Structure of the Earth
The Earth's structure is the geologic and atmospheric structure of earth.
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Shape
The Earth's shape is that of an oblate spheroid, with an average diameter of approximately 12,742 km. The rotation of the Earth causes the equator to bulge out slightly so that the equatorial diameter is 43 km larger than the pole to pole diameter. The largest local deviations in the rocky surface of the Earth are Mount Everest (8,850 m above local sea level) and the Mariana Trench (10,911 m below local sea level). Hence compared to a perfect ellipsoid, the Earth has a tolerance of about one part in about 584, or 0.17%. For perspective, this is less than the 0.22% tolerance allowed in pool balls. Due to the bulge, the feature farthest from the center of the Earth is actually Mount Chimborazo in Ecuador. The mass of the Earth is approximately 5,980 yottagrams (5.98 x 1024 kg).
Structure
The interior of the Earth, like that of the other terrestrial planets, is chemically divided into an outer siliceous solid crust, a highly viscous mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core. The liquid outer core gives rise to a weak magnetic field due to the convection of its electrically conductive material.
New material constantly finds its way to the surface through volcanoes and cracks in the ocean floors (see seafloor spreading). Many of the rocks now making up the Earth's crust formed less than 100 million (1×108) years ago; however the oldest known mineral grains are 4.4 billion (4.4×109) years old, indicating that the Earth has had a solid crust for at least that long [1].
Taken as a whole, the Earth's composition by mass [2] is:
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iron: 34 .1 % oxygen: 28 .2 % silicon: 17 .2 % magnesium: 15 .9 % nickel: 1 .6 % calcium: 1 .6 % aluminium: 1 .5 % sulfur: 0 .70 % sodium: 0 .25 % titanium: 0 .071 % potassium: 0 .019 % other elements: 0 .53 %
Interior
Interior heat
- Main article: geothermal (geology)
The interior of the Earth reaches temperatures of 5650 ± 600 kelvins [3] [4]. The planet's internal heat was originally generated during its accretion (see gravitational binding energy), and since then additional heat has continued to be generated by the decay of radioactive elements such as uranium, thorium, and potassium. The heat flow from the interior to the surface is only 1/20,000 as great as the energy received from the Sun.
Structure
Earth's composition (by depth below surface):
- 0 to 60 km - Lithosphere (locally varies 5-200 km)
- 0 to 35 km - Crust (locally varies 5-70 km)
- 35 to 60 km - Uppermost part of mantle
- 35 to 2890 km - Mantle
- 100 to 700 km - Asthenosphere
- 2890 to 5100 km - Outer Core
- 5100 to 6378 km - Inner Core
The core
The average density of the Earth is 5515 kg/m3, making it the densest planet in the Solar system. Since the average density of surface material is only around 3000 kg/m3, we must conclude that denser materials exist within the core of the Earth. In its earliest stages, about 4.5 billion (4.5×109) years ago, melting would have caused denser substances to sink toward the center in a process called planetary differentiation, while less-dense materials would have migrated to the crust. As a result, the core is largely composed of iron (80%), along with nickel and one or more light elements, whereas other dense elements, such as lead and uranium, either are too rare to be significant or tend to bind to lighter elements and thus remain in the crust (see felsic materials).
The core is divided into two parts, a solid inner core with a radius of ~1250 km and a liquid outer core extending beyond it to a radius of ~3500 km. The inner core is generally believed to be solid and composed primarily of iron and some nickel. Some have argued that the inner core may be in the form of a single iron crystal. The outer core surrounds the inner core and is believed to be composed of liquid iron mixed with liquid nickel and trace amounts of lighter elements. It is generally believed that convection in the outer core, combined with stirring caused by the Earth's rotation (see: Coriolis effect), gives rise to the Earth's magnetic field through a process described by the dynamo theory. The solid inner core is too hot to hold a permanent magnetic field (see Curie temperature) but probably acts to stabilise the magnetic field generated by the liquid outer core.
Recent evidence has suggested that the inner core of Earth may rotate slightly faster than the rest of the planet. In August 2005 a team of geophysicists announced in the journal Science that, according to their estimates, the earth's core rotated approximately 0.3 to 0.5 degrees per year relative to the rotation of the surface [5].
