What is relative and absolute age A rock

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.