Chapter 21 Review

1. Chapter 21 Review Fossils and the Rock Record

2. The oldest rock layer in an undisturbed rock sequence occurs • at the bottom of the sequence • below the sedimentary rock layer • below the unconformity • at the top of the sequence

3. The amount of time it takes for one-half of the original amount of an isotope to decay is known as its • correlation • half-life • period • radioactivity

4. Evolution is the process by which organisms • digest and grind their food • adapt to changes in their environments • become preserved by mineral replacement • leave behind evidence of their existence

5. On the geologic time scale, the smallest unit of time is called a(n) • era • eon • period • epoch

6. When part of the rock record is destroyed, the erosional gap that forms is a(n) • inclusion • key bed • unconformity • superposition

7. Which of the following is used to determine the absolute age of a rock? • relative-age dating • correlation • permineralization • radiometric dating

8. Index fossils are useful to geologists if the fossils • have lived over a short period of time • are not easily recognized • are not widely distributed geographically • are scarce

9. What are fossil insects preserved in hardened tree sap called? • trace fossils • gastroliths • coprolites • amber

10. Molds, casts, coprolites, and petrified wood are all example of • fossils • radioactive isotopes • unconformities • disconformities

11. The Phanerozoic Eon includes the Paleozoic, Mesozoic, and • Cenozoic Periods • Jurassic Periods • Tertiary Epochs • Cenozoic Eras

12. The Archean and Proterozoic are examples of • eons • periods • epochs • eras

13. Which of the following is marked by the appearance of organisms with hard parts? • the beginning of the Cenozoic • the end of the Precambrian • the end of the Mesozoic • the beginning of the Cretaceous

14. Which of the following lists the units of geologic time in order from shortest to longest? • era, eon, period, epoch • epoch, eon, period, era • eon, era, period, epoch • epoch, period, era, eon

15. An example of a fossil with original preservation includes a(n) • insect imbedded in amber • coprolite • dinosaur footprint • mold of a fish skeleton

16. An example of a fossil with altered hard parts includes • a mummified human body • a gastrolith • a saber-toothed cat in tar • petrified wood

17. An example of a trace fossil includes a • cast of a clam • worm trail • mud crack • raindrop impression

18. Which correctly orders units of geologic time, from most general to most specific? • eon, era, period, epoch • era, epoch, eon, period • period, era, eon, epoch • epoch, eon, period, era

19. What is the correct label for “C” on the diagram? • Paleogene • Paleozoic • Permian • Paleocene

20. The shaded area of which graph represents the percentage of geologic time that is part of the Precambrian? • A • B • C • D

21. The principle of superposition can be used to determine • the actual age of these rock layers • the relative densities of these rock layers • the actual temperatures of these rock layers • the relative age of these rock layers

22. What classification of unconformity occurs when sedimentary rock overlies igneous rock? • disconformity • nonconformity • angular unconformity • conformity

23. What characteristic of radioactive decay makes it useful for determining the absolute age of samples? • occurs at a constant rate • changes based on temperature • speeds up under high pressure • slows down in very dense samples

24. Using the graph, determine the half-life of thorium-232. • 7 billion years • 14 billion years • 28 billion years • 35 billion years

25. Radiocarbon dating can be used to determine the age of what type of materials? • igneous rock • living organisms • organic materials • lava flows

26. The shaded area of which of these graphs best represents the percentage of marine organisms that became extinct at the end of the Paleozoic? • A • B • C • D

27. Why are fossils of organism’s from the Phanerozoic eon much more likely to be found than fossils of organisms from any other eon? • Organisms present during the Phanerozoic eon had only soft body parts. • Organisms present during the Phanerozoic eon were all microscopic. • Organisms present during the Phanerozoic eon had hard body parts. • Organisms present during the Phanerozoic eon are the most ancient life forms.

28. Present time is part of the Mesozoic era. • True • False

29. Eons are longer units of time than Eras. • True • False

30. The Precambrian consists of the three earliest eons of geologic time. • True • False

31. The star on the time line indicates the beginning of the Precambrian. • True • False

32. Mammals and dinosaurs both emerged during the Paleozoic Era. • True • False

33. According to the principle of superposition, the youngest rock layers are closest to the surface. • True • False

34. Intrusions are younger than the rocks they cut across. • True • False

35. An unconformity between two sedimentary layers is classified as a disconformity. • True • False

36. A geologist who needed to correlate two rock formations in different areas could use a key bed for this work. • True • False

37. After one half life, the ratio of parent atoms to daughter atoms in a sample is 1:2. • True • False

38. The radioactive decay scale divides the history of Earth into time units that help geologists analyze the rock record. • True • False

39. The remains of once living plants or animals are called varves. • True • False

40. The principle of uniformitarianism states that sedimentary rocks are deposited in horizontal layers. • True • False

41. Dendrochronology is the science of comparing annual growth rings in trees to date changes in Earth's environments and past events. • True • False

42. Key beds are time markers that can be used to correlate rock layers across large areas. • True • False