Clues to Earth's Past

Fossil:

Remains, imprints, or traces of prehistoric organisms.

Permineralized remains:

Fossils in which the spaces inside are filled with minerals from groundwater.

Carbon film:

A thin film of carbon residue left, that forms a silhouette of the original organisms.

Mold:

A type of body fossil that forms in rock when an organism is buried, decays or dissolves and leaves a cavity in rock.

Cast:

Type of fossil that forms when sediments or water enter the cavity, form new rock, and produce a copy of the original object.

Index fossil: 

Remains of species that existed on Earth for relatively short periods of time, were abundant and were widespread geographically.

Principle of superposition:

States that in undisturbed rock layers, the oldest rocks are on the bottom and the rock become progressively younger toward the top.

Relative age: 

The age of something compared with other things.

Unconformity: 

Gaps in rock sequences in the rock layer due to erosion or periods without any deposition.

Absolute age:

The age, in years, of a rock or other object.

Radioactive decay:

Process in which some isotopes break down into other isotopes and particles.

Half-life: 

Time it takes for half of the atoms in the isotope to decay.

  

Radiometric dating: 

Process used to calculate the absolute age of rock by measuring the ratio of parent isotope to daughter product in a mineral and knowing the half life of the parent.

Uniformitarianism: 

Principle stating that Earth processes occuring today are similar to those that occured in the past.