1. The relative age of rocks
How old is the stuff around us?

2. What is relative age?
Relative age of a rock is established by comparing the rock to the other rocks around it. (An example is that one rock is older than another rock sitting right next to it).
The absolute age of a rock is established when the age of a rock is determined through scientific means. (Usually through carbon dating or argon testing).
One of the oldest rocks is a Moon rock called the “Genesis Rock” is dated at 4.1 billion years old.

3. The position of rock layers
The law of superposition states that relative age of rocks is established in horizontal layers by understanding the oldest layer is on the bottom. In rock that is layered, each higher layer is younger than the layer below it.

4. How to determine relative age?
Relative age is determined by geologist by studying extrusions, intrusions of igneous rock, faults, and gaps in the geologic record.
When lava cools on the surface of the Earth and becomes rock it is called an extrusion. Extrusion is always younger than the rock around it.
When magma pushes itself through rock layers and cools it is called an intrusion. Intrusions are always younger than the rock around them.
The faults found in the Earth is a crack or break in the rock which is younger than the rock because the rock is interrupted by the fault.
Gaps in the fossil record frequently are represented by a unconformity. An unconformity is when rock layers in the geologic record are lost because of erosion.
The top picture is an intrusion.
The bottom picture shows how faults can split rock.

5. Inclusions in rock and unconformity
An inclusion in rock is a small piece of rock or many pieces of rock which are suspended in a newer rock layer.
An unconformity is when layer of rock have been pushed up, then eroded so that layers of rocks on the surface have been eroded away.

6. Index Fossils
Fossils are used to tell the relative age of rocks by using index fossils.
These very special fossils are found all over the world and represent animals that have lived for a short period of time.
The index fossils can tell the age of the rock because the age of the animal is already known.