Cascadia's Fault - Jerry Thompson [43]
The scientists Sullivan consulted for his pre-eruption feature story had told him how the Cascadia Subduction Zone was thought to be a special case, different somehow from all the other continental collisions around the Pacific Rim. Cascadia’s volcanoes do form a line roughly parallel to the coast about a hundred miles (160 km) inland, and the mountain cones are spaced about forty-five miles (70 km) apart, like most other subduction zones with volcanic arcs. But in the minds of skeptics, that’s where the similarities ended.
Cascadia is not typical, Sullivan wrote, because “no coastal trench cuts in to the sea floor” at the point where the two tectonic plates converge and “no sloping zone of earthquakes” marks the descent of a seafloor slab beneath the coast. According to Sullivan’s sources, the Cascade volcanoes seem to have been created by an east-driving portion of the Pacific floor that had somehow run out of steam. The subduction process, wrote Sullivan, “is no longer vigorous enough to sustain a coastal trench and cause frequent earthquakes.”
So Cascadia’s smoking gun had run out of ammunition. No deep trench offshore and no deep quakes along the plate boundary—all because the movement of the eastbound Juan de Fuca plate had slowed down or even stopped. At least that’s what some experts thought at the time.
Trying to imagine how these huge plates float and slide over the curved surface of an imperfectly spherical planet, geophysicists came up with a sequence of events—a long geologic history—that seemed to fit the observable facts. As the floor of the Pacific Ocean spread apart along the Juan de Fuca Ridge, pushing the Juan de Fuca plate eastward, the rest of the Pacific plate (out on the western side of the ridge) was not moving due west but rotating in a more northerly direction toward Alaska.
At the same time, the North America plate was pushing westward and riding up over top of the eastbound Juan de Fuca slab. Around ten million years ago, so the theory went, the Juan de Fuca Ridge and plate started rotating clockwise, almost as if it were being spun by the angular movement of the two larger plates on either side of it. Think of a car going eastbound through an intersection when a northbound car passes just behind it, clipping the back fender. That northbound motion would make the eastbound car spin to the right, just as it got hit head on by a big westbound truck.
Five million years later, the Explorer plate had broken off the northern end of the Juan de Fuca, and some thought it might have fused or welded itself to the larger continental plate just north of Vancouver Island. There was further speculation that the Olympic Peninsula, on the northwest corner of Washington State, might also have been a piece broken off the Juan de Fuca plate and that it too had been fused to the continent, pressed against the outer edge of Vancouver Island, forcing up the Olympic Mountains in the process.
Two and a half million years later, on this hypothetical timeline, the North America plate had “disposed of” (subducted or recycled) a huge portion of the original Juan de Fuca plate. Down at the southern end, meantime, the smaller Gorda plate was breaking away as well and the spreading ridge offshore—the entire undersea mountain range—had rotated or been spun even farther to the right. At some point, according to this scenario, the relentless westward movement of North America would completely override what was left of the Juan de Fuca plate—and its spreading ridge offshore—just as it had apparently already done farther south in California.
Eventually, with the Juan de Fuca ridge and plate system gone, the boundary between the North America and Pacific plates would become