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  1. 1. Fossils and the Rock Record “ The Geologic Time Scale”
  2. 2. The Geologic Time Scale <ul><li>By studying the characteristics of rocks and the fossils within them, geologists can: </li></ul><ul><ul><li>interpret the environments the rocks were deposited in </li></ul></ul><ul><ul><li>Reconstruct Earth’s history </li></ul></ul><ul><ul><li>Possibly predict events or conditions in the future </li></ul></ul>
  3. 3. The Rock Record <ul><li>Geologists divided the history of Earth into time units based on the fossils contained within the rocks. </li></ul><ul><li>Geologic time scale is a record of Earth’s history from its origin 4.6 billion years ago to the present. </li></ul>
  4. 4. <ul><li>Names of the periods have not changed, but the years marking the beginning and the end of each unit of time are being changed. </li></ul>
  5. 5. Geologic Time <ul><li>The oldest division of time is at the bottom of the geologic time scale. </li></ul><ul><li>As you go up the younger the division. </li></ul><ul><li>Time scale is divided into units: </li></ul><ul><ul><li>Eons </li></ul></ul><ul><ul><li>Eras </li></ul></ul><ul><ul><li>Periods </li></ul></ul><ul><ul><li>epochs </li></ul></ul>
  6. 6. <ul><li>An eon is the longest time unit and is measured in billion of years. </li></ul><ul><ul><li>Archean </li></ul></ul><ul><ul><li>Proterozoic </li></ul></ul><ul><ul><li>Phanerozoic </li></ul></ul>
  7. 7. <ul><li>An era is the next-longest time unit and is measured in hundreds of millions to billions of years: </li></ul><ul><ul><li>Paleozoic (“old life”) </li></ul></ul><ul><ul><li>Mesozoic (“middle life”) </li></ul></ul><ul><ul><li>Cenozoic (“recent life”) </li></ul></ul>
  8. 8. <ul><li>Precambrian Time, which makes up approximately 90 percent of geologic time is divided into: </li></ul><ul><ul><li>Archean eons </li></ul></ul><ul><ul><li>Proterozoic eons </li></ul></ul>
  9. 9. Plants and Animals Evolve <ul><li>Paleozoic Era - the oceans became full of a wide diversity of plants and animals. </li></ul><ul><li>Cambrian Period – dominated by trilobites in the oceans. </li></ul><ul><li>End of the Paleozoic Era is marked by the largest marine extinction event </li></ul><ul><ul><li>90 % of all marine invertebrate </li></ul></ul>
  10. 10. <ul><li>The Mesozoic Era </li></ul><ul><ul><li>Dinosaurs emerged </li></ul></ul><ul><ul><li>Reef building corals </li></ul></ul><ul><ul><li>Large predatory reptiles developed in oceans </li></ul></ul><ul><ul><li>Amphibians began living on land </li></ul></ul>
  11. 11. <ul><li>Cretaceous Period </li></ul><ul><ul><li>Mammals evolved </li></ul></ul><ul><ul><li>Flowering plants and trees </li></ul></ul>
  12. 12. <ul><li>End of the Mesozoic- beginning of Cenozoic: </li></ul><ul><ul><li>large extinction </li></ul></ul><ul><ul><li>Remaining dinosaurs </li></ul></ul><ul><ul><li>Mammals increased (numbers and diversity) </li></ul></ul><ul><ul><li>Human ancestors developed </li></ul></ul><ul><ul><li>Grasses and flowering plants increased </li></ul></ul>
  13. 13. Periods of Geologic Time <ul><li>Periods are defined by the life-forms that were abundant or became extinct during the time in which specific rocks were deposited. </li></ul><ul><ul><li>Usually 10 of millions – 100 of millions </li></ul></ul><ul><ul><li>Some named for geographic region in which the rocks of that age were first observed </li></ul></ul><ul><ul><ul><li>Mississippian </li></ul></ul></ul><ul><ul><ul><li>jurassic </li></ul></ul></ul>
  14. 14. Epochs of Geologic Time <ul><li>Epochs are even smaller divisions of geologic time and are usually measured in millions of years to tens of millions of years. </li></ul><ul><ul><li>Rocks and fossils from this era are easily accessed and studied </li></ul></ul><ul><ul><li>Cenozoic divided into epochs – Paleocene and Oligocene. </li></ul></ul>
  15. 15. <ul><li>Regardless of how a geologic period was defined, each unit contains specific characteristics that set it apart from the rest of geologic history. </li></ul>
  16. 16. Relative-Age Dating of Rocks
  17. 17. <ul><li>Even into the 19 th century, people believed that the world was only 6,000 years old. </li></ul><ul><li>James Hutton attempted to explain the forces that continually change the surface features of Earth. </li></ul><ul><li>The principle of uniformitarianism states that the processes occurring today have been occurring since Earth formed. </li></ul>
  18. 18. <ul><li>Only the rate, intensity, and scale have changed. </li></ul><ul><li>Ex. Ocean shore </li></ul>
  19. 19. Principles for Determining Relative Age <ul><li>Concept of relative-age dating places the age of rocks and the events that formed them in order, but w/o exact dates. </li></ul><ul><li>Done by comparing rocks to each other. </li></ul>
  20. 20. Geologic Principles <ul><li>The principle of original horizontality states that sedimentary rocks are deposited in horizontal or nearly horizontal layers. </li></ul><ul><li>We may not know actual dates but we do know that the youngest layer is at the top and as you go down the layers get older. </li></ul><ul><li>This is an application of the principle of superposition. </li></ul>
