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Earth’s History

Earth’s History. Earth’s History. Planet Earth is approximately 4.5 X 10 9 years old Rocks of the crust provide clues to Earth’s past By analyzing these clues we can infer events from the past. Earth’s History. Principle of Uniformitarianism Major assumption in geology

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Earth’s History

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  1. Earth’s History

  2. Earth’s History • Planet Earth is approximately 4.5 X 109 years old • Rocks of the crust provide clues to Earth’s past • By analyzing these clues we can infer events from the past

  3. Earth’s History • Principle of Uniformitarianism • Major assumption in geology • Events in the past occurred the same way that they are occurring today. Examples Include: • Weathering/erosion • Deposition • Volcanism • Plate tectonics

  4. Geologic Time • Plate motions and mountain building events can be placed on the geologic time scale

  5. Geologic Time Geologists have divided Earth’s history into time units based on the the fossil record

  6. Geologic Time • A study of the fossil record shows • A great variety of plants, animals, and simpler life forms have lived on Earth in the past • That life forms have evolved through time • Most life forms of the geologic past have become extinct

  7. Geologic Dating

  8. There are two types of Geologic Dating • Absolute • Determines how many years old something is • Relative • Used to determine if one thing is younger or older than another

  9. Using radiometric dating Law of superposition Use of index fossils Correlation of rock layers Tools used to determine Absolute Age Relative Age

  10. Relative Dating

  11. Law of Superposition • Sediments are laid down underwater in horizontal layers and form sedimentary rocks

  12. Law of Superposition • In a series of sedimentary rocks the bottom layer is the oldest and the top layer is the youngest • Lower layers must be in place before younger rocks can be deposited on top of them • Exception: when something occurs to overturn layers

  13. Grand Canyon

  14. GRAND CANYON- LAW OF SUPERPOSITION YOUNGEST ON TOP OLDEST ON BOTTOM

  15. Law of Superposition • Rock layers are older than folds found in them • Layers were there before they were folded

  16. Folds/Tilted

  17. Law of Superposition • Rock layers are older than faults found in them • This is logical: you can’t break a rock if it does not exist; so rock containing a fault must be older than the fault

  18. Law of Superposition • Fossils are generally the same age as the rock layers in which they are found • Animal remains are deposited along with the sediments that will turn into sedimentary rocks

  19. Law of Superposition • Igneous intrusions are younger than the rock that they cut through or flow out of

  20. Igneous Intrusion - Cross Cutting

  21. Unconformity • When a new rock layer is formed atop an eroded surface

  22. Upper SilurianCarbonates Tilted Ordovician Shales and Sandstones unconformity Taconic Unconformity

  23. Practice: what happened here?

  24. Correlation • Matching similar rock layers in different locations to see if they formed at the same time

  25. f03_09_pg37

  26. Which fossil might be found in Devonian rock layers?

  27. Volcanic Ash Falls • Can also be used to correlate rock layers over a large area • Ash is a good indicator because: • The ash from one explosion has distinct characteristics • ash can be deposited around the globe • The event occurs at one, geologically brief, time

  28. Fossils • Naturally occurring preserved remains or impressions of living things • Generally only hard parts get preserved • Bones, teeth, shells • Hard parts are replaced by naturally occurring minerals

  29. Eurypterus NY State Fossil Silurian index fossil

  30. Fossils • Can be a mold (impression) • Ex: footprint • Or a cast • Ex: filled in footprint

  31. Fossils (Cont) • Other types • Ice • Tar • Carbonaceous film • Amber • Imprints • Petrified

  32. Fossils • Generally found in sedimentary rock layers Why don’t fossils exist in most igneous or metamorphic rock?

  33. Fossils • The living organism that made the fossil lived during the time the rock layer was forming • i.e. when the sediment was being deposited

  34. Fossils • Fossils can provide information about ancient environments • Marine fossils indicate a marine environment, wooly mammoths indicate a cold environment, etc…

  35. Fossils • A study of the fossil record shows: • A great variety of plants, animals and simpler life forms have lived on Earth in the past • That life forms have evolved through time • Most life forms of the geologic past have become extinct

  36. Index Fossils • Index Fossils--used to date rock layers (strata) • Fossils from creatures that existed for a geologically short period of time • Ex. less than 2.0 x 107 • Fossils from creatures that had a wide geographic distribution

  37. Which letter would make a good index fossil?

  38. Absolute Dating

  39. Radioactive Decay • When an unstable radioactive element changes into a stable element

  40. Example Unstable Radioactive Isotope Stable Decay Product Nitrogen 14 (N14) Carbon 14 (C14) Uranium 238 (U238) Lead 206 (Pb206) Parent Daughter

  41. Radiometric Dating Half life: The time required for 1/2 of a parent material to break down to daughter material

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