Damning Del Puerto Canyon, a Geological and Natural Treasure in our County

I awoke this morning mildly astonished to see my own words making up the headline of a Modesto Bee article about a proposed dam in Del Puerto Canyon, a deep gorge cutting into the heart of the Diablo Range in the western part of Stanislaus County. Del Puerto is one of the most unique landscapes of California's Coast Ranges, and as I noted, a geological and natural treasure. I was hugely dismayed to find that a proposal exists to build a large reservoir in the lower canyon, and my email to a colleague ended up being quoted in the Modesto Bee article linked above (hence my surprise at being quoted; I wasn't directly interviewed). The article accurately describes my concerns about the project. There are large landslides in the lower canyon that canyon that would almost surely be reactivated (or accelerated; they show evidence of recent motion) if the base is inundated by lake water. There are definite seismic concerns, as a probable active fault lies just east of the dam site. But my biggest concern is the effect the dam will have on the natural environment of the canyon.

The environmental impact report was published recently (read it here.). Comments on the Environmental Impact Report can be made at a public meeting from 4 p.m. to 6 p.m. Jan. 15 at the Hammon Senior Center, 1033 W. Las Palmas Ave., in Patterson. Written comments will be accepted until Jan. 27 at Del Puerto Water District, 17840 Ward Ave., Patterson 95363. If you appreciate the intrinsic value of our precious local canyon, I hope you will comment and make your voice heard.

I've written often about Del Puerto Canyon over the years (see many of the articles here), and to give you a feel for the unique nature of the canyon, I'm adapting an article from last May.

The strange and alien landscape in upper Del Puerto Canyon.

California has some really strange landscapes. A state that has beaches, mountains, volcanoes, forests, and deserts is going to offer many perspectives of the complex geological influences on the state. But for alien and otherworldly, few places in the state can compare to the journey you take when you follow Del Puerto Canyon from its mouth in the Great Valley to the headwaters in the Diablo Range. It's a journey into the middle of the world.

"Del Puerto" refers to "The Gate", the constriction of hard sandstone at the mouth of the canyon. This will be the site of the proposed dam. It will be more than 200 feet high.

I guess I should be a bit more specific. We journey to rocks that had once been part of the Earth's mantle, the 1,800 mile thick layer that lies just beneath the thin crust (3-50 miles thick). We can't reach the core of the planet, because no one can (despite sci-fi movies that say otherwise). Since mantle rock is very hot and is subject to convection, it is at least conceivable that the rocks we are exploring have once been close to the Earth's core.

This is an active landslide that will be partially inundated by the proposed reservoir. I am concerned about the effect of adding water to the slip plane. California's first discovery of dinosaur bones was at the top of this slope.

So how does one explore the Earth's mantle? Well, first one has to get through the crust, and the thinnest crust is that which makes up the ocean floors. It's nominally composed of basalt, but the details are more complex.

In Del Puerto Canyon, the ocean floor is covered by...a bit of sediment. About 25,000 feet of it! The sediments poured off the mountainous edge of the continent during the later part of the dinosaur era, the Cretaceous Period. There was a huge subduction zone that formed as oceanic crust plunged into the mantle beneath the edge of the North American continent. This so-called Cascadia Subduction Zone caused volcanoes to form where the Sierra Nevada is today, but the area offshore of the volcanic arc, the forearc basin, collected sediments. As the sediments accumulated, they pressed the crust downward and even more sediment piled on top. Eventually the layers reached a thickness of five miles.

The basin collected fossils as well. There were the usual shells of clams, snails and ammonites, a variety of shark teeth, and three groups of seagoing reptiles, the plesiosaurs (think Loch Ness), ichthyosaurs (think reptilian version of a dolphin), and 35-foot-long mosasaurs (think "swim for your life!"). Even dinosaur fossils have been found. The first dinosaur ever found in California, a Saurolophus, was discovered in the lower reaches of Del Puerto Canyon in 1935.

