In 2005, I was driving along winding roads through the Drakensberg Mountains in Lesotho, South Africa. Tall, cliff-like rock formations called escarpments punctuate the landscape, rising a kilometer or more into the air. Overwhelmed by the dramatic scenery, one question came to mind: How on earth did this landscape come into being?
The outer shell of our planet is broken into seven or eight large sections, or tectonic plates, on which the continents lie. We expect the continents to rise at the active boundaries of these plates, where volcanism and earthquakes are often concentrated.
But why – and how – do these spectacular structures form far from these boundaries? Our new theory, published in Nature after nearly two decades of thought and forensic work, explains how uplifts like those in the Drakensberg can occur in supposedly stable parts of continents.
The continents we know today were once united as single, large “supercontinents.” One example is Gondwana, which existed hundreds of millions of years ago and began to break apart in the age of the dinosaurs. We believe that the breakup of these supercontinents triggered a kind of shaking process beneath the continents that we now call a “mantle wave.” This movement deep within the Earth slowly ripples across the partially melted underside of the landmass, disturbing its deep roots.
The mantle is the 2,900 km thick layer of the Earth that lies beneath the outer crust on which we live. To study what happens when continents break apart, we developed sophisticated dynamic models to recreate the properties of the Earth’s crust and mantle and how they are physically stressed when forces act on them.
USGS
When continents separate, hot rock from the Earth’s mantle surges up and fills the gap. This hot rock rubs against the cold continent, cools, becomes denser, and sinks, much like a lava lamp.
What has gone unnoticed until now is that this movement not only disturbs the region near the so-called rift zone (where the Earth’s crust is being pulled apart), but also the nearby roots of the continents. This in turn triggers a chain of instabilities caused by heat and density differences that propagate inland beneath the continent. This process does not happen overnight – it takes many tens of millions of years for this “wave” to reach the deep interior of the continents.
This theory could have profound implications for other aspects of our planet. For example, if these mantle waves erode 30 to 40 kilometers of rock from the continental roots, as we suspect, this will trigger a cascade of heavy impacts at the surface. The loss of this rocky “ballast” will make the continent more buoyant, causing it to rise like a hot air balloon after it has shed its sandbags.
Andrew Mohamed
This uplift of the Earth’s surface, which occurs directly above the mantle wave, should lead to increased erosion by rivers. This happens because the uplift raises previously buried rock, makes slopes steeper and more unstable, and allows rivers to carve deep valleys. According to our calculations, the erosion should be one to two kilometers, or in some cases even more.
The innermost parts of the continents are some of the hardest and most stable parts of the Earth, so removing these regions by a few kilometers is no easy task.
But at the edges of these stable continental regions, the cratons, there are kilometer-high escarpments, such as in Lesotho. These huge escarpments enclose these regions and extend for thousands of kilometers. They are evidence of a fundamental disruption of the landscape at about the same time that the supercontinent Gondwana broke apart – that is, about 180 million years ago.
Mysterious plateaus
Inland from these great escarpments we find plateaus, such as the Central Plateau of South Africa, which rises more than a kilometre above sea level. The origins of these plateaus have long been a mystery and were not usually associated with the escarpments.
Some scientists had previously suggested a phenomenon known as mantle plumes – massive upwellings of hot, floating material from deep within the Earth – as a possible explanation for the plateaus.
Such plumes could potentially push the Earth’s crust upward and provide dynamic support. However, there is no evidence of such an internal continental plume feature in geological records from the surrounding continents or oceans from the time period in question. Could our mantle wave offer a new explanation?
To test our predictions, we turned to thermochronology – a science that helps us understand how rocks now at or near the surface have cooled over time. Certain minerals, such as apatite, are sensitive to temperature and time. Much like a black box, these minerals record a “cooling history,” providing snapshots of how the temperature of a particular rock has changed.
Nuntiya / Shutterstock
Here we used several existing measurements across South Africa. This analysis confirmed our model’s predictions: across the region, erosion occurred over several kilometers at approximately the times suggested by our models. Even more remarkable, erosion moved across South Africa in a pattern very similar to the mantle wave in our simulations.
To further investigate this relationship, we used another type of simulation called landscape evolution modelling, which looks at how water interacts with the landscape and how the Earth’s surface actually springs or “bends” in response to rivers shaping the landscape.
When we incorporated the mantle wave into our computer model, we saw how it could theoretically form a high plateau. Our results explain how vertical movements of continents can occur far from active tectonic plate boundaries, where most uplift is known to occur.
The massive erosion that occurs during these mantle wave events can cause intense chemical weathering of rocks, which removes carbon dioxide from the atmosphere and thus promotes global cooling. These uplifts can also physically separate flora and fauna, leading to speciation and shaping evolution. We have made great progress in understanding the processes that cause mountain ranges to form away from continental margins. And it still amazes me that all of this started with an impressive view of the Lesotho landscape.
