Geologic Time (Lessons 1-3)

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Across
  1. 2. Geologic time begins with a long span of time called the ___ time.
  2. 3. The first land vertebrates were ___.
  3. 7. A break in Earth's crust
  4. 9. Reptiles have __ skin and lay eggs with tough, leathery shells.
  5. 10. A great landmass or supercontinent where all continents were together
  6. 14. During radioactive __, the atoms of one element break down to form atoms of another element.
  7. 15. Animals without backbones
  8. 16. Geologists use the law of __ to determine relative ages of sedimentary rock layers
  9. 18. A vertebrate that controls its own body temperature and feeds milk to its young.
  10. 20. Oneof the most important events of the Cretaceous period was the evolution of ___ plants.
  11. 21. Animal with a backbone
  12. 25. The Devonian period is often called Age of the __.
  13. 26. During the Cambrian period, all animals lived in the ___.
  14. 27. The first vertebrates were ___ fish.
  15. 29. A gap in the geologic record
  16. 31. Geologists continually ___ the geologic time scale.
  17. 32. The geologic time __ is a record of the geologic events and evolution of life forms as shown in the rock and fossil records.
  18. 33. Scientists hypothesize that another mass extinction at the end of the Cretaceous period was caused when an ___ struck Earth.
Down
  1. 1. Geologists divide the time between the Precambrian and present into 3 long units of time called ___.
  2. 4. The Quarternary period is sometimes referred to as the Age of ____.
  3. 5. During the __ period, dinosaurs were the dominant land animals.
  4. 6. __ fossils help geologists match and date rock layers even if they're far apart.
  5. 7. The preserved remains or traces of living things
  6. 8. An animal that lives part of its life on land and part in water
  7. 11. __ age of a rock is the number of years that passed since that rock was formed.
  8. 12. During the Quarternary period, large sheets of ice covered the Earth in an ___ ___.
  9. 13. A shallow __ covered much of North America during the Silurian period.
  10. 17. Eras are subdivided into units of geologic time called ____.
  11. 19. Mammals first appeared in the ___ period.
  12. 22. Lava can harden on the surface of rocks to form an igneous ____.
  13. 23. ___ age compares a rock to ages of other rocks.
  14. 24. The first part of the Paleozoic era is the ___ period.
  15. 28. At the end of the Paleozoic era,there was a mass ___ event.
  16. 30. Scientists estimate between 90% of ___ animals and 70% of __ animals died in the mass extinction.