1. Topic VII Geologic History

2. How do you determine how old a rock is?
Uniformitarianism: “the present is the key to the past” (what is happening today—uplifting, erosion… has always happened)

3. Relative Age of Rock Layers: comparing rock layers in an outcrop—exposed rock (does not tell you an exact age—but can tell you which rocks are older)
“Relatives”

4. Absolute Age: finding the actual age of the rock (radioactive dating)

5. Six ways to determine the relative age of an outcrop—“Sandwich Rules”
Law of Original Horizontality: all sedimentary rocks formed in horizontal layers

6. Law of Superposition: oldest rock is on the bottom (in a sedimentary layer that has not been overturned when folded)

7. Igneous Intrusions (inside) / Extrusions (outside): are always younger than the rocks they are in or on
(toothpick)

8. Look for contact metamorphism (burn) marks (if you see them, the rock layer is older than the metamorphism)

9. Mineral Veins are younger than the rocks they are in

10. Sediments that make up sedimentary rocks are older than the rocks themselves

11. Folds, faults, and tilts are younger than the rocks they are in (cut)

12. Sequence of Geologic Events:
Events animation

13. But there is a problem….
Unconformities: buried erosional surfaces (rock layers are destroyed—usually indicated by a horizontal wavy line in the cross section)—creates a “Gap in Time”

14. More unconformities….

15. More unconformities….

16. 1 More unconformity….

17. What is correlation?
Correlation: matching up of rock and geologic events in two separate areas

18. How can rocks be correlated?
Similarity of rock layers (color, composition)

19. Fossils (remains of prehistoric life): seeing if fossils match up in separate rock layers—can tell the organisms environment (does not work for all fossils)

20. Fossil Correlation

21. Index Fossils: fossils that have a wide geographic distribution and a short period of existence (1 rock layer)

22. What is needed to make a fossil:
Hard parts (soft parts decay quickly)
Quick burial (slows decay)

23. Volcanic Ash: can be spread throughout the world and settle as a thin layer very quickly (very good time marker)

24. **Index Fossils are the best way to correlate rock layers

25. How can scientists determine the absolute age of rock layers?
Radioactive Dating: determining the age of something by comparing the amount of radioactive isotope to the stable decay product

26. meaning…
The INSTANT a rock is created, it contains 100% of some radioactive isotope. As a rock gets older, the radioactive isotope breaks down into a decay product….scientists can measure this breakdown.

27. But this can also be used to date the remains of once LIVING things…
All living things on planet Earth are part of the CARBON CYCLE… (think Carbon Dioxide)…meaning over the course of our life, we build-up Carbon in our bodies…the instant we die, we have 100% Carbon

28. So, how do you do this…
Half Life: amount of time it takes for a radioactive isotope to decay to ½ its original mass (occurs at a predictable rate)
ESRT Cover

29. Carbon-14: isotope used to date the remains of once living things—people, shirts, mammoths—less than 50,000 years old (this is because its half life is only 5700 years—very short)

30. The box method…
If 100 grams of pure carbon-14 (C-14) starts to decay, how much C-14 will be left after 11,400 years?

31. If 100 grams of pure carbon-14 starts to decay, how much nitrogen-14 (N-14) will be created in 11,400 years?

32. **half life never changes for a particular isotope no matter what happens!
Fire
Crushing

33. Geologic Time Scale
Geologic Time Scale: time-line that shows the history of the Earth (created by correlating fossil evidence throughout the world)

36. Problems with the geologic time scale:
Unconformities—time scale may be too short

37. What does the Geologic Time Scale show about life on Earth?
Simple life forms began first

38. Evolution: species have changed to increase their chance of survival (this leads to diversity of life)

39. Most life forms have become extinct