Glaciology is the study of ice and its effects. Since ice can appear on or in the earth as well as in its seas and other bodies of water and even its atmosphere, the purview of glaciologists is potentially very large. For the most part, however, glaciologists’ attention is direction toward great moving masses of ice called glaciers, and the intervals of geologic history when glaciers and related ice masses covered relatively large areas on Earth. These intervals are known as ice ages, the most recent of which ended on the eve of human civilization’s beginnings, just 11,000 years ago. The last ice age may not even be over, to judge from the presence of large ice masses on Earth, including the vast ice sheet that covers Antarctica. On the other hand, evidence gathered from the late twentieth century onward indicates the possibility of global warming brought about by human activity.
HOW IT WORKS
Ice, of course, is simply frozen water, and though it might appear to be a simple subject, it is not. Glaciologists classify differing types of ice, for instance, with regard to their levels of density, designating them with Roman numerals. The ice to which most of us are accustomed is classified as ice I. We will not be concerned with the other varieties of ice in the present context, but it should be noted that the ice in glaciers is quite different from the ice in an ice cube or even the ice on a pond in winter. These differences are a result of massive pressure, which reduces the air content of the ice in glaciers.
By definition, ice is composed of fresh water rather than saltwater. This is true even of icebergs, though they may float on the salty oceans. The reason is that water has a much higher freezing point than salt, and, therefore, when water freezes, very little of the salt remains joined to the water. Most of the salt is left behind in the form of a briny slush, and so much of Earth’s fresh water supply actually is contained in great masses of ice, such as the glaciers of Antarctica.
Glaciology is defined as the study of ice, its forms, and its effects. This means that the glaciologist has a much wider scope than a geologist, meteorologist, or oceanographer, each of whom is concerned primarily with the geosphere, atmosphere, and hydrosphere, respectively. Though ice commonly is associated with the hydrosphere, where it appears on Earth’s oceans, rivers, and lakes, it also is found on and even under the solid earth. There are even situations in which ice is found in the atmosphere.
Glaciology and Glaciers
Despite the wide distribution of ice on Earth and the many forms it takes, the work of most glaciologists is concerned primarily with ice as it appears in glaciers. A glacier is a large, typically moving mass of ice on or adjacent to a land surface. It does not flow, as water does; rather, it is moved by gravity, a consequence of its extraordinary weight.
Obviously, a glacier can form only in an extremely cold region—one so cold that the temperature never becomes warm enough for snow to melt completely. Some snow may melt as a result of contact with the ground, which is likely to be warmer than the snow itself, but when temperatures drop, it refreezes. A glacier starts with a layer of ice, on which snow gathers until refreezing gradually creates compacted layers of snow and ice.
AN INFRARED SATELLITE IMAGE OF ANTARCTICA, SHOWING ICE SHELVES PROJECTING FROM THE COASTLINE. THE
LARGEST ICE SHELF, THE ROSS, SITS TO THE LEFT OF THE TRANSANTARCTIC MOUNTAINS (LOWER CENTER).
As anyone who has ever held a snowball in his or her hand knows, snow is fluffy, or, to put it in more scientific terms, it is much less dense than ice. A sample of snow is about 80% air, but as ice accumulates over a layer of snow, the weight of the ice squeezes out most of the air. As the layers grow thicker and thicker, the weight reduces the air further, creating an extremely dense, thick layer of ice. Ultimately, the ice becomes so heavy that its weight begins to pull it downhill, at which point it becomes a glacier.
Glacial Temperature and Morphologic Characteristics
Glaciers may be classified according to either relative temperature or morphologic characteristics (i.e., in terms of its shape). In terms of temperature, a glacier may be “warm,” meaning that it is close to the pressure melting point. Pressure melting point is defined as the temperature at which ice begins to melt under a given amount of pressure. It is commonly known that water melts at 32°F (0°C), but only under conditions of ordinary atmospheric pressure at sea level. At higher pressures, the melting point of water is lower, which means that it can remain liquid at temperatures below its ordinary freezing point. (The melting point and freezing point of a substance are always the same.)