Eventually, one will reach the base of the oldest sediments, and encounter the ocean crust itself. Faulting obscures some of the relationships, and so in the picture below we see some of the oldest sediment on the right (somewhat brownish shale) and basaltic/andesitic volcanic rock on the left (greenish gray), separated by a fault. The volcanic rocks are harder, and the canyon takes on a more rugged aspect as we climb higher into the mountains.

The Coast Ranges of California are one of the youngest mountain systems in the world, having been uplifted mostly in the last 3 million years or so. The streams in this dry environment have not been able to downcut as fast as the mountains are rising, so they flow much of the way over bedrock. There are few floodplains in these mountains.

The water flows almost year-round and thus the canyon is a critical habitat for all kinds of wildlife. Dozens of mammals and reptile species are known, and nearly 200 bird species have been observed here.

Oceanic crust is basaltic in composition, but there are differences at depth. On the ocean floor, basalt flows form "pillows", globular masses of the volcanic rock. Beneath the pillow basalts, basaltic dikes fed the eruptions. Dikes occur when volcanic rock fills cracks and fissures in the surrounding rock. Since the surrounding rock is also dike material, the entire layer, a mile or two thick, is made of vertical sheet dikes. Feeding these dikes were magma chambers composed of...basalt! But some of the basalt was left at the base of the oceanic crust where it then cooled slowly to form a sparkling crystalline rock called gabbro. The entire suite of rocks is called an ophiolite sequence. The Coast Range Ophiolite sequence in Del Puerto Canyon is considered to be the second best exposed in the state, behind the Point Sal Ophiolite in southern California.

There is a spot in one of the most rugged parts of the canyon to investigate the gabbro where it was pierced by a vein of quartz (below). People have looked for gold here, but I doubt they found any.

Just a few more miles up the canyon we penetrate the uppermost part of the mantle. The rock originally consisted of ultramafic minerals like olivine and pyroxene, but here the rock has been metamorphosed into serpentine, California's state rock. The rock was sheared and faulted on its way to the surface, leaving shiny green and black polished surfaces (below).

And then we are there. In the uppermost part of the canyon, we reach the netherworld of mantle rock that was far less altered, so it retained some of its original appearance. In places we can see olivine and pyroxene crystals, as well as grains of chromite. These ultramafic rocks contain few nutrients needed by plant life, so only a few species can tolerate living on these slopes. Gray pines are among them, grasses generally are not. There are a number of wildflower species endemic to California that can be found here.

Looking at these shattered broken rocks from very deep in the Earth, one imagines hell freezing over. The forges of the demons and devils lie frozen in place, to be slowly removed by earthly weathering. They try to invade the surface realm, but they are defeated by the forces of the heavens, the water and ice falling from the sky.

It may have been a metaphorical battlefield, but in the end there is great beauty in the rarity of the flowers, plants and animals that thrive, or at least tolerate the conditions in the upper canyon.

Del Puerto Canyon is traversed (slowly) by Highway 130, originating in Patterson on the floor of the Great Valley. It can also be reached by way of Mines Road out of Livermore, a winding road out of the San Jose area over Mt. Hamilton and the Lick Observatory complex. It is not a fast way to go!

I know that there is a need for water in the Central Valley. But no matter how many dams get built, there will never be enough to meet the expressed needs and desire of agribusiness. But I feel that we need to keep some of the wild places, and Del Puerto is one of those especially unique places to learn about our planet. I hope you will make your voice heard about this project.

What to do on a Saturday? Let's Go to the Middle of the Earth (via Del Puerto Canyon)!

The strange and alien landscape in upper Del Puerto Canyon.

California has some really strange landscapes. A state that has beaches, mountains, volcanoes, forests, and deserts is going to offer many perspectives of the complex geological influences on the state. But for alien and otherworldly, few places in the state can compare to the journey you take when you follow Del Puerto Canyon from its mouth in the Great Valley to the headwaters in the Diablo Range. It's a journey into the middle of the world. Our Geology Club made the trip a few weeks ago as an ending of the semester celebration. Odd way to celebrate? Did you simply party? We traveled half-way to the center of the planet!

