Driving Through the Most Dangerous Plate Boundary in the World: Looking for the "Big One"

Tomales Bay just might be the most contradictory places in all of California. On the one hand, it is one of the most peaceful and serene places in the state, a largely undeveloped fifteen-mile long bay, protected from the worst of Pacific storms and waves by Inverness Ridge. On the other, it hides the trace of the San Andreas fault, and California's worst earthquake ripped through the bay in 1906.

We are just getting started on our journey across the most dangerous plate boundary in the world, but our journey is an exploration of a fossil convergent boundary, not an active one. But that doesn't mean it's not dangerous. A Mesozoic convergent boundary was present off of California for well over 100 million years, producing hundreds of thousands of magnitude nine earthquakes and tsunamis. Thousands of volcanoes would have erupted along the inland arc. But the subduction zone responsible for all the mayhem was replaced in late Cenozoic time by a transform boundary, the San Andreas fault system. The San Andreas has been responsible for plenty of mayhem itself, but the magnitude eight earthquakes that occur here are only one-thirtieth of the power of a nine.

It may seem like a large bay like Tomales would have been a major harbor and port along the California coast, but the waterway is mostly very shallow, and the mouth of the bay is subject to heavy wave activity and hidden shifting shoals. Getting in and out of the bay can be very hazardous (according to the state, in one year alone 13 boaters lost their lives there).

Once in the bay, the water is generally calm and is popular with small boaters, kayakers and fishing enthusiasts. A few small villages are present, including Olema, Inverness, Point Reyes Station and Marshall. Mostly the bay is undeveloped, and is a unique environment compared to the rest of the California coast. The calm water in the interior bay is the exception rather than the rule, and is an important natural habitat for a large number of species. A fair portion of the bay is protected as Tomales Bay State Park and much of the west shore is part of Point Reyes National Seashore. Several parcels along the eastern shore are part of the Golden Gate National Recreation Area.

Looking at Inverness Ridge from the east shore of Tomales Bay. Photo by Mrs. Geotripper.

Rocks within the San Andreas and related fault zones (the San Gregorio and Golden Gate faults in this instance) are highly sheared and ground up so they are easily eroded. Linear valleys will often form along fault zones, and such was the case here near Point Reyes. The valley extended from Bolinas Lagoon and Olema to the northern end of Tomales Bay, but when the last ice age ended around 12,000 years ago, sea level rose and flooded much of the stream floor. That was the origin of Tomales Bay. The rocks west of the fault, as noted in the last post, are composed of granitic and metamorphic rock related more to the Sierra Nevada and Mojave Desert than the North Coast. They have been carried north hundreds of miles by the lateral movement along the San Andreas fault.

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

The recurrence interval of massive earthquakes on the northern segment of the San Andreas fault is not exactly clear. Besides the well-known 7.8 magnitude tragedy in 1906, there is evidence of other major seismic events in 1522, 1686 and either 1748 or 1838, suggesting a return of giant quakes roughly every 125 years or so. Other studies have suggested longer intervals, and no one is suggesting that they occur at regular intervals. There is even some suggestion that major events are related to large seismic events (magnitude 8.5-9.0) on the Cascadia subduction zone off to the north. Long story short, the "Big One" in Northern California may happen tomorrow, or it could happen 300 years from now. And large quakes are possible on other faults in Northern California, especially the San Gregorio and Hayward fault zones. And the San Andreas fault in Southern California is on an entirely different rupture schedule. If you are going to live here on the west coast, the watchwords are "be prepared".

Bolinas Lagoon (and pinnipeds)

As we drive south from Bear Valley and the headquarters for Point Reyes National Seashore, we are following the trace of the San Andreas fault. In 1906, the fault rupture shifted the ground here as much as twenty feet, the largest such offsets recorded. After a few miles we reach the north end of Bolinas Bay.

