Fossils: Evidence of Past Life

Fossils• Fossils are the remains or traces of

prehistoric life.

Fossils• Unaltered Remains• Remains of organisms that haven’t changed, or

have barely changed over time• Name some examples or unaltered remains…

Fossils• Altered Remains

• The remains of an organism are likely to be changed over time.

• Fossils can often become petrified (turned to stone)• Molds and casts are also common fossils.

Fossils • Indirect Evidence

• Trace fossils are indirect evidence of prehistoric life.

Fossils• Conditions Favoring Preservation

• Two conditions are important for preservation: rapid burial and the possession of hard parts.

Dating with Radioactivity

Dating with Radioactivity

Radioactivity is the spontaneous decay of certain unstable atomic nuclei.

•What is radioactivity?

Radioactivity

Dating with Radioactivity • A half-life is the amount of time necessary

for one-half of the nuclei in a sample to decay to a stable isotope.

Dating with Radioactivity • Every isotope decays\changes at a

constant rate• Radiometric dating is calculating a rocks

age by examining the amount radioactive isotopes present

• Radioactive isotopes decay (or change) into atoms called daughter products• Examples:

• Carbon-14 decays to Carbon-12• 1 half-life = 5,730 years

• Unranium-235 decays to Lead-207• 1 half-life = 713 million years

• Urannium-238 decays to Lead-206• 1 half-life = 4.5 billion years

Dating with Radioactivity

• Radiometric dating only works if the fossil has remained in a closed system since it became a fossil

The Half-Life Decay Curve

Radioactive Isotopes Frequently Used in Radiometric Dating

• Radiocarbon dating is done by comparing the amount of carbon-14 (an isotope of carbon) to the amount of carbon-12 (another carbon isotope) in a sample.

Dating with Radioactivity