GEOLOGY/GEOPHYSICS 101 Program 27 Glaciers


Well, hello, again. I'm glad you could join us today for Program 27. Today we'll study glaciers, their formation, the dynamics, and the causes and effects of glaciation.
Today glaciers seem kind of remote and inaccessible. We don't see them very often because they're only found at high altitudes or high latitudes, but in the recent geologic past glaciers played a much more extensive role in geologic processes; in fact, the wide recognition came only about a hundred years ago that there had been recent massive and repeated episodes of glaciation on a global scale, set a new paradigm, and provided us a new model for the understanding Earth's history, but it also presented us with new problems to solve.
It, for example, forced us to recognize the radical global climatic changes and wonder about what caused them, and glaciers also created much of the modern landscape and geographic features that we see in the United States and Europe today.
By studying glaciers we can learn about the regional history and develop a recent timetable of geologic events in a region that was glaciated, and it also forces us to rethink the idea of uniformitarianism. The glacial age also had tremendous effects on the development of plants, animals, and also on mankind and our culture and also causes many mass extinctions of organisms of various types,
so today we are in Chapter 18 in the text, pages 405 to 431; that's Lesson 23 in the study guide, and again note that we did cover Chapter19 first; we reversed the sequence on these two chapters, so as usual, follow the study plan in the study guide, and when you're done go back to the learning objectives and be sure that you've learned each one of those objectives.
Well, before the video today I'd like to go over with you some of the features of glaciers and glaciation, and then after the video we'll come back and examine the great Ice Age and take a look at Hawaiian glaciation.
Well, let's first look at the origin of glacial ice. Glacial ice originates whenever winter snowfall exceeds the summer melting in a given area over a particular time interval, and even a small annual net gain of snow can create a thick layer of ice over time. Remember that geologic events that take place, even small, magnified over large amounts of time can cause radical changes.

Basically, the weight of the overlying snow compresses the lower portion of the snow and causes changes which are not that different from the changes that take place in rocks during metamorphism. Ice, after all, is a mineral. As a crystal structure it's a solid, naturally occurring, and so on. Only with ice, of course, these changes take place at much lower temperatures and pressures than metamorphic rock.

Basically the ice crystals in the snow recrystallize as compaction occurs, and the ice passes through fairly recognizable stages. There are words for this. "Neve and firn", for example, are two stages in this progression of ice. It's also significant that the ice may trap air bubbles as it recrystallizes, and these air bubbles can be read later on to determine both the composition and the age of the atmosphere that's been trapped. We can use this, among other things, to find out a base line for the amount of carbon dioxide that was in the atmosphere before man's industry appeared on the Earth and caused increases of carbon dioxide. We can also read air temperature by looking at oxygen isotope ratios and other things.

Okay, the ice actually becomes quite plastic, and it does this at fairly low pressures. The ice becomes plastic enough to flow when it accumulates to a depth of only about 60 feet or so, so when it does so, it flows downhill slowly, but measurably, very similar to the way a viscous liquid flows; in fact, it's not so different from the water in a stream as far as valley glaciers are concerned, or in the case of a continental ice sheet, it's like the ball of silly putty that collapses and flattens out under its own weight as the ice spreads out.

We might also note that this moving ice is a very effective erosional agent. It can pluck rocks off the bottom; it can abrade and scratch, and so rocks of all sizes can be incorporated into the ice that is frozen into the bottom layers or shove along ahead of the moving ice like a bulldozers, so as far the dynamics of a glacier goes, it is important to recognize that a glacier is a dynamic system; that means it's a system that's constantly undergoing change, so whether or not a glacier grows or shrinks depends upon a balance between accumulation of snow in one place and removal or "ablation" as geologists say it in another place.

We classify the zone of accumulation as simply the place where precipitation exceeds evaporation. In valley glaciers this might be at the head of the glacier up in the high mountain regions, and for an ice sheet, this may be some particular region within the interior of the continent which we see as an unusually large amount of snowfall. Ablation or the removal of ice occurs usually at some distance from a zone of accumulation, and the flow of ice moves the ice that's formed from the zone of accumulation into the zone of ablation. Ablation refers to the losing of the ice by several methods. It may evaporate, for example, change directly from the solid into the gas. It may melt, or pieces of the ice may break off at the front, and if this happens in an ocean or in the sea, then this calving off of chunks of ice may create icebergs which then float away.

Now, all modern glaciers show episodes of advance and retreat; that is, sometimes they advance, and sometimes they retreat, and their are hints of global patterns, but it's not quite that simple because some glaciers may advance while other glaciers in different parts of the world are retreating, and, by the way, the word "retreat" is kind of a misleading term. The glaciers don't really flow uphill; they simply shrink by melting away at the terminus or the end of the glacier, and if the rate of melting at the terminus exceeds the rate of supply in the zone of accumulation, then the glacier retreats upslope as it shrinks.

