Age of the Earth

By studying the geology in the form of rocks and fossils scientists have been able to track the events that have occurred on our planet since it was first formed approximately 4,600 million years ago.

Geological time scale

EON (Phanerozoic - Proterozoic)
Two or more geological eras form an Eon, which is the largest division of geologic time, lasting many hundreds of millions of years.

ERA (Proterozoic - Paleozoic - Mesozoic - Cenozoic)
Two or more geological periods comprise an era, which is hundreds of millions of years in duration.

PERIOD (Cambrian-Ordovician-Silurian-Devonian-Carboniferous-Permian-Triassic-Jurassic-Cretaceous-Paleogene-Neogene)
The period is the basic unit of geological time in which a single type of rock system is formed, lasting tens of millions of years.

EPOCH (Lower - Upper, Paleocene-Eocene-Oligocene-Miocene-Pliocene-Pleistocene-Holocene)
An epoch is a division of a geologic period; it is the smallest division of geologic time, lasting several million years.

AGE (cf. stratotype)
An age is a unit of geological time which is distinguished by some feature (like an Ice Age). An age is shorter than epoch, usually lasting from a few million years to about a hundred million years.

Below is a quick summary of some of the events that occurred in each period.

Geological Time periods

Quaternary 1.6 million years ago (mya)

Man arrived in Ireland during this period. At one point during this time Ireland went through an Ice age. Bogs also began forming. Marine life is similar to what it is today.

Tertiary 65 mya

There is volcanic activity in Ireland during this time and plants, birds and mammals are a common sight.

Cretaceous 135 mya

Dinosaurs become extinct at the end of this period and in Ireland chalk-producing algae deposit chalk in the sea off Ulster. Fish are evolving but reptiles dominate the sea.

Jurassic 205 mya

This is the period where the North Atlantic Ocean begins to form, Ireland is no longer covered by sea and flowering plants are formed. Primitive birds begin to appear and this is also the age of the dinosaurs.

Triassic 250 mya

All of the major continents are joined together; dinosaurs and flying reptiles began to appear, as did the first true mammals.

Permian 290 mya

In the south of Ireland mountains began to form while in the rest of the world trilobites – a common fossil found in recent times, begin to disappear.

Carboniferous 355 mya

Limestone is deposited in the shallow sub-tropical seas around Ireland. Many plants were fossilised form this period ands sharks and bony fish are very common.

Devonian 410 mya

Ireland is a very dry mountainous land with many amphibians. Around the world the first animals appear on land in the form of scorpions and insects.

Silurian 438 mya

The melting ice caps have probably caused the sea levels to rise. Sharks and bony fish begin to appear, as do the first land plants.

Ordovician 510 mya

Sea urchins and starfish appear at this time and fossilised spores of land plants have been dated back to now. Sediments are being deposited on the ocean floor creating sedimentary rock. All life is to be found in water.
Cambrian 544 mya

The first marine organisms with hard shells appear during this period and phytoplankton is abundant in the sea.

Precambrian 4,600 mya

In Ireland, the oldest rocks were formed about 1780 million years ago. The first record of life dates back to this period, approximately 3,400 mya bacteria and blue-green algae were present in the sea. The earth is believed to have originated around 4,600 mya.


 Geological time in more detail
Click on the image to view a larger version in another window.

Image © USGS http://pubs.usgs.gov/gip/geotime/time.html

Image © USGS http://pubs.usgs.gov/gip/geotime/time.html


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