"Del Puerto" refers to "The Gate", the constriction of hard sandstone at the mouth of the canyon.

I guess I should be a bit more specific. We journeyed to rocks that had once been part of the Earth's mantle, the 1,800 mile thick layer that lies just beneath the thin crust (3-50 miles thick). We couldn't reach the core of the planet, because no one can (despite sci-fi movies that say otherwise). Since mantle rock is very hot and is subject to convection, it is at least conceivable that the rocks we explored had once been close to the Earth's core.

So how does one explore the Earth's mantle? Well, first one has to get through the crust, and the thinnest crust is that which makes up the ocean floors. It's nominally composed of basalt, but the details are more complex.

In Del Puerto Canyon, the ocean floor is covered by...a bit of sediment. About 25,000 feet of it! The sediments poured off the mountainous edge of the continent during the later part of the dinosaur era, the Cretaceous Period. There was a huge subduction zone that formed as oceanic crust plunged into the mantle beneath the edge of the North American continent. This so-called Cascadia Subduction Zone caused volcanoes to form where the Sierra Nevada is today, but the area offshore of the volcanic arc, the forearc basin, collected sediments. As the sediments accumulated, they pressed the crust downward and even more sediment piled on top. Eventually the layers reached a thickness of five miles.

The basin collected fossils as well. There were the usual shells of clams, snails and ammonites, a variety of shark teeth, and three groups of seagoing reptiles, the plesiosaurs (think Loch Ness), ichthyosaurs (think reptilian version of a dolphin), and 35-foot-long mosasaurs (think "swim for your life!"). Even dinosaur fossils have been found. The first dinosaur ever found in California, a Saurolophus, was discovered in the lower reaches of Del Puerto Canyon in 1935.

Eventually, one will reach the base of the oldest sediments, and encounter the ocean crust itself. Faulting obscures some of the relationship, and so in the picture below we see some of the oldest sediment on the right (somewhat brownish shale) and basaltic/andesitic volcanic rock on the left (greenish gray), separated by a fault. The volcanic rocks are harder, and the canyon takes on a more rugged aspect as we climb higher into the mountains.

The Coast Ranges of California are one of the youngest mountain systems in the world, having been uplifted mostly in the last 3 million years or so. The streams in this dry environment have not been able to downcut as fast as the mountains are rising, so they flow much of the way over bedrock. There are few floodplains in these mountains.

The water flows almost year-round and thus the canyon is a critical habitat for all kinds of wildlife. Dozens of mammals and reptile species are known, and nearly 200 bird species have been observed here.

Oceanic crust is basaltic in composition, but there are differences at depth. On the ocean floor, basalt flows form "pillows", globular masses of the volcanic rock. Beneath the pillow basalts, basaltic dikes fed the eruptions. Dikes occur when volcanic rock fills cracks and fissures in the surrounding rock. Since the surrounding rock is also dike material, the entire layer, a mile or two thick, is made of vertical sheet dikes. Feeding these dikes were magma chambers composed of...basalt! But some of the basalt was left at the base of the oceanic crust where it then cooled slowly to form a sparkling crystalline rock called gabbro. The entire suite of rocks is called an ophiolite sequence. The Coast Range Ophiolite sequence in Del Puerto Canyon is considered to be the second best exposed in the state, behind the Point Sal Ophiolite in southern California.

We stopped in one of the most rugged parts of the canyon to investigate the gabbro where it was pierced by a vein of quartz (below). People have looked for gold here, but I doubt they found any.

Just a few more miles up the canyon and we penetrated the uppermost part of the mantle. The rock originally consisted of ultramafic minerals like olivine and pyroxene, but here the rock has been metamorphosed into serpentine, California's state rock. The rock was sheared and faulted on its way to the surface, leaving shiny green and black polished surfaces (below).

