Earth

Earth is the third planet from the Sun and the only body in the Solar System known to harbour life. It is the largest terrestrial object in the Solar System.

[ NASA / JPL / Caltech ]

 

Siderial Period	Perihelion (AU)	Aphelion (AU)	Inclination (degrees)	Axial Tilt (deg)	Axial Period (d, h, m)
365.26 d	0.98	1.02	0.0	23.45	0, 23, 56.1
Equatorial Diameter (km)	Oblateness	Mass (Earth = 1)	Density (water = 1)	Albedo (geom.)	No. of Satellites
12, 756	0.003	1.00	5.5	0.37	1

 

Plate Tectonics

Plate tectonics was first proposed by Alfred Wegener, a German meteorologist, in 1904. However, the idea did not catch on until the 1960's when fresh interpretation of this fundamental mechanism began.

The Earth is a very dynamic body. Plate tectonic processes are driven by the large amounts of internally-produced heat, changing the face of the planet more rapidly than any other body in the Solar System. The shape of oceans and land-masses can change completely over periods of only a few tens of millions of years.

Water plays a vital role not only in the preservation of life, but in plate tectonics as well. As oceanic crust is created along oceanic ridges (such as the Mid-Atlantic Rift or the East Pacific Rise), water is chemically combined with the molten rock materials and locked into it. The rock is consequently much denser than rocks formed on dry land. This causes oceanic plates to subduct beneath continental plates when they collide, which induces melting of the rock, rising of the magma, and eruption of the water-laden rocks onto the surface, releasing large amounts of water back into the atmosphere.

Geological Time Scale

Geologists have been able to study the Earth in great detail - it being our home planet helps! Consequently, a detailed history of the Earth has been built up, especially since the 1960's, to which the geological timescale is central. In the table below, time is measured in millions of years before the present (Ma). (Source: Harland et al., 1989)

Webmaster's note, June 2007: the timescale has been undergoing some expantion in recent days in the form of notes on each geological period; these detail important climatic/geological/biological events throughout Earth's history in order to put the timescale into some sort of perspective. This is a work in progress, so I beg for your patience while I finish the additions; I will also cite various references.

