Stranger in a Strange Land – the Barberton Greenstone Belt
If you were able to visit Archaean Earth you would indeed be a stranger in a strange land. Earth in the Archaean would have seemed like an ocean-covered alien planet; a dim sun hung in the sky, the atmosphere was toxic and almost without oxygen. There was no vegetation, volcanic eruptions filled the sky with ash, a hail of volcanic debris fell for kilometres around the many volcanoes. Torrential downpours lasting millions of years lashed the surface. The moon hung much larger in the night sky — it was closer and so tides were higher, violent, more frequent and the days were shorter.
The Archaean spanned some 1.5 billion years — ~2.5 - 4 billion years ago — and at its end the Earth had been transformed from a wholly oceanic planet to one with plate tectonic activity, ocean basins filled with sediment, volcanic island arcs, continental collisions and rifts tearing the newly-formed crust apart. Deep oceanic basins, evidenced by today’s banded iron formations, chert beds, chemical sediments and pillow lavas, had been formed by tectonic activity. Simple life forms — microbes — some of whose remains are found in the black cherts, had evolved. Some microbes formed mats, seen in sandstones along the trail, some used light for energy, others used sulphur escaping from deep vents on the seafloor or in hot springs on the surface.
The Barberton Greenstone Belt is the best-preserved example of a 350 million year sequence of Archaean Earth rocks — the Barberton Supergroup. These staggeringly ancient volcanic and sedimentary rocks are a unique record of Archaean Earth between about 3.2 and 3.57 billion years ago. The rocks are of three main groups:
Onverwacht — some sedimentary but mostly volcanic rocks ~14 km thick
Fig Tree — deep-water sedimentary rocks ~7 km thick
Moodies — shallow-water sedimentary rocks ~2.5 km thick
The rocks are a record of cataclysmic volcanic activity, a boundless ocean, racing tides, immense asteroid impacts and the birth of the first continent — the Kaapvaal Craton — and some of, or perhaps even the earliest, life on Earth. A growing global network of geologists descend annually on Barberton to search for clues to the Archaean eon. Over 30 years of research have helped to define, among other things: the evolution of Earth’s atmosphere; the origins of life; the growth mechanisms of continents and the composition of the earliest oceans. Although rocks of similar age and even older are known from other parts of the world, none combines the outstanding and diverse characteristics of the Barberton Greenstone Belt — a fame more significant and far longer-lasting than its colourful gold-rush past.
Visit the coordinates and post your photo of the EarthCache plaque
Logging requirements:
What kind of rock is the large isolated and patterned boulder within the circle of rock specimens?
What caused the patterning in this rock?
Which two rocks in this collection of specimens contain some of Earth's most ancient known microbes?
Which rock is the result of the oldest and hottest lava flows, and who discovered and named it?