A “warm” glacier, such as those that appear in the Alps, is relatively mobile, because it is at the pressure melting point. This kind of glacier contrasts with a “cold,” or polar, glacier, in which the temperatures are well below the pressure melting point; in other words, despite the extremely high pressure, the temperature is so low that the ice will not melt. As their name suggests, polar glaciers are found at Earth’s poles, which effectively means Antarctica, since the area of the North Pole is not a land surface. A third category of glacier, in terms of temperature, is a subpolar glacier, found (not surprisingly) in regions near the poles. Examples of subpolar glaciers, or ones in which the fringes of the glacier are colder than the interior, are found in Spitsbergen, islands belonging to Norway that sit in the Arctic Ocean, well to the north of Scandinavia.
In the classification of geologic sciences, glaciology often is grouped with geomorphology. The latter field of study is devoted to landforms, or notable topographical features, and the forces and processes that have shaped them. Among those forces and processes are glaciers, which can be viewed in terms of their shape, the locale in which they form, and their effect on the contour of the land.
Alpine or mountain glaciers flow down a valley from a high mountainous region, typically following a path carved out by rivers or melting snow in warmer periods. They move toward valleys or the ocean, and in the process they exert considerable impact on the surrounding mountains, increasing the sharpness and steepness of these landforms. The rugged terrain in the vicinity of the Himalayas and the Andes, as well as the alpine regions of the Cascade Range and Rocky Mountains in the United States, are partly the result of weathering caused by these glaciers.
The glacial forms found in Alaska, Greenland, Iceland, and Antarctica are often piedmont glaciers, large mounds of ice that slope gently. Iceland, Greenland, and Antarctica as well as Norway are also home to cirque glaciers, which are relatively small and wide in proportion to their length. Though they experience considerable movement in place, they usually do not move out of the basinlike areas in which they are formed.
Other Ice Formations
There are several other significant varieties of ice formation, including ice caps, ice fields, and ice sheets. An ice cap, though much bigger than a glacier, typically has an area of less than 19,300 sq. mi. (50,000 sq km). Nonetheless, its mass is such that it exerts enormous weight on the land surface, and this exertion of force allows it to flow.
At the center of an ice cap or an ice sheet is an ice dome, and at the edges are ice shelves and outlet glaciers. Symmetrical and convex (i.e., like the outside of a bowl), an ice dome is a mass of ice often thicker than 9,800 ft. (3,000 m). An outlet glacier is a rapidly moving stream of ice that extends from an ice dome. Ice shelves, at the far outer edges, extend into the oceans, typically ending in cliffs as high as 98 ft. (30 m). Ice fields are similar to ice caps; the main difference is that the ice field is nearly level and lacks an ice dome. There are enormous variations in size for ice fields. Some may be no larger than 1.9 sq. mi. (5 sq km), while at different times in Earth’s history, others have been as large as continents.
The most physically impressive of all ice formations, an ice sheet is a vast expanse of ice that gradually moves outward from its center. Ice sheets are usually at least 19,300 sq. mi. (50,000 sq km) and, like ice caps, consist of ice domes and outlet glaciers, with outlying ice shelves. Given their even greater size compared with ice caps, ice sheets exert still more force on the solid earth beneath them. They cause the rock underneath to compress, and, therefore, if an ice sheet ever melts, Earth’s crust actually will rise upward in that area.
An example of an ice sheet is the Antarctic ice sheet, which is permanently frozen—at least for the foreseeable future. The Antarctic ice sheet covers most of Antarctica, an area of about five million sq. mi. (12.9 million sq km), the size of the United States, Mexico, and Central America combined. Within it lies 90% of the world’s ice and more fresh water than in all the planet’s rivers and lakes combined. By contrast, the impressive Greenland ice sheet, at 670,000 sq. mi. (1,735,000 sq km), is dwarfed, as are smaller ice sheets in Iceland, northern Canada, and Alaska.
