Principles of Rocks

Metamorphic Rocks

A metamorphic rock is always older than the non metamorphosed rocks around it. The metamorphic rock must have formed before the surrounding rocks otherwise they would have been metamorphosed as well.

Principle of inclusion

A rock unit that contains inclusions must be younger than the rock unit from which the inclusions came.

Principle of crosscutting relations

An igneous rock unit or fault that cuts across another rock must be younger than the unit it cuts across. In other words, the one doing the cutting must come second, otherwise it wouldn't have anything to cut.

Sedimentary Rock

Formed by layers of sediment that after years of heat and pressure turned to rock

Igneous Rock

Formed from magma that cooled and hardened

Principle of faunal succession

Groups or assemblages of plant and animal fossils appear in the geologic record in a specific order. These assemblages can be used to identify certain periods of geologic time.

Principle of Superposition 1

In a sequence of undeformed sedimentary rock layers, the rocks get older from top to bottom. (bottom is oldest, top is youngest)

Principle of original horizontality 2

The principle of original horizontality states that sediments are deposited in horizontal layers that are parallel to the surface on which they were deposited. This implies that tilted or folded layers indicate that the crust has been deformed.

Principle of Superposition 2

The principle of superposition states that, in a series of undisturbed layers, the oldest layer is on the bottom and each overlying layer is progressively younger with the youngest layer on the top.

Principle of original horizontality 1

When sedimentary rock layers are being deposited, gravity forces them to be deposited as flat horizontal layers. Once the sediment has solidified and become rock, they can be tilted or folded.