Did everywhere start out like a mountain?
Two rare metals give us a clue, researchers suggest. Samantha Page reports.
Every place on Earth once started as part of a mountain, scientists at Rice University in the US are suggesting.
Due to the unique ratio on the Earth’s crust of two rare elements, niobium and tantalum, Ming Tang and his colleagues theorise that the Earth’s crust was mostly formed in continental arcs, as mountains were pushed up by continental collisions.
“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,” says Tang, lead author of a study published in Nature Communications.
“Today, most places are flat because that is the stable stage of the continental crust. But what we found was that when the crust formed, it had to start out with mountain-building processes.”
Niobium and tantalum are two rare metals that scientists think of as twins, because they occur together, usually in a one-to-one ratio. That’s not the case for rocks in the continental crust, though, which have less niobium than expected. This imbalance suggests to Tang and his colleagues that the rocks were formed in a unique process.
"On average, the rocks in continental crust have about 20% less niobium than they should compared to the rock we see everywhere else," Tang says. "We believe this missing niobium is tied to the mystery of continents. By solving or finding the missing the niobium, we can get important information about how continents form."
Tang says arclogites – volcanic rocks – captured the imbalance of niobium. Under very high pressure, rutile, another mineral, traps niobium and tantalum. The two rare minerals are trapped at an even ratio at temperatures above 1000 degrees Celsius, but below that, rutile begins to “prefer” niobium.
Those specific and rare conditions are found underneath continental arcs, like the Andes, Tang says. That means the arclogites could be responsible for taking up the “missing” niobium in the Earth’s crust – but that means the Earth’s crust was formed in the same volcanic process.
"Continental arcs are like a magic system that links everything together, from climate and oxygen concentrations in the atmosphere to ore deposits," says co-auther Cin-Ty Lee. "They're a sink for carbon dioxide after they die. They can drive greenhouse or icehouse, and they are the building blocks of continents."