Geology Reference
In-Depth Information
history from which the evidence has been carried away, moved, or reshaped from its
original form of deposition.
Fortunately, sediments from the ocean floor also contain evidence for ice ages and the
periods of warming and melting in between (called
interglacials
). Scientists combine in-
formation from the seafloor sediments with glacial features on land to estimate the time
and extent of glacial ice ages.
Cycling through ice ages
For most of the earth's long history (I explain the depth of geologic time in Chapter 16),
the planet has been too warm for large amounts of ice to cover the continents. In the
last 3 million years (relatively recent in geologic time), periods of extreme cold have
come and gone, causing ice sheets and glaciers to advance and retreat repeatedly — a
cycle of ice ages.
Scientists have determined two primary causes of ice age cycling: changes in the posi-
tion of the continents on the earth's surface and changes in the earth's position relative
to the sun.
Moving to colder regions
When the continental plates move around the earth's surface, sometimes they move
closer to the North or South Pole. (I explain the movement of continental plates in
Chapter 9.) As they approach a pole, these landscapes experience more extreme cold
and wetter winters — conditions that lead to the creation of glaciers and ice sheets.
The presence of ice on the land and ocean surface at the poles becomes a mirror that re-
flects the sun's heat instead of absorbing it. This reflection, or
albedo,
leads the earth's
entire climate to become colder and intensifies the cold of winter, creating larger, more
extensive sheets of ice.
Orbiting, spinning, and tilting around the sun
The other cause of ice age cycles is the position of the earth relative to the sun. As a
planet orbiting the sun, the earth experiences three different cycles that each occur
over thousands of years. These cycles are described as
eccentricity, obliquity,
and
preces-
sion.
Together they are named
Milankovitch cycles
after the astronomer Milutin Mil-
ankovi5, who first calculated them about a hundred years ago. They are illustrated in
Figure 13-6, and I describe them here:
