CLUES TO EARTH'S PAST

Fossil 


remains, imprints, or traces of prehistoric organisms that can tell when and were the organisms once lived and how they lived

permineralized remains

fossils in which the spaces inside are filled with minerals from groundwater

carbon film

thin film of carbon residue preserved as a soil

Mold
a type of body fossil that forms in rocks when an organism with hard parts is buried, decays or dissolves, and leaves a cavity in the rock

Cast

a type of body fossil that forms when crystals fill a mold or sediments wash into a mold and harden into rock

Index fossils

remains of species that existed on earth for relatively short period of time and can be used by geologists to assign the ages of rock

principle of superposition

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

relative age

the age of something compared with other things

unconformity

gap in the rock layer that is due to erosion or periods without any deposition 

absolute age

age, in years, of a rock or other object; can be determined by using properties of the atoms that make up materials. 

radioactive decay

process in which some isotopes break down into other isotopes and particles

half-life

time it takes for half the atoms of an isotope to decay

radiometric dating

process used to calculate rhe 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