Geology Reference
In-Depth Information
ton and into southern British Columbia, Canada. The Cascades are continental volcanic
arcs produced by the subduction and melting of what little remains of the Farallon Plate.
Only a small portion is left, and it is now called the Juan de Fuca Plate. It continues to
move toward and under the North American plate along the northwestern United States,
causing volcanic activity and earthquakes along the west coast.
Carving the Grand Canyon with uplift
Possibly the most famous geologic feature of modern North America is the Grand
Canyon. You can ind a photo of the Grand Canyon in this topic's color photo section,
and its location is indicated in Figure 21-3.
The rock layers of the Grand Canyon were formed over a period of at least 550 million
years (though many layers that should date to the last 250 million years are missing,
probably due to erosion). The canyon itself could only have been carved long after the
sediments that form the rocks were laid down, compressed, and lithified (turned to
stone).
Exactly how the deep canyon has formed is still being explored by many geologists.
Most accept that the removal of sediments by the Colorado River plays a large role.
However, the rate of river flow and sediment removal doesn't add up to the enormous
amount of eroded material from the canyon. Therefore, some scientists wonder if anoth-
er process is at play. What they have found is that this region of southwestern North
America is also experiencing a slow and gentle uplift (relative to other regions of the
American west). The uplift began during the Neogene period of the Cenozoic, lifting the
Colorado Plateau region more than 1,500 meters (more than 4,900 feet) above sea level.
The combination of slow uplift and stream erosion together could explain how such
deep canyons have formed in such a relatively short time (geologically speaking, of
course).
Icing over northern continents
During the Paleogene period (at the start of the Cenozoic), the global climate was still
fairly warm. But a dramatic change toward cooler conditions occurred in the middle
Cenozoic, at the beginning of the Neogene period, about 23 million years ago. This cool-
ing climate led to the multiple glaciations of the Quaternary period ice ages that oc-
curred after 2.8 million years ago. Some scientists have suggested that the uplift of the
Himalayan Plateau as India connected with Asia played a major role in shifting global cli-
mate conditions (see the sidebar “Causing global cooling”). Multiple hypotheses are still
being tested and explored to explain the changes seen as the Neogene began.
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