Relative Dating not to be confused with dating

Relative Dating (not to be confused with dating your relatives!)

Fossil Record • Fossils help to determine a rock layers’ relative age • An index fossil is used to help determine relative age 3 overall characteristics 1. Easily recognizable 2. Must be found everywhere 3. Lived for a short time period

Determining the fossil record by… • Relative Age • Absolute Age

Relative Age • Not an exact age; way of telling one layer of rock is older than another • Laws and rules govern how you determine the relative ages of rock layers and things that you find around them.

Laws and Rules • Law of Horizontality –Rock layers are laid down horizontally – never on a slant • Law of Superposition –each sedimentary rock layer is older than the overlying younger layer

Superposition is well illustrated by the strata in the Grand Canyon

Laws and Rules • Sometimes due to crustal movement sedimentary layers aren’t always horizontal • Law of Cross-cutting Relationships –States that a fault or intrusion is younger than rock layer it cuts through

Laws and Rules • Igneous Intrusion –occurs when magma squeezes into or between layers of preexisting rock.

Recognizing laws & rules

Laws and Rules • Included Fragments –Sometimes other things found in the rock layer give you clues about its age • pieces of rock found IN another rock must be OLDER (formed first).

Laws and Rules • Unconformity –a buried surface of erosion separating two rock masses representing a gap in geologic time. . .

Horizontality, Included Fragments, AND… an UNCOMFORMITY

Practice: what happened here?