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Dating the Earth

When geologists first began studying the earth, when they
knew it was very, very old. But they weren’t sure how old.
The geologists took a closer look at rock formations. They
found many fossils, the remains of ancient plants and animals, in
sedimentary rocks. Sedimentary rocks are formed in layers, with
the oldest layers at the bottom. Fossils within these layers show the
plants and animals that existed when the rocks formed. So,
geologists could figure out which parts of the earth were the oldest
based on the ages of fossils in the rock layer. This method is called
relative dating.
In the 1900s scientists began to understand radioactivity. If
something is radioactive, it breaks down, or decays, at a constant
rate, and scientists know this rate. For example, scientists know
that the element uranium breaks down into lead over a certain
amount of time. By measuring the amount of uranium and lead in a
rock, geologists can measure the age of the rock. They know how
long it takes for the lead to form. This method is called absolute
dating.

Comprehension Questions

I. Say which of the following statements are false and
which are true. Correct the false statements to make them
true.

1. When the geologists first began studying the earth they
knew at once how old it was.
2. To be sure about the earth’s age, the geologists took a
closer look at rock formation.
3. Sedimentary rocks are formed in layers, with the oldest
layer at the top.
4. In the 1900s scientists began to understand radioactivity.
5. The geologists have no methods to measure the age of the
rock.

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