Era	Period	Epoch	Dates from... (Ma)	Events
Cenozoic	Quaternary	Holocene	0.01
		Pleistocene	1.64
	Tertiary	Pliocene	5.20
		Miocene	23.30
		Oligocene	35.40
		Eocene	56.50
		Palaeocene	65.00
Mesozoic	Cretaceous	Gulf	97.00	As in the late Jurassic the global environment was warm and balmy in the Early Cretaceous, with the general drying of the climate slowly progressing. Conditions remained good for plants, with tropical and temperate conifer forests remaining in abundance, along with some dryer open plains. The continents continued to spread apart, with new seas and oceans appearing. These included the Atlantic, which opened some time after 130 Ma. The separation of the continents caused the isolation and subsequent independent evolution of many groups of animals and plants that were iniitially common between them. Two giant dinosaurs evolved in South America; the largest [titanosaur] sauropod, Argentinosaurus, and the largest [allosaur] theropod, Giganotosaurus. At ~100 Ma, in the mid-Cretaceous, the global climate began to cool, perhaps due to the spread of the Atlantic. The cooling process accelerated, particularly as the Cretaceous drew to an end. Flowering plants (or angiosperms) made their first appearance in the late Cretaceous as small shrubs and weeds. They proved to be a milestone in plant evolution as reproduction via flowers enabled plants to evolve more quickly; by the end of the period the broad-leaved angiosperms dominated forest canopies, as they do today. The tyrannosaurs enjoyed most success as a group in the late Cretaceous, with the well-known Tyrannosaurus rex evolving only ten million years or so before all dinosaurs became extinct. The ichthyosaurs began to decline, and became extinct ~90 Ma. The plesiosaurs survived until the K-T extinction event, as did a new group of marine reptiles that evolved ~95 Ma, the mosasaurs. Some sea turtles became huge; 4.5m long in some cases. Some pterosaurs also became huge, with wingspans of 9+ metres. Such animals may have been able to migrate large distances. However, they declined in number towards the end of the period, with only a few species living beyond ~70 Ma. The remainder perished in the extinction at the end of the Cretaceous. BIRDS...... Mammals began to increase in number (although they remained morphologically small), particularly as the Cretaceous drew to a close - maybe because they were well adapted to cooler conditions. At the end of the Cretaceous period an extinction event occurred which wiped out approximately 40% of all animal families, including the dinosaurs, pterosaurs, most marine and aquatic reptiles (except the crocodilians and turtles), some arthropods such as the ammonites, and some fish. Most groups that survived were badly affected and took some time to recover in the Tertiary period. The extinction seems to have taken place over a period of 2-3 million years, beginning at ~67 Ma, and was most likely initiated by the global cooling of the climate. However, the impact of an asteroid at 65 Ma - the crater of which sits on the edge of the Yucatan peninsula - finished off the process. The fallout from the impact left a thin tell-tale layer of rock containing iridium around the world, which marks the K-T (Cretaceous-Tertiary) boundary.
		Early	145.60
	Jurassic	Malm	157.10	Pangaea continued to break up...... As the continents separated the climate became warm and more humid, and monsoons would have periodically swept across lowland areas, creating large river basins and seasonal floods. Conifer forests dominated polar regions, and the tropics were ruled by conifers, ferns and cycads. Vegetation was clearly lush, prospering in the more tropical climate, as extensive coal beds were laid down in the Jurassic. Dinosaurs became the dominant land vertebrates. Being highly adaptable and diversifying hugely they spread to all continents, becoming a highly successful group. The pterosaurs (flying reptiles) also diversified and enjoyed great success in the Jurassic. The largest fish ever known evolved in the Jurassic. Leedsichthys, which was a 25m long filter-feeding teleost fish, evolved alongside rather less friendly giant carnivorous pliosaurs such as 25m long Liopleurodon - the largest marine predator ever known. Marine crocodilians also appeared. The first true mammals appeared at ~205 Ma in the early Jurassic, probably evolving from the cynodonts. The climate in the tropics became somewhat drier in the mid-Jurassic, and arid conditions spread in many areas. The drying of the climate resulted in extensive, almost savannah-like open areas appearing, but it was not dry enough to cause desertification. At around this time, ~190 Ma, a herbivorous dinosaur group called the sauropods appeared. They became a large group, particularly in late Jurassic and early Cretaceous times, and produced the largest land animals the world has ever known - including the longest (among the diplodocids), and the tallest and heaviest (among the brachiosaurids). All of the large [land-dwelling] carnivores from the early Jurassic until the end of the Cretaceous were members of the bipedal theropod group. The first tyrannosaurs appeared in the mid- to late-Jurassic.
		Dogger	178.00
		Lias	208.00
	Triassic	Late	235.00	The Earth remained hot and dry, although rainfall sustained much plant life. Even at the poles, temperatures remained above 10°C. Plant life was globally represented predominantly by ferns, gingkos and primitive conifers. There were few remaining large predators or many competitors for various niches, which meant that a few well adapted species came to represent rather high percentages of all vertebrate life on land. Due to the lack of variety and to take advantage of the available niches, animal evolution accelerated. This resulted a wide range of new species and genres. Among these were aquatic/marine reptiles (the sauropterygia), which came to dominate Mesozoic seas. These included the nothosaurs and more fish-like ichthyosaurs. In the Late Triassic, Pangaea began to break up into smaller continents; initially Laurasia and Gondwana, separated by the Tethys ocean. The intense volcanism associated with this event may have played some role in the Triassic-Jurassic extinction event. The previously dominant therapsid ('mammal-like') reptiles declined - apart from the cynodonts, which went on to become true mammals. A new group of reptiles called the archosaurs rose to dominance at ~240 Ma, whose success would last throughout the Mesozoic era. The group included the dinosaurs, pterosaurs (or 'flying lizards') and crocodilians. The dinosauria consisted of two major groups; the saurischia (lizard-hipped) and ornithischia (bird-hipped). Among the dinosaurs, the prosauropods (mostly herbivores) and theropods (carnivores) emerged in the Late Triassic. An extinction event at the end of the Triassic affected life in the oceans particularly badly: many invertebrate groups suffered great losses, the conodonts became extinct and all marine reptiles apart from plesiosaurs and ichthyosaurs disappeared. Life on land fared better, although several groups of small reptiles, primitive dinosaurs and some amphibians died out.
