Tuesday, July 21, 2015

Vagabonding on Dangerous Ground: Following the Cascadia Subduction Zone on Highway 101

Lake Crescent in Olympic National Park at the north end of Highway 101.
I'm sure that NO ONE in the various forms of mass media WOULD EVER stoop to using HYPERBOLE to increase their readership/viewership. You know, saying things like "THE BIG ONE IS COMING", that the GIGANTIC TSUNAMI is going to DESTROY EVERYTHING WEST OF I-5, that EVERYONE WILL DIE in the BIG EARTHQUAKE (or at some point decades afterward of old age). I would, for instance NEVER use capital letters in my opening paragraphs to try and catch the reader's eye. That being said, I really did have half of my trip completed before the media storm began concerning the chances for a magnitude 9 earthquake in the Pacific Northwest. I would have been describing my trip anyway, but the brouhaha over the possible earthquake and tsunami provides an opportunity to inject some level-headed geology into the discussion.
Route of Highway 101 through California, Oregon and Washington (red line). Interstate 5 runs parallel to Highway 101 just inland.
The fact is that the Pacific Northwest IS dangerous ground. But so is California. So is Oklahoma. And Minnesota. And Florida. And every bit of land across the world. There is nowhere that humans can live on this planet that is free of natural hazards. It doesn't work to go moving somewhere else, because no place is truly safe. The best we can do is to is to understand the nature of the threat, and to be prepared for the disasters when they happen.

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
So our journey quite literally was an exploration of nearly the entire subduction zone, as seen from U.S. Highway 101 and Highways 1 and 99 in British Columbia. As can be seen in the diagram above, the zone begins near Cape Mendocino in Northern California and extends to Vancouver Island in British Columbia, Canada. The highway parallels the coastline, and follows the coastal terraces and beaches in many places, especially in Oregon. We spent ten days making our way north from San Francisco to Canada and northern Washington, and three days heading home again along Interstate 5. We camped when spots were available, and stayed in hotels (if we had to).

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.
Source: http://crew.org/sites/default/files/cascadia_subduction_scenario_2013.pdf
I imagine you could be thinking "what's the difference between this blog series (Dangerous Ground), and my just completed series (Driving Across the Most Dangerous Plate Boundary)? The facetious answer is, of course, that in this series, we will be driving along the plate boundary instead of across it. But the more serious answer is that the ancient Cascadia Subduction Zone in Central California is extinct. It stopped activity tens of millions of years ago. The subduction zone of the Pacific Northwest is presently active, and capable of causing catastrophic damage. And the landscape is different in many ways. In the series I plan to talk about the geology, of course, but I also find it to be a land of majestic beauty. That's the thing about geological hazards; they may be dangerous and cause human catastrophes, but they also result in some of the most spectacular landscapes on the planet.
Post a Comment