Description of absolute dating and numerical age
The goal of this lecture is come to come to a scientific understanding of geologic time and the age of the Earth.
In order to do so we will have to understand the following: To better understand these concepts, let's look at an archeological example: Imagine we are a group of archeologists studying two different trash pits recently discovered on the Tulane University campus and at the Audubon Zoo (where they all aksed for you).
Look at the many photographs of the Grand Canyon in your textbook.
Note that you can follow the layers all along the walls of the canyon, and you can find the same layers on both sides of the canyon.
Principles of Stratigraphy Stratigraphy is the study of strata (sedimentary layers) in the Earth's crust.
Geologist in the 1800s worked out 7 basic principles of stratigraphy that allowed them, and now us, to work out the relative ages of rocks.
For example, the mudstone, sandstone and shale are cut by the basalt dike, so we know that the mudstone, sandstone, and shale had to be present before the intrusion of the basalt dike.
Principle of Inclusions If we find a rock fragment enclosed within another rock, we say the fragment is an inclusion.
If the enclosing rock is an igneous rock, the inclusions are called xenoliths.
Thus, in a sequence of layers that have not been overturned by a later deformational event, the oldest layer will be on the bottom and the youngest layer on top.
This is the same principle used to determine relative age in the trash pits discussed previously.