People mellow with age. At least, most of us do. The emotions of youth become less all-important and less demanding of our attention. Young people feel that every perceived slight needs addressing. The heat goes to the head and mistakes are made. The Earth, too, went through that phase, before it settled down in middle-age continentality. Volcanoes were everywhere and the lava they produced was hot. It was a restless time. It took the Earth a billion years to grow out of it.
The young, hot-headed earth was almost entirely a water planet. We are talking 4 billion years ago and the time of the late heavy bombardment had just ended. The first continental slivers were forming. The oldest of these that survives is at Amitsoq, south-east Greenland, a combination of volcanic and sedimentary rock. Plate tectonics was beginning although still in its infancy: full blown plates with their subductions would not exist until another billion years. But mantle plumes existed, probably more plentiful and faster than nowadays. These threw up volcanic plateaus and islands, and these became the cores of the new continents.
The best recorded of the origin stories of the continents comes from the Southern African craton. This was once the heart of Gondwana. But the region was much older than this. The oldest rock here is a staggering 3.6 billion years. It takes us back into a different era, called the Archean, a period lasting until 2.5Gyr ago. The Earth was a different planet. The atmosphere was not conducive to life, with high CO2, probably mixed with methane to give a photochemical haze in the air. But the ocean already existed. The oceanic crust was divided into numerous microplates, where everything was more active than now. The interior was hotter. There were no continents yet but there were plenty of oceanic islands.
Building a continent
The location of the oldest rocks of the South African craton are in Barberton. They are called the Barberton greenstone and they tell a story about how the continents formed.
The rocks form a belt stretching from northeast to southwest, roughly 100 by 60 km with rocks covering an age range of 500 million years. The region is at the border of Swaziland, just south of the famous Kruger park. The southern parts of the Kruger park are dense with wildlife and tourists. The river is full of hippos and crocodiles and the wooded area hides the elephants well. They have right of way, even if suddenly appearing from the bush, and they know it. They happily chase the drivers to let them know who is boss. The real Kruger is not here: that is much further north where the land gets wilder and the animals more dispersed. But if you want to quickly get your African safari in, the southern Kruger is hard to beat. The birdlife is also superb. Sometimes the animals get out of the park, and you may suddenly find a hippo in your pond. Nothing to do but call the park service and ask whether they do collect.
The Kruger park is rugged, with lots of granite ‘kopjes’ (small peaks). The landscape looks ancient. But it is nothing on the Barberton area where the rock are many times older. Not much has survived, of courses. The originally horizontal layers have turned on edge in timeless upheavals. This is why the belt is narrow. The whole area is embedded in ancient graniodorite which has survived better – partly because it was underground. It is all part of the story.
The Barberton mountains are a green land: in the wet season it is covered in bush and some forest. It is an ancient greenstone, but that does not refer to the vegetation. The ‘green’ in the greenstone comes from chlorite, and it makes rock rather unpalatable to vegetation. The chlorite is a later metamorphic addition to the rocks, but it shows its origin as oceanic crust.
The original rock consists of three groups, called Onverwacht, Fig Tree, and Moodies. Onverwacht is the oldest of the three. It is mostly volcanic. The rocks of the Onverwacht group show how volcanic rocks erupted onto an ocean floor. Just like in modern oceans, the water cools the surface of the lava and the pressure prohibits any explosive activity. The result is pillow lavas. Indeed, the Onverwacht group shows plenty of these pillows in the region. There is little doubt how this land began, deep under water.
The most common origin of pillow lavas is a mid-oceanic spreading centre. That has also been suggested for the Barberton rocks. But it has been questioned. The eruptions began 3.54 Gyr ago and lasted for 250 million years. That seems too long for a spreading centre. The layers also are extensive and not faulted. The suggestion is that this was a volcanic plateau or submarine shield volcano. It is not known whether plate tectonics in its modern form, with spreading centres and subduction zones, had already developed on Earth. It may have been an environment more like Venus, with areas of volcanic activity and deeper basins without, but without larger scale motion from one such area to another. The origin of plate tectonics is still debated.
