AGENDA Reading Preview 12.1 Fill-in Notes 12.1 Power notes Bill Nye Video 12.1 Work book

OBJECTIVES Describe the way fossils can form Identify the use of relative dating and absolute dating techniques

KEY CONCEPT Fossils are a record of life that existed in the past

Vocabulary Relative dating – estimate of age of a fossil based on the location of the fossil in strata. Radiometric dating – technique that measures the natural decay rate of isotopes to calculate the age of material. Isotope – form of an element that has the same number of protons but a different number of neutrons as another element Half-life – amount of time it takes for half of the isotope in a sample to decay into its product isotope

Fossils can form in several ways. Permineralization occurs when minerals carried by water are deposited around a hard structure.

A natural cast forms when flowing water removes all of the original tissue, leaving an impression.

Trace fossils record the activity of an organism.

Amber-preserved fossils are organisms that become trapped in tree resin that hardens after the tree is buried.

Preserved remains form when an entire organism becomes encased in material such as ice.

Specific conditions are needed for fossilization. Only a tiny percentage of living things became fossils.

Relative dating estimates the time during which an organism lived. It compares the placement of fossils in layers of rock. Scientists infer the order in which species existed.

Radiometric dating provides an accurate way to estimate the age of fossils. Radiometric dating uses decay of unstable isotopes. Isotopes are atoms of an element that differ in their number of neutrons. neutrons protrons

Radiometric dating uses decay of unstable isotopes. Isotopes are atoms of an element that differ in their number of neutrons. A half-life is the amount of time it takes for half of the isotope to decay.

Age of the Earth Scientists use radiometric dating on Meteorites thought to have been formed at the same time as the Earth. Uranium-to-lead isotope ratios Scientist estimate the Earth to be about 4.5 billion years old.