The History of Earth Chapter 6 Sections 1 and 2

How do scientists know the age of geologic formations on Earth? Paleontology is the study of these long dead organisms. Geologists rely on rocks and fossils to help them understand what happened in Earth’s past. Determining the age of an object or event is called relative dating.

Dating Rock Layers Scientists know that rock layers can be dated by where they are located. Rock layers near the bottom are older than rock layers near the top. The principle that states that “younger rocks are over older rocks” is called SUPERPOSITION.

Superposition Using relative dating principles and the position of layers within rock, it is possible to reconstruct the sequence of geologic events that have occurred at a site. In the image below, you can see the different layers that have been deposited over thousands of years.

1. Which is older--rock unit 2 or rock unit 3? 3. Which is older--pluton 10 or rock unit 23? 4.Which episode happened earlier— the folding of 3, 4, 5, and 6 or the intrusion of dike 11?

Geologic Column The arrangement of rock layers in which the oldest rocks are at the bottom and the youngest at the top is a geologic column. Scientists look at the layers to determine what has occurred in the history of this particular area. The layering that occurs is called stratification.

What is a unconformity? An unconformity is a missing rock layer that creates gaps in the rock sequence. In the picture, erosion has worn away rock layers and then sediment was deposited on top of the older rocks. As you can see in the picture, layers are missing.

What process disturbed these rock layers?

What process disturbed these rock layers?