Travels in Cascadia: Sitting Woman Falls. She's Sitting on Geological Pillows.

There are so many charming little corners to be found on Vancouver Island. We were well underway on our two-week field course on the geology and anthropology of British Columbia, and on this day we had already explored the Sooke Potholes and the base of the ocean crust at East Sooke Park (along with a couple of interesting petroglyphs). It was getting late in the afternoon and the crew was getting pretty tired, but I had heard that there was another site of interest at the end of a short trail, so we made one more stop. It was a place called Witty's Lagoon, which sounds like a theme park or something, but it actually is a geologically interesting section of coastline along the Salish Sea.

This area (and indeed all of Vancouver Island) was covered by glacial ice as recently as 13,000 years ago. When the ice melted, sea level rose to cover some of the previously exposed lands forming a series of bays and coves. Witty's Lagoon is a nice example of one of these, and it is largely unaffected by urban development. Metchosin Creek flows over a ledge of basalt to form Sitting Woman Falls at the upper end of the cove (above).

The basalt, part of what is called the Crescent Terrane, has an interesting story to tell. The basalt erupted on the floor of the Pacific Ocean around 50 million years ago at a divergent plate boundary. It originally melted because of the release of pressure at the mid-ocean ridge, and accumulated in plutons several miles beneath the ocean floor. As the crust spread apart, fractures formed and the basalt followed the breaks all the way up to the sea floor where it erupted out. When basalt erupts in water, it forms odd looking lumps about the size of old-fashioned down pillows, around two or three feet across. These pillows accumulated in layers hundreds of feet deep.

The sequence of gabbro plutons, sheet dikes (the filled fractures), and pillow basalts constitute an ophiolite sequence that in this area is called the Metchosin Igneous Complex. In our previous post we had a chance to see the gabbro as it was exposed on the shoreline of East Sooke and Becher Bay. We didn't get to see any good examples of the dikes, but the cliff at Sitting Woman Falls was composed of pillow basalts. I've zeroed in on the section of the cliff to the right of the falls in the picture above. The pillows are not really well exposed, so I've cheated by adding some pictures below of pillow basalts that we've seen elsewhere in Washington and California.

The picture above shows pillows exposed in the cliffs at Cape Disappointment at the mouth of the Columbia River in Washington. Below we can see some pillow basalts exposed near Nicasio on the Marin Headlands in California.

It's incredible to think of the forces involved in taking the oceanic crust from the bottom of the sea, and mashing it into the edge of the North American continent where it ended up being exposed at Witty's Lagoon, and indeed throughout the region, including the high peaks of the Olympic Peninsula across the Strait of Juan de Fuca.

The Disappointing Cape: Land's End and Rock Pillows

Just over 200 years ago, the first expedition of Americans reached the Pacific Ocean after following the Missouri River over the Rocky Mountains and down the Columbia River. Lewis and Clark and their crew arrived in November of 1805, and they knew they could not make the return trip until the winter snows abated.

They explored the area around the mouth of the Columbia for an decent campsite to stay in several months. They ultimately settled on the south side of the river at Fort Clatsop, but for a time they spent a week at "that dismal little nitch" on the north side of the river. They explored around the rocky peninsula that had already been given the name Cape Disappointment by fur trader John Meares who, because of a storm, turned his ship around in 1788, just missing the mouth of the Columbia. The credited discovery was four years later, although we know that history has a serious bias; the river was discovered more than 10,000 years ago by humans.

The peninsula is today a state park, and it has a well-developed campground complex that fronts a wide sandy beach, with a more modest sandy cove on the east side called Waikiki Beach (it memorializes a Hawaiian seaman who perished during a shipwreck).

The winter encampment of the Lewis and Clark expedition at Fort Clatsop was largely a miserable affair with constant hunger and lack of supplies. They had hoped to flag down a sailing ship coming down the coast to trade for supplies, but their fort was a couple of miles from the beach and they missed ships for lack of a good lookout point. One might wonder why they didn't set up camp on the wide sandy beaches of Cape Disappointment. The answer is pretty straightforward: the beach didn't exist when they were there. It was a rocky peninsula as can be seen on their sketched map (below)

The mouth of the Columbia River is a nightmare for shipping. The discharge is more than enough to support even large freighters as far upstream as Portland, but the river carries vast amounts of sand and silt, and the shifting bars have caused vast numbers of shipwrecks. In an effort to stabilize the shifting sand bars, jetties were constructed in 1886. One extends for several miles from Cape Disappointment. Although the rock wall helps to keep the river channel clear, it also serves as a barrier to the southward movement of sand along the coast north of the Cape. The sand started backing up immediately forming the wide flat beaches that can be explored today (see the map below...sand is the yellow unit marked "Qb").

