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Learning About Our Past. The Geologic Time Scale. Four Major Divisions of Time. Pre-Cambrian – simple marine life Paleozoic – invertebrates Mesozoic – dinosaurs & reptiles Cenozoic – mammals. Pre-Cambrian Era. 4.6 bya to 544 mya Longest unit of geologic time Not much known - PowerPoint PPT Presentation
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Four Major Divisions of Time
Pre-Cambrian – simple marine lifePaleozoic – invertebratesMesozoic – dinosaurs & reptilesCenozoic – mammals
Pre-Cambrian Era
4.6 bya to 544 myaLongest unit of geologic timeNot much knownSponge fossil record
Deeply buried and changed by heat and pressure
Pre-Cambrian Era (cont’d.)
Cyanobacteria – produced oxygenOzone began formingSoft bodied simple organisms like jellyfish
existed
Paleozoic Era544 mya to 245 myaWarm shallow seasMarine life forms
Trilobites, Brachiopods, & CrinoidsPangaea forms
Paleozoic Era (cont’d.)Fish appearedAppalachian Mountains formedPlant & Animal life moved to landMass extinctions, tectonic activity & glaciers
Mesozoic Era245 mya to 65 myaDinosaurs & reptiles dominateBirds appearLittle mammals (non-dominant)Pine trees and flowering plantsPangaea breaks up
Cenozoic Era
65 mya to presentDinosaurs become extinctClimate cools; ice ages occurTHE AGE OF MAMMALS
Paleontology
•Study of fossils•Almost all fossils are found in sedimentary rocks
•Usually only hard parts of organism are saved but occasionally whole organisms remain
Preservation of Organisms•Tar Beds: thick petroleum
•Freezing: Low temperatures protect and preserve organisms
Conditions to Form Fossils•Sedimentary Rock•Buried quickly so that scavengers and micro-organisms don’t disturb the remains
•Hard body parts
5 Types of Fossils•Mold
–A cavity in a rock that has the shape of remains that were trapped there; water dissolved the remains away, leaving its imprint
5 Types of Fossils•Cast
–A type of fossil formed when an earlier fossil in the rock is dissolved away leaving the impression (mold) and NEW sediments or mineral crystals fill the mold
5 Types of Fossils•Carbonaceous film:
–A fossil impression in a rock, consisting only of a thin carbon residue that forms an outline of the original organism
5 Types of Fossils•Petrified Remains
–Plant or animal remains that have been petrified or “turned to rock”; this happens when minerals carried in groundwater replace the original remains.
Proof of the Geologic Time Scale•The Fossil Record•Determining the fossil record by
•Relative Age•Absolute Age
Relative Age
•Not an exact age; way of telling one layer of rock is older than another
•Law of Superposition–each sedimentary rock layer is older than the overlying younger layer
Relative Age•Sometimes due to crustal mov’t sedimentary layers aren’t always horizontal
•Law of Cross-cutting Relationships–States that a fault or intrusion is younger than rock layer it cuts through
Relative Age•Fossils help to determine a
rock layers relative ageA fossil is
any evidence of earlier life preserved in
rock.
• An index fossil is used to help determine relative age
3 overall characteristics1. Easily recognizable
2. Must be found everywhere 3. Lived for short time period
Absolute Age•When round about age isn’t good enough but need the actual date of the event
1. Tree Rings & Varves2. Radioactive Decay
Radioactive Decay•Most rocks contain elements that are radioactive. Since these elements give off particles and energy they will eventually form new nonradioactive elements.
Radioactive Decay•By comparing the amount of radioactive and nonradioactive elements scientists can predict the absolute age of a specific rock
Examples:•Uranium is commonly found in rocks
•U-238 will decay to form U-234 (releases 2 protons & 2 neutrons)
•It will eventually decay all the way to produce Pb-206
Half-Life•The decay of U-238 to Pb-206 is a very slow process.
•Rate of decay is constant•It takes 4.5 billion yrs for half of U-238 to decay
Carbon Dating•C-14 is a radioactive element found in ALL living things.
•When plant/animal dies C-14 decays to form N-14
•The half-life of C-14 is about 5,730 years
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