Second period, Gondwana's formation: 900–550 million years ago
The second historical period started when Rodinia began to break apart around 900 million years ago. That split Namibia and its neighbours in Rodinia into three landmasses, namely the Kalahari Craton to the east, the Congo Craton to the west, and the Rio de la Plata Craton to their south. Two deep oceans formed as the craton landmasses drifted apart. The Adamastor Ocean extended along a north–south axis from present-day Kunene in the north to the Orange River in the south following a similar trend to Namibia’s current Atlantic coastline, while the Khomas Ocean cut a swathe through present-day central Namibia.
Great volumes of sediments were eroded off the landmasses and deposited in the oceans. At the same time, hot semi-fluid magma rose from the earth's upper mantle, becoming lava on the surface where it cooled and solidified into volcanic rock. Together with the sediments, successions of interbedded sedimentary and volcanic rocks were formed, which are today part of the Nosib Group in the Damara Orogen. Many saline lakes also formed within these large basins, much like the present-day lakes of the East African Rift Valley.
The denser oceanic crust between the three continental landmasses began to sink about 700 million years ago. This led to the convergence of the three landmasses and the closure of the oceans. By about 550 million years ago, the landmasses of the southern hemisphere had been consolidated into the supercontinent Gondwana. During the process of convergence, oceanic sedimentary rocks in the Khomas and Adamastor oceans were pushed up and metamorphosed into the extensive mountain belts of the Damara Orogen in central Namibia, and the Gariep Orogen and Kaoko Belt in southern and northern Namibia, respectively.
The compressive forces were enormous, heating, pressurising and melting rock, which then rose because it was lighter and thus more buoyant than the surrounding rock. The molten rock formed the intrusions of Damara Granites, such as the extensive Donkerhoek and Omangambo granites, the uranium-bearing Rössing Alaskite, and tourmaline-bearing pegmatites in central Namibia.
As colliding landmasses are pushed upwards to create mountain ranges, large basins form behind them as the colossal weight of the mountains causes the surrounding land to subside. Both the Nama Basin in the south and the Owambo Basin in the north of Namibia were thus formed during the Damara, Gariep and Kaoko mountain-building events. The Nama and Owambo basins were eventually filled with debris eroded off the newly built mountain belts, the debris respectively forming the Nama Group (figure 2.18) in southern Namibia, and the Mulden Group, most of which lies beneath the Owambo Basin and its Cuvelai drainage system.
Earth, 900 million years ago
Rodinia had not yet broken up. The three cratons that formed Rodinia – Congo, Kalahari and Rio de la Plata – then began to drift away from each other and, as the rifts between them widened, the Khomas and Adamastor oceans came into being.
2.18 Gondwana's formation, 900–550 million years ago
Most geological formations belonging to this period began as sediments in the Damara and Adamastor oceans before they were later pushed up to form the extensive mountain belts of the Damara Orogen in central Namibia and the Gariep Orogen in southern Namibia. The rock groups of these two formations are mapped in the various units shown in this map. The processes that led to the building of the mountain belts also led to the formation of Gondwana, the new supercontinent.
Photo: J Irish
Photo: J Kaufman
Photo: J Mendelsohn
Photo: J Mendelsohn
Photo: JB Dodane
Art: Helge Denker; Copyright: NamPost
Earth, 550 million years ago
The formation of the supercontinent Gondwana occurred when the Rio de la Plata, Congo and Kalahari cratons collided. This process also closed the Khomas and Adamastor oceans.