The Antarctic ice sheet is the Sahara of ice masses, though, in fact, it is almost 50% larger than the Sahara desert and a good deal more inhospitable. Whereas the Sahara is scattered with towns and oases and has a steady population of isolated villagers, nomads, and merchants in caravans, no one lives on the Antarctic ice sheet except scientists on temporary missions. And whereas people have lived in the Sahara for thousands of years (it became a desert only somewhat recently, during the span of human civilization), scientific missions to Antarctica became possible only in the twentieth century. As it is, researchers spend only short periods of time on the continent and then in heavily protected environments.
Ice shelves and glaciers
Just as the Antarctic ice sheet is the largest in the world, one of its attendant shelves also holds first place among ice shelves. The continent is shaped somewhat like a baby chick, with its head and beak pointing northward toward the Falkland Islands off the coast of South America and its two greatest ice shelves lying on either side of the “neck.”
Facing the Weddell Sea, and the southern Atlantic beyond it, is the Ronne Ice Shelf, which extends about 400 mi. (640 km) over the water. The world record-holder, however, is the Ross Ice Shelf on the other side of the “neck,” near Marie Byrd Land. About the same size as Texas or Spain, the Ross shelf extends some 500 mi. (800 km) into the sea and is the site of several permanent research stations.
The Antarctic is also home to a vast mountain range, the Transantarctic, which stretches some 3,000 mi. (4,828 km) across the “neck” between the Ross and Weddell seas. Included in the Transantarctic Mountains is Vinson Massif, which at 16,860 ft. (5,140 m) is the highest peak on the continent. The continent as a whole is largely covered with mountain ranges, between which lie three great valleys called the Wright, Taylor, and Victoria valleys. Each is about 25 mi. (40 km) long and 3 mi. (5 km) wide.
These are the largest continuous areas of ice-free land on the continent, and they offer rare glimpses of the rocks that form the solid-earth surface deep beneath Antarctica. They are also among the strangest places on the planet, forbidding lands even by Antarctic standards. The three are known as the “dry valleys,” owing to their lack of precipitation; indeed, if they lay in a more temperate zone, they would be deserts far more punishing than the Sahara. Geologists estimate that it has not rained or snowed in these three valleys for at least one million years. The reason is that ceaseless winds keep the air so dry that any falling snow evaporates before it reaches the ground. In this arid, brutally cold climate, nothing decomposes, and seal carcasses a millennium old remain fully intact.
The thickness of the ice
The dry valleys are exceptional, because most of Antarctica lies under so much ice that the rocks cannot be seen. The ice in Antarctica has an average depth of more than a mile: the depth averages about 6,600 ft. (2,000 m), but in places on the continent it is as thick as 2 mi. (3.2 km). Thus, “ground level” on Antarctica is equivalent to a fairly high elevation in the inhabitable portion of the planet. Denver, Colorado, for instance, touts itself as the “Mile-High City,” and its elevation has enough effect on a visiting flatlander that rival sports teams usually spend a few days in Denver before a game, adjusting themselves to the altitude.
The thickness of the ice has allowed glaciologists to take deep ice-core samples from Antarctica. An ice core is simply a vertical section of ice that, when studied with the proper techniques and technology, can reveal past climatic conditions in much the same way that the investigation of tree rings does. (See Paleontology for more about tree-ring research, or dendrochronology.) Ice-core samples from the Antarctic provide evidence regarding Earth’s climate for the past 160,000 years and show a pattern of warming and cooling that is related directly to the presence of carbon dioxide and methane in the atmosphere. These core samples also reveal the warming effects of increases in both gases over the past two centuries.
Because of the great thickness of its ice, Antarctica has the highest average elevation of any continent on the planet. Yet beneath all that ice, the actual landmass is typically well below sea level. The reason for this is that the ice weighs it down so much; by contrast, if the ice were to melt, the land would begin to spring upward. The melting of the Antarctic ice shelf would be a disaster of unparalleled proportions. If all that ice were to melt at once, it would raise global sea levels by some 200 ft. (65 m). This would be enough to flood all the world’s ports, along with vast areas of low-lying land. For instance, waters would swell over New York City and all ports on America’s eastern seaboard and probably would cover an area extending westward to the Appalachian Mountains. Even if only 10% of Antarctica’s ice were to melt, the world’s sea level would rise by 20 ft. (6 m), enough to cause considerable damage.
