Mohorovičić discontinuity
{{Short description|Boundary between the Earth's crust and the mantle}}
The Mohorovičić discontinuity ({{IPAc-en|ˌ|m|oʊ|h|ə|ˈ|r|oʊ|v|ɪ|tʃ|ɪ|tʃ}} {{respell|MOH|hə|ROH|vih|chitch}}; {{IPA|hr|moxorôʋiːtʃitɕ|lang}}){{cite book |first=Max|last=Mangold |title=Aussprachewörterbuch |language=de |date=2005 |publisher=Dudenverlag |edition=6th |location=Mannheim |isbn=9783411040667 |page=559}}{{snd}}usually called the Moho discontinuity, Moho boundary, or just Moho{{snd}}is the boundary between the crust and the mantle of Earth. It is defined by the distinct change in velocity of seismic waves as they pass through changing densities of rock.{{Citation|last1=Rudnick|first1=R. L.|chapter=3.01 – Composition of the Continental Crust|year=2003|chapter-url=http://www.sciencedirect.com/science/article/pii/B0080437516030164|title=Treatise on Geochemistry|editor-last=Holland|editor-first=Heinrich D.|publisher=Pergamon|doi=10.1016/b0-08-043751-6/03016-4|isbn=978-0-08-043751-4|access-date=2019-11-21|last2=Gao|first2=S.|volume=3|page=659|bibcode=2003TrGeo...3....1R|editor2-last=Turekian|editor2-first=Karl K.}}
The Moho lies almost entirely within the lithosphere (the hard outer layer of the Earth, including the crust).{{cite book|url=https://books.google.com/books?id=jFPMa4MxwJkC&pg=PA216|title=The changing Earth: exploring geology and evolution|author=James Stewart Monroe|author2=Reed Wicander|date=2008|publisher=Cengage Learning|isbn=978-0-495-55480-6|edition=5th|page=216}} Only beneath mid-ocean ridges does it define the lithosphere–asthenosphere boundary (the depth at which the mantle becomes significantly ductile). The Mohorovičić discontinuity is {{convert|5|to(-)|10|km|mi|sigfig=1}} below the ocean floor, and {{convert|20|to(-)|90|km|mi|sigfig=1}} beneath typical continental crusts, with an average of {{convert|35|km|mi}}.
Named after the pioneering Croatian seismologist Andrija Mohorovičić, the Moho separates both the oceanic crust 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 earthquakes had two sets of P-waves and S-waves, one set that followed a direct path near the Earth's surface and the other refracted by a high-velocity medium.{{cite book |title=Geological science |author=Andrew McLeish |page=122 |url=https://books.google.com/books?id=rhkgwEvrVe8C&pg=PA122 |isbn=978-0-17-448221-5 |date=1992 |edition=2nd |publisher=Thomas Nelson & Sons}}
Nature and seismology
Image:Gros Morne moho.jpg ophiolite in Gros Morne National Park, Newfoundland. This rock which formed the Ordovician Moho is exposed on the surface.]]
