A History Of Deep Time – Part 4
Prof Maarten de Wit (Published by University of KwaZuluNatal)
Karoo rocks
The following 1 700 million years of the earth’s history are not well represented in Mpumalanga. Earth’s destructive erosion has erased most of the evidence: whole chapters have been removed from the geological history book. But rocks about 300 million years old and younger are again well preserved and extensively distributed throughout the province. These ‘Karoo’ rocks tell a story of different ‘dynasties’: a time during which first reptiles and then dinosaurs ruled the planet, to be overtaken by mammals in turn, 65 million years ago. This is the time of the earth’s transformation into a mature planet and a period that heralds the dawn of modern earth, with flowering plants, grasslands, and the evolution of primates.
In the far north west of Mpumalanga, across the Springbok Flats, the ‘old’ rocks of the young Earth are covered by a thin blanket of these relatively recent Mesozoic Karoo rocks. But further south, along the roads to Witbank and Middelburg, Bethal, Hendrina, Ermelo, Carolina, and beyond the southern border of Mpumalanga, the Karoo rocks are thicker and contain the massive seams of the vast coal fields of South Africa, which, for so long, have provided it with cheap energy.
A section of the Komati River, host to fossilised remnants of the oldest life on earth.
The side effects of this resource have been particularly damaging for Mpumalanga in contemporary times. Unprecedented health risks and environmental destruction are rife in vast areas around Witbank and Middelburg, where the coal is burnt in great quantities to produce electricity and steel products, and the enormous volumes of toxic byproducts are polluting the ground and atmosphere.
The Mpumalanga coals formed in vast swamps from rotting forests in a 100-million-year period between 200 and 300 million years ago. Africa then was still attached to South America, India and Antarctica as part of the Gondwana supercontinent. Primitive plants, such as the famous Glossopteris
flora, had colonised the entire southern hemisphere, and mammal-like reptiles and later dinosaurs roamed across the landscape of Mpumalanga. Fossils of these animals are found in abundance and are commonly displayed in local and national museums.
Then, halfway through this period, about 250 million years ago, rapid global mass extinction struck the planet. More that 90 per cent of biodiversity across the world was destroyed at species level within less that 70 000 years, a mere blink of an eye in geological terms. Scientists studying this catastrophic event, recorded in the Karoo rocks of Mpumalanga and beyond, have shown that it was related to large swings in climate. Understanding the causes of this extinction event, and the subsequent recovery, is of interest both academically and for our management of a modern world now undergoing relatively rapid human-induced climate change.
About 300 million years ago Mpumalanga was covered in a thick ice sheet, at least the size of that covering Antarctica today, and probably even bigger.
By 250 million years ago the climate in Mpumalanga had changed to dry warm conditions, and by 200 million years ago that area was covered by a Saharalike desert and large tracts of dune. Then, suddenly, about 180 million years ago, as the Gondwana continent started to disintegrate, large volcanic lava fields poured across the landscape of sand dunes. These volcanic rocks are best preserved to the south of Mpumalanga, in the Drakensberg Mountains of the Free State, KwaZulu-Natal and Lesotho. But extensive volcanoes and lava flows must have covered Mpumalanga too, because their feeders occur abundantly, in the form of dykes cutting through the older rocks of the province. In addition, equivalent-age volcanic sequences are exposed along the northern extension of the Lebombo Mountains, and all along the eastern border of the Kruger National Park as far as its northern margin at the Olifants River.
Recent geological history
Rocks younger than 180 million years old are poorly preserved in Mpumalanga. Most of the province (and indeed most of southern Africa) had by then been lifted well above sea level, and the region became dominated by erosional processes that slowly sculpted the landscapes into the forms in which we see them today.
Occasional young volcanic plugs, some in the form of kimberlites, are encountered, but in general there is little to be found of geological
significance with which to reconstruct what happened during that period. The most recent history, however, is recorded in deposits found in cave systems that in places also yield rich archaeological treasures. These materials provide scientists with glimpses of rapid climate changes related to the recent ice ages that, in turn, may have stimulated early primate evolution and human development.
In the central parts of the Barberton Mountainland, such caves and other diggings also yield abundant evidence of indigenous mining of its
oldest rocks: the 3.4 billion-year-old iron deposits that had formed at the very beginning of Mpumalanga’s history.
The geological history inscribed in the rocks of Mpumalanga tells an extraordinary story, from the young Earth to a modern world, and with the
kind of detail that cannot be extracted from rocks anywhere else in the world.
Unravelling this history is a truly unique and enriching experience that should be accessible to all South Africans and other world citizens. Mpumalanga, therefore, carries a great responsibility to ensure that its deepest memories remain preserved for posterity.
Click here to read Part 1 – A History Of Deep Time
Click here to read Part 3 – A History Of Deep Time
