Travels in Cascadia: Threading the Needle on Hurricane Ridge

Leading a field studies trip is a stress-filled enterprise. There are the big things to worry about: auto accidents, injuries, conflicts with law enforcement, lost reservations, and those sorts of things. But those thankfully don't happen much. But weather does happen, and field studies trips tend to be tightly scripted affairs with not much room for weather-related complications. Yet they happen, especially on trips in the Pacific Northwest. We've had trips where we had just the one chance to see Mt. St. Helens, and it was completely fogged in. There was the one chance to see the Sea to Sky Highway in British Columbia, and it was raining the entire way. We've missed a lot.

This year was going to be different. We worked some flexibility into the schedule, spending two nights each at most of our localities, giving us the chance to postpone a particular plan for a day to allow the weather to clear up. But on our second day out, I was worried. Ever since the longest ten-day forecast, a storm was brewing out in the Pacific Ocean, one that was arriving in waves over several days. We had given ourselves two days on the Olympic Peninsula, and rain was falling on Hurricane Ridge the first day, so we elected to go to Neah Bay and Cape Flattery instead. But that left us just one more chance to have a clear view at Hurricane Ridge in Olympic National Park. The sunrise (above) was not promising. According to the forecast, we would have a brief window of maybe three or four hours before the storm closed in, but we drove through light showers on the road up to the ridge.

My concern grew as we continued up the road, rising from sea level to over 5,000 feet. The far ridges would appear for a moment and then become obscured, and I didn't know until we reached the top ridge if we would actually see anything...

… but we did! And no matter how many times I've been on Hurricane Ridge, nothing quite prepares me for the view from the end of the paved road. It is simply astonishing. As we emerged from the vehicles I felt the stress falling away, like dropping a particularly heavy load from my shoulders. We gathered the group and said a few words about the geology. We would save the longer presentations for later in the day down the hill. With an impending storm, I didn't want our students to miss any of the dramatic scenery. And it is dramatic.

The Olympic Mountains rise from sea level to nearly 8,000 feet and are extremely rugged. They capture prodigious amounts of rain and snow on the western flanks, so much so that temperate rainforests coat the western slopes. It was a nightmare for geologists who were trying to unravel the geologic history.

The mountains exist because of subduction. For most of 200 million years a convergent boundary has been active in the region, as the crust of the Pacific Ocean basin has been sinking against the edge of the North American Continent. In some places, like California, the subduction zone has been replaced by a transform boundary (the San Andreas fault). But in Northern California, Oregon, Washington, and part of British Columbia, the subduction zone is still active, still producing earthquakes, and still raising mountains. It's called the Cascadia Subduction Zone (from hence comes the name of this series).

Source: Geological Society of America

In a "normal" subduction zone, there are four parts: the trench, an accretionary wedge, a forearc basin, and a magmatic arc. The trench is the deepest part of the ocean floor where the oceanic crust sinks back into the mantle. The accretionary wedge is a collection of seafloor sediments and crust that has been scraped off the subducting plate and added to the edge of the continent. The forearc basin is a shallow sea that may develop inland of the accretionary wedge (California's Great Valley originated in this fashion). The magmatic arc is a system of volcanoes and intrusive plutons resulting from the melting of rocks in the lower crust and upper mantle above the descending slab (water released from the slab lowers the melting point of the rock, leading to the formation of the molten rock).

Looking at the thickly forested slopes below Hurricane Ridge, I cannot envy the geologists who originally mapped the Olympic Mountains. Simply finding an exposure of rock must have been challenging at times. What these geologists did was to take the rare rock exposures and extrapolate them into a semi-coherent map that reveals the structure of the Olympic Mountains. They did the equivalent of taking a few pieces of a jigsaw puzzle, putting them in the right location relative to the others, and then drawing in the remainder of the puzzle from scratch. I've been way too spoiled by the naked rock exposures of places like Death Valley and the Mojave Desert!

