The Mohorovičić discontinuity ( , ), usually referred to as the Moho discontinuity or the Moho, is the boundary between the
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...
's
crust and the
mantle
A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that.
Mantle may refer to:
*Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear
**Mantle (vesture), an Eastern Orthodox ve ...
. It is defined by the distinct change in velocity of
seismic waves as they pass through changing densities of rock.
The Moho lies almost entirely within the
lithosphere (the hard outer layer of the Earth, including the crust).
Only beneath
mid-ocean ridge
A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a div ...
s does it define the lithosphere–
asthenosphere
The asthenosphere () is the mechanically weak and ductile region of the upper mantle of Earth. It lies below the lithosphere, at a depth between ~ below the surface, and extends as deep as . However, the lower boundary of the asthenosphere is ...
boundary (the depth at which the mantle becomes significantly ductile). The Mohorovičić discontinuity is below the
ocean floor, and beneath typical continental crusts, with an
average
In ordinary language, an average is a single number taken as representative of a list of numbers, usually the sum of the numbers divided by how many numbers are in the list (the arithmetic mean). For example, the average of the numbers 2, 3, 4, 7 ...
of .
Named after the pioneering
Croatian seismologist
Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other ...
Andrija Mohorovičić, the Moho separates both the
oceanic crust
Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic ...
and
continental crust from the underlying mantle. The Mohorovičić discontinuity was first identified in 1909 by Mohorovičić, when he observed that
seismograms from
shallow-focus earthquake
An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, fr ...
s had two sets of
P-wave
A P wave (primary wave or pressure wave) is one of the two main types of elastic body waves, called seismic waves in seismology. P waves travel faster than other seismic waves and hence are the first signal from an earthquake to arrive at any ...
s and
S-waves, one set that followed a direct path near the Earth's surface and the other
refracted by a high-velocity medium.
Nature and seismology
The Moho marks the transition in composition between the Earth's rocky outer crust and the more plastic mantle. Immediately above the Moho, the
velocities of primary seismic waves (P-waves) are consistent with those through
basalt
Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...
(6.7–7.2 km/s), and below they are similar to those through
peridotite
Peridotite ( ) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high pr ...
or
dunite
Dunite (), also known as olivinite (not to be confused with the mineral olivenite), is an intrusive igneous rock of ultramafic composition and with phaneritic (coarse-grained) texture. The mineral assemblage is greater than 90% olivine, w ...
(7.6–8.6 km/s).
This increase of approximately 1 km/s corresponds to a distinct change in material as the waves pass through the Earth, and is commonly accepted as the lower limit of the Earth's crust. The Moho is characterized by a transition zone of up to 500 meters. Ancient Moho zones are exposed above-ground in numerous
ophiolites around the world.
Beginning in the 1980s, geologists became aware that the Moho does not always coincide with the crust-mantle boundary defined by composition.
Xenolith
A xenolith ("foreign rock") is a rock fragment ( country rock) that becomes enveloped in a larger rock during the latter's development and solidification. In geology, the term ''xenolith'' is almost exclusively used to describe inclusions in ig ...
s (lower crust and upper mantle rock brought to the surface by volcanic eruptions) and seismic-reflection data showed that, away from continental
craton
A craton (, , or ; from grc-gre, κράτος "strength") is an old and stable part of the continental lithosphere, which consists of Earth's two topmost layers, the crust and the uppermost mantle. Having often survived cycles of merging and ...
s, the transition between crust and mantle is marked by
basaltic
Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron ( mafic lava) exposed at or very near the surface of a rocky planet or moon. More than ...
intrusion
In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...
s and may be up to 20 km thick. The Moho may lie well below the crust-mantle boundary and care must be used in interpreting the structure of the crust from seismic data alone.
Serpentinization of mantle rock below slowly spreading
mid-ocean ridge
A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a div ...
s can also increase the depth to the Moho, since serpentinization lowers seismic wave velocities.
