1. Dating the Earth Chapter 12

2. A major goal of Geology is to interpret Earth’s History !! A pioneering geologist and 2 nd director of the USGS. Led an expedition down the Colorado River through the Grand Canyon.

3. The wonders of the Grand Canyon cannot be adequately represented in symbols of speech, nor by speech itself. The resources of the graphic art are taxed beyond their powers in attempting to portray its features. Language and illustration combined must fail. - John Wesley Powell -

4. Discovering Earth’s History Geologists use 3 basic assumptions when studying Earth’s history: 1) The rock record – Earth’s rocks record geological events and life forms of the past. 2) Uniformitarianism – the forces and processes observed on Earth today also acted in the past. 3) Earth’s age – Earth is very old and has changed over time.

5. Discovering Earth’s History James Hutton (1726 – 1797) The “Father” of Earth Science! Proposed Principle of Uniformitarian: The physical, chemical and biological processes that we observe today have been at work for a very long time. (In other words, the present is the key to the past!)

6. Relative Dating Enables geologists to place rocks in chronological order. Geologists use certain principles to determine the sequence in which events occurred. Relative dating does not tell us how long ago the events occurred!

7. Relative Dating Geologists use several principles to place rocks in chronological order:  Law of Superposition  Principle of Original Horizontality  Principle of Cross- Cutting Relationships  Inclusions  Unconformities Relative Dating Absolute Dating

8. Law of Superposition Each layer of sedimentary rock is older than the one above it, and younger than the one below it.

9. Law of Superposition Proposed by Nicolaus Steno (1669) Realized that rocks form by material settling In an undisturbed sequence of rocks, the youngest rocks will be on top and the oldest will be on bottom.

10. Principle of Original Horizontality Also discovered by Steno in mid 1600’s Realized that rocks are formed in flat layers States that rocks are originally deposited in horizontal layers that are only tilted after formation

11. Law of Original Horizontality

12. Law of Cross-Cutting Relationships Discovered by James Hutton A rock unit is older than any fault or igneous body that cuts across it

13. Principle of Cross-Cutting Relationships Faults and magma intrusions are younger than the surrounding rock.

14. Inclusions Rocks inside other rocks. Rocks containing inclusions are younger than the inclusions they contain.

15. Unconformities Long periods when deposition stopped, erosion removed previously formed rocks, and then deposition resumed.

16. Example 1 Use principles of relative dating to order these rock layers from oldest to youngest.

17. Example 2 Use principles of relative dating to order these rock layers from oldest to youngest.

18. Unconformities Long periods when deposition stopped, erosion removed previously formed rocks, and then deposition resumed.

19. Dating with Radioactivity Radioactivity – (radioactive decay) when large, unstable atoms break apart to form smaller, more stable atoms, releasing large amounts of energy.

20. Half-life The amount of time it takes for one-half a sample of radioactive material to decay. How much of the original sample will be left after 3 half-lives?

21. Radiometric Dating The procedure of calculating the absolute ages of rocks and minerals that contain radioactive isotopes.  Uranium – Lead  Potassium – Argon

22. Radiocarbon Dating Finding the age of organic materials by comparing the amount of carbon-14 to the amount of carbon-12 in a sample.

23. Radiometric Dating of Sedimentary Rock To determine the age of sedimentary rock, geologists must related the sedimentary rock to datable masses of igneous rock!

24. Radiometric Dating of Sedimentary Rock Example:  Applying Steno’s principles, geologists identify two igneous rock masses. One must be relatively older and the other relatively younger.  Then geologists use radiometric methods to date igneous rocks.  So, the age of the sedimentary rock must lie between the ages of the igneous rocks!