Pleistocene
Glacial Events
The Pleistocene
geological record gives evidence of 20 cycles of advancing and retreating
continental glaciers, though during most of the Pleistocene glaciers
were far more extensive than they are today. Much of this glaciation
occurred at high latitudes and high altitudes, especially in the Northern
Hemisphere. Up to 30% of the Earth's surface was glaciated periodically
during the Pleistocene. Large portions of Europe, North America (including
Greenland), South America, all of Antarctica, and small sections of
Asia were entirely covered by ice. In North America during the peak
of the Wisconsinan glaciation approximately 18,000 years ago, there
were two massive yet independent ice sheets. Both the eastern Laurentide
and the western Cordilleran ice sheets were over 3900 meters thick.
In Europe, ice covered Scandinavia, extended south and east across
Germany and western Russia, and southwest to the British Isles. Another
ice sheet covered most of Siberia. In South America, Patagonia and
the southern Andes mountains were beneath part of the Antarctic ice
sheet. Because so much water was taken up as ice, global sea level
dropped approximately 140 meters.
The causes of the Pleistocene cycle of glacial and interglacial episodes
are still being debated. It appears that continental positions, oceanic
circulation, solar-energy fluctuations, and Earth's orbital cycles
combined to generate these glacial conditions, so perhaps it is inappropriate
to pinpoint any single cause. Some scientists have calculated that
changes in the concentration of greenhouse gases were a partial reason
for large (5-7° C) global temperature swings between the ice ages
and interglacial periods.
Two scientists greatly influenced how Pleistocene glaciations were
interpreted. In the 1800s, geologists were studying widespread surface
deposits called diluvium. This archaic term referred to deposits
that could not be explained by the normal action of rivers and seas,
but instead were believed to have been produced by extraordinary floods
of vast extent. Louis Agassiz, a Swiss geologist who initially worked
on fossil fish, demonstrated that diluvium was actually a ground moraine
formed by continental glaciation. The other influential figure, the
Yugoslav mathematician M. Milankovitch, showed that variation in Earth's
orbital motions could explain periodic climate changes, including
continental glaciation. |
