Geologic Time Scale

Era

a major division of geological time. Divided up into smaller divisions called periods.

Period

subunit of era

Cenozoic

Era including the Paleogene and Neogene periods, marked by mammals, angiosperms and humans, began 65.5 M years ago

Paleozoic

an era occurring between 570 million and 230 million years ago, characterized by the advent of fish, insects, and reptiles

Mesozoic

An era with the Triassic, Jurassic and Cretaceous periods from 251 to 65.5 M years ago, marked by dinosaurs, gymnosperms and angiosperms, as well as the mass extinction at the end of the period

Eon

the largest division of geologic time

Epoch

subunit of a period

Radioactive Decay

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

Half-life

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

Radiometric dating

determination of absolute ages of rocks and fossils through calculations based on a radioactive isotope's fixed rate of decay

Fossil

preserved remains

Fossil dating

Comparing the fossil to the dates of known fossils found in the same rock layer or those found in the layer above and below

Precambrian

Name for the time in earths early history that accounts for ninety percent of earth's time, but only cellular organisms lived.

Cambrian explosion

large hard-bodied forms of animals appeared in the fossil record 535-525 million years ago.

Natural Selection

Survival of the fittest

Evolution

Change in a kind of organism over time; process by which modern organisms have descended from ancient organisms.

Index fossil

a fossil known to have lived in a particular geologic age that can be used to date the rock layer in which it is found

Principle of Superposition

oldest rocks are found below younger rocks.

Principle of Cross-cutting relationships

Anything that cuts through a rock layer is younger/newer than the rock layer.

Principle of horizontality

layers of sediment are generally deposited in a horizontal position. All rock layers are originally flat.

Principle of uniformitarianism

the same geologic processes that happened in the past still happen today.

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

Igneous intrusion

magma cutting through formed layers of sedimentary rock that then cools and hardens

Carbon-14

Radioactive form of the element carbon that has been used to determine the absolute age of recent fossils and geologic events.

Principle of included fragments

Pieces contained in a rock are older than the rock layer itself.

What are the 3 Eras in the Phanerozoic Eon

Paleozoic, Mesozoic, Cenozoic