Mohorovičić discontinuity

The Mohorovičić discontinuity (Croatian pronunciation: [moxorôʋiːt͡ʃit͡ɕ]),[1] usually referred to as the Moho, is the boundary between the Earth's crust and the mantle.

Named after the pioneering Croatian seismologist Andrija Mohorovičić, the Moho separates both the oceanic crust and continental crust from underlying mantle. The Mohorovičić discontinuity was first identified in 1909 by Mohorovičić, when he observed that seismograms from shallow-focus earthquakes had two sets of P-waves and S-waves, one that followed a direct path near the Earth's surface and the other refracted by a high-velocity medium.[2]

The Moho lies almost entirely within the lithosphere; only beneath mid-ocean ridges does it define the lithosphere–asthenosphere boundary. The Mohorovičić discontinuity is 5 to 10 kilometres (3–6 mi) below the ocean floor, and 20 to 90 kilometres (10–60 mi) beneath typical continental crusts, with an average of 35 kilometres (22 mi).[3]

Subduction-en
Earth's crust and mantle, Moho discontinuity between bottom of crust and solid uppermost mantle

Nature

Refraction of P-wave
Two paths of a P-wave, one direct and one refracted as it crosses the Moho.[2]
Gros Morne moho
Ordovician ophiolite in Gros Morne National Park, Newfoundland. This rock which formed the Ordovician Moho is exposed on the surface.

Immediately above the Moho, the velocities of primary seismic waves (P-waves) are consistent with those through basalt (6.7–7.2 km/s), and below they are similar to those through peridotite or dunite (7.6–8.6 km/s).[4] That suggests the Moho marks a change of composition, but the interface appears to be too even for any believable sorting mechanism within the Earth. Near-surface observations suggest such sorting produces an irregular surface.[5] The Moho is characterized by a transition zone of up to 500 m thick.[6] Ancient Moho zones are exposed above-ground in numerous ophiolites around the world.

Mohomap
The Moho maintains a relatively stable average depth of 10 km under the ocean sea floor, but can vary by more than 70 km below continental land masses.

History and Exploration

The Moho has played an integral part in the fields of geology and earth science for well over a century. Because of its refractive nature, the Moho allowed scientists to hypothesize about the layers of the earth without technology by measuring differences in time between refracted and non-refracted P-waves. These early studies gave rise to modern seismology.[7] During the late 1950s and early 1960s the executive committee of the U.S. National Science Foundation funded drilling a hole through the ocean floor to reach this boundary. However the operation, named Project Mohole, never received sufficient support and was mismanaged; the United States Congress canceled it in 1967. Soviet scientists at the Kola Institute pursued the goal simultaneously; after 15 years they reached a depth of 12,260 metres (40,220 ft), the world's deepest hole, before abandoning their attempt in 1989.[8]

Reaching the discontinuity remains an important scientific objective. One proposal considers a rock-melting radionuclide-powered capsule with a heavy tungsten needle that can propel itself down to the Moho discontinuity and explore Earth's interior near it and in the upper mantle.[9] The Japanese project Chikyu Hakken ("Earth Discovery") also aims to explore in this general area with the drilling ship, Chikyū, built for the Integrated Ocean Drilling Program (IODP).

Plans called for the drill-ship JOIDES Resolution to sail from Colombo in Sri Lanka in late 2015 and to head for the Atlantis Bank, a promising location in the southwestern Indian Ocean on the Southwest Indian Ridge, to attempt to drill an initial bore hole to a depth of approximately 1.5 kilometres.[10] The attempt did not even reach 1.3 km, but researchers hope to further their investigations at a later date.[11]

