ScienceEarth’s Changing

SurfaceChapter 4

Section 2

Finding the Relative Age of Rocks Relative means compared to others The relative age of a rock does not provide

its absolute age The absolute age of a rock is the # of yrs

since the rock formed

The Position of Rock Layers The Law of Superposition is used to

determine the relative ages of sedimentary rock layers In horizontal sed rock layers, the oldest layer is

@ the bottom Each higher layer is younger than the layers

below it.

Other Clues to Relative Age Extrusion is lava that hardens on the

surface The rock layers below an e are always older

Intrusion is where the magma cools & hardens into a mass of igneous rock The i is always younger than the rock layers

around or beneath it

A fault is a break in E’s crust

Gaps in the Geological Record The surface where new rock layers meet a

much older rock surface beneath them is called unconformity This is a gap in the geological record

Using Fossils to Date Rocks Index fossils help geologists match rock

layers To be useful as an index, a ‘f’ must be

widely distributed & rep a type of org that existed only briefly

I.F. are useful because they tell the relative ages of the rock layers in which they occur