The geologic map reveals the basic structure of the Olympics. A "horseshoe" of basalt and sedimentary rocks (the Peripheral Rocks, or Crescent Formation) partially surrounds the "Core Rocks", an assemblage of lightly metamorphosed sandstone and shale layers. The Core Rocks are characteristic of the types of deposits that form from underwater landslides ("turbidity currents") within the trench and accretionary wedge of a subduction zone. The fact that these rocks are now thousands of feet above sea level is the interesting conundrum. Accretionary wedges are generally below sea level, or exist as small islands. They can be pushed higher. For instance, the rocks of the Cascadia accretionary wedge are exposed in the Coast Ranges of  Washington, Oregon and California, but nowhere are the exposures as spectacular as the Olympic Mountains.

Convergent boundaries can be exceedingly complex places. Bits and pieces of continents and island arcs may randomly arrive at the subduction zone, mucking up the subduction process the way too many sheets of paper at once can muck up a paper-shredder. In the case of the Olympics, there was a mass of land north (Vancouver Island) and an accreted terrane to the south (the North Cascades), and a bend in the subduction zone itself. In essence, too much material was being stuffed into the subduction zone, so the excess material went the only way it could, which was up. The mountains have been rising for around 15 million years. They would be higher, but the incredible amount of precipitation tears the mountains down at a roughly equivalent rate.

Pillow basalt from the subducted oceanic crust is exposed along the Hurricane Ridge Road and along trails near the viewpoint. When we went down the road later on, we found that a small rockfall had dumped some of the pillows onto our highway. So, as it turned out, we managed to miss having rocks fall on our vans, i.e., one of the hazards I mentioned at the start of the post!

We could easily observe the glaciers that scour the upper reaches of the mountains. Glaciers technically shouldn't exist here. Although we were at a high enough latitude, the nearby Pacific Ocean moderates the climate, keeping things warmer than they would otherwise be (the Olympics are at the same latitude as Great Falls, Montana, or St. Paul, Minnesota). But temperature isn't the only factor in glacier development. The sheer amount of snowfall in combination with temperatures that are just cold enough allows glaciers to exist at these unusually low elevations.

We had a good introduction to the basic features of alpine glaciation as we gazed across the valley to Mt. Olympus. There were horns, aretes, and cirques as well. Glaciers were going to be a big part of the story of British Columbia, and Hurricane Ridge provided a spectacular setting for the first discussion of how they worked.

It was nearly noon and the storm clouds were building. We were rained on as we descended back down to the lowlands. It was time to prepare for the ferry ride across the Strait of Juan de Fuca to Vancouver Island.

A Look Back at Ten Years of Geotripping: Driving Through the Most Dangerous Plate Boundary in the World

Driving through the most dangerous (kind of) plate boundary in the world is actually not very easy to do. Subduction zones, with the exception of the volcanoes, are mostly deep under the sea. But Central California is a unique case, being an ancient subduction zone that has been uplifted and exposed by erosion, so that interested parties can literally drive through what once was miles underground or at the bottom of the deepest oceans. I got the idea for this blog series when I spent an afternoon driving the winding road that travels over the Coast Ranges at Lick Observatory, and down through Del Puerto Canyon into the Great Valley. It's the equivalent of driving twenty or thirty miles into the Earth's crust. Looking back over the titles, I'm worried that I am using up my lifetime supply of bad jokes...

I've been reviewing the archives this week to find some of my favorite posts from ten years of geoblogging. This compilation appeared on July 4, 2015.

Without a doubt, subduction zones are the most dangerous plate boundaries on the planet. Divergent plate boundaries produce earthquakes and occasional volcanoes, but nothing on the fearsome scale of the calderas and stratovolcanoes and magnitude 9 earthquakes experienced at convergent boundaries. Transform boundaries produce earthquakes, but they are magnitudes smaller than those produced at convergent boundaries (despite what certain Hollywood movies have asserted recently). Hot spots, while not a plate boundary, can produce huge caldera complexes like Yellowstone, but such monsters have not had much of an effect on human history of the last few thousand years. It is the subduction zones of our planet that have caused the most human misery, in the form of massive earthquakes, tsunamis, and violent volcanic eruptions.

We have been driving through an example of one of the most dangerous plate boundaries in the world, but our particular example has been inactive for a very long time. Central California was a subduction zone complex for more than 150 million years, primarily during the Mesozoic era, but it changed into a transform boundary only a few tens of millions of years ago. The San Andreas fault is the resulting feature, and it is capable producing damaging earthquakes, but even the most destructive quakes, like 1906 in San Francisco (death toll 3,000), is but 1/30 of the energy of a magnitude 9 quake like that of Indonesia in 2004 (where the resulting tsunami killed 200,000 people).

