1. Geologic Time (Rev.) Chapters 12 & 13

2. Scientists John Powell –Explored the Grand Canyon –Determined Earth’s history could be found in rocks –Realized Earth is much older than previously thought James Ussher –Made the first geologic and biologic timeline –Earth was created in 4004 BC

3. Scientists James Hutton –Published Theory of the Earth, within he stated that the ‘physical, chemical, and biological laws that operate today have also operated in the geologic past’ –…Uniformitarianism!! –Some processes today may have had less importance in the geologic past Nicolaus Steno –Described geologic observations based upon relative dating

4. Concepts – Steno’s Obs. Relative Dating –Identifying which rocks formed first –Tells us the sequence in which events occurred Law of Superposition –Each bed of rock in a sequence of layers is older than the layer above it –Rule also applies to lava flows and layers of ash

5. Which layer came first? A B C

6. Steno’s Observations Principle of Original Horizontality –Layers of sediment are generally deposited in a horizontal position –The Grand Canyon is a prime example –Sideling Hill is a “syncline” (folded, U-shaped layers)Sideling Hill –Other forces acted on the rock layers

7. Steno’s Observations Principle of Cross-Cutting Relationships –When a fault (or magma) cuts through rock layers, we can assume the fault or intrusion is younger than the rocks affected.

8. Steno’s Observations Inclusions –Pieces of one type of rock that are contained within another

9. Unconformities Disconformity – Two sedimentary rock layers that are separated by an erosional surface Nonconformity – Erosional surface separates older metamorphic or intrusive igneous rocks from younger sedimentary rocks Angular unconformity – Tilted or folded rocks are overlain by younger, flat layers

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11. Fossils Fossils are the remains or traces of prehistoric life Only found in sedimentary rock Three types: 1.Unaltered: teeth, bones, and shells. Flesh and other soft parts decompose 2.Altered: Remains can be petrified (turned into stone) 3.Indirect Evidence: Trace fossils (Impressions or footprints)

13. Fossils Two conditions are needed for fossils: –Rapid burial –Hard parts (bones, shells, etc.) Fur, flesh, & organs are eaten or decomposed Index fossils are used to correlate rocks of similar age in different regions. –An index fossil is a fossil associated with a particular span of time

14. Radioactive Dating Radioactivity –An atom of the same element can have different atomic masses –The number of neutrons can vary, the variants are called isotopes –Radioactivity is a process when the nucleus of an atom break apart, or decay –Half-life – amount of time necessary for ½ of the nuclei in a sample to decay to its stable isotope

15. Radioactive Dating Radiometric Dating –Use radioactive isotopes to calculate the age of a rock –Examples: Radioactive Daughter Half-life Parent Product Values (years) U-238 Pb-206 4.5 billion U-235 Pb-207 713 million Th-232 Pb-208 14.1 billion Rb-87 St-8747.0 billion

16. Carbon-14 Dating Carbon-14 is the radioactive isotope of carbon and is produced in the upper atmosphere When an organism dies the amount of C- 14 decreases ½ life = 5730 years Used to date recent geologic events!

17. Precambrian Time Time = 0 to 4 billion years Rocks –Mining of diamond, gold, silver, copper, and uranium provides rock samples from Precambrian time Early Atmosphere –The original atmosphere was composed of water vapor, carbon dioxide, nitrogen, and other gases –No oxygen!

18. Precambrian Time Evolution of atmosphere supporting life –The planet cooled  torrential rains  increased evaporation  increased cooling of the planet  oceans formed  water and carbon dioxide levels decreased  increased nitrogen levels First Lifeforms –Primitive plants released oxygen –By the end of Precambrian time ocean- dwellers required oxygen

19. Precambrian Time Fossils –Stromatolites are the most common Precambrian fossil Mounds of calcium carbonate Deposits from algae –Cyanobacteria Life beyond 3.5 bya Found in chert

20. Paleozoic Era Early Life (Cambrian, Ordovician, Silurian) –Cambrian Period Formation of the Appalachian Mtns. Age of the trilobites Hard parts (shells, etc…) –Ordovician Period Appearance of brachiopods –Attached to seafloor Cephalopods were the first large organisms –Silurian Period North America covered by shallow seas Corals and coral reefs

22. Paleozoic Era Late Life (Devonian, Carboniferous, Permian) –Devonian Period Age of fishes (armor plated organisms) –Carboniferous (Pennsylvanian, Mississippian) Formation of tropical swamps Large forests and trees Age of Amphibians (frogs, toads, etc…) –Permian Formation of Pangaea Mass extinction ended the Paleozoic Era

23. Paleozoic Era Paleozoic Extinction –Pangaea supercontinent formed –Changes in the world climate Drier climate –Most land and sea animals went extinct –Reptiles prospered Each period ends with a mass extinction: –Large and rapid die off of organisms

24. Mesozoic Era 248 million years ago to 65 mya Also known as the Age of Reptiles The Mesozoic Era includes the Triassic, Jurassic, and cretaceous periods This also noted the breakup of Pangaea At the start of the Mesozoic Era most of the earth was above sea level –Therefore there are few marine fossils

25. Mesozoic Era Tectonic activity started to increase As a result the Earth experienced many mountain building events, aka Orogeny –Orogeny, greek for “mountain generating” The orogenies continued through the Cretaceous period Gymnosperms – first seed-bearing plants that did not rely on oceans –Dominant plant during the Mesozoic

26. Mesozoic Era Dinosaurs dominated the earth for 100 million years Tyrannosaurus – large carnivore Brontosaurus – large herbivore Archaeopteryx – flying dinosaur It is believed a large meteor caused a mass extinction.

28. Chixculub Crater, Yucatan Peninsula

29. Cenozoic 65 mya to today Known as the age of Mammals or the Era of recent life. Two periods: Tertiary and Quarternary –Tertiary lasted 63 million years –Quaternary lasted 2 million years This is when mammals started to dominate life on land

30. Cenozoic Era Angiosperms- flowering plants with covered seeds Within the tertiary and quaternary periods are smaller periods called epochs –Holocene, Pleistocene, Pliocene, Miocene, Oligocene, Eocene, and Paleocene Large mammals started to dominate the Oligocene epoch –Mammoths, saber-toothed tigers, etc

31. Cenozoic Era Many large mammals went extinct in the late Pleistocene epoch Scientists are puzzled over why these animals disappeared One theory is early humans hunted them to extinction