Geoscience Reference
In-Depth Information
A planetary body scan
The clearest clues to the internal structure of the Earth come from seismology. Earthquakes
send out seismic shock waves through the planet. Like light being refracted by a lens or
reflected by mirrors, seismic waves travel through the Earth and reflect off different lay-
ers within it. Seismic waves travel at different velocities depending on how hot or soft the
rock is. The hotter and therefore softer the rock, the slower the wave travels. There are two
main types of seismic wave, primary, or P, waves, which are the faster and thus the first
to arrive at a seismograph, and secondary, or S, waves. P waves are pressure waves with
a push-pull motion; S waves are shear waves and cannot travel through liquids. It was by
studying S waves that the molten outer core of the Earth was first revealed. Detecting these
seismic waves on a single instrument would not tell you much, but today there are networks
of hundreds of sensitive seismometers, strung out around the planet. And every day there
are many small earthquakes to generate signals. The result is a bit like a body scanner in
a hospital, in which the patient is surrounded by X-ray sources and sensors and computers
use the results to build up a 3D image of her internal organs. The hospital version is known
as a CAT scan, standing for Computer Assisted Tomography. Its whole-Earth equivalent is
called seismic tomography.
The global network of seismographs is best at seeing things on a global scale. It will reveal
the overall layering in the mantle and changes in seismic velocities due to high or low tem-
perature on scales of hundreds of kilometres. There are also more closely spaced arrays,
originally set up to detect underground nuclear tests, and they, together with new arrays
being deployed by geophysicists in geologically interesting regions, have the potential to
see structure deep in the mantle on a scale of a few kilometres. And it seems that there is
structure on every scale. The clearest things in these whole-Earth body scans are the lay-
ers. Below 2,890 kilometres, the depth of the liquid outer core of our planet, S waves will
not pass. But several features stand out within the mantle. There is, as we've mentioned,
the Mohorovicic discontinuity at the base of the crust, and another at the base of the hard
lithosphere. The asthenosphere beneath is softer so the seismic velocities are slower. There
are clear layers 410 kilometres down and 660 kilometres down, with another less distinct
layer after about 520 kilometres. At the base of the mantle is another, probably discontinu-
ous, layer called the D” or D double prime layer, which varies from nothing to about 250
kilometres thick.
Seismic tomography also reveals more subtle features. Essentially, colder rock is harder
so seismic waves travel through it more quickly than they do through hotter, softer rock.
Where old, cold ocean crust dives beneath a continent or into an ocean trench, reflections
from the descending slab reveal its passage down into the mantle. Where Earth's hot core
bakes the underside of the mantle, it appears to soften and rise in a huge plume.