What Glaciers Do to Earth’s Surface
Periodically over the past billion years, Earth’s sea levels have advanced or retreated dramatically in conjunction with the beginning and end of ice ages. The latter will be discussed at the conclusion of this essay; in the present context, let us consider simply the geomorphologic effects of glaciers and ice masses. For example, as suggested in the discussion of Antarctica’s ice sheet, when glaciers melt, thus redistributing their vast weight, Earth’s crust rebounds. At the end of the last ice age, the crust rose upward, and in parts of North America and Europe this process of crustal rebounding is still occurring.
Glaciers move at the relatively slow speeds one would expect of massive objects made from ice: only a few feet or even a few inches per day. Friction with Earth’s surface may melt the layer of ice that comes in contact with it, however, and, as a result, this layer of meltwater becomes like a lubricated surface, allowing the glacier to move much faster. The entire body of ice experiences a sudden increase of speed, called surging.
Plowing through the land
A glacier is like a huge bulldozer, plowing though rock, soil, and plants and altering every surface with which it comes into contact. It erodes the bottoms and sides of valleys, changing their V shape to a U shape. The rate at which it erodes the land is directly proportional to the depth of the glacier: the thicker the ice, the more it bears down on the land below it. As it moves, the ice pulls along rocks and soil, which are incorporated into the glacier itself. These components, in turn, make the glacier even more formidable, giving it greater weight, cutting ability, and erosive power.
The sediments left by glaciers that lack any intervening layer of melted ice are known by the general term till. In unglaciated areas, or places that have never experienced any glacial activity, sediment is formed by the weathering and decomposition of rock. On the other hand, formerly glaciated areas are distinguished by layers of till from 200 to 1,200 ft. (61-366 m) thick. Piles of till left behind by glaciers form hills called moraines, and the depressions left by these land-scouring ice masses are called kettle lakes.
North America abounds with examples of moraines and kettle lakes. Illinois, for instance, is covered with ridges, called end moraines, left behind by the melting near the conclusion of the last ice age. Visitors can take in a splendid view of moraine formations at Moraine View State Recreation Area, located astride the Bloomington moraine in central Illinois. Likewise Minnesota, Wisconsin, the Dakotas, and Wyoming are home to many a moraine. As with the Illinois recreation area, Kettle Lakes Provincial Park, near Timmins, Ontario, provides an opportunity to glimpse gorgeous natural wonders left behind by the retreat of glaciers—in this case, more than 20 deep kettle lakes. Park literature invites visitors to boat, fish, or swim in the lakes, though it would take a hardy soul indeed to brave those icy waters.
The glacier transports material from the solid earth as long as it is frozen, but wholly or partially melted glaciers leave behind sedimentary forms with their own specific names. In addition to moraines, there are piles of sediment, called eskers, left by rivers flowing under the ice. In addition, deposits of sediment may wash off the top of a glacier to form steep-sided hills called kames. If the glacier runs over moraines, eskers, or kames left by another glacier, the resulting formation is called a drumlin.
Just as rivers consist of main bodies formed by the flow of tributaries (for example, the many creeks and smaller rivers that pour into the Mississippi), so there are tributary glaciers. When a glacial tributary flows into a larger glacier, their top elevations become the same, but their bottoms do not. As a result, they carve out “hanging valleys,” often the site of waterfalls. Examples include Yosemite and Bridalveil in California’s Yosemite Valley.
THE KENNICOTT GLACIER IN THE WRANGELL MOUNTAINS OF ALASKA.
The glaciers that exist today are simply the remnants of the last ice age, a time in which the size of the ice masses on Earth dwarfed even the great Antarctic ice sheet. When people speak of “the Ice Age,” what they mean is the last ice age, which ended about 11,000 years ago. Yet it is one of only about 20 ice ages that have taken place over the past 2.5 million years, roughly coinciding with the late Pliocene and Pleistocene epochs. Actually, periods of massive glaciation (the covering of the landscape with large expanses of ice) have occurred at intervals over the past billion years. Their distribution over time has not been random; rather, they are concentrated at specific junctures in Earth’s history.
