You’re standing at the grocery-store checkout. You put a bag of potato chips on the conveyor, and off they go, followed by a case of Pepsi, three loaves of bread, a watermelon, a box of Ho-Hos, and a sack of potatoes. Then, you realize that there is no bagger working and that everything is piled up at the end, in a BIG mess. That mess is a good model for the Olympic Peninsula and the whole coast from there up to Alaska.
The rocks of the Olympic Peninsula are a mixture of sea-floor basalts and the sorts of sediments that accumulate today off the coast and fill the trench there. Rivers draining the peninsula and other parts of the west coast carry great loads of sediment down to the ocean. Much of that sediment piles onto the sea floor that is slowly moving beneath the continent, a conveyor belt that drags some of the water-saturated sediment down to melt and then erupt from volcanoes. But, most of those sediments are “scraped off” on the way down, just as at the grocery store. The Olympic Peninsula is the offscrapings. Most of the rocks have been bent and twisted from the attempt to shove them under the continent (think of the potato chips after the milk jug hits them!). Some of the Olympic rocks have been heated a good bit—the conveyor belt took them part way down, but then they were squeezed back out.
And here's another fun way to understand subduction zones.
Video: Olympic Subduction Zone (:40)
Dr. Richard Alley: You may think that this looks like an ordinary sandwich cookie but in fact, this is a subduction zone demonstrator. Here is the Pacific Seafloor and on top of it is sediment, wind-blown dust and dead shells, and other things sitting on top of the basaltic sea floor and it's going down the subduction zone under the state of Washington and Oregon, and as it does some of the sediment is scraped off and is making a pile and that pile is the coast ranges and the Olympic Peninsula.
Our emerging picture of plate tectonics is that the earth is heated inside, softening the deep rocks of the asthenosphere enough that they can move in great, slow convection currents that transfer heat from deep in the earth to near the surface. Heat is conducted through the upper rocks, or is erupted through them by volcanoes, and eventually is lost to space. But, the upper rocks in most places are cold enough that they tend to break rather than flow—they are brittle. These brittle rocks form the lithosphere, which includes the crust and the uppermost mantle. The rocks of the crust in continents are rich in silica (often like andesite or granite in composition), making them light in color and low in density so that they float on the deeper rocks and are rafted around on them by the moving convection currents. The sea floor rocks in the crust are between the continents and the mantle in composition and typically are basalt. The sea-floor rocks are usually intermediate between continents and mantle in density as well, but if the sea-floor rocks are cold enough, they will be slightly denser than the hot mantle. Then, the sea-floor lithosphere consisting of the sea-floor crust plus a little attached mantle will sink into the asthenosphere of the deeper mantle.
Geologic Plates
The lithosphere is broken into a few big rafts, called plates—eight big ones plus some smaller ones, depending a little on how you define “big” and “small”—that float around on the convection cells below. Plate boundaries include spreading ridges where the plates move apart (remember Death Valley and the mid-ocean ridges), and subduction zones where the plates come together and one side sinks under the other. You might imagine that if plates can come together or pull apart, they must be able to slide past each other as well, which is what happens at the San Andreas Fault in California (we met it when we were discussing earthquakes); such slide-past boundaries are often called transform boundaries or transform faults (see the figure below). You might worry that sometime, two continents would run together; we’ll meet that soon when we visit the Great Smoky Mountains.
The lithosphere and asthenosphere are solids, but a little melted rock may occur in places in the asthenosphere, and some of this may leak out where plates are pulled apart, feeding basaltic volcanoes. And, the water taken down subduction zones can stimulate a little melting, feeding andesitic volcanoes that line up in arcs above the downgoing slabs of the subduction zones; examples of these volcanic arcs include the Cascades, Aleutians and Andes. Continents are a collection of scum formed from freezing of material that melted in the mantle and then moved upward and froze; continents are too low in density to sink back into the mantle. Continents grow as the conveyor belt from the mid-ocean ridge to the subduction zone brings in sediments and islands and what-not, or when andesitic volcanoes erupt on continents, or when andesitic volcanoes form an arc in the ocean that then collides with a continent (sometimes the site of subduction moves, and the volcanoes find themselves on the conveyor belt, or they hit a different continent). Because much of the sediment comes from the continents themselves, the growth of continents is not fast—material eroded from the continents falls on the conveyor and is added back at Olympic or erupted back at Crater Lake.