Geology Reference
In-Depth Information
As a result of glaciation during the Ice Age, Earth's
crust was depressed into the mantle and has since
risen, sea level fl uctuated widely, and lakes were pres-
ent in areas that are now quite arid.
A current widely and accepted theory explaining the
onset of ice ages points to irregularities in Earth's
rotation and orbit.
summers; colder winters; and the persistence of sea ice for long
periods in Greenland, Iceland, and the Canadian Arctic islands.
And because of the cooler, wetter summers, growing seasons
were shorter, which accounts for several widespread famines.
During the coldest part of the Little Ice Age (1680-1730),
the growing season in England was fi ve weeks shorter than
during the 1900s, and in 1695 Iceland was surrounded by sea
ice for most of the year. Occasionally, Eskimos following the
southern edge of the sea ice paddled their kayaks as far south
as Scotland, and the canals in Holland froze over in some
winters. In 1607, the fi rst Frost Fair was held in London, Eng-
land, on the Thames River, which began to freeze over nearly
every winter. In the late 1700s, New York Harbor froze over,
and 1816 is known as the “year without a summer” when un-
usually cold temperatures persisted into June and July in New
England and northern Europe. (The eruption of Tambora in
1815 contributed to the cold spring and summer of 1816.)
Many of you have probably heard of the Ice Age and have
some idea of what a glacier is, but it is doubtful that you
know much about the dynamics of glaciers, how they form,
and what may cause ices ages. In any case,
glaciers
are moving
bodies of ice on land that are particularly effective at erosion,
sediment transport, and deposition. They deeply scour the
■
■
Scientists know that Earth's surface temperatures have
increased during the last few decades, and even though
the evidence for a connection between greenhouse gases,
especially carbon dioxide (CO
2
), and global warming is
increasing, the human contribution to the problem has not
been fully resolved. We know from the geologic record that
an Ice Age took place between 1.8 million and 10,000 years
ago, and since that time, Earth has experienced several
entirely natural climatic changes. During the Holocene Maxi-
mum about 6000 years ago, the average temperatures were
slightly higher than they are now, and some of today's arid
regions were much more humid. The Sahara Desert of North
Africa had suffi cient precipitation to support lush vegetation,
swamps, and lakes. Indeed, Egypt's only arable land today
is along the Nile River (see the Geo-Focus in Chapter 12),
but until only a few thousands of years ago, much of North
Africa was covered by grasslands.
After the Holocene Maximum was a time of cooler temper-
atures, but from about
A
.
D
. 1000 to 1300, Europe experienced
the Medieval Warm Period during which wine grapes grew 480
km farther north than they do now. Then a cooling trend be-
ginning in about
A
.
D
. 1300 led to the
Little Ice Age
, which lasted
from 1500 to the middle or late 1800s. The Little Ice Age was
a time of expansion of glaciers (
◗
Figure 14.1); cooler, wetter
◗
Figure 14.1
Glacier Expansion During the Little Ice Age
Image not available due to copyright restrictions
b
The same glacier today. Its terminus is hidden behind a rock
projection at the lower end of its valley.
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