And then we were there. In the uppermost part of the canyon, we reached the netherworld of mantle rock that was far less altered, so that it retained some of its original appearance. In places we could see olivine and pyroxene crystals, as well as grains of chromite. These ultramafic rocks contain few nutrients needed by plant life, so only a few species can tolerate living on these slopes. Gray pines are among them, grasses generally are not. There are a number of wildflower species endemic to California that can be found here.

Looking at these shattered broken rocks from very deep in the Earth, one imagines hell freezing over. The forges of the demons and devils lie frozen in place, to be slowly removed by earthly weathering. They tried to invade the surface realm, but they were defeated by the forces of the heavens, the water and ice falling from the sky.

It may have been a metaphorical battlefield, but in the end there is great beauty in the rarity of the flowers, plants and animals that thrive, or at least tolerate the conditions in the upper canyon.

We turned around and headed back to more familiar habitats.

Del Puerto Canyon is traversed (slowly) by Highway 130, originating in Patterson on the floor of the
Great Valley. It can also be reached by way of Mines Road out of Livermore, or  a winding road out of the San Jose area over Mt. Hamilton and the Lick Observatory complex. It is not a fast way to go!

Unusual Unconformity on California's Lost Coast

Will someone save the unconformity? It's going to gone soon!

Well, okay, there's not much to be done about it, being that the exposure seen here is on the shoreline of one of the most violent storm-ridden coasts in California, and just a half mile or so from the northernmost land exposure of the San Andreas fault. It's at Shelter Cove on California's Lost Coast, one of the longest undeveloped stretches of coastline in the nation. Except for the small town of Shelter Cove, there is wilderness for a distance of about fifty miles, from Fort Bragg to Ferndale.

The underlying rock is part of the Franciscan Complex, a mixture of graywacke sandstone and shale that was deposited in the trench that once existed off the coast of California in Mesozoic and early Cenozoic time. The gray rocks were uplifted and eroded, and after a stretch of time, were covered by the tan-colored breccia or conglomerate. It was part of the wave-cut cliffs, but was isolated by a fluke of erosion. It's in the active wave zone, so it won't be long before it disappears. The wave-cut bench on which it sits may become a future unconformity if it is ever covered by sediment.

Driving Through the Most Dangerous Plate Boundary in the World: Exploring the Belly of the Beast in the Diablo Range

A long time ago (a few weeks; that's years in internet time) I began this series about driving through the most dangerous plate boundary in the world. Convergent boundaries with their subduction zones or gigantic thrust faults are the most dangerous boundaries that exist on the planet. We need no more reminder than the horrible tragedy that unfolded this week in Nepal.

California was once dominated by such a boundary, from about 200 million years to just a few tens of millions years ago. The subduction zone was replaced by a transform boundary known to most as the San Andreas fault. The San Andreas is dangerous in its own way, but the biggest quakes it causes pack only a thirtieth of the punch of a really big subduction zone quake (magnitude 8 compared to a magnitude 9+). The subduction zone still exists in the northern part of the state; it's called the Cascadia Subduction Zone.

The subduction complex includes three main features, an accretionary wedge, a forearc basin, and a magmatic arc. We have been traveling through parts of the accretionary wedge as we made our way through the Marin Headlands and Golden Gate. We've now entered another section of the wedge, the core of the Diablo Range. It's the domain of the Franciscan Complex, the "belly of the beast".

In the last post I likened the Diablo Range as being the structural equivalent of the alien baby popping out of John Hurt's chest in the movie Alien. Bear with me on this one. Material scraped from the ocean crust along with sediments washed in from the nearby continent accumulate in the trench, and get carried deep into the lithosphere where the rocks were subjected to intense pressure and elevated temperatures. The appearance and composition of the rocks was changed, and some of the minerals serve as proxies for estimating the depth the rocks reached: as much as 20 miles (32 kilometers)! The rocks churned upwards again, reaching close to the surface, but in the aftermath of the end of subduction the rocks were covered by the sediments of the Great Valley Group.

