The area of the Pacific Northwest I live in is seriously dangerous. When the Cascadia subduction zone rips, it will cause massive destruction: think the Indian Ocean earthquake and tsunami in 2004 or the Tōhoku earthquake and tsunami in Japan in 2011. In the meantime, it fuels ferocious volcanoes that erupt on sometimes monumental scales. And smaller faults throughout the region keep the ground shaking, sometimes causing a fair amount of damage on their own, like the 2001 Nisqually Earthquake. It can be scary, living here.
But when all's quiet on the tectonic front, it's absolutely gorgeous.
If you're in the Seattle area on a clear day, and want to take in some pretty sweeping views of what the Cascadia subduction zone has to offer, you'll want to hop aboard the Kingston Ferry in Edmonds, WA. It's a short trip, but you'll see a remarkable amount of geology going there and back again. It's not the subduction zone roller coaster I'd like to build, but it's close enough.
When you leave the ferry at Edmonds, you're sailing towards the subducting Juan de Fuca plate. You're not going to be able to see that far, or even out to the ocean, because there's a very important feature of subduction zones in the way: the accretionary wedge, also known as an accretionary prism or accretionary complex.
An accretionary wedge is basically a hodge-podge collection of various sediments and rocks, scraped up and squished together where two tectonic plates collide and one plate subducts underneath another. The downgoing plate is always an oceanic plate (continental plates don’t really subduct as continental crust is too buoyant), but the overriding plate can be either another oceanic plate (such as the Japan subduction zone) or a continental plate (such as the Cascades subduction zone). The sediments in an accretionary wedge are mostly marine sediments scraped off of the downgoing oceanic plate. Most of the marine sediments on the oceanic plate actually subduct down into the mantle. However, some of the marine sediments pile up and are accumulated into a wedge or prism-shaped pile of sediments where the downgoing plate meets the overriding plate. This scraped-off marine sediment is mixed with other material such as sediments weathered/transported from the overriding plate and fragments of rock broken off of the colliding tectonic plates.
The accretionary wedge isn't always visible on land. But when it is, you get sights like this:
The subducting Juan de Fuca plate has scraped a remarkable assemblage of seafloor basalts and sediments onto the edge of the North American continent, and the collision between the two plates has shoved them high above the sea. Imagine the forces that build mountains nearly 8,000 feet high! The Olympic Mountains are a marvelous place to go see subduction in action. And they make a tremendously lovely backdrop to the sky as you sail across the Sound.
The Olympic Mountains have some volcanic rocks in them, but they themselves aren't volcanic. That's not what accretionary wedges are about. However, with every great subduction zone comes some equally great volcanic action, and its results are all around you here. You get a sweeping view of the Cascade volcanic arc here, with three, yes three, visible volcanoes.
Mount Baker is the first you'll notice. It stands out to the north, and is visible from the park as well as on the ferry itself. Stand on the starboard side of the ferry for the trip out, and you can watch it pace you the entire way.
Mount Baker is the youngest of the Cascades volcanoes, born about 140,000 years ago, but older volcanoes preceded it. And it doesn't stand as alone as it sometimes looks: it's very much part of the North Cascades.
The Cascade Range is considered a volcanic arc, but not all of the mountains in it are volcanic. In fact, many aren't: especially in the North Cascades, they're largely enormous masses of metamorphic and sedimentary rocks, plus the intrusive igneous rocks of a rather substantial batholith, that have been raised by the pressures of colliding plates. And they are enormous: not only tall, but long. If you stand at the bow of the ferry on the way back from Kingston, you can see the arc crossing the horizon from north to south. As far as you can see, it's volcanic arc!
And if you look really, really closely in that last photo, you'll see one of the most overlooked volcanoes in the Pacific Northwest. It's nestled in with the other peaks, so it's really easy not to notice. But when the snows have largely melted from the other mountains, it stays anomalously snow-capped. It's Glacier Peak, and it's really freaking dangerous.
In fact, only Mount St. Helens can beat it for sheer explosive mayhem, at least in the last 15.000 years. When it wakes up, it's likely to be with a bang, and Seattle and the surrounding communities will be in trouble.
But of course, Glacier Peak isn't the Cascades volcano that presides over Seattle in prominent glory. That's Mount Rainier, which you can see to the south as you leave Kingston.
This is the highest peak in the Cascade Range, and it's not nestled within the other mountains. It rises from nearly sea level to its summit of 14.410 feet. It used to be even taller, but a sector collapse like the dramatic landslide at Mount St. Helens shaved off quite a bit of elevation nearly six thousand years ago. The people who lived here then must have been awestricken.
As you sail out over the Sound, you can turn back for a look at the Olympics, now quite close:
And you're once again traversing a neat subduction zone feature: the forearc basin. All of the Puget Lowland in which Seattle rests is part of this structural low between the accretionary wedge and the volcanic arc.
Amazing, isn't it? You can read more about the structure of the state from the trench to the backarc basin here.
Once you get back to Edmonds, don't forget to pause and view the majestic Olympics once more. If you've arrived just after sunset, the view is utterly captivating.
Subduction zones create some of the most beautiful landscapes on Earth. I'm willing to live with the danger just to see these views every day.
Dedicated to my cherished friend and adventuring partner Suzanne Buck, who adored sunsets and volcanoes.