It may take another 10 million years, but the African continent is not going to remain in one piece. It is being split apart, with the larger continent on one side and parts of Ethiopia and Somalia floating off on the other.
Modern-day Africa was the keystone of Gondwana, the aggregated mass of southern continents that co-existed for nearly 400m years.
That supercontinent has since split apart, creating the land masses we now recognize as South America, Australia, India, Madagascar, Antarctica and, of course, Africa. And now, a new study published in Nature Geosciences is shedding light on how Africa is breaking up as well.
For the greater part of the Phanerozoic Eon, 500m years ago up to the current era, the Gondwana continents shared a common history and were populated by the same plants and animals. These continents were also marked by a climatic history that indicates the changing “paleogeographic” position (location of the continents over time) of this enormous landmass – roughly twice the size of modern Eurasia.
Things began to change approximately 180m years ago as the modern South Atlantic Ocean, Southern Ocean, and Indian Oceans began to form. One by one, Africa’s Gondwanan partners began drifting away.
The modern-day East African Rift, an active tectonic plate boundary and one of the geological wonders of the world – is the site of the most recent of these separations, and provides a glimpse of how continents break apart. This is the focus of the current paper in Nature Geosciences by sedimentologist Eric Roberts of James Cook University and colleagues at several Australian and American institutions.
The first stages in the splintering of a stable continent are marked by an increase in surface elevation. Like the proverbial floating iceberg, which is 90% underwater, the elevation of continents obeys the principle of isostasy: less-dense materials float on top of denser materials. The elevation of continents reflects the thickness of their crust, which is composed of less-dense material than the mantle rocks below. Mountains are high because of their thick root, which is approximately twice as thick as for the average continent. Southern and eastern Africa are different, as they have an ordinary thickness of crust, and yet are elevated by more than 1km above the average continent.
So why did the continent rise? Well, it’s down to deep-seated mantle processes occurring far below the root of the continent. Here, upwellings along the margins of a “thermochemical pile”.
This uplift, which began in the mid-Cretaceous period (roughly 100m years ago), is causing the African continent to literally break apart. The cracks that form will eventually be filled with dense, mafic rock, a material rich in magnesium and iron – that forms the modern oceanic crust. When this uplift occurred is an enduring question for geologists and one that Roberts and colleagues have begun to answer. From their study of the Rukwa Rift Basin (RRB) in Tanzania, the western, presently-non-volcanic branch of the East African Rift system; Roberts and colleagues were able to reconstruct the shifting course of rivers that mark this dynamic landscape. The researchers first matched the pattern of magnetic reversals found in the sediments deposited by these rivers to the known history of Earth’s geomagnetic field, known as the Global Polarity Timescale.
Image via IFL Science