Basin and Range
The Earth Generated and Anatomized
I read Basin and Range, John McPhee’s celebration of geology, during a long plane ride from Portland to Boston. I am not a morning person, but I managed to reach the airport long before the flight (and almost before dawn) to secure a “geologist’s seat”—by the window, before the wing and on the left side—all for a view of Mt. St. Helens. I was not disappointed.
Volcanoes, with their symmetry and gently parabolic slopes, are perhaps the most stunning objects of a world dominated by irregular topography. If volcanoes tower above the local landscape—as they do in the northwestern United States—the effect is even more striking. When I flew by in early March, the snow had melted at lower elevations, leaving all the surrounding hills brown. But three gleaming white volcanic peaks stood out above them—Mt. St. Helens, with its cloud of steam still rising a thousand feet into the air, Mt. Adams beside it, and the granddaddy of the Cascade volcanoes, Mt. Rainier, behind. It was the most awesome sight I have ever seen from an airplane window.
We have been poor at assimilating the great lessons that geology teaches—the earth’s ceaseless motion and immensity of time (or “deep time” as McPhee calls it), I well remember the catechism I learned in grade school: Mt. Lassen, which erupted in 1914, is the only active volcano in the United States (Alaska and Hawaii were still colonial possessions at the time). Written history recorded no eruptions, so we declared that the internal fires had been quenched irrevocably. Lassen is but one peak in the Cascade chain; St. Helens is another. Every one of those volcanoes is potentially active—Shasta, Hood, Rainier, all of them. Mt. Rainier may bury Seattle before another earthquake levels San Francisco.
Geology presents its irreducible beauty in raw appearances—and who would gainsay it. But another, perhaps deeper, beauty lies in understanding. The Cascade volcanoes extend for hundreds of miles in a linear belt. Why are they so aligned? And why do they stop in northern California? Why do volcanoes tend to come in linear arrays anyway? Until the theory of “plate tectonics” revolutionized geology by constructing a new earth, these questions had no adequate answers. But now we recognize that dense oceanic rock is pushed downward (subducted) beneath lighter continental rock when two “plates”—the large, thin “wafers” that form the earth’s upper layer—push into each other, one with oceanic rock and the other with continental rock at its margin. As oceanic rock slides into the earth below a continent, friction induces partial melting of the sinking plate and, perhaps, some of the surrounding mantle rock as well. This molten material may rise to form a chain of volcanoes on the earth’s surface right above the sinking edge of the oceanic plate.
California struck (and either consumed or removed) a ridge on the ocean floor many millions of years ago. Hence, from San Francisco …
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