  21. 21. <ul><li>The principle of cross-cutting relationships states that an intrusion or a fault is younger than the rock it cuts across. </li></ul>
  22. 22. Inclusions <ul><li>Relative age also can be determined where an overlying rock layer contains particles of rock material from the layer beneath it. </li></ul><ul><li>The bottom layer was eroded, and the loose material on the surface became incorporated in the newly deposited top layer. </li></ul>
  23. 23. <ul><li>These particles, called inclusions, indicate that the rocks in the lower layer are older than those on top. </li></ul>
  24. 24. Other Means of Determining Relative Age <ul><li>The fact that Earth is constantly changing as a result of processes such as weathering, erosion, earthquakes, and volcanism makes it difficult to find an undisturbed sequence of rock layers. </li></ul>
  25. 25. <ul><li>An erosional surface might become buried by the deposition of younger rocks. </li></ul><ul><li>This buried erosional surface results in a gap in the rock record and is called an unconformity. </li></ul><ul><li>Horizontal sedimentary rocks overlie horizontal sedimentary rocks, it is called disconformity. </li></ul>
  26. 26. <ul><li>A different type of unconformity exists when sedimentary rocks overlie nonsedimentary rocks such as granite or marble. </li></ul><ul><li>The contact point between the nonsedimentary and sedimentary rock is called a nonconformity. </li></ul><ul><li>Figure 21-7 pg. 561 </li></ul>
  27. 27. Correlation of Rock Strata <ul><li>Correlation is the matching of outcrops of one geographic region to another. </li></ul><ul><li>Geologists examine rocks for distinctive fossils and unique rock or mineral features to help correlate the rock layers. </li></ul>
  28. 28. Absolute-Age Dating of Rocks
  29. 29. Vocabulary <ul><li>Radioactive decay </li></ul><ul><li>Radiometric dating </li></ul><ul><li>Half-life </li></ul><ul><li>Dendrochronolgy </li></ul><ul><li>Varve </li></ul><ul><li>Key bed </li></ul>
  30. 30. <ul><li>Absolute-age dating enables scientists to determine the actual age of a rock, fossil, or other object. </li></ul><ul><li>Scientists use the decay rate of radioactive isotopes. </li></ul>
  31. 31. <ul><li>Radioactive substances emit nuclear particles </li></ul><ul><li>The number of protons and neutrons lower with each emission. </li></ul><ul><li>Thus the element is changed to a different element. </li></ul>
  32. 32. <ul><li>The old radioactive element is called the “parent” </li></ul><ul><li>The new is called the “daughter” </li></ul><ul><li>The emission of radioactive particles and the resulting change into other elements over time is called radioactive decay. </li></ul>
  33. 33. Use of Radioactive Isotopes <ul><li>In a process called radiometric dating, scientists attempt to determine the ratio of parent nuclei to daughter nuclei within a given sample of a rock or fossil. </li></ul><ul><li>Because it often takes a long time for the entire amount of an isotope to decay, geologists use the length of time it takes for one-half of the original amount to decay. </li></ul><ul><li>This period of time is called the half-life . </li></ul>
  34. 34. Carbon - 14 <ul><li>Commonly used radioactive isotope in determined the absolute age of an object. </li></ul><ul><li>Especially used for materials of organic origin. </li></ul><ul><li>C-14 decays into non-radioactive N-14 . </li></ul><ul><li>It has a half-life of 5730 years. </li></ul><ul><li>Accurate up to 75,000 years. </li></ul>
  35. 35. <ul><li>Different radioactive elements are used based on the relative age of the object. </li></ul>
  36. 36. Other Ways to Determine Age <ul><li>Naturally occurring materials, such as trees, lake-bottom sediment, and volcanic ash can also be used to determine age of objects. </li></ul>
  37. 37. Tree Rings <ul><li>The age of a tree can be determined by counting the number of annual tree rings in a cross section of the tree. </li></ul><ul><li>The widths of tree rings are directly related to the climatic conditions during growth periods. </li></ul>
  38. 38. <ul><li>Dendochronology is the science of comparing annual growth rings in trees to determine events and changes in past environments. </li></ul>
  39. 39. Seasonal Climatic Changes <ul><li>Summer deposits are generally light-colored and relatively thick compared to the thinner, organically enriched and dark-colored sediments of winter. </li></ul><ul><li>These bands of alternating light and dark-colored sediments of sand, clay and silt are called varves . </li></ul>
  40. 40. Distinctive Sediment Layers <ul><li>66 million years ago an asteroid 10 km in diameter hit the Yucatan peninsula in Mexico. </li></ul><ul><li>It thru dust and crushed rock into the atmosphere </li></ul><ul><li>This dust and rock then settled onto the Earth’s surface and became a sediment layer found in many places around the world. </li></ul>
  41. 41. <ul><li>This layer became a key bed. </li></ul><ul><li>A key bed is a distinct layer that can be used to correlate rock layers across large areas. </li></ul><ul><li>These layers are easy to recognize, like a coal bed. </li></ul><ul><li>Mt. St. Helen – the ash will eventually become a thin, clay layer that will mark the date of the eruption. </li></ul>