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Birth of Earth

Birth of Earth

 

The Earth is thought to have been formed about 4.6 billion years ago by collisions in the giant disc-shaped cloud of material that also formed the Sun.Gravity slowly gathered this gas and dust together into clumps that became asteroids and small early planets called planetesimals. These objects collided repeatedly and gradually got bigger, building up the planets in the Solar System, including the Earth.​Artwork showing the early Earth

The Earth forms

 

The History of the Earth concerns the development of the planet Earth from its formation to the present day. Nearly all branches of natural science have contributed to the understanding of the main events of the Earth's past. The age of Earth is approximately one-third of the age of the universe. An immense amount of biological and geological change has occurred in that time span.

Earth formed around 4.54 billion (4.54×109) years ago by accretion from the solar nebula. Volcanic outgassing probably created the primordial atmosphere, but it contained almost no oxygen and would have been toxic to humans and most modern life. Much of the Earth was molten because of extreme volcanism and frequent collisions with other bodies. One very large collision is thought to have been responsible for tilting the Earth at an angle and forming the Moon. Over time, the planet cooled and formed a solid crust, allowing liquid water to exist on the surface.

The first life forms appeared between 3.8 and 3.5 billion years ago. The earliest evidences for life on Earth are graphite found to be biogenic in 3.7 billion-year-old metasedimentary rocks discovered in Western Greenland and microbial mat fossils found in 3.48 billion-year-old sandstone discovered in Western Australia. Photosynthetic life appeared around 2 billion years ago, enriching the atmosphere with oxygen. Life remained mostly small and microscopic until about 580 million years ago, when complex multicellular life arose. During the Cambrian period it experienced a rapid diversification into most major phyla.

Geological change has been constantly occurring on our planet since the time of its formation and biological change since the first appearance of life. Species continuously evolve, taking on new forms, splitting into daughter species, or going extinct in response to an ever-changing planet. The process of plate tectonics has played a major role in the shaping of Earth's oceans and continents, as well as the life they harbor. The biosphere, in turn, has had a significant effect on the atmosphere and other abiotic conditions on the planet, such as the formation of the ozone layer, the proliferation of oxygen, and the creation of soil.

We humans have only been around for a tiny fraction of the Earth's 4.6-billion-year history, but we have still managed to build up our knowledge of what happened so long ago.

This Earth timeline highlights some of the important events that have shaped our world. The pages in the timeline contain interesting video clips from BBC television series such as Earth: Power of the Planet, Horizon, How the Earth Made Us, and Bang Goes the Theory.

 

Various theories have been proposed to explain how the Moon formed. The most widely accepted scenario begins shortly after the Earth formed about 4.6 billion years ago.

At this time, an object about the size of Mars struck the Earth. This early planet, which has been named Theia, was partially absorbed into the Earth, but a large amount of debris was also sprayed out into space. Gravity pulled the debris into orbit around our planet and, as the numerous fragments collided, they began to clump together. The Moon was formed as these clumps grew larger and larger.

Rock samples gathered by the astronauts gave us a better understanding of the Moon and provided evidence used to support the so-called "giant impact hypothesis" described above.

Artwork showing the impact that may have formed the Moon

The Moon forms

 

The giant impact hypothesis, sometimes called the Big Splash, states that the Moon was formed out of the debris left over from an indirect collision between the Earth and an astronomical body the size of Mars, approximately 4.5 billion years ago, in the Hadean eon; about 20–100 million years after the solar system coalesced. The colliding body is sometimes called Theia, for the mythical Greek Titan who was the mother of Selene, the goddess of the Moon. An alternative name for the colliding body is Orpheus, for the legendary musician, poet, and prophet in ancient Greek religion and myth.

The giant impact hypothesis is currently the favoured scientific hypothesis for the formation of the Moon. Supporting evidence includes the Earth's spin and Moon's orbit having similar orientations, Moon samples indicating the surface of the Moon was once molten, the Moon's relatively small iron core, lower density compared to the Earth, evidence of similar collisions in other star systems (that result in debris disks), and that giant collisions are consistent with the leading theories of the formation of the solar system. Finally, the stable isotope ratios of lunar and terrestrial rock are identical, implying a common origin.

