Following the Paleozoic, the Mesozoic extended roughly 186 million years, from 252.2 to 66 million years ago when the Cenozoic Era began. This time frame is separated into three geologic periods. From oldest to youngest:

The lower (Triassic) boundary is set by the Permian-Triassic extinction event, during which approximately 90% to 96% of marine species and 70% of terrestrial vertebrates became extinct. It is also known as the “Great Dying” because it is considered the largest mass extinction in the Earth’s history. The upper (Cretaceous) boundary is set at the Cretaceous-Tertiary (KT) extinction event (now more accurately called the Cretaceous–Paleogene (or K–Pg) extinction event), which may have been caused by the impactor that created Chicxulub Crater on the Yucatán Peninsula. Towards the Late Cretaceous large volcanic eruptions are also believed to have contributed to the Cretaceous–Paleogene extinction event. Approximately 50% of all genera became extinct, including all of the non-avian dinosaurs.

Compared to the vigorous convergent plate mountain-building of the late Paleozoic, Mesozoic tectonic deformation was comparatively mild. The sole major Mesozoic orogeny occurred in what is now the Arctic, creating the Innuitian orogeny, the Brooks Range, the Verkhoyansk and Cherskiy Ranges in Siberia, and the Khingan Mountains in Manchuria. This orogeny was related to the opening of the Arctic Ocean and subduction of the North China and Siberian cratons under the Pacific Ocean. Nevertheless, the era featured the dramatic rifting of the supercontinent Pangaea. Pangaea gradually split into a northern continent, Laurasia, and a southern continent, Gondwana. This created the passive continental margin that characterizes most of the Atlantic coastline (such as along the U.S. East Coast) today.

By the end of the era, the continents had rifted into nearly their present form. Laurasia became North America and Eurasia, while Gondwana split into South America, Africa, Australia, Antarctica and the Indian subcontinent, which collided with the Asian plate during the Cenozoic, the impact giving rise to the Himalayas.

The Triassic was generally dry, a trend that began in the late Carboniferous, and highly seasonal, especially in the interior of Pangaea. Low sea levels may have also exacerbated temperature extremes. With its high specific heat capacity, water acts as a temperature-stabilizing heat reservoir, and land areas near large bodies of water—especially the oceans—experience less variation in temperature. Because much of the land that constituted Pangaea was distant from the oceans, temperatures fluctuated greatly, and the interior of Pangaea probably included expansive areas of desert. Abundant red beds and evaporites such as halite support these conclusions, but evidence exists that the generally dry climate of the Triassic was punctuated by episodes of increased rainfall. Most important humid episodes were the Carnian Pluvial Event and one in the Rhaetian, few million years before the Triassic-Jurassic extinction event.

Sea levels began to rise during the Jurassic, which was probably caused by an increase in seafloor spreading. The formation of new crust beneath the surface displaced ocean waters by as much as 200 m (656 ft) more than today, which flooded coastal areas. Furthermore, Pangaea began to rift into smaller divisions, bringing more land area in contact with the ocean by forming the Tethys Sea. Temperatures continued to increase and began to stabilize. Humidity also increased with the proximity of water, and deserts retreated.

The climate of the Cretaceous is less certain and more widely disputed. Higher levels of carbon dioxide in the atmosphere are thought to have caused the world temperature gradient from north to south to become almost flat: temperatures were about the same across the planet. Average temperatures were also higher than today by about 10°C. In fact, by the middle Cretaceous, equatorial ocean waters (perhaps as warm as 20°C in the deep ocean) may have been too warm for sea life, and land areas near the equator may have been deserts despite their proximity to water. The circulation of oxygen to the deep ocean may also have been disrupted. For this reason, large volumes of organic matter that was unable to decompose accumulated, eventually being deposited as “black shale”.

Not all of the data support these hypotheses, however. Even with the overall warmth, temperature fluctuations should have been sufficient for the presence of polar ice caps and glaciers, but there is no evidence of either. Quantitative models have also been unable to recreate the flatness of the Cretaceous temperature gradient.

Different studies have come to different conclusions about the amount of oxygen in the atmosphere during different parts of the Mesozoic, with some concluding oxygen levels were lower than the current level (about 21%) throughout the Mesozoic, some concluding they were lower in the Triassic and part of the Jurassic but higher in the Cretaceous, and some concluding they were higher throughout most or all of the Triassic, Jurassic and Cretaceous.

The dominant land plant species of the time were gymnosperms, which are vascular, cone-bearing, non-flowering plants such as conifers that produce seeds without a coating. This is opposed to the earth’s current flora, in which the dominant land plants in terms of number of species are angiosperms. One particular plant genus, Ginkgo, is thought to have evolved at this time and is represented today by a single species, Ginko biloba. As well, the extant genus Sequoia is believed to have evolved in the Mesozoic.

