Geological history of the Earth

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A geological history of the Earth

The Earth was formed 4600 mya (million years ago), at the same time as the Sun and the rest of the Solar System.

Geologists (Greek for those who study the Earth) divide the history of the Earth into eons, eras, periods and epochs.

The Archaean (Greek for ancient) eon lasted from 4600 mya until 2500 mya. For most of this eon there was no life of any sort; the earliest living organisms began to appear at the very end of it.

The Proterozoic (Greek for first animals) eon lasted from 2500 mya until 600 mya. For most of this eon only the very simplest one-celled organisms existed.

The Phanerozoic (Greek for visible animals) eon began 600 mya and has lasted until the present day. During this eon all the more complex life forms have evolved.

For all but the most specialised purposes the Archaean and Proterozoic eons are not usually divided further; instead all rocks from these eons, and the fossils found in them, are normally classified just as pre-Cambrian - see below.

The Phanerozoic eon is divided into three eras.

The Paleozoic (Greek for ancient animals) era lasted from 600 mya until 248 mya. During this era many classes of invertebrate, and also primitive fishes, amphibians, reptiles and mammals, began to appear.

The Mesozoic (Greek for middle animals) era lasted from 248 mya until 66 mya. It is sometimes referred to as the age of dinosaurs.

The Cenozoic (Greek for recent animals) era began 66 mya and has lasted until the present day. It is sometimes referred to as the age of mammals.

Each era of the Phanerozoic eon is divided into a number of periods.

The Paleozoic era is divided into seven (or six) periods. They are named mostly for places where rocks of these ages were first discovered. Starting with the earliest they are the

The Cambrian saw an evolutionary explosion unlike anything before or since: at its beginning only the most primitive organisms existed; at its end primitive forms of almost all the present-day animal and plant phylla were in evidence.

In some books the Mississipian and Pensylvanian periods are grouped together as the Carboniferous period. It was during this period, about 300 mya, that the plants grew whose fossilised remains form today's coal seams.

The Mesozoic era is divided into three periods.

The Triassic period lasted from 248 - 208 mya. Its name comes from Greek, because the first rocks of this period discovered in Germany were in three layers. This period saw the first dinosaurs.

The Jurassic period lasted from 208 - 148 mya. Its name comes from Jura, a place on the French-Swiss border where many early dinosaur fossils were found. During this period the dinosaurs flourished.

The Cretaceous period lasted from 148 - 66 mya. It gets its name from the Greek for chalk, because during this period the great chalk deposits forming the Downs and White Cliffs of Dover were laid down. The dinosaurs diversified during this period and birds evolved. Then 66 mya the Cretaceous period ended very suddenly with the rapid extinction of all the dinosaurs except birds, and about 80% of all other animal species.

Much of the great work on dinosaurs and other fossils and the rocks in which they are found has been done by Germans. The German for chalk is Kreibe, so the very important and clearly defined boundary between Cretaceous and Tertiary rocks (see next paragraph) is usually referred to by geologists as the K-T rather than the C-T boundary.

We often refer to the sudden end of the Cretaceous period as the Great Extinction, but it must be borne in mind that there have been several other such extinctions, perhaps the greatest of all being at the end of the Permian.

Most geologists now believe that the Great Extinction at the end of the Permian was caused by a series of massive volcanic eruptions in the area of modern Siberia, and that at the end of the Cretaceous was caused by a giant metereorite impact in the area of Mexico: this is discussed on the Page on Shooting Stars and Meteorites.

The Cenozoic era is divided into two periods, the Tertiary and Quaternary. In the early days of modern geology, when it was thought the Earth was only a few thousand years old, its history was divided into four Ages: Primary, Secondary, Tertiary and Quaternary (from the Latin for first, second, third and fourth). Once it was realised that the Earth was thousands of millions of years old, the nomenclature was completely changed and eons, eras, periods and epochs were introduced, but Tertiary and Quaternary remained in use as the names of periods in the Cenozoic era.

The Tertiary period lasted from 66 mya until 2 mya. It is divided into five epochs, the

The Quaternary period started 2 mya. It is divided into two epochs, the Pleistocene and the Holocene. Today we are living in the Holocene epoch of the Quaternary period of the Cenozoic era of the Phanerozoic eon.

You can read more about this by visiting The Web Time Machine site. This gives much more information about each eon, era, period and epoch.

To find out how the Earth's crust looked during these times visit the Plate Tectonics web site Link to Plate Tectonics website



© Barry Gray December 2008