Source: http://en.wikipedia.org/wiki/Geology
(This extensive article contains a wealth of information, part of which is extracted here.)
CONTENTS
Hadean Eon. Arcaean Eon. Proterozoic Eon. Phanerozoic Eon.
Geology (from the Greek γή, gê, “earth” and λόγος, logos, “study or speech”) is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which they change.
The geologic time scale encompasses the history of the Earth. It is bracketed at the old end by the dates of the earliest solar system material at 4.567 Ga, (giga-annum: billion years ago) and the age of the Earth at 4.54 Ga at the beginning of the informally recognized Hadean eon. At the young end of the scale, it is bracketed by the present day in the Holocene epoch.
The table of geologic time spans presented here agrees with the dates and nomenclature set forth by the International Commission on Stratigraphy, and uses the standard colour codes of the United States Geological Survey.
The largest unit of geological time is the EON, of which there are four: Hadean, Archaean, Proterozoic and Phanerozoic. These are divided in turn into separate ERAs, each of which is again subdivided into several PERIODs (from Greek περίοδος), totalling 13. Most periods are subdivided into yet smaller units called EPOCHs.
In the Earth Sciences rocks and especially the sequences of rocks called stratum (plural: strata) arrayed in an ordered “rock column” occurring during a timespan are the focus of study so the above time units are paired with corresponding Rock strata units whose characteristics define such points elsewhere that occurred concurrently as the local rock layers were laid down as sediments.
Each unit of strata, no matter how interrupted the record recorded in the local rock column, is mapped into the overall geologic record and classified carefully into chronological units of geologic time based on world wide efforts of the International Commission on Stratigraphy working to correlate the world’s local stratigraphic record into one uniform planet-wide benchmarked system, in an steady effort ongoing since 1974. While paleontologists often refer to faunal stages rather than geologic periods, they are often used in popular presentations of paleontology or plate reconstructions.
http://en.wikipedia.org/wiki/Hadean_eon
The Hadean is the first geologic eon of Earth and lies before the Archean. It began with the formation of the Earth about 4,567.17 million years ago and ended roughly 4,000 million years ago, though the latter date varies according to different sources. The name “Hadean” derives from Hades, the Greek name for the underworld. The name is in reference to the “hellish” conditions on Earth at the time: the planet had just formed and was still very hot due to high volcanism, a partially molten surface and frequent collisions with other Solar System bodies. The geologist Preston Cloud coined the term in 1972, originally to label the period before the earliest-known rocks on Earth. Other, older texts simply refer to this eon as the Pre-Archean.
Since few geological traces of this period remain on Earth there is no official subdivision. However, the Lunar geologic timescale embraces several major divisions relating to the Hadean and so these are sometimes used in a somewhat informal sense to refer to the same periods of time on Earth.
The Lunar divisions are:
Pre-Nectarian, from the formation of the Moon’s crust up to about 3,920 million years ago
Nectarian ranging up to about 3,850 million years ago, in a time when the Late Heavy Bombardment, according to that theory, was in a stage of decline.
There is a recently proposed alternative scale published in 2010 by Solid Earth, a new open access journal. The article proposes the addition of the Chaotian and Prenephelean Eons preceding it and divides the Hadean into three eras with two periods each. The Paleohadean era consists of the Hephaestean (4.5-4.4 Ga) and the Jacobian periods (4.4-4.3 Ga). The Mesohadean is divided into the Canadian (4.3-4.2 Ga) and the Procrustean periods (4.2-4.1 Ga). The Neohadean is divided into the Acastan (4.1-4.0 Ga) and the Promethean periods (4.0-3.9 Ga).
In the last decades of the 20th century geologists identified a few Hadean rocks from Western Greenland, Northwestern Canada, and Western Australia. Rock formations in Greenland comprise sediments dated around 3,800 million years ago and are somewhat altered by a volcanic dike that penetrated the rocks after they were deposited. Individual zircon crystals redeposited in sediments in Western Canada and the Jack Hills region of Western Australia are much older. The oldest dated zircons date from about 4,000 million years ago—very close to the hypothesized time of the Earth’s formation.