The extinction of nearly all animal species at the end of the Permian Period allowed for the radiation of many new lifeforms. In particular, the extinction of the large herbivorous pareiasaurs and carnivorous gorgonopsians left those ecological niches empty. Some were filled by the surviving cynodonts and dicynodonts, the latter of which subsequently became extinct. Some plant species had distributions that were markedly different from succeeding periods; for example, the Schizeales, a fern order, were skewed to the Northern Hemisphere in the Mesozoic, but are now better represented in the Southern Hemisphere.

Recent research indicates that the specialized animals that formed complex ecosystems, with high biodiversity, complex food webs and a variety of niches, took much longer to reestablish, recovery did not begin until the start of the mid-Triassic, 4M to 6M years after the extinction and was not complete until 30M years after the Permian-Triassic extinction event. Animal life was then dominated by large archosaurian reptiles: dinosaurs, pterosaurs, and aquatic reptiles such as ichthyosaurs, plesiosaurs, and mosasaurs.

The climatic changes of the late Jurassic and Cretaceous provided for further adaptive radiation. The Jurassic was the height of archosaur diversity, and the first birds and eutherian mammals also appeared. Angiosperms radiated sometime in the early Cretaceous, first in the tropics, but the even temperature gradient allowed them to spread toward the poles throughout the period. By the end of the Cretaceous, angiosperms dominated tree floras in many areas, although some evidence suggests that biomass was still dominated by cycad and ferns until after the Cretaceous–Paleogene extinction.

Some have argued that insects diversified with angiosperms because insect anatomy, especially the mouth parts, seems particularly well-suited for flowering plants. However, all major insect mouth parts preceded angiosperms and insect diversification actually slowed when they arrived, so their anatomy originally must have been suited for some other purpose.

As the temperatures in the seas increased, the larger animals of the early Mesozoic gradually began to disappear while smaller animals of all kinds, including lizards, snakes, and perhaps primates, evolved. The Cretaceous–Paleogene extinction event exacerbated this trend. The large archosaurs became extinct, while birds and mammals thrived, as they do today.

In North America, the era began with deep sedimentary basins along the eastern, southeastern, and western sides of the continent, while the interior was dry land. As the era proceeded, the marginal seas periodically washed over the stable interior, leaving sedimentary deposits to mark their incursions. During the early part of the era, the area of exposed Precambrian, or shield, rocks in central Canada were eroding, supplying sediment to the basins from the interior. Beginning in the Ordovician Period, mountain building intermittently proceeded in the eastern part of the Appalachian region throughout the rest of the era, bringing in new sediments. Sediments washing from the Acadian Mountains filled the western part of the Appalachian basins to form the famous coal swamps of the Carboniferous Period. In North America carboniferous is not generally used instead the time is divided between Mississippian and Pennsylvanian period because of differences in the sedimentary rock deposited in this time. The Mississippian is characterized by limey sediments deposited in shallow seas, typically with abundant crinoidal fossils as in the Burlington fm. The Pennsylvanian typically is characterized by terrestrial sediments such as sands, shale and most importantly coal. Most of our oil and gas are obtained from Pennsylvanian sediments. Where this has been stripped, as in the Ozark domal region, oil is not typically available.

Paleoclimatic studies and evidence of glaciers indicate that central Africa was most likely in the polar regions during the early Paleozoic. During the early Paleozoic, the huge continent Gondwanaland had either formed or was forming. By mid-Paleozoic, the collision of North America and Europe produced the Acadian-Caledonian uplifts, and a subduction plate uplifted eastern Australia. By the late Paleozoic, continental collisions formed the supercontinent Pangaea and resulted in some of the great mountain chains, including the Appalachians, Urals, and mountains of Tasmania.

Geologically, the Paleozoic starts shortly after the breakup of a supercontinent called Pannotia. Throughout the early Paleozoic, the Earth’s landmass was broken up into a substantial number of continents. Towards the end of the era, the continents gathered together into a supercontinent called Pangaea, which included most of the Earth’s land area.

The Ordovician and Silurian periods were warm greenhouse periods, with the highest sea levels of the Paleozoic (200 m above today’s); the warm climate was interrupted only by a 30 million years cool period, the Early Palaeozoic Icehouse, culminating in the Hirnantian glaciation.

