Geologic Time Scale

Front 1

Permian/Triassic Boundary

Back 1

The time break when up to 90% of all species (especially those in the marine realm) went extinct at around 251 Ma. This constitutes the most significant mass extinction in Earth history.

Front 2

Cretaceous/Tertiary Boundary

Back 2

The time break at which the dinosaurs and many other organisms went extinct, perhaps as the result of a bolide impact (to make the Chicxulub Crater). This is the infamous "KT boundary"!

Front 3

PreC/Cambrian Boundary

Back 3

The time break at which many complex, multi-cellular animals with shells and skeletons first appeared in the fossil record.

Front 4

Hadean

Back 4

The first time slice (Eon) of the Precambrian. We know very little about this very old time slice because very few rocks exist from it (no Earth rocks--just a few meteorites and zircon mineral grains)! It begins with the origin of the Earth and Moon, differentiation of the core and mantle, and formation of the earliest atmosphere.

Front 5

Phanerozoic

Back 5

The Eon when life became extremely diverse and abundant, with millions of different forms living in the sea and then on land. The word literally means "visible life".

Front 6

Archean

Back 6

An Eon of the Precambrian. During this lengthy time, life got started (with single-celled fossils and stromatolites), continental crust formed and cratons developed.

Front 7

Proterozoic

Back 7

Another Eon during Precambrian time. During this time, and oxygenated atmosphere formed (recorded by BIF's), ancient super-continents (such as Rodinia) assembled and broke up, continental glaciations ebbed and flowed, and sex (!!) came about.

Front 8

Precambrian (PreC)

Back 8

The time slice that makes up the vast majority of geologic time (88+%). Life, the atmosphere, oceans, tectonics, and continents originated during this time.

Front 9

Cenozoic

Back 9

Mammals become the dominant life form during this "new life" Era. We are currently living in this Era, as well. Ice caps come and go and the Alpine-Himalayan Chain formed and continues to do so.

Front 10

Mesozoic

Back 10

The Era the occurred between 251 and 65 Ma. Its root words mean "middle life". Reptiles, especially dinosaurs, ruled during this time. Pangea was intact at the beginning but broke apart during this time.

Front 11

Paleozoic

Back 11

Shelled organisms (invertebrates) and the first land plants all appeared during this "old life" Era. The age of fishes and amphibians occurred during this time.

Front 12

Ordovician

Back 12

The Period when fish show up in the fossil record. They began to radiate at this time. A major extinction event occurred near the end of this period. Also, the Taconic Orogeny occurs, signaling the beginning of Appalachian Mountain formation.

Front 13

Cambrian

Back 13

The Period that began about 542 Ma. An "explosion" of life occurred during this first Period of the Paleozoic (all major phyla of animals, except one, make their appearance during this time).

Front 14

Carboniferous

Back 14

A period of extensive plant-life preservation in coastal swamps, especially in Pennsylvania (but also in Europe). Many carbon-rich coal beds were deposited during this time. Reptiles also make their appearance in the fossil record.

Front 15

Silurian

Back 15

The Period that marks the appearance of vascular plants. Large reefs grew in shallow seas. Also, fishes were proliferating during this time.

Front 16

Devonian

Back 16

The Period that marks the appearance of animals (insects and amphibians) onto land. A major extinction occurred near the end of this Period (environment got bad...). Also, the Acadian Orogeny occurred, marking the second major phase of Appalachian Mountain building.

Front 17

Permian

Back 17

The Period when Pangea was fully assembled into a supercontinent to make the Appalachian Mountains. This orogeny is called the Alleghanian. Also, a super-mega-major mass extinction occurred at the end of this Period.

Front 18

Triassic

Back 18

Dinosaurs (and other reptiles), as well as mammals, became important life forms in this Period.

Front 19

Jurassic

Back 19

A movie was made, making this the most well known Period of all!! Stegosaurus was around at this time, but Tyrannosaurus Rex was not. Also, Pangea was breaking up at this time, and the Atlantic Ocean began to open.

Front 20

Cretaceous

Back 20

The last Period that dinosaurs (such as T-Rex) were alive and kicking. The Rocky Mountains began forming during this time, as the result of the Sevier and Laramide orogenies. Flowering plants also emerged.

Front 21

Quaternary

Back 21

The Period we are currently living in. This Period began about 2.6 million years ago. Its root means "fourth", and the named of this time slice is a relict from the 19th century when the time scale was first devised.

Front 22

Tertiary

Back 22

The first Period after the extinction of the dinosaurs. The Alps and Himalayas started to rise as the Tethys Sea closed and continents collided. The Laramide Orogeny ended in the west, with plate motions changing to cause extension and shearing that formed the Basin and the Range and San Andreas. Also, primates, horses, whales and other mammals diversified and climates fluctuated. This Period is now split into the Paleogene and Neogene periods.

Front 23

Pleistocene

Back 23

The Epoch of widespread glaciation known as the "Ice Ages". Many landforms from this event can be found throughout North America, including the moraines of northwestern Pennsylvania. Climates changed wildly, sea level rose and fell, and organisms tried to keep up with the changes.

Front 24

Holocene

Back 24

The Epoch we (being Homo sapiens) are currently living in. By the way, insects rule! Some consider this a time of human-induced mass extinction, thus the proposal to break out a time slice called the Anthropocene. Will the end of this time slice be defined by our own extinction? Who will define it?