Travels in Cascadia: Walking Under the Ocean Floor at East Sooke Bay, British Columbia

We continued our explorations of British Columbia with a hike at the south end of Vancouver Island at East Sooke. It's an unusual place, out of place with the rocks that make up most of the island. The majority of Vancouver Island is made up of metamorphic rocks of the Wrangellia Terrane. These rocks originated as island arcs and continental fragments in the Pacific Ocean which added to the west coast of North America as the ocean crust sank beneath the continent at the Cascadia Subduction Zone.

Source: Chris Yorath

But the south tip of the island is made up of rocks related to the Olympic Peninsula which lies 20 miles away across the Strait of Juan de Fuca. These rocks are parts of the ocean crust, and are called the Metchosin Igneous Complex, or the Metchosin Ophiolite. They formed in Eocene time, around 50 million years ago as vast amounts of basaltic lavas spilled out on the Pacific Ocean floor

The Olympic Mountains seen from East Sooke across the Strait of Juan de Fuca

Ophiolites are generally considered to be slices of oceanic crust that form at divergent plate boundaries. The oceanic crust is pulled apart by extensional forces, relieving pressure on the underlying asthenosphere where the rocks are close to the melting point. The loss of pressure causes some melting to take place, and the resulting basaltic magma rises through fractures caused when the sea floor is pulled apart. An ophiolite has three distinct parts, with pillow basalts making up the ocean floor (more on pillows in a coming post), sheet dikes (the fractures), and gabbro plutons at the base. Gabbro is a coarse-grained igneous rock with the same composition as basalt (it cools slowly, allowing for crystal growth). A pluton is any kind of rock that has been intruded into the crust.

East Sooke Regional Park lies a few miles west of Victoria along the shoreline of the Salish Sea. For a coastal park it has an unusual 'feel'. Because the Salish Sea consists of a series of straits and inlets, wave energy is considerably diminished, at least at the times that I've been able to visit. The waves barely register and the shoreline seems more like a large lake, much like Lake Tahoe in my own home region. But the water is definitely salty!

My goal for our class this day was to get a close look at the rocks of the gabbro pluton portion of the ophiolite. In other words, we were going to go walking beneath the ocean floor. The class, a combined dyad of geology and anthropology students had other ideas. The anthropologists slightly outnumbered the geologists, so they were intent on finding some reported petroglyphs in the region. We went hiking on the Alyard Farm Trail, which was a loop of about two miles, first along the rocky shore, and ending in a thick conifer forest.

Luckily, the petroglyphs had been carved out of boulders of the gabbro, so we got the best of both worlds, with some glacial grooves as icing on the cake. Can you see the first one in the picture below? Without looking ahead, can you tell what it was meant to be (remember the landscape setting)?

I'm told that this is the representation of a sea lion. One source on the internet (the arbiter of all truth) mentions the following myth about the petroglyph: "Long years ago a great supernatural animal like a sea lion killed many of the Becher Bay Indians while they were canoeing. The tribe nearly became extinct; the remaining members were afraid to go on the water until one day a mythical man caught the sea lion and turned him into the stone representation on Alldridge Point" (Anonymous, Report of BCPM, 1928).

Note the grainy nature of the rock in the picture below. Gabbro is a dark-colored plutonic rock that has the same composition as basalt, but the individual grains are visible because of the slow rate that the magma cooled. The gray minerals are plagioclase feldspar, while the black minerals are mostly a variety of pyroxene, perhaps augite. Small grains of olivine are scattered throughout the rock.

There is a second petroglyph nearby of a salmon (below), but it has seriously faded. Both petroglyphs are attributed to the T'Sou-ke First Nation people, but the age of the rock carvings is not known. They quite likely are thousands of years old, based on the amount of weathering.

I'll probably say something similar to this many more times as we continue our exploration of British Columbia, but here we go: if you ever have the opportunity to visit Victoria and Vancouver Island, set aside some time to explore the East Sooke area. In addition to the beautiful coastal trail, there is also the East Sooke Potholes, a series of deep pools eroded out of the rocks after the last ice age. We didn't have the time to explore further up the coast, but the guides mention a number of fascinating places to investigate.

