Mohorovic̆ić discontinuity (Moho) The Earth's internal structure can be thought of as a series of concentric shells, each of which transmit seismic waves at a different speed. The boundaries between shells are marked by seismic discontinuities at which the speed of transmission of seismic waves makes a sudden jump. Distinct seismic discontinuities occur between the inner and outer core and between the outer core and the mantle, and these reflect major changes in composition. There are two well-marked seismic discontinuities within the mantle, which are believed to represent depths at which the pressure is great enough to cause minerals in the mantle to adopt a denser structure (a phenomenon described as a phase change) although there is no difference in overall chemical composition on either side of the discontinuity. The shallowest seismic discontinuity that is global in its extent is the one known as the Mohorovic̆ić discontinuity (or Moho for short) after the talented Croatian meteorologist and geophysicist Andrija Mohorovic̆ić (1857–1936) who, in 1909, was the first to recognize it.
The Moho represents the base of the Earth's crust, and represents a difference in composition rather than a phase change. Both crust and mantle are made of silicate rock and the compositional difference between them is slight compared to the difference between mantle and core. Even so, it is sufficient to allow compressional seismic waves (P-waves) to travel at about 8 km s
−1 in the upper mantle, as compared with only 6 or 7 km s
−1 in the lower crust. The effect of this is to allow seismic waves from a crustal earthquake to reach a sufficiently distant detector faster by passing through the base of the crust and into the mantle before being refracted back up through the crust towards the detector than waves that have travelled directly through the crust to the detector. Mohorovic̆ić proved this by analysing signals collected by seismometers at the Zagreb observatory and elsewhere, that had emanated from a destructive local earthquake.
Mohorovic̆ić's technique relied on refraction of naturally occurring seismic waves at the crust–mantle boundary, but the Moho can be pinpointed more precisely by seismic reflection. The conventional method of mapping the Moho along the line of a seismic traverse is to detonate powerful explosive charges just below the surface and to record the reflections by an array of detectors known as geophones. An alternative seismic reflection technique capable of detecting the Moho and resolving its fine-scale structure is to use a truck-mounted vibrator as a source; this has the advantage of providing a pure signal of known (and variable) frequency.
Continental crust is thicker than oceanic crust, so the Moho is usually at a depth of about 35 km below the continents, though it can be as shallow as 25 km where the crust has been thinned and stretched or as deep as 90 km below major mountain belts. The Moho is only about 7–10 km deep below oceanic crust, which is thinner than the continental variety.
The Moho was formerly assumed to represent a sharp discontinuity, but detailed seismic studies have revealed that in at least some places it is complex and probably layered, with peridotite from the mantle interleaved with gabbro (below the oceans) or diorite (below the continents) over a depth range of perhaps a few hundred metres. In collision zones the Moho may also be duplicated by compressional faulting.
It is a common misconception that the Moho marks the base of the Earth's tectonic plates, but this is not so. The Moho is a firmly welded interface across which crust and upper mantle are held together. The crust and uppermost mantle together constitute the Earth's lithosphere, its outer strong and rigid layer, which is able to move across the deeper mantle (the asthenosphere) because the pressure and temperature are such that the latter is relatively weak. Because of the great depth to the Moho, erosion is nowhere able to expose it at the Earth's surface. However, in rare instances a slice of oceanic crust and upper mantle, known as an ophiolite, has escaped subduction during ocean closure and has instead been thrust over the edge of a continental plate (a process described as obduction). Here the ‘fossil’ Moho within the ophiolite can be examined (Fig.1). In such places the mantle is revealed mostly as varieties of peridotite such as harzburgite (the main minerals of which are olivine and orthopyroxene) or lherzolite (main minerals as for harzburgite with the addition of clinopyroxene), which are evidently residua from which the basaltic magma that has gone to make the oceanic crust has been extracted. The contact between the harzburgite or lherzolite mantle and the overlying rocks is described as the ‘petrological Moho’, because it divides rocks that are petrogenetically part of the mantle from rocks that are petrogenetically part of the crust. Most of the deep oceanic crust is gabbroic in composition, but there are layers, especially near its base, consisting of varieties of peridotite such as wehrlite (main minerals olivine and clinopyroxene) that are not residual but instead have crystallized from magma that has had the remaining melt extracted (perhaps squeezed out) before it was fully solidified. Petrogenetically peridotite of this type belongs to the crust, but seismic waves would travel just as fast through it as through any other peridotite. If there is a well-marked interface between overlying gabbros and underlying magmatic peridotites, this is described as the ‘seismic Moho’, because it corresponds to the Moho that can be detected
in situ round the globe by seismology.
The only seismic data we have for any planetary body other than the Earth comes from a network of seismometers installed on the Moon by the
Apollo landings. This network was able to use seismic refraction calculations to show that there is a clear crust–mantle interface beneath the near side of the Moon, at a depth of about 60 km between the
Apollo 12 and
Apollo 14 sites and about 75 km near the
Apollo 16 site. By analogy with the Earth, this interface is usually referred to as the lunar Moho. Doubtless seismometers on Mars will before long determine the depth to the martian Moho.
In 1957 the main civilian science funding agency in the United States of America, the National Science Foundation, began a flirtation with the idea of attempting to drill a borehole right through the Earth's crust and into the mantle. The scheme acquired the splendid name of Project Mohole, and began promisingly. It was realized that because the oceanic crust is so much thinner than continental crust, the best chance of success would be to drill the ‘Mohole’ through oceanic crust. Despite the difficulties of drilling from a platform floating on some thousands of metres of water, this would be easier than on-land drilling of the much deeper hole required to reach the Moho below continental crust. In 1961, using a drilling rig mounted on a converted naval barge, a trial hole reached basalt at a depth of 200 m below 3.5 km of water off the coast of Mexico. After this promising start Project Mohole ran into political difficulties and it was terminated in 1966 amid an aura of financial scandal. However, the techniques and expertise that had been developed in Project Mohole sowed the seeds for the international Deep Sea Drilling Project (DSDP) and Ocean Drilling Project (ODP) in the following decades, which although never attempting to reach the Moho itself, have provided a wealth of information about the ocean basins and their history by drilling less ambitious holes at well-chosen sites around the globe.
David A. Rothery
Bibliography
Brown, G. C. and and Mussett, A. E. (1993) The inaccessible Earth (2nd edn). Chapman and Hall, London