In the modern world, pillow lavas produce basalt. But although the Onverwacht group has a minor basaltic component, the majority were a different, much hotter type of lava. This type was first identified in the Barberton belt, along the southern part where the river Komati flows. It thus was given the name komatiite. They don’t make it any more. Nowadays, oceanic basalt is melted in shallow magma reservoirs at temperatures of at most 1300 C. The komatiite was formed several hundred degrees hotter, up to 1650 C. They came from a mantle that was notably hotter than nowadays.
The eruptions were not continuous: they were interrupted by epochs when thin layers of sediments collected, composed of iron-rich and silica-rich mud and volcanic tuffs. After the pillow lavas stopped forming, the sedimentation kept forming, now including carbonate layers which buried the lavas. The sedimentary layer is called the ‘middle marker’ and is a widespread part of the Onverwacht group. The carbonate shows that the ocean was much like that of today, with abundant calcium carbonates. The most notable is a dacitic layer, 2 km thick, dated at 3.45 Gyr ago. This has been interpreted as the subsiding volcanic peak, in fairly shallow water.
Now a second pile of volcanic rocks formed, but with a composition of basalt and dacite. This is the upper Onverwacht group which formed around 3.42 Gyr ago and there are similarities with modern-day island arcs. The magma did not come from as hot a chamber as the earlier komatiites. The dacite suggests that the magma chambers had time to cool and differentiate. It has been suggested that the newly formed oceanic crust was subjected to some subduction, partly melted, and percolated back into the crust.
Now things quieted down for a while, until the Fig Tree group formed between 3.26 and 3.23 Gyr ago. It is a volcaniclastic sedimentary sequence that is capped by felsic volcanic rocks that formed in deep- to shallow-water to alluvial environments. Some of the deposits show evidence for turbidites: underwater landslides. In these slides, the coarse material (sand) reaches farthest and mud the least: mud therefore forms the upper slope. The mud stones are quite black from the amount of graphite. This suggests that the volcanic islands were bounded by a trench into which the slides went down.
The Moodies Group was deposited next, between 3.23 Gyr, and 3.11 Gyr ago. It consists of shallow-marine to fluvial sandstone and conglomerate with minor shale and banded iron-formation. It shows some banding that is typical of tidal bays, where even the neap-spring tide cycle can be seen. Counting the layers shows that there were only 18 days in a tidal month, a combination of the Earth spinning faster and the Moon being closer than nowadays.
What happened? The island arc and trench suggest that plate tectonics was beginning to behave more like the modern Earth. The partial melt had formed a graniodorite (felsic) which had been emplaced in the oceanic crust. This reduced the density of the crust, thickened it, and caused the area to rise. When the oceanic floor began to subduct, this region was too buoyant for that. This lower density material formed the core of the island and morphed it into young continental crust. The lithosphere thickened and formed the 100-km deep keels of the modern continents. The Barberton area was not a single terrane: it was several distinct regions which formed in different places and only came together during the last phase of the Moodie group.
This process went on not just in the Barberton area. Similar events took place elsewhere in the world. The Pilbara area is an example, with the same age as the area here. The next phase came when series of these new island arcs began to amalgamate. This formed the core of the new cratons. The earlier granite was remelted, and now formed large emplacements of over 60 kilometers in size. The process would continue for another 500 million years. By the end, the world was full of microcontinents. The first continental collisions occurred. Mountain building began. 2.5 Gyr ago, the Archaean ended and the modern world took shape.
There was another event which left its scars. At 3.26 Gyr ago, the Barberton area shows a layer of spherules. The spherules are the size of sand grains, and formed from condensation of rock vapour in the atmosphere. Effectively, these were liquid rock drops which formed in the atmosphere. It rained rock. There are four such layers in the Barberton greenstone, but this layer (number 2) stands out, at the border between the Onverwacht and Fig Tree groups. The layer has a thickness between 20 cm and 3 meters, of which between 10% and 50% consists of the spherules. It is present in the southern area of Barberton. The same layer is seen in the Pilbara area. It contains chromium indicative of an extraterrestrial origin. This was a meteorite impact, from a carbonaceous chondrite. And not just any impact.
