Early Geologists Unravel Earth's Mysteries Through Fossils and Sedimentary Rocks

1. Chapter 2 Early Geologists Tackle History's Mysteries

2. Fossils Fossils are the remains or traces of prehistoric life. They can tell us much about the history of the Earth and life.

3. Early Interpretation of Fossils Around 450 BC, the Greek philosopher, Herodotus, noticed fossil seashells in outcrops of sedimentary rocks that were far from the sea, and high above sea level. He concluded that the area had once been beneath the sea. Seas had once covered the continents.

4. Early Interpretation of Fossils To some, the seashells suggested the Biblical flood of Noah. • But why would animals that lived in the water be killed by a flood? • Why do we see evidence of many floods occurring at different times and in different places? • Why are different types of fossil animals and plants found in the different sedimentary layers?

5. As geologists acquired a better understanding of the immensity of geologic time, they were able to use relative dating to produce a more valid interpretation of fossils and sedimentary rocks. The fossils are the record of plants and animals which lived and died over an immense span of time.

6. How do fossils form?

7. Sediment Sediment is deposited worldwide. Sediment includes: • Gravel, sand, silt, mud and clay from rivers and streams • Clay and organic debris settling to the seafloor • Dust and volcanic ash carried by the wind and deposited

8. Sedimentary Rocks Most sedimentary rocks occur in the form of layers called beds or strata. Each layer is the result of the deposition of sediment during some natural event (such as a flood or storm).

9. When plants and animals are covered by sediment and buried, they may become preserved as fossils.

10. Fossil Preservation These characteristics are favorable for fossil preservation: • Rapid burial with sediment to prevent destruction of the dead organism by scavenging or bacterial decay. • Presence of hard parts like bones, teeth, or shell.

11. Preservation of Soft Parts Soft parts may be preserved as fossils by: • Freezing (wooly mammoths) • Desiccation (drying or mummification) • Preservation in tree sap (amber) • Preservation in tar (LaBrea tar pits) • Preservation in peat bogs (Lindow man – England; Tollund man – Denmark) • Carbonized imprints in fine-grained sediment.

12. Steno’s Laws • Principle of Superposition - Oldest rocks on the bottom Younger rocks on top • Principle of Original Horizontality - Sediments are deposited in flat, horizontal layers. • Principle of Original Lateral Continuity Sediments are deposited over a large area in a continuous sheet.

13. Stratigraphy Steno’s Laws are the basis of stratigraphy, the study of layered rocks. Stratigraphy allows geologists place rock units into a sequence, to help interpret Earth history.

14. Principle of Cross-Cutting Relationships - Faults Where a fault cuts across a sequence of sedimentary rock, the fault is younger than the rocks it cuts. The sedimentary rocks are older than the fault which cuts them, because they had to be there first, before they could be faulted.

15. Principle of Cross-Cutting Relationships - Intrusions Where an igneous intrusion cuts across a sequence of sedimentary rock, the sedimentary rocks are older than the igneous rock which intrudes them. The intrusion is younger than the rocks it cuts.

16. Principle of Cross-Cutting Relationships - Unconformities The irregular erosional surface is an unconformity. The unconformity is younger than the rocks that have been eroded.

17. The Principle of Inclusions – Sedimentary Rocks Fragments of eroded rock overlie the unconformity. These are gravel clasts or inclusions. The pieces of gravel are older than the bed in which they are found.

18. The Principle of Inclusions –Igneous Rocks A xenolith is a fragment of the surrounding rock which has broken off during an intrusion and fallen into the magma. The xenolith is older than the igneous rock which contains it.

19. Comparison of inclusions in a sedimentary rock (A) with inclusions in an igneous rock (B). Which are gravel clasts and which are xenoliths?

20. Interpreting a Sequence of Events Determine the order in which the geologic events occurred.

21. European Researchers Unravel the Succession of Strata • John Strachey (1671-1743) used superposition and lateral continuity to interpret the stratigraphy of coal-bearing rocks in England. He also described an unconformity.

22. European Researchers Unravel the Succession of Strata 2. In Italy, Giovanni Arduino (1713-1795) classified mountains as: • "Primary mountains" composed of crystalline rocks. These were interpreted to be oldest. • "Secondary mountains" composed of fossiliferous sedimentary rocks. • "Tertiary mountains" composed of beds of gravel, sand, and clay, that were youngest.

23. Neptunists and Plutonists Clash Professor Abraham Gottlob Werner (1749-1817) was a prominent German geologist who insisted that all rocks formed from a great ocean. For this reason, he and his followers were called Neptunists, after Neptune, the Roman god of the sea. He thought that the Earth had changed little since its beginning.

24. Werner contributed to the geologic time scale. He recognized: • "Primitive rocks" (called "primary“ by Arduino). He thought these rocks were deposited by a hot, mineral-rich ocean. They formed the cores of mountain ranges. • "Transition rocks“ - Flat-lying sandstones, shales, coals, limestones, and old lava flows. He thought these rocks were deposited when Earth became suitable for life. • "Alluvium", - loose gravel, sand, and clay that overlay the transition rocks.

25. Werner’s ideas were criticized because he could not explain what had happened to such an immense volume of water, and because he insisted that lava flows were precipitated from water.

26. Plutonists Other geologists showed the volcanic origin of lava flows. These were the Plutonists, named for the Roman god of the underworld. Plutonists said that "fire" or heat, rather than water, was involved in the origin of "primitive" igneous rocks. James Hutton was a prominent Plutonist who disagreed with Werner.

