A Geologic Time Chart from Rockman

Rockman's Geologic Time Chart

People who study rocks are called geologists. Those who study fossils are called paleontologists. The one thing they have in common is an interest in learning more about the age of the rocks that make up the earth. By doing so, they are better able to learn about and to understand how and when the earth was formed and about what kinds of plants and animals were present during different periods of time. The study of Geologic Time in relation to the age of the earth is called Geochronology.

Fossils are the remaining evidence of life that once flourished. As a general rule, fossils are found in layers of sedimentary rocks originally deposited by the erosive action of water. For additional information about how different types of rocks are formed, visit our Rock Cycle page.

Own a genuine piece of geologic time, check out our Fossil Page.

The use of modern technology in measuring the rate of radioactive decay in a rock allows scientists to fairly accurately determine its age. While discrepancies of 2 to 10 million years in determining the age of a rock are not unusual, in terms of geologic time, such discrepancies are relatively minor. Correlating that information with knowledge about fossil remains found in layers of sedimentary rock has allowed us to develop what we commonly refer to as the Geologic Time Scale

The chart presented here is a representation of current consensus about geologic time. We hope it will be useful.

Geologic Time is subdivided into a number of categories. The major categories (presented here) are:

The most recent Geologic Time is at the top of the chart; the oldest at the bottom. Located in the lower right hand corner of some boxes on the chart you will find small numbers. Those numbers represent the number in millions of year ago that one geologic time period ended and another began. For example - in the corner of the box labeled "Pleistocene", you will find the number 1.8 which indicates that the Pleistocene epoch began 1.8 milion years ago. The epoch following the Pleistocene (the Pliocene) began 1.8 million years ago and ended about 5 million years ago.

The Cenozoic Era - is the most recent of the three major subdivisions of geologic and animal history. It spans about 65 million years from the end of the Cretaceous Period to the present and is divided into two main subdivisions: the Quaternary and Tertiary Periods.
The Quanternary Period- the present to 1.8 million years ago (MYA) and divided into two Epochs; the Holocene (present - 11,000 years ago) and the Pleistocene (11,000 years ago-1.8 MYA). Since it is believed that humans first appeared during the Holocene Epoch, the Quanternary Period is commonly referred to as the "Age of Man."
The Tertiary Period - 1.8 to 65 MYA; the longest of the Cenozoic periods.
The Pliocene Epoch- 1.8 to 5 MYA; the Panamanian land-bridge between North and South America appeared; accumulation of ice at the poles led to the extinction of most species living there; the advance of glaciers and the beginning of the ice ages.
The Miocene Epoch- 5 to 23 MYA; warming global temperatures; enormous spread of grasslands and savannas.
The Mesozoic Era- 65 to 245 MYA; three periods: the Cretaceous (65-146 MYA), Jurassic (146-208 MYA) and Triassic (208-245 MYA); world fauna changed drastically; reptiles abundant.
The Cretaceous Period- 65-146 MYA; although new dinosaurs appeared during this time, known as the last of the "Age of Dinosaurs"; first primates, angiosperms appear; breaking apart of the world-continent Pangaea begins.
The Jurassic Period- 146-208 MYA; great plant-eating dinosaurs; lush growths of ferns and palm-like cycads; smaller but vicious carnivores stalking the great herbivores; oceans full of fish, squid, and coiled ammonites; first frogs, salamanders, crocodiles, flying reptiles, birds; oilfields of the North Sea are Jurassic in age.
The Triassic Period- 208-245 MYA; Pangaea in existence altering global climate and ocean circulation; the appearance of modern conifers, cycadeoids; first turtles, lizards, mammals, dinosaurs.
The Paleozoic Era- 245-544 MYA; two of the most important events in the history of animal life occurred during this period: (1) at the beginning of the period a dramatic "explosion" of diversity in animal life occurred and (2) near the end of the period, the largest mass extinction in history wiped out approximately 90% of all marine animal species; six major continental land masses existed.
The Pennsylvanian Period- 286-325 MYA; first conifers, insects, reptiles.
The Mississippian Period- 325-360 MYA; first seed ferns; belemnoids
The Cambrian Period- 505-544 MYA; gastropods, bivalves, trilobites, brachiopods appear.
The PreCambrian Eon- 544 MYA to the beginning of time; makes up roughly seven-eighths of the Earth's history; geologists believe the Earth was formed during this Eon of time and that the first life appeared; tectonic plates were present and began moving; the atmosphere became enriched in oxygen and just before the end of the Precambrian Eon (based on fossil evidence), complex organisms including the first animals appeared.
The Proterozoic Era- 544 MYA to 2.5 BYA (billion years ago); stable continents appeared; first abundant fossils of living organisms (primarily bacteria and archaeans) are found in layers of rocks from this Era; first evidence of oxygen build-up in the atmosphere appeared causing an enormous global catastrophe that doomed many existing bacterial groups; first sponges, jellyfish.
The Archaean Era- 2.5 to 3.8 BYA; true beginning of geological history of the Earth; the atmosphere was likely composed of methane, ammonia, and other toxic gases; the Earth's crust began cooling and rocks and continental plates began to form; the oldest known fossils come from South Africa and Western Australia and date to roughly 3.5 BYA. They consist of bacteria microfossils which were probably the Earth's only inhabitants for more than a billion years of this Era.
The Hadean Era- Hadean time is not a geological period as such. During the first 800 million or so years of its history, the Earth would likely have been molten and was constantly being bombarded by meteorites and asteroids. With the exception of meteorites that probably struck the Earth as it was being formed, no rocks on the Earth are this old. Some igneous rocks have been Radiometrically dated from the time of solidification of the Earth. This explains why the oldest meteorites and lunar rocks are about 4.5 billion years old, but the oldest Earth rocks currently known are 3.8 billion years old. Once the formation of the Earth was complete, solid rock formed, and geological history began.