Driving Across the Most Dangerous Plate Boundary in the World: These Rocks are All Wrong!

The view from Muir Beach Overlook, midway between San Francisco and Point Reyes National Seashore

Point Reyes National Seashore: Is it land's end, or ocean's end? From a human point of view, it is the former. This is the far west of North America, and you can't go any farther without a boat or plane. But if we consider the oceanic crust that was being subducted beneath the continent here, it is the latter. It's where the ocean floor came to an end by being consumed and presumably melted within the Earth's mantle, the subcrustal layer that extends to the core some 1,800 miles down. In any case, it is the starting of our journey across the most dangerous plate boundary in the world. As I've pointed out before, it isn't presently the most dangerous boundary; it changed long ago, and the stripping effect of erosion has revealed the inner workings of the subduction zone that produced much of California's present-day landscape.

Drake's Beach, with outcrops of Purisima Formation that reminded early sailors of Dover.

But the rocks at Point Reyes are all wrong! As was mentioned in the first post of this series, an ocean-continent convergent boundary (a subduction zone) consists of three structural features: an accretionary wedge, a forearc basin, and a magmatic arc. The wedge deposits should be found nearest the coastline, and the granitic or volcanic rocks of the magmatic arc would be found far inland. As we look around at our journey's starting point at the lighthouse at Point Reyes, we find rocks related more to the magmatic arc. There are exposures on the Point Reyes peninsula of granitic rock, metamorphic rocks, and the silica rich sediments that have been eroded from them. Why are things out of order here?

It's due to the structural changes that resulted in the cessation of subduction and the beginning of movement on the San Andreas fault. At transform boundaries like the San Andreas, the crust and upper mantle (the lithosphere) are shifting laterally. The rocks west of the fault, Point Reyes, the Central Coast's Salinian Block, Los Angeles, San Diego and the Baja Peninsula are moving as a unit to the northwest, more or less towards Alaska.

Around 30 million years ago the rocks of Point Reyes were in Southern California. Around 29 million years ago a major restructuring of plate boundaries took place. Granites and metamorphic rocks related to the Sierra Nevada were sliced off and started their northward journey and we find them today underlying the lighthouse at Point Reyes. And that's where we now start our journey across the most dangerous plate boundary.

Source: USGS (http://pubs.usgs.gov/of/2005/1127/)

I'm afraid we've got a bit of a walk before we can get in a car and start driving. The lighthouse at Point Reyes is situated midway up the cliff above the sea, and we've got a climb of about 300 steps to reach our road. The rocky headland where the lighthouse is located is composed of granitic rock and some overlying late Cenozoic sedimentary rocks.The rocks are resistant to erosion and stand as a high rocky point.

Photo by Mrs. Geotripper

Take a few deep breaths and start climbing...

The cliffs of granitic rock are steep and nearly vertical. If you listen carefully, you may hear the barking of sea lions in the small coves below. During the right time time of year you may spot some of the migrating whales offshore.

The climb, though, is worth the effort, because when we reach the top of the hill there is a marvelous view to the north towards one of the longest unbroken sandy beaches in central or northern California. One might think it would be a popular swimming beach, but the fierce winds, high and unpredictable waves, and cold water make for uncomfortable conditions.
From this lofty perch we can make out the mountainous terrain of Inverness Ridge, composed mostly of the granite and metamorphic rocks of the Salinian Block. The gentle westward slopes expose sedimentary rocks of Miocene and Pliocene age, deposited during the long journey northwest from Southern California when the rocky basement was submerged beneath the ocean waves. Active dune fields can be found along the extensive beaches of the peninsula.

The peninsula is protected from development as Point Reyes National Seashore, and is a haven for wildlife. On our drive towards the "mainland" we are likely to see numerous bird species, including California Quail. A herd of Tule Elk graze the grasslands along the road.

The road climbs over the crest of Inverness Ridge and descends to the village of Inverness and the shores of Tomales Bay. We've reached the San Andreas fault, the second most dangerous plate boundary in our narrative. Maybe the rocks will be "right" when we find a way to cross the boundary between the Pacific and North American plates.

