Geologic Age Determination

http://www.nps.gov/archive/grca/photos/images/T501.jpg

Uniformitarianism: (uniform = the same) Geological processes occurring today, like plate movement, mountain formations, etc., have been occurring since the Earth was formed.

Correlation: The matching of equivalent rocks in different geographic areas.

http://earthsci.org/processes/geopro/introgeo/rockcycle.gif

• Relative time – Places geologic events in order of occurrence. Determining relatively how old rock units are based upon the rocks that surround it. What happened first? What happened last? These subdivisions of the geologic time scale are given names based on places where particular rocks were first described.

(Example: Any teacher in this school is “older” relative to the students no matter how old the teachers actually are)

Geological Views of Time

http://www.geo.ucalgary.ca/~macrae/timescale/timescale.html

• Absolute time -- Numerical ages in "millions of years ago". These dates are most commonly obtained from radiometric dating methods using carefully chosen rock samples.

(Example: Most students in my sophomore Ecology class are 16 years old, while I am 24 years old)

Geological Views of Time

http://www.geo.ucalgary.ca/~macrae/timescale/timescale.html

Geologic Timescale

http://www.geo.ucalgary.ca/~macrae/timescale/timescale.html

The Law of Superposition is one of the most basic principles of geology . Younger rock layers are deposited on top of older layers, during normal conditions of deposition. The basic principle of stratigraphy (the study of sedimentary rock layers), is still the single best method that geologists have for determining the relative ages of rock sequences.

http://www.astro.virginia.edu/class/oconnell/astr121/im/GrandCan-geolog-column.jpg

Law of Superposition

Notice the different

color bands that indicate different layers of

rock.

Principle of Original Horizontality

http://www.cacegypt.org/Sinai/homework/GREG/pages/Q2_04/Q2Images/PlateTec_Pics/foldMnt.jpg

http://www.canyondave.com/Layers.html

Sedimentary rock layers (and lava flows) are always formed in a horizontal orientation. Any folding or tilting must have happened sometime after the layers were originally formed.

Before After

**If there is any folding, tilting, or faulting, you know that it occurred after the rock was layered because of the Principle of Original Horizontality

http://www.litho.ucalgary.ca/transect_info/snorcle/photos/granite.html

The Principle of Cross-Cutting Relationships states that an igneous intrusion (or a fault) is always younger than the rock it cuts across.

http://ceed.utpb.edu/media/images/west-texas-geology/1ig_chisosdikes.jpg

Cross-Cutting

In the diagram at the left, the igneous intrusion (E) must be younger than rock layers A, B, C, and D because it contains inclusions from those layers.

The Principle of Inclusions states that the rock unit that contains inclusions is younger than the rock that the inclusions came from.

Gneiss in Granite

http://www.gpc.edu/~pgore/geology/historical_lab/relativedating.htm

http://www.msstate.edu/dept/geosciences/CT/TIG/WEBSITES/LOCAL/Summer2003/Schuster-Loomis_Rea/gneiss%20black%20inclusions.jpg

• Which is older, the fault or the rock?

From oldest to youngest:

1. E

2. C

3. B

4. D

5. A

1. C

2. A

3. L

4. B

5. G

6. F