Laboratory Manual for Introductory Geology 4e

EXERCISE 12.8
What Numerical Ages Can Tell Us About Sedimentary Rocks (continued)
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The two geologists also disagree on the source region from which the clasts in the sandstone were eroded. Geologist A
thinks the source area contained igneous rocks of several different ages, but Geologist B says the source area was a huge
batholith set in place over a short period.
(b) What do the zircon data contribute to this debate?
12.5 Correlation: Fitting Pieces
of the Puzzle Together
Imagine how difficult it would be for an alien geologist visiting the Earth 200 million
years from now to reconstruct today’s geography. Plate-tectonic processes
could have moved some continents, split some apart and sutured others together,
or opened new oceans and shrunk or closed others. In what is now North America,
there would be evidence of mangrove swamps, forests, grassy plains, large lakes
and rivers, shoreline features, alpine glaciers and deserts, and active volcanoes and
other mountains. Numerical dating would show rocks that today range from more
than 3 billion years old to as little as 1 year old.
This is the challenge facing geologists today as we try to read the record of Earth
history. There is no single place where all 4.56 billion years of Earth history are
revealed—not even in the Grand Canyon. Geologists working in California can work
out the history of their field areas using all of the skills you’ve learned to interpret
the formation and deformation of rocks, but how can they compare their results
with those of someone working in Maine? In Japan? In Africa? Before we understood
index fossils and learned how to date rocks numerically, the best we could do
was to recognize that a sequence of rocks in Japan looked like a sequence of rocks
in Britain—but were the rocks the same age, or did they just represent the same
processes acting hundreds of millions of years apart?
With numerical dating tools, we can show that different kinds of rock from different
areas around the globe are actually the same age—a process called correlation—and
then compile paleogeographic maps showing where ancient shorelines
and continents were located at that time. Exercise 12.9 examines a smaller-scale
version of this problem.
EXERCISE 12.9
Correlation
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Five years ago, geologists determined the relative ages of rocks in two areas of midcontinental North America, but the
sequences (shown in the figure on the next page) were not identical. Unfortunately, similar rocks appear at several places
in each sequence, making it difficult to know exactly which limestone unit, for example, in the western sequence correlates
with a particular limestone unit in the eastern sequence. It is better to compare sequences of units, which reflect sequences
of depositional environments, than to look only at similarities between individual rock units.
(continued)
320 CHAPTER 12 INTERPRETING GEOLOGIC HISTORY