It's taken a couple of months to work through our journey, so I've compiled all of the posts here so one can catch the continuity of the story. Here goes...

A New Blog Series

The introduction to the new series, a geological transect from the California Coast to Yosemite Valley, crossing an ancestral subduction zone that once caused geological havoc in a zone from Mexico to Canada (and still is in a few places).

Reconnaissance

An overview (in the most literal sense) of the lands we will traverse on our journey. We have a look at central California from above.

These Rocks are All Wrong!

Granite is exposed in the rocks of the Point Reyes Peninsula. But the arrangement of rock and sediment in subduction zones suggests that granite shouldn't be anywhere near here. It's the San Andreas fault. In California, it's always the San Andreas' fault.

Looking for the Big One

The peace and serenity of Tomales Bay belies a violent past. The San Andreas fault slices right through the bay, and produces large earthquakes with disturbing irregularity. The epicenter of the San Francisco quake in 1906 was not far from here.

Welcome to Geology's Junk Drawer

The Marin Headlands began as a giant collector of geological flotsam and jetsam from the crust of the Pacific Ocean. The jumble of rocks accumulated in an accretionary wedge, and were later lifted up into the mountains of the Marin Peninsula.

Geology's Junk Drawer on the Marin Headlands

Exploring the hidden corners of the Marin Headlands, we find Redwood forests, beautiful views of San Francisco, and disturbing reminders of World War II.

Terra Fatale on the Marin Headlands

Although the subduction zone that formed the rocks of the Marin has been extinct for a long time, there are still plenty of hazards remaining in the region, both geological and nautical. The Point Bonita Lighthouse has been present in one form or another for 160 years. It hasn't always worked, as there are upwards of 300 shipwrecks in the area. There are other hazards too.

The Alien Bursts Forth in the Diablo Range!

We head across San Francisco Bay and start an arduous journey through the Diablo Range. The range formed as an exotic (read "alien") terrane pushed upwards through the sediments of the Great Valley Group, piercing the surface and rising into the sky. John Hurt would no doubt approve...

Exploring the Belly of the Beast in the Diablo Range

We make the first part of a long drive through a rugged portion of the Diablo Range, one of the largest sub-ranges in the Coast Ranges of California. Along the way we cross through two exotic terranes of rocks that had been carried miles deep into the crust in the accretionary wedge of the subduction zone.

At the Portal of Hell in Diablo Range

Making our way down Del Puerto Canyon in the Diablo Range, we do the equivalent of traveling through the crust of the Earth all the way into the mantle, finding outcrops of peridotite and dunite, rocks containing the mineral olivine among others. For the mercury miners, this truly was a portal to Hell.

Exploring the Ocean Crust without Unobtanium

Del Puerto Canyon cuts a swath through the Coast Range Ophiolite, the remnants of the ocean crust that once lay at the bottom of the Pacific Ocean. Though mostly in private ownership, the canyon is one of the most scenic in the Coast Ranges, and a pair of county parks in the upper reaches invite exploration.

Into the Realm of the Drowning Dinosaurs

Sediments accumulated in a deep trough along the west coast of North America called a forearc basin for upwards of 100 million years. The layers reached a depth of 5 miles! In those waters swam mosasaurs (yes, like in the recent Jurassic World movie, but they forty feet long, not a hundred), plesiosaurs, ammonites, and occasionally a drowning dinosaur. The first discovery of a dinosaur in California happened here in 1936.

The Sea Floor that became the Greatest Agricultural Region on Earth

The American Serengeti is a vast plain 400 miles long and 50 miles or so wide that once was the sea floor. The grasslands of the Great Valley once supported millions of migratory birds and grazing animals. It still supports millions of organisms, but these days, those organisms are humans. 95% of the original prairie has been developed for agriculture and the region produces a quarter of the nation's produce. And all of the almonds and walnuts.

In the Pleistocene, a Different Kind of Danger

The Great Valley would have been a most dangerous region for a different reason in the Pleistocene. Among the great herds of grazing animals there were predators, and they were adapted to bringing down giant prey, not the small game we find today. Gigantic Short-faced Bears, Saber-tooth cats, American Lions, Jaguars, and Dire Wolves.

