Laboratory Manual for Introductory Geology 4e

EXERCISE 1.9
Name:
Course:
How Long Does It Take to Make an Ocean? To Erode a Mountain?
Section:
Date:
(a) Rates of uplift and erosion. The following questions will give you a sense of the rates at which uplift and erosion
take place. We will assume that uplift and erosion do not occur at the same time—that mountains are first uplifted,
and only then does erosion begin—whereas the two processes actually operate simultaneously.
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If mountains rose by 1 mm/yr, how high would they be (in meters) after 1,000 years? m
10,000,000 years? m 50 million years? m
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The Himalayas now reach an elevation of 8.8 km, and radiometric dating suggests that their uplift began about
45 million years ago. Assuming a constant rate of uplift, how fast did the Himalayas rise? km/yr
m/yr
mm/yr
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Evidence shows that there were once Himalaya-scale mountains in northern Canada, in an area now eroded nearly
flat. If the Earth were only 6,000 years old, as was once believed, how fast would the rate of erosion have had to be
for these mountains to be eroded to sea level in 6,000 years? m/yr mm/yr
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Observations of modern mountain ranges suggest that they erode at rates of 2 mm per 10 years. At this rate, how
long would it take to erode the Himalayas down to sea level?
years
(b) Rates of seafloor spreading. Today the Atlantic Ocean is about 5,700 km wide at the latitude of Boston. At one
time, however, there was no Atlantic Ocean because the east coast of the United States and the northwest coast
of Africa were joined in a huge supercontinent. The Atlantic Ocean started to form “only” 185,000,000 years ago,
as modern North America split from Africa and the two continents slowly drifted apart in a process called seafloor
spreading.
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Assuming that the rate of seafloor spreading has been constant, at what rate has North America been moving away
from Africa? mm/yr km per million years
GEOTOURS EXERCISE 1
Scaling Geologic Time in the Grand Canyon
Name:
Course:
Section:
Date:
Exploring Geology Using Google Earth
1. Visit digital.wwnorton.com/geolabmanual4
2. Go to the Geotours tile to download Google Earth Pro and the accompanying
Geotours exercises file.
Check and double-click the Geotour01 folder icon in Google Earth
to fly to the Grand Canyon of northern Arizona. Here, erosion by the
Colorado River treats visitors to one of the most spectacular exposures
of geologic history on the planet (ranging from ∼270-million-yearold
Kaibab Limestone capping the Grand Canyon’s rim to the ∼2000
million-year-old Vishnu Schist paving the Inner Gorge). Right-click on
the Bright Angel Trail (red path), and select Show Elevation Profile to
plot a graph of elevation change (y-axis) versus distance along the trail
(x-axis). Please note that axis values can vary slightly depending on window
size, so students should use the numbers provided for their calculations.
(continued)
22 CHAPTER 1 SETTING THE STAGE FOR LEARNING ABOUT THE EARTH