Geology Reference
In-Depth Information
Although changes in
axial tilt
have little effect on equatorial
latitudes, they strongly affect the amount of solar radiation
received at high latitudes and the duration of the dark period
at and near Earth's poles. Coupled with the third aspect of
Earth's orbit, precession of the equinoxes, high latitudes
might receive as much as 15% less solar radiation, certainly
enough to affect glacial growth and melting.
Precession of the equinoxes
, the last aspect of Earth's orbit
that Milankovitch cited, refers to a change in the time of the
equinoxes. At present, the equinoxes take place on about
March 21 and September 21 when the Sun is directly over
the equator. But as Earth rotates on its axis, it also wobbles
as its axial tilt varies 1.5 degrees from its current value, thus
changing the time of the equinoxes. Taken alone, the time of
the equinoxes has little climatic effect, but changes in Earth's
axial tilt also change the times of
aphelion
and
perihelion
,
which are, respectively, when Earth is farthest from and clos-
est to the Sun during its orbit (Figure 14.21c). Earth is now at
perihelion, closest to the Sun, during Northern Hemisphere
winters, but in about 11,000 years perihelion will be in July.
Accordingly, Earth will be at aphelion, farthest from the Sun,
in January and have colder winters.
Continuous variations in Earth's orbit and axial tilt
cause the amount of solar heat received at any latitude to
vary slightly through time. The total heat received by the
planet changes little, but according to Milankovitch, and now
many scientists agree, these changes cause complex climatic
variations and provided the triggering mechanism for the
glacial-interglacial episodes of the Pleistocene.
Climatic events with durations of several centuries, such as
the Little Ice Age, are too short to be accounted for by plate
tectonics or Milankovitch cycles. Several hypotheses have been
proposed, including variations in solar energy and volcanism.
Variations in solar energy could result from changes
within the Sun itself or from anything that would reduce the
amount of energy Earth receives from the Sun. The latter
could result from the solar system passing through clouds
of interstellar dust and gas or from substances in the atmo-
sphere reflecting solar radiation back into space. Records
kept over the past 90 years indicate that during this time the
amount of solar radiation has varied only slightly. Although
variations in solar energy may infl uence short-term climatic
events, such a correlation has not been demonstrated.
During large volcanic eruptions, tremendous amounts
of ash and gases are spewed into the atmosphere, where they
refl ect incoming solar radiation and thus reduce atmospheric
temperatures. Small droplets of sulfur gases remain in the at-
mosphere for years and can have a signifi cant effect on climate.
Several large-scale volcanic events have occurred, such as the
1815 eruption of Tambora, and are known to have had climatic
effects. However, no relationship between periods of volcanic
activity and periods of glaciation has yet been established.
Chapter Summary
Glaciers currently cover about 10% of the land surface
and contain about 2.15% of all water on Earth.
A glacier forms when winter snowfall exceeds summer
melt and therefore accumulates year after year. Snow is
compacted and converted to glacial ice, and when the ice
is about 40 m thick, pressure causes it to fl ow.
Glaciers move by plastic fl ow and basal slip.
Valley glaciers are confi ned to mountain valleys and fl ow
from higher to lower elevations, whereas continental gla-
ciers cover vast areas and fl ow outward in all directions
from a zone of accumulation.
The behavior of a glacier depends on its budget, which
is the relationship between accumulation and wastage.
If a glacier has a balanced budget, its terminus remains
stationary; a positive or negative budget results in the
advance or retreat of the terminus, respectively.
Glaciers move at varying rates depending on slope, dis-
charge, and season. Valley glaciers tend to fl ow more rap-
idly than continental glaciers.
Glaciers effectively erode and transport because they
are solids in motion. They are particularly effective at
eroding soil and unconsolidated sediment, and they can
transport any size sediment supplied to them.
Continental glaciers transport most of their sediment in
the lower part of the ice, whereas valley glaciers may carry
sediment in all parts of the ice.
Erosion of mountains by valley glaciers yields several
sharp, angular landforms including cirques, arêtes, and
horns. U-shaped glacial troughs, fi ords, and hanging val-
leys are also products of valley glaciation.
Continental glaciers abrade and bevel high areas, producing
a smooth, rounded landscape known as an ice-scoured plain.
Depositional landforms include moraines, which are
ridgelike accumulations of till. The several types of
moraines are terminal, recessional, lateral, and medial.
Drumlins are composed of till that was apparently
reshaped into streamlined hills by continental glaciers or
fl oods.
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