Sunday, July 26, 2015

Vagabonding on Dangerous Ground: The End is Coming (of the Cascadia Subduction Zone)!

Sugarloaf Rock at Cape Mendocino. Somewhere out to sea beyond the cape is where the Cascadia Subduction Zone is being destroyed, bit by bit.
The end is coming! I'm not referring to the possible destruction caused by the possible magnitude 9 quake that's been in the news of late. I'm talking about the actual destruction and disappearance of the Cascadia Subduction Zone itself. But if you are anxious to see this happen, you'll need to be patient. About 10 or 20 million years patient.

And you might want to be careful what you wish for anyway...

As mentioned in previous posts of this series, the Cascadia Subduction Zone is a spot where oceanic crust of the Pacific Ocean is being driven beneath the North American continent. The subducting plate, the Explorer-Gorda-Juan de Fuca is one of the smallest crustal plates on the planet. And it is being subducted faster than it is being created. It is indeed being the incredible (geologically, anyway) rate of a couple of inches per year. It will take tens of millions of years to consume the remaining plate material. When it is consumed, the subduction zone will cease to exist. The good people (or whatever species has evolved by then) of Portland and Seattle can stop worrying about subduction zone earthquakes.

As for being careful what you wish for? As the subduction zone is being progressively destroyed, it is replaced by the newly formed San Andreas fault/transform boundary. Although the San Andreas fault can't cause earthquakes as powerful as the subduction zone shakers (they top out at around magnitude 8.0, about 1/30th the energy of a magnitude 9.0), they are capable of serious mischief, as the quakes are often shallower, and that's a bad thing. The tragedy at Haiti a few years ago is a horrific example. The magnitude 7 quake killed nearly a quarter million people, despite having only 1/30th the energy of a magnitude 8 tremor. The focus was just a few miles deep.

The beach and sandbar at the mouth of the Mattole River, one of the very few undammed rivers in California
How can we know the subduction zone is actually ending? In the case of northern California, we can see that the plate has actually been disappearing for a long time, about 28 million years. It was once active in southern and central California. The rocks left behind by the collision ultimately became the Franciscan Complex, the rocks making up the core of the California Coast Ranges.  We explored those rocks in some detail in the last blog series, Driving the Most Dangerous Plate Boundary in the World.

Crowded Mattole Beach
The spot where one plate boundary ends and another begins is called the Mendocino Triple Junction (three plates are in contact, the Pacific, the Gorda, and the North American). It lies some distance offshore of Mattole Beach and Cape Mendocino, one of the most isolated spots in California. We drove winding Mattole Road for 70 miles or so between Humboldt Redwoods State Park and the small town of Ferndale. This isn't your average California coastline. There are just two very small villages, Petrolia and Honeydew, and only a handful of people ever wander the beaches. Mattole Beach is the trailhead for the Lost Coast trail and King Range, a 28 mile route along the only truly roadless coast in California.

This is one of the most geologically active corners of California, and California as a whole is a very geologically active place. Mattole Road is winding and steep because the land here has been uplifted very quickly, and the intense rainfall erodes the mountains at an astounding rate. Honeydew is one of the wettest places in the state with rainfall in some years exceeding 100 inches (2,550 mm). The dark rocks in the picture above are former sea stacks (small rocky islands) that were lifted up out of the water by the recurring earthquakes that raised the shoreline by a couple of inches or feet every couple of decades. A particularly damaging quake took place in 1992, the magnitude 7.2 Petrolia event.
So our journey along the Cascadia Subduction Zone began at the end, in a manner of speaking.  The road above Cape Mendocino was particularly steep, providing a marvelous view of the uplifted coastal terrace. The flat area used today as grazing lands was in the geologically recent past the wave zone. The heavy surf is eating away at the terrace, forming a new wave-cut bench, which will itself eventually be raised up out of the surf.

We took a leisurely day exploring the beautiful coastline before grabbing some dinner in Fortuna and heading back to the campsite in Humboldt Redwoods. We would head north in the morning.


Anonymous said...

why are the western edges of the plates abrubt straight lines? We don't know exactly where the edges are?

DSchwartz78 said...

Check the bathymetry for the plate based on sonar imagery and it really is a collection of straight lines. Or mostly. And it helps that the western boundary is a divergent fault which tend to be fairly straight.