The Dr. Who of Mountain Ranges

At least three Sierra Nevada ranges have existed throughout time. They might have even once been higher than today. The distorted deformed metamorphic rocks tell the story of the earlier ranges.

A Gentle Landscape Belies a Fiery Past

The Valley Springs formation, exposed throughout much of the Sierra Nevada foothills region, forms gentle grass and oak covered slopes, but the rock is made of volcanic ash that originated in monumental explosions millions of years ago. The ash came from gigantic calderas, some of which were hundreds of miles away in central Nevada.

A Landscape Buried in Hot Mud, and a 6-foot Long Saber-tooth Salmon

A lot of volcanoes in the world are made of mud. Lots and lots of mud. But this mud formed in violence. The Mehrten formation of the Sierra Nevada has other surprises too: gigantic tortoises, and six-foot long salmon...with fangs...

A Tale of Two Subduction Zones

There have actually been at least two subduction zones in California. Remains of the older one still make up the rocks of the Sierra Nevada Mother Lode, and those rocks were the source of gold in the Gold Rush.

Exploring the Underside of the Volcano

We wrap up the series in the heart of the ancient magmatic arc: Yosemite Valley. Walking among the towering cliffs, we are reminded that the rocks were actually formed within the magma chambers of volcanic systems, perhaps similar to Lassen Peak, Mt. Shasta, or even at times, Yellowstone.

Driving Through the Most Dangerous Plate Boundary in the World: A Compilation of Fear(somely cool geology)

Without a doubt, subduction zones are the most dangerous plate boundaries on the planet. Divergent plate boundaries produce earthquakes and occasional volcanoes, but nothing on the fearsome scale of the calderas and stratovolcanoes and magnitude 9 earthquakes experienced at convergent boundaries. Transform boundaries produce earthquakes, but they are magnitudes smaller than those produced at convergent boundaries (despite what certain Hollywood movies have asserted recently). Hot spots, while not a plate boundary, can produce huge caldera complexes like Yellowstone, but such monsters have not had much of an effect on human history of the last few thousand years. It is the subduction zones of our planet that have caused the most human misery, in the form of massive earthquakes, tsunamis, and violent volcanic eruptions.

We have been driving through an example of one of the most dangerous plate boundaries in the world, but our particular example has been inactive for a very long time. Central California was a subduction zone complex for more than 150 million years, primarily during the Mesozoic era, but it changed into a transform boundary only a few tens of millions of years ago. The San Andreas fault is the resulting feature, and it is capable producing damaging earthquakes, but even the most destructive quakes, like 1906 in San Francisco (death toll 3,000), is but 1/30 of the energy of a magnitude 9 quake like that of Indonesia in 2004 (where the resulting tsunami killed 200,000 people).

It's taken a couple of months to work through our journey, so I've compiled all of the posts here so one can catch the continuity of the story. Here goes...

A New Blog Series

The introduction to the new series, a geological transect from the California Coast to Yosemite Valley, crossing an ancestral subduction zone that once caused geological havoc in a zone from Mexico to Canada (and still is in a few places).

Reconnaissance

An overview (in the most literal sense) of the lands we will traverse on our journey. We have a look at central California from above.

These Rocks are All Wrong!

Granite is exposed in the rocks of the Point Reyes Peninsula. But the arrangement of rock and sediment in subduction zones suggests that granite shouldn't be anywhere near here. It's the San Andreas fault. In California, it's always the San Andreas' fault.

Looking for the Big One

The peace and serenity of Tomales Bay belies a violent past. The San Andreas fault slices right through the bay, and produces large earthquakes with disturbing irregularity. The epicenter of the San Francisco quake in 1906 was not far from here.

Welcome to Geology's Junk Drawer

The Marin Headlands began as a giant collector of geological flotsam and jetsam from the crust of the Pacific Ocean. The jumble of rocks accumulated in an accretionary wedge, and were later lifted up into the mountains of the Marin Peninsula.

Geology's Junk Drawer on the Marin Headlands

Exploring the hidden corners of the Marin Headlands, we find Redwood forests, beautiful views of San Francisco, and disturbing reminders of World War II.

