Finding the Relative Age of Rocks
Relative and Absolute Ages
_________ Age The age of a rock compared to the ages of rock layers.
 
_____________ Age The age of a rock given as the number of years since the rock formed. 

LAW OF ______________________________ in horizontal sedimentary rock layers, the oldest layer is at the bottom. Each higher layer is younger than the layers below it.

Other Clues to Relative Age
•	Clues from Igneous Rocks:
	1.  Lava that hardens on the surface is called an _____________________
             
  2.  Magma that cools and pushes into bodies of rock and hardens is called an ____________________________
•	Clues from Faults:

____________: is a break in the Earth’s crust. 
•	Forces inside the Earth cause movement of the rock on _____________ sides of a fault.
•	Fault is _____________________ than the rock it cuts through. 

Review
Geologists use the Relative and Absolute Age of rocks to determine ____________.
Gaps in the geologic record occurs when sedimentary rock layers are not ________________.
_____________________ is a gap in the geologic record. An unconformity shows where some rock layers have been lost because of _________________.

Using Fossils to date rocks
Certain fossils, called ________________________help geologists match rock layers.
Index fossils – fossils of widely distributed organisms that lived only during one __________________________.
Example of an Index Fossil:
•	__________________ (hard shelled animals whose bodies had three distinct parts. 
•	Trilobites evolved in _______________ seas more than 500 million years ago.

To become an Index Fossil … ____________________________________________________________________________________________________________________________________

Example – trilobite type with large eyes
These large-eyed trilobites survived for a time AFTER other bites became _______________.
If a geologist finds large-eyed Trilobites in a rock layer, the geologist can infer that those rocks are younger than rocks containing other types of trilobites

How do scientists measure the absolute age of rocks?

________________________ the process where an unstable element becomes stable

______________________________ looks at how much radioactive decay has occurred in an object to determine _________.
Half life = the time it takes for half of the ____________________atoms to decay