An ice age is only a term used to describe one of two things: (1) whole glacial epochs such as the Pleistocene, or (2) a single glacial stage such as the Wisconsin or Illinoisan (see attached map) which are within glacial epochs, and they can last about 19,000 to 100,000 years. Within these periods the Earth was covered by large glaciers called ice sheets. Basically, glaciers are large masses of mobile, and permanent ice formed on land by the union and recrystallization of snowflakes. Today most of the glacial ice is located in the Antarctic, and glaciers hold about 75% of the Earth's fresh water supply (Nichols 1). On average glaciers move about a meter each day due to the sliding over the bedrock beneath them and the internal deformation of the ice. Ice sheets are dome-shaped glaciers that cover about 19,300 square miles, and they move in all directions. A glacier can be classified as large or small. Large glaciers are called medial moraines and crevasses, and small glaciers are classified as glacier tables and cryoconite holes. Medial moraines are surface ridges of material near the middle of a glacier, and crevasses are wedge-like cracks in the surface of a glacier or of an ice sheet. A glacier table is a large block of stone resting on an ice pedestal, and cryoconite holes are a result of small quantities of dust on the ice that does not insulate, but instead conducts heat to the ice beneath it and creates an almost vertical hole (Nichols 2). Glaciation is the covering of land by glacial ice, and evidence of glaciation has been found in at least five stretches of geologic time: in the middle of the Huronian era in Precambrian time; at the end of the Proterozoic Era; the middle of the Paleozoic Era between the Ordovician and Silurian Periods; the late Carboniferous and early Permian Periods in the late Paleozoic Era; and in the Pleistocene Epoch. The first known ice age occurred during Precambrian times, and is the largest geologic period. In Precambrian times the Earth was first formed and occurred about 4.6 billion years ago (the same as the Earth's estimated age), and the Huronian Era in Precambrian times took place about 1,700 to 2,300 million years ago. The major events during Precambrian times were: initial accumulation of cosmic material to form the Earth, and the separation of the planet into an inner molten metallic core and an outer silicate mantle. The only signs of life during this period are fossilized algae or bacteria in South Africa, and have an estimated age of 3 billion years. The second great ice age occurred during the Proterozoic Era which also occurred during Precambrian times, and started about 670 million years ago. The third great ice age occurred during the Paleozoic Era that started from 600 million to 225 million years ago, and is the era in which most fossil records have been extracted from, thus the name of the era which means "ancient life" in Greek. It is also the era in which gondwanaland existed; gondwanaland is the supercontinent that consisted of Africa, South America, Australia, Antarctica, and India in the Jurassic Period within the Paleozoic Era. During the first three periods of this era (Cambrian, Ordovician, and Silurian) no life occurred on land. The third great ice age within the Paleozoic Era was between the Ordovician and Silurian Periods. The Ordovician Period that spanned 500 to 425 million years ago is the second period within the Paleozoic Era. "The name is derived from the Ordovices, an early Celtic tribe that once inhabited the area of northwest Wales where the characteristic strata were first described" (Mintz 1). The interesting fact about this period is that part of the ice age that occurred was in the region know today as the Sahara Desert, when it was located in the region of the South Pole. Life during this period consisted of colonial corals, moss animals, lampshells, nautiloids, trilobites, echinoderms, and graptolites. The most interesting of these animals were trilobites. Trilobites are extinct marine arthropods , and their closest living relatives are spiders, crustaceans, and insects. Trilobites probably were predators and scavengers, and burrowed in sands and mud scurried on the seafloor (Lieberman 1). The fourth great ice age took place about 420 million years ago between the Carboniferous and the Permian Period in the late Paleozoic Era. During the Carboniferous Period the six different continents (one was gondwanaland) collided and joined to form Pangaea (or Pangea which means "all land" in Greek) this took place about 225 million years ago. Because the Earth was suffering these great changes the once hot climate in the early stages of the Carboniferous Period began to cool rapidly producing an ice age in the southern hemisphere. The last great ice age occurred when glaciers and ice sheets covered most of
and Europe in the Pleistocene Epoch which began about 1.7 million years ago, and the Recent Epoch (Holocene) together comprise the Quaternary Period. In the Pleistocene Epoch the Earth went through several important changes during the past 1.5 million years. As many as 30 or more repeated glaciations altered the biogeographic distribution patterns of marine and terrestrial plants and animals alike. Another was the displacement of climatic zones by as much as twenty to thirty degrees Fahrenheit. In the last ice age (70,000 years ago, and ended 10,000 years ago) most of North America was covered by ice, linking Alaska with Asia which produced human migration about 40,000 years ago, and that is why an "isolated" continent was inhabited. The earlier ice ages lasted about 10 million years, and most of the more recent ones lasted about a million years. Some of the great ice sheets, such as the Ordovician-Silurian and the Permo-Carboniferous sheets, appear to have migrated back and forth repeatedly across a large paleocontinent over a period of 100 million years (Goldthwait 1). Ice ages are