Named for Kimberley, South Africa where they were first found, kimberlite pipes are large, conical rock formations known to host diamonds.
Diamonds, as investors already know, are formed nearly 100 miles beneath the earth’s surface in the rock of the mantle. At this depth in the Earth’s interior, carbon is subjected to immense pressures which are necessary to transform carbon into diamond.
Of course, diamond mining does not involve expeditions to the centre of the Earth – and that is where kimberlite pipes come in.
How are kimberlite pipes formed?
Kimberlite pipes are formed when bouyant magmas formed deep within the Earth ascend along paths of weakness towards Earth’s surface. Along the way the kimberlite magma breaks off pieces of the rocks it passes through. If it happens to go through a diamond bearing rock the diamonds will be entrained and brought up to the surface as “passengers” in the kimberlite. Kimberlite magmas are rich in volatiles (a fancy word for gases) which are in liquid form. As the magmas ascend closer to surface the pressure decreases (due to less overlying rock) and the gases come out of solution – changing from liquid to gas. This results in an explosion which actually forms a crater, jettisoning out the country rock. The crater is then filled in with kimberlite magma ascending from below and mixed kimberlite and country rock eroding in from above. The kimberlite pipes range in size from over 200 hectares to not much larger than the chair you are sitting in.
One rule of thumb in diamond exploration is that the best chance of finding an economic diamond mine is near the centre of an archean craton. An archean craton is an ancient (over 2.6 billion years old) mass of rocks which often form parts of our present day continents. This is due to these areas of the crust being extremely thick (over the 150 kilometers needed to form diamond) and relatively cool (as if the temperatures are too high carbon is stable as graphite and not diamond) due to the rocks having had such a long time to cool since formation.