Geologic History

Law of Superposition

The geologic principle that states that in horizontal layers of sedimentary rock, each layer is older than the layer above it and younger than the layer below it.

Cross Cutting Principle

When something cuts across a body of rock, that "something" is younger than the rock it cuts across.

igneous intrusion

where magma cut through other rock layers underground

fault

a crack in the earth's crust

Unconformity

A break in the geologic record created when rock layers are eroded or when sediment is not deposited for a long period of time.

inclusions

Pieces of one rock unit that are contained within another; the rock embedded are older than the layer that contains them.

radioactive decay

The breakdown of a radioactive element, releasing particles and energy

parent isotope

the isotope that undergoes decay; its unstable and radioactive

daughter isotope

the stable isotope produced by the radioactive decay of the parent isotope

half-life

length of time required for half of the radioactive atoms in a sample to decay

fossils

Preserved remains of once-living organisms

Conditions for fossilization

- Suitability of the remains (hard parts)

- Quick burial

- Favorable medium (what its preserved in)

- Minerals must be present

- time

Environments for fossilization

-Marine environments

-Volcanic ash layers

-Tar pits and peat bogs

index fossil

fossils of widely distributed organisms that lived during only one short period

Molds and Casts

Shells often leave empty cavities called molds within hardened sediment. When sand or mud fills a mold and hardens, a natural cast forms.

amber fossil

small organisms become trapped in tree sap and over time the sap hardens

ice fossilization

organisms trapped in ice

petrified fossils

fossils in which minerals replace all or part of an organism

Index Fossil Correlation

Even though rock layers some distance away from each other might have a different name, if there is the same index fossil in each of those layers, it indicates that it is the SAME age rock layer.