Like the great mass extinctions of the past (see Paleontology), ice ages are among the markers geologists use in separating one interval of geologic time from another. In fact, there have been connections between ice ages and mass extinctions, particularly those that resulted from a recession of the seas. For example, the mass extinction that took place near the end of the Ordovician period (about 435 million years, or Ma, ago) came about as a result of a drop in the ocean level, which was caused, in turn, by an increase of glaciation that coincided with that phase in Earth’s history.
The late Ordovician/early Silurian ice ages (between 460 to 430 Ma ago) are among four major phases of glaciation during the past 800 million years. The first of these occurred during the late Proterozoic eon, toward the end of Precambrian time (between 800 and 600 Ma ago). Another happened during the Pennsylvanian subperiod of the Carboniferous and extended throughout the Permian period, thus lasting from about 350 to 250 million years ago. The last period of glaciation is the one in which we are living, beginning in the late Neogene period and extending into the current Quaternary.
Atmosphere: In general, an atmosphere is a blanket of gases surrounding a planet. Unless otherwise identified, however, the term refers to the atmosphere of Earth, which consists of nitrogen (78%), oxygen (21%), argon (0.93%), and other substances that include water vapor, carbon dioxide, ozone, and noble gases such as neon, which together comprise 0.07%.
Crustal rebounding: An upward movement by Earth’s crust in response to the melting of a glacier, which redistributes its vast weight and causes Earth to rebound.
Geomorphology: An area of physical geology concerned with the study of landforms, with the forces and processes that have shaped them, and with the description and classification of various physical features on Earth.
Geosphere: The upper part of Earth’s continental crust, or that portion of the solid Earth on which human beings live and that provides them with most of their food and natural resources.
Glaciation: The covering of the landscape with large expanses of ice, as during an ice age.
Glacier: A large, typically moving mass of ice on or adjacent to a land surface.
Glaciology: An area of physical geology devoted to the study of ice, its forms, and its effects.
Hydrosphere: The entirety of Earth’s water, excluding water vapor in the atmosphere but including all oceans, lakes, streams, groundwater, snow, and ice.
Ice age: A period of massive and widespread glaciation. Ice ages usually occur in series over stretches ofseveral million years, or even several hundred million years.
Ice cap: An ice formation bigger than a glacier but smaller than an ice sheet. An ice cap typically has an area of less than 19,300 sq. mi. (50,000 sq km) and, like an ice sheet, consists of an ice dome, with ice shelves and outlet glaciers at the edges.
Ice core: A vertical section of ice, usually taken from a deep ice sheet such as that in Antarctica. When studied with the proper techniques and technology, ice cores can reveal past climatic conditions in much the same way that the investigation of tree rings does.
Ice dome: A symmetrical, convex (i.e., like the outside of a bowl) mass of ice, often thicker than 9,800 ft. (3,000 m), usually found at the center of an ice cap or an ice sheet.
Ice field: A large ice formation,similar to an ice cap except that it is nearly level and lacks an ice dome. There are enormous variations in size for ice fields. Some may be no larger than 1.9 sq. mi. (5 sq km), while at different times in Earth’s history, some have been as large as continents.
Ice sheet: A vast expanse of ice, usually at least 19,300 sq. mi. (50,000 sq km), that moves outward from its center. Like the smaller ice caps, ice sheets consist of ice domes and outlet glaciers, with outlying ice shelves.
Ice shelf: An ice formation at the edge of an ice cap or ice sheet that extends into the ocean, typically ending in cliffs as high as 98 ft. (30 m).
Landform: A notable topographical feature, such as a mountain, plateau, or valley.
Ma: An abbreviation used by earth scientists, meaning million years or megayears. When an event is designated as, for instance, 160 Ma, it usually means 160 million years ago.
Mass extinction: A phenomenon in which numerous species cease to exist at or around the same time, usually as the result of a natural calamity.
