Wednesday January 4, 2012 (A Brief History of Geology; Relative Dating)

The Launch Pad Wednesday, 1/4/12 What is “geology ?” Geology is the study of the Earth, the materials of which it is made, the structure of those materials, and the processes acting upon them. It includes the study of organisms that have inhabited our planet. An important part of geology is the study of how Earth’s materials, structures, processes and organisms have changed over time.

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Geology

A Brief History of Geology During the seventeenth and eighteenth centuries, the doctrine of catastrophism strongly influenced people’s thinking about Earth. Catastrophists believed that Earth’s landscapes had been developed primarily by great catastrophes. James Ussher, in the mid- 1600s, concluded that the Earth was only a few thousand years old, having been created in 4004 B.C.

A Brief History of Geology Modern geology began in the late 1700s, when James Hutton, a Scottish physician, published his Theory of the Earth. Hutton put forth a fundamental principle that is a pillar of geology today: uniformitarianism.

A Brief History of Geology This principle simply states that the physical, chemical, and biological laws that operate today have also operated in the geologic past. This means that the forces and processed that we observe presently shaping our planet have been at work for a very long time.. This idea is commonly expressed as “The present is the key to the past.”

Determining Age How old someone or something is can be thought of in two different ways: relatively or numerically. For instance, consider these two scenarios: – Frank is older than John, and John is older than Mary. – Frank is 18 years old, John is 16, and Mary is 10. The first statement is relative; it lists the order of the ages, but doesn’t provide actual numerical information. The second statement is numerical, as it gives the actual ages of the three. Geologists use both relative and numerical dating when accessing the age of rocks and formations.

Relative Dating Relative dating means placing rocks in their proper sequence of formation. This technique cannot tell us how long ago something took place, only that events took place one after the other. Again this is not a numerical dating procedure!

Relative Dating Principles and rules of relative dating: The Law of Superposition states that in an undeformed sequence of sedimentary rocks, each bed is older than the one above it and younger than the one below. In other words, the oldest rocks are on the bottom and the youngest on the top.

Superposition is Well Illustrated in the Grand Canyon Figure 11.3