We May Finally Know Where Continents Come From
If you can overlook the insignificant lifeforms on its surface, the history of Earth is the slow expansion of its continents. Billions of years ago our planet was largely a water world with a few small islands poking above the surface. Then, as now, the floor of the oceans bore more resemblance to the composition of the other rocky planets than it does to continental rocks. The question of how continents came to cover so much of the planet has puzzled geologists, but a new theory gives the credit to mountain ranges.
The idea is counter-intuitive. After all, if there were no continents, where would the mountains sit? Nevertheless, Dr Ming Tang of Rice University said in a statement: “If our conclusions are correct, every piece of land that we are now sitting on got its start someplace like the Andes or Tibet, with very mountainous surfaces.”
Many of the world’s mountain ranges are eroded continental arcs, located at the margin where a continental plate rode over an oceanic one. Tang proposes these arcs are factories for the distinctive continental rocks.
Tang’s hypothesis, presented in Nature Communications, is based on the distribution of niobium and tantalum in rocks from around the world. Although the two metals’ names sound like they come from science fiction and fantasy books respectively, the elements are so chemically similar they usually turn up in the same places, leaving a very constant ratio.
However, the continental crust has 20 percent less niobium, proportionally, than the rest of the planet. The missing niobium has troubled geologists for decades.
Tang studied databases of the elemental composition in rocks, and came to the conclusion the continental average is being dragged down by arclogites, materials that collect at the base of continental arcs. Although often deeply buried, arclogites can be brought to the surface by volcanoes.
Tang had arclogites from the High Sierras tested and confirmed those made of rutile crystals are high in niobium. He thinks these crystals trapped a lot of niobium, but not tantalum, and most of them sunk into the mantle, leaving low niobium crust behind.
Rutile only forms under high pressures, such as is provided at the base of the crust with a mountain range above. While the niobium content provided the clue that allowed Tang to recognize what was happening, the more important aspect of the minerals formed by the continental arcs is that their low iron content makes them lighter, and therefore more buoyant than oceanic basalts.
Where the first continental arc came from is unclear, but once it appeared, it began a self-reproducing cycle. The arc produced continental rock, which rode over any oceanic plates it encountered, producing more continental arcs, and therefore more continents.