Okay, basically there are only two different type of glaciers, and these refer to the locations in which we find the glaciers. There are "alpine" or "valley" glaciers and "continental" glaciers, sometimes called ice sheets.

The two types of glaciers are related, and they produce similar results, but also quite distinct results, so let's look at valley glaciers first. Valley glaciers generally flow downhill from high mountain regions in existing stream valleys, and like water in a stream, they flow faster in the center and slowest at the edges. Crevices may form from internal stresses near the surface where the ice is brittle. Keep in mind that the ice is flowing plastically at the lower regions, but the surface, which is not under pressure is also moving and, therefore, it's subjected to brittle fracture. These crevices may form patterns on the surface like this glacier, the blue glacier on Mt. Olympus in Washington State, and in many cases, these crevices may be covered by snow, which makes it very dangerous and very difficult to walk over the surface of the glacier.

Okay, the zone of accumulation for valley glaciers in the high mountains may feed several different valley glaciers, and some of the glaciers may join tributaries to form a larger glacier downslope. In the process, they scour out the bottom of usually a "v" shaped valley into a broad, smooth sided "u" shaped valley, and at the same time "colluvium" from mass wasting, that is rock falls and so forth accumulate along the edges of the glacier giving the glacier a striped appearance on the side.

As far as continental glaciers go, the zone of accumulation is generally in regions in the interior of the continent where snowfall exceeds melting over a fairly large region over a fairly long period of time. Presently, continental glaciers cover only the continents of Antarctica and the continent of Greenland. They spread rapidly from accumulation center, and glaciologists call these "spreading centers," use the same term that we use in plate tectonics for the spreading of plates, and the centers may be multiple; that is, there may be several different spreading centers, and the spreading centers are not necessarily at the Earth's North and South Poles.

The actual locations of the spreading centers depend upon various climatic variables. The continental ice sheets are characterized by covering broad areas. Hundreds, and even thousands, and in many cases millions of square miles, and the ice may move very far from the source. For example, the great pleistocene glaciers, which we'll come back to and talk about later, in North America spread from Hudson Bay more than a thousand miles south and also eastward from the Rockies in both the U.S. and Canada, and by the way, many alpine glaciers or valley glaciers in the rocky mountains are remains of these pleistocene large ice sheets from before.

Okay, continental and valley glaciers do have some common and some distinct features, so let's look at some of the common features. One of the common features is the presence of grooves and polished rocks. As the glacier moves across the underlying bedrock, as it gouges out rocks, the rocks become stuck in the bottom like a giant sheet of sandpaper, so grooves of polished rocks like this outcrop in the Sierra Nevada are quite common, and in this picture we notice the size of the boulders that are strewn along this outcrop. We also find that area that's regions that have been glaciated are characterized by rounded hills and ridges in the bedrock, and again like these in the granite of the Sierra Nevada, and by the way, in this picture the lake at the base of the cliff formed from a depression which was gouged in the soft rocks by the movement of the glacier.

Okay, valley glaciers tend to develop fairly unique features. One of these are called "cirques." A "cirques" is an amphitheater headed depression at a valley head. This is where the ice accumulates and exerts the most pressure. This is a deep depression that's scoured out below the level of the valley floor. These are often filled with water to form glacial lakes after the glacier melts away. In adjacent valleys if glaciers are flowing into adjacent valleys, the erosion sideways of the glacier may form either "horns" or "arete."

These are both erosional remnants which protruded from the top of the glacier. A "horn" is a faceted peak where a glacier formed on three or more sides of a peak. A most famous example of this is the Matterhorn in Switzerland, and "arete" is a sharp ridge between glaciated valleys where again the glacier has scoured out and left the peak standing beside.

The sides of these aretes are often smooth with rounded ridges. Another common features in glaciated valleys is called "hanging valleys." These are simply valleys which are far above the level of a main valley, a tributary far above the level of a main valley, and these may represent either glaciated or unglaciated valleys. After the glacier melts away, they often leave spectacular waterfalls plunging over smooth cliffs more than a thousand feet high like this one in Yosemite Valley.

Glaciers also leave distinctive depositional features. One of these is in front of the glacier. As the water melts away, we find what's called an "outwash plane." This consists of sediment usually poorly sorted rocks and rock flower, all sizes, which represents the material dropped by the melt water from the glacier as it flows down hill. It may included many braided streams. Another depositional feature are called "moraines".

"Moraines" are basically ridges of poorly sorted rock of all sizes which are simply left behind when the ice melts. This is generally material that was either on top of the glacier or material imbedded in the glacier. When the ice melts away, it simply drops the rocks, and so forth on the ground.

There are several different types of "moraines."

