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 We've been slowly conducting a journey across an ancient subduction zone, one that is no longer active.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The subduction zone is no longer active, but this isn't to say the land isn't still dangerous.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: Geology's Junk Drawer on the Marin Headlands

We continue our drive through the most dangerous plate boundary in the world (past tense, since it became inactive 20 million-plus years ago), and we've arrived at the Marin Headlands, one of the most beautiful coastlines in the world, and a marvelous exposure of what I've called geology's junk drawer, the accretionary wedge of a subduction zone. The rocks of the wedge are called the  Franciscan Complex, and it's appropriate, given the chaotic nature of the rocks

We are taking a little detour from our drive from Point Reyes, and tooling about the headlands on Conzelman Drive. But even before that, I didn't want to miss a gem of a national monument tucked away in the depths of the headlands, Muir Woods.

Muir Woods preserves one of the small remnants of old-growth Redwood forest left in California. More than 95% of the original forests have been cut down, and the northern Coast Ranges have been deeply altered, and the ecosystems shredded. The native range of the Coast Redwoods and the extent of the Franciscan Complex correspond closely, but I don't think the relationship is necessarily biological. The trees need the climate afforded by the proximity of the Pacific Ocean, and the Franciscan Complex is a young rock unit that formed parallel to the same coastline.

As pointed out in the previous post of the series, the Marin Headlands are somewhat unique, having a higher than usual percentage of sea-floor ribbon chert exposures and the underlying pillow basalts of the ocean crust. As we start up the Conzelman Road, we get some marvelous views of the Golden Gate and the orange bridge that crosses it (see the top photo). In the parking areas people use to stare at the bridge, there are marvelous exposures of the chert and basalt.

The chert and basalt have been metamorphosed to some extent and deeply deformed, but the rocks still show much of their original nature. In some of the roadcuts, the chert can be seen lying directly on the sea-floor basal. Geologists have figured out that the sequence preserves about 100 million years of deposition (from 200 to 100 million years ago). This is an extraordinarily long record of slow travel of the plate across the Pacific Ocean.

The scenery is extraordinary along the road. The Golden Gate rises out of the sea in bold cliffs, and though the headlands have been altered by road-building and military activity, the region today exudes beauty and serenity.
 
Wait, what? Military activity on these beautiful cliffs? Why?

When World War II was being fought, San Francisco Bay was one of the most important strategic strongholds for the entire Pacific Theater of operations. There was the harbor itself through which much of the fleet operated, factories and ammunition depots, and the inland agricultural areas. That and a huge civilian population. The hills above the Golden Gate were studded with huge cannons and guns that could wreak devastation on any enemy flotilla that tried to enter the harbor. Below is Battery Mendell, just north of the Point Bonita Lighthouse.

The Marin Headlands region is a stunningly beautiful place with a fascinating geological story. The area is protected as part of the Golden Gate National Recreation Area, which is more or less equivalent to a national park. It is certainly a popular destination, but it's usually possible to find some quiet corners, especially if you are willing to hike a little. In our next post, we'll head down to the beaches of the Marin.

Some Spring Wildflowers from the Coast Ranges, and a mini-quiz

Lindley was apparently a busy botanist. Being the non-botanist I am, I was trying to identify some of the spring wildflowers I photographed last week in Del Puerto Canyon in California's Diablo Range. I tracked down two of them, and both were named for Lindley. I don't know if it was for the same person, but I do see on the Google that there was a John Lindley who was an English botanist in the 1800s.

But first there are the thistles we saw. I know that some of the thistles are invasive weeds, and in fact the star thistle is devastating pastures all over the state. And killing horses in the process. I don't know if the other thistles are natural or invaders. Hopefully my botanist readers can clue us in.

 Still, the thistles are a bright addition to the landscape.

Then there is the first Lindley, the Lindley's Blazing Star (Mentzelia lindleyi). From a distance, I tend to mistake them for poppies, but there is no mistaking the difference up close. This is one of the wildflowers that is tolerant of serpentine soils.

 The other Lindley is not actually a flower, but a seed pod. It is the dandelion-like Silver Puff (Uropappus lindleyi).  The actual flower is much like a dandelion too.

The next flower is not a Lindley, but it is very strange looking. It is apparently called the Chia (Salvia columbariae). The seeds of this flower were an important component of the diet of a number of Native American groups in California and the southwest, and also had medicinal uses. It also appears to be a serpentine-tolerant species.

Then there are the two that I couldn't identify after a brief search. There is the purple flower below...

 ...and what appears to me to be some kind of Monkey Flower, but as in all things botanical, I am open to correction!

I took three different trips to three very different localities in the last couple of weeks. The first was a spur of the moment exploration of the eastern Great Valley and lower Sierra Nevada foothills near New Hogan Reservoir and Valley Springs, followed by a pair of laboratory field trips to Del Puerto Canyon in the Coast Ranges where I took these wildflower pictures. Then, on Saturday, it was another field trip to Yosemite Valley, where we saw Half El Capitan, Half Cathedral Rocks, Half Yosemite Falls, and No Dome. Check out the different kinds of scenery and rocks...

The Great Valley with Mount Diablo in the distance

The lower reaches of Del Puerto Canyon, showing thick layers of the Great Valley Group,
and a pretty incredible landslide (slump and earthflow)

Some tightly folded radiolarian cherts in the middle stretches of Del Puerto Canyon

Olivine peridotite from the upper end of Del Puerto Canyon

An overcast day in one of the best places in the world to see granitic rock exposed in all its splendor

It's hard to imagine any geological region more different than these three places. The utterly flat and mostly featureless Great Valley, the soaring granite cliffs of Yosemite Valley and the High Sierra Nevada, and the (usually) muted slopes of the Coast Ranges, in this instance the Diablo Range. And yet there is a single geologic entity that joins all of them together.

This isn't exactly a mystery to be solved by the readers, more a lead-in to the next post, but you are welcome to comment if you can answer what exactly it is that links these disparate features of California's landscape.

Photo of the Day - Feeling Crushed?

I took my physical geology students geotripping this week instead of their lab sections...Del Puerto Canyon cuts a deep swath through the eastern Diablo Range of Coast Ranges of California. In this single gorge, one can pass through a 6 kilometer thick section of the Great Valley Group (deposited within a forearc basin), a thick and nearly complete ophiolite sequence and underlying mantle peridotite (the Coast Range ophiolite), and a section of the Franciscan Complex (part of an accretionary wedge complex). How many places are there in the world where one can drive right through the crust into a subduction zone, and into the underlying mantle? The picture above is a section of highly deformed cherts of the Franciscan Complex.

Somewhere, Miss Frizzle and her magic school bus must be jealous....