I had three main resources for the geology in and around the city of Victoria and East Sooke:
The Geology of Southern Vancouver Island by Chris Yorath
Roadside Geology of Southern British Columbia by Bill Mathews and Jim Monger
Geology of British Columbia, A Journey Through Time by Sydney Cannings, JoAnne Nelson, and Richard Cannings.

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: Exploring the Oceanic Crust Without Unobtainium

One of the movies I most love to hate is "The Core". There are plenty of ridiculous parts, there are several earnest geologist characters, there are the many gory deaths while the heroes save planet Earth from being severely microwaved or something like that, and there is of course unobtainium, the magical element that makes everything else possible, including "Avatar". But mostly I scream at the end when the heroes make it though the oceanic crust at the "plate boundary" near Hawaii. Just like physicists yelling "you can't hear explosions in space", I yell "there's no plate boundary at Hawaii". But there is oceanic crust, and it is very hard to explore. In fact most oceanic crust is pretty much inaccessible except by the use of expensive drilling rigs at sea, and they haven't penetrated the deepest parts of the crust yet. Oh, and unobtainium doesn't really exist...

But many flowers do exist in Del Puerto, at the right time of year.

The thing is, we can explore the oceanic crust, and for us in Central California, it involves simply driving up Del Puerto Canyon in the portion of the Coast Ranges called the Diablo Range. We began our exploration in the last post of our series about driving through the most dangerous plate boundary in the world. We were journeying through the depths of hell as represented by the peridotite and serpentine of the Earth's mantle. That portion is in the uppermost parts of the canyon. As we descend towards the Great Valley, we make our way through a section of oceanic crust called the Coast Range Ophiolite.

We saw some elements of an ophiolite sequence while we explored the Marin Headlands in this post a few weeks ago. The big difference is that the rocks of the Marin formed a few thousand miles from the California coast as much as 200 million years ago. The Coast Range Ophiolite is thought to have originated much closer to the margin of the continent around 140 million years ago.

The upper canyon has some nice exposures of highly contorted deep-sea chert similar to those of the Marin Headlands (above), but pillow basalts are a bit harder to find in Del Puerto Canyon. Other parts of the oceanic crust are well-represented however. Pillow basalts form during eruptions onto the sea floor. The rocks of Del Puerto formed much deeper in the crust. The eruptions of basalt on the sea floor were fed by numerous sheet dikes in the mid-levels of the crust, which were in turn supplied by plutons of basaltic magma that later cooled slowly to form gabbro plutons. Gabbro looks like no other rock in the coast ranges (below).

The blocky exposures reveal a rock full of mafic (dark iron-rich) minerals. The "black granite" of many stone countertops is not granite at all, but gabbro. The gabbro of Del Puerto is somewhat finer grained than most rock used in countertops, but under a microscope it is very pretty. It often contains small crystals of green olivine (the gemstone peridot).

The gabbro exposed in the middle part of Del Puerto has been split by a quartz-rich dike that captures the attention of most canyon travelers. It is probably related to hydrothermal fluids (hot water solutions) developed during faulting and uplift of the range.

The quartz vein is very resistant to erosion and forms a wall of rock running up the cliff. It has caught the attention of gold seekers, although I am unaware of any economical ores in the region. This hasn't stopped the speculators. The vein was recently staked for mining, even though no one has found anything there in 90 years of easy access to the outcrop.

This stretch of the canyon gets a fair amount of precipitation and reasonably good soils develop on the slopes. Some water is usually present on the valley floor all year. The vegetation is some of the most diverse to be found anywhere in Coast Ranges, and several hundred bird species have been sighted in the canyon at one time or another.

