West coast tectonics

I had an interesting question come across my GeoMania website a while back: “If Mt. St. Helens and the whole west coast is a convergent plate boundary with subduction, how come there are no volcanoes along the San Andreas fault?”

Before we start, you may want to click here for some general background information on plate tectonics and plate boundary interactions.

Click here for a more detailed summary of global tectonic processes from the Josephine Ophiolite GeoTour.

The San Andreas is what is called a “transform” fault: a place where sections of the earth’s crust slide side-by-side past each other. They are usually found along tectonic “zones of divergence,” and represent horizontal offsets due to the reality of trying to fit a linear zone of faulting onto a spherical earth (the most famous “spreading center” is the Mid-Atlantic Ridge” that is separating the new world from the old, but there are others). The San Andreas is a classic example of one of these “strike-slip” faults — probably because it’s on land and in California.

But something doesn’t make sense: if subduction is what is supposed to be happening along the Pacific coast, where did this transform fault come from?

Unlike South America, the entire west coast of North America isn’t a convergent plate boundary. It used to be, but somewhere around 30,000,000 years ago — give or take a week — things started to get somewhat crazy: the North American plate actually caught up with the spreading center that was in the Pacific Ocean and started to eat the spreading axis itself. The way it works now is a bit complex:

Starting in the South Pacific, there is spreading taking place along the “East Pacific Rise,” with oceanic crust moving westward to Asia and Australia, and east until it runs into South America (where it is subducted and leads to the formation of the volcanic Andes Mountains). This spreading and subduction continues north along the length of South and Central America and the west coast of Mexico, where it runs up the Gulf of California. It is this divergence that is separating Baja California from mainland Mexico, and has led to the formation of the Imperial Valley and Salton Sea.

Anyway, if you follow the spreading axis where it enters the Gulf of California you’ll see that it isn’t a very clean break — trying to fit a two-dimensional fault onto a three dimensional sphere makes for some pretty weird geometry, and there are many offsets along the ridge. These transform faults actually stagger the axis of spreading, and are very common along all known zones of divergence (feel free to check the global tectonic map at the top of this post if you don’t believe me).

The San Andreas is simply one of these transform faults. On the south it joins the East Pacific Rise where it comes out of the Gulf of California, and the north end runs off the coast at Cape Mendocino on the northern California coast. North of the “Mendocino Triple Junction” the sense of motion reverts back to spreading, and there is actually a relatively short segment of the original East Pacific Rise still lurking beneath the waters of the Pacific Northwest — happily making seafloor basalt that then moves to the east until it runs into northern California, Oregon, and Washington.

Where they meet the basaltic seafloor crust is subducted along the Cascadia Subduction Zone, giving rise to the volcanic mountains of the Cascade Range, along with massive earthquakes greater than 9 on the Richter scale. The last rupture along the Cascadia may have been way back in January of 1700, but that doesn’t mean it’s dead, simply that the stresses continue to build (and sooner or later they WILL rupture — hang on tight).

To close the circle on our original question: Volcanic activity is common at both spreading centers and subduction zones, but because there is no ripping apart or subduction taking place along a transform fault, there isn’t any magma formation to lead to volcanoes. (This is one more reason why the 1997 disaster movie “Volcano” — which takes place in Los Angeles — doesn’t make much sense from a geological perspective.)

However, the intensity of the faulting — the 1906 quake that nuked the Bay Area was estimated at an 8 on the Richter Scale — easily makes up for this apparent lack of volcanic eruptions.