There remain several questions concerning the best current models of the giant impact hypothesis, however. The energy of such a giant impact is predicted to have heated Earth to produce a global 'ocean' of magma; yet there is no evidence of the resultant planetary differentiation of the heavier material sinking into Earth's mantle. At present, there is no self-consistent model that starts with the giant impact event and follows the evolution of the debris into a single moon. Other remaining questions include when the Moon lost its share of volatile elements and why Venus, which also experienced giant impacts during its formation, does not host a similar moon.

About 4 to 3.8 billion years ago a period of intensecomet and asteroid bombardment is thought to have peppered all the planets including the Earth. Many of the numerous craters found on the Moon and other bodies in the Solar System record this event.

One theory holds that a gravitational surge caused by the orbital interaction of Jupiter and Saturn sentNeptune careening into the ring of comets in the outer Solar System. The disrupted comets were sent in all directions and collided with the planets. These water-rich objects may have provided much of the water in the Earth's oceans.

The record of this event is all but lost on the Earth because our planet's tectonic plate system and active erosion ensure that the surface is constantly renewed.

Artwork showing the Late Heavy Bombardment

The Late Heavy Bombardment ends

 

The Late Heavy Bombardment (abbreviated LHB and also known as the lunar cataclysm) is a hypothetical event thought to have occurred approximately 4.1 to 3.8 billion years ago (Ga), spanning the Neohadean and Eoarchean eras. During this interval, a disproportionately large number of asteroids apparently collided with the early terrestrial planets in the inner solar system, including Mercury, Venus, Earth, and Mars. The LHB happened after the Earth and other rocky planets had formed and accreted most of their mass, but still quite early in Earth history.

Evidence for the LHB derives from lunar samples brought back by the Apollo astronauts. Isotopic dating of Moon rocks implies that most impact melts occurred in a rather narrow interval of time. Several hypotheses are now offered to explain the apparent spike in the flux of impactors (i.e., asteroids and comets) in the inner Solar System, but no consensus yet exists. The Nice model is popular among planetary scientists; it postulates that the gas giant planets underwent orbital migration and scattered objects in the asteroid and/or Kuiper belts into eccentric orbits, and thereby into the path of the terrestrial planets. Other researchers argue that the lunar sample data do not require a cataclysmic cratering event near 3.9 Ga, and that the apparent clustering of impact melt ages near this time is an artifact of sampling materials retrieved from a single large impact basin. They also note that the rate of impact cratering could be significantly different between the outer and inner zones of the Solar System.

Exactly when the first life on Earth - the ancestors of modern bacteria - began is a subject of debate, but evidence suggests it could be as much as 3.5 billion years ago.

Early bacterial life introduced oxygen to theatmosphere. As the first free oxygen was released through photosynthesis by cyanobacteria, it was initially soaked up by iron dissolved in the oceansand formed red coloured iron oxide, which settled to the ocean floor. Over time, distinctive sedimentary rocks called banded iron formations were created by these iron oxide deposits. Once the iron in the oceans was used up, the iron oxide stopped being deposited and oxygen was able to start building up in the atmosphere about 2.4 billion years ago.Stromatolites

Early life: Oxygen enters the atmosphere

 

The Great Oxygenation Event (GOE), also called the Oxygen Catastrophe, Oxygen Crisis, Oxygen Revolution, or Great Oxidation, was the biologically induced appearance of dioxygen (O2) in Earth's atmosphere. Geological, isotopic, and chemical evidence suggest that this major environmental change happened around 2.3 billion years ago (2.3 Ga).

Cyanobacteria, which appeared about 200 million years before the GOE, began producing oxygen by photosynthesis. Before the GOE, any free oxygen they produced was chemically captured by dissolved iron or organic matter. The GOE was the point when these oxygen sinks became saturated and could not capture all of the oxygen that was produced by cyanobacterial photosynthesis. After the GOE, the excess free oxygen started to accumulate in the atmosphere.