Driving Through the Most Dangerous Plate Boundary in the World: Reconnaissance

We're headed on a blog adventure through the most dangerous kind of plate boundary in the world. To make things clear, the boundary we are exploring is not currently the most dangerous in the world, although it is certainly very hazardous. As described in my introduction yesterday, most subduction zones are not easy to explore. Most parts lie underwater or deep in the crust. We are instead traveling through the fossil subduction zone in California that was active from about 200 million years to about 29 million years ago.

An active subduction zone, like those that lie offshore of Indonesia, Peru/Chile, Japan, or the Philippines, is capable of producing monstrous earthquakes ranging as high as magnitude 9.5. A quake of that size can rupture the sea floor over distances of 800-900 miles (1,300-1,500 kilometers), with offsets in the range of 50 or 60 feet (15-20 meters). Such quakes, happening five or six times in a century around the world, have killed hundreds of thousands of people. The worst volcanic disasters of recent history (Krakatoa in 1883, Tambora in 1815, Pinatubo in 1991, and Mount Pelée in 1902) wiped out several hundred thousand lives as well, and have even altered world climate.

The San Andreas fault on the San Francisco Peninsula. It runs along the linear valley containing Crystal Springs and San Andreas Reservoir (yes, that's where the fault got its name). The fault runs out to sea near Pacifica, but emerges on land again at Point Reyes National Seashore to the north.

California once was this kind of geologic environment. It's unimaginable the number of disastrous earthquakes and eruptions that took place over nearly 200 million years that were never witnessed or felt by any human being. The rocks produced by this intense geologic activity underlie most of California, and the rocks are a complicated mess. In Central California, though, there is a certain organization that still exists. Looking at the geologic map above one can see three broad strips of rock or sediment trending roughly north-west, the mostly green Coast Ranges, the yellow Great Valley, and the red, blue and green of the Sierra Nevada. These three belts correspond roughly to the accretionary wedge, forearc basin, and magmatic arc of the now inactive subduction zone.

The Golden Gate Bridge and the entrance to San Francisco Bay. The Marin Headlands are on the left, and San Francisco is on the right. The San Andreas would cross the scene just below the bottom margin of the photograph, underwater.

The California we know of today is dominated by a different kind of plate boundary, a transform. The name San Andreas is known to most, a fault zone famous for the San Francisco earthquake of 1906, but the state is split by dozens of other active faults. They mostly trend to the northwest, and are causing the movement of Baja California, Los Angeles, and Monterey as a large landmass towards Alaska at the stunning rate of about 2 inches (5 cm) per year (be thankful it isn't faster). The largest earthquakes expected on this type of boundary fall within the range of magnitude 7.8-8.0. Such earthquakes are deadly, capable of killing thousands of people, but an 8.0 releases only about 1/30 of the energy of a magnitude 9.0 earthquake. Another way to understand the difference is to realize that the one quake in Japan in 2011 (magnitude 9.0) released more energy than all of California's earthquakes over the last 150 years combined.

The Sacramento-San Joaquin River Delta is one of the most complex regions in the state. The rivers split into multiple channels, forming nearly three dozen islands. The rich farmlands are protected from flooding by poorly built levees and dikes. The area is vulnerable to liquefaction damage in the event of earthquakes.

Everyone who has started out on a major journey wishes they had an accurate map, and I'll wager that the pioneers who set out for California in the Gold Rush days wished they could have had an aerial view of their route (if they had seen their route, they probably would have stayed home back east). And that's what we are doing today. Getting across the rocks of the subduction zone is not far as the crow flies, perhaps a hundred miles, but the varied kinds of rock are problematic. Rugged topography complicates road-building in both the Coast Ranges and the Sierra Nevada. Rivers and floodplains complicate road-building in the Great Valley. For a long time it wasn't easy getting around at all, and in some places it is still difficult traveling.

The East Bay hills, the Diablo Range, and the Carquinez Strait, where the Sacramento River flows into the bay. These hills are underlain by the accretionary wedge deposits of the Franciscan Complex.

The Coast Ranges are the most complicated part of our journey. A major river system, the Sacramento-San Joaquin drains the waters of the Sierra Nevada and Great Valley at the delta and the Carquinez Straits. The accretionary wedge deposits, called the Franciscan Complex, contain many kinds of rock, and each rock produces a different kind of topography. Compare the pictures above of the different corners of San Francisco Bay.