The Greenland sediments include banded iron beds. They contain possibly organic carbon and imply some possibility that photosynthetic life had already emerged at that time. The oldest known fossils (from Australia) date from a few hundred million years later.
A sizeable quantity of water would have been in the material which formed the Earth. Water molecules would have escaped Earth’s gravity more easily when it was less massive during its formation. Hydrogen and helium are expected to continually leak from the atmosphere.
Part of the ancient planet is theorized to have been disrupted by the impact that created the Moon, which should have caused melting of one or two large areas. Present composition does not match complete melting and it is hard to completely melt and mix huge rock masses. However, a fair fraction of material should have been vaporized by this impact, creating a rock vapour atmosphere around the young planet. The rock vapour would have condensed within two thousand years, leaving behind hot volatiles which probably resulted in a heavy CO2 atmosphere with hydrogen and water vapour. Liquid water oceans existed despite the surface temperature of 230 °C (446 °F) because of the atmospheric pressure of the heavy CO2 atmosphere. As cooling continued, subduction and dissolving in ocean water removed most CO2 from the atmosphere but levels oscillated wildly as new surface and mantle cycles appeared.
Study of zircons has found that liquid water must have existed as long ago as 4,400 million years ago, very soon after the formation of the Earth. This requires the presence of an atmosphere. The Cool Early Earth theory covers a range from about 4,400 to 4,000 million years ago.
It is unlikely that life could have formed and established itself in the extreme, volatile conditions of the Hadean. If life had begun to form at this time, it most likely would have been destroyed several times, being forced to start over again. It is probable, however, that the building blocks necessary for life as humans know it were formed at some point during this time. Life would be granted a true start in the succeeding Archean Eon, after conditions on Earth began to stabilize.
http://en.wikipedia.org/wiki/Archean_Eon
The Archean (also spelled Archaean; formerly Archaeozoic) is a geologic eon before the Proterozoic Eon, before 2.5 Ga (billion years, or 2,500 Ma) ago. The Archean era is generally agreed to have started at 3.8 billion years ago, but this boundary is informal.
Instead of being based on stratigraphy as all other geological ages are, the beginning of the Archean eon is defined chronometrically. The lower boundary (starting point) of 3.8 billion years has not been officially recognized by the International Commission on Stratigraphy.
The Archean customarily starts at 3.8 Ga—at the end of the Hadean Eon. In older literature, the Hadean is included as part of the Archean. The name comes from the ancient Greek Αρχή (Arkhē), meaning “beginning, origin”.
The Archean is one of the four principal eons of Earth history. When the Archean began, the Earth’s heat flow was nearly three times higher than it is today, and it was still twice the current level at the transition from the Archean to the Proterozoic (2,500 Ma). The extra heat was the result of a mix of remnant heat from planetary accretion, heat from the formation of the Earth’s core, and heat produced by radioactive elements.
Most surviving Archean rocks are metamorphic or igneous. Volcanic activity was considerably higher than today, with numerous lava eruptions, including unusual types such as komatiite. Granitic rocks predominate throughout the crystalline remnants of the surviving Archean crust. Examples include great melt sheets and voluminous plutonic masses of granite, diorite, layered intrusions, anorthosites and monzonites known as sanukitoids.
The Earth of the early Archean may have supported a tectonic regime unlike that of the present. Some scientists argue that, because the Earth was much hotter, tectonic activity was more vigorous than it is today, resulting in a much faster rate of recycling of crustal material. This may have prevented cratonisation and continent formation until the mantle cooled and convection slowed down. Others argue that the oceanic lithosphere was too buoyant to subduct, and that the rarity of Archean rocks is a function of erosion by subsequent tectonic events. The question of whether plate tectonic activity existed in the Archean is an active area of modern research.
