| Popis: |
One of the few windows through the earth’s relatively thin crust into its underlying mantle is the diatreme or pipe that occasionally has pierced the crust to bring mantle material such as diamond to the surface (Fig. 9–1). Experimental work has shown that most synthetic diamonds generally must have very high temperatures (2,000°C) and pressures (70,000 kg/cm2) to form. In nature these conditions exist only at depths of at least 200 km below the earth’s surface. Material formed at these depths comes to the surface in pipes, which may be thought of as a very special type of volcano that taps depths as great as 350 km below the earth’s surface. The volcanic rock type found in these pipes is called kimberlite, named for its type area of Kimberley, South Africa. Since its discovery in 1871, the Kimberley area has yielded more than 200 million carats of diamond and a wealth of geologic information about kimberlite pipes and the earth’s upper mantle. |