History
Croatian seismologist
Andrija Mohorovičić is credited with first discovering and defining the Moho. In 1909, he was examining data from a local earthquake in
Zagreb
Zagreb ( , , , ) is the capital (political), capital and List of cities and towns in Croatia#List of cities and towns, largest city of Croatia. It is in the Northern Croatia, northwest of the country, along the Sava river, at the southern slop ...
when he observed two distinct sets of P-waves and S-waves propagating out from the focus of the earthquake. Mohorovičić knew that waves caused by earthquakes travel at velocities proportional to the density of the material carrying them. As a result of this information, he theorized that the second set of waves could only be caused by a sharp transition in density in the Earth's crust, which could account for such a dramatic change in wave velocity. Using velocity data from the earthquake, he was able to calculate the depth of the Moho to be approximately 54 km, which was supported by subsequent seismological studies.
The Moho has played a large role in the fields of
geology
Geology () is a branch of natural science concerned with Earth and other Astronomical object, astronomical objects, the features or rock (geology), rocks of which it is composed, and the processes by which they change over time. Modern geology ...
and earth science for well over a century. By observing the Moho's refractive nature and how it affects the speed of P-waves, scientists were able to theorize about the earth's composition. These early studies gave rise to modern
seismology
Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other ...
.
In the early 1960s,
Project Mohole was an attempt to drill to the Moho from deep-ocean regions.
After initial success in establishing deep-ocean drilling, the project suffered from political and scientific opposition, mismanagement, and
cost overruns, and it was cancelled in 1966.
[Mohole, LOCO, CORE, and JOIDES: A brief chronology](_blank)
Betty Shor, The Scripps Institution of Oceanography, August 1978, 7 pp. Access date 25 June 2019.
Exploration
Reaching the discontinuity by drilling remains an important scientific objective.
Soviet
The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a List of former transcontinental countries#Since 1700, transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, ...
scientists at the
Kola Superdeep Borehole pursued the goal from 1970 until 1992. They reached a depth of , the world's deepest hole, before abandoning the project. One proposal considers a rock-melting
radionuclide-powered capsule with a heavy
tungsten
Tungsten, or wolfram, is a chemical element with the symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identified as a new element in 1781 and first isol ...
needle that can propel itself down to the Moho discontinuity and explore
Earth's interior
The internal structure of Earth is the solid portion of the Earth, excluding its atmosphere and hydrosphere. The structure consists of an outer silicate solid crust, a highly viscous asthenosphere and solid mantle, a liquid outer core whose ...
near it and in the upper mantle. The Japanese project
Chikyu Hakken ("Earth Discovery") also aims to explore in this general area with the drilling ship, ''
Chikyū
is a Japanese scientific drilling ship built for the Integrated Ocean Drilling Program (IODP). The vessel is designed to ultimately drill beneath the seabed, where the Earth's crust is much thinner, and into the Earth's mantle, deeper than any ...
'', built for the
Integrated Ocean Drilling Program (IODP).
Plans called for the
drill-ship ''
JOIDES Resolution'' to sail from
Colombo
Colombo ( ; si, කොළඹ, translit=Koḷam̆ba, ; ta, கொழும்பு, translit=Koḻumpu, ) is the executive and judicial capital and largest city of Sri Lanka by population. According to the Brookings Institution, Colombo me ...
in
Sri Lanka
Sri Lanka (, ; si, ශ්රී ලංකා, Śrī Laṅkā, translit-std=ISO (); ta, இலங்கை, Ilaṅkai, translit-std=ISO ()), formerly known as Ceylon and officially the Democratic Socialist Republic of Sri Lanka, is an ...
in late 2015 and to head for the
Atlantis Bank, a promising location in the southwestern
Indian Ocean
The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or ~19.8% of the water on Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by ...
on the
Southwest Indian Ridge, to attempt to drill an initial bore hole to a depth of approximately 1.5 kilometres.
The attempt did not even reach 1.3 km, but researchers hope to further their investigations at a later date.
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See also
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Notes
References
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External links
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Map of the Moho depth of the European plate
{{DEFAULTSORT:Mohorovicic discontinuity
Plate tectonics
Structure of the Earth
Croatian inventions
Earth's crust