Okay, two other features that the video will mention but I don't think explain very well are eskers and drumlins. These are features mostly of large continental glaciers, and "eskers" are basically long sinuous ridges of gravel which are deposited by streams flowing under a melting glacier; those streams flowing in a tube under a melting glacier deposits sediment much like a stream. A "drumlins," on the other hand is an inverted spoonshaped ridge, usually of gravel, and it's not exactly clear how drumlins form, but it seems that they're probably old terminal moraines which have now been overridden by a new advance of the glacier, and both of these features generally tend to run parallel to the direction of movement of the glacier.

Okay, glaciers also leave behind features called "kettles" and "kettle lakes." This happens if a large block of the glacier breaks off and is covered by later sediment. When that ice melts, it leaves behind a depression which may either be filled with water or not. If it's filled with water, it's called a kettle lake; if it's not, it's simply called a kettle, so I think this should be sufficient background to relate to the video so keep this stuff in mind and let's watch the video.

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Some of the most picturesque landscapes on Earth owe their existence to glaciers. There are many examples. spectacular mountain ranges such as the Alps, Himalayas, and the Rockies were sculpted by repeated glaciation.

Yosemite Valley here in the Sierra Nevada Mountains would have been just another nondescript river valley if glaciers hadn't carved it to its present shape. Many of the world's most beautiful lakes were gouged out of hard rock by glaciers. These include North America's own Great Lakes and the famous lochs of Scotland. Even the great expanses of rich agricultural soils that blanket China, and the Soviet Union, Canada, and the United States owe their existence to glaciers.

Moving glacial ice pulverizes the underlying rock into silt-sized fragments. This silt was eventually transported and concentrated by the wind into the vast fertile soils of today.

Early scientists didn't really appreciate the important geological role of glaciers. Even geologists were convinced that glaciers had never existed outside of their present locations over the last one million years. A breakthrough came in 1836 when Swiss scientist, Lewis Agasse, reported evidence that the inhabitance of medieval villages in Europe had moved their towns to keep pace with advancing glaciers. Further study revealed that glaciers leave behind a distinctive deposit of sediment like these boulders as they melt back and retreat. Geologically recent examples of these sedimentary deposits found hundreds of kilometers from the nearest glacier demonstrated to Agasse that vast portions of the continents of the Northern Hemisphere had been recently covered with glacial ice.

Observations like these led to the realization that glaciers are active and powerful agents of landscape evolution. Glaciers are large longlasting masses of ice which slowly flow across the land. While most places from time to time have temperatures cold enough for snow and ice to form, only in a few of these places do conditions permit the growth of glaciers. Glaciers come from the accumulation of snow either in polar regions, or in high altitudes, or even at the Equator at very high elevations on the tops of mountains, so wherever you have precipitation that falls in the form of snow, you can get a glacier, and there's one other requirement, and that is that more snow has to fall in the winter than melts in the summer, so that in every twelve- months period, some of the previous winter's snowfall is left over. So as snow accumulates year after year a glacier begins to form.

The actual manner in which snow is converted into glacial ice involves compaction and rearrangement of snow crystals. There's a process that goes on as each layer of snow is added year by year, that's residual snow from the previous winter. It compacts under the weight of new snowfall, and as that compaction takes place, the snow crystals are pressed closer together. The air in the original snow pack is generally expelled by this compaction, and the snow crystals join together to form an intermediate substance between snow and ice which we call "firn." It's an old Swiss term that's still used today, so eventually the firn itself is more compact,more recrystallized, and it becomes glacier ice.

Subject to extreme instantaneous stress ice shatters like glass, but if stress such as gravity is applied gradually over a long period of time, the ice bends. This process called "plastic deformation" explains how glaciers move. Generally ice must accumulate to a thickness approximately 20 meters before movement starts.

Pulled by gravity, ice in a glacier typically shifts downslope a few millimeters per day. To study glacial flow, Lewis Agasse and his students built a hut on the ice itself. They observed that the center of the glacier moved most quickly while friction slowed down movement along its sides. A similar phenomenon is observed in rivers and streams. Scientists like Agasse also wanted to understand how glaciers flow internally. But it wasn't until early in the post World War II era that glaciologists were able to drill a hole through a Swiss glacier, and this was a whole several hundred meters deep, maybe a couple of inches in diameter or smaller, but they put an aluminum tube in that hole right down to the bottom of the glacier.

The scientists discovered that the tube bent as it shifted downslope, so just as friction slows movement of ice at the sides of glaciers, it slows movement at the base as well. Scientists also discovered that glaciers not only creep over the bedrock but in places break free to glide across it. Such basal slip is lubricated by water melting from the ice. Streams of this subglacial melt water commonly pour from the smelts of glaciers.

There are two common types of glaciers. In mountainous regions at high altitudes, glaciers fill stream valleys. Their movement downslope is confined by the paths of the valley, and so these rivers of ice are known as valley glaciers.

