Geology Reference
In-Depth Information
Figure 14.18 Valley Train and a Kettle
b A kettle in a moraine in Alaska.
A valley train in Alaska made up of stratifi ed drift.
a
Figure 14.19 Kames and Eskers
b An esker near Dahlen, North Dakota.
This small hill in Wisconsin is a kame.
a
For more than a century, scientists have attempted to
develop a comprehensive theory explaining all aspects of ice
ages, but they have not yet been completely successful. One
reason for their lack of success is that the climatic changes
responsible for glaciation, the cyclic occurrence of glacial-
interglacial episodes, and short-term events such as the Little
Ice Age operate on vastly different time scales.
Only a few periods of glaciation are recognized in the geo-
logic record, each separated from the others by long intervals
of mild climate. Such long-term climatic changes probably
result from slow geographic changes related to plate tectonic
activity. Moving plates carry continents to high latitudes where
glaciers exist, provided they receive enough precipitation as
snow. Plate collisions, the subsequent uplift of vast areas far
above sea level, and the changing atmospheric and oceanic
circulation patterns caused by the changing shapes and posi-
tions of plates also contribute to long-term climate change.
The Milankovitch Theory
Changes in Earth's orbit as a cause of intermediate-term
climatic events were fi rst proposed during the mid-1800s, but
the idea was made popular during the 1920s by the Serbian
astronomer Milutin Milankovitch. He proposed that minor
irregularities in Earth's rotation and orbit are suffi cient to
alter the amount of solar radiation received at any given lati-
tude and hence bring about climate changes. Now called the
Milankovitch theory , it was initially ignored but has received
renewed interest since the 1970s and is now widely accepted.
 
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