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Push up some mountains. Cut them down. Drown the land under the sea. Push up some more mountains. Cut them down. Push up a third set of mountains, and let the river cut through them. “Unconformity” is the geologic term for an old, eroded land surface buried under younger rock layers. Put your outspread hand over the Carlin Canyon, Nevada unconformity and your fingers span roughly forty million years- the time that it took to bevel down the first set of mountains and deposit the younger layers on top. What is forty million years? Enough time for a small predatory dinosaur to evolve into a bird. Enough time for a four-legged, deer-like mammal to evolve into a whale. And far more than enough time to turn an ape-like creature in eastern Africa into a big-brained biped who can marvel at such things. The Grand Canyon’s Great Unconformity divides 1.7 billion-year-old rock from 550 million-year-old rock, a gap of more than one billion years. One billion years. I earn my salary studying the Earth and teaching its history, but I admit utter helplessness in comprehending such a span. A billion pages like those of this book would stack up more than forty miles. I had lived one bullion seconds a few days before my thirty-second birthday. A tape measure one billion inches long would stretch two-thirds of the way around the Earth. Such analogies hint at what deep time means- but they don’t get us there. “The human mind may not have evolved enough to be able to comprehend deep time, " John McPhee once observed, “it may only be able to measure it. .Keith Meldahl
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Earth processes that seem trivially slow in human time can accomplish stunning work in geologic time. Let the Colorado River erode its bed by 1/100th of an inch each year (about the thickness of one of your fingernails.) Multiply it by six million years, and you’ve carved the Grand Canyon. Take the creeping pace of which the continents move (about two inches per year on average, or roughly as fast as your fingernails grow). Stretch that over thirty million years, and a continent will travel nearly 1, 000 miles. Stretch that over a few billions years, and continents will have time to wander from the tropics to the poles and back, crunching together to assemble super-continents, break apart into new configurations- and do all of that again several times over. Deep time, it could be said, is Nature’s way of giving the Earth room for its history. The recognition of deep time might be geology’s paramount contribution to human knowledge. .Keith Meldahl
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Each scenario is about fifteen million years into the future, and each assumes that the Pacific Plate will continue to move northwest at about 2.0 inches per year relative to the interior of North America.In scenario 1, the San Andreas fault is the sole locus of motion. Baja California and coastal California shear away from the rest of the continent to form a long, skinny island. A short ferry ride across the San Andreas Strait connects LA to San Francisco.In scenario 2, all of California west of the Sierra Nevada, together with Baja California, shears away to the northwest. The Gulf of California becomes the Reno Sea, which divides California from Nevada. The scene is reminiscent of how the Arabian Peninsula split from Africa to open the Red Sea some 5 million years ago. In scenario 3, central Nevada splits open through the middle of the Basin and Range province. The widening Gulf of Nevada divides the continent form a large island composed of Washington, Oregon, California, Baja California, and western Nevada. The scene is akin to Madagascar’s origin when it split form eastern Africa to open the Mozambique Channel.Keith Meldahl