unusual and short episodes of the Earth's climatic history because the combined length of glacial stages is only about 50 to 200 million years, or only 1 to 4% of Earth's history (Goldthwait 1). Why scientists believe in an ice age? The answer begins with Louis Agassiz, who was born in 1807 in Switzerland, and his last 25 years teaching at Harvard, died in 1873 . Agassiz became a teacher of natural science, and he knew much about the glaciers of his native Alps. He observed how they rubbed the valley floors and side, carried rocks, and left mounds of gravel as they melted (Agassiz 1). He noticed also that boulders of granite could be found hundreds of miles from any solid granite formations. Finally, bedrock far from the Alps showed grooves and scratches, such as would be made if glaciers had pushed small rocks over it. It the glaciers had been big enough to do this, they must have covered most of Northern Europe. How do scientists know when an ice age occurred, and that the Earth suffered climactic changes called ice ages? The answer is simple. By the use of isotope dating of igneous rocks, and fossils above and below the glacial layers we can determine the geologic period in which a certain ice age occurred. Beyond 10 million years ago, these dates may vary by a million years; between 290 million and 650 million years ago, by as much as 10 million years (Goldthwait 1). Another form of evidence or proof of an ice age lies in glaciation which lies in the widespread deposition of a unique kind of sediment called till that can be observed under all glaciers today. Till can lie on surfaces of glaciation, such as grooved, striated, or polished bedrock pavement. Tills also contain a variety of rock types called erratics which are rocks fragments carried by glacial ice, that derive from widely disparate areas. The rock surfaces of tills have facets caused by the abrasion of the rocks in the bottom layer of ice (Kimer 1). Most of what scientists know about ice ages is due to the Pleistocene ice ages because most of the evidence left behind of an ice age can be dated back to this era. An example of such evidence are varved deposits. "Varved deposits are thinly bedded, alternating coarse and fine grained sediment layers formed as annual accumulations at the bottom of a lake" (Ashley 1). Composed of coarse grained sand or silt layer and an overlying fine grained silt or clay layer, and are usually 1 to 5 cm thick. Most varved deposits were formed during the melting of the great continental glaciers during the Pleistocene ice age, and form in glacial lakes near the ice margin. When the lake freezes in the winter, the fine silt and clay brought in during summer runoff continue to settle out from the quiet water column, creating a uniformly thick layer over the entire lake bottom (Ashlye 1). Loess were another significant factor in evidence of ice ages because they are usually associated to the Pleistocene Era. A Loess (German for loose) is a loose surface sediment originally formed by wind action d}ring the Pleistocene ice ages. Loess deposits are often 10 to 15 meters thick, and they usually are not layered. Loess contain silt-size grains, mostly of quartz but also of clay minerals, feldspar, mica, hornblende, pyroxene, and sometimes carbonate minerals (Kay 1). In North America loess occurs in an area extending east from the Rocky Mountains to Pennsylvania and south to the Mississippi delta (Kay 1). The older the rock record of an ice age the less it is preserved, because rocks are altered by metamorphism over long periods of time. We will try to understand the cause of an ice age, but there are several theories about glaciation and ice ages. Several conditions are necessary for an ice age to occur. The only adequate source of water for such massive amounts of ice are the oceans, and wind and weather patterns have to be specific. For example, to preserve snow year around the summers have to be cool. This can be accomplished by dust accumulating in the stratosphere which was ten times dustier during glacial times (Goldthwait 3). Dust absorbs radiation and reflects some of the Sun's heat, thus cooling the Earth's surface. At first scientists thought it was volcanic dust, but it has not been any evidence of massive volcanic dust accumulation during ice ages. Another theory is based on evidence in dunes and loess blankets. The evidence indicates that winds were more intense in glacial times, and the belts of westerlies were pushed toward the equator (Goldthwait 3). Which would result in intensified heat exchange that would produce more clouds and precipitation. The increased would be about 80% effective in reflecting solar radiation; therefore the Earth's surface would become 2 to 4 degrees (Fahrenheit) cooler. Another interesting cause may have been amounts of carbon dioxide in the lower atmosphere. Carbon dioxide can increase short-wave sunlight coming in, and prevents long-wave heat radiation from passing out of the atmosphere, thus raising the temperature between glaciations (Goldthwait 3). The result is that seawater would have been about 8 degrees Fahrenheit cooler, and the cooler water could absorb carbon dioxide, thus cooling the air by 2 to 4 degrees Fahrenheit. Another theories relating to ice ages is the Ewing-Donn theory of glaciation. Basically, when the sea level was lower surface currents no longer delivered heat to the far north (or south in earlier ice ages); this dropped high latitude temperature. The ice surface would have then cooled the air masses contacting it by more than 5 degrees centigrade; this would have caused the further extension of sea ice that is recorded in the sandy sediments on the ocean floor (Goldthwait 3). Sea ice would have reflected the energy from the Sun, thus cooling the Earth and preventing access of moisture to the air. Finally, the ice sheets begin to shrink because of lack of moisture, and rising sea levels, and warm ocean