Fossilization and Rock

Dating

Fossils• 99.999% of organisms do not

become fossils, instead they decompose

• Fossil = preserved remains of ancient organisms

• Usually found in sedimentary rock

FossilsConditions needed for fossilization:

1. Made of hard parts (bone, shell)2. Quickly buried (mud, sand, volcanic

ash)3. Die in a low oxygen environment

Very specific criteria!! This is the reason why the fossil record is biased!!

FossilsTypes:

1. Petrified (wood, old bones)

2. Frozen (mammoths via glaciers)

Fossils3. Molds and Casts

-impressions left = mold

- impressions filled = cast

4. Amber- tree resin (sap) traps bugs, pollen, small

animals and turns to rock (Jurassic Park)

Fossils5. Asphalt/Tar

- oil near surface (saber tooth tigers, giant sloths)

6. Coprolites- fossilized excrement (can determine

size and diet)

Rock DatingEarth has changed through time!!

• Geologic record preserved in rock gives us the evidence

• Need to look through rock layers (strata) to decipher age relationships between rocks and fossils

Rock DatingMethods of dating rock and fossils include:

1. Relative Dating

2. Absolute/Radiometric Dating

Rock Dating1. Relative Dating

- use of location of rock layers to determine

age

- law of superposition = lower strata are older

while younger strata are higher up if strata are horizontal

- not as accurate

Rock DatingIndex/Guide Fossils

• Useful when trying to determine the age of a rock

• Each fossil assemblages stay in similar aged strata

Rock Dating2. Absolute/Radiometric Dating

- using radioactive isotopes found in rock to determine a rock or fossils age

- very accurate but must use correct isotopes and half-life

Rock DatingHalf-life

• Radioactive isotopes decay over time

• Decay rate depends on each isotope

• Half-life = the number of years it requires for half of radioactive isotopes to decay

Rock DatingRadioactive isotopes commonly used:

- Carbon-14 (5,730 yrs.)- Potassium-40 (1.227 billion

years)- Rubidium-87 (48.8 billion

years)