Free oxygen is toxic to obligate anaerobic organisms, and the rising concentrations may have wiped out most of the Earth's anaerobic inhabitants at the time. Cyanobacteria were therefore responsible for one of the most significant extinction events in Earth's history. Additionally, the free oxygen reacted with atmospheric methane, a greenhouse gas, greatly reducing its concentration and triggering the Huronian glaciation, possibly the longest snowball Earth episode in the Earth's history.

Eventually, aerobic organisms began to evolve, consuming oxygen and bringing about an equilibrium in its availability. Free oxygen has been an important constituent of the atmosphere ever since.

Snowball Earth describes a theory that for millions of years the Earth was almost entirely or wholly covered in ice, stretching from the poles to the tropics.

This freezing happened over 650 million years ago in the Pre-Cambrian, though it's now thought that there may have been more than one of these global glaciations. They varied in duration and extent but during a full-on snowball event, life could only cling on in ice-free refuges, or where sunlight managed to penetrate through the ice to allow photosynthesis.

Artwork showing Snowball Earth

Snowball Earth

 

The Snowball Earth hypothesis posits that the Earth's surface became entirely or nearly entirely frozen at least once, some time earlier than 650 Ma (million years ago). Proponents of the hypothesis argue that it best explains sedimentary deposits generally regarded as of glacial origin at tropical paleolatitudes, and other otherwise enigmatic features in the geological record. Opponents of the hypothesis contest the implications of the geological evidence for global glaciation, the geophysical feasibility of an ice- or slush-covered ocean, and the difficulty of escaping an all-frozen condition. There are a number of unanswered questions, including whether the Earth was a full snowball, or a "slushball" with a thin equatorial band of open (or seasonally open) water.

The geological time frames under consideration come before the sudden appearance of multicellular life forms on Earth known as the Cambrian explosion, and the most recent snowball episode may have triggered the evolution of multi-cellular life on Earth. Another, much earlier and longer, snowball episode, the Huronian glaciation, which occurred 2400 to 2100 Ma may have been triggered by the first appearance of oxygen in the atmosphere, the "Great Oxygenation Event."

Life became more diverse and abundant in the seas during the Cambrian time period, which started about 545 million years ago. Fossils in Pre-Cambrian rocks are of simple life forms such as bacteria, with more complex soft-bodied creatures appearing towards the beginning of the Cambrian. Cambrian rocks show large numbers of many different types animals, many with hard shells.

This important shift is often described as an "explosion", but new evidence suggests that it may have been a more gradual change. Part of the difficulty is that the fossil record is not complete - some life forms were more likely to be fossilised than others, and the conditions that allow fossilisation to occur were also not constant.

Trilobite fossils

Life 'explodes'

 

The Cambrian explosion, or less popularly Cambrian radiation, was the relatively short evolutionary event, beginning around 542 million years ago in the Cambrian Period, during which most major animal phyla appeared, as indicated by the fossil record. Lasting for about the next 20 million years, it resulted in the origin of the body plan of modern metazoans. Additionally, the event was accompanied by major diversification of other organisms.[note 1] Prior to the Cambrian explosion,[note 2] most organisms were simple, composed of individual cells occasionally organized into colonies. Over the following 70 or 80 million years, the rate of diversification accelerated by an order of magnitude[note 3] and the diversity of life began to resemble that of today. Many of the present phyla appeared during this period, with the exception of Bryozoa, which made its earliest known appearance in the Lower Ordovician.

The Cambrian explosion has generated extensive scientific debate. The seemingly rapid appearance of fossils in the “Primordial Strata” was noted as early as the 1840s, and in 1859 Charles Darwin discussed it as one of the main objections that could be made against the theory of evolution by natural selection. The long-running puzzlement about the appearance of the Cambrian fauna, seemingly abruptly and from nowhere, centers on three key points: whether there really was a mass diversification of complex organisms over a relatively short period of time during the early Cambrian; what might have caused such rapid change; and what it would imply about the origin of animal life. Interpretation is difficult due to a limited supply of evidence, based mainly on an incomplete fossil record and chemical signatures remaining in Cambrian rocks.

Phylogenetic analysis has been used to support the view that during the Cambrian radiation metazoa evolved monophyletically from a single common ancestor: flagellated colonial protists similar to modern choanoflagellates.

