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We land-dwellers tend to take for granted the continents upon which we live out our days and nights. But Earth’s continents are in fact a planetary oddity. No other planet in our solar system—or outside of it, as far as we know—hosts large land masses like Africa or Australia, separated from one another by vast oceans.

Although other rocky planets and moons in our solar system have topographic highs and lows, the continents on Earth are not simply higher in elevation than the ocean basins; they are made of something entirely different: granite, a low-density rock rich in silicon, sodium, potassium, and uranium—elements found in much lower concentrations in the mantle, the ultimate source of all crustal rocks.

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But how exactly these granite continents formed—and survived—has remained a longstanding mystery.

Certainly they did not form all at once. The early Earth is thought to have been a water world with only basaltic crust and no emergent land. The earliest foundations of the continents, formed 4 billion years ago, would have erupted from submarine volcanoes and never risen above sea level. But distilling these magmas, rich in rare elements, would require many cycles of melting and refining. 

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Now a new paper in Nature suggests it was only after these land masses rose out of the sea, and not before, that they became the strong and durable continents we know today. The authors propose that weathering from rain and wind concentrated radioactive elements in the emergent rock, producing sufficient heat to transform it into granite. The counter-intuitive finding tells us that the atmosphere and hydrosphere shaped the Earth’s continents and provides insight into how habitable Earth-like planets might evolve elsewhere in the universe.

In daring to rise above the ocean waves, the continents assured their long-term survival.

As silica-rich magmas melted out of the mantle, these would have accumulated into thicker bodies, buoyant proto-continents that began to stand higher than the global ocean, argue co-authors Jesse Reimink and Andrew Smye, both professors of geoscience at Penn State University. One might imagine that exposure to the destructive forces of weathering and erosion would work against continental growth—that when the continents came up for air, they would have sown the seeds of their own destruction.

But, instead, Reimink and Smye suggest that exposure-related breakdown of rock was in fact the key to a burst of granite production that solidified “cratons” into the durable cores of the modern continents. These cratons have been compared to the keel of a boat—they keep the continents floating at or near sea level across vast timescales.

The surprising insight grew out of a “thought experiment,” according to Reimink and Smye. The two professors, who share an interest in the long-term evolution of the solid Earth, were musing about how the emergence of continents and onset of erosion might have changed the Earth system.  “We asked ourselves: How would the appearance of sediments—the products of erosion—have altered the way the world works?” said Reimink.

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While erosion flattens topography, it tends to redistribute, rather than remove, mass from the continents. Most sediments accumulate either on the submerged flanks of land masses—the continental shelves—or in low-lying inland basins. As sediments move from their rocky sources to ultimate sites of deposition—a process that is mostly driven by rivers—they interact with water and atmospheric carbon dioxide, which changes their chemistry.  Easily dissolved elements like sodium are carried away in solution, leaving a residue of quartz and clay minerals, which are then deposited as sandstones and shales—the latter also known, less elegantly, as mudstones.  

Although they don’t tend to form the dramatic landscapes that sandstones often do—think of the rock arches and slot canyons of the American Southwest—mudstones are more abundant, representing about 70 percent of all sedimentary rocks by volume.  Mudstones also tend to contain high concentrations of insoluble trace elements, most notably uranium and thorium, both of which are radioactive and therefore generate heat.  Reimink and Smye wondered how the “invention” of large amounts of heat-producing shale or mudstone via weathering might have altered the crust. 

Earth’s primordial inventory of radioactive elements has been decaying since its formation. Three billion years ago, Earth harbored significantly more uranium and thorium, and so any given volume of mudstone would have been even hotter than it is today.  Reimich and Smye realized that their thought experiment pointed to a previously unrecognized source of heat for a worldwide episode of crustal melting that occurred between about 3 and 2.5 billion years ago called the “Neoarchean granite bloom.” During this time, large volumes of granites were thrust into the nascent continental masses, “gluing” them together into sturdy cratons.

Previous hypotheses for the cause of the granite “bloom” had invoked heat rising from the mantle, but even the young continents would have been thick enough to insulate themselves against heat emanating from below. Reimink and Smye suggest, instead, that “hot” mudstones created by weathering became buried or tectonically folded into the early continental crust, and then heated it from within.

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The authors present the results of computer models showing that for plausible volumes of shale and higher amounts of uranium and thorium in Archean time, radioactive decay could have heated existing crustal rocks—both granites and sediments—to their melting temperatures, producing large volumes of new granite and solidifying the cratons. That is, the shales were hot enough to “cook” themselves and the surrounding rocks to the point of melting, and those magmas, long since cooled, form the nearly indestructible crust beneath our feet.   

In other words, we may owe our land-dwelling existence to lowly mud.

Reimink and Smye’s work suggests that in daring to rise above the ocean waves, the continents assured their long-term survival by giving up heat-producing sediments that then acted to forge them into something stronger. This new study contributes to a growing recognition of unexpected connections between the interior and exterior of this planet—how tectonic processes within the Earth and hydrologic processes on its surface collaborate to maintain habitable conditions over billion-year timescales.

And it’s a reminder not to take granite—or mud—for granted.

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Lead image: Bisams / Shutterstock

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