On land masses near the poles, such as Greenland and Antarctica, single giant glaciers cover vast regions. These glaciers are called "continental glaciers" or "ice sheets." Ice sheets are actually spread across the continent like broad domes. The thickest part of the dome is at the center of the glacier where the greatest snowfall takes place and causes ice to build up to its greatest thickness. The weight of this thicker central region forces the glacier to radiate in all directions as opposed to the relatively straight downhill movement of the valley glacier.

Regardless of type, all glaciers move ice from its point of origin to areas where it melts. In a typical valley glacier, ice builds up year after year at the head of the glacier in the so-called "zone of accumulation." Downslope the ice melts away faster than it can build up at that lower, warmer altitude. This is the so-called "zone of wastage." These two zones are divided by the snow line which can actually be seen on some glaciers during the summer months.

Downslope of the snow line melting snow exposes old silty firn in the zone of wastage. Upslope the glacier is permanently covered with fresh white snow. The snow line shifts up or down the surface of the glacier from year to year. During cool years the snow line lies at lower elevations than in warm years. If ice moves into the zone of wastage faster than it melts away, the snout of the glacier will advance farther downslope, but if not enough moves into the zone of wastage to compensate for melting, the snout of the glacier will retreat upslope. Whether the snout of the glacier advances or retreats, ice within the glacier itself is continuously flowing downslope to melt away. As it does so, the ice carries tons of rock, silt, and other debris. Some of this material comes from the mechanical weathering and toppling of rocks on the surface of the glacier; other material is plucked and scraped out by the ice as it flows across the bedrock. Well, glaciers erode in a very characteristic style. They polish. They grind. They gouge. They pluck away at the rocks that are there totally destroying any topography that was already present. Any streams in its path are obliterated by the oncoming ice.

The ice itself is hardly pure; it's dirty; it's filled with everything that the ice has previously scratched and ground away at, and so a glacier is moving like ice filled sandpaper and uses the rocks and sand debris within itself to further gouge and scrape the earth. The tremendous erosive power of moving ice is evident in areas where glaciers have melted away exposing the bedrock. These surfaces were polished smooth by glacial ice. Grit caught beneath the glacier carved long scratch marks called "striations" in the polished rock as the ice flowed across it.

On a more impressive scale, the valleys once occupied by glaciers are carved out acquiring steep walls and broad gently sloping floors. This "u" shaped valley profile contrasts greatly with the "v" shaped profiles typical of unglaciated stream valleys.

At the edges of the glacier, eroded material is dumped out by the melting ice. This unsorted rock debris is known as "till." If the snout of the glacier remains in about the same position for a long time, a very great mound of till called an "end moraine" may form.

Moraines can also accumulate at other places next to glaciers. Lateral moraines, for example, grow where till is deposited along the sides of glaciers. Where two glaciers combine, lateral moraines merge forming "medial" moraines. Understanding moraine deposition is of great importance to geologists because it can be used to reconstruct the history of glacial movement.

For example, overlapping moraines not only show the position of a glacier at various times, but also indicate the glaciers typically advance and retreat over and over. It was evidence like this that helped geologists recognize that the Earth has experienced repeated cycles of glaciation or ice ages over the last two million years. An ice age consists of a gradual cooling of the climate and growth of glaciers worldwide terminated by a warm inter- glacial climate during which glaciers melt back and retreat.

During the glacial history of the Earth, we now understand that changes in sea level and the evolution of life are also linked with glacial cycles. What's less clear, however, is the cause of ice ages, and a number of theories attempt to explain the processes responsible for glacial cycles. There have been about ten ice age cycles over the last one million years, and any theory that is advanced to explain these cycles must take into account the regular repetition of glacial activity and the link between glacial cycles and global climate. The repetitive nature of ice ages during the past two million years suggests that the world may again be moving toward a period of deep freeze.

This is not likely to begin within our lifetimes but may commence within the next few thousand years.Most scientists believe ice ages are tied to changes in the position of Earth as it orbits the sun. As the Earth rotates if occasionally it tilts a bit, then a good portion of the Earth will undergo sudden cooling. We know that happens. Another change occurs, and we know this happens as well. With a not perfect revolutionary period of the Earth's rotation around the sun, it's more elliptical, and in certain stages it becomes more elliptical than other times. We know that happens.

The theory for the global cooling of the Earth and the formation of the ice age is that on occasion both the wobble and noncircular rotation or revolution of the Earth's passage around the sun occur together, and we get global cooling that is unique to that time. Recent data support this orbital explanation for ice ages. Drill cores from deep sea sediments contain fossils of microscopic plants that were sensitive to ocean temperatures, indicators of past ice ages and the warm times in between.

