Geological explanations - Secret Valleys

Wednesday 6 February 2008

Even geologists rarely appreciate the great age of East Africa's Rift Valley System. Rifts are formed and are later completely filled-in by erosion as their shoulders collapse and wash down into the valley below. Eventually the rifts are eroded beyond easy recognition, and new rifts open, whether in the same place or elsewhere in the region, to accommodate renewed stresses in the Earth's crust as tectonic plates shift.

It appears that rifting had occurred in what is now East Africa as far back as 550 million years ago, permitting rock, fluids, and heat to flow upwards in a unique geological episode known as the Pan-African Event. This one-of-a-kind "heat pulse" is associated with the formation of deposits of colored gemstones right across the southern continents and even in far-away Brazil where it is also called the "Pan-African Event".

One rift of Pan-African age, hundreds of kilometers long, has been so severely eroded that it is now entirely flat. That means you can walk right "off" the shoulder of the rift, across it, and on to the other side without ever encountering hills or bumps any higher than the local termite mounds. Only the type of rock changes, from 1a) thin-bedded rocks a bit like a layer cake, to 2) massive, monotonous, valley-fill rocks, and back to 1b) thin-bedded rocks on the other side of what had been a Rift Valley in times long before the dinosaurs.

Ancient geological faults which cut these secret or disguised valleys turn out to be good places to prospect for deposits of gemstones, but no one knows quite why. One guess is that these ancient faults, which are also eroded and difficult to detect, had intersected the Earth's "deep plumbing system", thereby permitting rocks, fluids and heat to rise from unusually great depths.

John Saul