Mantle
- Main article: Mantle (geology)
Earth's mantle extends to a depth of 2890 km. The pressure, at the bottom of the mantle, is ~140 GPa (1.4 Matm). It is largely composed of substances rich in iron and magnesium. The melting point of a substance depends on the pressure it is under. As there is intense and increasing pressure as one travels deeper into the mantle, the lower part of this region is thought solid while the upper mantle is plastic (semi-molten). The viscosity of the upper mantle ranges between 1021 and 1024 Pa·s, depending on depth [6]. Thus, the upper mantle can only flow very slowly.
Why is the inner core thought solid, the outer core thought liquid, and the mantle solid/plastic? The melting points of iron-rich substances are higher than that of pure iron. The core is composed almost entirely of pure iron, whereas iron-rich substances are more common outside the core. So, surface iron-substances are solid, upper mantle iron-substances are semi-molten (as it is hot and they are under relatively little pressure), lower mantle iron-substances are solid (as they are under tremendous pressure), outer core pure iron is liquid as it has a very low melting point (despite enormous pressure), and the inner core is solid due to the overwhelming pressure found at the center of the planet.
The Crust
The crust ranges from 5 to 70 km in depth. The thin parts are oceanic crust composed of dense (mafic) iron magnesium silicate rocks and underlie the ocean basins. The thicker crust is continental crust, which is less dense and composed of (felsic) sodium potassium aluminium silicate rocks. The crust-mantle boundary occurs as two physically different events. First, there is a discontinuity in the seismic velocity, which is known as the Mohorovičić discontinuity or Moho. The cause of the Moho is thought to be a change in rock composition from rocks containing plagioclase feldspar (above) to rocks that contain no feldspars (below). Second, there is a chemical discontinuity between ultramafic cumulates and tectonized harzburgites, which has been observed from deep parts of the oceanic crust that have been obducted into the continental crust and preseved as ophiolite sequences.
Hydrosphere
- Main article: Ocean
Earth is the only planet in our solar system whose surface is known to have liquid water. Water covers 71% of Earth's surface (97% of it being sea water and 3% fresh water [7]); the surface is divided into five oceans and seven continents. Earth's solar orbit, vulcanism, gravity, greenhouse effect, magnetic field and oxygen-rich atmosphere seem to combine to make Earth a water planet.
Earth is actually beyond the outer edge of the orbits which would be warm enough to form liquid water. Without some form of a greenhouse effect, Earth's water would freeze. Paleontological evidence indicates that at one point after blue-green bacteria (Cyanobacteria) had colonized the oceans, the greenhouse effect failed, and Earth's oceans may have completely frozen over for 10 to 100 million years in what is called a snowball Earth event.
On other planets, such as Venus, gaseous water is destroyed (cracked) by solar ultraviolet radiation, and the hydrogen is ionized and blown away by the solar wind. This effect is slow, but inexorable. This is one hypothesis explaining why Venus has no water. Without hydrogen, the oxygen interacts with the surface and is bound up in solid minerals.
In the Earth's atmosphere, a tenuous layer of ozone within the stratosphere absorbs most of this energetic ultraviolet radiation high in the atmosphere, reducing the cracking effect. The ozone, too, can only be produced in an atmosphere with a large amount of free diatomic oxygen, and so also is dependent on the biosphere (plants). The magnetosphere also shields the ionosphere from direct scouring by the solar wind.
Finally, vulcanism continuously emits water vapor from the interior. Earth's plate tectonics recycle carbon and water as limestone rocks are subducted into the mantle and volcanically released as gaseous carbon dioxide and steam. It is estimated that the minerals in the mantle may contain as much as 10 times the water as in all of the current oceans, though most of this trapped water will never be released.
The total mass of the hydrosphere is about 1.4×1021 kg, ca. 0.023% of the Earth's total mass.
Atmosphere
- Main article: Earth's atmosphere
Earth has a relatively thick atmosphere composed of 78% nitrogen, 21% oxygen, and 1% argon, plus traces of other gases including carbon dioxide and water vapor. The atmosphere acts as a buffer between Earth and the Sun. The Earth's atmospheric composition is unstable, and is maintained by the biosphere. The large amount of free diatomic oxygen is maintained through solar energy by the Earth's plants, and, without the plants supplying it, the oxygen in the atmosphere will over geological timescales combine with material from the surface of the Earth. Free oxygen in the atmosphere is a signature of life.
The layers, troposphere, stratosphere, mesosphere, thermosphere, and the exosphere, vary around the globe and in response to seasonal changes.
The total mass of the atmosphere is about 5.1×1018 kg, ca. 0.9 ppm of the Earth's total mass.