Moraine: A hill-like pile of till left behind by a glacier.
Morphology: Structure or form, or the study thereof.
Outlet glacier: A rapidly moving stream of ice that extends from an ice dome.
Physical geology: The study of the material components of Earth and of the forces that have shaped the planet. Physical geology is one of two principal branches of geology, the other being historical geology.
Pressure melting point: The temperature at which ice begins to melt under a given amount of pressure. The higher the pressure, the lower the temperature at which water can exist in liquid form.
Relief: Elevation and other inequalities on a land surface.
Sediment: Material deposited at or near Earth’s surface from a number of sources, most notably preexisting rock.
Till: A general term for the sediments left by glaciers that lack any intervening layer of melted ice.
Topography: The configuration of Earth’s surface, including its relief as well as the position of physical features.
Humans and the ice ages
In fact, many scientists question whether the last ice age has ended and whether we are merely living in an interglacial period. Certainly crustal rebounding is still taking place, as noted earlier.
Inasmuch as “glacial period” refers to a time when glaciers cover significant portions of Earth and when the oceans are not at their maximum levels, we are indeed still in a glacial period.
It seems as though human existence has been bounded by ice, both in its onset and its recession. Ice ages have been a regular feature of the two million years since Homo sapiens came into existence, and the species had much of its formative experience in times of glaciation. The latter part of the last ice age created a land bridge that made possible the migration of Siberian peoples to the Americas, so that they are known now as Native Americans. (The name is well deserved: the ancestors of the Native Americans moved east from Siberia about 12,000 years ago, whereas less than half that much time has elapsed since the Indo-European ancestors of Caucasian Europeans moved west from what is now Russia. Certainly no one today questions whether Germans, Italians, British, French, and other groups are “native” Europeans.)
As an indication that ice ages have not ceased to occur, there is the Little Ice Age, which lasted from as early as 1250 to about 1850. This was a period of cooling and expansion of glaciers in the temperate latitudes on which Europe is located. Glaciers destroyed farmlands and buildings in the Alps, Norway, and Iceland, while Norse settlements in Greenland became uninhabitable. Europe as a whole suffered widespread crop failures, with a resulting loss of life. Evidence for this ice age, and indeed for all ice ages, can be discerned from the “footprints” left by glaciers from that time. Another telling sign is the transport of materials, such as rocks and fossils, from one part of Earth’s surface to another.
Understanding ice ages
What caused the Little Ice Age? The answer, or rather the attempt at an answer, goes to the core question of what causes ice ages in general. Earth scientists have cited both extraterrestrial and terrestrial factors. Among the leading extraterrestrial causes are an increase in sunspot activity (see Sun, Moon, and Earth for more about sunspots) as well as changes in Earth’s orientation with respect to the Sun.
Contenders for a terrestrial explanation include changes in ocean circulation, as well as meteorites and volcanism. Either of these last two could have caused the atmosphere to become glutted with dust, choking out the Sun’s light and cooling the planet considerably. (Such a calamity has been blamed for several instances of mass extinction, most notably, the one that wiped out the dinosaurs some 65 million years ago. See Paleontology.)
Though it appears that we are living in an interglacial period and that Earth could undergo significant cooling again thousands of years from now, there is also an even more frightening prospect of human-induced warming. As noted earlier, the Antarctic ice core reveals an increase of carbon dioxide and methane in the atmosphere during the past two centuries. Though these gases can be produced naturally, this excess in recent times appears to be a by-product of industrialized society. The glaciers of Europe are receding, and whether this could be the result of human activity or simply part of Earth’s natural change as it comes out of the last ice age remains to be seen.
In the meantime, ice offers a great deal of potential for understanding our own planet and others. It may yet turn out that the ice sheets covering Mars contain single-cell life-forms. Furthermore, in August 1996, the National Aeronautics and Space Administration (NASA) reported that a meteorite found on the Antarctic ice may provide evidence of life on Mars. It seems that the 4.1-lb. (2 kg) meteorite contains polycyclic aromatic hydrocarbons that may have existed on that planet several billion years ago.