Using these fossils, scientists have been able to chart the temperature changes of the world's oceans going back nearly a half million years. Scientists have also shown that these changes can be correlated to variations in the distance between Earth and sun which occur with regularity, but these cyclic changes in Earth-sun geometry have been going on for the entire history of Earth, and glacial epics are uncommon events. Perhaps the positions of the continents themselves play a role in triggering ice ages.

Throughout Earth's history, plate tectonics has shifted continents, changed their shapes, and altered the pattern of ocean currents around them. All of these are critical factors in regulating Earth's climate, and if land masses are near the Poles, they can support ice sheets that could not grow otherwise, so ice ages could be triggered by a combination of Earth's orbital pattern and the movement of land masses into polar latitudes.

The last ice age left striking marks still visible on the landscape. Coastlines the world over shifted because much of the water that had been available to the oceans was frozen instead. We see the sea level drop as much as 300, 400 feet below present day positions, and with that dropping of sea level all the streams of all the continents of the world being that much higher above present day seas and needing to move down toward the seas started eroding, digging down, running faster, so all the rivers begin to run faster throughout the world. There was more erosion due to streams.

As the last ice age ended, melting icecaps discharged vast quantities of water into the ocean flooding the lower valleys of these streams. Many have become inlets, harbors, or estuaries. Some have filled with sediment becoming flat, coastal valleys. The retreating glaciers have had another though less obvious effect on the landscape. The other big effect happened at the end of the ice age. As the ice began to melt, a tremendous weight was taken off the continents of the world, and the lass masses began to rise.

We've already seen how sea level has dropped during the ice age, and now at the end the lands begin to rise. Today the Hudson Bay region has risen as much as 400 feet and is still rising today. Across the interiors of continents, the ice sheets left moraines hundreds of kilometers long. Streamlined mounds of till known as "drumlins" were sculpted by flowing and melting ice. Thousands of lake filled basins called "kettles" were caused by the melting of ice buried in till.

Sinuous ridges of sediments left by subglacial streams known as "esters" wind across the land. Most important to our modern civilization, however, are the windblown deposits of glacial silt or loess, which have weathered to form rich farmland soil. Among the most profound effects of the ice ages on our world was the formation of land bridges. The dropping sea level exposed low lying coastal areas in some cases linking land masses that are presently separate.

The most famous land bridge between Asia and North America allowed animals including humans to migrate between the two continents. Evidence of ice ages is not only recorded by the landscape. Scientists have also discovered important clues to Earth's past within glaciers themselves. Glaciers contain within their stratigraphic layers all of the material that fell with the snowfall that incorporated within it. One can liken glaciers as a depositional environment, a glacial environment, and in the deposits contain events of Earth's history that occur simultaneously during the depositional process. You see, ice sheets are essentially atmospheric processes. Practically any material passing through the atmosphere may be trapped in glaciers as they form Pollen, volcanic ash, and meteorites have been discovered in the ice. Even bubbles of an ancient air have been found representing a valuable record of Earth's past atmosphere and climate.

Since the early 1970s an international project has been underway to drill into Greenland's vast continental glacier and obtain ice core samples. Called the "Greenland Ice Core Program," this effort involves scientists from countries around the world. The prime difficulties in an operation, a field operation such as drilling in ice relate primarily to the climate, the weather. It's not the best place to work, but it's overcome somewhat by the technique used today to excavate trenches and cover the tops of the trenches with some form of a material and to work, if you will, underground with what power sources available and so forth, so we can eliminate the inclement weather problems and work as we do on a 24 hour basis.

After all, it does take up to three years to drill through the bedrock in any location whether it be Greenland or Antarctica, and that length of time requires a lot of stamina from the crew involved. The cores from Greenland date back over 100,000 years. The age of the ice is determined by counting bands which form seasonally. High density ice bands are typical of winter months; low density of summer. In deep ice the bands are compressed together making them impossible to see, so chemical methods including Carbon14 dating are used instead to determine the age of the ice. Not all of the ice core analysis is done in the field. Ultimately, the samples are carefully packed and transported to several resource institutions. One of the foremost centers for the study of ice cores is the State University of New York at Buffalo.

An important goal of this project is to determine the composition of gases trapped in the ice. In general, the content of nitrogen and oxygen in bubbles shows little change relative to our modern atmosphere, but the levels of such important greenhouse gases as methane and carbon dioxide rise significantly in bubbles trapped after the last ice age.

Scientists aren't sure whether the rise in greenhouse gases ended the ice age, or the end of the ice age simply allowed these gases to increase because of the greater biological activity. Another indicator of past climate is "stable oxygen isotopes," which are atomic variations of the element oxygen that do not decay. These isotopes are frozen within the ice itself at the time of its formation. A very interesting specimen here, Yes. The 018/016 ratio of a snow sample or an ice sample in a core reflects the temperature at which the precipitation formed in a precipitating cloud above. This is preserved in the ice cores, and by measuring continuously the 018/016 using vast petrometers, one determines summer and winter layers.

