What is relative and absolute age?

• A rock layer’s relative age is its age compared to the ages of other rock layers.

• Generally (relative) versus specifically (absolute)…

• Example: you– 11 – 13 years

– birthday

What is the Law of Superposition?

• In horizontal sedimentary rock layers, the oldest layer is at the bottom. Each higher layer is younger than the layers below it

• EX: (your experience, new question, or WU answered.)

What other clues help to find relative age?

• Clues from igneous rock: extrusions (hardens on surface) and intrusions (hardens beneath surface) are always younger.

• EX: (your experience, new question, or WU answered.)

• Clues from faults: a fault is always younger than the rock it cuts through.

• EX: (your experience, new question, or WU answered.)

• Clues from an unconformity: where an old, eroded rock surface is in contact with a newer rock layer.

• An unconformity is a gap in the geologic record where some rock layers have been lost because of erosion.

• EX: (your experience, new question, or WU answered.)

• Clues from fossils: index fossils (widely distributed, existed only briefly)

To date rock layers, geologists first give a relative age to a layer of rock at one location. Then they can give the same age to matching layers of rock at another location.