The eastern margin of the Diablo Range in the Coast Ranges near Patterson. The parallel stripes of rock are the sedimentary rocks of the Great Valley Sequence, which were deposited in a forearc basin

The eastern part of the Coast Ranges show more organization. The linear strips seen in the picture above are the tilted sediments of the Great Valley Sequence. These are the rocks that underlie the floor of the Great Valley, sometimes to depths of five miles (8 kilometers). They were deposited in a forearc basin setting.

The San Joaquin portion of the Great Valley near Patterson and Modesto. The floodplain of the San Joaquin River is the gray streak across the middle of the photograph.

The floor of the Great Valley is one of the most altered landscapes on planet Earth. The soils that developed in the semiarid climate are rich with nutrients, and more than 500 kinds of crops, fruits and nuts are grown there. 95% of the landscape has been co-opted by agriculture. Irrigation makes it possible, so the rivers have been altered as well. Some no longer flow to the sea (although efforts are being made to change that). Vast amounts of water, equivalent to a large natural river, are pumped and carried south through the canals of the California Water Project. Much of it ends up in the Los Angeles basin.

The town of Colusa in the northern Sacramento Valley. The Sacramento River winds through the area.

We then reach the foothills of the Sierra Nevada. The Sierra is a huge westward tilted block of granite and metamorphic rock 400 miles long, and 50-60 miles wide. The mountains start gently enough, an almost imperceptible change of slope, but the bedrock is close to the surface so groundwater is not available for irrigation purposes. The prairie is mostly used as cattle range, and remains much as it was hundreds of years ago, aside from barbed wire fences and exotic European grasses that have crowded out the native bunchgrasses. Thousands of acres have recently been converted to almond groves with uncertain water sources.

The Sierra Nevada foothills near Sonora and Oakdale. Highway 108 crosses the middle of the photograph. The rocks are mostly composed of volcanic lahars (mudflows) of the Mehrten formation, dating to around 10 million years.

We finally reach the alpine landscape of the high Sierra Nevada. The rocks exposed here are the granitic plutons that once fed the volcanoes of the magmatic arc of the subduction zone. More than a hundred individual intrusions have been mapped, ranging in age from about 200 to 80 million years. Each of the intrusions probably fed a volcanic field miles above, but erosion has stripped away those miles of overlying rock. The remnants of the ancient subduction zone volcanoes can be seen as cobbles in the rocks of the Great Valley Sequence.

The high country of the Sierra Nevada at the headwaters of the San Joaquin River.

As has been mentioned before, the subduction zone is still active in Northern California. The active volcanic centers at Mt. Shasta and Lassen Peak still threaten the small towns in the region. Eruptions took place at Lassen in 1914-17, and at Shasta in 1786.

Mt. Shasta and the hummocky topography of a gigantic debris avalanche that spread northward from an ancient cone of Shasta about 350,000 years ago. Several active glaciers can be seen around the summit region of the 14,180 foot (4,,322 meters) high mountain.

We've now seen the aerial view of the main elements of our coming blog journey. I don't know yet the precise route we will be following, but I suspect it will begin in the Marin Headlands and cross the Golden Gate Bridge onto the San Francisco Peninsula. From the South Bay, we'll cross the interior Coast Ranges at Mt. Hamilton and Del Puerto Canyon. We'll cross the San Joaquin Valley, with stops on the floodplain of the San Joaquin River, and then make our way through the Sierra Nevada foothills and to the crest of the range. It's a drive that could be done in a day, but it's a route I would prefer to savor over several days.

Driving Through the Most Dangerous Plate Boundary in the World: A New Blog Series

Source: adapted from National Park Service and R. J. Lillie. 2005, Parks and Plates

Before I get accused of "cable-newsing/click-baiting" with my choice of a headline, I'll amend it to say "Driving through the most dangerous kind of plate boundary in the world".

Where in the world do we find the worst earthquakes, and many of the worst volcanic eruptions? Looking at maps of earthquake epicenters and volcanic eruptions, it doesn't take long to realize that there are specific zones where disasters and human misery occur. They follow oceanic trenches and their associated volcanic arcs (curving series of active volcanoes). Horrific events like the Sumatra earthquake of 2004, the 2011 Tohoku earthquake near Japan, and the 1991 eruption of Mt. Pinatubo in the Philippine Islands were the result of oceanic crust and upper mantle (the lithosphere) sliding beneath the adjacent continental or island landmasses. It is the sliding and grinding of the oceanic plate in these subduction zones that produces the huge earthquakes, and it is the complicated interaction of extreme heat and volatile materials in the mantle that leads to the formation of magmas and resulting volcanic eruptions.

