Plateaus around the world are in mysterious locations, and it could be due to waves beneath the continents

Earth’s topography is a result of what happens beneath the surface. In places like South Africa, Brazil and India, plateaus are in locations where plateau formation would be unusual, until now.

Plateaus
Plateaus across the world have interesting beginnings, and a new theory could explain.

The theory of plate tectonics was introduced in the early 1900s from ideas of meteorologist Alfred Wegener. His hypothesis of rock masses the size of continents drifting around the globe once seemed foolish, but geologists know today that continents float on hot rocks in the mantle. Now, researchers are proposing an addition to the plate tectonics theory.

An amendment

The theory of plate tectonics might be getting an upgrade, according to researchers. These researchers say the ancient breakup of tectonic plates makes churning waves in the mantle that tumble in slow motion under continents over tens of millions of years, sculpting the topography in continental interiors. Researchers are currently getting better reception than Wegener did.

The interior of continents, called cratons, are stable bodies of rock far from tectonic plates and are responsible for Earth’s topography. Some cratons have tall plateaus with interesting origins. The Brazilian highlands northwest of Rio de Janeiro, the Western Ghats of southern India and the central plateau of South Africa are all arranged in a “suspiciously systematic” way, according to Tom Gernon, geologist at University of Southampton. This means the plateaus begin at the coasts and as you move inland, they appear to gradually get younger.

Gernon proposed that kimberlite, diamond-containing volcanic deposits, eruptions are triggered by “long, slow mantle waves rippling inward from continental edges”. Gernon wondered if this could also apply to plateaus.

The making of the hypothesis

The study used a model that mimics movements in Earth’s mantle after a continental breakup. When hot mantle rock rises into the gap between the diverging tectonic plates, like a lava lamp, the rock cools. The rock then becomes dense and sinks in a circular matter, called convection.

The 2D slice of Earth for this model is wider than other simulations. The convection propagated laterally outwards on each side of the rift, similar to a wave. These waves then rippled across underbellies of continents at a rate of only 15 to 20 kilometers every million years.

The waves roll past continents, scraping off rocks to include them. This leaves behind a lighter region of continent which then rises up to create plateaus 1-2 kilometers high. Wind and water begin to erode the rock, lessening the load and allowing it to rise another half kilometer, similar to how a hot air balloon drops its sandbags and rises up. Gernon says the topographic uplifts in Brazil, India and South Africa are “a ghost of a long past rifting episode”.

Data reveal

Gernon and the research team analyzed geological data that covered 47 highland sites in South Africa and 20 in Brazil. These regions separated when the supercontinent Gondwana rifted apart 140 million years ago. The data showed the temperature history of rocks once buried at each site and when they were lifted and cooled.

The moment of maximum cooling at each site, when the highland part was being lifted up the fastest, tracked the modeled progression of the mantle waves as they migrated into the continent. In South Africa, uplift and cooling was most recent in Lesotho, which has many dramatic land features.

The theory explains how continental interiors can be dynamic, says Claire Curie, a geophysicist at University of Alberta. This is all happening below the surface. The theory could also do away with attributing plateaus to plumes, which are columns of mantle rock rising from deep within the Earth.

Mantle plumes are a viable theory for explaining uplift under the Colorado Plateau in the US and the East Africa Rift zone. Convective mantle waves are a better explanation for continental highlands in Gernon’s study in Brazil, India and South Africa. It is not likely for there to be plumes under the plateaus regularly.

What this brings to the table

This new theory could also explain more than just plateaus and diamond eruptions. Gernon is soon to publish a paper examining how wave-driven uplift could help explain biogeochemical cycles in geologic past, with increased erosion bringing sediments into the ocean that consumed oxygen, leading to marine extinctions. The waves could also influence timing and location of major climatic events, like ice ages, by increasing the exposure of rocks that absorb carbon dioxide. Ice sheets form and accumulate on cold and uplifted jagged surfaces.

Waves could help explain earthquakes that occur in continental interiors that are assumed to be stable, like in West Africa. Physical barriers created by uplift might cause flora and fauna to speciate, says Folarin Kolawole, a structural geologist at Columbia University.

Researchers can’t say for sure that each episode of continental rifting generates the Earth-shaping ripples. This study only used data from the breakup of Gondwana. The researchers are hopeful this theory will be tested by others using different land masses and points throughout time.