Then, as the pressures rose as a result of regional compression along bends in the San Andreas fault, the rocks of the Franciscan Complex were thrust upwards, piercing through the cover of Great Valley sediments, somewhat like the beast in Alien. They were exposed to erosion for the first time. And it is those rocks we traverse as we drive from Mt. Hamilton to the headwaters of Del Puerto Canyon.

After the narrow deserted road up the flank of Mt. Hamilton, the Lick Observatory complex felt positively urban.

The rocks themselves are unremarkable to look at, being composed mostly of a clay-rich sandstone called graywacke (a German term for "gray barren rock"), and black mudstone and shale. A few interesting chunks of rock are exposed here and there, red or green chert sequences and pillow basalts like those of the Marin Headlands, and some other blocks called "blueschist knockers" (please don't ask). The blueschist is a form of metamorphic basalt or other volcanic rock that has been altered under high pressure into a bluish-gray rock containing the minerals glaucophane and lawsonite.

The observatory complex is really a small village with dormitories and tourist parking.

The earliest geologists to study the Franciscan Complex had a real challenge on their hands. The rocks didn't make much sense. They didn't "follow the rules" of normal stratigraphy. They were intensely faulted and folded, sometimes upside down, and thus were difficult to map. The geologists had no known process in their knowledge base that could explain such rocks. It wasn't until the formulation of the theory of plate tectonics in the late 1960s that these rocks could be explained as the product of deformation and metamorphism in the accretionary wedge of a subduction zone.

The views from the summit of Mt. Hamilton are expansive, to say the least.

Although the rocks are similar in appearance, detailed mapping and chemical analysis allowed the researchers to distinguish several unique zones, or terranes, within the Franciscan, rocks that had formed together at a particular time. There are two of these sequences in the core the Diablo Range, the Eylar and Burnt Hills terranes. The Eylar is the older of two. It contains some fragments of the volcanoes that once existed at the summit of the ancestral Sierra Nevada. The Burnt Hills terrane includes bits of granite, indicating that the ancestral Sierra had undergone significant erosion by late Cretaceous time.

Some of the Manzanita bushes near the summit were more like small trees

San Antonio is a peaceful valley owned by some ranch outfit called "No Trespassing". His signs were everywhere.

So, the thing is, I've been living here in the Great Valley for 25 years, and I've been up Del Puerto Canyon dozens of times, but until last month I had never traveled State Route 130 to Lick Observatory and San Antonio Valley. I finally did so, and I took the pictures gathered together in this post. There aren't all that many rocks in the pictures, because graywacke (the German "gray barren rock") is gray. And barren. The chaparral and oak woodlands were far more interesting, at least on a macro-scale. In a petrographic microscope, all kinds of interesting things have happened to these rocks due to their deep burial.

An American Wigeon in a stock pond in San Antonio Valley

After all the tight curves and winding roads on the ridge of Mt. Hamilton, San Antonio Valley was a refreshing change with straight stretches of pavement and wide open oak woodlands and meadows. It was late March and the spring wildflowers were blooming, though maybe not for long in this drought year.

And then an abrupt transformation. The gentle landscape of San Antonio Valley is rimmed in on the east by the ridge of Red Mountain, and the bleached tailings piles from the old mercury mines indicated a major change in the rock structure. We are crossing the boundary between the rocks of the Franciscan Complex and the base of the oceanic crust and upper mantle. Del Puerto Canyon, lying a short distance east over a low pass, cuts a deep swath through the Coast Range Ophiolite and the overlying Great Valley Sequence. We'll be talking about that region in the next post.

Mercury was mined from rocks of the Earth's mantle in the Red Mountain Mining District.

Driving Through the Most Dangerous Plate Boundary in the World: Terra Fatale on the Marin Headlands

In the old film noir movies, the femme fatale was a staple character, an attractive and seductive woman, especially one who brought disaster to any man who became involved with her. To be fair, there were plenty homme fatale characters in movies over the years too. But how many movies have a terra fatale character, a seductive and beautiful geographical region that brought disaster on those who chose unwisely to visit or settle in? A few quickly come to mind, Pompeii and Vesuvius, for instance (although Pompeii was not the greatest movie ever made), and Southern California (Volcano, Earthquake, 2012). Some lands are simply more hazardous than others.