The waves coming into Bolinas Bay tend to refract as they encounter Duxbury Point and swing around so they carry sand northwest. This has caused the formation of a sand spit that closes off Bolinas Lagoon, forming one of the nicer tourist beaches in the San Francisco Bay Area, Stinson Beach. Three different faults cross this sand bar, the San Andreas, the San Gregorio, and the Golden Gate. I wonder if this kind of information is reported on deeds of sale for the string of homes on the sand bar.

We continue our journey southward along the coast on Highway 1 as it winds its way along the cliffs beyond Stinson Beach. The road climbs quickly onto the rugged slopes of the peninsula called the Marin Headlands. We have crossed the San Andreas fault, and are now on the North American Plate. The rocks here are very different, as we have reached the "most dangerous plate boundary in the world" (past tense). We will explore further in the next post.

Stinson Beach, Bolinas Lagoon, and the Point Reyes Peninsula.

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.

Where the Sierra Nevada Rises From the Sea: Tomales Bay and Bolinas Lagoon

Let's cross back onto North America for a short bit. In this off and on series about the most beautiful coastline in the world we have been jumping back and forth between the North American and Pacific plates as we followed the transform boundary called the San Andreas fault. In the last post on the Point Reyes Headland we were standing on the granitic rocks of the Sierra Nevada which have been displaced northward at least 200 miles along California's iconic fault line.

Source: http://pubs.usgs.gov/of/2005/1127/

We pick up our journey northward along the coast on Highway 1 as it winds its way along the cliffs beyond Muir Beach (and don't miss Muir Woods National Monument if you are ever in the region). We are on the east side of the San Andreas fault, meaning we are on the North American Plate (the fault is just offshore).

The rocks east of the fault are a melange of many different kinds of rock that accumulated in the trench deposits (the accretionary wedge) of the vast subduction zone that controlled the tectonic development of California for upwards of 200 million years. There are sequences of graywacke sandstone, dark shale, limestone, chert, and volcanic rocks that were jumbled and churned as the Pacific Plate sank into the mantle beneath the edge of the North American Plate. The earliest geologists who attempted to map these rocks, which today are called the Franciscan Complex, were generally stymied in their efforts because the rocks refused to follow the familiar "rules" of stratigraphy, like superposition or lateral continuity.The mess of rocks only makes sense when you realize they're supposed to be a mess (below).

The highway descends the cliffs and Bolinas Lagoon comes into view. Rocks within the San Andreas and related fault zones (the San Gregorio and Golden Gate faults in this instance) are highly sheared and ground up so they are easily eroded. Linear valleys will often form along fault zones, and such was the case here near Point Reyes. The valley extended from Bolinas Lagoon to the northern end of Tomales Bay, but when the last ice age ended around 12,000 years ago, sea level rose and flooded much of the stream floor. This is the origin of Bolinas Lagoon and Tomales Bay.

Waves in this region come mostly out of northwest, causing the majority of beaches in the area to transport sand southward. The waves coming into Bolinas Bay tend to refract as they encounter Duxbury Point and swing around so they carry sand northwest. This has caused the formation of a sand spit that closes off Bolinas Lagoon, forming one of the nicer tourist beaches in the San Francisco Bay Area, Stinson Beach.

The lagoon is shallow and has developed numerous mudflats that are exposed at low tide (and which make for a nice sheltered refuge for the seals and sea lions in the region). Three major faults cross the lagoon (as noted above, they are the San Andreas, San Gregorio, and Golden Gate), which must cause at least some consternation for the dozens of homeowners who live on the sandy spit.

A drive a few miles north through Olema Valley brings the traveler to Tomales Bay, the other drowned river valley that has developed along the San Andreas fault. The bay might have been a major harbor and port along the California coast except that the 12 mile long estuary is mostly very shallow, and the shoals and sneaker waves at the mouth of the bay are extremely hazardous to boaters (according to the state, in one year alone 13 boaters lost their lives there).

