CHAPTER 12 GEOLOGIC TIME
<ul><li>Earth’s history is recorded in the  rock record  such as the rock layers of the Grand Canyon. </li></ul>
<ul><li>Principle of Uniformitarianism – the forces and processes we observe on Earth today are the  same  forces and proc...
<ul><li>The law of  superposition  – each bed of rock is older than the one above it.  The oldest rock is at the bottom, t...
<ul><li>The principle of  horizontality  – layers of sediment are deposited in horizontal layers or strata. </li></ul>
<ul><li>The principle of  cross-cutting  relationships – a fault or intrusion that cuts the rock layer must be younger tha...
<ul><li>Unconformity  – a long time in which deposition stopped, erosion removed layers and then deposition began again </...
 
<ul><li>Fossils are any remains or traces or prehistoric life </li></ul><ul><li>Conditions favoring fossilization:  hard p...
<ul><li>Unaltered  remains – frozen in ice, shells </li></ul>
<ul><li>Petrified  – mineral-rich water fill the ore spaces.  The minerals precipitate out and harden </li></ul>
<ul><li>Mold  – forms when a hard part is buried in sediment and then the hard part dissolves, accurately represents the s...
 
<ul><li>Carbonization  – organism is buried in fine sediment, the liquid and gases are squeezed out and leaves of thin res...
 
<ul><li>Animal footprints made in soft sediment are  compacted  and cemented </li></ul><ul><li>Burrows of animals in rock ...
 
 
<ul><li>Principle of  fossil succession  – fossil organisms succeed each other in a definite order </li></ul><ul><li>Age o...
<ul><li>Index  fossils – useful in dating other fossils found in the same rock layer.  Index fossils are limited to a shor...
 
<ul><li>Radioactivity – the spontaneous breakdown of an unstable nuclei into a different element </li></ul><ul><li>Half-li...
 
<ul><li>Radiometric  dating – using the rate of decay in the surrounding rock to determine the age </li></ul><ul><li>Radio...
 
<ul><li>Geologic Time Scale  </li></ul><ul><li>Eons  are the greatest expanse of time.  Eons are divided into  eras .  Era...
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  1. 1. CHAPTER 12 GEOLOGIC TIME
  2. 2. <ul><li>Earth’s history is recorded in the rock record such as the rock layers of the Grand Canyon. </li></ul>
  3. 3. <ul><li>Principle of Uniformitarianism – the forces and processes we observe on Earth today are the same forces and processes that occurred throughout Earth’s history. </li></ul><ul><li>We study the present to understand the past. </li></ul><ul><li>Relative Dating tells of the sequence of events, not how long ago the events occurred. </li></ul>
  4. 4. <ul><li>The law of superposition – each bed of rock is older than the one above it. The oldest rock is at the bottom, the youngest rock at the top. </li></ul>
  5. 5. <ul><li>The principle of horizontality – layers of sediment are deposited in horizontal layers or strata. </li></ul>
  6. 6. <ul><li>The principle of cross-cutting relationships – a fault or intrusion that cuts the rock layer must be younger than the rock it cuts. </li></ul>
  7. 7. <ul><li>Unconformity – a long time in which deposition stopped, erosion removed layers and then deposition began again </li></ul><ul><li>Angular unconformity – indicates that a period of folding or tilting occurred (continental collisions, dune build-up, or at end of delta) </li></ul><ul><li>Disconformity – sedimentary layers that a separated by an erosion </li></ul><ul><li>Nonconformity – an erosional surface separates older metamorphic or igneous rock from younger sedimentary rock </li></ul>
  8. 9. <ul><li>Fossils are any remains or traces or prehistoric life </li></ul><ul><li>Conditions favoring fossilization: hard parts , buried quickly, and separated from the environment </li></ul>
  9. 10. <ul><li>Unaltered remains – frozen in ice, shells </li></ul>
  10. 11. <ul><li>Petrified – mineral-rich water fill the ore spaces. The minerals precipitate out and harden </li></ul>
  11. 12. <ul><li>Mold – forms when a hard part is buried in sediment and then the hard part dissolves, accurately represents the shape and surface markings </li></ul><ul><li>Casts – created when hollow spaces of a mold are later filled with mineral matter </li></ul>
  12. 14. <ul><li>Carbonization – organism is buried in fine sediment, the liquid and gases are squeezed out and leaves of thin residue of carbon. Black shale shows the outlines of leaves </li></ul>
  13. 16. <ul><li>Animal footprints made in soft sediment are compacted and cemented </li></ul><ul><li>Burrows of animals in rock and wood later filled with mineral matter and preserved </li></ul><ul><li>Coprolites – fossilized dung </li></ul><ul><li>Gastroliths – fossilized polished stomach stones </li></ul>
  14. 19. <ul><li>Principle of fossil succession – fossil organisms succeed each other in a definite order </li></ul><ul><li>Age of Trilobites, Age of Fishes, Age of Coal Swamps, Age of Reptiles, Age of Mammals </li></ul>
  15. 20. <ul><li>Index fossils – useful in dating other fossils found in the same rock layer. Index fossils are limited to a short time span, are geographically widespread, and occur in large numbers </li></ul><ul><li>Fossils can also be used to interpret past environments. Shells, for example, would indicate shallow seas, and shorelines. </li></ul>
  16. 22. <ul><li>Radioactivity – the spontaneous breakdown of an unstable nuclei into a different element </li></ul><ul><li>Half-life – the time it takes for one-half of the nuclei to decay. The half-life ratio can be used to determine the age of an object </li></ul>
  17. 24. <ul><li>Radiometric dating – using the rate of decay in the surrounding rock to determine the age </li></ul><ul><li>Radiocarbon-14 dating – using the ratio of carbon-14 to carbon-12 to determine the age. The half-life of carbon-14 is 5730 years and is used to date objects up to 75,000 years ago. </li></ul>
  18. 26. <ul><li>Geologic Time Scale </li></ul><ul><li>Eons are the greatest expanse of time. Eons are divided into eras . Eras divided into periods and periods are divided into epochs </li></ul><ul><li>Phanerozoic Eon – means “ visible life ” </li></ul><ul><li>Paleozoic Era – means “ ancient life ” </li></ul><ul><li>Mesozoic Era – means “ middle life ” </li></ul><ul><li>Cenozoic Era – means “ recent life ” </li></ul><ul><li>Precambrian – 88% of Earth’s history, few life forms </li></ul>
  19. 27. In books on Page 365

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