Science Puzzle
How Do Diamonds Reach the Surface?
Diamonds are made of carbon, squeezed into crystals by huge pressure and heat about 150 to 200 kilometres underground. The deepest mine ever dug goes down about 4 kilometres. Most diamonds are between 1 and 3.5 billion years old.
There's a catch. At the surface, diamond isn't carbon's comfortable form; heated without that enormous pressure, it turns into soft grey graphite, the stuff in pencils. So how do diamonds get to the surface, still as diamonds?
The Answer
They're blasted up in rare, extremely fast volcanic eruptions of a magma called kimberlite, which rises from deep inside the Earth and carries the diamonds to the surface before they have time to change.
Kimberlite magma starts at least as deep as the diamonds, and it's rich in gas and very runny. As it forces its way upwards it rips off chunks of the rock it passes through, diamonds included, and races to the surface at speeds often estimated at tens of kilometres an hour. The whole journey may take hours to days. Slow rising magma would give the diamonds time to turn into graphite or dissolve on the way; a fast trip cools them quickly while they're still diamonds.
The eruptions leave behind carrot-shaped pipes of kimberlite, narrowing as they go down, and most diamond mines dig into these pipes. Kimberlite eruptions are so rare that nobody has ever seen one, and most of the pipes are tens to hundreds of millions of years old.
A diamond in a ring is, in a sense, still trying to become graphite. At room temperature that change is so slow that it will never happen in practice, so the ring is safe.
The principle: Rapid ascent preserves deep minerals. Kimberlite eruptions carry diamonds up from deep in the mantle so quickly that they cool before they can turn into graphite.