The Moho marks the transition in composition between the Earth's crust and the lithospheric mantle. 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).{{cite book |title=Macro-engineering: a challenge for the future |page=169 |author=RB Cathcart |author2=MM Ćirković |name-list-style=amp |editor=Viorel Badescu |editor2=Richard Brook Cathcart |editor2-link=Richard Brook Cathcart |editor3=Roelof D Schuiling |url=https://books.google.com/books?id=5bZBEM31K1MC&pg=PA169 |isbn=978-1-4020-3739-9 |date=2006 |publisher=Springer}} 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.{{Cite book |last=Mckenzie |first=D. P. |title=Geophysical Monograph Series |date=2013-03-25 |publisher=American Geophysical Union |isbn=978-1-118-66897-9 |editor-last=Hart |editor-first=Pembroke J. |location=Washington, D. C. |pages=660–664 |doi=10.1029/gm013p0660}} Ancient Moho zones are exposed above-ground in numerous ophiolites around the world.{{Cite journal|last1=Korenaga|first1=Jun|last2=Kelemen|first2=Peter B.|date=1997-12-10|title=Origin of gabbro sills in the Moho transition zone of the Oman ophiolite: Implications for magma transport in the oceanic lower crust|journal=Journal of Geophysical Research: Solid Earth|language=en|volume=102|issue=B12|pages=27729–27749|doi=10.1029/97JB02604|bibcode=1997JGR...10227729K}}
File:Global Moho depth map of the Earth.png
Beginning in the 1980s, geologists became aware that the Moho does not always coincide with the crust-mantle boundary defined by composition. Xenoliths (lower crust and upper mantle rock brought to the surface by volcanic eruptions) and seismic-reflection data showed that, away from continental cratons, the transition between crust and mantle is marked by basaltic intrusions 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.{{cite journal |last1=O'Reilly |first1=Suzanne Y. |last2=Griffin |first2=W.L. |title=Moho vs crust–mantle boundary: Evolution of an idea |journal=Tectonophysics |date=December 2013 |volume=609 |pages=535–546 |doi=10.1016/j.tecto.2012.12.031|bibcode=2013Tectp.609..535O }}
Serpentinization of mantle rock below slowly spreading mid-ocean ridges can also increase the depth to the Moho, since serpentinization lowers seismic wave velocities.{{cite journal |last1=Minshull |first1=T. A. |last2=Muller |first2=M. R. |last3=Robinson |first3=C. J. |last4=White |first4=R. S. |last5=Bickle |first5=M. J. |title=Is the oceanic Moho a serpentinization front? |journal=Geological Society, London, Special Publications |date=1998 |volume=148 |issue=1 |pages=71–80 |doi=10.1144/GSL.SP.1998.148.01.05|bibcode=1998GSLSP.148...71M |s2cid=128410328 }}{{cite journal |last1=Mével |first1=Catherine |title=Serpentinization of abyssal peridotites at mid-ocean ridges |journal=Comptes Rendus Geoscience |date=September 2003 |volume=335 |issue=10–11 |pages=825–852 |doi=10.1016/j.crte.2003.08.006|bibcode=2003CRGeo.335..825M |url=https://comptes-rendus.academie-sciences.fr/geoscience/articles/10.1016/j.crte.2003.08.006/ }}
History
Croatian seismologist Andrija Mohorovičić is credited with discovering and defining the Moho.{{Citation|last1=Braile|first1=L. W.|title=The continental Mohorovičič Discontinuity: Results from near-vertical and wide-angle seismic reflection studies|date=1986|work=Geodynamics Series|volume=13|pages=257–272|editor-last=Barazangi|editor-first=Muawia|publisher=American Geophysical Union|language=en|doi=10.1029/gd013p0257|isbn=978-0-87590-513-6|last2=Chiangl|first2=C. S.|editor2-last=Brown|editor2-first=Larry}} In 1909, he was examining data from a local earthquake in Zagreb when he observed two distinct sets of P-waves and S-waves propagating out from the focus of the earthquake.{{cite journal |last1=Mohorovičić |first1=A. |title=Potres od 8.x.1909; Das Beben vom 8.x.1909. |journal=Godisnje Izvjesce Zagrebackog Meteoroloskog Opservatorija za godinu 1909 - Jahrbuch des Meteorologischen Observatoriums in Zagreb für das Jahr 1909 [Yearbook of the Meteorological Observatory in Zagreb for the year 1909] |date=1910 |volume=9 |issue=4 |pages=1–63 |trans-title=The earthquake of 8 October 1909 |language=Croatian, German}} 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 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.{{Cite book|last1=Prodehl|first1=Claus|last2=Mooney|first2=Walter D.|date=2012|title=Exploring the Earth's Crust – History and Results of Controlled-Source Seismology|language=en|doi=10.1130/mem208|isbn=9780813712086}}
In the early 1960s, Project Mohole was an attempt to drill to the Moho from deep-ocean regions.{{Cite book |first1=Edward L. |last1=Winterer |date= 2000 |chapter=Scientific Ocean Drilling, from AMSOC to COMPOST |title=50 Years of Ocean Discovery: National Science Foundation 1950–2000|location=Washington, D.C.|chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK208831/ |publisher=National Academies Press (US) }} 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.[https://library.ucsd.edu/scilib/hist/shor-mohole-chron.pdf Mohole, LOCO, CORE, and JOIDES: A brief chronology] 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 scientists at the Kola Superdeep Borehole pursued the goal from 1970 until 1992. They reached a depth of {{convert|12260|m|ft}}, the world's deepest hole, before abandoning the project.
