How supercontinent ruptures leave behind geological orphans

Why pieces of continents keep appearing in the middle of the oceans

It turns out that continental ruptures are just as complicated as human ruptures, with events leaving fragments scattered far from home.

By Evan Howell edited by Andrea Thompson

Across the globe, from the Red Sea to the deep ocean ridges of the Atlantic, more than a dozen geological misfits lurk. These scraps of continental crust are found in the middle of the oceans, sometimes hundreds of kilometers from the nearest continent. Scientists have been perplexed for decades as to how they got there; the fragments “have even been used as an argument against plate tectonics,” says João Duarte, a geologist at the University of Lisbon in Portugal.

But a recent study in Natural geosciences suggests that these misplaced fragments fit perfectly with our understanding of plate tectonics and actually date back to the chaotic early moments of the breakup of ancient supercontinents. As a continent begins to open up – as is currently the case in the Red Sea –narrow fault zones can isolate small pieces of continental crustabandoning them on a raft of newly formed oceanic crust.

When continents break apart, they form new plate boundaries at what are called mid-ocean spreading centers: gurgling conveyor belts of magma that create new ocean crust and separate continents. The black expanse of thin, dense, and relatively young basalt in these centers contrasts sharply with the thicker, more buoyant continental crust, which is primarily granite but contains the full range of sedimentary, metamorphic, and igneous rocks and may be billions of years old. So when scientists discovered shards of much older continental crust surrounded by younger oceanic material, something went wrong.

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Examining the displaced pieces of continental crust, the researchers eventually noticed a trend: Continental debris appeared at transform faults, where mid-ocean ridges bend at right angles and blocks of crust slide past each other perpendicular to the ridges. To understand what was happening, Attila Balázs, a geophysicist at the Swiss Federal Institute of Technology in Zurich, and his colleagues used high-resolution three-dimensional computer models to reconstruct the continents.

When several land masses collided to form the supercontinent Pangaea long before the dinosaurs, the Earth’s crust broke into discrete blocks and folded like a carpet pushed against a wall, rising to form mountains stretching from the Appalachians to the Atlas Mountains. “It’s a bit like breaking a plate or dropping a glass. There will be many fractured and weak areas,” says Balázs. When the tectonic plates began to break apart millions of years later, these ancient faults at their edges awoke, reincarnating themselves as transform faults.

The continental wrecks that had baffled geologists formed under specific conditions as the continents separated: the first requirement was that the continents slowly split at an oblique angle to each other. As the continents sheared and twisted unevenly, local forces squeezed together thin ribbons of crust and popped them up like geological meerkats, isolating and cutting them apart. Eventually, some magma emerged in the process, but not enough to melt the shards. Once these rare conditions were met, pieces of continental crust moved along these rejuvenated faults and drifted into new ocean basins. According to Balázs, the process takes up to 30 million years to unfold.

Susanne Buiter, a geophysicist at the GFZ Helmholtz Geoscience Center in Germany, who was not involved in the study, says the new three-dimensional model helps solve a long-standing mystery. The classic theory of plate tectonics assumed that continents separated cleanly, but “maybe the break isn’t always that clean,” she says.