Mohorovičić discontinuity

Source: Wikipedia, the free encyclopedia.
Earth's crust and mantle, Moho discontinuity between bottom of crust and solid uppermost mantle

The Mohorovičić discontinuity (

seismic waves as they pass through changing densities of rock.[2]

The Moho lies almost entirely within the

ocean floor, and 20 to 90 kilometres (10–60 mi) beneath typical continental crusts, with an average
of 35 kilometres (22 mi).

Named after the pioneering

S-waves, one set that followed a direct path near the Earth's surface and the other refracted by a high-velocity medium.[4]

Nature and seismology

Two paths of a P-wave, one direct and one refracted as it crosses the Moho[4]
Ordovician 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).[5] 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.[2] The Moho is characterized by a transition zone of up to 500 meters.[6] Ancient Moho zones are exposed above-ground in numerous ophiolites around the world.[7]

As shown in the figure, 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.

Beginning in the 1980s, geologists became aware that the Moho does not always coincide with the crust-mantle boundary defined by composition.

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.[8]

Serpentinization of mantle rock below slowly spreading mid-ocean ridges can also increase the depth to the Moho, since serpentinization lowers seismic wave velocities.[9][10]

History

Croatian seismologist Andrija Mohorovičić is credited with discovering and defining the Moho.[11] 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.[12] 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.[13]

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.[13]

In the early 1960s, Project Mohole was an attempt to drill to the Moho from deep-ocean regions.[14] 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.[15]

Exploration

Reaching the discontinuity by drilling remains an important scientific objective.

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.[18] The attempt did not even reach 1.3 km, but researchers hope to further their investigations at a later date.[19]

See also

Notes

  1. ISBN 9783411040667
    .
  2. ^
    ISBN 978-0-08-043751-4
    , retrieved 2019-11-21
  3. ISBN 978-0-495-55480-6
    .
  4. ^
    ISBN 978-0-17-448221-5
    .
  5. ISBN 978-1-4020-3739-9
    .
  6. ^ D.P. McKenzie – The Mohorovičić Discontinuity
  7. doi:10.1029/97JB02604
    .
  8. doi:10.1016/j.tecto.2012.12.031
    .
  9. S2CID 128410328
    .
  10. doi:10.1016/j.crte.2003.08.006
    .
  11. ISBN 978-0-87590-513-6
  12. ^ Mohorovičić, A. (1910). "Potres od 8.x.1909; Das Beben vom 8.x.1909" [The earthquake of 8 October 1909]. 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] (in Croatian and German). 9 (4): 1–63.
  13. ^
    ISBN 9780813712086
    .
  14. ^ Winterer, Edward L. (2000). "Scientific Ocean Drilling, from AMSOC to COMPOST". 50 Years of Ocean Discovery: National Science Foundation 1950–2000. Washington, D.C.: National Academies Press (US).
  15. ^ Mohole, LOCO, CORE, and JOIDES: A brief chronology Betty Shor, The Scripps Institution of Oceanography, August 1978, 7 pp. Access date 25 June 2019.
  16. ^ "How the Soviets Drilled the Deepest Hole in the World". Wired. 2008-08-25. Retrieved 2008-08-26.
  17. ^ 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.
    S2CID 918850
    .
  18. ^ Witze, Alexandra (December 2015). "Quest to drill into Earth's mantle restarts". Nature News. 528 (7580): 16–17.
    PMID 26632566
    .
  19. ^ 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
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