See also

Notes

  1. ^ Mangold, Max (2005). Aussprachewörterbuch (in German) (6th ed.). Mannheim: Dudenverlag. p. 559. ISBN 9783411040667.
  2. ^ a b Andrew McLeish (1992). Geological science (2nd ed.). Thomas Nelson & Sons. p. 122. ISBN 978-0-17-448221-5.
  3. ^ James Stewart Monroe; Reed Wicander (2008). The changing Earth: exploring geology and evolution (5th ed.). Cengage Learning. p. 216. ISBN 978-0-495-55480-6.
  4. ^ RB Cathcart & MM Ćirković (2006). Viorel Badescu; Richard Brook Cathcart & Roelof D Schuiling (eds.). Macro-engineering: a challenge for the future. Springer. p. 169. ISBN 978-1-4020-3739-9.
  5. ^ Benjamin Franklin Howell (1990). An introduction to seismological research: history and development. Cambridge University Press. p. 77 ff. ISBN 978-0-521-38571-8.
  6. ^ D.P. McKenzie - The Mohorovičić Discontinuity
  7. ^ Prodehl, Claus; Mooney, Walter D. (2012). "Exploring the Earth's Crust—History and Results of Controlled-Source Seismology". doi:10.1130/mem208. Cite journal requires |journal= (help)
  8. ^ "How the Soviets Drilled the Deepest Hole in the World". Wired. 2008-08-25. Retrieved 2008-08-26.
  9. ^ Ozhovan, M.; F. Gibb; P. Poluektov & E. Emets (August 2005). "Probing of the Interior Layers of the Earth with Self-Sinking Capsules". Atomic Energy. 99 (2): 556–562. doi:10.1007/s10512-005-0246-y.
  10. ^ Witze, Alexandra (December 2015). "Quest to drill into Earth's mantle restarts". Nature News. 528 (7580): 16–17. Bibcode:2015Natur.528...16W. doi:10.1038/528016a.
  11. ^ Kavanagh, Lucas (2016-01-27). "Looking Back on Expedition 360". JOIDES Resolution. Archived from the original on 2016-07-09. Retrieved 2016-09-21. We may not have made it to our goal of 1300 m, but we did drill the deepest ever single-leg hole into hard rock (789 m), which is currently the 5th deepest ever drilled into the hard ocean crust. We also obtained both the longest (2.85 m) and widest (18 cm) single pieces of hard rock ever recovered by the International Ocean Discovery Program and its predecessors! [...] Our hopes are high to return to this site in the not too distant future.

References

External links

1909 in science

The year 1909 in science and technology involved some significant events, listed below.

Abduction (novel)

Abduction is a 2000 novel written by Robin Cook.

Andrija Mohorovičić

Andrija Mohorovičić (23 January 1857 – 18 December 1936) was a Croatian meteorologist and seismologist. He is best known for the eponymous Mohorovičić discontinuity and is considered as one of the founders of modern seismology.

Boring (earth)

Boring is drilling a hole, tunnel, or well in the earth.

Boring is used for various applications in geology, agriculture, hydrology, civil engineering, and mineral exploration. Today, most earth drilling serves one of the following purposes:

return samples of the soil and/or rock through which the drill passes

access rocks from which material can be extracted

access rocks which can then be measured

provide access to rock for purposes of providing engineering supportUnlike drilling in other materials where the aim is to create a hole for some purpose, often the case of drilling or coring is to get an understanding of the ground/lithology. This may be done for prospecting to identify and quantify an ore body for mining, or to determining the type of foundations needed for a building or raised structure, or for underground structures, including tunnels and deep basements where an understanding of the ground is vital to determining how to excavate and the support philosophy. Drilling is also used in vertical and inclined shaft construction.

Conrad discontinuity

The Conrad discontinuity corresponds to the sub-horizontal boundary in continental crust at which the seismic wave velocity increases in a discontinuous way. This boundary is observed in various continental regions at a depth of 15 to 20 km, however it is not found in oceanic regions.