		Mid	241.10
		Scythian	245.00
Palaeozoic	Permian	Zechstein	256.10	Cool, dry conditions pervaded at the beginning of the Permian. Ferns and primitive conifers quickly moved into open areas left by retreating tropical forests. Amphibians survived the change in climate but were not well suited to the new conditions. The reptiles fared rather better and, forced to innovate in the cool climate, rose to dominate life on land. Late in the Permian period the continents all met up to form a supercontinent called Pangaea, parts of which extended to the poles. The climate became hot and dry, leading to extensive desert environments. A group of 'mammal-like' reptiles, the therapsids, adapted well to the desert conditions and survived into the Triassic period (and beyond). By the close of the Permian the climate had become extreme. This, combined with the problem of a single landmass (and therefore a limited set of ecosystems), caused conditions for life to worsen. Intense competition amongst organisms of all kinds in the limited habitats available began to result in many extinctions. Eventually, 96% of all marine life and over 70% of land vertebrates became extinct; the most severe extinction event in the history of life on Earth.
		Rotliegendes	290.00
	Carboniferous	Pennsylvanian	322.80	The Earth had a globally hot, humid climate that extended into the Arctic and Antarctic circles. The atmospheric concentration of oxygen was very high, allowing the evolution of giant arthropods. Densely forested swamps dominated lowland areas; lycopsid trees (up to 50m tall), tree-sized ferns and horsetails were common. Huge coal deposits were laid down during the Carboniferous due to prolific plant life. Amphibians were ideally suited to such humid, swampy environments and thrived, dominating vertebrate life on land. Reptiles first appeared ~340 Ma. They are adapted to be less reliant on wet environments than amphibians, and have a great advantage in the ability to lay their eggs away from water. However, they were not particularly successful until the Permian period. Sharks and bony fish dominated life in the oceans, and coral reefs began a slow recovery. An ice age began at the end of the Carboniferous at ~290 Ma, with ice caps extending from both poles. The ice age dramatically altered climatic conditions around the world, forcing a retreat of tropical forests and their associated fauna, leading to the extinction of many species.
		Mississippian	362.50
	Devonian	Late	377.40	Pioneer vegetation soon developed into primitive forests that followed rivers and estuaries. They included the first tree-like plants. Millipedes and the precursors to spiders, the trigonotarbids, dominated land-dwelling animals. In the sea, the evolution of the jaw in fish lead to a rapid increase in predatory varieties; giant placoderms (armoured fish, including the 8m-long Dunkleosteus) evolve alongside the first sharks and teleost (bony) fish. Sharks would prove to be more successful than the placoderms, and may have forced their extinction (by 355 Ma, in the early Carboniferous). Some [jawed] fish make the move onto land, and the first amphibians evolve. Towards the end of the Devonian period a series of extinction events (or one prolonged event) occurred, beginning at ~364 Ma. This lasted for approximately 3 million years and primarily affected life inhabiting warm, shallow seas. Trilobites fared badly but a few survived until the Permian extinction. Reef-building organisms were particularly badly affected and did not recover fully until the Mesozoic era. -> LINK <-
		Middle	386.00
		Early	408.50
	Silurian	Pridoli	410.70	Arthropods continued to dominate sea life. Some became capable of briefly emerging onto land. Complex reef systems developed in the tropics. Towards the end of the Silurian, small plants and fungi pioneered the colonisation of land - but remained near to streams and rivers. Permanently land-dwelling arthropods soon followed (millipedes were relatively common). Most were herbivores, a few were carnivores. At ~420 Ma, fish evolved armour plating and a movable lower jaw.
		Ludlow	424.00
		Wenlock	430.40
		Llandovery	439.00
	Ordovician	Bala	463.90	Animal life remained in the sea, dominated by the arthropods. Trilobites were numerous. The first scorpions evolved. Land remained largely barren. Some lichens and slime moulds inhabited river and stream banks. Extinction event, perhaps due to global cooling and... Gondwanaland glaciation ??? - --> LINK <-- -
		Dyfed	476.10
		Canadian	510.00
	Cambrian	Merioneth	517.20	Animals evolved hard parts, aiding fossilisation and therefore also our knowledge of such forms of life. The complex eye also evolved. This is often credited with increasing the rate of evolution of animals as complex predator-prey relationships resulted. Life remained confined to the sea, but several major animal groups emerged, including arthropods, molluscs, echinoderms and vertebrates (in the form of primitive jawless fish).
		St. David's	536.00
		Caerfai	570.00
Eon	Era	Period	Dates from... (Ma)	Events
Proterozoic 2500 Ma to 570 Ma	Sinian	Vendian	610.00	Complex multi-cellular (but soft-bodied) life emerged in warm, shallow coastal waters following the thaw at ~600 Ma. Evidence of this stage of life exists as Ediacaran fossils.
		Sturtian	800.00	There is much evidence that from ~750 Ma until ~600 Ma a severe ice age occurred in which the average global temperature dropped to -40°C and all the oceans froze over - the controversial 'Snowball Earth' theory.
	Riphean	Karatau	1050.00
		Yurmatin	1350.00
		Burzyan	1650.00	Eukaryotic (complex-celled) life evolved ~1,500 Ma. Not long after, simple multi-cellular organisms appeared such as jellyfish-like and filter feeding sponge-like animals.
	Animikian		2200.00	By 2,000 Ma the atmosphere contained a persistent level of oxygen (i.e. it was being produced in sufficient quantities that it could not all react with the environment), some of which formed an ozone layer.
	Huronian		2450.00
Archean 4000 Ma to 2500 Ma	Randian		2800.00
	Swazian		3500.00
	Isuan		3800.00	Cyanobacteria evolved sometime before 3,500 Ma. This form of life could extract energy from sunlight (photosynthesis). The first fossil evidence of life dates from this era; colonies of cyanobacteria form stromatolites.
	Hadean	Early Imbrian	3850.00	Life first appeared ~3,850 Ma in the primitive oceans, existing as simple prokaryotic bacteria.
		Nectarian	3950.00
Priscoan 4560 Ma to 4000 Ma		Basin Groups 1-9	4150.00
		Cryptic	4560.00
Key Brown Earth's evolution Purple Atmosphere, climate and ecosystems Yellow Precambrian organism evolution Blue Sea-dwelling animal evolution Orange Land-dwelling animal evolution Green Plant evolution Red Extinction event

 

Life

Life probably evolved spontaneously on Earth, but many astronomers are convinced that it is extra-terrestrial in origin. Many outer Solar System natural satellites contain large amounts of ice consisting of not only water but also hydrocarbons. Spectral analysis of material in cometary tails and comas has also revealed the presence of hydrogen, carbon, oxygen and nitrogen, which could have reached Earth very easily.