There are two schools of thought concerning the amount of continental crust that was present in the Archean. One school maintains that no large continents existed until late in the Archean: small protocontinents were the norm, prevented from coalescing into larger units by the high rate of geologic activity. The other school follows the teaching of Richard Armstrong, who argued that the continents grew to their present volume in the first 500 million years of Earth history and have maintained a near-constant ever since: throughout most of Earth history, recycling of continental material crust back to the mantle in subduction or collision zones balances crustal growth.
Opinion is also divided about the mechanism of continental crustal growth. Those scientists who doubt that plate tectonics operated in the Archean argue that the felsic protocontinents formed at hotspots rather than subduction zones. Through a process called “sagduction”, which refers to partial melting in downward-directed diapirs, a variety of mafic magmas produce intermediate and felsic rocks. Others accept that granite formation in island arcs and convergent margins was part of the plate tectonic process, which has operated since at least the start of the Archean.
An explanation for the general lack of Hadean rocks (older than 3800 Ma) is the efficiency of the processes that either cycled these rocks back into the mantle or effaced any isotopic record of their antiquity. All rocks in the continental crust are subject to metamorphism, partial melting and tectonic erosion during multiple orogenic events and the chance of survival at the surface decreases with increasing age. In addition, a period of intense meteorite bombardment in the period 4.0-3.8 Ga pulverized all rocks at the Earth’s surface during the period. The similar age of the oldest surviving rocks and the “late heavy bombardment” is thought to be not accidental.
The Archean atmosphere is thought to have nearly lacked free oxygen. Astronomers think that the sun had about 70–75% of the present luminosity, yet temperatures appear to have been near modern levels even within 500 Ma of Earth’s formation, which is puzzling (the faint young sun paradox). The presence of liquid water is evidenced by certain highly deformed gneisses produced by metamorphism of sedimentary protoliths. The equable temperatures may reflect the presence of larger amounts of greenhouse gases than later in the Earth’s history. Alternatively, Earth’s albedo may have been lower at the time, due to less land area and cloud cover.
By the end of the Archaean c. 2500 Mya, plate tectonic activity may have been similar to that of the modern Earth. There are well-preserved sedimentary basins, and evidence of volcanic arcs, intracontinental rifts, continent-continent collisions and widespread globe-spanning orogenic events suggesting the assembly and destruction of one and perhaps several supercontinents. Liquid water was prevalent, and deep oceanic basins are known to have existed by the presence of banded iron formations, chert beds, chemical sediments and pillow basalts.
Although a few mineral grains are known that are Hadean, the oldest rock formations exposed on the surface of the Earth are Archean or slightly older. Archean rocks are known from Greenland, the Canadian Shield, the Baltic Shield, Scotland, India, Brazil, western Australia, and southern Africa. Although the first continents formed during this eon, rock of this age makes up only 7% of the world’s current cratons; even allowing for erosion and destruction of past formations, evidence suggests that continental crust equivalent to only 5-40% of the present amount formed during the Archean.
In contrast to Proterozoic rocks, Archean rocks are often heavily metamorphised deep-water sediments, such as graywackes, mudstones, volcanic sediments, and banded iron formations. Carbonate rocks are rare, indicating that the oceans were more acidic due to dissolved carbon dioxide than during the Proterozoic. Greenstone belts are typical Archean formations, consisting of alternating units of metamorphosed mafic igneous and sedimentary rocks. The meta-igneous rocks were derived from volcanic island arcs, while the metasediments represent deep-sea sediments eroded from the neighbouring island arcs and deposited in a forearc basin. Greenstone belts represent sutures between protocontinents.
Fossils of cyanobacterial mats (stromatolites, which were instrumental in creating the free oxygen in the atmosphere) are found throughout the Archean, becoming especially common late in the eon, while a few probable bacterial fossils are known from chert beds. In addition to the domain Bacteria (once known as Eubacteria), microfossils of the domain Archaea have also been identified.
