Earth Science Lesson

There are several basic principles that geologists use to figure out the history of a rock:

1. Uniformitarianism
2. Original horizontality
3. Superposition
4. Cross-cutting relationships
5. Walther’s Law

This EarthCache will focus on two of these principles.

Uniformitarianism

The principle of uniformitarianism states that processes that alter the earth’s crust are the same processes that occurred millions of years ago. Furthermore, the results of processes today are the same as the results of the same processes millions of years ago. This means that we can take our observations of processes that occur today, and observations of the results and know the process that formed it when we see that same result in the rock record. For example, you can look in a stream and see ripple marks in the sand, formed by the flow of water over the sand. If you see ripple marks in the rock record, you can know that a similar process was at work.

Original Horizontality

The principle of original horizontality states that sediment is deposited horizontally. This is sometimes easier to envision with liquids: imagine pouring water into a cup. The surface of the water is perfectly flat - horizontal. If you dump that water into a bowl, the surface remains flat. Now imagine that you have a jello mixture in the bowl - if you chill it and it solidifies, and then pour a different color on top, You have the two flat layers of jello, one on top of the other. This is similar to how sedimentary rocks form. As water moves sediment from high regions, like mountains, to low regions, like the ocean, the energy of the system decreases until the sediments are deposited in a basin, like a lake or an ocean. More sediment is deposited on top, and over time the whole sequence lithifies (sort of like the jello did in the fridge). The rocks remain horizontal until a force acts on them, pushing (or pulling) them out of their original orientation.

For Additional Clarification:

Tools Needed:  It might help to have a straight edge tool such as a straight aluminum hiking pole to hold up at arm’s length, horizontal and parallel to the rock layers.  This will help in answering the questions below.  A protractor might also be useful here in determining the angle of change.

Superposition

This principle states that a sequence of rocks in their original orientation will have the oldest rock on the bottom and the youngest rock on the top. A simple way to think about this is that for something to be on top of something else, for example in order to put a book on top of a table, the table has to be there. If the table isn’t already there and you put the book down, it falls to the floor (and note! The floor had to be there for the book to land on it.). The same is true of rocks. In order to deposit a sandstone on top of a limestone, the limestone has to already be there. Knowing this, geologists can figure out the relative ages of rocks on top of each other.

Cross-Cutting Relationships

Similarly to the principle of superposition, a rock must already be in place to be cut by a fault, igneous intrusion or erosion. By carefully examining which rock units are cut by faults or intrusions, or which rock units have been weathered, geologists can further determine the relative ages of rocks.

Walther's Law

Walther’s law is a little different from the previously discussed geologic principles, but it is just
as important. Instead of dealing only with relative time, Walther’s law deals with relative space through time. Walther’s law states that depositional environments that are laterally adjacent on the surface of the earth will also appear in succession in a stratigraphic sequence. If there is something missing, there is missing time, or an unconformity.

Logging Tasks

Questions for Logging Credit:

1. The sediment that formed these limestone rock layers here were originally deposited horizontally.  What has happened to the rock layers?
2. Estimate the angle of change in the rock layers from their original position.  A protractor will be helpful here. (Remember that these rock layers were once straight and horizontal).
3. What Geology Principle best describes how the material that formed these rocks were originally deposited?
4. From your answer in questions #1 and #3, what has happened to these rocks since they were originally formed?  Explain how this might have happened.
5. Looking at the angled layers, which layer is the oldest? Why? (color)
6. Which principle helps you determine the answer to question 5?
7. Post a photo of yourself, or a proxy, at GZ (No spoilers please).

References:

1. Penn State's College of Earth and Mineral Sciences, "The Principles of Geology" https://www.e-education.psu.edu/marcellus/node/872
2. https://en.wikipedia.org/wiki/Principle_of_original_horizontality http://www.thisoldearth.net/Geology_Online-1_Subchapters.cfm?Chapter=4&Row=2