The lands near a subduction zone are the most dangerous on Earth. To live along a convergent boundary, in the zone of the magmatic arc, or on an island of an accretionary wedge is courting disaster. The worst earthquakes, and some of the most violent volcanic eruptions occur near such boundaries. We've been slowly conducting a journey across an ancient subduction zone, one that is no longer active. It's exposed in the Coast Ranges, Great Valley, and Sierra Nevada of California, where the rocks from the heart of the zone have been exposed by erosion. The rocks have been severely changed by heat and burial deep in the crust of the Earth, and now provide valuable information to geologists. In our last two posts we took a look at some of the strange rocks found there, the chert and pillow basalt of the Franciscan Complex. Today we are looking at some of that seductive scenery, the terra fatale.

The subduction zone is no longer active, but this isn't to say the land isn't still dangerous. The San Andreas fault lurks just offshore, and the steep terrain invites landsliding and severe shoreline erosion. The western side of the Marin Headlands has never been developed to the extent of the more sheltered eastern side along San Francisco Bay. But in the extreme ruggedness, we can find great beauty. A system of roads and trail explore this spectacular landscape. Conzelman Road winds across the slopes above the Golden Gate, offering a view of the Point Bonita Lighthouse (top picture). The road ends at Rodeo Beach, a small lagoon that has been blocked off from the open sea by a baymouth bar. The color of the sand looks off to those who are used to white sand beaches in such places as Florida. A closer look reveals why: the sand has very few clear quartz grains. The grains are mostly composed of the chert eroded from nearby cliffs.

Another road winds across the ridge between Rodeo Beach and Point Bonita, providing stunning views of the steep coastal cliffs, Rodeo Beach, and the lighthouse at Point Bonita. Bird Island, seen in the second picture of the post, and on the far left side of the picture below, is a resting and roosting site for Cormorants and Brown Pelicans.

Although the Marin Headlands are protected as a natural area under the administration of the Golden Gate National Recreation Area, there are signs and hints of intense human use. As mentioned previous posts, numerous cannons and gun emplacements dotted the cliffs, especially during World War II. Incredibly, some 2,000 acres were sold in the 1960s for housing developments that would have housed 30,000 people. Thankfully, the project failed, and the land was preserved as parkland.

The Point Bonita Lighthouse has stood on this rocky bluff for 160 years guiding ships into San Francisco Bay. A lot of ships missed. Around 300 ships have run aground over the years, including the wreak of the steamship City of Rio de Janeiro in 1901, with a loss of 128 lives.

With the end of this brief exploration of the Marin Headlands, we now cross the Golden Gate Bridge, passing through San Francisco, and driving through the urban center of San Jose. We're ready to take on the interior of the accretionary wedge, by crossing the Coast Ranges at Mt. Hamilton and Del Puerto Canyon. That will be in the next post...

Driving Through the Most Dangerous Plate Boundary in the World: Welcome to Geology's Junk Drawer

View from Muir Beach Overlook towards the Marin Headlands

I'm pretty sure every household has a junk drawer. It's that place where objects that we can't or won't get rid of accumulate. It's a mixed up drawer full of screws, paperclips, old receipts, coupons, expired credit cards, coins...they all end up collected in that one place. And even in the most organized of households, this is the one place where chaos reigns supreme. There is a similar kind of place in the Earth's crust.

One of the great intellectual leaps in the science of geology was the recognition that the Earth is organized, that there are geological processes that follow certain natural laws. Principles like superposition, lateral continuity and original horizontality allowed the early geologists to decipher the geologic history of a region, leading to the first "golden age" of the science in the early 1800s. From the careful analysis of the stratigraphic relationships in a given region, earth scientists were able to construct the geologic time scale, and solve some hugely mysterious problems about how the crust of the Earth operates. They were finally able to understand the origins of most kinds of rocks.

