Geoscience Reference
In-Depth Information
Over time, they can often undergo slow subsidence. Lake Eyre in Australia and the Great
Lakes of North America occupy such basins. The southern African craton, by contrast, has
been uplifted by the buoyant rocks of a mantle plume beneath.
Profile of a continent
The same sort of principle that revealed the structure of the Earth as a whole, seismic tomo-
graphy, can study the deep interior of continents in great detail. To get the high resolution
required, the technique relies not on random natural earthquakes on the other side of the
world, detected by widely spaced seismometers, but creates artificial seismic waves and
picks up their reflections using nearby, closely spaced arrays of detectors. It is very expens-
ive and at first was the monopoly of the oil exploration companies, which jealously guarded
the results. But now there are many national projects which are sharing their data. The most
advanced of these are in North America, where the Consortium for Deep Continental Re-
flection Profiling in the USA and Lithoprobe in Canada have built up a detailed series of
profiles. To create the shock waves they employ a small fleet of purpose-built trucks which
use hydraulic rams to shake the ground with heavy metal plates. Deep vibrations are mon-
itored by a network of sensors over many miles which record the reflections from numerous
layers beneath the ground. Computer analysis reveals each discontinuity or sudden change
in density. These profiles go far deeper than the sedimentary basins of most interest to the
oil prospectors. They reveal the ancient sutures between continents that merged with one
another long ago. They have revealed reflections from a layer descending into the mantle
beneath the Lake Superior region of Canada that could be the oldest subduction zone yet
found, with the floor of a lost ocean about 2.7 billion years old. The profiles reveal how
basalt magma rising from the mantle and unable to break through the thick continent under-
plates it with sheets of basalts known as dykes. They also reveal how, when continental
rocks get buried deep enough, they begin to melt so that they rise up through the continent
to recrystallize as granite.
