|Lake Crescent in Olympic National Park at the north end of Highway 101.|
|Route of Highway 101 through California, Oregon and Washington (red line). Interstate 5 runs parallel to Highway 101 just inland.|
The latest tempest involves a New Yorker article on the coming magnitude 9 earthquake along the Cascadia Subduction Zone. The basics of this potential earthquake have been known for nearly half a century, ever since the theory of plate tectonics came to be accepted by geologists. The details of a massive earthquake that took place in 1700 came to be known more than a decade ago. But it seems there is sometimes a tipping point when the public becomes aware of such hazards, and the New Yorker article seems to have done the trick. People in the northwest are talking earthquakes and tsunamis. The Cascades volcanoes must be feeling very neglected right now; they were once the main topic of any discussion of geologic hazards in the region.
|The Juan de Fuca and Gorda crustal plates, among the world's smallest. The Cascadia Subduction Zone is the black line with the converging arrows. Many of the most important Cascade volcanoes are shown as red triangles.Source: NASA|
Interstate 5 is a freeway from Mexico to Canada. It's a fast and efficient way to get from one place to another (except for the traffic jams in Seattle and Portland). But it's also not very interesting over much of its length. Highway 101 is more a patchwork of freeways, highways, and two-lane roads, and because of the challenges of the geology it is a route to be followed in a more leisurely manner. It is far more scenic, and for that manner, more dangerous in the geologic sense. It's never a surprise to see signs referring to falling rocks and tsunami evacuation routes.
|Side view of the Cascadia Subduction Zone (source: U.S. Geological Survey)|
The source of all the geological mayhem is one of the smallest crustal plates in the world. The Juan de Fuca plate forms a few hundred miles out in the Pacific Ocean where new basaltic lavas emerge from an oceanic divergent boundary. The plate moves east at a rate of few inches per year. It then dives beneath the North American Continent, where the buildup of frictional stress results in earthquakes, both big and small. Eventually the plate begins to melt, releasing plutons of molten rock that sometimes leak out at the surface, forming the Cascades volcanoes like Rainier, St. Helens, Crater Lake and Shasta.