Life was probably present throughout the Archean, but may have been limited to simple non-nucleated single-celled organisms, called Prokaryota (formerly known as Monera). There are no known eukaryotic fossils, though they might have evolved during the Archean without leaving any fossils. No fossil evidence has been discovered for ultramicroscopic intracellular replicators such as viruses.
http://en.wikipedia.org/wiki/Proterozoic
The Proterozoic is a geological eon representing the time just before the proliferation of complex life on Earth. The name Proterozoic comes from Greek and means “earlier life”. The Proterozoic Eon extended from 2,500 Ma to 542.0±1.0 Ma (million years ago), and is the most recent part of the informally named “Precambrian” time. It is subdivided into three geologic eras (from oldest to youngest): the Paleoproterozoic, Mesoproterozoic, and Neoproterozoic.
The well-identified events of this eon were the transition to an oxygenated atmosphere during the Mesoproterozoic; several glaciations, including the hypothesized Snowball Earth during the Cryogenian period in the late Neoproterozoic; and the Ediacaran Period (635 to 542 Ma) which is characterized by the evolution of abundant soft-bodied multicellular organisms.
The geologic record of the Proterozoic is much better than that for the preceding Archean. In contrast to the deep-water deposits of the Archean, the Proterozoic features many strata that were laid down in extensive shallow epicontinental seas; furthermore, many of these rocks are less metamorphosed than Archean-age ones, and plenty are unaltered. Study of these rocks shows that the eon continued the massive continental accretion that had begun late in the Archean, as well as featured the first definitive supercontinent cycles and wholly modern orogenic activity.
The first known glaciations occurred during the Proterozoic; one began shortly after the beginning of the eon, while there were at least four during the Neoproterozoic, climaxing with the Snowball Earth of the Sturtian and Marinoan glaciations.
One of the most important events of the Proterozoic was the gathering up of oxygen in the Earth’s atmosphere. Though oxygen was undoubtedly released by photosynthesis well back in Archean times, it could not build up to any significant degree until chemical sinks — unoxidised sulphur and iron — had been filled; until roughly 2.3 billion years ago, oxygen was probably only 1% to 2% of its current level. Banded iron formations, which provide most of the world’s iron ore, were also a prominent chemical sink; most accumulation ceased after 1.9 billion years ago, either due to an increase in oxygen or a more thorough mixing of the oceanic water column.
Red beds, which are coloured by haematite, indicate an increase in atmospheric oxygen after 2 billion years ago; they are not found in older rocks. The oxygen buildup was probably due to two factors: a filling of the chemical sinks, and an increase in carbon burial, which sequestered organic compounds that would have otherwise been oxidized by the atmosphere.
Throughout the history of the Earth, there have been times when the continental mass came together to form a supercontinent, followed by the break-up of the supercontinent and new continents moving apart again. This repetition of tectonic events is called a Wilson cycle. It is at least clear that, about 1,000–830 Ma, most continental mass was united in the supercontinent Rodinia. Rodinia was not the first supercontinent; it formed at about 1.0 Ga by accretion and collision of fragments produced by breakup of the older supercontinent, called Nuna or Columbia, which was assembled by global-scale 2.0–1.8 Ga collisional events. This means plate tectonic processes similar to today’s must have been active during the Proterozoic.
After the break-up of Rodinia about 800 Ma, it is possible the continents joined again around 550 Ma. The hypothetical supercontinent is sometimes referred to as Pannotia or Vendia. The evidence for it is a phase of continental collision known as the Pan-African orogeny, which joined the continental masses of current-day Africa, South-America, Antarctica and Australia. It is extremely likely, however, that the aggregation of continental masses was not completed, since a continent called Laurentia (roughly equivalent to current-day North America) had already started breaking off around 610 Ma. It is at least certain that by the end of the Proterozoic eon, most of the continental mass lay united in a position around the south pole.
