A lithosphere (Ancient Greek: λίθος [lithos] for "rocky", and σφαίρα [sphaira] for "sphere") is the rigid, outermost shell of a terrestrial-type planet or natural satellite that is defined by its rigid mechanical properties. On Earth, it is composed of the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater. The outermost shell of a rocky planet, the crust, is defined on the basis of its chemistry and mineralogy.
The study of past and current formations of landscapes is called geomorphology.
Earth's lithosphere includes the crust and the uppermost mantle, which constitute the hard and rigid outer layer of the Earth. The lithosphere is subdivided into tectonic plates. The uppermost part of the lithosphere that chemically reacts to the atmosphere, hydrosphere and biosphere through the soil forming process is called the pedosphere. The lithosphere is underlain by the asthenosphere which is the weaker, hotter, and deeper part of the upper mantle. The Lithosphere-Asthenosphere boundary is defined by a difference in response to stress: the lithosphere remains rigid for very long periods of geologic time in which it deforms elastically and through brittle failure, while the asthenosphere deforms viscously and accommodates strain through plastic deformation.
History of the concept
The concept of the lithosphere as Earth’s strong outer layer was described by A.E.H. Love in his 1911 monograph "Some problems of Geodynamics" and further developed by Joseph Barrell, who wrote a series of papers about the concept and introduced the term "lithosphere". The concept was based on the presence of significant gravity anomalies over continental crust, from which he inferred that there must exist a strong, solid upper layer (which he called the lithosphere) above a weaker layer which could flow (which he called the asthenosphere). These ideas were expanded by Reginald Aldworth Daly in 1940 with his seminal work "Strength and Structure of the Earth." They have been broadly accepted by geologists and geophysicists. These concepts of a strong lithosphere resting on a weak asthenosphere are essential to the theory of plate tectonics.
There are two types of lithosphere:
- Oceanic lithosphere, which is associated with oceanic crust and exists in the ocean basins (mean density of about 2.9 grams per cubic centimeter)
- Continental lithosphere, which is associated with continental crust (mean density of about 2.7 grams per cubic centimeter)
The thickness of the lithosphere is considered to be the depth to the isotherm associated with the transition between brittle and viscous behavior. The temperature at which olivine begins to deform viscously (~1000 °C) is often used to set this isotherm because olivine is generally the weakest mineral in the upper mantle. Oceanic lithosphere is typically about 50–140 km thick (but beneath the mid-ocean ridges is no thicker than the crust), while continental lithosphere has a range in thickness from about 40 km to perhaps 280 km; the upper ~30 to ~50 km of typical continental lithosphere is crust. The mantle part of the lithosphere consists largely of peridotite. The crust is distinguished from the upper mantle by the change in chemical composition that takes place at the Moho discontinuity.
Oceanic lithosphere consists mainly of mafic crust and ultramafic mantle (peridotite) and is denser than continental lithosphere, for which the mantle is associated with crust made of felsic rocks. Oceanic lithosphere thickens as it ages and moves away from the mid-ocean ridge. This thickening occurs by conductive cooling, which converts hot asthenosphere into lithospheric mantle and causes the oceanic lithosphere to become increasingly thick and dense with age. In fact, oceanic lithosphere is a thermal boundary layer for the convection in the mantle. The thickness of the mantle part of the oceanic lithosphere can be approximated as a thermal boundary layer that thickens as the square root of time.
Here, is the thickness of the oceanic mantle lithosphere, is the thermal diffusivity (approximately 10−6 m2/s) for silicate rocks, and is the age of the given part of the lithosphere. The age is often equal to L/V, where L is the distance from the spreading centre of mid-oceanic ridge, and V is velocity of the lithospheric plate.
Oceanic lithosphere is less dense than asthenosphere for a few tens of millions of years but after this becomes increasingly denser than asthenosphere. This is because the chemically differentiated oceanic crust is lighter than asthenosphere, but thermal contraction of the mantle lithosphere makes it more dense than the asthenosphere. The gravitational instability of mature oceanic lithosphere has the effect that at subduction zones, oceanic lithosphere invariably sinks underneath the overriding lithosphere, which can be oceanic or continental. New oceanic lithosphere is constantly being produced at mid-ocean ridges and is recycled back to the mantle at subduction zones. As a result, oceanic lithosphere is much younger than continental lithosphere: the oldest oceanic lithosphere is about 170 million years old, while parts of the continental lithosphere are billions of years old. The oldest parts of continental lithosphere underlie cratons, and the mantle lithosphere there is thicker and less dense than typical; the relatively low density of such mantle "roots of cratons" helps to stabilize these regions.
Geophysical studies in the early 21st century posit that large pieces of the lithosphere have been subducted into the mantle as deep as 2900 km to near the core-mantle boundary, while others "float" in the upper mantle, while some stick down into the mantle as far as 400 km but remain "attached" to the continental plate above, similar to the extent of the "tectosphere" proposed by Jordan in 1988.
