Geological Timeline of the Earth

Geology is the study of the Earth. This includes the composition, structure and properties of the Earth, including the processes that produce geological changes, and the history of how these changes have affected the Earth. The Earth has a very long history, and its geology has changed continuously over its history, and still continues to change.

Earth from Space

A geological timeline of the Earth summarizes the history of these changes. It is not the same as the timeline of life on Earth, although geology affects life and life has influenced geology. Many of the geological stages of the Earth, specially in the more recent past when life was plentiful, are often defined at least in part by the strata in which different fossils have been found.

The Earth is about 4.56 billion years old. The Sun and all planets, asteroids, comets, etc. condensed from the solar nebula. This nebula was itself the result of an earlier supernova explosion in the vicinity. The shockwave from the supernova explosion may have initiated the process of contraction of the solar nebula. Gradually, gravitational forces within the nebula caused the matter to aggregate and clump together. As the matter contracted, it started to spin, and took the form of an accretion disk.

Most of the material aggregated at the center, forming the proto-Sun. As gravity compressed this matter further, its temperature rose to a point where fusion was ignited at the core, and the Sun was born. The violent solar wind blew away most of the material (specially lighter elements) outwards, leaving a mixture rich in heavier elements, which gradually coalesced into the inner rocky planets. This process was relatively rapid - it's estimated that it took about a hundred million years to form the Sun and the planets from the solar nebula.

The Earth formed soon after the Sun. How soon is a matter of debate, since no rocks from the earliest period exist on Earth anymore. The oldest meteorites are made of the same material that condensed into the planets. Some of these date within 1-2 million years of the formation of the solar system, and remarkably, some are basaltic meteorites. Since basalts can only form under high temperatures and pressures, it seems likely that clumps of material large enough to have sufficient pressure and temperature at their cores formed at a very early age.

The Earth continued to grow as it accumulated more material from impactors. One of these impactors was possibly a Mars-sized planetesimal (known as Theia), which may have resulted in the formation of the moon. It seems likely that Earth reached almost its full present mass (99%) within the first 60 million years, that is, during the Cryptic period of the Hadean.

Mean Radius 6371 km
Equatorial Radius 6378.1 km
Polar Radius 6356.8 km
Surface Area 510,072,000 sq km
Mass 5.9736 x 10E24 kg
Mean Density 5.5153 g/cc
Sidereal Rotation Period 23H 56M 4.09054S
Axial Tilt 23.439281 degrees
Aphelion 152,097,701 km
Perihelion 147,098,074 km

The internal structure of the Earth is layered. The outermost layer is lithosphere (literally "rocky sphere") which is a region about 60 km thick composed predominantly of solid rock. The outermost 35 km of the lithosphere are also known as the "crust". Note that these numbers are averages, and the actual depth of the lithosphere or crust varies considerably by region. It is thinnest under the ocean floors. The next layer inwards is the mantle, which is solid (but plastic and highly viscous) rock. The mantle is 2900 km thick, and forms about 70% of the bulk of the Earth. The crust can simply be considered the outermost layer of the mantle, with some chemical differences. The crust was formed by repeated melting and solidification of the mantle, and is therefore a somewhat crystalline melt-product of the mantle. Due to the periodic re melting, the heavier elements (such as magnesium) sunk below into the mantle and lighter elements (such as silicon and aluminum) predominate in the crust.

Below the mantle is the Earth's core, which consists of the inner and outer cores. The outer core is liquid, while the inner core is solid. These are regions of very high pressures and temperatures, containing predominantly iron. The inner and outer cores of the Earth are responsible for generating the Earth's magnetic field.

Geological Ages of the Earth

Here's a table of the Geological Ages of the Earth. Note that this is just a rough representation - the durations of different periods are not to scale.

Hadean Cryptic 4.56b - 4.50b
Basin Groups 4.50b - 3.95b
Nectarian 3.95b - 3.85b
Early Imbrian 3.85b - 3.80b
Archean Eoarchean 3.80b - 3.60b
Paleoarchean 3.60b - 3.20b
Mesoarchean 3.20b - 2.80b
Neoarchean 2.80b - 2.50b
Proterozoic Paleoproterozoic Siderian 2.50b - 2.30b
Rhyacian 2.30b - 2.05b
Orosirian 2.05b - 1.80b
Statherian 1.80b - 1.60b
Mesoproterozoic Calymmian 1.60b - 1.40b
Ectasian 1.40b - 1.20b
Stenian 1.20b - 1.00b
Neoproterozoic Tonian 1.00b - 850m
Cryogenian 850m - 635m
Ediacaran 635m - 542m
Phanerozoic Paleozoic Cambrian 542m - 490m
Ordovician 490m - 442m
Silurian 442m - 415m
Devonian 415m - 359m
Carboniferous Mississippian 359m - 318m
Pennsylvanian 318m - 299m
Permian 299m - 251m
Mesozoic Triassic 251m - 199m
Jurassic 199m - 145m
Cretaceous 145m - 65m
Cenozoic Paleogene Paleocene 65m - 56m
Eocene 56m - 34m
Oligocene 34m - 23m
Neogene Miocene 23m - 5.33m
Pliocene 5.33m - 2.588m
Pleistocene 2.588m - 11700y
Holocene 11700y - present

Click to see each division in detail. A much more detailed chart prepared by the International Commission on Stratigraphy can be found at their web site (as pdf) or here (gif).

The following is a brief summary of the eons of geologic time. You can select an eon here or click directly in the table to the left.

Hadean

This was the age when the Earth first formed, accreting from the solar disk. The moon formed early in this period, possibly due to collision with a planet-sized impactor. The surface of the Earth cooled rapidly, and liquid water was probably present within the first 100 million years of the Earth's formation. This period is marked by heavy bombardment by asteroids and comets from space. Although the first generally accepted evidence of life comes from a later age, it's possible that life established itself as early as 4 billion years ago, during this eon.

Archean

This was a 1.3 billion year period in which the Earth's geology was established. The oldest rocks generally belong to this period, and it marks the beginning of the formation of continental plates. Life evolved during this period, with the earliest evidence from about 3.5 billion years ago. Photosynthesis appeared towards the end of the Archean, about 2.8 billion years ago, but oxygen levels in the atmosphere remained negligible due to the presence of several oxygen sinks in the rocks and oceans.

Proterozoic

This was a period of great geological changes in the Earth. Plate tectonics finally became established during this period. The first evidence of multicellularity and sexual reproduction dates to about 1.2 billion years ago, in the middle of the proterozoic. Towards the end, the Earth cooled rapidly, and for a time, may have been completely frozen. It soon recovered, and the first complex multicellular life appeared.

Phanerozoic

This is most recent eon, which continues to this day. In this time, multicellular life spread from the oceans to land, the first plants, trees, and forests appeared. All modern phyla existed early in the phanerozoic, and continued to evolve into the life forms we see today. This includes the age of the dinosaurs (the Jurassic and Cretaceous), the age of mammals (Cenozoic), the emergence of birds, modern reptiles, etc. This is also the period of the formation and breakup of the most recent supercontinent, Pangaea, and the most severe mass extinction (the end-Permian extinction).

To read the timeline in chronological order, proceed to the Hadean Eon. Or you may click any period in the table above.