HOW IS A FOSSIL FORMED?

1. SedimentAn animal is buried by sediment, such as volcanic ash or silt, shortly after it dies. Its bones are protected from rotting by the layer of sediment.

4. ErosionErosion from rain, rivers, and wind wears away the remaining rock layers. Eventually, erosion or people digging for fossils will expose the preserved remains.

2. LayersMore sediment layers accumulate above the animal’s remains, and minerals, such as silica (a compound of silicon and oxygen), slowly replace the calcium phosphate in the bones.

3. MovementMovement of tectonic plates, or giant rock slabs that make up Earth’s surface, lifts up the sediments and pushes the fossil closer to the surface.

FIVE MAIN TYPES OF FOSSILS

PetrifiedFossils

Molds andCasts

CarbonFilms

TraceFossils

PreservedRemains

Relative age

• Age of rocks and geologic features compared with other rocks and features nearby

10.3 Absolute age dating

Relative vs. Absolute Age Dating

Relative• Determines ages of rocks in

order that they were formed but without exact dates

Absolute• Determine absolute date of

the rock

Radiometric dating• Isotopes = atoms of the

same element that have the same number of protons but different number of neutrons

Carbon-14 dating• Radioactive decay: Some isotopes are

unstable, emit radiation and decay into “daughter” isotope– Carbon-14 is unstable isotope and decays into

nitrogen-14

• Over time: – The amount of “parent” isotope, carbon-14,

decreases– The amount of “daughter” isotope, nitrogen-14,

increases– The ratio of the two indicates the age of the rock

• Half life: time required for half of parent isotope to decay into daughter isotopes– E.g. Half life of carbon 14 is approx. 5730 years

Types of dating

• Radiocarbon dating: organic material (bones, wood, charcoal)

• Rock dating: living tissues replaced by minerals (e.g. U 235 Pb-207)

Types of dating

• Radioactive dating game• Quiz next class