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Raising the Rockies

I hereby recommend the new book Assembling California by John McPhee, to every Alaskan with any interest in geology. I do this partly because McPhee is a fine writer and because unraveling the geologically messy puzzle of California makes an entertaining story. The real reason, however, is that McPhee supports northern chauvinism; he credits Alaska with raising the Rocky Mountains.

The idea would have been considered outrageous a few decades ago. To make any sense at all, the suggestion has to be based in the theory of plate tectonics, and that theory became respectable only toward the end of the 1960s. Prior to that, the image of great segments of the earth's crust skidding about on the underlying mantle, like so many bumper cars on a well-waxed floor, ran counter to geologists' reasoning. New technologies changed their perception and gave new reasons for why the earth appears as it does. Plate tectonics explains a lot, from what causes most great earthquakes to why identical kinds of fossils can be found on lands separated by an ocean's breadth.

In his story, McPhee incorporates a kind of primer on basic plate tectonics. New crustal plates form at spreading centers, such as the ridge running the length of the Atlantic Ocean. The crust moves away from this source, bearing whatever continental bits it may carry along its travels. For example, Kodiak has docked at Alaska only recently (geologically speaking) after a long sail northwestward on the shoulders of the Pacific plate. Plates are born, but they also die. They dive into trenches at continental edges, sinking down again into the mantle from which they rose. The islands and other landmasses they carry are usually scraped off in the process---think of Kodiak again---though sometimes they seem to be partially swallowed into a trench, jamming it. Then a new trench opens farther offshore, and the abandoned island becomes a new part of the mainland. In a nutshell, that is the basic geological history of California, as new landmass after landmass has come sailing in, stuck fast to the land already there, and slowly built the coastline from the vicinity of Salt Lake City westward to Santa Barbara.

Now, envision what would happen if a really large block of dry land was borne by its plate into a collision with an even more massive block. This kind of slow-motion impact is under way right now. India was once part of a great southern continent, but hundreds of millions of years ago it was sundered from Africa and skidded northward. Once it was going at a brisk clip (speaking in geologically appropriate terms) of up to eight centimeters a year. As it grinds into and under the belly of Asia, India has slowed way down. It's now moving north at about two centimeters a year.

In the process, the Indian impact is lifting the Himalayas. The entire Tibetan plateau is a by-product of continental collision. (Think of it as the sprung hood on a pickup truck that's smacked into an eighteen-wheeler.)

Having come that far with the story, McPhee pauses to puzzle over what could have hit North America to lift the Rocky Mountains, He suggests that a land mass far to the north slipped along the fault system lining North America's western shore. Alaska, he says, though itself an array of cemented-together continental bits and island arcs, once smacked into this continent with enough force to lift great mountains high.

So, in effect, the Rockies are the crumpled fenders left as memorials to the exuberant arrival of the Alaskan bumper car at North America. Do you suppose this ancient encounter still echoes in the driving styles of people in both states?