Geology Reference
In-Depth Information
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and Volcanism
The Pleistocene Epoch is best known for widespread gla-
ciers, but it was also a time of continuing tectonic activity
and volcanism. Orogeny continued in the Himalayas and
the Andes, and deformation at convergent plate boundar-
ies proceeded unabated in the Aleutian Islands, Japan, the
Philippines, and elsewhere. Interactions between the North
American and Pacifi c plates persisted along the San Andreas
fault (a transform plate boundary), yielding folds, faults,
and a number of basins and uplifted areas (
What Would You Do
As the only resident of your community with any background
in geology, you are asked by a curious person why western
North America has volcanoes, earthquakes, mountain ranges,
and small valley glaciers, whereas these same features are
absent or nearly so in the eastern part of the continent. How
would you explain this disparity? Can you think of how the
situation might be reversed—that is, what kinds of geologic
events would lead to these kinds of phenomena in the east?
Figure 23.13a).
For example, several east-west-trending mountain ranges in
southern California owe their existence to stresses created
along a bend in the San Andreas fault.
Ongoing subduction of remnants of the Farallon plate
beneath Central America and the Pacifi c Northwest accounts
for Pleistocene and present-day volcanism in these two
regions. Although the Cascade Range began evolving far
back in the Paleocene, the large composite volcanoes such as
Mount Shasta and Mount Rainier, as well as the lava dome
known as Lassen Peak, are mostly Pleistocene and Recent
(see Geo-inSight on pages 650 and 651). Volcanism also took
place at many other locations in the western United States
(Figure 23.13b), and huge calderas formed in what is now
Yellowstone National Park in Wyoming.
◗
Most of the Gulf Coastal Plain was dominated by detri-
tal deposition, but in the Florida section of the region and
the Gulf Coast of Mexico, signifi cant carbonate deposition
took place. Florida was a carbonate platform during the Cre-
taceous and continued as an area of carbonate deposition
into the Early Paleogene; carbonate deposition continues
even now in Florida Bay and the Florida Keys.
The east coast of North America includes the
Atlantic Coastal
Plain
and extends seaward across the continental shelf, slope,
and rise (Figure 23.12b). It is a classic example of a passive
continental margin. When Pangaea began fragmenting dur-
ing the Triassic, continental crust rifted, and a new ocean ba-
sin began to form. Remember that the North American plate
moved westerly, so its eastern margin was within the plate,
where a passive continental margin developed.
The Atlantic continental margin has a number of Meso-
zoic and Cenozoic basins, formed as a result of rifting, in which
sedimentation began by Jurassic time. Even though Jurassic-age
rocks have been detected in only a few deep wells, geologists
assume that they underlie the entire continental margin. The
distribution of Cretaceous and Cenozoic rocks is better known,
because both are exposed on the Atlantic Coastal Plain, and
both have been penetrated by wells on the continental shelf.
Sedimentary rocks on the broad Atlantic Coastal Plain,
as well as those underlying the continental shelf, slope, and
rise, were derived from the Appalachian Mountains. Nu-
merous rivers and streams transported sediments toward
the east, where they were deposited in seaward-thickening
wedges (up to 14 km thick) that grade from terrestrial depos-
its on the west to marine deposits further east. For instance,
the Calvert Cliffs in Maryland consist of rocks deposited in
marginal marine environments (Figure 23.1d).
In 1837, the Swiss naturalist Louis Agassiz argued that large
boulders (erratics), polished and striated bedrock, U-shaped
valleys, and deposits of sand and gravel in parts of Europe
resulted from huge glaciers moving over the land. Although
the idea initially met with considerable resistance, scientists
fi nally came to realize that Agassiz was correct and accepted
the idea that an Ice Age had taken place in the recent geo-
logic past.
The Distribution and Extent of Pleistocene Glaciers
We know today that the Pleistocene, or what is commonly
called the Ice Age, began 1.8 million years ago and ended
about 10,000 years ago. During this time, several intervals
of widespread continental glaciation took place, especially
on the Northern Hemisphere continents, each separated
by warmer interglacial periods (
Figure 23.14). In addi-
tion, valley glaciers were more common at lower elevations
and latitudes, and many extended much farther than their
shrunken remnants do today (see Chapter 14 Geo-Focus on
pages 364 and 365). Unfortunately, scientists do not know
whether we are still in an interglacial period or entering an-
other glacial interval.
As one would expect, the climatic effects responsible for
Pleistocene glaciers were worldwide. Nevertheless, Earth was
not as frigid as portrayed in movies and cartoons, nor was
the onset of the climatic conditions leading to glaciation very
rapid. Indeed, evidence from several types of investigations
indicates that the climate gradually cooled from the Eocene
through the Pleistocene. Furthermore, evidence from oxygen
◗
HOLOCENE EPOCHS
The
Pleistocene
and
Holocene
or
Recent
epochs constitute
the most recent 1.8 million years of geologic time. The
Pleistocene, which began 1.8 million years ago and ended
10,000 years ago, is the focus of our discussion here be-
cause it was the time of extensive glaciation; hence, the
Pleistocene is commonly called the
Ice Age
.
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