The Ice Age


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 North America 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
ocean 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. 


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