By providing data about past temperatures, stable isotope analysis has helped answer a long standing question about the ice ages, whether or not they occur simultaneously in both the Northern and Southern Hemispheres. I think the major contribution of stable isotope analyses in ice cores if you look at it from a bipolar point of view, the Antarctic and the Greenland ice cores, the results from both records show us that approximately 10,700 years ago the ice age ended in both North and Southern Hemispheres.

That wasn't too many years ago that climatologists and meteorologists were even sure that wind system crossed the Equator. Glaciers also preserve evidence of human impact on the atmosphere. Since the start of the Industrial Revolution, the levels of carbon dioxide, sulphate, and other pollutants have sharply increased in glacial ice.

The Industrial Revolution, which has been attributed to occurring in the early 1800s,1820, 1830 produced a major change in the gaseous composition of the atmosphere, and this was brought about primarily by fossil fuel, high sulphur content fossil fuel consumption, coal, soft coal, and the productivity of the factories and what have you. The degree to which industrial activity is affecting our global climate remains controversial.

Understanding natural climate cycles through ice core research will help put human impact in perspective. It will also help us make the adjustments needed to sustain our civilization in a world of continual dramatic change. Glaciers and glaciation are among the first subjects that geologists attempted to study, and understanding them is now more important than ever. Global climate is linked with glaciation, and changes in glaciers are used to make long term predictions of rainfall patterns and of extreme weather such as hurricanes and drought. Understanding the cause of glacial cycles is also critical to the current debate over global warming and the Greenhouse Effect.

Our interglacial climate is currently undergoing a warming phase, which is at least in part natural. Some of this warming, however, may be due to the 25 percent in atmospheric carbon dioxide that has occurred since 1850. Only by understanding the natural process of interglacial warming can we accurately monitor and predict any human impact on the climate.

Glacial ice volume is the principal factor controlling sea level. Sea level rose about 12 centimeters over the last century, again due to the warming of the inter-glacial climate and retreat of the glaciers worldwide, but the current rate of rise is about three times as fast, and estimates of sea level rise over the next century vary from 30 centimeters to over 2 meters. This may not sound very threatening, but 30 centimeters of sea level rise would correspond to 500 meters of coastal flooding in some areas of the world.

For this reason, glaciation is tied to the future of some of the world's great coastal cities. Glaciers are responsible for fascinating land forms and for cycles of great change on the Earth's surface. The regular cycles of glacial advance and retreat are in a sense to pulse of the Earth or a clock by which we measure portions of geologic time, but even more important, glacial cycles contain vital clues, clues to the conditions that future inhabitants of the Earth will someday inherit.

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Major funding for "Earth Revealed" was provided by the Annenberg C.P.B. Project.


Well, the video shows us lots of features of glaciation, and it talks a little bit about the great ice age or the plisticine, but I'd like to elaborate a little bit on the ice age, especially the discovery of the ice age because this discovery and recognition that took place over a hundred years is one of the great stories in geology and it ranks right up there with the discovery of plate tectonics as far as our understanding of the Earth that actually began with James Hutton, who suggested back in 1795 that many of the gravels and boulders of the great alpine valleys of Switzerland had been put there by ancient extensions of alpine ice.

The valleys are rounded and "u" shaped, and there's no ice in them now except at high elevations, and even in Hutton's time the glaciers had been observed to advance and retreat seasonally and over time, but history was discounted in the 1800s.

In 1815, another Swiss geologist named Shaupentier heard a mountaineer in one of the alpine valleys mention that he thought the origin of these so-called erratic boulders in the Swiss valleys was from the advance of the glacier in prehistoric times, but Shaupentier didn't believe him; in fact, he ignored the information. It was commonly believed in those times that these erratic boulders which were not confined just to the will valleys were found many places in the world. It was believed that these boulders had been moved by the Great Flood, the Great Flood of Noah, and of course, no one wanted to counter the Bible's interpretation at the time, but after 12 years Shaupentier after walking around the Swiss valleys was convinced, and he, then, caused an earlier paper which had been presented on this topic to be published and set out himself to collect evidence of the moving ice.

In 1834 Shaupentier, himself, published a paper on the probable cause of the transport of these erratic rocks in Switzerland. This paper was ignored. One of the people who saw this or heard this paper being presented was a young fellow named Lewis Agasse, Agasse at that time was a professor of natural history at the College of New Shatelle, and he heard Shaupentier's theory and, like many people who heard the theory, thought it was ridiculous and absurd.

Now, Agasse had earned a considerable reputation as a paleontologist mainly studying ancient fish, and at Shaupentier's insistence, he went along with Shaupentier on a field trip strolling through the upper Rone valley in Switzerland, and Agasse after viewing the evidence in the field firsthand said that his life was altered and changed as radically as the ice had altered the valley, and, in fact, after seeing the evidence, he had no doubt at all that Shaupentier was correct, so Agasse then visited similar landscapes in other regions in the Alps, and at a spark of intuition he saw ice covering more than just the valleys of the Alps, and the idea of continental glaciation fell into place.

