Interpreting Earth's History
Imagine your on a trip with your family and you saw this road-cut cliff. How would you interpret the relative history of how that cliff was made?
How building a sandwich is similar to earth's relative history
Principle 1: Original Horizontality
This sandwich is formed in layers much like sedimentary rock is formed in layers. These layers are also flat like sedimentary rocks. Therefore, if it is squished, the sandwich was made first, then squished later.
Principal 2: Cross Cut Relations
Principle 3: Original Continuity
Principle 4: Super Position
Principle 5: Unconformity
Utah's Geological Cross Sections - Capitol Reef, UT
Interpreting the history of the earth through the rock layers of the earth is much like interpreting how a sandwich (and even a doughnut) was made.
How was each sandwich & doughnut made?
How was each sandwich & doughnut made?
The first sandwich was made first by:
* Notice that the older is on the bottom and the youngest was on the top! |
The second sandwich was made first by:
*Notice that the cut was last! |
This jelly filled doughnut was made first by:
* Notice that the jelly was filled in last! |
Now that we know how a sandwich is made, let's see how this relates to how a geologist interprets how the earth was made by interpreting which rock layer and geological processes came first, second & third.
*Notice that we cannot say exactly how old each layer is. We can only determine its relative age by saying "older" or "younger". We will need to use another method to put age ranges on the rock layers - which will be shown on another page.
These relative dating techniques are shown in more detail below.
- Rock layers are formed first in flat layers. Therefore, if they're are tilted because it got squished, then the tilting came after it was laid down.
- The oldest layer is on the bottom and the youngest layer is on top.
- If the layers are cut by a fault, the cut is the youngest process.
- If the layers are cut or filled, from an igneous intrusion, the igneous intrusion is the youngest process.
*Notice that we cannot say exactly how old each layer is. We can only determine its relative age by saying "older" or "younger". We will need to use another method to put age ranges on the rock layers - which will be shown on another page.
These relative dating techniques are shown in more detail below.
Principles of Relative Dating
Relative Dating Techniques of a Geologist
Use the 5 different principles of relative dating on the 8 sites for your activity on page 27-29 "which came first". The sites are posted below.
The five principles of relative dating:
The five principles of relative dating:
- Original Horizontality: (Originally Flat) Sediments form horizontally flat layers in water bodies.
- Original Continuity: (Originally it was one continuous flat sheet) The layer of dirt on one side of the canyon is the same layer of dirt on the other side of the canyon.
- Super Position: (Above Placement) The oldest layer is on the bottom, and the youngest layer is on top. i.e. if on Monday, I wore jeans and a T-shirt and threw it in the laundry basket before I went to bed, then Tuesday I wore a polo and slacks and threw them in the laundry basket, then wore shorts and a tank-top on Wednesday, which article of clothing would be found on the bottom? The Jeans and T-shirt would be found on the bottom and has been there the longest.
- Unconformity: (Gap in Geologic Time - or missing layer of dirt) this is formed when you have depositional layering, followed by uplift & erosion, then it goes back to a depositional environment. For instance, if I took the last layer of shorts & my tank-top out and re-wore it to go camping in - but lost it out in the forest, but came back wearing overalls, I'd end up putting the overalls back into the laundry basket - but I'd be missing my shorts and tank top. Therefore, I'm missing a layer of clothes from the laundry basket.
- Cross cut relations. If a rock is cut by a fault or igneous intrusion, it is the youngest event. If I had a steak and a knife, and I cut the steak with the knife, which had to be there first - the cut or the steak? Of course the steak. You can't make a cut with out it. Therefore, the cut is the youngest event, and the steak being the older. In other words, the rock is the older event followed by the fault or igneous intrusion.
Click here to learn more about Utah Geology: UGS
Weathering & erosion in action
Geologic Cross Sections - History Interpretations Activity (Homework pg 25)
Each layer of rock tells us a little bit about that regions geological history. By analyzing the rocks within that layer we can tell what geologic event took place, the sequence of the geological event, what the environment looked like and what critters lived during that time. By piecing together each of these different layers, we can piece together that regions history.