Earth History 8.E.2

1. Earth History 8.E.2

2. Chapter 15 The Principle of Uniformitarianism • Scientist James Hutton, the author of Theory of the Earth, proposed that geologic processes such as erosion and deposition do not change over time. • Uniformitarianism is the idea that the same geologic processes shaping the Earth today have been at work throughout Earth’s history. • The next slide shows how Hutton developed the idea of uniformitarianism.

3. Chapter 15

4. Chapter 15 The Principle of Uniformitarianism, continued • Uniformitarianism Versus Catastrophism Hutton’s theories sparked a scientific debate by suggesting the 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.

5. Chapter 15 The Principle of Uniformitarianism, continued • To explain Earth’s history, most scientists supported the principle of catastrophism. • Catastrophism is the principle that geologic change occurs suddenly. • Supporters of this theory thought that mountains, canyons, seas, and other features formed during rare, sudden events called catastrophes.

6. Chapter 15 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.

7. Chapter 15 Modern Geology—A Happy Medium • During the late 20th century, scientists such as Stephen J. Gould challenged Lyell’s uniformitarianism. They believed that catastrophes occasionally play an important role in shaping Earth’s history. • Today, scientists realize that most geologic change is gradual and uniform, but catastrophes that cause geologic change have occurred during Earth’s long history.

8. Chapter 15 Paleontology—The Study of Past Life • The history of the Earth would be incomplete without knowledge of the organisms that have inhabited our planet and the conditions under which they lived. • The science involved with the study of past life is called paleontology. • Paleontologist study fossils, which are the remains of organisms preserved by geologic processes.

9. How do fossils form? • Fossils are the preserved remains or traces of living things. • Fossils provide evidence of how life has changed over time. • Fossils also help scientists infer how Earth’s surface has changed. • Fossils are clues to what past environments were like.

10. How do fossils form? • Most fossils form when living things die and are buried by sediments. The sediments slowly harden into rock and preserve the shapes of the organisms.

12. How do fossils form? • Scientists who study fossils are called paleontologists.

13. How do fossils form? • Fossils are usually found in sedimentary rock. • Sedimentary rock is the type of rock that is made of hardened sediment.

14. What are the different kinds of fossils? • Fossils found in rock include petrified fossils, molds and casts, carbon films, and trace fossils. • Other fossils form when the remains of organisms are preserved in substances such as tar, amber, or ice.

15. What are the different kinds of fossils? • Petrified Fossils • A fossil may form when the remains of an organism become petrified. • Petrified means “turning to stone” • Petrified fossils are fossils in which minerals replace all or part of an organism.

16. What are the different kinds of fossils? • Molds and casts • A mold is a hollow area in sediment in the shape of an organism or part of an organism. • A mold forms when the hard part of the organism such as a shell, is buried in sediment. • A cast is a copy of the shape of an organism. • Water carrying dissolved minerals and sediment may seep into the empty space of a mold. If the water deposits the minerals and sediment there, the result is a cast.

17. Molds and Casts

18. What are the different kinds of fossils? • Carbon films • An extremely thin coating of carbon on rock. HOW DOES A CARBON FILM FORM? • When sediment buries an organism, some of the materials that make up the organism can become gases. These gases escape from the sediment, leaving carbon behind. Eventually, only a thin film of carbon remains.

19. Carbon Films

20. What are the different kinds of fossils? • Trace fossils • Trace fossils provide evidence of the activities of ancient organisms. • A fossilized footprint is on example of a trace fossil. • Other examples of trace fossils include the trails that animals followed or the burrows that they lived in.

21. Trace Fossils

22. What are the different kinds of fossils? • Preserved Remains • Some processes preserve the remains of organisms with little or no change. • Some remains are preserved when organisms become trapped in tar. • Ancient organisms also have been preserved in amber. Amber is the hardened resin, or sap, of evergreen trees. • Freezing is another way in which remains can be preserved.

23. Preserved Remains

24. What do fossils tell about how organisms have changed over time? • The fossil record provides evidence about the history of life on Earth. The fossil record also shows that different groups of organisms have changed over time. • The fossil record reveals a surprising fact: fossils occur in a particular order. • Older rocks contain fossils of simpler organisms. Younger rocks contain fossils of more complex organisms. • In other words, the fossil record shows that life on Earth has evolved, or changed.

