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– Time
Κόσμος – The Universe


Γη – Earth
Βίος – Life
Λόγος – Mind

The Earth
View of Earth as seen from Apollo 17
The following material has been copied and edited from Wikipedia. It is hoped that the page will be progressively customised with more material pertaining to processes through geological time.

Earth (often referred to as the Earth, Terra, or Planet Earth) is the third planet in the solar system in terms of distance from the Sun, and the fifth largest. It is also the largest of its planetary system's terrestrial planets, making it the largest solid body in the solar system, and it is the only place in the universe known to support life. Accepted scientific theory states that the Earth was formed around 4.57 billion years ago [1] and its largest natural satellite, the Moon, was orbiting it shortly thereafter, around 4.53 billion years ago.

Since it formed, the Earth has changed through geological and biological processes that have hidden traces of the original conditions. The outer surface is divided into several tectonic plates that gradually migrate across the surface over geologic time spans. The interior of the planet remains active, with a thick layer of convecting yet solid Earth mantle and an iron core that generates a magnetic field.

The outer layer of the Earth can be divided into several compartments: the hydrosphere (or sphere of water), the lithosphere (or sphere of soils and rocks), and the atmosphere (or sphere of the air), a biosphere (the totality of living organisms), and more recently and perhaps metaphorically a Noosphere.

The Earth's atmospheric conditions have been significantly altered by the presence of life forms, which create an ecological balance that modifies the surface conditions. About 71% of the surface is covered in salt-water oceans, and the remainder consists of continents and islands.

There is significant interaction between the Earth and its space environment. The relatively large moon provides ocean tides and has gradually modified the length of the planet's rotation period. A cometary bombardment during the early history of the planet is believed to have played a role in the formation of the oceans. Later, asteroid impacts are understood to have caused significant changes to the surface environment. Changes in the orbit of the planet may also be responsible for the ice ages that have covered significant portions of the surface in glacial sheets.



See menu below for more detailed coverage:

Magnetosphere Exosphere Atmosphere
Ionosphere Thermosphere
Hydrosphere Biosphere Stratosphere
Pedosphere Lithosphere Crust Geosphere
Upper Crust
Lower Crust
Upper Mantle Lithospheric Mantle Mantle
Deeper Upper Mantle
Lower Mantle
Outer Core Core
Inner Core
Topics: Climate | Composition of the Earth | Formation of the Earth | Gaia Hypothesis | Geography | History of the Earth | Plate tectonics | Structure of the Earth

History of the Earth

Main article: History of the Earth

Based on the available evidence, current scientists have been able to reconstruct detailed information about the planet's past. Earth is believed to have formed around 4.57 billion years ago out of the solar nebula, along with the Sun and the other planets. Initially molten, the outer layer of the planet cooled when water began accumulating in the atmosphere when the planet was about half its current radius, resulting in the solid crust. The moon formed soon afterwards, possibly as the result of the impact with a Mars-sized object known as Theia. Outgassing and volcanic activity produced the primordial atmosphere; condensing water vapor, augmented by ice delivered by comets, produced the oceans [2]. The highly energetic chemistry is believed to have produced a self-replicating molecule around 4 billion years ago, and half a billion years later, the last common ancestor of all life lived [3]

The development of photosynthesis allowed the sun's energy to be harvested directly; the resultant oxygen accumulated in the atmosphere and gave rise to the ozone layer. The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes [4]. Cells within colonies became increasingly specialized, resulting in true multicellular organisms. Aided by the absorption of harmful ultraviolet radiation by the ozone layer, life colonized the surface of Earth.

Over hundreds of millions of years, continents formed and broke up as the surface of Earth continually reshaped itself. The continents have migrated across the surface of the Earth, occasionally combining to form a supercontinent. Roughly 750 million years ago (mya), the earliest known supercontinent Rodinia, began to break apart. The continents later recombined to form Pannotia, 600–540 mya, then finally Pangaea, which broke apart 180 mya [5].

Since the 1960s, it has been hypothesized that severe glacial action between 750 and 580 mya, during the Neoproterozoic, covered much of the planet in a sheet of ice. This hypothesis has been termed "Snowball Earth", and is of particular interest because it preceded the Cambrian explosion, when multicellular lifeforms began to proliferate [6].

Since the Cambrian explosion, about 535 mya, there were five mass extinctions [7]. The last occurred 65 mya, when a meteorite collision probably triggered the extinction of the (non-avian) dinosaurs and other large reptiles, but spared small animals such as mammals, which then resembled shrews. Over the past 65 million years, mammalian life has diversified, and several mya, a small African ape gained the ability to stand upright. This enabled tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain. The development of agriculture, and then civilization, allowed humans to influence the Earth in a short timespan as no other life form had, affecting both the nature and quantity of other life forms, and the global climate.

Internal structure

Main article: Structure of the Earth
Earth cutaway from core to exosphere. Partially to scale

The interior of the Earth, like that of the other terrestrial planets, is chemically divided into layers. The Earth has an outer silicate solid crust, a highly viscous mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core.

