August 15, 2016
by Lauren Burch
There are plenty of volcanoes in the Cascade Mountains of the Pacific Northwest, produced by the subduction of the Juan de Fuca Plate beneath the North American plate along the Cascadia Subduction Zone (CSZ). The nearby Olympic Mountains of Washington State, however, do not have any volcanoes. This raises the question: why are the Cascade Mountains volcanic, but the Olympic Mountains are not? They both lie along the same section of the CSZ, and they’re only a few hundred kilometers apart, separated by the Puget Sound lowlands. What makes the two mountain ranges so different?
Subduction zones like the CSZ tend to produce the same basic pair of geologic features: volcanoes and accretionary wedges. Volcanoes form above the zone where the subducting slab reaches about 100 km depth and begins to release water into the overlying mantle rock. Just like salt lowers the melting temperature of ice, water and other contaminants lower the melting temperature of solid rock, allowing magma to form without an increase in temperature. The magma eventually migrates toward the surface and erupts as lava from a volcano. Until the slab reaches that specific depth, the temperature and pressure conditions aren’t quite right for melting to happen, which is why our Cascade volcanoes are so far inland from the Cascadia Fault.
The Olympic Mountain Range is non-volcanic because it is actually a huge accretionary wedge. Unlike volcanoes, accretionary wedges form near the shallow end of the subducting slab as pieces of oceanic crust and marine sediment get plowed into a “wedge” by the overlying plate (imagine using your hand to sweep sand or dirt across a table). Often, accretionary wedges are hidden below sea level or produce gentle coastal ranges, but the Olympics are a rare exception - the slab beneath the Olympic Peninsula is colder and dips at a shallower angle than other sections of the CSZ, which may explain the unusually high elevation of the Olympic Range. Because it is an accretionary wedge, the western side of the Olympic Peninsula is largely made of up ocean sediments scraped off the Juan de Fuca Plate. If you were to hike to the tip of Hurricane Ridge, you might even find shelly limestones and sandstones with marine fossils in them!
June 15, 2016
by Steve Malone
May 30, 2016
by Lauren Burch
All the mountains, oceans, and islands on Earth exist because of plate tectonics. Different plate boundaries produce different geologic features: divergent boundaries spread apart to form mid-ocean ridges and rift valleys, transform boundaries slide past one another to form strike-slip faults like the San Andreas, and convergent boundaries collide to form tall mountains, deep trenches, and volcanoes. This type of plate boundary is responsible for the numerous volcanic arcs around the Pacific Rim (often called the “Ring of Fire”), and formed our iconic Cascade Volcanoes. Here in the Pacific Northwest, the Juan de Fuca plate is subducting beneath the North American plate along a convergent plate boundary called the Cascadia Subduction Zone (CSZ). Subduction zones like this are the only fault systems capable of producing very large megathrust earthquakes, but they only do so occasionally - over the last 100 years, there have been 84 earthquakes of magnitude 8.0 or greater worldwide, and only 4 of them were greater than an M9.
The simplest answer to the question “Will there be another large earthquake on the CSZ?” is yes. However, the question of “when” is much more difficult to answer. Seismologists don’t know exactly when the next large earthquake will occur on the CSZ, but we do have a good picture of when they have happened over the past 10,000 years. If we divide 10,000 years by the number of ~M9 earthquakes found in that time period, the average recurrence rate for M9 earthquakes along the CSZ is roughly 550 years. We are 316 years past the last great CSZ earthquake in 1700, and we estimate that there is about a 15 % chance that an M9 will occur on this fault within the next 50 years. However, research on submarine landslide deposits shaken loose by big earthquakes indicate that M8+ earthquakes occasionally strike off the coast of Oregon in between “full rip” M9 events. This research suggests that there is a greater probability of reoccurence of a great earthquake in Southern Oregon than off the Washington coast, but there is not a consensus within the geophysical community as to specifically how much greater the hazard is.