25. What do fossils tell about how organisms have changed over time? • The fossil record provides evidence to support the theory of evolution. • A scientific theory is a well-tested concept that explains a wide range of observations. • Evolution is the gradual change in living things over long periods of time. • The fossil record shows that millions of types of organisms have evolved. • But many others have become extinct. • A type of organism is extinct if it no longer exists and will never again live on Earth.

26. What do fossils tell about how organisms have changed over time? • Paleontologists use fossils to build up a picture of Earth’s environments in the past. • Fossils also provide evidence of Earth’s climate in the past. • Scientists can use fossils to learn about changes in Earth’s surface.

27. How do geologists determine the relative age of rocks? • When you look at a rock containing a fossil, your first question may be, “How old is it?” • The relative age of a rock is its age compared to the ages of other rocks. • The relative age of a rock does not provide its absolute age • The absolute age of a rock is the number of years since the rock formed.

28. How do geologists determine the relative age of rocks? • The position of Rock Layers • Geologists use the law of superposition to determine the relative ages of sedimentary rock layers. • According to the law of superposition, in horizontal sedimentary rock layers the oldest layer is at the bottom. Each higher layer is younger than the layers below it. • To determine the age of most sedimentary rocks, scientists study the fossils they contain. • How would a geologist find the relative age of a rock? • By observing the rock’s position in relation to the rock layers above and below it.

31. How do geologists determine the relative age of rocks? • Other clues to Relative Age • Clues from Igneous Rock • Igneous rock forms when magma or lava harden. • Lava that hardens on the surface is called an extrusion. The rock layers below an extrusion are always older than the extrusion. • The magma cools and hardens into a mass of igneous rock called an intrusion. • An intrusion is always younger than the rock layers around and beneath it.

32. How do geologists determine the relative age of rocks? • Clues from Faults • A fault is a break in Earth’s crust. Forces inside Earth cause movement of the rock on opposite sides of a fault. • A fault is always younger than the rock it cuts through. To determine the relative age of a fault, geologists find the relative age of the most recent rock layer through which the fault slices.

34. How do geologists determine the relative age of rocks? • Gaps in the Geological Record • The geologic record of sedimentary rock layers is not always complete. • Deposition slowly builds layer upon layer of sedimentary rock. • Some of these layers may erode away, exposing an older rock surface. • Then deposition begins again, building a new rock layer.

35. How do geologists determine the relative age of rocks? • The surface where new rock layers meet a much older rock surface beneath them is called an unconformity. • An unconformity is a gap in geologic record. • An unconformity shows where some rock layers have been lost because of erosion. • Unconformities show where an old, eroded surface is in contact with a newer rock layer.

37. How are index fossils useful to geologists? • If a type of organism existed for only a short period of time, its fossils would show up in a limited range of rock layers. These fossils are called index fossils. • Index fossils are fossils that are found in the rock layers of only one geologic age, and can be used to establish the age of the rock layers.

38. How are index fossils useful to geologists? • To date rock layers, geologists first give a relative age to a layer of rock at one location. • Then they can give the same age to matching layers of rock at other locations. • Certain fossils, called index fossils, help geologists match rock layers. • To be useful as an index fossil, a fossil must be widely distributed and represent a type of organism that existed only briefly. • Index fossils are useful because they tell the relative ages of the rock layers in which they occur.

39. The Trilobite • One example of an index fossil is a trilobite. • Trilobites were a group of hard-shelled animals whose bodies had three distinct parts. • They evolved in shallow seas more than 500 million years ago.

40. Examples of Index Fossils

41. What is “B”? • Place the layers in order from oldest to youngest

42. Put the events in order

43. Put the events in order

45. Chapter 15 Absolute Age • Absolute dating is any method of measuring the age of an event or object in years. • To determine the absolute ages of fossils and rocks, scientists analyze isotopes of radioactive elements. • Atoms of the same element that have the same number of protons but different numbers of neutrons are called isotopes.

46. Absolute Age • Think of determining the absolute age of a rock layer as finding the rock layer’s birthday. • Often impossible to do • Uses Radioactive Dating

47. Chapter 15 Absolute Age: Radioactive Decay • Most isotopes are stable, meaning that they stay in their original form. • Other isotopes are unstable. Scientists call unstable isotopes radioactive.

48. Chapter 15 Radioactive Decay, continued • Radioactive isotopes tend to break down into stable isotopes of the same or other elements in a process called radioactive decay.

49. Chapter 15 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 the object’s age. • 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.

50. Chapter 15 Radioactive Decay, continued • The unstable radioactive isotope is called the parent isotope. • The stable isotope produced by the radioactive decay of the parent isotope is called the daughter isotope.