Based on the thickness of the layer, the impactor has been estimated as between 37 and 55 km in diameter. This is considerably larger than then the KT event. The impact caused dikes to form in the Onverwacht group. Some of the spherules found their way into these dikes, suggesting they were still open when the rock rain arrived. Gradation of the spherules suggests passing tsunami waves. The area was still deep under water, but this did not protect against currents caused by the tsunamis, or the earthquakes. The moment of the earthquakes has been estimated at a minimum of M10.8. A crater of some 500 km across may have formed. However, this would have been on Archean oceanic crust of which little survives, thousands of kilometers away from the Barberton rocks.
There was a second impact of similar size 30 million years later. These are likely the two largest impacts the Earth has suffered over the past 3.5 billion years ago. Were they related? That seems plausible. Perhaps a large asteroid had broken in two following a collision, and both fragments ended up with orbits with intersected the Earth. A game of russian roulette followed which only had losers.
One more thing is worth pointing out. The S2 layer is exactly at the change-over between the Onverwacht and Fig Tree group. In fact, so is the S3 layer, as the change happened a little later in the north of Barberton where the S3 layer is seen. The Fig Tree group is one of felsic volcanisms, and the onset of internal melt which formed continental crust. Perhaps these impact had a role in this. They formed large cracks where magma chambers could collect, may even have induced melt themselves, and ended the epoch of komatiite. Was this a worldwide change? It probably would have happened over time any way, but these massive impacts may have accelerated the change.
But what is komatiite, and what was different about the Earth to make it form?
Komatiite is an ultramafic magma. ‘Mafic’ stands for a magma that is rich in magnesium and iron. Basalt does this: it is a property of mantle material, and mafic magma thus indicates that there is a conduit to the mantle. The mantle is not normally melted, so heat needs adding. That can be done either in a spreading centre (where mantle material can upwell from deep because of a lack of pressure above), or it can through heat from a hot spot. The hot spot can be shallow (most are) or it can be a proper mantle plume. The lack of silica makes the magma to be of low viscosity: it flows easily, and over long distances. It is also dense, and that makes it harder to erupt over the less dense continental crust without some significant heat input.
Ultramafic is even closer to the composition of the mantle, and has very low silicate content. In the modern world, this happens when magma chambers collect olivine from the mantle and slowly become more and more mafic. The result is picrite basalt, and this can erupt in places like Hawai’i. But in the early Earth, the olivine from the mantle could erupt directly and this is komatiite. It is silicate and aluminium poor and has a very high melting temperature. Its viscosity approaches that of water, so it flowed every more easily than basalt. But there haven’t been any significant komatiite eruptions since 3 billion years. The aluminium is not always the same: some komatiites are depleted in aluminium, but others are not. The undepleted ones are older.
The melting temperature of komatiite depends on its precise composition, and it appears those changed over the archaean. For a MgO fraction of 30% (typical for the early archaean) and chromite content over 1500 ppm, the melting temperature is around 1600C. The melt happens at 400 km depth, at a mantle temperature of 2000 C. This is believed to be appropriate for the Al-rich komatiites of Barberton, and those elsewhere in the world at that time. The mantle was hotter than it is now, due to higher level of radioactives, but not this hot. So where did the komatiites come from? The temperatures of a few hundred degrees above the normal mantle suggests the presence of mantle plumes. The komatiites are believed to have formed in the tails of superplumes. This puts the evidence for the extensive pillow lava under new light. These regions may have been the ancient, subsea equivalents of modern flood basalts.
The hotter mantle would have had lower viscosity than the modern mantle, making plumes easier to form and faster to rise. But as the mantle cooled, things changed. The komatiites show lower MgO, less Al and Cr. They would have melted at lower temperatures, 1400C, and lower depth of 150-200 km. By the end of the archaean, 2.5 Gyr ago, conditions for komatiites were only found in few places and the fraction of such lavas decreased dramatically. The youngest komatiites erupted 100 million years ago on Gorgona island. But these are lower MgO, and the melting temperature was much lower than hat of the pure komatiites of the Archaean. They are part of our new, mature world. The hotheaded days of the Earth are well and truly over. All that is left is a memory of greenstone belt. Barberton is the graffiti of our youth.
Albert, February 2019