Post by cjm on Jan 3, 2016 13:22:58 GMT
McCathy & Rubidge: Earth & Life, Struik (2005), p 269
…
THE AFRICAN SUPER SWELL
The massive uplift of southern Africa in the last 20
million years is unusual. In fact, it has resulted in a
topographic anomaly of global significance (figure
9.19). Whereas areas of similar geology and geo-
logical history elsewhere on Earth, such as Western
Australia or central Canada, today lie at elevations
of around 300 m to 400 m above sea level,
most of southern Africa lies at elevations
above 1 000 m. This topographic anomaly,
which has been called the African Super
Swell, has recently become the focus of
intense research.
While the Theory of Plate Tectonics can
explain most of the topographic features of
the Earth, such as the Himalayan Mountains
and the Mariana Trench, the African Super
Swell poses a problem, because it lies far
from tectonic plate boundaries, where most
topographic features occur. Hence the inter-
est from scientists. To investigate this feature,
earth scientists have been using a technique
called seismic tomography, analogous to a
medical CAT (or CT) scan, to image the inte-
rior of the Earth. Rather than X-rays, as used
in CAT scans, earth scientists use seismic
waves from distant earthquakes, which are
recorded by sensitive seismometers. By
measuring the small differences in the trav-
el times of the energy pulses from the earth-
quakes to the various seismometers, it is
possible to build up images of the Earth’s
interior. These images highlight areas that
are more dense and probably colder (pulses
travel faster), distinguishing them from areas
that are less dense and probably hotter
(pulses travel more slowly).
It turns out that deep in the mantle beneath
southern Africa there is a huge blob of hot material,
I nearly 2 000 km in diameter, which appears to be
l rising towards the surface. Like a rising bubble in
thick syrup, the Earth’s surface is being pushed up,
creating the Super Swell. Why this is happening is
not known. The blob has a tail (like a giant tadpole),
which rises closer to surface under the East African
Rift Valley, and is undoubtedly responsible for the
rifting in that region. Perhaps the blob will express
itself by major rifting of southern Africa some time
in the future, as it nears the top of the mantle.
...