Source: US Geological Survey and https://nwgeology.wordpress.com/the-fieldtrips/pillow-lava-sites-in-washington/pillows-in-the-crescent-formation-cape-disappointment-state-park/

One of the coolest things about exploring Cape Disappointment is the privilege to actually see the rocks. The wave-carved cliffs are not totally covered by vegetation as is so often the case in western Washington. And the rocks are weird. They are not layered, and instead are marked by strange globular masses of what turns out to be weathered and oxidized orange basalt (we would normally expect basalt to be black like the lava flows in Hawaii). How did they get this way?

Note the cormorant for scale (upper left)

When basalt erupts underwater, it forms these globs that are about the size of thick down pillows. It looks like toothpaste being squeezed out and then getting pinched off. Although these rocks are on land today, they were once on the bottom of the Pacific Ocean. They formed in eruptions at the mid-ocean ridge on the seafloor. They traveled like a conveyor belt for hundreds of miles (at the stunning rate of several inches a year) and were scraped off and added to the edge of the continent in the subduction zone that forms the western boundary of the North American Plate in Washington and Oregon. These rocks, the Crescent Formation, are Eocene in age (around 40 million years ago).

Cape Disappointment is a fascinating place to explore (it's neither dismal or disappointing). Watch the weather, though. Not only for the incessant rain, but for fog. It's said to be the foggiest place in the country which helps explain why the peninsula has not one, but two lighthouses.

Cormorant and Pillow Basalt Above Waikiki Beach (but not the one you think)

I'm still on the road so posts are few, but I occasionally get to a computer. Today's picture is a Cormorant flying in front of a wave-cut cliff exposing pillow basalt at Waikiki Beach. I've been writing a lot about Hawai'i of late, so you are forgiven for thinking that this is one the islands, but it's not. It's at Cape Disappointment on the Columbia River in Washington state. It got the name from an event in 1811 when a shipwreck led to the death of a Hawaiian crewmember whose body washed up on the beach here (I don't know why they didn't just use his name).

The pillow basalt is part of the Crescent Formation, a unit that preserves sea-floor sediments and basalt of the ocean crust that was swept into the subduction zone along the west coast of North America. Pillows are globular masses of basalt that form when the lava flows into water. The lava dates from the middle Eocene epoch, about 40 million years ago.

Cape Disappointment is steeped in history, being the last point of solid bedrock along the Columbia River where it flows into the Pacific Ocean. The name came from the failure of early explorers to recognize this spot as the mouth of the Columbia River (hazardous and constantly shifting sand bars made navigation upstream extraordinarily difficult). It was also the place where the Lewis and Clark Expedition finally reached the Pacific Ocean.

The construction of a huge jetty to make navigation on the Columbia River a bit safer caused sand to back up against the rock barrier, forming an extensive beach where none existed before. The site is now a state park and national historical park. There is a nice campground where we enjoyed some rare sun the other day (Cape Disappointment is said to be the foggiest place in the United States).

There are two lighthouses on the peninsula, due to the aforementioned hazards. Around 200 ships were wrecked in the immediate vicinity over the years.

Driving to the Center of the Earth in Del Puerto Canyon...Piercing the Ocean Crust

Rugged terrain in the upper Del Puerto Canyon just beyond the Tesla-Ortigalita fault (on the right near the people)

As in the last post, I'm exaggerating a little bit. We're not going to the center of the Earth, we are instead using California's unique geology to explore the mantle, the layer that extends from 20 miles to about 1,800 miles depth, about half way to the center. These rocks have been on a long journey to reach the Earth's surface, and they are not often seen by casual explorers.

Volcanic rocks in Del Puerto Canyon. These are either pillow basalts or highly jointed rocks.