In a stunning moment of realization, he saw that ice many thousand feet thick had been more or less continuous from Ireland all the way into Russia, so Agasse wrote this up and presented a paper to the Holvetic Society, which was a scientific society in Switzerland in 1837. It's kind of an interesting paper for several reasons. In the first place, Agasse was scheduled to present a paper on paleozoology about the descriptions of some ancient fish, but instead he presented he presented this paper on the glacial age in which he outlined at great length the evidence and the chronology of the glacial history of Northern Europe.

He called this the "Epoch Glaciere." Well, to say that he did not overwhelm his colleagues is an understatement at best. In fact, he was attacked more than defended because of this theory largely because like many other scientists who put forth unusual theories, he didn't quite know where to stop; in fact, he concluded that the ice of the great ice sheet was broken up by the emergence of the newborn Alps arising up through the ice. Von Humboldt who was a well established very respected geologist at the time, after whom we know the Humboldt current and various things urged Agasse to forget about glaciers and go back to cataloging fossil fish.

Agasse's response was to go out into the field and seek more evidence. He visited glaciers on the rim of the Matterhorn, and he learned that a cabin that had been build on the glacier was now more than 2,000 feet down the mountain from where it had been built, so he and his team of workers drove a row of stakes across the glacier to discover its movement was not unlike that of water in a stream, and it's also worthy of note here that Agasse nearly drowned while being lowered on a rope 120 feet into a crevice hen he slipped off the ice and fell into a pool of water in the crevice.

Well, in addition to all this work in Europe, he also visited England, and Scotland, and Ireland, and Wales, and ultimately came here to the United States where he studied the shores of Lake Superior, paddled in a canoe, walked around the Hudson Islands in upstate New York, and there he observed the same features that he had seen in the Rone Valley. The difference was that these were not mountainous regions. These are flatlands, and it's hard to explain how there could have been glaciers in flatlands without some sort of major event.

Well, I don't want to get into the great detail on how Agasse was able to convince his compatriots and his colleagues of this, but both Charles Lyell and Charles Darwin ultimately became convinced, both by talking to Agasse and examining the evidence in the field themselves, and, in fact, Darwin noted that Agasse was worth any three scientists, both because of the extent of his knowledge and the quality of his scientific work.

Well, Agasse ultimately became a professor of natural history at Harvard University, where he died in 1873, and it's worthy of note here that when Agasse died, Harvard did indeed hire professors to replace him, and it's also interesting to note that his successor in the chair of geology, the person who succeeded Agasse published a paper nine years later debunking the ice age as a "myth." He said in his paper, in fact, "It may now be rejected without hesitation. The Glacial Epic was a local phenomenon." Well, like so many ideas in science, they're not accepted by people at the time the person puts them forth, but this theory has now been illuminated by more than a century of expanded research.

In fact, we now can identify glacial outwash at the mouth of the Mississippi River and identify huge lakes which were formed from water trapped between the mountains and retreating ice, and, in fact, the Great Lakes in Northern United States are now all that remain of this original lake.

This idea of a glacial age also challenges Hutton's idea of uniformitarianism. Uniformitarianism, you may remember, is based upon the idea that geological processes work in the same way and at the same rate as they did in the past, so the presence of large glaciers which sweep over the surface of the Earth is not uniformitarianism; in fact, it's not strictly uniformitarianism, at least as envisioned by Hutton.

What we do see is that we have a set of processes called glaciers that's remained constant through time, the processes themselves; but the rate, the duration, and the location of these processes are not uniform but exist within limits and vary over time, so like many controversies in science, the real story is a little bit of both.

Yes, it's uniformitarianism on the average, but there are these catastrophic events that take place over fairly short time periods that radically alter the surface of the Earth in short times in geologic terms anyway, and we saw in the program on "Evolution" that this is exactly the way evolution takes place, too; in fact, in evolutionary terms, this is called "punctuated equilibrium" where we find relatively quiet periods of more or less constant change punctuated by intense change, and by the way, this is also a common story in the development of scientific theories that people argue about "Is it 'A' or is it 'B' and eventually they find out that it's a little bit of both one way or the other.

The extent of the continental glaciation was also worthy of note. Now, as I mentioned earlier, currently only Antarctica and Greenland are covered by ice sheets, but these ice sheets are as much as 10,000 feet thick. In the plisticine period, four major advances of the ice sheet covered the Northern Hemisphere to about 40 degrees latitude; that's about the latitude of New York, and Cleveland, and San Francisco. The advances of these ice sheets formed the boundaries of what we call the "plisticine epic," and it occurred over a million years or so. It covered New England, New York, and the northern Great Plains, and created such features as Long Island, and Cape Cod, and the Great Lakes. The last retreat of the ice was about 10,000 years ago, and it's no coincidence that this happens to go along with the appearance of the species "homosapiens"; that's us, and it also coincides with the rise in human culture.

