Relative Dating

Earth Science Grade 8

The Principle of Superposition: the rocks on the bottom of the column are older than the rocks at the top. The Principle of Original Horizontality: most sedimentary layers of rock are deposited in a horizontal (flat) position.

Youngest Rocks

Oldest Rocks

The Principle of Cross Cutting Relationships: Sometimes magma pushes, or intrudes, into cracks in existing rocks. When the melted rock cools and solidifies, the resulting feature is called an igneous intrusion.

Plate boundaries: Motion along faults causes rock layers to be offset from each other. This can cause the flat layers of rock to shift off kilter or cause “bending” of the rock.

Compression or mountain buildingConvergent boundary

Strike/Slip Transform Plate Boundary

Unconformities: Sometimes, an area where layers of sediments have been deposited is uplifted, and the surface experiences erosion. If more layers of sediments are subsequently added on top of this erosion surface, the resulting feature is called an unconformity.

Notice how the layers are labeled oldest (A) to youngest (E)

See if you can identify which layers are the oldest to youngest:

A

BC

DE

F

Oldest Youngest

See if you can identify which layers are the oldest to youngest:

A

BC

DE

F

Oldest Youngest

E -> C -> B -> A -> D -> F

ABDEC

ADCBE

BAC

EIDHBFACG

EBIFGCAHD