The rest of the bay is generally calm and is popular with small boaters, kayakers and fishing enthusiasts. Only a few small villages are present, including Olema, Inverness, Point Reyes Station and Marshall. Mostly the bay is undeveloped, and is a unique environment compared to the rest of the California coast. The calm water in the interior bay is the exception rather than the rule, and is an important natural habitat for a large number of species. A fair portion of the bay is protected as Tomales Bay State Park and much of the west shore is part of Point Reyes National Seashore. Several parcels along the eastern shore are part of the Golden Gate National Recreation Area.

We've nearly completed our off and on again web series on the "granite coast" of California, the place where the Sierra Nevada rises from the sea. The final post will be a visit to Bodega Bay...and..."The Birds"...

Where the Sierra Nevada Rises from the Sea: Point Reyes National Seashore

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

Around 30 million years ago major parts of Central California were in Southern California. It's about that "simple". Around 29 million years ago a major restructuring of plate boundaries was taking place along the Pacific Coast. A subduction zone (convergent boundary) that had influenced the California margin for nearly 200 million years, forming the granitic rocks of the Sierra, was changing over to something new. It was transforming, so to speak.

For many millions of years, oceanic crust and exotic terranes were carried eastward towards the continent, only to be forced beneath the edge of North America (subducted). Eventually the source of oceanic crust, the East Pacific spreading center, made contact with the convergent margin and subduction ended. Because of the relative motion of the two plate edges, a new fault system developed, the San Andreas, and a portion of Southern California began to move northwestward along the new fault. Such zones that divide crustal plates from one another are called transform boundaries. Granitic rock that had formed as part of the Sierra Nevada Batholith was sliced off from the south end of the province and was carried north several hundred miles (see below). The displaced granitic rocks are referred to as the Salinian Block.

The Point Reyes Peninsula is one of the more intriguing corners of California. It's connected to the California Coast Ranges after a fashion but it is a world apart. Tomales Bay, Olema Valley, and Bolinas Lagoon mark the location of the San Andreas fault. The epicenter of the infamous 1906 San Francisco Earthquake was close to Point Reyes, and offsets totaled close to 20 feet during the magnitude 7.8 seismic event.

The peninsula is protected from development as Point Reyes National Seashore, and is a haven for wildlife. During my visit several years back I captured my best shot ever of a California Quail. On our way to the beaches, we saw a herd of Tule Elk.

The Peninsula has several distinct geological sections. The mountainous terrain of Inverness Ridge is composed mostly of the granite and metamorphic rocks of the Salinian Block. Much of the western and southern parts expose sedimentary rocks of Miocene and Pliocene age. During the journey northwest from Southern California, the peninsula was submerged beneath the ocean waves. Active dune fields can be found along the extensive beaches of the peninsula.

Drake's Estero is an interesting feature. The branching arms of the bay suggest that it is a submerged river valley. The shallow water has proven to be a good spot for oyster farming, and the 80 year old business is embroiled in a battle with the park service over land-use issues.

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

From the headland of Point Reyes, Point Reyes beach stretches northward for around 10 miles, making it one of the longest stretches of sandy beach in Central or Northern California. As popular as it might have been, the fierce waves and very cold water make for uncomfortable water play.

The rocky headland where the lighthouse is located is composed of granitic rock and some of the overlying late Cenozoic sedimentary rocks. The lighthouse is visible from as far as 24 miles away, but fog obscures the view for around 140 days a year. Winds are nasty (highest recorded was 133 mph), and the temperature generally hovers between 50 and 60 degrees Fahrenheit all year.

Reaching the lighthouse requires descending 300 steps, but during the rare clear days, the spot is one of the best in California for whale-watching. Expect crowds on the nice days...

Steps leading to the Point Reyes Lighthouse

In the next post, we'll explore the more inland part of Point Reyes National Seashore, mainly Tomales Bay and Bolinas Lagoon.

Point Reyes Lighthouse, constructed in 1870