GEOLOGIC HISTORY relative age vsabsolute age 1 ABSOLUTE

GEOLOGIC HISTORY relative age -vsabsolute age 1

ABSOLUTE AGE DATING 2

Absolute (or actual) Age Dating CLASSZONE. COM absolute age Ø actual age in years. 3

Age can be determined by…. Ø counting lake varves (annual lake sediment). 4

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Ø counting tree rings. 6

Ø most common method is using radiometric dating. (measuring the amount of a particular radioactive element in a rock). 7

Radioactive Decay is… Øthe natural and spontaneous breakdown, Øof the nucleus, Øof unstable (parent) atoms into more stable (daughter) atoms. 8

During radioactive decay Øenergy and subatomic particles are released. 9

Location of nuclear power plants 10

Øthis occurs at a constant rate, which can not be changed. 11

The radioactive isotope (Parent material) will break down naturally into another element called the decay product (Daughter material). The rate of radioactive decay is measured in terms of half-lives. 12

half-life Øthe amount of time it takes for half of the radioactive element to decay. 13

The age of a rock can be determined by comparing…. Øthe relative amount of the Undecayed substance (radioactive “parent” material) Øto the relative amount of Decay product (stable “daughter” material). 14

COMPLETE EXAMPLE Ø Color in the bars. Shade in the part of the bar that represents the amount of parent material (undecayed). Ø Using fractions, write down the amount of parent and daughter in each bar Ø Use the number of half-lives to determine the age of the sample. 15

# of Half-lives 1 2 3 4 1/2 1/4 1/8 1/16 0 1/2 3/4 7/8 15/16 0 5, 700 11, 400 17, 100 22, 800 0 4. 5 x 109 9. 0 x 109 13. 5 x 109 16 0 1 1 1/2 1/4 1/8 0 parent daughter C-14 Age U-238 Age 18 x 109

Graph Example: 100 % parent 0 % daughter 50 % parent 50 % daughter 25 % parent 75 % daughter 0 half-life 1 half-life 2 half-lives Draw Graph 100 parent = percent 75 daughter = 50 25 0 0 2 17

The more daughter material in the sample, the older the sample. The amount of time for a half-life is different for each radioactive substance. See ESRT. 18

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Carbon -14 ……. . decays to …… Nitrogen-14 half-life = 5700 years. Uranium-238……. decays to……… Lead-206 half-life = 4, 500, 000 years. 21

Radioactive substances with short half-lives, such as C-14, are good for dating recent organic remains. 22

Those with longer halflives, such as U-238, are useful for dating older rocks. 23

PRACTICE QUESTIONS • ABSOLUTE AGE DATING 24

GEOLOGIC HISTORY 25

Geologic History from the Rock Record • some life forms exist only during specific intervals of time. • therefore, fossils in rock can be used to determine relative age. • ex) dinosaurs existed only during the Mesozoic Era. 26

Geologic Time Scale • geologists have used fossil evidence to divide 4. 6 billion years of Earth’s history into smaller units. • it is these divisions that make up the “Geologic Time Scale”. • see; ESRT p. 8 & 9, “Geologic History of New York State”. 27

WORKSHEET “THE GEOLOGIC TIME SCALE” 28

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WORKSHEET “Landscape Regions and Bedrock Geology” 38

SYRACUSE Landscape region Erie-Ontario lowlands Age of bedrock Silurian Rock types L. S. , Shale, S. S. , cong. Fossils Hexameroceras, Eucalyptocrinus 39

JAMESTOWN Landscape region Allegheny Plateau Age of bedrock Devonian Rock types L. S. , Shale, S. S. , cong. Fossils Phacops, Centroceras 40

WATERTOWN Landscape region Erie-Ontario lowlands Age of bedrock Ordovician Rock types L. S. , Shale, S. S. , dolostone Fossils Cryptolithus, Valcouroceras 41

OLD FORGE Landscape region Adirondack Mountains Age of bedrock Middle Proterozoic Rock types Gneisses, quartzites, and marbles Fossils none 42

Locate the Adirondack Mountains. List two reasons for why you would not be able to find any fossils at this location. 1 – They are composed of metamorphic rocks. Only sedimentary rocks contain fossils. 2 – Precambrian life was mostly single cell and/or lacked hard parts = not easily preserved as fossils. 43

Precambrian Ø represents 88% of Earth’s history. 44

Ø fossils in these rocks are rare and difficult to identify; 1) earliest life was small and lacked hard parts. 2) these rocks may have been eroded away. cyanobacteria 45

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Uniformity Ø“the present is the key to the past”. Øassumes that the geologic processes occurring today also occurred in the past. Øtherefore, we can interpret past events by examining what is happening today. 47

EVOLUTION OF EARTH AND LIFE Fossils Ø a wide variety of life forms have lived over time. Anomalocaris canadensis Recent stromatolites at Hamelin Pool, Aus Burgess Shale Hallucigenia sparsa 48

Ø most of these life forms are now extinct. Since, the chances of fossilization are low…. Ø most forms of past life probably have not been identified. Ø we compare fossils to similar life forms that exist today. 49

Environmental change. fossil evidence; Ø corals = shallow, warm ocean water. 50

Ø coal deposits = swamps. 51

Ø salt and gypsum = evaporating oceans. Dead Sea 52

Ø Ø Ø environments have changed as the Earth’s lithospheric plates have moved. as a landmass moves closer to the equator, its climate gets warmer. plate collisions form mountains. 53

Evolution and Extinction organic evolution Ø Ø Ø theory that life forms change through time. offspring with favorable variations will survive. offspring with unfavorable variations will become extinct. 54

Mass Extinctions Ø Ø periods of time when large numbers of species went extinct. believed to have been caused by catastrophic events…. impact events. example) K-T boundary, 65 mya, 70% of all species went extinct (including the dinosaurs). CLASSZONE. COM Ø extinctions allowed other species (mammals) to thrive and evolve. 55