To The Victoria Falls
Formation of the Victoria Falls
The Formation of the Victoria Falls
The origins of the formation of the Victoria Falls we see today begins some 200-150 million years ago, during the Jurassic or Upper Karoo Period, when the present-day land masses forming South America, African, India, Australasia and Antarctica were all joined as one huge super-continent known as Gondwanaland. Tectonic activity caused break up of this landmass, with huge cracks or fissures forming in the earth's crust from which erupted huge volumes of molten basalt.
From Kazungula on the Botswana border to the Matetsi river, an isolated area of this basalt remains exposed, over which the present-day Zambezi flows and has cut its path, forming the falls and gorges. The depth of the basalt is not known, perhaps up to 1,000 metres - but at least 300 meters in depth at the Falls region. As successive layers of larva formed over each other they created the layers of darker blue-black rock, sandwiched between narrow reddish seams which can be seen in the vertical cross-sections created by the gorge, as can be seen for example in the second and third gorges down from the Victoria Falls.
As the molten eruptions slowed and stopped, and the region stabilised, the central area of southern Africa subsided, perhaps under its own weight, to form what is often described as an inverted dish effect, with a raised border around most of the coastal areas and a shallow depressed central area forming a vast internal drainage network across what is known as the Southern African Plateau (see History of the Zambezi). Most of this ancient plateau is now heavily eroded, but in areas the flat-topped hills which are left by the erosive processes are isolated remnants of the original plateau.
After this period, there is a long gap of over 100 million years during which the geographic record tells us little or no information, when any deposits which may have formed over the basaltic surface have been completely eroded and removed. Only after this time did deposits begin to accumulate and over these today sits a thick mass of unconsolidated reddish, Kalahari, sand - formed under desert conditions and wind blown across the majority of the southern continent. These sands flank the wide and shallow Zambezi valley above the Victoria Falls, the river bed sitting on the exposed basalt rock. Downstream from the Falls the river valley continues in the same manner and height, with the gorge cutting deep into the old basalt bed of the river.
A unique feature of the formation of the Victoria Falls is the zigzagging nature of the gorge in which it flows, with at least eight east-west sections, all of similar width to the present-day Falls, connected by shorter north-south channels through which the river now flows. There are numerically numbered downstream from the Falls, which plummet into the First Gorge. These sections become less clear as one travels downstream of the Falls, but are more clearly seen from the air.
Satellite view of the Victoria Falls (NASA image)
This east-west and north-south pattern is explained by weaknesses created in the basaltic rock as it solidified, and re-enforced but subsequent structural movements across the continental landmass. This results in cracks or joints in the solid rock which are aligned roughly along east-west and north-south axis. Over time these joints became in-filled with weaker sedimentary rocks or clays, created from the overlying deposits.
These are the two clues to the spectacular formation of the Victoria Falls and the gorges below that we see today. The lines of east-west weakness in the basalt rock have been eroded by the Zambezi as it travelled over them in a southerly direction. As the erosive power of the river exploited these weaknesses, the broad and deep east-west gorges where formed. As the huge flow of water became concentrated over these falls, the erosive power of the water has broken at lines of north-south weakness, cutting backwards upstream and in doing so concentrating the flow of water an increasing its erosive power. This sequence of exploiting wide east-west lines of weakness across the width of the river bed, followed by erosion along north-south weakness lines create the gorge pattern we see today.
The eight east-west sections of the gorge are therefore old versions of the Victoria Falls, which have been successively cut upstream, each of which was formed by the river exploiting an east-west line of weakness and creating a waterfall across the width of its bed, then gradually eroding upstream at a narrow point, capturing more water from the falls as it downcuts through the rock, and eroding headwards until it finds another line of east-west weakness which is then exploited.
Formation of the Victoria Falls
As one travels downstream through the gorges the fall-lines become older and have been affected by the weathering effects of geographical and biological erosion, distorting and concealing their formation. However nearer the Victoria Falls, for example in the second gorge, it is much clear to see how the process has worked, with the Boiling Pot the point at which the north-south line of weakness has broken through and stolen the water flow from the east-west fall-line, the near vertical sides of the gorge and the exposed solid basalt rock carved by the erosive forces of the mighty Zambezi.