Chapter 21 Vocabulary

Edit Answers
12345678910111213141516171819
Across
  1. 6. the only indirect fossil evidence of an organism; traces of worm trails, footprints, and tunneling burrows.
  2. 10. record of Earth’s history from its origin 4.6 bya to the present.
  3. 11. gap in the rock record caused by erosion or weathering.
  4. 13. longest time unit in the geologic time scale.
  5. 14. occurs when an unusually large number of organisms disappear from the rock record at about the same time.
  6. 16. third-longest time unit in the geologic time scale, measured in tens of millions of years.
  7. 17. time unit in the geological time scale, smaller than a period, measured in hundreds of thousands to millions of years.
  8. 18. fossil that can form when a shelled organism decays in sedimentary rock and is removed by erosion or weathering, leaving a hollowed-out impression.
  9. 19. establishing the order of past geologic events.
Down
  1. 1. the principle that, in an undisturbed rock sequence, the oldest rocks are on the bottom and each consecutive layer is younger than the layer beneath it.
  2. 2. method that enables scientists to determine the actual age of certain rocks and other objects.
  3. 3. period of time it takes for a radioactive isotope, such as carbon- 14, to decay to one-half of its original amount.
  4. 4. unit of geo- logic time consisting of the first three eons during which Earth formed and became hospitable to life.
  5. 5. emission of radioactive particles and its resulting change into other iso- topes over time.
  6. 7. second-longest time unit in the geologic time scale, measured in tens to hundreds of mil- lions of years, and defined by differences in life- forms that are preserved in rocks.
  7. 8. remains of plants or animals that were abundant, widely distributed, and existed briefly that can be used by geologists to correlate or date rock layers.
  8. 9. fossil formed when an earlier fossil of a plant or animal leaves a cavity that becomes filled with minerals or sediment.
  9. 12. process used to determine the absolute age of a rock or fossil by determining the ratio of parent nuclei to daughter nuclei within a given sample.
  10. 15. determines the age of relatively young organic objects; objects that are alive or were once alive.