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)