Following the "explosion" of life during the Cambriangeological time period, the fossil record suggests that life has become increasingly diverse through time. However, this general trend is punctuated by periods of time when large numbers of organisms became extinct. The largest five of these events are called major mass extinctions. Climate change,volcanoes and asteroid impacts have all been suggested as causes of these events. The event thatwiped out the dinosaurs about 65 million years ago is one of the "big five" and experts now favour the theory that it was one or more asteroid impacts that killed off these famous creatures and many other organisms.Artwork depicting the asteroid impact that may have wiped out the dinosaurs

Major mass extinctions begin

 

An extinction event (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the amount of life on Earth. Such an event is identified by a sharp change in the diversity and abundance of macroscopic life. It occurs when the rate of extinction increases with respect to the rate of speciation. Because the majority of diversity and biomass on Earth is microbial, and thus difficult to measure, recorded extinction events affect the easily observed, biologically complex component of the biosphere rather than the total diversity and abundance of life.

Over 98% of documented species are now extinct, but extinction occurs at an uneven rate. Based on the fossil record, the background rate of extinctions on Earth is about two to five taxonomic families of marine invertebrates and vertebrates every million years. Marine fossils are mostly used to measure extinction rates because of their superior fossil record and stratigraphic range compared to land organisms.

Since life began on Earth, several major mass extinctions have significantly exceeded the background extinction rate. The most recent, the Cretaceous–Paleogene extinction event, which occurred approximately 66 million years ago (Ma), was a large-scale mass extinction of animal and plant species in a geologically short period of time. In the past 540 million years there have been five major events when over 50% of animal species died. Mass extinctions seem to be a Phanerozoic phenomenon, with extinction rates low before large complex organisms arose.

Estimates of the number of major mass extinctions in the last 540 million years range from as few as five to more than twenty. These differences stem from the threshold chosen for describing an extinction event as "major", and the data chosen to measure past diversity.

Pangaea (sometimes spelled Pangea), the most recent of a series of supercontinents on Earth, formed about 270 million years ago and broke apart about 200 million years ago. At this time most of the dry land on Earth was joined into one huge landmass that covered nearly a third of the planet's surface. The giant ocean that surrounded the continent is known as Panthalassa.

The movement of Earth's tectonic plates formed Pangaea and ultimately broke it apart.

Pangaea existed during the Permian and Triassicgeological time periods, which were times of great change. The Permian mass extinction, which wiped out an estimated 96% species about 248 million years ago, was a major event during this time.Artwork showing the Earth at the time Pangaea broke up

Pangaea begins to break up

 

Pangaea or Pangea (/pænˈdʒiːə/) was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It formed approximately 300 million years ago and then began to break apart after about 100 million years. Unlike the present Earth, much of the land mass was in the Southern Hemisphere. Pangaea was the first reconstructed supercontinent and its global ocean was accordingly named Panthalassa.

About 2.6 million years ago at the start ofPleistocene epoch, large ice sheets up to several kilometres thick began to appear in the northern hemisphere. These ice sheets would advance during cooler glacial periods and retreat during warmer interglacials.

We are living during an interglacial period called theHolocene that started about 11,500 years ago.

Ice ages are powerful evidence of the natural climate change that has occurred on the Earth in the geological past. In the 21st century the effect that humans are having on this natural cycle is an area of active scientific investigation.Artwork depicting a herd of mammoths

Glaciers advance and retreat

 

Quaternary glaciation also known as the Pleistocene glaciation or the current ice age, refers to a series of glacial events separated by interglacial events during the Quaternary period from 2.58 Ma (million years ago) to present. During this period, ice sheets were established in Antarctica and perhaps Greenland, and fluctuating ice sheets occurred elsewhere (for example, the Laurentide ice sheet). The major effects of the ice age are erosion and deposition of material over large parts of the continents, modification of river systems, creation of millions of lakes, changes in sea level, development of pluvial lakes far from the ice margins, isostatic adjustment of the crust, and abnormal winds. It affects oceans, flooding, and biological communities. The ice sheets themselves, by raising the albedo, effect a major feedback on climate cooling.

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