A Convergence of Wonders: Day 3, Driving Across the Subduction Zone

So what is convergence? The word can have many meanings, but in geology, convergence refers to regions where the plates of the Earth's lithosphere (the solid layer of crust and upper mantle) are forced together. When two continents converge, huge mountain ranges like the Himalayas and the Alps are formed. When oceanic lithosphere is forced into continental lithosphere, the denser and thinner oceanic rock is forced under the continent in subduction zones, where parts of the plate melt and form magma bodies (plutons) which rise into the continental crust, ultimately forming volcanoes and masses of plutonic rock like granite and diorite (like those of the Sierra Nevada). Besides volcanism, subduction zones are responsible for huge earthquakes like those that ravaged Japan this year and Indonesia in 2004.

This series of posts is called a Convergence of Wonders because much of the landscape we traversed on our trip was influenced by the subduction zone (the convergent boundary) that has been active off the coast for the last 200-300 million years, right up to the present day. It is called the Cascadia Subduction Zone. On the third day of the trip we got as close to the subduction zone as we could, crossing the Cascades and setting up camp on the coast at the mouth of the Columbia River. Our destination at the end of the day was the ominously named Cape Disappointment.

Our first stop was a delightful spot I had not seen before, Smith Rock State Park near Bend, Oregon. I've been guilty of thinking of parts of central and eastern Oregon as being relatively featureless basalt flows in monotonous horizontal layers. This isn't actually true, and Smith Rock is one of those incredible exceptions. The sheer vertical cliffs are composed of rhyolite, a volcanic rock that is the polar opposite of basalt. It doesn't so much flow as explode, in clouds of hot ash that can bury a landscape instantly. The ash can be so hot and so thick when it lands that it is melded into a solid rock more or less instantly. These hardened deposits are called welded tuff. That is the material that forms the remarkable cliffs of Smith Rock State Park. It makes for a climber's paradise.

The cliffs of Smith Rock are part of a caldera, a feature we saw in the last post at Newberry Volcano, but this caldera formed in a far more violent fashion. Because it formed tens of millions of years ago, the shape and form of the caldera has been lost to erosion. We wouldn't see a "modern" (in the geologic sense) caldera until we reached Yellowstone a week down the road.

The stratovolcanoes of the High Cascades provide a backdrop for the tan-colored rhyolite cliffs. The snowcapped peak in the first picture is South Sister (10,358 ft; 3,157 m). Mt. Jefferson (10,497 ft; 3,199 m), the second tallest volcano in Oregon is nicely framed over the balanced boulder in the pictures below and above.

It was hard to turn away, but we had a mountain range to cross. I took one last shot of the steep cliffs, and we trundled into the vans and started to head out to the wilds...of the Portland Freeway system. The archaeologists had something they wanted to see in Vancouver.

We crossed the high ridge on the south flank of Mt. Hood (10,358 ft; 3,157 m), and started down the long hill into town.

Braving the traffic, we crossed the bridge over the very swollen Columbia River into Washington. Mt. Hood loomed in the distance.

Fort Vancouver was a fur trading outpost in the 1840s and later a military installation. An active archaeological dig was taking place as the U.S. Park Service was trying to better understand the arrangement of private housing units adjacent to the fort. Employees of the Hudson Bay Company were provided with some benefits, but they and their families had to grow much of their own food.

The hour was growing late, and we had another hour of driving. As we crossed the Columbia at Astoria, I took a single picture of the ghostly presence of Mt. St. Helens in the far distance. As will be seen, it was the only time we would see the mountain on the trip.

We arrived in camp at Cape Disappointment State Park, on the north side of the mouth of the Columbia River. It was a beautiful place, with campsites only a few hundred feet from the beach. I walked out for a look. The beach was wide and sandy (due to human construction, actually; more tomorrow).

I turned my attention to the cliffs at the northern end. They had an unusual aspect to them. I knew they must be part of the accretionary wedge of the Cascadia Subduction Zone, material scraped off the ocean crust as it dived beneath the North American Continent. I took a closer look.

The cliffs were pillow basalts! Pillow basalts form as basalt pours out onto the ocean floor at divergent boundaries. If convergent boundaries destroy the crust, divergent boundaries are places where new crust is made. They balance each other out in the long run. These basalts traveled thousands of miles at an inch or two a year, before being scraped off to become a part of the coastal complex of rocks.

The bright sunset carried the promise of clear weather. The long-range weather forecasts didn't. That was a problem for another day. The next day, specifically.

I headed back into the forest to enjoy dinner and a relatively warm evening. There were even hot showers! We settled down for a peaceful night. The rain started in the wee hours of the morning.