A History Of Deep Time - Part 1

Prof Maarten de Wit

Mpumalanga hosts one of the two oldest and bestpreserved 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 the earth’s earliest continent. Rocks from these areas provide the only direct information from which the earliest history of our planet can be confidently reconstructed.

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 to infinity, and takes us back to when the earth was 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 for scientists interested in how the young earth worked 3 500 000 millennia ago, and in searching for new clues to the origin of life. In a real sense, Mpumalanga represents the cradle of life.

The earth is about 4 550 million years old, but no terrestrial rocks that old have ever 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 more than 4 000 million years in 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 all been ‘cooked and stirred’ under such high temperatures and pressures that their original features have been destroyed, and most traces of their origins erased. In contrast, the slightly younger rocks of the Barberton Mountainland, 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 the earth’s history with an unrivalled degree of accuracy; from all over the world, geologists make the scientific pilgrimage to Barberton to study this small fossilised remnant of the young earth and its earliest inhabitants.

Along the road from Badplaas to Elukwatini and from there into the Songimvelo Nature Reserve, or along the Komati River from the Nkomazi Wilderness 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, then young researchers at the University of the Witwatersrand, discovered an entirely new class of volcanic rock along these river sections. These rocks are now known as ‘komatiites’ and have been age-dated between 3 470 and 3 482 million years.

The Mpumalanga komatiites became famous almost overnight 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 crystallised 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 hotly debated.

As long as the enigmatic spinifex textured komatiites of Mpumalanga retain their secrets about our early earth, interest in them will not wane. Unfortunately, many of the most important outcrops of the Mpumalanga komatiites have been damaged during the past two decades. If they are to remain accessible to future generations, it is vital that they be recognised soon as a fundamental heritage, before they lose further value.

Click here to read Part 2 - A History Of Deep Time

Reference: ‘A history of deep time’.  Mpumalanga: history and heritage (edited by. P Delius). University of KwaZuluNatal Press, 2007