27. Uniformitarianism James Hutton: • Saw Earth as a dynamic, ever changing place where rocks and mountains form slowly, and are slowly weathered and eroded. • Recognized that "the present is the key to the past". • Recognized uniform natural laws govern geologic processes, later called uniformitarianism.

28. James Hutton - continued • Published Theory of the Earth in 1785. • Realized the immensity of geologic time. "No vestige of a beginning, no prospect of an end." • First interpretation of sequence of events in an unconformity. • Saw a world dominated by cycles.

29. Uniformitarianism Uniformitarianism means that geologic processes are uniform through time. Physical and chemical laws that govern nature are uniform. Uniform natural laws govern weathering, erosion, glacial movement, earthquakes, volcanic eruptions, and the transport of sediment by moving water.

30. Some events which occurred in the past, and left a record in the rocks, ARE NOT OCCURRING TODAY, or have not occurred in the human lifespan. These include: • Huge meteorite impacts • Extensive volcanism • Large glacial ice sheets accompanied by much lower sea levels • Differences in atmospheric chemistry - Earth's original atmosphere lacked oxygen, so chemical process acting in the weathering environment did not include oxidation

31. Actualism • Many geologists prefer to use the term actualism, to emphasize the importance of natural laws to the concept of uniformitarianism. • Actualism is the principle that natural laws governing past and present processes on Earth have been the same.

32. Unconformities An unconformity is an ancient surface of erosion (or non-deposition), separating older rocks from younger rocks. If the older rocks are folded or tilted, the unconformity is referred to as an angular unconformity.

33. James Hutton recognized the significance of the unconformity at Siccar Point in Scotland

34. Principle of Fossil Succession • William Smith (1769-1839) was an English surveyor and civil engineer who was working to site canals to transport coal in England. • He saw that layers of rocks occurred in a definite order, and that rock units could be differentiated on the basis of the fossils they contain.

35. Principle of Fossil Succession Fossils occur in a consistent vertical order in sedimentary rocks all over the world. This is the Principle of Fossil Succession. Geologists interpret fossil succession to be the result of evolution - the natural appearance and disappearance of species through time.

36. Fossil Succession Confirmed Baron Georges Leopold Cuvier (1769-1832), a French vertebrate paleontology expert (along with Alexander Brongniart), confirmed William Smith's findings that fossils display a definite vertical succession in the rock record, and that the succession is basically the same in widely separated areas.

37. Catastrophism Cuvier also noted that the fossils changed across unconformities. He concluded that there had been a series of catastrophic floods. This viewpoint became known as catastrophism.

38. Uniformitarianism vs. Catastrophism English geologist, Sir Charles Lyell (1797-1875) held the opposing viewpoint, that of uniformitarianism. Seemingly abrupt changes in the fossil record were interpreted to result from missing strata that were eroded along the unconformity. The ancestors of the "new" fossil groups were actually present in the underlying strata.

39. Uniformitarianism vs. Catastrophism Both uniformitarianism and catastrophism probably operate jointly. Uniformitarianism dominates the day-to-day processes. Catastrophism comes into play episodically with events such as asteroid impacts, or sudden, severe climatic changes that cause extinctions, and other unusual events.

40. It has been documented that fossils appear in a particular sequence in the rock record. That is without question. But why is this so?

41. Evolution: How Organisms Change Through Time English biologist and geologist, Charles Darwin (1809-1882) provided a hypothesis to account for the observed fossil succession. He served as a naturalist in a 5-year mapping expedition around the world on the H.M.S. Beagle. He gathered volumes of data to support his hypothesis of the evolution of organisms by natural selection.

42. Natural selection is based on the following observations: • A given species produces more offspring than can survive to maturity. • Variations in morphology (form and structure) and physiology (organs and functions) exist among individuals of a species. • The individuals of a species must compete with one another for food and habitat. • Individuals with the most favorable traits are more likely to survive to reproduce. • Beneficial traits are passed on to the next generation.

43. Cause of Variation Darwin did not know the cause of the variations among individuals in a species. Many years later, after Darwin's death, scientists determined that variations within a species are caused by new gene combinations that occur during reproduction, and from genetic mutation.

44. Earth History in America

45. Ice Age Hypothesis Louis Agassiz (1807-1873) was a Swiss paleontologist who arrived in North America in 1846 and founded the Harvard Museum of Comparative Zoology. He studied glaciers and proposed that ice sheets once covered much of North America and Europe("Ice Age hypothesis").

46. Ice Age Hypothesis Evidence includes: • Glacial striations (scratches on rock) • Glacial erratics (huge boulders transported by ice) • Glacial moraines (mounds of rock debris deposited by melting glaciers) • Lakes scoured by glacial erosion

47. Ice Age Hypothesis The "Ice Age" was recognized as the main event of the Pleistocene Epoch (1.5 million years ago to 8000-10,000 years ago). Ice blanketed one third of Earth's land area.

48. Appalachian Mountain Sedimentary Deposition James Hall (1811-1898) was the director of New York's first geological survey.He recognized that fossils in 40,000 ft (7.5 mi) thick sedimentary rock sequence in NY were deposited in shallow water, and concluded that the seafloor had subsided during deposition. The Appalachian Mountains were later raised from a marine basin.

49. Geology in the Western US Ferdinand V. Hayden (1829-1887) - mapped geology of Badlands of South Dakota and other areas in the west. Helped convince Congress to establish Yellowstone National Park, the oldest national park in the US.

50. Geology in the Western US John Wesley Powell (1834-1902) - journeyed by boat through Grand Canyon on Colorado River to map geology despite having lost an arm in Civil War. Directed the U.S. Geological Survey.