INTERPRETING GEOLOGIC HISTORY

INTERPRETING GEOLOGIC HISTORY

a record as possible, geologists combine evidence from FROM FOSSILS many localities. To do this, rocks of the same age from

Fossils allow geologists to interpret geologic history. For different localities must be matched in a process called example, the remains of marine animals in rocks of the

correlation. But how do we correlate rocks over great Canadian Rockies or near the top of Mount Everest tell

distances?

us that these places once lay submerged beneath the sea. If you follow a single continuous sedimentary bed Therefore, we infer that later tectonic processes raised

from one place to another, then it is clearly the same these regions to their present elevations.

layer in both localities. But this approach is impractical The theory of evolution states that life forms have

over long distances and where rocks are not exposed. changed throughout geologic time. Fossils are useful in

Another problem arises when correlation is based on determining relative ages of rocks because different ani-

continuity of a sedimentary layer. When rocks are corre- mals and plants lived at different times in the Earth’s his-

lated for the purpose of building a geologic time scale, tory. For example, trilobites lived from 535 million to

geologists want to show that certain rocks all formed at 245 million years ago, and the first dinosaurs appeared

the same time. Suppose that you are attempting to trace about 220 million years ago.

a beach sandstone that formed as sea level rose. The In a sequence of sedimentary rocks that formed over

beach would have migrated inland over time, and as a re-

a long time, different fossils appear and then vanish from sult, the sandstone would have become younger in a bottom to top in the same order in which the organisms

landward direction (Fig. 9–12).

evolved and then became extinct through time. Rocks Over a distance short enough to be covered in an containing dinosaur bones must be younger than those

hour or so of walking, the age difference may be unim- containing trilobite remains. The principle of faunal

portant. But if you trace a similar beach sand laterally succession states that fossil organisms succeeded one

over hundreds of kilometers, its age may vary by mil-

Figure 9–11 The mummified remains of this Bronze Age hunter were recently discovered pre- served in glacial ice near the Austrian-Italian border. (Sygma)

Absolute Geologic Time 145

sea level

Sea level falls or Original

continent rises leaving a sea level

continuous blanket of beach sand (a)

Old sea level

Later sea level

(b)

(c)

Figure 9–12 As sea level rises (a and b) or falls (c) slowly through time,a beach migrates laterally,forming a single sand layer that is of different ages in different geographic localities.

lions of years. Thus, there are two kinds of correlation: atmosphere. Wind carries the ash over great distances be-

Time correlation means age equivalence, but lithologic

fore it settles. Some historic ash clouds have encircled correlation means continuity of a rock unit, such as the

the globe. When the ash settles, the glass rapidly crys- sandstone. The two are not always the same because

tallizes to form a pale clay layer that is incorporated into some rock units, such as the sandstone, were deposited

sedimentary rocks. Such volcanic eruptions occur at a at different times in different places. To construct a record

precise point in time, so the ash is the same age every- of Earth history and a geologic time scale, geologists

where.

must find other evidence of the geologic ages of the rocks.