An international team of researchers has used sound waves to estimate the composition of the Earth’s upper mantle and they think that about a quadrillion tons of diamonds are buried somewhere over 160 kilometers (100 miles) deep beneath our feet.
The new research is reported in the journal Geochemistry, Geophysics, Geosystems and is based on decades of seismic activity data. Seismographers register the waves from all the movements within the Earth, such as earthquakes. They are mechanical waves moving at certain speeds depending on the material they go through.
Based on the data, the researchers have tried to reconstruct what the interior of the Earth is like. And that’s where they stumbled upon something interesting. There are certain areas in which the seismic waves tend to speed up without explanation. These regions are called cratonic roots and are the oldest and most immovable sections of continental tectonic plates. These are sometimes seen as upside-down mountains stretching as deep as 320 kilometers (200 miles).
To explain the seismic data, the team started playing with the possible composition of these roots. They arrived at the conclusion that if these structures contain between 1 and 2 percent diamond, then the speed of the seismic waves makes sense. This might not seem much in terms of the total amount of material in the crust and mantle, but it is 1,000 times more diamonds than expected.
"This shows that diamond is not perhaps this exotic mineral, but on the [geological] scale of things, it's relatively common," senior author Dr Ulrich Faul of MIT said in a statement. "We can't get at them, but still, there is much more diamond there than we have ever thought before."
"It's circumstantial evidence, but we've pieced it all together," Faul added. "We went through all the different possibilities, from every angle, and this is the only one that's left as a reasonable explanation."
Diamonds are formed under extremely high-temperature and high-pressure conditions found below Earth’s crust. The reason why they are rare is that they only get close to the surface in specific eruptions that carve out geological pipes made of a rock called kimberlite. A large concentration of them deep below the crust, then, is not too bizarre of a hypothesis.