1. Relative Dating & Deforming the Earth’s Crust
Folds, Faults and Deformities…
OH MY!!

2. Is this the correct principle?
Uniformitarianism
Geologic processes that occurred in the past can be explained by current geologic processes
Erosion and weathering that happens today must have been the same millions of years ago
Very slow, gradual, predictable process
Is this the correct principle?

3. Why was this the accepted principle?
Catastrophism
Geologic change happens suddenly
Mountains, canyons and oceans formed during rare events
These rare events are called catastrophes
Up until the mid-1800’s, this was the accepted principle
Why was this the accepted principle?

4. Modern Geology Uniformitarianism Catastrophism
Can geologic processes take a very long time? (think Tectonic Plates, mountains growing)
YES!
Mountains forming/growing
Grand Canyon forming from erosion
Moving of the continents
Can events happen very quickly? (Think meteor causing dinosaur extinction)
Large Volcanic eruptions forming islands and causing extinctions
Meteor strikes creating craters and causing extinctions
Earthquakes, Tsunamis, etc…

5. Relative Dating
Relative Dating – a method used to determine whether an object or event is older or younger than another object or event
Who is Older or Younger?

6. Absolute Dating
Absolute Dating – any method used to determine the age of an event or object in numbers
Certain molecules called Isotopes decay over time
Isotope – An atom that has the same number of protons, but a different number of neutrons
We can use this decay to measure how old something is (rocks, fossils, etc…)
Half-life – the amount of time it takes for half of a radioactive substance to decay

7. Disturbances
Unfortunately not all rock layers are laid out perfectly. Some things that can make things more difficult are…
Unconformity – missing part of a geologic column
Missing rock layer(s) due to either Erosion or Non- deposition
Deformations – Altering of rock layers after they are laid down, but not removing them

8. Deformations

9. Deformations
Tectonic plates move, causing stress to build up in the lithosphere
Stress = force (three types)
Compression = squeeze together
Tension = stretch apart
Shearing = sliding past
Stress from tectonic plates cause rock layers in the lithosphere to change

10. Folding Definition: bending rock layers due to stress
Older rock starts on bottom
Anticline = upward arch (looks like an A)
Syncline = downward, U shaped (think sink)

11. SYNCLINE ANTICLINE

12. Folding Folds can be large or small
Some measured in kilometers or centimeters

13. MONOCLINE

17. Faults
Definition: surface along which rocks break and slide past each other
Can be seen on the earth’s surface
Usually have a Hanging wall and Footwall

18. Type of Stress?
Which wall falls down?
Are rock layers disturbed?

19. Type of Stress?
Which wall falls down?
Are rock layers disturbed?

20. Type of Stress?
Which wall falls down?
Are rock layers disturbed?

21. Rock layers aren’t always perfect
Relative Dating
Faults, Unconformities and Deformations raise a very important problem…
Rock layers aren’t always perfect
So what do we do when rock layers aren’t perfect?
What happens when they look more like…

22. This. How do we find Relative Age?

23. Relative Dating
One way scientists calculate relative age is by using a set of rules
Superposition – if undisturbed, rock layers will always be older as you go down (youngest on top)
Original Horizontality – Layers of rock are always formed in relatively even horizontal layers
Cross-Cutting Rule – if there is a fault cutting through layers of rock, the rock must be older
Deformation Rule – if there is a deformation (fold, tilt, intrusion) the rock that is deformed must be older

24. How do we find Relative Age?

25. Relative Dating
Another way geologists can figure out disturbed layers of rock is with the Geologic Column
Geologic Column – a complete set of the rock layers with all known fossils and layers in their proper order
Geologic Columns are used to find Unconformities
When a layer of rock is missing from Erosion or Non- Deposition the geologic column can tell geologists what SHOULD be there

27. Which layers are missing from the second outcropping?