currents begin to melt the sea ice. The Milankovitch theory is the most widely accepted theory in the world about ice ages. In 1930, Milutin Milankovitch a Yugoslavian geophysicist proposed that the Earth's orbit about the Sun caused the cycles of glaciation. As Paul A. Kay in his summary of the Milankovitch theory states: Three features of the Earth-Sun geometry undergo long-period changes. The obliquity of the ecliptic-the tilt of the Earth's axis from a direction perpendicular to the plane of the Earth's orbit-varies by about 1.3 deg about the mean of 23.1 deg over a period of about 41,000 years, changing the contrast between the seasons. The shape of the orbit varies from circular to elliptical (eccentricity, 0.00 to 0.06) over a period of about 97,000 years, resulting in a seasonal variation of 20 to 30 percent in net solar radiation received. The axis also changes its alignment among the stars (precession of the equinoxes), altering the season of closest approach to the Sun (perihelion) over a period of 22,000 years. ("Milankovitch theory" 1) The Milankovitch theory is widely accepted because studies of deep-sea cores, fossils, and modern radiolarian assemblages in the oceans have shown that temperatures change according to Milankovitch cycles of 450,000 year intervals. A more modern theory states that stardust is responsible for ice ages. Interglacials are warm periods occurring every 100,000 years during the past 2 million years (see attached table). It started when Gordon MacDonald of the University of California at San Diego challenged the Milankovitch theory when he found that the tilt of the Earth's orbit was the only true aspect of Milankovitch's theory, thus he concluded that the insufficient change in solar input was not enough to give ice sheets their marching order. Then, MacDonald contacted Richard A. Muller, a physicist at the University of California's Berkeley National Laboratory, who also concluded that the tilt of the Earth's orbit was not sufficient or logical to create an ice age. Muller reached his conclusion after measuring the tilt of the plane of Earth's orbit in relation to the larger plane of Jupiter's orbit (Wilson 2). To visualize the Muller's research Jim Wilson states "a crude but useful way to visualize this is to imagine the Sun as the hub of a car wheel, the plane of Jupiter's orbit extending to the sidewall of the tire and the plane of Earth's orbit as the hubcap. Simply put, the hubcap wobbles slightly." ("Stardust and ice ages" 2). The main idea is that the plane of the Earth's orbit wobbled in a way that would cause it to stay in long periods in stardust rich areas the amount of solar energy would be reduced. The only problem with this theory is that interplanetary dust clouds that account for the rise and fall of helium-3 have not been found, because helium-3 comes from space and can be found in oceanic sediments. To understand better the effects of ice ages we must first discuss paleoclimatology. Paleoclimatology is the study of past climates throughout geological time and of the causes of their variations. The climatic evidence of paleoclimatology, and its interpretation is highly speculative and can be interpret in different ways. One is the nature of the evidence is such that the farther into the past one looks, the less information is obtained. The geologic record consists of incomplete data accumulated and integrated over long periods of time. Interpretations, therefore, tend to be qualitative and of low resolution, and sometimes ambiguous. Closer to the present, much ephemeral (that is, short time-scale) evidence is still in existence, and it is possible to make out more detail (Kay 1). Thus, while an early Paleozoic glaciation might be identified and its extent roughly estimated, Cenozoic glacial-interglacial events can be clearly recognized and the character of fluctuations within them elucidated (Kay 1). The apparent increase in variability closer to the present is a result of the preservation of high-resolution data. Past geological periods and epochs probably experience variable climates throughout their duration. The greenhouse effect is said to be related to ice ages. Water vapor, carbon dioxide, and methane keep ground temperatures at a global average of about 15 degrees C. The gasses have this effect because as incoming solar radiation strikes the surface, the surface gives off infrared radiation, that the gases trap and keep near ground level. The effect is comparable to the way in which a greenhouse traps heat, hence the term. Even a limited rise in average surface temperature might lead to at least partial melting of the polar icecaps and hence a major rise in sea level, along with other severe environmental disturbances (Anthes 1). This melting of the icecaps can create an artificial or at least speed up the process of an ice age. Water vapor is a major reason why humid regions experience less cooling at night than do dry regions. However, variations in the atmosphere's CO2 content are what have played a major role in past climatic changes. Fossil fuels contribute to the global increase in atmospheric CO2. Numerous scientists have maintained that the rise in global temperatures in the 1980s and early 1990s is a result of the greenhouse effect. A report issued in 1990 by the Intergovernmental Panel on Climate Change, prepared by 170 scientist worldwide, further warned that the effect could continue to increase markedly (Anthes 1). Most major Western industrial nations have pledged to stabilize or reduce their CO2 emissions during the 1990s. The U.S. pledge thus far concerns only chlorofluorocarbons (CFCs). CFCs attack the ozone layer and contribute thereby to the greenhouse effect, because the ozone layer protects the growth of ocean phytoplankton (Anthes 1). That is why ice ages are interesting and important subjects. When we understand the Earth's past we understand our future on this planet.