The first advanced single-celled, eukaryotes and multi-cellular life, Francevillian Group Fossils, roughly coincides with the start of the accumulation of free oxygen. This may have been due to an increase in the oxidized nitrates that eukaryotes use, as opposed to cyanobacteria. It was also during the Proterozoic that the first symbiotic relationships between mitochondria (for nearly all eukaryotes) and chloroplasts (for plants and some protists only) and their hosts evolved.
The blossoming of eukaryotes such as acritarchs did not preclude the expansion of cyanobacteria; in fact, stromatolites reached their greatest abundance and diversity during the Proterozoic, peaking roughly 1200 million years ago.
Classically, the boundary between the Proterozoic and the Phanerozoic eons was set at the base of the Cambrian period when the first fossils of animals including trilobites and archeocyathids appeared. In the second half of the 20th century, a number of fossil forms have been found in Proterozoic rocks, but the upper boundary of the Proterozoic has remained fixed at the base of the Cambrian, which is currently placed at 542 Ma.
http://en.wikipedia.org/wiki/Phanerozoic
The Phanerozoic is the current geologic eon in the geologic timescale, and the one during which abundant animal life has existed. It covers roughly 542 million years (541.0 ± 1.0) and goes back to the time when diverse hard-shelled animals first appeared. Its name derives from the Ancient Greek words φανερός and ζωή, meaning visible life, since it was once believed that life began in the Cambrian, the first period of this eon. The time before the Phanerozoic, called the Precambrian supereon, is now divided into the Hadean, Archaean and Proterozoic eons.
The time span of the Phanerozoic includes the rapid emergence of a number of animal phyla; the evolution of these phyla into diverse forms; the emergence of terrestrial plants; the development of complex plants; the evolution of fish; the emergence of terrestrial animals; and the development of modern faunas. During this timespan tectonic forces caused the continents to move and eventually collect into a single landmass known as Pangaea, which then separated into the current continental landmasses.
The Proterozoic-Phanerozoic boundary happened 541.0 ± 1.0 million years ago. In the 19th Century, the boundary was set at the first abundant animal (metazoan) fossils. But several hundred groups (taxa) of metazoa of the earlier Proterozoic era have been identified since systematic study of those forms started in the 1950s. Most geologists and paleontologists would probably set the Proterozoic-Phanerozoic boundary either at the classic point where the first trilobites and reef building animals (archaeocyatha) such as corals and others appear; at the first appearance of a complex feeding burrow called Treptichnus pedum; or at the first appearance of a group of small, generally disarticulated, armoured forms termed ‘the small shelly fauna’. The three different dividing points are within a few million years of each other.
The Phanerozoic is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic. In the older literature, the term Phanerozoic is generally used as a label for the time period of interest to paleontologists, but that use of the term seems to be falling into disuse in more modern literature.
It has been demonstrated that changes in biodiversity through the Phanerozoic correlate much better with the hyperbolic model (widely used in demography and macrosociology) than with exponential and logistic models (traditionally used in population biology and extensively applied to fossil biodiversity as well). The latter models imply that changes in diversity are guided by a first-order positive feedback (more ancestors, more descendants) and/or a negative feedback arising from resource limitation. The hyperbolic model implies a second-order positive feedback. The hyperbolic pattern of the world population growth arises from a second-order positive feedback between the population size and the rate of technological growth.
The hyperbolic character of biodiversity growth in the Phanerozoic can be similarly accounted for by a feedback between the diversity and community structure complexity. It is suggested that the similarity between the curves of biodiversity and human population probably comes from the fact that both are derived from the interference of the hyperbolic trend with cyclical and stochastic dynamics.
The second and third timelines are expanded subsections of their preceding timeline as indicated by red and green asterisks.
