How and why do ‘stable’ parts of continents gradually rise to form some of Earth's greatest topographic features like the large plateaus of southern Africa? It’s a question that has puzzled geoscientists for decades. Now, though, thanks to a new study published in the journal Nature, we might have an answer.
A team of researchers, led by Prof Thomas Gernon, discovered that when tectonic plates break apart, they trigger powerful waves deep within the Earth that can cause continental surfaces to rise by over a kilometre.
This process explains the formation of dramatic landforms such as great escarpments – steep topographic features that form long, dramatic cliff-like edges along continental margins – and expansive plateaus, which significantly influence climate and biodiversity.
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"Scientists have long suspected that steep kilometre-high topographic features called Great Escarpments – like the classic example encircling South Africa – are formed when continents rift and eventually split apart," Gernon said.
"However, explaining why the inner parts of continents, far from such escarpments, rise and become eroded has proven much more challenging"
The research team used advanced computer models and statistical methods to analyse how the Earth's surface responded to the continental breakup that kicked off hundreds of millions of years ago.
Before this, Earth’s continents were stuck together in a large landmass known as Pangaea. This splintered into Gondwanaland and Laurasia, which gradually split further into the contents we know today.
They found that when continents split, the stretching of the continental crust causes stirring movements in Earth's mantle, creating a "sweeping motion" that disturbs the continents' deep foundations.
“It’s like stretching a piece of toffee,” Gernon told BBC Science Focus. “You get this deformation in the middle where the crust gets thinner. This causes an upwelling of hot material from below.”
Gernon explained that as the hot material wells up, it hits the cold continent and cools, thus sinking again. Through this, a kind of convective cycle of swirling material is generated.
The instability caused by these convection cycles then begins to disturb nearby material, causing it to behave in a similar way.
This disturbance sets off a chain reaction: a "deep mantle wave" travels along the continent's base at a pace of 15-20 kilometres per million years. This sounds dreadfully slow, but it’s enough to begin removing layers of rock from the continental roots.
As a result, the continent rises through a process called isostasy, similar to how a hot-air balloon rises when it sheds weight.
But wait, there’s more. The uplift triggers a wave of surface erosion from weathering – which also takes many millions of years – that moves across the continent, removing huge amounts of rock from the surface (as opposed to from beneath as a result of the deep mantle wave). This causes further land surface rise as more weight is lost, forming elevated plateaus.
This process explains why parts of continents previously thought of as stable experience substantial uplift and erosion.
Gernon added, “That’s what’s really fascinating here. These regions are the ancestral hearts of the continents, which have survived for billions and billions of years.
“They’ve lived through the major events in Earth’s history but for some reason after the continents broke up they went through this major disturbance.”
Interestingly, the team's findings also link this continental reshaping process to the formation of diamonds.
“As you disturb the roots of the continent, ancient material which is heavy with all the ingredients you need to produce diamonds melts and shoots up,” Gernon said.
Yet this continent-shaping process is far more consequential than pretty gems appearing in the ground. As Gernon put it, “It’s hard to underestimate the importance of this process”.
While Africa is used as a model, the same processes occurred the world over – from South and North America to parts of Northern Europe, Antarctica and Greenland. Wherever you have landmass rising hundreds of metres, Gernon explained, ecosystems are fundamentally changed, forcing species to adapt and evolve in the process.
About our expert
Thomas Gernon obtained his B.Sc. (Hons) in geology from University College Dublin, and a Ph.D. in physical volcanology from the University of Bristol. After a period as a lecturer in geology at Trinity College Dublin (2008 to 2009), he joined the University of Southampton as a lecturer (Assistant Professor) in Earth Science and was promoted to associate professor in 2016. Gernon has led or co-authored over 60 peer-reviewed papers in international scientific journals and has delivered over 50 lectures at international conferences, workshops, seminars and public talks.
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