Discovering Earth's History: Studying Geologic Time and Rock Records

1. Chapter12 Geologic Time • Sedimentary Rocks: Rocks formed from the weathered products of preexisting rocks that have been transported, deposited, compacted, and cemented.

2. Chapter12 Geologic Time • Igneous Rocks: • Rocks formed by the crystallization of molten magma

3. Studying Earth’s History 12.1 Discovering Earth’s History • Major John Wesley Powell- Civil war vet and geologists- led scientific expedition down the Colorado River, through the Grand Canyon. • To him, the walls of the Canyon provided evidence of Earth’s long history. • His work led scientists to believe geologic time grew from thousands to many millions of years.

4. Studying Earth’s History 12.1 Discovering Earth’s History • In studying Earth’s history, geologists make use of three main ideas: • the rock record provides evidence of geological events and life forms of the past • Processes overserved on Earth in the present also acted in the past • Earth is very old and has changed over geologic time.

5. Studying Earth’s History 12.1 Discovering Earth’s History • Late 1700’s- Scottish physician- James Hutton published his Theory of the Earth. • Hutton put forth the fundamental principal of Uniformitarianism: • means that the physical, chemical, and biological processes that we observe today have been at work for a very long time.

6. Relative Dating—Key Principles 12.1 Discovering Earth’s History • 1800’s- William Hutton- worked to interpret Earth’s rock record, known as relative dating • Method used to place rocks in chronological order. Identifies which rock units formed first, second, etc.

7. Relative Dating—Key Principles 12.1 Discovering Earth’s History • In relative dating, geologists follow several principals: • Law of Superposition • Principle of Original Horizontality • Principle of Cross-Cutting Relationship.

8. Relative Dating—Key Principles 12.1 Discovering Earth’s History • 1636- 1686 : Nicolaus Steno- Danish geologists • Steno developed the Law of Superposition: • States that in an under-formed sequence of sedimentary rocks, each layer is older than the one above it and younger than the one below it.

9. Ordering the Grand Canyon’s History • Law of Superposition

10. Relative Dating—Key Principles 12.1 Discovering Earth’s History • Steno also developed the Principle of Original Horizontality: • States that layers of sediment are generally deposited in a horizontal position. • If rock layers appear flat, it means they’ve remained in their original horizontal position.

11. Disturbed Rock Layers • Principle of Original Horizontality

12. Relative Dating—Key Principles 12.1 Discovering Earth’s History  Principle of Cross-Cutting Relationships • The principle of cross-cutting relationships states that when a fault cuts through rock layers, or when magma intrudes other rocks and crystallizes, we can assume that the fault or intrusion is younger than the rocks affected. Fault: a fracture in Earth along which movement has occurred Dike: a tabular-shaped intrusive igneous feature that cut across preexisting rock layers.

13. Applying Cross-Cutting Relationships Fault: a fracture in Earth along which movement has occurred Dike: a tabular-shaped intrusive igneous feature that cut across preexisting rock layers.

14. Reading the Rock Record 12.1 Discovering Earth’s History • Applying Steno’s principles, geologists read geologic features to reconstruct Earth’s history. • Geologists use three methods to interpret rock records: • Inclusions • Unconformities • Correlate rock layers •

15. Reading the Rock Record 12.1 Discovering Earth’s History • Inclusions: are pieces of one rock unit that are contained within another. • Inclusions made of older igneous rock can be found within younger sedimentary rock layers on top of the weathered igneous rock.

16. Formation of Inclusions

17. Relative Dating—Key Principles 12.1 Discovering Earth’s History  Unconformities • represents a long period during which deposition stopped, erosion removed previously formed rocks, and then deposition resumed.

18. A Record of Uplift, Erosion, and Deposition

19. Formation of an Angular Conformity 3 types of unconformities: 1. angular- layers of sedimentary rock form over older sedimentary rock that are tilted or folded. 2. disconformities – two sedimentary rock layers are separated by an erosional surface. 3. nonconformities – an erosional surface separates older metamorphic or igneous rocks from younger sedimentary rocks.

20. Correlation of Rock Layers 12.1 Discovering Earth’s History  Correlation is establishing the equivalence of rocks of similar age in different areas.

21. Correlation of Strata at Two Locations

22. Fossil Formation 12.2 Fossils: Evidence of Past Life  An extinct organism is one that no longer exists on Earth.  Fossils are the remains or traces of prehistoric life. They are important components of sediment and sedimentary rocks. The type of fossil that is formed is determined by the conditions under which organism died and how it was buried.

23. Types of Fossils: Petrified 12.2 Fossils: Evidence of Past Life • Petrified Fossils: Organisms remains become petrified. • Petrified means “turned into stone.” • Process: • Mineral-rich water soaks into small cavities and pores of the original organism. • Minerals precipitate from the water and fill the spaces. • Minerals sometimes replace the cell walls or solid material of the organism.

24. Types of Fossils: Petrified 12.2 Fossils: Evidence of Past Life

25. Types of Fossils: Molds and Casts 12.2 Fossils: Evidence of Past Life • Molds • Process: • Mold is created when a shell or other structure is buried in sediment and then dissolved by underground water. • Mold reflects only the shape and surface marking of the organism. • Cast fossils are created if the hollow spaces of a mold are later filled with mineral matter.

