Evidence About Earth’s Past (Book)

• The study of rock layers is called stratigraphy. Laws of stratigraphy help scientists determine the relative ages of rocks. The main law is the law of superposition. This law states that deeper rock layers are older than layers closer to the surface.
• An unconformity is a gap in rock layers. They occur where older rock layers eroded away completely before new rock layers were deposited.
• Other clues help determine the relative ages of rocks in different places. They include key beds and index fossils.
• Scientists use the geologic time scale to illustrate the order in which events on Earth have happened.
• The geologic time scale was developed after scientists observed changes in the fossils going from oldest to youngest sedimentary rocks. They used relative dating to divide Earth’s past in several chunks of time when similar organisms were on Earth.
• The geologic time scale is divided into eons, eras, periods, and epochs.

Points to Consider

In this lesson, you read how scientists determine the relative ages of sedimentary rock layers. The law of superposition determines which rock layers are younger or older than others.

• What about the actual ages of rocks? Is there a way to estimate their ages in years?
• And what about other kinds of rocks? For example, is there a way to estimate the ages of igneous rocks?