Geologically speaking, Namibia is undoubtedly one of the most interesting countries in the world. Here the visitor finds the oldest desert, the second largest canyon, the highest dunes and the largest meteorite in the world. Besides these superlatives, Namibia offers a large diversity of lesser known, but also fascinating geological attractions.

Geological features are immediately obvious and dominate the character of the country. The particularly sharp and clearly visible rock formations allow a view into the history of the earth, which reaches from the present day back to the beginning of our planet. As relics of this geological history, Namibia's bizarre rock formations and beautiful landscapes show evidence of various important geological processes: from ongoing weathering and erosion, previous huge mountain building events and sea level changes to gigantic volcanoes and the collision of whole continents. In addition, the rocky remnants exhibit evidence of all imaginable climatic extremes, which Namibia has experienced in its history; from ice ages and sub-polar conditions to hot tropical humid climates and hot dry deserts. Therefore, Namibia represents a unique 'geological Eldorado' for professional geologists as well as for the nonprofessional.

From 2,600 to 850 million years ago: the oldest rocks

Even though the earth's history goes back 4,600 million years, most of what can be seen on the surface in Namibia today was formed during the last 1,000 million years. Rather little is known of how the land masses were arranged before then. Some rocks in the Kunene Region formed about 2,600 million years ago, but they were then obscured by complex rock-forming events that lasted until about 1,800 million years ago. These oldest rocks are now included in the Epupa Metamorphic Complex, which consists of a mixture of igneous and metamorphic rocks. Some economically valuable deposits of base and precious metals - such as gold, copper, lead and zinc - may be found in them. Other volcanic rocks (the Khoabendus Group) and related granites (the Fransfontein Granite Suite) are in north-western Namibia as well, while other old metamorphic complexes are to be found in north-eastern and central Namibia.

Evidence suggests that two or more huge land masses collided to form the great continent of Rodinia (a name derived from the Russian word for 'motherland') between roughly 1,400 and 1,200 million years ago. The collisions produced enormous volcanic upheavals, sedimentary basins and a huge mountain belt in what is now southern Africa. Remnants of the many different rocks formed then remain in an arc that extends from South Africa north-west through Karasburg and Solitaire on the edge of the Namib Desert and then swings north-east to Rehoboth and into Botswana. The rocks are called the Namaqua Metamorphic Complex. Elsewhere they and other rocks related to the formation of Rodinia, lie buried below deposits of sands and younger rocks where their presence is revealed to us by magnetic survey information.

The break-up of Rodinia

Shifts in the earth's mantle (the layer between the crust and the core) started to break the continent of Rodinia apart about 850 million years ago. Africa split from South America, and Namibia broke up in two land masses. At first, giant rift valleys formed between these continental fragments. Large rivers flowed into the valleys, depositing various sediments. Many highly saline lakes also formed in the valleys, much like the lakes of the East African Rift Valley today.

From 750 to 550 million years ago: the Adamastor and Khomas oceans

The Adamastor and Khomas oceans formed between the three land masses as they continued moving apart. These were deep oceans. Windhoek was submerged beneath the sea, and may have been thousands of kilometers from the land mass on which Rehoboth is situated today. Vast quantities of sediments from the surrounding land masses were deposited in the oceans, with different sediments being deposited in various depths. Sediments rich in organic matter and calcium carbonate, which would later become limestone and dolomite, accumulated in the calm, shallow seas on the margins of the land masses. The climate at the time was probably rather warm and dry since warm conditions are required for the extensive formation of limestone, and also because the sediments in the shallow seas contained very little material of the kind normally deposited by rivers. Environmental conditions in the shallow waters were probably rather similar to the tropical conditions experienced today along the Great Barrier Reef off the northern coast of Australia.

Other sediments were deposited in much deeper water from material that had first begun to accumulate on the edges of continental shelves. After they had reached critical masses and shook loose by tremors and earthquakes, these sediments slumped down in great mudslides to the ocean depths below. Rocks that formed later from these deep-water sediments are called turbidites.