In our first post, we had driven through the five mile (8 km) thick sequence of sedimentary rocks laid down on the floor of a relatively shallow ocean (the Great Valley Group). We reached a major fault near the base of the sedimentary rocks and the canyon changed in a major way. The smooth gentle slopes of grass gave way to rocky slopes covered with brush, scrub oak and the occasional cypress tree. The rocks had changed. We had reached the ancient oceanic crust, known here as the Coast Range Ophiolite.

An ophiolite sequence is a unique series of rocks that are usually understood to represent a cross-section of oceanic crust. The top of an ophiolite is composed of pillow lavas, lumpy chunks of basalt that form as molten rock encounters cold ocean water (see some forming in this short video). Those might be some pillow basalts in the picture above, but I've never been able to get close enough to confirm it. It could also be highly jointed volcanic rocks.

Beneath the pillow basalts, one might expect to find sheet dikes, fractures that have been filled with volcanic rock that had been on the way upwards to the ocean floor. The sheet dikes are not obvious in Del Puerto, although they can be picked out in a couple of places (we didn't stop in the right places on this trip).

There is a prominent dike in the canyon, but it is not actually part of the ophiolite. The rugged ridge is composed almost entirely of quartz. It probably formed millions of years after the others as hot hydrothermal fluids flowed through cracks and fractures. It's been investigated for gold mineralization, but I don't think anyone has found any ores worth mining (not that they wouldn't try; it's still under claim).

Quartz vein and gabbro outcrops in Del Puerto Canyon

Beneath the sheet dikes, one would expect to find the plutons that once fed the eruptions of basalt and other lavas on the seafloor. The magma that remained cooled slowly over thousands of years, forming a coarse-grained rock composed of crystals of amphibole, pyroxene, plagioclase feldspar, and maybe some olivine. The plutons are usually composed of a dark rock called gabbro, but some of the rocks are lighter-colored, a variety called diorite.

Highly jointed gabbro and diorite in Del Puerto, rocks of the lowest part of the oceanic crust

The presence of diorite and some silica-rich volcanic rocks in the upper parts of the sequence throws  a wrench in the normal interpretation of ophiolite, especially those that occur in California. Ophiolites that form at oceanic ridges (divergent boundaries) are usually poor in silica (composed almost entirely of basalt and gabbro). The ophiolite in the Coast Ranges of California may have formed in a more complex tectonic setting, in and near an island arc (a chain of volcanic islands like the Aleutian Islands today) associated with an oceanic trench.

Diorite in Del Puerto Canyon

In any case, we've penetrated the oceanic crust, a thickness of around three or four miles (6-7 km). These rocks don't often see the light of day, because when you think about it, what does it take to bring the ocean floor and crust to the slopes of a mountain range on land? Geologists have been trying for years to drill a hole through the oceanic crust, unsuccessfully so far, but a new effort has begun this year. Del Puerto Canyon is a place where we can literally walk from the base of the oceanic crust to the underlying mantle.

And that's what we'll do in the next post!

Gabbro near the quartz vein in Del Puerto Canyon

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.

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...

Northern Convergence: Vancouver Island, the Plan That Was, Part II

Ah, the lost opportunities! I don't want to make it sound like we had a bad Northern Convergence trip; we actually had a great time taking our students through an exploration of the geology of Vancouver Island,  but the trip we made was different than the one we planned. In the last post we talked about the Sooke Potholes, and in today's post we are talking about the opportunity to understand why the basalt exists.

East Sooke Regional Park is a large wilderness park just 20 kilometers or so outside of Victoria at the south end of Vancouver Island. It has a nice network of trails that explore rocky coastlines and deep rainforests. I was really looking forward to exploring it, but time was conspiring against us. That opening picture? That's where the sun was in the western sky as we arrived in the park on our little scouting trip a few days before we were to meet the students. Sunsets take awhile at these northern latitudes, but there are still a finite number of hours of daylight, and we had used most of ours up.

I ran down the trail to Becher Bay because East Sooke Park is said to have marvelous exposures of the oceanic crust that had been pushed into the subduction complex along the western coast of North America, and lifted up above sea level. The ocean crust includes pillow basalts, which form as lava spills out on the ocean floor, sheet dikes, which were the conduits that fed the seafloor eruptions, and gabbro plutons, the magma chambers that fed the eruptions. These rock associations are called ophiolites, and they are often of great interest to geologists, not just for the knowledge they give us about oceanic processes, but because they often are a source of valuable ores, such as copper, mercury, chrome, and platinum.

