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 destroyed...at 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.

Geotripper Actually Gets Courageous and Drives (one of) the Most Dangerous Plate Boundaries in the World

So I spent the last several months producing a blog series on the most dangerous plate boundary in the world, but had to point out many times that the subduction zone was extinct, and that central California, as dangerous as it might be, was not the most geologically hazardous place in the world. That dubious honor goes to active subduction zones. I'm on the road again (most happily), and my first stop was an active subduction zone. It's not that far away from our previous journey; it's in northern California where the San Andreas transform boundary ends, and the Cascadia Subduction Zone begins. It happens at Cape Mendocino on California's Lost Coast.

It's not an easy place to get to. We followed winding, rutted Mattole Road from Humboldt Redwoods State Park over the coastal mountains to the tiny hamlets of Honeydew and Petrolia and checked out the remote coastline at Mattole Beach (the estuary of the Mattole River), and the stretch of beach between McNutt Creek and Sugarloaf Island at Cape Mendocino.

As you can see in the picture above, Mattole Beach was horrifically crowded, and we could hardly find a bare patch of sand to call our own. Well, okay, not so crowded. The small campground was half full, though. This region is where three plate boundaries come together: a triple junction. It is a geologically active zone, with a number of magnitude 7+ earthquakes over the last century, including a particularly nasty event in 1992 (magnitude 7.2) that caused serious damage in Petrolia, Ferndale, and Eureka.

The event lifted the coastline several feet in places. The coastal plain near Sugarloaf Island (below) is actually a wavecut bench that has been lifted several tens of feet over the last few thousand years. Waves once scoured the surface at the ranch buildings. 

Sugarloaf Island is situated just offshore of Cape Mendocino. From here north, the Cascadia Subduction Zone rules the geology. Quakes along the zone can be huge, as great as magnitude 9. The last occurred just over 300 years ago in 1700. It generated a tsunami that caused damage in Japan.

At this point, Mattole Road left the coast and climbed steeply over a series of ridges to Ferndale. Each of the ridges were fault blocks separated by thrust faults. This is a dangerous landscape!

Posts will be sketchy over the next few days or weeks as we camp our way north through Cascadia.

The Lost Coast Found, and An Answer to Friday's Foto Mystery

Actually today's post is the answer to three questions...there's Friday's question, and these: What's the westernmost spot in the contiguous United States California? And, at what spot you can stand really close to a triple junction? All three questions are related to California's Lost Coast.
The Lost Coast is one of the few parts of California's coastline that is not traversed by either Highway 1 or Highway 101 (the area around Vandenburg is one of the others). For some seventy miles between Ferndale and Rockport, the coastline is a wilderness explored only by trails or a few rarely traveled country roads. One of these roads provides access to the small village of Shelter Cove. The other is Mattole Road, which connects the town of Ferndale with the hamlets of Petrolia and Honeydew, and then joins Bull Creek Flats Road in Humboldt Woods State Park.

Reading about the backpacking along the Sinkyone Coast and the King Range captured my imagination; how many places can you backpack to a deserted coast in California, and worry about bears (and probably Bigfoot...)? Unfortunately we didn't have that kind of time. We were in Fortuna and had five hours before my family gathering. I looked at the map, saw the looping road and "73 miles", and said to myself that there was plenty of time. Uh-huh...