The formation of the continent of Gondwana

The three land masses started moving together again about 700 million years ago, and the movements culminated in the formation of Gondwana about 550 million years ago. As the land masses converged, sediments were also pushed up into mountain belts in the same way as the Himalayas rose when India collided with Asia. The remains of all the deep- and shallow-water deposits make up the rock formations of the Damara Supergroup and Gariep Complex, many of which are now prominent features. Thus, the Khomas Hochland and other large areas of west-central and coastal Namibia were formed from deep-water deposits. Shallow-water deposits, by contrast, produced belts of limestone and dolomite hills, including the Otavi-Grootfontein-Tsumeb hills extending west to Outjo and north to Ruacana. Another belt of limestone lies between the Gamsberg and near Steinhausen. Otjikoto and Guinas lakes near Tsumeb, and the Arnhem Cave near Seeis, formed in these lime stones. One giant piece of limestone went its own way when the continents collided, sliding south-west for about 120km to form the Naukluft Mountains.

Volcanic rocks produced by a mid-oceanic ridge in the Khomas Ocean possibly formed the Matchless Belt, from which copper has been extracted at the Otjihase and Matchless mines. The Belt is between 500 and 3,000 m wide, and extends for 350km from near Gobabeb in a north-easterly arc to Windhoek and beyond. Similar volcanic rocks in southern Namibia form part of the Gariep Complex.

Where continents collide, their margins generally fold upwards into mountains, and basins often form away from their ridges, almost as if the earth's crust counters uplift in one area by subsidence behind it. This happened when the Khomas Hockland and the northern dolomite hills were forced upwards, producing behind them the Nama and Owambo basins, respectively. Both basins were later filled with sediments carried in by wind, rivers and glaciers, and this is how the shales, sandstones, lime stones and conglomerates of the Nama Group were formed. The compressive forces of the colliding continents also forced molten rock upwards from under the earth's crust as intrusions of igneous rocks, which formed the Damara Granite Intrusions.

Namibia from 300 to 200 million years ago

The whole sequence of Karoo Supergroup rocks started to form when ice sheets and glaciers covered most of Gondwana during an ice age lasting from 300 to 280 million years ago. The ice age may have been due to a combination of changes in the nature of the earth's orbit around the sun and a shift in the positions of continents in relation to the poles. An extensive sheet of moving ice covered the Karoo Basin (a basin overlying the Nama Basin in southern Namibia) while smaller ice sheets and glaciers formed in the north and north-west. The sheets of ice flattened the landscape, while glaciers cut through the land surface to gouge out new valleys. One of the best-known of these is the Kunene Valley. It was only much later that it became a large river with a catchment area draining a large part of Angola. Some glacial deposits lie deep below the sands that now cover the Owambo Basin.

Other sediments of the Karoo Supergroup accumulated once the glaciers started to melt and retreat. Large rivers flowed out from the retreating glaciers, depositing sediments into big deltas and lakes that spread over the Karoo Basin. These sediments are now shales and sandstones, and the abundant reserves of water in the Stampriet aquifer are held in some of these sandstones.

Another large basin also formed about 280 to 250 million years ago. Most of this (the Parana Basin) was in what is now South America, but part of it also extended into Namibia, where it is known locally as the Huab Basin. Sediments deposited by rivers into this basin now remain as limestones, shales and sandstones.

Parts of Gondwana started to splinter and form rift valleys between 250 and 230 million years ago. The only one of these valleys in Namibia was situated in the vicinity of the Waterberg, where it is called the Waterberg Basin, but the basin extended north-east to the Okavango Delta and perhaps further. Arid river systems flowed into the Basin form the north-west, leaving deposits which are now called the Omangonde Formation. Numerous fossils of mammal-like reptiles and other animals have been found in these rocks.

Namibia from 200 to 132 million years ago

Two seas of sand formed over parts of Gondwana. The first of these covered much of southern Africa during an arid phase between 200 and 170 million years ago. Remnants of these dunes and sands that formed the Waterberg Basin now cap the Waterberg Plateau, Mount Etjo and several other hills in this area. The other sea of sand formed largely in what is now South America, covering a massive area of about 1,600,00kmē, which is almost twice the size of Namibia. Part of it extended into north-western Namibia, where remnants of this sand now remain in fossilized dunes such as those at Twyfelfontein. The life of this gigantic area of sand in Namibia was, however, cut short when Gondwana started to break apart 132 million years ago.

