Sections 8.2 and 8.3 Page

 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

 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.

 My sister is 18 years old. I am 23.  A) Absolute age  B) Relative age

 My brother is younger than my sister.  A) Absolute age  B) Relative age

 That dinosaur fossil is much older than that fish fossil.  A) Absolute age  B) 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

 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.

ABCABC A B C

ABCABC A B C

 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.

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

 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.