There is a chunk of Earth’s crust on South Africa’s eastern escarpment so ancient that it beggars the imagination. We as humans work on another time scale entirely – our short lives are measured in decades and a time span of 2000 years seems so very very long. Five million years spans all of human evolution and even those numbers seem improbable. So to now tell you that rocks which underlie this piece of land have been dated at 3 538 000 000 years might indeed stretch your imagination. That number does indeed read as three billion, five hundred and thirty eight thousand million. Whew! To set our own lives against this kind of timescale makes them appear entirely insignificant, and the history of mankind not much more so. But there is worse to come – they have discovered ancient gneisses in the Slave Province of Canada dated at 4 billion years old, and a single zircon crystal from Australia has been dated at 4.3 billion years. But this is no dent to our national pride – we are still up there with the front runners and Barberton remains an earth scientist’s Mecca.
It is one of the few places on Earth where the Archaean (rocks of 2500 million years and older) is exceptionally well preserved and easily accessible. At one stage the Barberton Mountain Land held the record for the oldest rocks on Earth. But let us not indulge in hubris here, remember the insignificance of our allotted three score and ten years when compared against the background of this vast, 3.5 billion year sweep of geological time.
The Barberton Mountain Land is characterised by rugged topography and deeply incised river valleys. It is shaped roughly like a giant ice-cream cone, with the mandatory cherry on top located at Badplaas in the southwest and Komatipoort forming the tip of the cone (well, almost). It extends from the high African hinterland down to the Lowveld and includes a portion of northwest Swaziland in its sweep.
The towns of Steynsdorp, Havelock, Horo, Kaapmuiden, Louw’s Creek and of course Barberton have the honour of being founded on this veritable Methuselah of the Earth’s crust and those who walk on it are treading on hallowed ground. Moving swiftly from the realm of the metaphor to that of the mundane, the limits of the zone and its simplified geology are shown in the accompanying figure.
To peer through the swirling mists of the deep abyss of geological time in an attempt to unravel what went before is an almost impossible task. To have remnant samples from this deep abyss preserved relatively unaltered is a gift in the process of unravelling the complexities of Earth history.
Without boring you unduly with lessons in geology, suffice it to say that the earth is in a constant state of flux. Mountains are thrust upward and then ground down by the inexorable mill of erosion. The crust beneath the ocean basins is created by fire at the mid-oceanic ridges and is then carried away towards destructive plate margins on massive convection cells at a rate ranging from 2.5 (the rate at which your finger nails grow) to 15 cm per year. There is no hurried agenda to the slow grind of Earth’s workings – she has all the time in the world to crank the workings of her machine. On average, oceanic plates will last no longer than 100 million years – they will be carried back down into Earth’s mantle along subduction zones – known as destructive margins – the most famous of which is the Ring of Fire encircling the Pacific. Similarly mountain ranges such as the Alps and the even mightier Himalaya were thrust skyward approximately 65 to 40 million years ago – the mountain building process in these regions is still ongoing. But they are being worn down by the agents of erosion, and in 100 million years will be reduced remnants of their former glory, nothing more than a detritus of sand and mud. Throw another hundred million years into the pot and there will be little left of the world which we currently know.
Two hundred million years may seem an immense span of time, but considering how crust, both continental and oceanic, will doubtless be destroyed and recycled in this time, then to have a 3.5 billion year remnant preserved right here in our own back yard is miraculous indeed.
The Barberton Mountain Land is indeed a remnant – a remnant adrift in a sea of granite, cast to the sharks of time and erosion, leaving her chewed and frayed around the edges. Two geologist brothers, Viljoen and Viljoen, wrote a paper in 1969 titled grandly “The Geology and Geochemistry of the Lower Ultramafic Unit of the Overwacht Group and a proposed new class of igneous rocks Komatiite, Upper Mantle Project.“ This if course all sounds very esoteric to most of us, but one has to read a little between the lines to understand the significance of what they had found. Some of you might remember in the mid 1990’s the discovery of a 90 kg, cowlike animal called a Saola in the jungles between Vietnam and Laos. This was the first land vertebrate of this size to be discovered for more than fifty years. Well, the discovery of a new type of igneous rock in the Komati Valley, and an extruded lava at that, was perhaps the geological equivalent of the discovery of the Saola, although it didn’t get anywhere near as much coverage in the popular press. The exciting thing is that the chemistry and mode of occurrence of this rock shed light on the geological conditions prevailing during the formation of Earth’s crust way back then, which until this discovery had been open to nothing but pure conjecture. Appropriately this rock was called a komatiite and several other occurrences have since been found elsewhere on the planet’s face, but it was here near Barberton that this ancient rock type was first brought to the attention of the geological world.