The Conrad discontinuity (named after the seismologist Victor Conrad) is considered to be the border between the upper continental crust and the lower one. It is not as pronounced as the Mohorovičić discontinuity, and absent in some continental regions. Up to the middle 20th Century the upper crust in continental regions was seen to consist of felsic rocks such as granite (sial, for silica-aluminium), and the lower one to consist of more magnesium-rich mafic rocks like basalt (sima, for silica-magnesium). Therefore, the seismologists of that time considered that the Conrad discontinuity should correspond to a sharply defined contact between the chemically distinct two layers, sial and sima.However, from the 1960s onward this theory was strongly contested among geologists. The exact geological significance of the Conrad discontinuity is still not clarified. The possibility that it represents the transition from amphibolite facies to granulite facies metamorphism has been given some support from observations of the uplifted central part of the Vredefort crater and the surrounding Kaapvaal Craton.

Earth's crust

The Earth's crust is a thin shell on the outside of the Earth, accounting for less than 1% of Earth's volume. It is the top component of lithosphere: a division of Earth's layers that includes the crust and the upper part of the mantle. The lithosphere is broken into tectonic plates that move, allowing heat to escape from the interior of the Earth into space.

The crust lies on top of the mantle, a configuration that is stable because the upper mantle is made of peridotite and so is significantly denser than the crust. The boundary between the crust and mantle is conventionally placed at the Mohorovičić discontinuity, a boundary defined by a contrast in seismic velocity.

The crust of the Earth is of two distinctive types:

Oceanic: 5 km (3 mi) to 10 km (6 mi) thick and composed primarily of denser, more mafic rocks, such as basalt, diabase, and gabbro.

Continental: 30 km (20 mi) to 50 km (30 mi) thick and mostly composed of less dense, more felsic rocks, such as granite.Because both continental and oceanic crust are less dense than the mantle below, both types of crust "float" on the mantle. This is isostasy, and it's also one of the reasons continental crust is higher than oceanic: continental is less dense and so "floats" higher. As a result, water pools in above the oceanic crust, forming the oceans.

The temperature of the crust increases with depth, reaching values typically in the range from about 200 °C (392 °F) to 400 °C (752 °F) at the boundary with the underlying mantle. The temperature increases by as much as 30 °C (54 °F) for every kilometer locally in the upper part of the crust, but the geothermal gradient is smaller in deeper crust.

Ivrea zone

The Ivrea zone is a tectonic terrane in the Italian Alps, that consists of a steeply dipping piece of the Earth’s lower crust of the Apulian plate. The zone is named after the Italian city of Ivrea.

Geologically the Ivrea zone is considered a part of the Southern Alps. Most rocks in the zone are sedimentary, for example limestones that have been turned into marble by metamorphism. Most of the zone has been through granulite facies metamorphism and was intruded by mafic plutons. This is the type of rock common in the lower regions of the crust. When the terrane was uplifted during the formation of the Alps, the upper crust was eroded off so that these rocks are now at the surface.

Geophysical research shows the mantle is relatively close under the surface at the Ivrea zone. Some geologists think the boundary between pyroxenites and lherzolites, that is also found in outcrops in the Ivrea zone, represents the Mohorovičić discontinuity (the Moho). The Moho is seismologically defined as the boundary between the crust and mantle.

List of Croatian inventions and discoveries

Croatian inventions and discoveries are objects, processes or techniques invented or discovered, by people from Croatia.

Lithosphere

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 layer under the lithosphere is known as the asthenosphere.

Magmatic underplating

Magmatic underplating occurs when basaltic magmas are trapped during their rise to the surface at the Mohorovičić discontinuity or within the crust. Entrapment (or 'stalling out') of magmas within the crust occurs due to the difference in relative densities between the rising magma and the surrounding rock. Magmatic underplating can be responsible for thickening of the crust when the magma cools. Geophysical seismic studies (as well as igneous petrology and geochemistry) utilize the differences in densities to identify underplating that occurs at depth.

