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?
Click here for background information on plate margins.
The San Andreas is what is termed a Transform Fault: a place where sections of the earth's crust slide side-by-side past each other. As mentioned in the background information, they are usually associated with spreading centers, and represent horizontal offsets due to the reality of trying to fit a linear zone of faulting onto a spherical earth. The San Andreas Fault 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 things got pretty crazy. It was about that time that the North American plate actually caught up with the spreading center which was in the Pacific Ocean and started subducting the spreading axis itself. The way it works now is relatively complex:
Starting in the south Pacific, there is spreading taking place along the "East Pacific Rise" spreading center, with oceanic crust moving west 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 up the west coast of Mexico, where it runs up the Gulf of California. It is this spreading motion which is separating Baja California from mainland Mexico, and has led to the formation of the Imperial Valley and Salton Sea in southernmost California.
If you follow along the spreading axis, you'll see that it isn't a very clean break - a two-dimensional fault on a three dimensional sphere makes for some 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.
The San Andreas fault is simply one of these transform faults which offset a spreading center (in this case, separating the Pacific Plate to the west from the North American Plate to the east). On the south it joins with the East Pacific Rise where it comes out of the Gulf of California, and the north end runs off the coast at Cape Mendicino on the northern California coast. North of Cape Mendicino, 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 - making seafloor basalt which then moves to the east until it runs into northern California, Oregon, and Washington. At that point the seafloor is subducted along the Cascadia Subduction Zone, giving rise to the volcanic mountains of the Cascade Range.
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. However, the intensity of the faulting easily makes up for this apparent lack of natural disasters.
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