Biology Reference
In-Depth Information
This erosion continued until thirty million years ago, when Africa collided with Europe. The
African plate ceased to drift, pinned possibly by its collision in the Mediterranean, or by the giant
volcanic plumes that rose through the earth's mantle to create the Cape Verde and Afar archipela-
goes. Only once Africa became static did the continent we know begin to take its shape. This large,
flat expanse of deeply weathered land rested above a hot area, a convection circulation in the under-
lying mantle. Like a sheet of metal slowly warping above a flame, the continent changed its shape
over millions of years. Plumes of less dense material rose from the hot mantle, pressing against the
crust, at once thinning it by partial melting and pushing it up, warping its surface, initiating volcanic
activity, and reactivating ancient faults.
The old Africa of low, eroded plains lifted into the landscape that geographers and historians
have remarked on since Herodotus's time, a continent of long swells and basins. Whereas most con-
tinents have areas of extremely high elevation and others almost at sea level, very little of Africa is
extremely high or low. The continent bulges from ocean to ocean, rising and falling in successive
sweeps 125 to 1,250 miles in length. On the resulting tablelands are peaks, ridges, and escarpments
created by millions of years of wind and water erosion. The elevation drops to both the north and
west, marking the paths of the Nile and the Congo, the continent's two largest, though dramatically
different, rivers.
The Congo is unlike other rivers in Africa. The gradual slope of the landscape usually results
in slow drainage and immense deltas, as with the Nile, Niger, Zambezi, and Limpopo, all of which
carried large quantities of sediment down from areas of the continent that were lifted. But Africa's
basins and swells also cause internal drainage. As the continent was reshaped, water that was unable
to reach the ocean formed immense lakes, then spilled out into a new river, to another set of lakes,
slowly working closer to the coast.
At first glance, the Congo basin fits this description, draining internally. For millions of years,
it most likely formed a massive inland lake as the result of a geologic swell along the coast that
blocked access to the ocean. But at some point in the last thirty million years, the Congo River cut
through the highlands beyond the Pool Malebo and created an outlet, a 220-mile descent of narrow,
violent rapids that possess, Adam Hoschchild writes, “as much hydroelectric potential as all the
lakes and rivers of the United States combined.” Over millions of years, the water rushing out from
the land created the world's largest submarine river canyon, 497 miles long and 3,900 feet deep on
the floor of the Atlantic, as well as an abyssal fan, a river delta composed of millennia of sediment,
hidden beneath the ocean.
But the landscape where I found myself listening to roosters crow across Djolu's clustered
homesteads is more ancient than much of the rest of Africa. The Congo basin is a craton, an ancient
continental core dating back 2 to 3.6 billion years. Whereas much of the earth's crust is composed of
relatively new material from plumes and rifting, cratons are typically thicker and deeply rooted, res-
istant to volcanism, which will occur at their edges but not their centers. While the lands of Africa
swelled and broke open around it, the Congo craton held its place. East Africa lifted, drying out and
losing its forests as it gained altitude, but the Congo remained lush. The waters drained from the
newly raised lands, pouring into the low, immovable basin and making it one of the wettest places
in Africa. Today, most of the DRC lies within what Alden Almquist, in
Zaire
, describes as a “vast
hollow . . . the shape of an amphitheater, open to the north and northwest and closed in the south
and east by high plateaus and mountains.”
