1. History of Geologic Theory

2. Humans have always wanted to know how the earth around them formed.
We’ve talked about the specifics of some of these with constructive and destructive geologic processes.
Now let’s talk THEORY. Remember, in science, Theory means a consistent, well- supported explanation that explains what we see. It’s not “just an idea”. 2

3. Abraham Gottlob Werner, 1787
Neptunism
Abraham Gottlob Werner, 1787
Earth covered in water then rock settled
All rocks precipitated from ocean water
Problems?
older rock found on top of younger rock
igneous rock formed from the cooling of magma
Earth was covered in water due to Biblical references. Much like strictly horizontality However, scientist found older rock (primitive rock or harder) on the newer sedimentary rock. How did it mix up. Found igneous rock was formed by cooling magma and not the primitive rock that was already there.

4. Great flood

5. Baron Georges Cuvier, Late 18th – Mid 19th Century
Researched fossils and extinction
Series of six major catastrophes
“All of these facts, consistent among themselves, and not opposed by any report, seem to me to prove the existence of a world previous to ours, destroyed by some kind of catastrophe.”
Problems?

6. Catastrophic Problems?
tilted sedimentary rock must have been uplifted by some geologic process

7. James Hutton and Gradualism
The changes on earth are the result of gradual processes (like deposition, erosion, weathering).
“The past history of our globe must be explained by what can be seen to be happening now.”
Plutonism - igneous rocks result from the cooling of hot magma (Takes time)
What kinds of processes did Hutton imagine had shaped the world in the past?

9. WEIRD GEOLOGY OF JEDBURGH, SCOTLAND
Photo credit: Dave Souza, Wikimedia Commons

10. Charles Lyell and Uniformitarianism
Principles of Geology
“The present is the key to the past.”
Much better writer than Hutton; disseminated idea of Uniformitarianism to a much wider audience
The geologic processes we see today are the same ones that occurred in the past, at the same rate. 10

12. From Uniformitarianism/Gradualism to “Actualism”
Uniformitarianism was a huge step in the right direction
Evidence exists that there have been different intensities of geological activity during the past
Deccan Traps near Mumbai, India
Series of intense volcanic eruptions ~66 mya

13. Does Uniformitarianism mean that nature is ALWAYS constant?
Uniformitarianism explains most of what we see today.
BUT, catastrophic events have happened that shape the earth.
In reality, the earth is shaped by a mixture of gradual and catastrophic events.

14. We know the earth is extremely old (how?)
Why is Uniformitarianism a more logical approach to understanding Earth history?
Constancy of laws: In order to meaningfully study the past, we must assume the same or similar mechanisms operated then as now. Otherwise, how can we know anything about the past?
Parsimony: “Occam’s razor”. The simplest explanation is oftentimes the correct one.
We know the earth is extremely old (how?)

15. Journal Questions:
Give an example of how gradual geologic processes have shaped earth and explain
Give an example of how catastrophic events have shaped earth and explain.