Learning About Our Past

The Geologic Time Scale

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

swartpuntia

Fossil Seed Fern

330 mya

Crocodile Jaw

Prehistoric Wasp

95 mya

Theropod Tracks

Caturus 140 mya

Trex 65 mya

Coccolithicus pelagus

45 mya

Learning About Our Past

Fossils: Types and Conditions

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•Mummification: drying an organism so bacteria will not destroy it.

Preservation of Organisms•Amber: hardened tree sap that captures and preserves insects.

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.

Preservation of Organisms

• .

5 Types of Fossils•Original Remains

–Bones, shells, teeth

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

Which layer is older?

Which layer is younger?

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

Examples:

•U-238 = parent element

•Pb-206 = daughter element

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

Half-Life

•In conclusion = the more Pb-206 in a rock the older it will be .

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

Organism Dies

No New C-14

Living Organism

C-14

5,730 yrs later

50% C-14 11,600 yrs later

25% C-14

Let’s take some time to practice learning what is

meant by an element’s half-

life