The sediments of the Great Valley Group were deposited on the Coast Range Ophiolite. The boundary between the ophiolite and sedimentary rocks along Del Puerto Canyon road is a fault, the Tesla-Ortigalita. It's a little hard to see in the photo below, but note the color change in the rocks from lower right to upper left. Some years ago, about 1996, I stood here with Al Bennison, who as a child discovered the first dinosaur ever found in California. He went on to a career in paleontology, and mapped much of the region. He mentioned that the shale on the right side of the fault sometimes yielded up ammonite fossils. He walked over, looked at the rock a moment, then pulled out an ammonite specimen. I have gone back to that exposure a dozen times or more, staring for hours at the rocks, hammering away, and I have never found one.

A final note about exploring Del Puerto Canyon. It is a marvelous habitat for many wonderful creatures, but there are some that one may enjoy better from a distance. In 2008 we saw this rattlesnake.

And in 2012 a flipped rock revealed this small arachnid. Explore carefully!

This post is part of a continuing series about the ancient subduction zone complex exposed in Central California. It is no longer active as such, but once was a zone of earthquakes, tsunamis, volcanoes and many dinosaurs must have perished when it was active in Mesozoic time. For a preview of the series, check out this original post for the series.

The Other California: A Journey to the Center of the Earth (kind of...)

How many of you tried to dig a tunnel to China in the backyard when you were a kid? Given the soil conditions in the yard I grew up in, I'm probably lucky to be alive. I dug tunnels looking for buried treasures, gemstones, fossils and sometimes I was just curious what was down there. Geologists, I've found, are the kids who tried to find all those things, and never really grew up.

So how far do these overgrown kids get? It turns out that the deepest tunnels that humans have ever been able to dig reach depths of about 12,800 feet, a little over 2-1/2 miles. That might seem like a lot from our point of view, but the depth to the center of the Earth is around 4,000 miles. We've barely scratched the surface, yet the temperatures of the rock at these depths is well over 100 degrees, and the rocks are under so much pressure that explosions of rocks from the walls are a constant danger to the miners. Kids, there's got to be a better way to see what lies deep below. And there is, in the Other California, one of those places not found on the postcards. The adventure lies in the Klamath Mountains, and the most dangerous thing you have to face is slipping on a slick river rock, because geological processes have brought the rocks many miles up from the depths. You need only explore the rivers flowing off the mountains to see what the deep interior of the earth looks like.

The Klamath Mountains are a collection of bits and pieces of the earth's crust that have been carried great distances from their point of origin and slammed (at geologic speeds of inches per year) into the western edge of the North American continent. A huge variety of igneous and metamorphic rocks are found around the province, and some of the most interesting are those that once resided deep in the Earth's mantle, a layer that extends from just below the crust, from maybe 15 or 20 miles beneath our feet, to a depth of about 1,800 miles. Here are a couple of bits of the Earth's deep hidden places that I found on a short trip to the Eastern Klamath Terrane in the vicinity of Gazelle.

The oceanic crust is usually described as being made of basalt, but a few miles down in the crust the basaltic magma cools slowly to form a coarse-grained basaltic rock called gabbro. Sometimes, as can be seen above, the crystals that form are huge, with black hornblende and white feldspar crystals several inches long. Igneous rocks with such large crystals are called pegmatites.

Going even "deeper" into the interior, we pass the Mohorovicic Discontinuity, the dividing line between the crust and mantle. The upper part of the mantle is composed of olivine-rich rocks like dunite or peridotite. Olivine is best known to most people as the green gemstone peridot. That's right, much of the Earth's interior is made up of gems! The rock in the picture above is dunite, in part slightly altered to serpentine.

In many parts of the Klamath Mountains, the mantle rocks are completely altered to serpentine, the state rock of California. These ultramafic rocks are fairly rich in a number of unusual metal ores, including platinum, nickel, magnesium and mercury. One of the most important ores is chromite, which is the only significant source we have for chromium, the metal that puts the "stainless" in stainless steel. We import most of the chromium that we need from foreign sources, but in wartime (especially the two World Wars), the ores were mined domestically, and a number of operations were present in the Klamaths. The black semi-metallic crystals in the picture above are chromite, with green serpentine across the top.

In our next post we are going to "climb" into the underside of a volcano...