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.