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 We've been slowly conducting a journey across an ancient subduction zone, one that is no longer active.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The subduction zone is no longer active, but this isn't to say the land isn't still dangerous.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

On the Most Inaccessible Mountain Range on the Planet: Walking the Mid-Ocean Ridge in the Marin Headlands

What is the most inaccessible mountain range on planet Earth? The Himalaya? The southern Andes? Some mountain range in the Antarctic wastes? Nah. The most inaccessible mountain range is so difficult to visit that only a handful of humans have ever even laid eyes on it. It lies at the bottom of the sea, and is called the Mid-ocean ridge. It dwarfs the other major mountain ranges around the planet, extending for around 45,000 miles, enough to nearly circle the planet twice.

The Mid-ocean Ridge is not a high mountain range, maybe 5,000-10,000 feet high, but it lies thousands of feet beneath sea level except in extraordinary places like Iceland. Iceland is part of the Mid-ocean Ridge, but probably also includes magmas rising from a hot spot, giving it the extra elevation to rise above the waves.

So mountaineering on the Mid-ocean Ridge is beyond most mortal folk, but you can explore the surface of the ridge in a few places around the world where oceanic crust has been scraped off the sea-floor and accreted to the edge of a continent. One such place is on the Marin Headlands just north of San Francisco (I noticed in the geoblogosphere that Chuck Bailey at W&M Blogs has recently described another such spot in the somewhat more exotic Wadi Jezzi in Oman - Check it out!).

Some of the most spectacular scenery along the most beautiful coastline in the world can be found on the Marin Headlands (this post is actually a continuation of a blog series I started before the Christmas holidays). When you cross the Golden Gate Bridge headed north, take the Alexander Avenue offramp and follow Conzelman Road onto the headlands. Immediately you are treated with a gorgeous view of the Golden Gate Bridge, but the real story is behind you in the roadcuts. There are ribbon cherts (a form of deep ocean sediment derived from myriads of one-celled creatures called diatoms) lying directly on ocean-floor basalt. Lay your hands on the contact between the two and you are touching the surface of what once was part of the greatest mountain range on planet Earth!

The Marin Headlands don't lack for inaccessibility themselves. The coastline through this region is exceptionally rugged, with steep cliffs that slope directly into the surf. San Francisco Bay lies hidden behind the peninsula, and the ruggedness of the region prevented its discovery for many years (nearby Monterey Bay was visited as early as 1542, but San Francisco Bay was not discovered until 1769).

The odd looking cement structures found around the Marin Headlands are kind of a jarring reminder that the history of this region is not that of long-protected wilderness sanctuary. 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 a level of protection that 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. While in the area, don't pass up Muir Woods National Monument. It's one of the most-visited monuments in the country, but wandering through the Redwood forest, it's not hard to see why.