We'll come back to that a little later. The causes of these periods of glaciation are not known for sure, but we do know that they're rare events in geologic history, and this rarity suggests that there's a combination of factors that contribute to the causes; in fact, there have only been three recorded episodes of glaciation in the past billion year's of Earth's history: the plisticine, one about 300 hundred million years ago in the paleozoic, and one about 700 million years ago in the late precambrian.

Now, there have been several models that have been put forth to suggest the causes of the glacial advances in the glacial periods, but the models don't really tell us much because the models themselves even disagree on the causes, and, in fact, both models which suggest that warming has initiated a glacier and models which suggest that cooling has initiated the glaciers both seem to work reasonably well. It's kind of hard to figure that, but one of the theories says that, for example, if the Earth gets warmer, then more water evaporates from the ocean, that puts more water into hydrologic cycle, which then is available to precipitate, and once it precipitates, it covers with snow that changes the balance of solar radiation, which causes more cooling, and so on.

Another model says that cooling initiates it in the first place because cooling, then changes the heat balance of the continents, which causes the snow to stay around longer, which causes ice to accumulate, so forth. Okay, so it's not clear which one of these, either warming or cooling, causes it. There are several causes that have been proposed, and these were detailed in the video, so I won't go into great details, but astronomical cycles is one; variations in the Earth orbit or variations in the sun's energy output; atmospheric events of various types, dust from meteorite impacts, for example, or the presence of large amounts of volcanic dust in the atmosphere.

Many people like to subscribe to the idea of plate movements as one of the causes. Plate movements, as you are aware, affect the location of the continents, and many models suggest that during time when plate movements have put a large percentage of the continental areas near the Poles, that this might initiate it, but movement of continents can also change ocean and atmospheric circulation patterns.

It's been observed, for example, that the closing of Central America between North America and South America occurred just about the beginning of the plisticine and must have affected ocean circulation because now the Gulf Stream carries a tremendous amount of heat away from the Equator toward the Poles. Before Central America closed, that current would have flowed along the Equator, and so the diversion of that current may have had a cause, but whatever the cause is, it's very clear that it has happened; in fact, we know that major disruptions in any system cause oscillations as a new equilibrium has reached.

Okay, it also serves as a notice to us that major climatic changes are possible, and in long linear systems like the Earth's climate, small changes in one place may create relatively large changes in another place.

Okay, well let's move on to our last topic for today, and that is glaciation here in Hawaii. Ordinarily, we wouldn't think of glaciation taking place in Hawaii, but during the plisticine epic when the Earth's climate was much colder, there was significant glaciation here in Hawaii but only at high altitudes above 11,000 feet; in fact, only Mauna Kea and Mauna Loa are high enough for glaciations to have occurred, and either Mauna Loa wasn't glaciated, or the evidence has been covered by later eruptions; in fact, the only evidence we have is on Mauna Kea where we find grooved and polished rock.

I have an example of grooved and polished rock here. This is a little piece taken from about the 12,000 foot level on Mauna Kea, and you can very clearly see the grooves, and you can notice that arete in various places here, the sort of shiny colors, where the rock has been polished. We also find on Mauna Kea terminal moraines like this one in the photograph that occurs near about 12,000 feet. Plisticine glaciation in Hawaii also had profound effects on sea level and sea level throughout the Hawaiian Islands.

During sea level rises, for example, coral and alluvium were deposited in the valleys to create flat valley floors like in Manoa Valley here on Oahu and Waipio Valley on the Island of Hawaii. Sea cliffs with bases above present sea level and land with present shoreline are found here on Oahu.

Well, I think that's probably enough. It's worthy to note that there are Hawaiian glaciers, but we don't know the precise causes of the episodes of glaciation, but we do know they have occurred. The question is "Is the glaciation finished or are we inter-glacial?" And we simply can't say for sure.

The prognosis isn't good either way. If sea level rises it's bad luck. If sea level falls, it's bad luck.

The chances are that it's going to do one or the other because there's much more icecaps present on the Earth now than there have been at any time throughout Earth's history, so we might note here that human culture evolved as a response to the harsh conditions of plisticine glaciation. We adapted to it as a species in the past at the dawn of humanity, but then there were few people, poorly defined political boundaries, and no worldwide economic system. Such a change would create social stresses that would make our current political problems seem petty by comparison, and I have to ask you "Are we up to it if it happens?"

Well, those are questions which should concern us all, and with those thoughts to ponder, study hard, and I'll see you next time.

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