Mt. Shasta and Little Glass Mountain, in the active part of California's subduction zone

Subduction zones are complicated places, and it can be difficult to study active systems. We may get highly accurate maps of the seafloor, and geophysical data may reveal the broad outlines of what lies beneath the bottom of the sea, but direct sampling and observation of the deep crust is mostly beyond our technology. So how to study and understand the dynamics of these zones of terror?

Maybe you have noticed that I sometimes say nice things about the geology of California. Once in a while, anyway. The state has such a rich variety of geologic and tectonic landscapes that one could spend a lifetime exploring them all (which for the record is what I am currently doing, although I am known to explore other places as well). As such, the state provides a nice outdoor laboratory for looking in the active parts of a convergent boundary, as well as a marvelous place to observe the deep interior of a fossil subduction zone complex.

California's Great Valley may seem to be a monotonous flat valley (clarification: it is a monotonous flat valley), but it hides a violent and complicated past.

Besides the trench, there are three structures within many subduction zone complexes: an accretionary wedge, a forearc basin, and a magmatic arc (see the diagram at the top of the post).

The accretionary wedge is a gathering place for the flotsam and jetsam of the seafloor and oceanic crust, as well as sediments from the continent. These trench deposits include a clay- rich sandstone called graywacke, dark colored shale, pillow basalt, deep-ocean chert, and the occasional volcano or coral reef. These rocks are carried deeper and deeper into the subduction zone, and are put under tremendous pressure. The rocks are churned up, faulted, and deformed into a chaotic mass called a mélange (from the French word for mixing).

A forearc basin forms in a relatively shallow sea between the crest of the accretionary wedge and the volcanic arc inland. Sediments, primarily sandstone, siltstone, and shale derived from the continent, accumulate to depths of tens of thousands of feet. This can happen in a shallow basin because the weight of the sediment pushes the crust downward, making room for more sediments. The foundation of the forearc basin is ocean crust rocks collectively called an ophiolite sequence.

A magmatic arc is a chain of volcanoes fed by magma generated when the subducting slab of oceanic crust reaches the semi-molten layer within the mantle called the asthenosphere (from the Latin "weak shell"). Water in the subducted slab serves as a catalyst to lower the melting points of the silica-rich minerals, causing the rock to melt and form plutons of magma that rise through the continental crust. If the magma reaches the surface and erupts, it may form andesite, dacite, or rhyolite lava. If it cools slowly deep in the crust, it will form a variety of granitic rock, such as actual granite, granodiorite, tonalite, quartz monzonite, or diorite.

Part of the Diablo Range, a subdivision of the Coast Ranges, from the summit of Mt. Hamilton, which houses the Lick Observatory complex.

California's complicated geological history includes a period of nearly 200 million years when the entire state was influenced by a subduction zone. Beginning about 29 million years ago, the subduction zone was progressively replaced by a transform boundary, a series of lateral faults known as the San Andreas fault system (yes, that San Andreas). The process is not yet complete, as the subduction zone still exists in the northern part of the state where it feeds the eruptions of Mt. Shasta and Lassen Peak. The remains of the ancient subduction complex now make up the Sierra Nevada, the Great Valley, and the Coast Ranges. One can conveniently explore this incredible complex in a car or on foot without the threat of magnitude 9 earthquakes, or catastrophic rhyolite caldera eruptions. Probably.

I hope you'll join me on this coming blog journey across California and through the guts of an ancient subduction zone. I got the seed of an idea for this series when I finally drove the winding road from San Jose to the Great Valley past Lick Observatory and down Del Puerto Canyon (I've been in Del Puerto many times, but never drove beyond the head of the canyon). I can't believe it took me this long to get around to it, but that's what happens sometimes.

This series is also meant to coincide with the long-awaited opening of our Great Valley Museum of Natural History, which opens to the public on April 4. Information is available on Facebook at  https://www.facebook.com/GreatValleyMuseum?pnref=story, and at http://www.mjc.edu/instruction/sme/gvm/. I hope to see you there!