Mohorovičić discontinuity

World map showing the depth of the Moho.

The Mohorovičić discontinuity (Croatian pronunciation: [moxorôʋiːt͡ʃit͡ɕ]), 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 Moho lies almost entirely within the lithosphere; only beneath mid-ocean ridges does it define the lithosphere–asthenosphere boundary. 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.

The Mohorovičić discontinuity is 5 to 10 kilometres (3–6 mi) below the ocean floor, and 20 to 90 kilometres (10–60 mi), with an average of 35 kilometres (22 mi), beneath typical continents.

Nature

Earth cross-section showing location of the Mohorovičić discontinuity (labelled as 2)

Two paths of a P-wave, one direct and one refracted as it crosses the 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). 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.

The Moho is characterized by a transition zone of up to 500 m thick. Ancient Moho zones are exposed above-ground in numerous ophiolites around the world.

Exploration

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.

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. The Japanese project Chikyu Hakken ("Earth Discovery") also aims to explore this general area with the drilling ship, Chikyū, built for the Integrated Ocean Drilling Program (IODP).

See also

• Brittle–ductile transition zone
• Core–mantle boundary
• Kola Superdeep Borehole
• Lehmann discontinuity

References

• Harris, P. (1972). "The composition of the earth". In Gass, I. G.; et al. Understanding the earth: a reader in the earth sciences. Horsham: Artemis Press for the Open University Press. ISBN 0-85141-308-0. 
• "Schlumberger Oilfield Glossary". Schlumberger. Retrieved 2008-07-17. 
• Dixon, Dougal (2000). Beginner's Guide to Geology. New York: Bounty Books. ISBN 0-7537-0358-0. 

External links

• Britt, Robert Roy (2005-04-07). "Hole Drilled to Bottom of Earth's Crust, Breakthrough to Mantle Looms". Imaginova. Retrieved 2008-07-17. 
• "Digging a Hole in the Ocean: Project Mohole, 1958-1966". National Academy of Sciences. Retrieved 2008-07-17. 
• Map of the Moho depth of the European plate