Words to Know to Interpret the Relative Ages of a Geologic Profile
Law of Superposition states that the rock layer on the bottom is the oldest and the layers get
younger as you move up the profile. This is true for any undisturbed rock exposure.
Igneous Intrusions are formed when magma is injected into older rock layers in the crust.
Therefore they are younger than the rock they are found in. Look for contact metamorphic rock
in the layer above and below the intrusion.
Igneous Extrusions are rocks that formed from lava on the surface of the earth. They are
younger than the rock layers below. Look for contact metamorphic rock on the bottom only.
Folds are bends in the rock layers. They occur after the rock layers formed.
Faults are cracks in the rock layers where some movement has taken place. Faults produce
offset layers.
Unconformity, a buried erosion surface, is formed when an area of the crust was uplifted above
sea level and then eroded. Some time after that the area subsided below sea level and new
sediments were deposited on top of the eroded surface
______________________________________________________________________
Uniformitarianism: the present is the key to the past
Original Horizontality: sedimentary layers are initially deposited in horizontal layers
Superposition: the layer on the bottom is the oldest, and they get younger as they pile up.
Cross-cutting relationships: the thing being cut is older than the thing doing the cutting.
______________________________________________________________
ANGULAR UNCONFORMITIES
DISCONFORMITIES
NONCONFORMITIES