Biology Reference
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years ago. As this occurred the boiling mantle, heat driven by radioactivity and coalitions,
started forming encrustations that bonded into larger units called cratons, first described in
1921 by a German geologist whose name escapes me. Like petals or lily leaves on a pond,
these joined together to form continents. As a child, it was fascinating watching molten lead in
a heating pan; hardly a recommended practice. As it steamed areas of the molten lead would
start hardening and multiple encrusted areas would fuse to form a larger continent. Then the
underneath molten lead would sometimes breakthrough the “continent” and form new blobs, or
if the hardened lead cracked a line would form (fissure). Currents within the molten lead and
upwelling heat would move the hardened mass around or recycle it. Drops of water on it
would instantly vaporize into clouds of steam, pieces of wood would turn to charcoal and
catch alight, and pieces of metal or stones would float on the denser lead. Hence, expensive
heavier metals like gold, platinum, and tungsten, if left on a floating iron cauldron would tend
to sink through the ancient earth's mantle towards the center of the earth. These metal mineral
deposits may have arrived later, possibly from meteorites or asteroids.
Similar to molten lead, these encrustations on the earth's mantle formed cratons, penetrating
some 150-200 miles into the mantle, floating like giant icebergs that joined together into a
massive continent called Pangaea. Later Pangaea broke up into Laurasia, with the continents of
the Americas and Asia, and Gondwanaland, made up of Africa, India, Antarctica and
Australia. In Southern Africa, the Kaapvaal craton dominates and extends from the Kalahari
Desert in the west to the Indian Ocean in the east. Part of the eastern part broke off to form the
western part of Australia [Pilbara craton, see Zegers, de Wit, Dann, White, in Terra Nova,
1998]. The two cratons make up what was originally the Vaalbara craton. To the north of the
Kaapvaal craton is the Zimbabwe craton and to the north east of this is the Tanzania craton and
to the north-west the Congo craton. Between these cratons are now the Zambezi river (north of
Zimbabwe Craton) and the Limpopo river to the south; an area between the Kaapvaal and
Zimbabwe Craton (together the Kalahari craton), called the Intermediate Limpopo Belt that is
characterized by less granite and more sand and sediments and the Limpopo Basin.
The Limpopo Belt that the Limpopo Basin rests on is divided into three zones: the Northern
Marginal Zone (NMZ), the Central or Intermediate Zone (CMZ) and Southern Marginal Zone
(SMZ). It is characterized by supracrustal granulite facies rocks situated among the granite-
greenstone rocks of the two cratons on either side of the CMZ, as summarized from Chinoda,
Moyce, and Matura. The Bubye River lies in the CMZ while the Great Zimbabwe Dyke is not
far to the North. The CMZ area here has mostly grey granitoid rocks and quartz, feldspar, and
some garnet and has been termed the Beitbridge Complex.
We are now driving down the Intermediate Limpopo Belt. The Kaapvaal craton formed
some 3.7 billion years ago. In the center of is a giant basin that once upon a time was an inland
sea surrounded by high mountain ranges—the platinum rich Bushveld Igneous Complex. Indeed
South Africa produces 58% of the world's platinum (about 7 million troy ounces annually) and
has the largest reserves of Africa's 80% and also among the largest reserves of aluminum,
chrome, coal, diamonds, gold, manganese, vanadium, vermiculite, and zirconium. These
rimming mountains eroded over time and filled the basin with sand and sediments, including
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