Striations and grooves

Gerelateerde afbeelding

They were transported from Canada, and by reading the fabric of the till and taking bearings from striations and grooves in the underlying rock-and by noting the compass orientation of drumlin hills, which look like sculptured whales and face in the direction from which their maker came-anybody can plainly see that the direction from which the ice arrived in this region was something extremely close to 045°, northeast. At least one pipe containing gem diamonds must exist somewhere near a line between Indianapolis and the Otish Mountains of Quebec, because the ice that covered Indiana did not come from Kimberley-it formed and grew and, like an opening flower, spread out from the Otish Mountains. With rock it carried and on rock it traversed, it narrated its own journey, but it did not reveal where it got the diamonds. There is a layer in the mantle, averaging  flexplek huren tilburg about sixty miles below the earth’s surface, through which seismic tremors pass slowly. The softer the rock, the slower the tremor-so it is inferred that the lowvelocity zone, as it is called, is close to its melting point. In the otherwise rigid mantle, it is a level of lubricity upon which the plates of the earth can slide, interacting at their borders to produce the effects known as plate tectonics. The so-termed lithospheric plates, in other words, consist of crust and uppermost mantle and can be as much as ninety miles thick. Diamond pipes are believed to originate a good deal deeper than that-and in a manner which, as most geologists would put it, “is not well understood.” After drawing fuel from surrounding mantle rock-compressed water from mica, in all likelihood, and carbon dioxide from other minerals-the material is thought to work slowly upward into tl1e overlying plate. Slow it may be at the start, but a hundred and twenty miles later it comes out of the ground at Mach 2. The result is a modest crater, like a bullet hole between the eyes. No one has ever drilled a hundred and twenty miles into the earth, or is likely to. Diamond pipes, meanwhile, have brought up samples of what is there. It is spewed all over flexplek huren groningen the landscape, but it also remains stuck in the throat, like rich dense fruitcake. For the most part, it is peridotite, which is the lowest layer of the subcontinental package and is believed to be the essence of the mantle. There is high-pressure recrystallized basalt, full of garnets and jade. There are olivine crystals of incomparable size. The whole of it is known as kimberlite, the matrix rock of diamonds.

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