And what were conditions like 3.5 billion years ago? Well, we must add a little more flesh to the geological skeleton, otherwise we will be getting ahead of ourselves. Sitting on top of the ancient komatiites is an accumulation of volcanic lavas, limestones and shales. These rocks make up the Onverwacht Group, the first of three broad subdivisions of the geology of the Barberton Mountain Land. Overlying the Onverwacht is the Fig Tree Group; a sedimentary pile of material laid down in submarine conditions. Finally all is overlain by the Moodies Group which comprises quartzite (metamorphosed sandstone), sandstone and shales with occasional volcanic lavas.
The interpretation of conditions prevailing in the Archaean has been made based on studies of the BML and other greenstone belts around the world. Mantle heat flows were higher then than now. The chemistry and mineralogy of the komatiites indicate that they were erupted at relatively high temperatures compared to those associated with the eruption of younger basaltic lavas. Richard Fortey in his recent book titled Earth: An Intimate History, sums things up beautifully, describing prevailing Archaean conditions thus: “The nascent masses of continental crust had not yet congealed to their present size. Instead, smaller rafts of lighter rocks formed the nuclei of what would become more stable continental areas..…. The sea – and there was certainly an ocean – was whipped up by storms, reducing all land newly elevated by tectonics to sedimentary waste. Slabs of oceanic rock were covered with sediments derived from the rapid weathering of the protocontinents. Unprotected by any cloak of plants, the wind and rain worked fast upon the naked rocks. This was a world of tempests and flash floods, of jagged crags and dunes. Clays and grits, the mucky progeny of erosion, slumped down into deep water. Volcanic rocks were erupted over the sea floor….. Buoyed up by their less dense composition, the growing continents bobbed onwards, as would rafts of cork on a mill pool, while heavy ocean crust was created and then destroyed in the early cycles of plate tectonics.”
These interpretations of the processes which formed these ancient greenstone belts might quicken the pulse of earth scientists everywhere, but these events took place so far back in Earth’s history to be almost irrelevant to most of us here in the early part of the 21st Century. But two really important conditions prevailed which do affect us all. Firstly sedimentary structures in the form of sand ripples within the sandstones of the Moodies Group indicate deposition on tidal flats, and to have a tide, we must have a moon. So this is the first evidence of for the existence of the moon in orbit early on in our geological history. Secondly, and perhaps most importantly, on pondering our origins and our significance on this small blue planet of ours, it may be useful to remember that some of the first evidence of unicellular life occurs within the Onverwacht and Fig Tree Groups. Spheroidal or rod-shaped micro-organisms formed algal mats called stromatolites – a form of blue-green algae which changed the world forever. Back then the atmosphere was almost devoid of oxygen with hydrogen sulphide and methane being the dominant gases. Blue green algae, stromatolites included, added via the process of photosynthesis oxygen to the atmosphere, molecule by tiny molecule to create an atmosphere which sustains us all. But the evolution of life must have predated the deposition of the Fig Tree sediments and currently it is thought to have occurred around 3.2 billion years ago.
Having said all that, other than being of interest in terms of the origin of life and the fact that the moon was winging its elliptical path around our Earth 3.5 billion years ago, what other relevance does all this have to our lives? Well, internationally, Archaean Greenstone belts, as they are also known, are home to many of the world’s gold mines.
The four operating in the Barberton region are have the romantic names of Sheba, Consort, Fairview and a somewhat more down to earth name, Agnes. Gold was discovered in the headwaters of the Blyde Rivier in 1873 by “Wheelbarrow” Patterson which led to a gold rush and the founding of the town of Pilgrim’s Rest. The largest gold nugget (perhaps the term boulder would be more appropriate here), discovered in 1875, weighed in at 8.02 kilograms. But the gold rush days soon passed by when gold was discovered in the Witwatersrand gold fields 12 years later.
For an amazing tour up the pass and back into the abyss of time, go visit the official website of the Barberton Greenstone Belt Geotrail here.