Page 120 - Introduction to Petroleum Engineering

106 PETROLEUM GEOLOGY
6.1.1 Formation of the Rocky Mountains

The movement of tectonic plates across the surface of the globe generated forces
powerful enough to form the structures we see today. We illustrate the concepts by
describing the formation of the Rocky Mountains.
Fossils of marine life—such as the spiral‐shaped, invertebrate ammonites—found
in the Rocky Mountains are evidence that the central plains of the United States and
the foothills of the Rocky Mountains were below sea level in the past. The Western
Interior Seaway, which is also known as the Cretaceous Sea or North American
Inland Sea, was a sea that extended from the Arctic Ocean through the Great Plains
of North America and south to the Gulf of Mexico. The inland sea split North America
into two land masses: Laramidia and Appalachia.
Scientists believe the Rocky Mountains were formed by the subduction of plates
beneath the western edge of the North American plate. The subduction created stress
that caused buckling of the North American plate along a line of weak rock that
paralleled the North American Inland Sea. The buckling uplifted the central part of
the United States and formed the Rocky Mountains. The flatirons and Red Rocks
near Denver, Colorado, show the degree of structural buckling.
The Rocky Mountains run north–south through much of North America and are
part of the North American plate. The shapes of fossilized leaves indicate that the
Rocky Mountains were once twice as high as they are today. The shape of leaf edges
in fossilized leaves can be used to estimate temperature and elevation. Edges of the
leaves of some plants are jagged at lower temperatures and smooth at higher temper-
atures. Since atmospheric temperature decreases as elevation above sea level
increases, we are more likely to find leaves with jagged edges at higher elevations
and cooler temperatures than leaves with smooth edges. Based on these observations
and the study of fossilized leaves, the elevation of the Rocky Mountains today
appears to be lower than it was in the past. The loss of material can be attributed to
weathering and erosion. Much of the eroded material was collected in valleys and
basins or was transported away from the region by rivers.
The Rocky Mountains have also been shaped by the movement of glaciers. A river
can carve a V‐shaped valley through mountains. A glacier moving through the
valley can carve out the sides of a V‐shaped valley so the valley acquires a U shape.
The glacier can move large boulders over great distances. When the glacier melts,
it leaves behind debris, such as boulders, that was carried by the glacier.

Example 6.2 Erosion
How long would it take to completely erode a mountain that is 1 mi high?
Suppose the rate of erosion is 3 mm per century. Express your answer in years.
Answer
Mountain height is 1 mi = 5280 ft = 1609 m. The rate of erosion is 3 mm per 100
years or 0.03 mm per year. Thus, the time to erode a mile high mountain is
about 1609 m/0.03 mm/yr or 54 million years.