1. The Principle of Uniformitarianism Scientist James Hutton, the author of Theory of the Earth, proposed that geologic processes such as erosion & deposition do not change over time. Uniformitarianism - idea that same geologic processes shaping Earth today have been at work throughout Earth’s history.

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3. The Principle of Uniformitarianism, continued Uniformitarianism Versus Catastrophism Hutton’s theories sparked a scientific debate by suggesting Earth was much older than a few thousand years, as previously thought. A few thousand years was not enough time for the gradual geologic processes that Hutton described to have shaped the planet.

4. The Principle of Uniformitarianism, continued A Victory for Uniformitarianism Catastrophism was geology’s guiding principle until the work of geologist Charles Lyell caused people to reconsider uniformitarianism. Lyell published Principles of Geology in the early 1830s. Armed with Hutton’s notes and new evidence of his own, Lyell successfully challenged the principle of catastrophism.

5. Modern Geology -- A Happy Medium Late 20th century, Stephen J. Gould challenged Lyell’s uniformitarianism. They believed that catastrophes occasionally play important role in shaping Earth’s history. Today, scientists realize most geologic change is gradual & uniform, but catastrophes that cause geologic change do occur during Earth’s long history.

6. Radioactive Decay Absolute dating - any method of measuring the age of an event or object in years. To determine absolute ages of fossils & rocks, scientists analyze isotopes of radioactive elements. Isotopes - Atoms of the same element that have the same number of protons but different numbers of neutrons.

7. Radioactive Decay, continued Most isotopes are stable, meaning they stay in their original form. Other isotopes are unstable. Scientists call unstable isotopes radioactive.

8. 8 Radioactive Decay, continued Radioactive isotopes tend to break down into stable isotopes of same or other elements - process called radioactive decay.

9. Radioactive Decay, continued Because radioactive decay occurs at a steady rate, scientists can use the relative amounts of stable and unstable isotopes present in an object to determine object’s age.

10. 10 Radioactive Decay, continued Dating Rocks — How Does It Work? In radioactive decay, an unstable radioactive isotope of one element breaks down into a stable isotope. The stable isotope may be of the same element or of a different element. Unstable radioactive isotope - parent isotope. Stable isotope produced by radioactive decay of parent isotope - daughter isotope. 10

11. Radioactive Decay, continued Rate of radioactive decay is constant, so scientists can compare amount of parent material w/amount of daughter material to date rock. The more daughter material there is, the older the rock is. If you know rate of decay for a radioactive element in a rock, you can figure out absolute age of rock.

12. Radioactive Decay, continued Half-life - the time needed for half of a sample of a radioactive substance to undergo radioactive decay. After every half-life, the amount of parent material decrease by one-half.

13. 13 Types of Radiometric Dating Scientists use different radiometric-dating methods based on estimated age of an object. There are four main radiometric-dating techniques; you’ll need to know two. Potassium-Argon Method Potassium-40 has a half- life of 1.3 Gy, & it decays leaving a daughter material of argon. This method is used mainly to date rocks older than 100,000 years. 13

14. 14 Types of Radiometric Dating, continued Carbon-14 Method Carbon is normally found in three forms, the stable isotopes carbon-12 & carbon-13, and the radioactive isotope carbon-14. Living plants & animals contain a constant ratio of carbon-14 to carbon-12. Once a plant or animal dies, no new carbon is taken in. The amount of carbon-14 begins to decrease as plant or animal decays. 14

15. 15 Types of Radiometric Dating, continued The half-life of carbon-14 is 5,730 years. The carbon-14 method of radiometric dating is used mainly for dating things that lived within the last 50,000 years. 15

16. 16 A bit of review!! 16

17. A Record of Earth’s History Discovering Earth’s History  Rocks record geological events and changing life forms of the past.  Because rocks are constantly recycled, the Earth’s rock record is neither complete nor continuous.

18. A Brief History of Geology Discovering Earth’s History  Uniformitarianism is the concept that the forces and processes that we observe today have been at work for a very long time. “The present is the key to the past”  The surface and interior of the Earth have been slowly changed over immense stretches of time by the same geological processes that continue today.

19. Fossil Formation Fossils: Defined  Fossils are the remains or traces of prehistoric life found in sediment and sedimentary rocks.  The type of fossil that is formed is determined by the conditions under which an organism died and how it was buried.

20. Fossil Formation Fossils: How Do They Form  Refer to Figure 2, page 287 Most fossils are preserved in low to moderate energy, water environments with active deposition (swamps, lakes, ocean, rivers). Two conditions are important for preservation: rapid burial and the possession of hard parts. Some Exceptions – Burgess Shale

21. Fossils: Altered Remains  Altered Remains Fossils become petrified or turned to stone by mineral rich water solutions through the process of replacement. Molds/casts copy the shape of ancient organisms Molds – Hollow area in sediment Cast - Minerals fill in the mold Carbonization occurs when an organism is buried under fine sediment preserving leaves and delicate animals.

22. Fossil Formation Fossils: Trace Fossils  Indirect Evidence Trace fossils are indirect evidence of prehistoric life as they preserve the activities of ancient organisms. Examples: footprints, animal/insect tracks, animal burrows

23. Fossil Formation Fossils: Unaltered Remains  Unaltered Remains Some remains of organisms—such as teeth, bones, and shells—may have changed only slightly over time as a result of the preservation method. Examples: La Brea Tar Pits Insects in Amber Frozen Remains

24. Types of Fossilization

25. The Fossil Record  The Fossil Record Fossils provide evidence about the history of life on Earth and how that life has changed over time. Fossils occur in a particular order Oldest rocks contain fossils of simple organisms Youngest rocks contain more fossils of more complex organisms. This gradual change in the remains of organisms in the fossil record supports the Theory of Evolution.

26. Fossil Formation Fossils: Interpreting Past Environments  Interpreting Environments Fossils can also be used to interpret and describe ancient environments. By understanding modern depositional environments, we can reconstruct ancient depositional environments. Refer to Figure 8, Page 292

27. Depositional Environments