
Often near the terminal moraine is a small lake or pond called a tarn. A tarn often acts as a catchment for melt waters from the glacier. When glaciers completely melt, and are no longer permanent bodies of ice, often their legacy is found in the carved U-shaped valley, and melted waters can be found in a tarn at the base of cirques.
Glaciers are found on the surface of the Earth in two types of locations, those found in the alpine regions in temperate zones, and polar regions near the north and south poles, such as the large ice sheets that are found in Greenland and Antarctica. Study of glaciers has demonstrated a record of their retreat as the Earth has warmed during the past century. To form, a glacier needs to be continually added to through the accumulation of snow and ice during the winter, which must be greater than the ablation or melt of snow and ice during the summer. If the ablation of the ice and snow in the summer is greater than the amount added in the winter, the glacier will retreat. This forms two regions of a glacier, near the head or source will be the accumulation area, while the ablation area will be near the terminus or tail of the glacier. Between them is the equilibrium line which is the boundary line between the area of accumulation and ablation. This can often be observed in satellite images of glaciers, where the accumulation areas are crisp white, with fresh snow, and ablation areas are darker colors as the surface melts and exposes rocks and melt water on the surface. Each year the glacier will undergo growth during the winter months, and melt during the summer, and exhibit a changing mass balance depending on how much new ice was contributed to the glacier, and how much was ablated by melt.
Most glaciers on Earth today are retreating, and shrinking. These shrinking glaciers are leaving behind piles of till, which record their retreat over time. As the glacier retreats up the valley, the piles of till form a continuous record of a terminal moraine, which records the retreat of the glacier from its greatest extend. Documenting and determining the age of these rocks at the terminal moraine is most often done using the radioactive isotope beryllium-10, which accumulates in quartz minerals, that contain oxygen atoms which when are exposed to sunlight can change to radioactive isotope beryllium-10, that will decay back to oxygen, but the amount of this isotope can be used to determine how long the rock was exposed to light, and how long ago it was unburied from the ice that carried it to the terminal moraine deposit. Globally, glaciers reached their last maximum extent about 25,000 years ago, during the last Ice Age. This is about the same time that according to the work of Milutin Milankovic, that Earth’s orbit would produce a more variable and cooler climate for Earth. Scientists refer to these periods as Glacial and Inter-Glacial periods in the recent geological history of Earth.
The Great Ice Age
[edit | edit source]For Louis Agassiz the studies of glaciers in the Swiss Alps suggested that much of the Earth was once covered by ice sheets and large glaciers, but fellow scientists at the time were skeptical. One of his greatest skeptics was William Buckland, who came to visit the Swiss Alps. Buckland was a professor in Oxford, England, and is more famous today for naming the first dinosaur, Megalosaurus in 1824. But during his tenure as lecturer of geology, he was a noted advocate for the idea of Earth’s surface had been shaped by a great flood. Buckland’s deluge model, suggested that all boulders and rocks on the surface of the Earth had been transported by an ancient large flood. The two scientists met in Switzerland and despite their differing opinions became friends, as Agassiz argued for an Earth shaped by ice, while Buckland argued for an Earth shaped by water.
Agassiz demonstrated how thick layers of ice flow slowly over the landscape, due to a unique property of ice and the influence of gravity. Like a stack of playing cards, the internal crystalline structure of the ice undergoes deforming stress promotes the motion of the ice downslope. This ice can freeze and thaw particularly near its contact with the rock floor and walls of the mountain valley. This can produce glacial grooves or striations on the rock surface as the ice drags grains plucked from the rock’s surface and dragged over it. This melt is enhanced at great thicknesses by the increased pressure the ice is subjected to at this depth, which decrease the melting/freezing point below 0° C. This high pressure from very thick sheets of ice allow the glacier to skate on this narrow floor of melt waters. This freeze/thaw cycle at the ice/rock interface produces what is called rock flour, finely ground minerals that provides important nutrients to forests downslope from the glacier.
While Buckland began to be convinced of the evidence gathered by Agassiz that glaciers and ice had shaped the European Alps, he invited Agassiz to Scotland to see what evidence they could gather there for evidence of ice shaping the surface of Scotland. The two scientists embarked on this trip, each discussing at depth the shapes of valleys, mountains and features on the landscape of Scotland. They observed fjords, where ice craves a large U-shape valley that later is filled with ocean water, and found examples of piedmonts (foot mountains) where glacial till accumulated on the flanks of mountain ranges. They found drumlins scattered across Scotland, narrow hills which were all oriented in the same direction, as if a giant ice sheet rode over these hills shaping them like a wood carver, as well as esker, narrow sinuous hills formed by the motion of meltwaters beneath gigantic ice sheets. Most stunning of all were the erratics, large angular boulders that appeared to have been dropped out of long-ago melted ice that carried them great distances. These monolithic stones were scattered across the Scottish landscape. Agassiz saw the evidence of great ice sheets that once covered Scotland and England. In time, Buckland was convinced too of a great Ice Age in the ancient times. His deluge model for the surface of Earth fell out of his mind, as he realized the evidence they had gathered implicated a great frozen history to Earth.
Agassiz’s ideas of an Ice Age in Earth’s history were further cemented when he moved to the United States in 1846. Agassiz initially intended to visit the country to study evidence of glaciers in North America, and continue his work on fossil fish, but enjoyed the country so much he decided to stay. To Agassiz’s eyes, North America was carved by great glaciers. The Hudson River of New York, a flooded U-shaped valley formed by massive ice sheets, the Finger Lakes sculpted by ice. Most astonishing were the recently mapped Great Lakes, which formed through the motion of thick layers of ice that scooped out these areas, and filled them with water. Along the northeastern coast he observed how Long Island was a gigantic terminal moraine as ice dumped poorly sorted sediments and till forming the unique coastline of the United States. From Boston to New York Agassiz observed striations on boulders and erratics that were scattered across the landscape of New England. Agassiz was appointed a professorship at Harvard University, and in 1859 founded the Museum of Comparative Zoology on the campus. During the American Civil War between 1864 to 1865 he lectured on Glaciers and the Ice Period in Earth’s past. His ideas would likely have carried less influence had not his second wife Elizabeth Cabot Cary, a prolific writer, promoted his research. Elizabeth Cary was a professor during a time when few women were permitted to teach, she published an influential textbook, called A First Lesson in Natural History in 1859, under a pseudonym, using the genus name for baneberry (Actaea), a beautiful flowering plant, but producing highly toxic and poisonous berries. Elizabeth Cary taught young ladies near Boston in a women’s college, and was a leading supporter for the instruction of blind students in the study of natural history. Together they are buried near Boston, under a unique tombstone, a boulder shipped to the United States having been selected from the glaciers that inspired their mutual admiration for the power and beauty of a planet carved and shaped by ice.
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h. Earth’s Endangered Lakes and the Limits of Freshwater Sources. |
a. Journey to the Center of the Earth: Earth’s Interior and Core. |