The road climbs out of Ferndale, and mostly has two lanes of pavement, except for the occasional landslide that has deposited portions of the road downslope. It climbs through an attractive forest, and bright displays of wildflowers.
We were surprised by the laid back attitude of the coyote on the slope above the road. He watched us for a few moments, and then laid down for a nap...
Lupines were especially common along the road, from the highest ridges right down to the coast. Since I love lupines in all their many incarnations, this was a wonderful sight.
We crossed a series of high ridges and at least four fault zones (see the geologic map at the bottom of the post). Some of the land was forested, but much was given over to cattle grazing. The views were quite stunning. In the picture below, we had reached the valley of Bear Creek and the settlement of Capetown, which was actually a ranch headquarters and little else.
The crossing of Bear Creek gave us a tantalizing view of the beach, but we had another mountain to get over before the road reached the coast. Does it seem a bit overcast in some of these pictures? This is one of the wettest corners of the state, averaging somewhere around 100 inches of rain every year.
We crossed Cape Ridge, and dropped rather precipitously down a few switchbacks to the coastline at the mouth of Singley Creek. Here we find the answer to one of our questions of the day. Cape Mendocino is the westernmost piece of land in the contiguous United States California. It is not reached by any roads or trails that I know of due to private property (I'm open to correction on this!), but the last switchback down to the coast represents the farthest one can go in a westward direction, and is only about a quarter mile short of being the westernmost point. The rock in the photo below is Sugarloaf Island, and it lies just offshore of Cape Mendocino.
The road reaches the coastal plain and follows the shoreline for six miles or so. There are few pullouts or parking spots, but there is a wide spot in the road at Steamboat Rock that provides access to the beach and some excellent birdwatching. There were hundreds of them on the unusually shaped sea stack.
We continued rolling south along the beach and soon encountered our mystery rock from Friday's Foto Blog (that's it on the left side of the photo below). There were some great observations in yesterday's comments, and most centered on the possibility of pillow basalts, and I think that is correct (I wasn't able to get samples, so if some Humboldt friends are reading, and I'm wrong, let me know). The regional bedrock is the Coast Range unit of the Franciscan Complex, the mix of rocks that formed in the churning mess of an accretionary wedge in the subduction zone that has existed off the coast of California for several hundred million years (see the geologic map below). Most of the complex is composed of graywacke sandstone and shale with bits and pieces of materials scraped off the ocean floor, including basalts that formed at oceanic ridges. Basalt that erupts on the sea floor forms the unique pillow shaped masses. This particular outcrop was harder than the surrounding sedimentary rocks, and would have been a sea stack a few centuries ago before this section of coast was lifted up by fault activity.
With the intense amount of yearly rainfall, and the rapidly uplifted mountains composed of weak sedimentary rocks, mass wasting is inevitable. We saw a recent slump on the road. I don't envy the road crew that will be bringing in heavy equipment to fix things...
As many of you know by now, I am not the most attentive of birdwatchers, but I'm trying harder to get to know them. I had never noticed this unique shorebird before, and learned that it is called the Black Oystercatcher (Haematopus bachmani).
As we came to the south end of the beach, we saw another somewhat rare feature for northern California beaches: sand dunes. Most of the dunes were stabilized by a thick growth of grasses, but some were active.
Finally, the road turned east and UP. It was time for the climb up to the little town of Petrolia, which got its name from the distinction of being the first place in California where oil was ever produced, in 1865. It was a pleasant little village of 500 or so people. The town was badly damaged by the shaking from the magnitude 7.2 Petrolia quake in 1992, which brings us to the answer to the third question. A triple junction is a point where three tectonic plates meet. In this case, the North American Plate and Pacific Plate are separated by the San Andreas fault, which can be seen to pass the region just offshore (there were offsets here from the 1906 in San Francisco!). The San Andreas curves out into the Pacific, and just a few miles offshore meets a subduction zone that divides the North American Plate from the Gorda Plate. It is clear from the earthquake activity and ongoing mountain-building in the region that this is a focus of intense geologic activity. As far as I know, when standing on the coast here, you are standing practically as close to a triple junction as you can anywhere in the world, except for the Afar region in Africa, or at Mt. Fuji in Japan (anyone else know of triple junctions on land? It's late and my mind is fuzzy!)There is much more one can say about this road, but this is a long post already. I haven't even mentioned the Mattole River and Humboldt Redwoods, which dominate the last thirty miles of the road. Suffice to say it was spectacular. It is certainly worth mentioning that the road was not crowded. It doesn't go anywhere in particular, which means that the value of following it is the journey, not the destination. I certainly want to go back when I have some time to explore some of the corners and canyons, especially in the higher parts of the Humboldt Redwoods.

A nice road description:
http://www.amwest-travel.com/awt_lost_coast.html
Geologic Maps and Descriptions of the region:
http://pubs.usgs.gov/mf/2000/2336/cmmap.pdf
http://pubs.usgs.gov/mf/2000/2336/
http://pubs.usgs.gov/mf/2000/2336/ceghexpl.pdf

By the way, I think this was my 800th blog post. Who figured?