Geoscientists can directly study the nature of the subcontinental mantle by examining mantle xenoliths brought up in kimberlite, lamproite, and other volcanic pipes. The histories of these xenoliths have been investigated by many methods, including analyses of abundances of isotopes of osmium and rhenium. Such studies have confirmed that mantle lithospheres below some cratons have persisted for periods in excess of 3 billion years, despite the mantle flow that accompanies plate tectonics.
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- ^Daly, R. (1940) Strength and structure of the Earth. New York: Prentice-Hall.
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The structure of the Earth is divided into layers. These layers are both physically and chemically different. The Earth has an outer solid crust, a highly viscousmantle, a liquid outer core, and a solid inner core.The shape of the earth is an oblate spheroid, because it is slightly flattened at the poles and bulging at the equator
The boundaries between these layers were discovered by seismographs which showed the way vibrations bounced off the layers during earthquakes. Between the Earth's crust and the mantle is a boundary called the moho. It was the first discovery of a major change in the Earth's structure as one goes deeper.
- The crust is the outermost layer of the Earth. It is made of solid rocks. It is mostly made of the lighter elements, silicon, oxygen, aluminium. Because of this, it is known as sial (silicon = Si; aluminium = Al) or felsic.
- The mantle is the layer of the Earth right below the crust. It is made mostly of oxygen, silicon and the heavier element magnesium. It is known as sima (Si for silicon + ma for magnesium) or mafic. The mantle itself is divided into layers.
- The uppermost part of the mantle is solid, and forms the base of the crust. It is made of the heavy rock peridotite. The continental and oceanic plates include both the crust proper and this uppermost solid layer of the mantle. Together this mass makes up the lithosphere. The lithosphere plates float on the semi-liquid aesthenosphere below.
- Upper aesthenosphere: magma
- Lower aesthenosphere
- Lower mantle
- The Earth's core is made of solid iron and nickel, and is about 5000–6000oC.
- Outer core is a liquid layer below the mantle,
- Inner core, is the very center of the Earth. It is very hot and, due to the high pressure, it is solid.
A full explanation of these effects is not yet clear. It seems that with the increasing heat and pressure comes changes in the crystallization of minerals, so that the composition might be a kind of changing mixture of liquid and crystals.
The moho[change | change source]
The moho, properly called the Mohorovičić discontinuity, is the boundary between the Earth's crust and the mantle. It was discovered by Croatianseismologist Andrija Mohorovičić in 1909. He discovered that seismograms of earthquakes showed two kinds of seismic waves. There is a shallow slower wave which arrives first, and a deep faster wave which arrives second. He reasoned that the deeper wave changed speed as it got just below the mantle. The reason it went faster was that the material of the mantle was different from that of the crust.
The discontinuity lies 30–40 km below the surface of continents, and less deep below the ocean floors.
Drilling holes[change | change source]
Geologists have been trying to get at the Moho for years. During the late 1950s and early 1960s Project Mohole did not get enough support, and was cancelled by the United States Congress in 1967. Efforts were also made by the Soviet Union. They reached a depth of 12,260 metres (40,220 ft) over 15 years, the world's deepest hole, before abandoning the attempt in 1989.
Reaching the discontinuity is still an important scientific target. A more recent proposal considers a self-descending tungsten capsule. The idea is that the capsule would be filled with radioactive material. This would give off enough heat to melt the surrounding rock, and the capsule would be pulled down by gravity.
The Japanese project Chikyū Hakken ("Earth discovery") plans to use a drilling shop to drill down through the thinner ocean crust. On 6 September 2012 Scientific deep sea drilling vessel Chikyu set a new world record by drilling down and obtaining rock samples from deeper than 2,111 meters below the seafloor off Shimokita Peninsula of Japan in the northwest Pacific Ocean.
Macquarie Island[change | change source]
Macquarie Island, off Tasmania, is at the meeting-point of two huge oceanic plates: the Pacific Plate and the Indo-Australian Plate. The island is made of material pushed up from deep in the Earth's mantle. It is thought that the green ophiolite rock was formed at the moho, and was brought up by a mid-oceanic ridge. Now it comes to the surface because the two plates are scrunching together. It is the only place on Earth where this is happening at present. There are other places where ophiolite is found, but they were brought up many millions of years ago. Ophiolites are found in all the major mountain belts of the world.
References[change | change source]
- ↑ 1.01.1Levin H. 2006. The Earth through time. 8th ed, New York: Wiley. Chapter 7, p184. ISBN 0-471-69743-5
- ↑"How the Soviets drilled the deepest hole in the world". Wired 2008. Retrieved 2008-08-26.
- ↑Ozhovan M. et al 2005. Probing of the interior layers of the Earth with self-sinking capsules. Atomic Energy. 99, 556–562. doi:10.1007/s10512-005-0246-y.
- ↑A report on the findings does not appear to be published yet. The following link is to the planning proposal, April 30 2012. 
- ↑That is, the junction between the bottom of the Earth's crust and the top of the Earth's mantle.
- ↑Macquarie Island - UNESCO World Heritage Centre. Whc.unesco.org. Retrieved on 2013-07-16.
- ↑Ben-Avraham Z. et al 1982. The emplacement of ophiolites by collision. Journal of Geophysical Research: Solid Earth (1978-2012)87 (B5) 3861-3867.