A Crack in the Edge of the World by Simon Winchester

its bulk from their picture windows, it is simply a piece of scenery, an eternal and unyielding part of the view. Very few people ever stop to wonder why this particular mountain is where it is; what forces caused the land to slope upward as it does. And yet those forces, their complicated workings all encapsulated in the geologic history of this one mountain, are part of the same set of forces that caused the destruction of San Francisco. To understand this, to understand Mount Diablo, is to begin to understand why California, perched precariously at the edge of the North American world, has been destined by its geology to be both so beautiful and so dangerous.
    Like most, I had given precious little thought to such things on that summer’s evening. As I drove up through the gloaming along the winding mountain roads, my interest in the hills, such as it was, related simply to my need to muster sufficient horsepower to overcome them. To me that night—and to countless others who were seeing the hillside every day, and who might have paused to wonder why it might be and what it might be— volcano was perhaps the explanation that most readily came to mind, especially in the American West, where such things are much more commonly seen. The shape of the mountain when it is viewed from afar—a low double cone, which from the Central Valley side seems to rise spectacularly alone out of the plains like an Etna, a Vesuvius, or a Mount St. Helens—rather reinforces the impression.
    But Mount Diablo is most certainly not a volcano. Few places inthe world are as geologically complicated as California, and few parts of California’s underside are as raggedly confused as Mount Diablo. And though the story that will unfold in later chapters has to do with geological structures and happenstances that involve San Francisco, Santa Rosa, San Jose, the San Andreas Fault, and a host of other faults besides that are some distance away from this rather peaceful-looking mountaintop, the saga of why Diablo is where it is and what it is has in fact great relevance to the geology and the geological processes that once destroyed San Francisco and that may yet destroy it again.
    THE GEOLOGY OF the northern half of California—whether we are talking about San Francisco Bay or the Central Valley, the Coast Range or the Sierra, the Monterey headlands or the coast of Humboldt County, or Mount Diablo itself—is all interlinked, subtly, confusingly, and, for the geological mapmakers, often maddeningly. These links go far beyond the borders of the state—political lines that pay no heed, in this case, to the absolutes of geology. * They spread far, far beyond—as we shall discover, they reach up to Alaska, they percolate across to Wyoming and Montana, they reach back west across two oceans as far, in fact, as India and Australia. One might say, indeed, that the story of what makes California so complex and so interesting and so dangerous—and what makes Diablo so similarly geologically alluring—has implications for, and connections to, the planet in its entirety.
    About 170 million years ago—in the early to middle part of the Jurassic, when dinosaurs were the dominant large land creatures in other parts of the world—the floor of what we now call the Pacific Ocean began to spread outward, eastward and westward, from a centralsuture line. The section of the floor that moved east did so with such force and speed that about 50 million years later—120 million years ago—it collided, gently but powerfully, with the cliffs and mountains at the western edge of what we now call North America. When that almighty crash occurred, two things appear to have taken place.
    First, a sliver of the ocean floor, which happened to be made of rocks that were somewhat heavier than those of the cliffs and hills of North America (thus causing it to lie so low that the ocean was able to accumulate above it, while the