26. Types of Fossils: Molds and Casts 12.2 Fossils: Evidence of Past Life

27. Types of Fossils: Carbon Films 12.2 Fossils: Evidence of Past Life • Carbon films preserve delicate details of leaves and animal parts. • Process: • An organism is buried under fine sediment • Over time, pressure squeezes out liquids and gases to leave behind a thin film of carbon. • Sometimes impressions of the organism remain.

28. Types of Fossils: Carbon Films 12.2 Fossils: Evidence of Past Life

29. Types of Fossils: Preserved Remains 12.2 Fossils: Evidence of Past Life • Fossilization preserves ALL or PART of an organism with little change to its remains. • Preservation can occur in amber- the hardened resin, or sap, of an ancient tree. • Preservation can also occur in tar- a thick petroleum that collects in pools at the surface.

30. Types of Fossils: Preserved Remains 12.2 Fossils: Evidence of Past Life

31. Types of Fossils: Trace Fossils 12.2 Fossils: Evidence of Past Life • Indirect evidence of prehistoric life • Some of the oldest known trace fossils are believed to be worm burrows. • Coprolites are fossils of dung (poop) and stomach contents (throw-up). • Gastroliths are highly polished stomach stones that were used in grinding of food by extinct reptiles.

32. Types of Fossils: Trace Fossils 12.2 Fossils: Evidence of Past Life

33. Types of Fossilization

34. Conditions for Fossilization 12.2 Fossils: Evidence of Past Life  Two conditions that favor preservation of an organism as a fossil are rapid burial and the possession of hard parts. • Dead organisms with shells, bones, and teeth (hard parts) have a better chance at being preserved. • Dead organisms without hard parts, unless covered by sediment immediately, were likely to be eaten or decomposed by bacteria.

35. Fossils and the History of Life 12.2 Fossils: Evidence of Past Life • 1700-1800’s: Two major developments helped scientists explain fossil records: • The Principle of Fossil Succession • The Theory of Evolution

36. Fossils and the History of Life 12.2 Fossils: Evidence of Past Life • Fossil Succession: Developed by William Smith • states that fossil organisms succeed one another in a definite and determinable order. • Smith found that fossils weren’t randomly distributed through rock layers. • Rather, each layer contained an assortment of fossils that did not occur in the layer above or below it.

37. Fossils and the History of Life 12.2 Fossils: Evidence of Past Life • Theory of Evolution: Developed by Charles Darwin • 1859- Theory of Evolution- states that life forms have changed over time, or evolved, from simpler to more complex forms. • Natural Selection- individuals that are better adapted to their environment are more likely to survive and reproduce than others. • Adaptations- traits possessed by organisms that affect their ability to survive and reproduce. (drives evolution)

38. Interpreting the Fossil Record 12.2 Fossils: Evidence of Past Life  Geologists used fossils to improve the correlation of rock layers and reconstruct past environments. • Index fossils are widespread geographically, are limited to a short span of geologic time, and occur in large numbers. • The presence of an index fossil in rock layers at different locations means that the layers are roughly the same age.

39. Overlapping Ranges of Fossils

40. Fossils and Past Evidence 12.2 Fossils: Evidence of Past Life  Interpreting Environments • • Fossils can be used to interpret and describe ancient environments. • Fossils can also be used to indicate the former temperature of the water in which organisms lived.

41. Dating with Radioactivity 12.3 Dating with Radioactivity • Today, geologists can determine the absolute age of a rock. • A rock’s absolute age is the approximate number of years before the present that the rock formed. • EX) Earth is about 4.56 byo • EX) Dinosaurs became extinct 65 mya

42. Basic Atomic Structures 12.3 Dating with Radioactivity  Orbiting the nucleus are electrons, which are negative electrical charges.  Atomic number is the number of protons in the atom’s nucleus.  Mass number is the number of protons plus the number of neutrons in an atom’s nucleus.

43. What is Radioactivity? 12.3 Dating with Radioactivity • Radioactivity is the process by which atoms decay, causing a releasing of energy. • Recall: each atom has a nucleus made up of protons and neutrons. • During radioactive decay, unstable atomic nuclei spontaneously break apart, or decay, releasing energy.

44. Common Types of Radioactive Decay

45. Half-Life & Isotopes 12.3 Dating with Radioactivity • An isotope is a different form of an element. • Half-Time is a common way of expressing the rate of radioactive decay. • A half-life is the amount of time necessary for one-half of the nuclei in a sample to decay to a stable isotope.

46. The Half-Life Decay Curve

47. Radiometric Dating 12.3 Dating with Radioactivity  Each radioactive isotope has been decaying at a constant rate since the formation of the rocks in which it occurs. • Radiometric dating is the way of calculating the absolute ages of rocks and minerals that contain radioactive isotopes. • In radioactive dating, scientists measure the ratio between the radioactive parent isotope and the daughter products in a sample.

48. Radiometric Dating 12.3 Dating with Radioactivity  As a radioactive isotope decays, atoms of the daughter product are formed and accumulate.  An accurate radiometric date can be obtained only if the mineral remained in a closed system during the entire period since its formation.

49. Radioactive Isotopes Frequently Used in Radiometric Dating

50. Dating with Carbon-14 12.3 Dating with Radioactivity  Radiocarbon dating is the method for determining age by comparing the amount of carbon-14 to the amount of carbon-12 in a sample.  When an organism dies, the amount of carbon-14 it contains gradually decreases as it decays. By comparing the ratio of carbon-14 to carbon-12 in a sample, radiocarbon dates can be determined.