tall mountains may have once been low, flat ground. These kinds of changes take place over
many millions of years, but they are still slowly going on today. The place where you live
right now may look very different in the far away future.
Relative and Absolute Age Dating of Rocks
The clues in rocks help scientists put together a picture of how places on Earth have changed.
Scientists noticed in the 1700s and 1800s that similar layers of sedimentary rocks all over
the world contain similar fossils. They usedrelative datingto order the rock layers from
oldest to youngest. In the process of relative dating, scientists do not determine the exact
age of a fossil but do learn which ones are older or younger than others. They saw that the
fossils in older rocks are different from the fossils in younger rocks. For example, older rock
layers contain only reptile fossils, but younger rock layers may also contain mammal fossils.
Scientists divided Earth’s history into several chunks of time when the fossils showed similar
things living on the Earth. They gave each chunk of time a name to help them keep track of
how Earth has changed. For example, one chunk of time when many dinosaurs lived is called
the Jurassic. We find fossils of Earth’s first green plants from the chunk of time named the
Ordovician. Many of the scientists who first assigned names to times in Earth’s history were
from Europe. As a result, many of the names they used came from towns or other local
places where they studied in Europe.
Ordering rock layers from oldest to youngest was a first step in creating the geologic time
scale. It showed the order in which life on Earth changed. It also showed us how certain
areas changed over time in regard to climate or type of environment. However, the early
geologic time scale only showed the order of events. It did not show the actual years that
events happened. With the discovery of radioactivity in the late 1800’s, scientists were able
to measure the exact age in years of different rocks. Measuring the amounts of radioactive
elements in rocks let scientists useabsolute datingto give ages to each chunk of time on
the geologic time scale. For example, they are now able to state that the Jurassic began
about 200 million years ago and that it lasted for about 55 million years.
Geologic Time Scale
Today, the geologic time scale is divided into major chunks of time calledeons. Eons may be
furtherdividedintosmallerchunkscallederas,andeacheraisdividedintoperiods.Figure
12.1shows you what the geologic time scale looks like. We now live in the Phanerozoic eon,
the Cenozoic era, and the Quarternary period. Sometimes, periods are further divided into
epochs, but they are usually just named “early” or “late,” for example, “late Jurassic,” or
“early Cretaceous.” Note that chunks of geologic time are not divided into equal numbers of
years. Instead, they are divided into blocks of time when the fossil record shows that there
were similar organisms on Earth.