Another fusty old tale undone by plate tectonics:
“Plate tectonics is the great unifying theory of geology, which makes it all the more amazing that it has only been accepted for about 50 years. If you think we’ve got it all figured out by now, a paper published this week in Science may surprise you. And you'd be wrong if you were expecting to read about some dusty rock cores. The new information comes from a much shinier source: diamonds.
Contrary to popular culture, diamonds are not formed from the metamorphosis of coal under tremendous heat and pressure. It makes for nice poetry, but it’s not true. The real story is actually a bit more interesting than that.
Diamonds form in the upper mantle, about 125 to 175 km below the surface, where they form along the undersides (or “keels”) of continents. After they crystallize, they commonly spend more than a billion years before coming to the surface in spectacular fashion. Author Bill Bryson describes that process best in A Short History of Nearly Everything: "What happens is that deep in the Earth there is an explosion that fires, in effect, a cannonball of magma to the surface at supersonic speeds." Poetry in motion, if you will. ”