The continent of Gondwana

Gondwana was a huge continent, consisting of India and all the present-day continents of the southern hemisphere. It remained stable for 350 million years. Then in a rather similar fashion to the previous breakup of Rodinia, Gondwana started to split apart about 180 million years ago. The split meant that the earth's crust - a layer some 30-40 km thick - broke apart. Enormous bursts of volcanic activity accompanied the break, with some of the earliest eruptions occurring when Antarctica and South Africa separated. These eruptions produced the Kalkrand Basalts and associated dykes and sills, and also the layers of basalt that cap the Drakensberg range in South Africa.

Gondwana continued to break apart over a period of 125 million years. In fact, the movements of the earth's crust that started the split so long ago continue today, with South America and Africa drifting further apart. The last major rift occurred about 55 million years ago, when India split away from Madagascar and moved northwards.

Namibia and South America split apart

The split between South America and southern Africa started about 132 million years ago when volcanic activity bombarded this part of the world over a period of between one and five million years. The volcanism produced a pile of lava and volcanic ash up to 1,6 km thick over vast areas of Brazil and north-western Namibia. Molten rock erupted from many volcanoes, including the Messum and Doros craters. Large areas of the sea of lava - known as the Etendeka Group lavas - remain visible in the area between the Huab and Hoanib rivers and in other areas north into Angola.

Many other noteworthy features of Namibia's landscape appeared the same time. Great masses of molten rock were forced up, into and through the earth's crust as volcanoes or as relatively subdued intrusions of igneous rock, producing the Erongo Mountains, Brandberg, Spitzkoppe and the Paresis Mountains. Hundreds of dykes and sills were also formed, as were many kimberlite pipes that intruded into the crust. Many of these pipes are in the Gibeon area and in the southern parts of the Khaudum Game Park, but no diamonds worth mining have yet been found in them. Brukkaros was also produced by more recent volcanic activity only 80 million years ago, but the circumstances leading to its later appearance are unknown.

The Kalahari and recent deserts

The margins of southern Africa lifted up when Namibia and South America finally parted ways about 132 million years ago. Erosion then slowly ate away the edge of southern Africa, so that the broad coastal plain that is now the Namib Desert had been formed in the region of 70 to 65 million years ago. This left a gigantic basin in the centre of southern Africa. Part of this is the Kalahari Basin, which now extends from the Northern Cape, north through Namibia, Botswana and Zambia, and into the area around the Congo River. The Kalahari Basin has progressively filled up with sand and water-borne deposits, the natures of which vary according to whether the area was going through a phase of high of low rainfall. The deposits of sands, clays and calcretes that make up the Kalahari Group cover the eastern parts of the Nama Basin and almost all of the Owambo Basin. The large sand seas of the Namib Desert have approximately the same age as the Kalahari sands.

Some of the river systems in the vast area covered by the Kalahari Group remain active (such as the Okavango River), some flow intermittently (for example, the Cuvelai Drainage System and the Nossob and Aub rivers), while others are essentially dormant at present (most of the Omuramba Omatako, for example).

Dune fields have likewise come and gone, and many of the neatly arranged linear dunes in various areas were formed during much drier times long ago. The alignment of dunes reflects the direction of the prevailing winds when the dunes were formed. In fact, the present sea of sand in the Namib provides a picture of what much of northern and eastern Namibia could have looked like during drier periods.

Present evidence suggests that the Namib probably formed between 16 and 15 million years ago, about the same time that the Antarctic became as cold as we know it today. A wetter period, lasting perhaps from 12 to 7 million years ago broke this long, dry period. The dry climate on the west coast is partly due to the cold Benguela Current, which flows north from the Antarctic Ocean. However, since the cold Benguela Current only formed about 5 million years ago, the dry environment of the Namib Desert must have been associated with other climatic circumstances.

References:

Atlas of Namibia by John Mendelsohn

Namibia Fascination of Geology by Nicole Grunert