The geologic component layers of the Earth [8] are at the following depths below the surface:

Depth Layer
Kilometres Miles
0–60 0–37 Lithosphere (locally varies between 5 and 200 km)
0–35 0–22 ... Crust (locally varies between 5 and 70 km)
35–60 22–37 ... Uppermost part of mantle
35–2890 22–1790 Mantle
100–700 62–435 ... Asthenosphere
2890–5100 1790–3160 Outer core
5100–6378 3160–3954 Inner core

Tectonic plates

Main article: Plate tectonics
A map pointing out the Earth's major plates.

According to plate tectonics theory currently accepted by the vast majority of scientists working in this area, the outermost part of the Earth's interior is made up of two layers: the lithosphere comprising the crust, and the solidified uppermost part of the mantle. Below the lithosphere lies the asthenosphere, which comprises the inner, viscous part of the mantle.

The lithosphere essentially floats on the asthenosphere. The lithosphere is broken up into what are called tectonic plates. These plates move in relation to one another at one of three types of plate boundaries: convergent, divergent, and transform. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along plate boundaries.


Main article: Landforms

The Earth's terrain varies greatly from place to place. About 70% of the surface is covered by water, with much of the continental shelf below sea level. If all of the land on Earth were spread evenly, then water would rise to an altitude of more than 2500 metres (approximately 8000 ft.). The remaining 30% not covered by water consists of mountains, deserts, plains, plateaus, etc.


Main article: Hydrosphere

The abundance of water on Earth is a unique feature that distinguishes the "Blue Planet" from others in the solar system. Approximately 70.8 percent of the Earth is covered by water and only 29.2 percent is terra firma.

The Earth's hydrosphere consists chiefly of the oceans, but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters. The average depth of the oceans is 3,794 m (12,447 ft), more than five times the average height of the continents. The mass of the oceans is approximately 1.35 × 10^18 tonnes, or about 1/4400 of the total mass of the Earth.


Main article: Atmosphere

The Earth's atmosphere has no definite boundary, slowly becoming thinner and fading into outer space. Three-quarters of the atmosphere's mass is contained within the first 11 km of the planet's surface. This lowest layer is called the troposphere. Further up, the atmosphere is usually divided into the stratosphere, mesosphere, and thermosphere. Beyond these, the exosphere thins out into the magnetosphere. An important part of the atmosphere for life on Earth is the ozone layer.


Main article: Biosphere

The totality of life on Earth is sometimes said to form a "biosphere". This biosphere is generally believed to have begun evolving about 3.5 billion (3.5 x 109) years ago. Earth is the only place in the universe officially recognized by the communities of Earth where life is absolutely known to exist, and some scientists believe that biospheres might be rare. It includes all living matter on the planet or that portion of the planet occupied by life. It reaches well into the other three spheres, although there are no permanent inhabitants of the atmosphere. Relative to the volume of the Earth, the biosphere is only the very thin surface layer which extends from 11,000 meters below sea level to 15,000 meters above.

The biosphere is divided into a number of biomes, inhabited by broadly similar flora and fauna. On land primarily latitude and height above the sea level separates biomes. Terrestrial biomes lying within the Arctic, Antarctic Circle or in high altitudes are relatively barren of plant and animal life, while most of the more populous biomes lie near the Equator.


Main article: Climate

The most prominent features of the Earth's climate are its two large polar regions, two narrow temperate zones, and a wide equatorial tropical region. Precipitation patterns vary widely, ranging from several metres of water per year to less than a millimetre.

Ocean currents are important factors in determining climate, particularly the spectacular thermohaline circulation which distributes heat energy from the equatorial oceans to the polar regions.


Main article: Pedosphere

The pedosphere is the outermost layer of the Earth that is composed of soil and subject to soil formation processes. It exists at the interface of the lithosphere, atmosphere, hydrosphere and biosphere.


[1] G.B. Dalrymple, 1991, "The Age of the Earth", Stanford University Press, California, ISBN 0-8047-1569-6.

[2] A. Morbidelli et al, 2000, "Source Regions and Time Scales for the Delivery of Water to Earth", Meteoritics & Planetary Science, vol. 35, no. 6, pp. 1309–20.

[3] W. Ford Doolitte, "Uprooting the Tree of Life", Scientific American, Feb. 2000.

[4] L. V. Berkner, L. C. Marshall, 1965, "On the Origin and Rise of Oxygen Concentration in the Earth's Atmosphere", Journal of the Atmospheric Sciences, Vol. 22, No. 3, pp. 225–61.

[5] J.B. Murphy, R.D. Nance, "How do supercontinents assemble?", American Scientist, vol. 92, pp. 324–33.

[6] J.L. Kirschvink, 1992, "Late Proterozoic Low-Latitude Global Glaciation: The Snowball Earth", The Proterozoic Biosphere, pp 51–52.

[7] D. Raup & J. Sepkoski, 1982, "Mass extinctions in the marine fossil record", Science, vol. 215, pp. 1501–03.

[8] T. H. Jordan, "Structural Geology of the Earth's Interior", Proceedings National Academy of Science, 1979, Sept., 76(9): 4192–4200.</ref>


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Earth menu adapted from the German Wikipedia Earth Sciences portal

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