The electric Moho
Alan G. Jones and Ian J. Ferguson
Since Mohorovicic discovered a dramatic increase in compressional seismic
velocity at a depth of 54 km beneath the Kulpa Valley in Croatia, the “Moho”
has become arguably the most important seismological horizon in the Earth
owing to its role as defining the crust-mantle boundary. It is now known to
be a ubiquitous feature of Earth, being found beneath both the continents and
the oceans, and is commonly assumed to separate lower-crustal mafic rocks from
upper-mantle ultramafic rocks. Electromagnetic experiments conducted to date,
however, have failed to detect a corresponding change in electrical conductivity
at the base of the crust, although one might be expected on the basis of
laboratory measurements. Here we report electromagnetic data from the Slave
craton, northern Canada, which show a step-change in conductivity at Moho depths.
Such resolution is possible because the Slave craton is highly anomalous,
exhibiting a total crustal conductance of less than 1 Siemens -- more than an
order of magnitude smaller than other Archean cratons. We also found that the
conductivity of the uppermost mantle directly beneath the Moho is two orders of
magnitude more conducting than laboratory studies on olivine would suggest,
inferring that there must be a connected conducting phase.
Nature, 409, 331-333, 2001.
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Alan G Jones / 10 June 2004 /