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Diamonds
I believe Diamonds are only formed where molten rock is present (maybe wrong). can volcanic eruption form diamonds, or does it have to be depth below the earth's surface, ie pressure form above?
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Although the magma (molten rock) in and of itself does not form the diamonds, they are carried to the earth's surface by formations known, geologically, as kimberlite pipes. These pipes are the remnants of deep volcanic eruptions, as you have surmised. They are usually quite narrow and quite deep. All diamonds are extremely old, approaching 2 to 3 billions of years. If the diamond was not transported to the earth's surface extremely rapidly, it would be nothing more than pencil lead (graphite)...
Just to add to Clanad's answer - (sorry folks, here come the geologists!! - gen2, Mr. Pahoehoe, you out there too?)
At around 1000 - 1200�C, the stable form of carbon is graphite. Diamond will only form at this sort of temperature if the pressure is over c. 45 kilobars (the pressure in the deepest oceanic crust is about 1 kilobar (kb), under the Antarctic ice-sheet (about 5km of ice) is about 0.5 kb.) This equates to something like at least 150km below the Earth.
This means that diamond can't form in volcanic eruptions, but can be brought up (rapidly) from within the mantle by upwellings of magma - classically in the form of kimberlite pipes as mentioned above.
Incidentally, the mantle is only about 15% molten , and it's the more liquid parts that are generally brought up in the type of volcano that erupts molten rock - (the mantle is a bit like a 'slush puppy' - a mixture of crushed ice and water, both solid and liquid ) - though occasionally lumps of the 'solid stuff' can be brought up aswell.
And finally, at lower pressures (ie. at the surface), diamond is unstable and reverts to graphite (in theory anyway) - luckily, it requires a lot of energy to start this process off, (called the activation energy), so it doesn't actually happen - so diamond is referred to as meta-stable.
So remember, as my old chemistry teacher used to say. "a diamond is a girl's meta-stable best friend."
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