GEOLOGIC HISTORY relative age -vs- absolute age

ABSOLUTE AGE DATING

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

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

counting tree rings.

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

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

During radioactive decay energy and subatomic particles are released.

Location of nuclear power plants

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

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.

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

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

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

# of Half-lives 1 2 3 4 1 1/2 1/4 1/8 1 1/2 1/4 1/8 1/16 parent 1/2 3/4 7/8 15/16 daughter C-14 Age 5,700 11,400 17,100 22,800 U-238 Age 4.5x109 9.0x109 13.5x109 18x109

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

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.

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

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

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

PRACTICE QUESTIONS ABSOLUTE AGE DATING

GEOLOGIC HISTORY

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.

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”.

“THE GEOLOGIC TIME SCALE” WORKSHEET “THE GEOLOGIC TIME SCALE”

“Landscape Regions and Bedrock Geology” WORKSHEET “Landscape Regions and Bedrock Geology”

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

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

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

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

Locate the Adirondack Mountains 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.

Precambrian represents 88% of Earth’s history.

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

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

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

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

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

coal deposits = swamps.

salt and gypsum = evaporating oceans. Dead Sea

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.

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

Mass Extinctions 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). extinctions allowed other species (mammals) to thrive and evolve. CLASSZONE.COM