The Age of Rocks and Fossils Sections 8.2 and 8.3 Page 325-334 March 1, 2011

Students should be able to answer: How do geologists determine the relative and absolute ages of rocks? How are index fossils useful to geologists? What is the law of superposition? What is radioactive dating? Key terms: relative age, absolute age, law of superposition, extrusion, intrusion, fault, unconformity, index fossil, radioactive decay, half-life

Relative Age vs. Absolute Age The relative age of a rock is its age compared to the ages of other rocks. The absolute age is the number of years since the rock formed.

Is this absolute age or relative age? My sister is 18 years old. I am 23. A) Absolute age B) Relative age

Is this absolute age or relative age? My brother is younger than my sister. A) Absolute age B) Relative age

Is this absolute age or relative age? That dinosaur fossil is much older than that fish fossil. A) Absolute age B) Relative age

Is this absolute age or relative age? That dinosaur fossil is about 70 million years old, but that fish fossil is only 500,000 years old. A) Absolute age B) Relative age

How do rock layers help us determine the age of rocks? The law of superposition says that the oldest rock layers are on the bottom of horizontal rock layers. Each higher layer is younger than the layer below it.

Which layer would be the oldest?

Which rock layer would be the youngest?

Intrusions and extrusions Magma cools and hardens into an intrusion. An intrusion is always younger than the rock layers around and beneath it.

Lava that hardens on the surface is called an extrusion. An extrusion is always younger than the extrusion below it.

Faults A fault is a break in the Earth’s crust. A fault is always younger than the rock it cuts through.

Unconformity The surface where new rock layers meet a much older rock surface beneath them is called an unconformity. Layers may have been lost to erosion.