Project Mohole

Project Mohole was an attempt in the early 1960s to drill through the Earth's crust to obtain samples of the Mohorovičić discontinuity, or Moho, the boundary between the Earth's crust and mantle. The project was to provide an Earth Science complement to the high-profile Space Race. While such a project was not feasible on land, drilling in the open ocean would be more feasible, because the mantle is much closer to the sea floor. The project was initially led by a group of scientists called the American Miscellaneous Society with funding from the National Science Foundation. The project suffered from political and scientific battles, mismanagement, and cost over-runs, and the U.S. House of Representatives discontinued funding for the project in 1966. By then a program of sediment drilling had branched from Project Mohole to become the Deep Sea Drilling Project of the National Science Foundation.

Structure of the Earth

The internal structure of the Earth is layered in spherical shells: an outer silicate solid crust, a highly viscous asthenosphere and mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core. Scientific understanding of the internal structure of the Earth is based on observations of topography and bathymetry, observations of rock in outcrop, samples brought to the surface from greater depths by volcanoes or volcanic activity, analysis of the seismic waves that pass through the Earth, measurements of the gravitational and magnetic fields of the Earth, and experiments with crystalline solids at pressures and temperatures characteristic of the Earth's deep interior.

Tectonics

Tectonics (from Latin tectonicus; from Ancient Greek τεκτονικός (tektonikos), meaning 'pertaining to building') is the process that controls the structure and properties of the Earth's crust and its evolution through time. In particular, it describes the processes of mountain building, the growth and behavior of the strong, old cores of continents known as cratons, and the ways in which the relatively rigid plates that constitute the Earth's outer shell interact with each other. Tectonics also provides a framework for understanding the earthquake and volcanic belts that directly affect much of the global population. Tectonic studies are important as guides for economic geologists searching for fossil fuels and ore deposits of metallic and nonmetallic resources. An understanding of tectonic principles is essential to geomorphologists to explain erosion patterns and other Earth surface features.

The Mohole Mystery

The Mohole Mystery is a juvenile science fiction novel, the eleventh in Hugh Walters' Chris Godfrey of U.N.E.X.A. series. It was published in the UK by Faber in 1968, in the US by Criterion Books in 1969 under the title The Mohole Menace. It was also published in French as Pionniers des ténèbres, (literally "Pioneers of Darkness") by Éditions de l'Amitié in 1973 and as A ameaça de Mohole in Portuguese by Edições Dêagã.

Travel to the Earth's center

Travelling to the Earth's center is a popular theme in science fiction. Some subterranean fiction involves traveling to the Earth's center and finding either a Hollow Earth or Earth's molten core. Planetary scientist David J. Stevenson suggested sending a probe to the core as a thought experiment. Humans have drilled over 12 kilometers (7.67 miles) in the Sakhalin-I. In terms of depth below the surface, the Kola Superdeep Borehole SG-3 retains the world record at 12,262 metres (40,230 ft) in 1989 and still is the deepest artificial point on Earth.

Undersea mountain range

Undersea mountain ranges are mountain ranges that are mostly or entirely underwater, and specifically under the surface of an ocean. If originated from current tectonic forces, they are often referred to as a mid-ocean ridge. In contrast, if formed by past above-water volcanism, they are known as a seamount chain. The largest and best known undersea mountain range is a mid-ocean ridge, the Mid-Atlantic Ridge. It has been observed that, "similar to those on land, the undersea mountain ranges are the loci of frequent volcanic and earthquake activity".

Upper mantle (Earth)

The upper mantle of the Earth begins just beneath the crust (at about 10 km (6.2 mi) under the oceans and about 35 km (22 mi) under the continents) and ends at the top of the lower mantle at 670 km (420 mi). Temperatures range from approximately 200 °C (392 °F) at the upper boundary with the crust to approximately 900 °C (1,650 °F) at the boundary with the lower mantle. Upper mantle material which has come up onto the surface is made up of about 55% olivine, 35% pyroxene and 5 to 10% of calcium oxide and aluminum oxide minerals such as plagioclase, spinel or garnet, depending upon depth.

Wave base

The wave base, in physical oceanography, is the maximum depth at which a water wave's passage causes significant water motion. For water depths deeper than the wave base, bottom sediments and the seafloor are no longer stirred by the wave motion above.

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