The early Cambrian climate was probably moderate at first, becoming warmer over the course of the Cambrian, as the second-greatest sustained sea level rise in the Phanerozoic got underway. However, as if to offset this trend, Gondwana moved south with considerable speed, so that, in Ordovician time, most of West Gondwana (Africa and South America) lay directly over the South Pole. The early Paleozoic climate was also strongly zonal, with the result that the “climate”, in an abstract sense became warmer, but the living space of most organisms of the time—the continental shelf marine environment—became steadily colder. However, Baltica (Northern Europe and Russia) and Laurentia (eastern North America and Greenland) remained in the tropical zone, while China and Australia lay in waters which were at least temperate. The Early Paleozoic ended, rather abruptly, with the short, but apparently severe, late Ordovician ice age. This cold spell caused the second-greatest mass extinction of Phanerozoic time. Over time, the warmer weather moved into the Paleozoic Era.

The middle Paleozoic was a time of considerable stability. Sea levels had dropped coincident with the ice age, but slowly recovered over the course of the Silurian and Devonian. The slow merger of Baltica and Laurentia, and the northward movement of bits and pieces of Gondwana created numerous new regions of relatively warm, shallow sea floor. As plants took hold on the continental margins, oxygen levels increased and carbon dioxide dropped, although much less dramatically. The north–south temperature gradient also seems to have moderated, or metazoan life simply became hardier, or both. At any event, the far southern continental margins of Antarctica and West Gondwana became increasingly less barren. The Devonian ended with a series of turnover pulses which killed off much of Middle Paleozoic vertebrate life, without noticeably reducing species diversity overall.

The late Paleozoic was a time which has left us a good many unanswered questions. The Mississippian began with a spike in atmospheric oxygen, while carbon dioxide plummeted to unheard-of lows. This destabilized the climate and led to one, and perhaps two, ice ages during the Carboniferous. These were far more severe than the brief Late Ordovician Ice; but, this time, the effects on world biota were inconsequential. By the Cisuralian, both oxygen and carbon dioxide had recovered to more normal levels. On the other hand, the assembly of Pangaea created huge arid inland areas subject to temperature extremes. The Lopingian is associated with falling sea levels, increased carbon dioxide and general climatic deterioration, culminating in the devastation of the Permian extinction.

While macroscopic plant life appeared early in the Paleozoic and possibly late in the Neoproterozoic, it mostly remained aquatic until sometime in the Silurian and Devonian, when it began to transition onto dry land. Terrestrial flora reached its climax in the Carboniferous, when towering lycopsid rainforests dominated the tropical belt of Euramerica. Climate change caused the Carboniferous Rainforest Collapse which fragmented this habitat, diminishing the diversity of plant life in the late Carboniferous and Permian.

A noteworthy feature of Paleozoic life is the sudden appearance of nearly all of the invertebrate animal phyla in great abundance at the beginning of the Cambrian. The first vertebrates appeared in the form of primitive fish, which greatly diversified in the Silurian and Devonian. The first animals to venture onto dry land were the arthropods. Some fish had lungs and strong, bony fins and could crawl onto the land also. The bones in their fins eventually evolved into legs and they became the first tetrapods. Amphibians were the dominant tetrapods until the mid-Carboniferous, when climate change greatly reduced their diversity. Later, reptiles prospered and continued to increase in number and variety by the late Permian.

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 accumulation of oxygen in the Earth’s atmosphere. Though oxygen is believed to have been released by photosynthesis as far back as Archean times, it could not build up to any significant degree until chemical sinks — unoxidized sulfur 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; their 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 colored by hematite, 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

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.

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 4 billion years is officially recognized by the International Commission on Stratigraphy.

The Archean customarily starts at 4 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 as high as 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. Some think that the similar age of the oldest surviving rocks and the “late heavy bombardment” is not coincidental.

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 Ma (million years ago), 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 metamorphized 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 neighboring 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.

Note : The above story is based on materials provided by Wikipedia

Since few geological traces of this eon 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:

There is a recently proposed alternative scale that includes the addition of the Chaotian and Prenephelean Eons preceding the Hadean, 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).

A sizeable quantity of water would have been in the material that 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 escape (even to the present day) due to atmospheric escape.

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 vapor atmosphere around the young planet. The rock vapor would have condensed within two thousand years, leaving behind hot volatiles which probably resulted in a heavy CO2 atmosphere with hydrogen and water vapor. 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.

A September 2008 study of zircons found that Australian Hadean rock holds minerals that point to the existence of plate tectonics as early as 4,000 million years ago. If this is true, the time when Earth finished its transition from having a hot, molten surface and atmosphere full of carbon dioxide, to being very much like it is today, can be roughly dated to about 4.0 billion years ago. The action of plate tectonics and the oceans traps vast amounts of carbon dioxide, thereby eliminating the greenhouse effect and leading to a much cooler surface temperature and the formation of solid rock, and possibly even life.