Geoscience Reference
In-Depth Information
landscape. Prior to Pleistocene glaciation, the areas now occu-
pied by the Great Lakes were large river valleys that were cut
into soft shale deposits. Very resistant granites, limestones, and
sandstones underlie the areas that are now regional landmasses,
such as the upper and lower peninsulas of Michigan.
As the Laurentide Ice Sheet oscillated back and forth across
the region during the Pleistocene, the glacier preferentially eroded
the softer rocks. In this way, the former river valleys gradually
changed into enormous glacial troughs. Because these troughs
progressively enlarged and deepened during subsequent gla-
ciations, they became the preferential pathways of the ice front
whenever it advanced back into the region. The modern Great
Lakes are the legacy of that complex geomorphic history. More
resistant rocks were much less eroded and became the foundation
for landforms such as the Upper and Lower Peninsulas of Michi-
gan, the Bruce Peninsula in Ontario, the Keweenaw Peninsula in
Upper Michigan, and a variety of islands.
increased levels of atmospheric carbon dioxide (and other
greenhouse gases) caused by human industrial activity. Re-
gardless of the cause, Earth appears to be in a distinct warm-
ing phase that has been linked to the increased incidence of
drought, species migration and loss, and rise in sea level.
Global warming is also having a major impact on glaciers
around the world.
As previously discussed in this chapter, glacial ad-
vance and retreat are controlled by fluctuations in the gla-
cial mass budget (see Figure 17.4). When snowfall in the
source area exceeds melting in the zone of ablation, gla-
ciers thicken and expand, either down valleys in alpine set-
tings or across landmasses in continental glaciers. Glaciers
naturally recede during periods when the rate of melting in
the zone of ablation exceeds the accumulation of snow in
the source area. During the Pleistocene Epoch, the budget
of global ice masses was controlled largely by fluctuations
in Earth-Sun geometry as described in the Milankovitch
theory. The most recent noteworthy episode of glacial ad-
vance occurred during the Little Ice Age, which was a time
of global cooling that occurred between about A.D. 1500
and 1850.
Since the end of the Little Ice Age, glaciers have gener-
ally been retreating around the world (Figure17.30a). Many
scientists have attributed this rapid rate of glacial retreat
to human-induced global warming. Although some of the
world's glaciers have actually advanced in the past few de-
cades, such as the Perito Moreno Glacier in Argentina, the
vast majority are melting at rapid rates. Glacier retreat has
been very noticeable in most alpine environments. In the
Swiss Alps, for example, the Rhône glacier has retreated
about 2.5 km (1.5 mi) in approximately the past 140 years.
In Peru, the Ururashraju Glacier retreated 500 m (1640 ft)
between 1986 and 1999 alone! At Glacier National Park in
Montana, glacier retreat has been dramatic. In 1850 approxi-
mately 150 glaciers were present in the area and most were
still there in 1910 when the park was established. By 2010,
however, only 25 glaciers larger than 10 hectares (25 acres)
remained. Repeat photography of the Grinnell Glacier, for
example, is representative of the kind of change seen else-
where in the park. Notice the dramatic difference in that
system between 1900 (Figure 17.30b) and 2008 (Figure
17.30c). If such rate of melting continues as environmental
models suggest, the Grinnell Glacier, as well as the remain-
ing glaciers in the park, will disappear entirely in the next
few decades.
Rapid melting of glaciers has also been observed on the
Greenland and Antarctic ice sheets. On the Greenland Ice
Sheet, the area on the edge of the glacier affected by melting
increased by 16% between 1979 and 2002. You can see how
the area affected by melting has changed in the past 20 years
in Figure 17.31. On the Antarctic Ice Sheet, the rate of glacial
melting is even more impressive. An example of this melting
is the collapse of the Larsen B Ice Shelf that occurred over a 35-
day period beginning January 31, 2002. An ice shelf is a plate of
glacial ice that extends from the landmass into the water; many
KEY CONCEPTS TO REMEMBER ABOUT
THE HISTORY OF GLACIATION ON
EARTH
1.
Several periods of intense glaciation occurred during Pre-
cambrian times in which most of the planet was covered
by ice. Such a period is referred to as a snowball Earth.
2.
In the context of geologic time, the most recent pe-
riod of major continental glaciations is the Pleistocene
Epoch, which lasted from approximately 1.6 million to
10,000 years ago.
3.
Until recently, it was believed that four major glaciations
occurred during the Pleistocene Epoch. Oxygen isotope
records derived from ice cores and marine sediments in-
dicate that more than 20 major glacial periods occurred
during the Pleistocene. The best understood of these
glaciations are the Illinoisan and Wisconsin periods,
which occurred most recently. All other glacial periods
are lumped into one category called the Pre-Illinoisan.
4.
Most glacial landforms in the upper Midwest, the north-
eastern United States, and Canada are products of the
Wisconsin glaciation, which occurred between about
110,000 and 10,000 years ago. The most distinctive
large-scale landforms are the Great Lakes, which are
glacial troughs cut into soft bedrock.
Probable Human Impact
on Glaciers
With an understanding of past glaciations in mind, a key
question to consider now is the status of contemporary gla-
ciers with respect to the issue of ongoing climate change.
Although this period of warming may be part of a natural
cycle, most climatologists believe that it is associated with