Terra Fatale on the Marin Headlands

Although the subduction zone that formed the rocks of the Marin has been extinct for a long time, there are still plenty of hazards remaining in the region, both geological and nautical. The Point Bonita Lighthouse has been present in one form or another for 160 years. It hasn't always worked, as there are upwards of 300 shipwrecks in the area. There are other hazards too.

The Alien Bursts Forth in the Diablo Range!

We head across San Francisco Bay and start an arduous journey through the Diablo Range. The range formed as an exotic (read "alien") terrane pushed upwards through the sediments of the Great Valley Group, piercing the surface and rising into the sky. John Hurt would no doubt approve...

Exploring the Belly of the Beast in the Diablo Range

We make the first part of a long drive through a rugged portion of the Diablo Range, one of the largest sub-ranges in the Coast Ranges of California. Along the way we cross through two exotic terranes of rocks that had been carried miles deep into the crust in the accretionary wedge of the subduction zone.

At the Portal of Hell in Diablo Range

Making our way down Del Puerto Canyon in the Diablo Range, we do the equivalent of traveling through the crust of the Earth all the way into the mantle, finding outcrops of peridotite and dunite, rocks containing the mineral olivine among others. For the mercury miners, this truly was a portal to Hell.

Exploring the Ocean Crust without Unobtanium

Del Puerto Canyon cuts a swath through the Coast Range Ophiolite, the remnants of the ocean crust that once lay at the bottom of the Pacific Ocean. Though mostly in private ownership, the canyon is one of the most scenic in the Coast Ranges, and a pair of county parks in the upper reaches invite exploration.

Into the Realm of the Drowning Dinosaurs

Sediments accumulated in a deep trough along the west coast of North America called a forearc basin for upwards of 100 million years. The layers reached a depth of 5 miles! In those waters swam mosasaurs (yes, like in the recent Jurassic World movie, but they forty feet long, not a hundred), plesiosaurs, ammonites, and occasionally a drowning dinosaur. The first discovery of a dinosaur in California happened here in 1936.

The Sea Floor that became the Greatest Agricultural Region on Earth

The American Serengeti is a vast plain 400 miles long and 50 miles or so wide that once was the sea floor. The grasslands of the Great Valley once supported millions of migratory birds and grazing animals. It still supports millions of organisms, but these days, those organisms are humans. 95% of the original prairie has been developed for agriculture and the region produces a quarter of the nation's produce. And all of the almonds and walnuts.

In the Pleistocene, a Different Kind of Danger

The Great Valley would have been a most dangerous region for a different reason in the Pleistocene. Among the great herds of grazing animals there were predators, and they were adapted to bringing down giant prey, not the small game we find today. Gigantic Short-faced Bears, Saber-tooth cats, American Lions, Jaguars, and Dire Wolves.

The Dr. Who of Mountain Ranges

At least three Sierra Nevada ranges have existed throughout time. They might have even once been higher than today. The distorted deformed metamorphic rocks tell the story of the earlier ranges.

A Gentle Landscape Belies a Fiery Past

The Valley Springs formation, exposed throughout much of the Sierra Nevada foothills region, forms gentle grass and oak covered slopes, but the rock is made of volcanic ash that originated in monumental explosions millions of years ago. The ash came from gigantic calderas, some of which were hundreds of miles away in central Nevada.

A Landscape Buried in Hot Mud, and a 6-foot Long Saber-tooth Salmon

A lot of volcanoes in the world are made of mud. Lots and lots of mud. But this mud formed in violence. The Mehrten formation of the Sierra Nevada has other surprises too: gigantic tortoises, and six-foot long salmon...with fangs...

A Tale of Two Subduction Zones

There have actually been at least two subduction zones in California. Remains of the older one still make up the rocks of the Sierra Nevada Mother Lode, and those rocks were the source of gold in the Gold Rush.

Exploring the Underside of the Volcano

We wrap up the series in the heart of the ancient magmatic arc: Yosemite Valley. Walking among the towering cliffs, we are reminded that the rocks were actually formed within the magma chambers of volcanic systems, perhaps similar to Lassen Peak, Mt. Shasta, or even at times, Yellowstone.