{{cite magazine|date=2008-08-25|title=How the Soviets Drilled the Deepest Hole in the World|url=https://www.wired.com/science/discoveries/multimedia/2008/08/gallery_kola_borehole|magazine=Wired|access-date=2008-08-26}} 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.
{{cite journal |last= Ozhovan |first= M. |author2= F. Gibb |author3= P. Poluektov |author4= E. Emets |name-list-style= amp |date= August 2005
|title= Probing of the Interior Layers of the Earth with Self-Sinking Capsules |journal= Atomic Energy |volume= 99 |issue= 2 |pages= 556–562 |doi= 10.1007/s10512-005-0246-y|s2cid= 918850 }}
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.
{{cite journal
|last= Witze |first= Alexandra
|date= December 2015
|title= Quest to drill into Earth's mantle restarts
|journal= Nature News
|doi=10.1038/528016a
|pmid= 26632566
|volume=528
|issue= 7580
|pages=16–17
|bibcode=2015Natur.528...16W|doi-access= free}}
The attempt did not even reach 1.3 km, but researchers hope to further their investigations at a later date.
{{cite web
|url = http://joidesresolution.org/node/4399
|title = Looking Back on Expedition 360
|last1 = Kavanagh
|first1 = Lucas
|date = 2016-01-27
|website = JOIDES Resolution
|access-date = 2016-09-21
|quote = 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.
|url-status = dead
|archive-url = https://web.archive.org/web/20160709120936/http://joidesresolution.org/node/4399
|archive-date = 2016-07-09
}}
See also
- {{annotated link|Brittle–ductile transition zone}}
- {{annotated link|Core–mantle boundary}}
- {{annotated link|Lehmann discontinuity}}
- Gutenberg discontinuity
Notes
{{Reflist}}
References
- {{cite book |last=Harris |first=P. |chapter=The composition of the earth |editor=Gass, I. G. |display-editors=etal |title=Understanding the earth: a reader in the earth sciences |publisher=Artemis Press for the Open University Press |location=Horsham |date=1972 |isbn=978-0-85141-308-2 |chapter-url-access=registration |chapter-url=https://archive.org/details/understandingear00igga }}
- {{cite web |url=http://www.glossary.oilfield.slb.com/default.cfm |title=Schlumberger Oilfield Glossary |access-date=2008-07-17 |publisher=Schlumberger |archive-date=2008-07-17 |archive-url=https://web.archive.org/web/20080717034259/http://www.glossary.oilfield.slb.com/default.cfm |url-status=dead }}
- {{cite book |last=Dixon |first=Dougal| author-link =Dougal Dixon|title=Beginner's Guide to Geology |publisher=Bounty Books |location=New York |date=2000 |isbn=978-0-7537-0358-8}}
External links
- {{cite news |first=Robert Roy |last=Britt |title=Hole Drilled to Bottom of Earth's Crust, Breakthrough to Mantle Looms |url=http://www.livescience.com/technology/050407_earth_drill.html |publisher=Imaginova |date=2005-04-07 |access-date=2008-07-17}}
- {{cite web |url=http://www.nas.edu/history/mohole/ |title=Digging a Hole in the Ocean: Project Mohole, 1958–1966 |access-date=2008-07-17 |publisher=National Academy of Sciences |archive-url=https://web.archive.org/web/20151102230813/http://www.nas.edu/history/mohole/ |archive-date=2015-11-02 |url-status=dead }}
- [http://www.seismo.helsinki.fi/mohomap/ Map of the Moho depth of the European plate]
{{physical oceanography|expanded=other}}
{{Earthsinterior}}
{{DEFAULTSORT:Mohorovicic discontinuity}}