I literally only had minutes, and didn't have time to walk to the petroglyphs, which would have offered exposures of all three kinds of rock, but I consoled myself by realizing I would be back in a few days with my students and more time. Which didn't happen, as it turned out (as I said, we saw great things, but we didn't make it to East Sooke). I looked at the beautiful small bay, snapped a few pictures of the Olympic Mountains across the Strait of Juan de Fuca, and headed back to the car. As mentioned in the previous post, much of the Olympic Mountain periphery rocks were composed of the same basaltic rocks.

In the dying light of the evening, I was going to try to make one more stop, Witty's Lagoon, to see what are said to be the best exposures of pillow basalts in the region. And I missed the turn. As a geologist and a life-long map-o-phile, I'm embarrassed even now to say that. And by the time I realized it, it was dark.  But we enjoyed a wonderful evening, with a late dinner courtesy of Red Robin in Victoria.

By the way, while I'm making confessions, about coming into Canada: If you want to not look like a complete doofus, learn how to pronounce the names of the places you are going to visit. At immigration, the polite officer asked why I was visiting Vancouver Island and I told him I was scouting out "Sookie" and "Na-na-emo". Eye-roll. It's "Sooke" with one syllable, and "Na-nymo" when you try to say Nanaimo. No wonder American tourists have a bad reputation....

Now THIS is why I love California beyond words (but I will use words anyway!)

California gets a lot of crap from around the country for whatever reasons (San Francisco liberals, Berzerkely, L.A. riots and floods, earthquakes, the whole nine yards), but you know what? I think people are just jealous, because my fair state is one of the most beautiful and interesting corners of planet Earth. Today provides a fine example.

I've been blogging a lot about Yosemite National Park of late, and frankly, lots more is coming, but I had an errand in the San Francisco Bay area this afternoon. Yosemite is a two hour drive from where I live. The Golden Gate Bridge is a two hour drive from where I live. My home town routinely comes up 49th or 50th out of 50 on those "best places to live in America" lists, and it has its problems, but I bet a lot of my geology friends would give up a lot just to be able to play in the snow for a few hours before heading to the beach to work on that sunburn. Or to put it more, eh, sophisticated-like, to go from the granite intrusions of a Mesozoic magmatic arc to the deep heart of a fossil subduction zone in the space of a few hours.

So, a few pictures of what we did after we finished our errands. We headed out onto the Marin Headlands, part of the Golden Gate National Recreational Area. The headlands are those dramatic hills at the north end of the Golden Gate Bridge. There's a winding narrow road that traverses the cliffs above the bridge, providing incredible views of the sea cliffs and San Francisco, which just has to be one of the prettiest cities in the world. The road ends at Rodeo Beach (picture above), which is just another typical northern California beach, meaning utterly breathtaking.

The rocks are strange; deep red brown and black, occasionally gray and green. The deep red layers in the picture above are ribbon cherts from the ocean floor, while the black bulbous looking rocks in the picture below are pillow basalts. They are the remains of deep ocean sediment and oceanic crust that were swept into the subduction zone that lay off the coast of central California from around 200 million years to as little as 10-15 million years ago (it's still there in the northernmost part of the state). The destruction of the subduction zone was the origin of the San Andreas fault, which lies just offshore of here.

If you are looking for the white sand beaches of the Bahamas or Florida, you won't find them in this part of California. Those beaches are made of quartz grains or coral fragments. In Northern California, the sand is composed of the same grand selection of minerals that are found in the rocks of the nearby cliffs, and almost any color can be picked out of a handful (for my sed-pet friends, yes, this is a gravel, not a sand sample; but it shows the colors better!).

An exploration of the roads around the tops and sides of the seacliffs reveals a bunch of strange looking concrete structures that are clearly old, and clearly abandoned. It turns out that if the Japanese Navy had ever decided to enter San Francisco Bay to cause havoc during World War II, they would have faced a devastating barrage of artillery fire coming from these concrete bunkers.

Today, these cliffs and rocky islands are home to hundreds of thousands of birds. The Marin Headland, clear-cut of lumber in the 19th century, covered with structures of war in the 20th century, is now a place of tranquility and contemplation. Sometimes we do a few things right...

I love California...