Crustal spreading

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“This is a direct result of the crustal spreading,” Deffeyes says. “It brings hot mantle up near tl1e surface. There is probably a fracture here, through which the water is coming up to this row of springs. The water is rich in dissolved minerals. Hot springs like these are the source of vein-type ore deposits. It’s the same story that I told you about the hydrothermal transport of gold. When rainwater gets down into hot rock, it brings up what it happens to find there-silver, tungsten, copper, gold. An ore-deposit map and a hot-springs map zakelijke energie vergelijken will look much the same. Seismic waves move slo lwly through hot rock. The hotter the rock, the slower the waves. Nowhere in the continental United States do seismic waves move more slowly than they do beneath the Basin and Range. So we’re not woofing when we say there’s hot mantle down there. We’ve meas1ured the heat.” The basin-range fault blocks in a sense are floating on the mantle. In fact, the earth’s crust everywhere in a sense is floating on the mantle. Add weight to the crust and it rides deeper, remove cargo and it rides higher, exactly like a vessel at a pier. Slowly disassemble I the Rocky Mountains and carry the material in small fragments to the Mississippi Delta. The delta builds down. It presses ever deeper on the mantle. Its depth at the moment exceeds twenty-five thousand feet. The heat and the pressure are so great down there zakelijke energie that the silt is turning into siltstone, the sand into sandstone, the mud into shale. For another example, the last Pleistocene ice sheet loaded two miles of ice onto Scotland, and that dunked Scotland in the mantle. After the ice melted, Scotland came up again, lifting its beaches high into the air. Isostatic adjustment. Let go a block of wood that you hold underwater and it adjusts itself to the surface isostatically. A frog sits on the wood. It goes down. He vomits. It goes up a little. He jumps. It adjusts. Wherever landscape is eroded away, what remains will rise in adjustment. Older rock is lifted to view. When, for whatever reason, crust becomes thicker, it adjusts downward. All of this-with the central image of the basin-range fault blocks floating in the mantle-may suggest that the mantle is molten, which it is not.

The road builders

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Geologists on the whole are inconsistent drivers. When a roadcut presents itself, they tend to lurch and weave. To them, the roadcut is a portal, a fragment of a regional story, a proscenium arch that leads their imaginations into the earth and through the surrounding terrane. In the rock itself are the essential clues to the scenes in which the rock began to form-a lake in Wyoming, about as large as Huron; a shallow ocean reaching westward from Washington Crossing; big rivers that rose in Nevada and fell through California to zakelijke energie the sea. Unfortunately, highway departments tend to obscure such scenes. They scatter seed wherever they think it will grow. They “hair everything over” -as geologists around the country will typically complain. “We think rocks are beautiful. Highway departments think rocks are obscene.” “In the North it’s vetch.” “In the South it’s the god-damned kudzu. You need a howitzer to blast through it. It covers the mountainsides, too.” “Almost all our stops on field trips are at roadcuts. In areas where structure is not well exposed, roadcuts are essential to do geology.” “Without some roadcuts, all you could do is drill a hole, or find natural streamcuts, which are few and far between.”
“We as geologists are fortunate to live in a period of great road building.” “It’s a way of sampling fresh rock. The road builders slice through indiscriminately, and no little rocks, no softer units are allowed to hide.” “A roadcut is to a geologist as a stethoscope is to a doctor.” “An X-ray to a dentist.” “The Rosetta Stone to an Egyptologist.” “A twenty-dollar bill to a hungry man.” “If I’m going to drive safely, I can’t do geology.” In moist climates, where vegetation veils the earth, streamcuts are about the only natural places where geologists can see exposures of rock, and geologists have walked hundreds of thousands of miles in and beside streams. If roadcuts in the moist world zakelijke energie vergelijken are a kind of gift, they are equally so in other places. Rocks are not easy to read where natural outcrops are so deeply weathered that a hammer will virtually sink out of sight-for example, in piedmont Georgia. Make a fresh roadcut almost anywhere at all and geologists will close in swiftly, like missionaries racing anthropologists to a tribe just discovered up the Xingu.