Melange deposits along Highway 1 just north of the Marin Headlands

But then geologists came to California, and all the rules seemed to get tossed out the window. While some rock sequences in the state followed the "rules" and formed decipherable sequences (in the Basin and Range Province, for instance), in other places the rocks made no sense at all. In parts of California's Coast Ranges, sequences of rocks were exposed that were upside down, and so chopped up by intense faulting that the layers couldn't be followed for more than a few hundred yards. The continuation of these layers couldn't be found. Rocks showed evidence of extreme depth of burial, 15 or 20 miles, and yet showed little evidence of having been heated to any great extent (the Earth is hot enough in many parts of the crust to melt rocks at much lesser depths).

Most of the time, geologic mapmakers define "formations" and "groups", well-demarcated strata that can be mapped, and be clearly distinguished from adjacent layers. They construct geologic maps of these rocks. But faced with the strange rocks of the Coast Ranges, they more or less threw up their hands and called these chaotic rocks the Franciscan Complex. And "complex" was right. There was an entire region on the geologic map of California given over to a disorganized mess of rocks that could barely be subdivided any further (it's the blue stuff on the map below). The rocks found in the complex include a clay-rich sandstone called graywacke, dark colored shale, pillow basalt, serpentine, deep-ocean chert, and the occasional volcano or coral reef.

Geologists had discovered geology's junk drawer.

It wasn't until the theory of plate tectonics was developed (and accepted by the geologic community) in the 1960s that the origin of the Franciscan Complex could be at least partly understood. The outer shell of the Earth called the lithosphere (composed of the the continental and oceanic crust as well as the uppermost part of the mantle), slides slowly over the nearly molten layer in the mantle called the asthenosphere. In some places, the lithosphere splits open, producing volcanic eruptions and new oceanic crusts. These are divergent boundaries. At convergent boundaries, the crust comes together and is destroyed, either by crumpling upwards into high mountain ranges, or by one plate sliding beneath the other at subduction zones. At transform boundaries, the plates of the lithosphere slide past each other. The subduction zone is the site of the largest earthquakes and most intense volcanic eruptions on the planet. They are the most dangerous plate boundaries in the world. Luckily, the subduction zone in central California is extinct and erosion has exposed its interior parts. That's why we are on this driving journey through the old subduction zone.

I've spent the first part of this blog series on the other side of California's transform boundary, the San Andreas fault. Driving south from Stinson Beach, we now reach the rocks of the Franciscan Complex on the Marin Headlands.

Source: http://3dparks.wr.usgs.gov/goga/html/geologic_maps.html

The trenches of subduction zones are the collecting place for sediments of the deep ocean as the crust slides into the mantle. Volcanoes and associated coral reefs are sometimes scraped off as well. Sandstone, silt and shale eroded from the nearby continent will also accumulate in the trench. The large chaotic wedge-shaped deposit is called an accretionary wedge.  The Marin Headlands include some of the less common rocks of the Franciscan Complex, ribbon chert and pillow basalt (green and orange on the map above).

NPS geologist William Elder discussing pillow basalts of the Marin Headlands near Point Bonita

The pillow basalts of the Marin Headlands formed at a divergent boundary around 200 million years ago, perhaps more than a thousand miles away, close to the equator. Lava erupting onto the seafloor produces the distinctive lobate shapes that are about the size of down pillows (but are considerably harder!).

Ribbon chert of the Franciscan Complex on Conzelman Road on the Marin Headlands

As this ocean crust moved on its long journey towards California, small single-celled creatures called radiolarians lived and died in the ocean waters above. As they died, their silica shells sank to the seafloor, slowly accumulating in dense layers of the quartz-rich rock called chert. They formed into distinctive layers about an inch thick, separated by thin layers of clay. The layering was about the thickness of ribbon, giving rise to the name "ribbon chert".

We'll have an exploration of the Marin Headlands in the next post...