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The Geology of Nkomazi : A history of deep time

As part of the Baberton Mountain Land in Mpumalanga, Nkomazi Game Reserve hosts one of the two oldest and best-preserved rock sequences found anywhere on our planet. The other is in Pilbara, Australia and there is good geological evidence to support the hypothesis that these two areas were once fragments of Earth’s earliest continent. Rocks from these areas provide the only direct information from which the earliest history of our planet can be reconstructed with confidence. The solid bedrock of Mpumalanga tells a rich story of immense antiquity; a story that projects so far back into the past that it is almost impossible to comprehend. It involves a time scale stretching out towards infinity, and takes us back to when the Earth was youthful, a mere 20 per cent of its present age. Africa’s oldest known rocks come from Mpumalanga and adjacent Swaziland, exposed in the rugged Barberton mountain chains that run all the way from Elukwatini and Tjakastad to Komatipoort, straddling the Swaziland border. Because the world’s oldest fossils have been found here the area is a Mecca of scientists interested in how the young Earth worked 3 500 000 millennia BC and in searching for clues to the origin of life.

Earth is about 4 550 million years old, but no terrestrial rocks that old have been found: the oldest known rocks are from Canada, where, in a small area near the Arctic Circle, the Acasta gneisses have been dated to just over 4 000 million years of age. The next oldest rocks, from Greenland, are about 3 800 million years old. Although these ancient rocks provide important glimpses into the deep past of our planet, they are of limited value because their exposures are relatively small, and because they have been ‘cooked and stirred’ under such high temperatures and pressures that their orginal features have been thoroughly destroyed, and most traces of their origins have been erased, In contrast, the slightly younger rocks of the Barberton Mountain Land, which ‘clock in’ at between 3 000 and 3 600 million years ago, are exquisitely preserved, in many places as if they originated only yesterday. Here researchers can ‘read’ and interpret the early chapters of Earth’s history with an unrivalled degree of accuracy; and from all over the world, geologists make the scientific pilgrimage to Barberton to study this small fossilized remnant of the young Earth and its earliest inhabitants.

Along the road from Emanzana (previously known as Badplaas) to Elukwatini and from there into the Songimvelo Nature Reserve, or along the Komati River from the Nkomazi Game Reserve area, just east of Tjakastad, to Kromdraai and beyond, one can visit large outcrops of these old rocks. In the late 1960s the twin brothers Morris and Richard Viljoen, who were then young researchers at the University of Witwatersrand, discovered an entirely new class of volcanic rock along these river sections. These rocks are now known as ‘komatiits’ and have been age-dated between 3 470 and 3 482 million years old. The Mpumalanga komatiites became an almost overnight celebrity because they are confined to the history of the early Earth and are unknown from volcanoes of the modern Earth. Komatiites often display spectacular textures of skeletal crystals (known as spinifex textures) which branch out like fern leaves. From these textures, and the chemical make-up of the rocks, it can be deduced that komatiite lavas crystallized exceptionally rapidly from very hot and probably water-rich molten magma. For such komatiitic magmas to have reached the Earth’s surface as lava flows, the internal conditions of our young planet must have been significantly different from those measured today. Yet more than 35 years since their discovery, the precise origin of komatiite is still hotely debated. As long as the enigmatic spinifex textured komatiities of Mpumalanga retain their secrets about our early Earth interest in them will surely not wane.

Volcanic basalts associated with the komatiities are also of great value interest. These rocks often display spectacular bulbous features known as pillow lavas. The outer rims of the pillows comprise volcanic glass in which microscopic traces of the world’s oldest life forms have recently been discovered in the form of hair-like tubes. These tubular structures are also found today in glassy margins of pillow lavas that cover the modern oceans in great abundance and are constructed by bacteria that consume chemical ingredients from the rock-glass to sustain their metabolism: these bacteria literally ‘eat’ rock. The new Mpumalanga finds confirm without doubt that simple life was rife on Earth about 3 500 millions years ago, and that most of its upper oceanic crust was teeming with bacteria. Other microscopic fossils have also been discovered in the Barberton Mountain Land in numerous other rock types, suggesting that there was a significant diversity of bacteria. Thus between 3 200 and 3 500 million years ago primitive life in Mpumalanga was already exploring a number of ecological niches and has established the world’s oldest biodiversity hot spots.

Numerous other important observations of an experiment on the rocks from the Barberton Mountains have provided insight into the hostile environmental conditions that prevailed on Earth in those early days. For example, tiny spherules have been found in thin rock layers that are widely spread throughout the central parts of Barberton Mountains. These are the rock equivalents of hailstones that settled out from clouds of molten rock droplets spewed into the atmosphere as a result of the extreme heat and mechanical energy liberated during periods of intense meteorite bombardment and volcanic activity at the time. In addition, detailed structures preserve in the sedimentary rocks of the imposing ridges that dominate the road sections between Barberton and Josefdal, and across the Swaziland border into Piggs Peak, illustrate that the earth-moon tidal system was already operational. From these rock ‘inscriptions’ one can calculate also that the Moon was considerably closer to Earth, that the Earth was spinning faster, and that terrestrial days were consequently shorter than they are today.

A particularly intriguing metalliferous rock mass near Bon Accord close to Malelane, was made up almost entirely of iron and nickel, probably representing a chunk of the inner core of the early Earth that was embedded in its rock near the surface at a time of dynamic upheaval. Scientists have also tested rocks from the central parts of Barberton Mountain Land to prove that the Earth at that time had a self-generating magnetic field, although this field was probably not as strong as it is today.

These and many other geological treasures provide tiny glimpses into the workings of our young planet. But the record is fragmentary and there is still much that we can not understand. Scientists have only just scratched the surface of Mpumalanga and more remains to be discovered. The importance of studying the geological history of the Barberton Mountain Land cannot be over emphasized: it has allowed South African scientists in the past 50 years to define the very cutting edge of research into the beginnings of Earth history and, in doing so, has helped to shape the future of new emerging sciences like geo-biology and astro-biology, and will continue to play a role in planning future missions to Mars in the quest for signs of life on other planets.

Between 3 200 and 3 100 million years ago, the world’s oldest gold deposits formed along the northwestern edge of the Barberton Mountains. Some of these exceptionally rich and are still explained at the Fairview and Consort mines located close to the road between Barberton and Malelane. In that same period, the Barberton rocks were extensively deformed and uplifted by plate tectonic processes similar to those operative on the present Earth, and a substantial continental mass has stabilized the Mpumalanga region by 3 000 million years BC. The hard evidence for the existence of this ancient continent comes from the escarpment to the south and west of the Barberton Mountain Land, where its rocks were engulfed by a vast volume of numerous different types granite between 2 800 and 3 450 million years ago. These granites were at depths of 10 to 30 km below the surface of the crust at that time, yet today, the road from Carolina to Oshoek, and beyond into Swaziland, meanders across them on a relatively flat and extensive Highveld surface – a peneplain. The age of this peneplain can be deduced from what lies directly on top of it. In the Machadodorp-Kaapsehoop region, for example, the granites and the Barberton rocks that, further northward, also make up much of the great and spectacular Drakensburg escarpment. These rocks comprise the Transvaal Sequence, a flat lying blanket of sedimentary rocks, up to 3 000 metres thick, which formed in a shallow sea between 2 700 and 2 400 million years ago. Yet, before this ancient sea could transgress across the flat granite surface, erosion must have planned off the early continent to great depths to expose this granite close to sea level. This dates the remarkably extensive Mpumalanga peneplain to earlier than 2 700 million years B C, and as such it is the oldest mature continental erosion surface of the world.

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