The Olympic Mountains: Accretionary Wedge Geology
Introduction
A few months ago, we followed US101 along the Washington coast and reported on Eocene (56-34 Ma) sedimentary rocks that were deposited in the trench that fronts the Cascadia subduction zone. Today I am expanding my investigation into the heart of the Olympic Mountains, which comprise the northern extent of the sedimentary wedge known as the Coast Range (see Fig. 1).
Figure 1. This schematic representation of a subduction zone beautifully encapsulates the Olympic Mountains, which are labeled on it as well–even Olympic National Park. The Coast Range (shown in green) is the exhumed remains of the sedimentary wedge, as shown in purple and labeled as the Trench. It is useful to note that, as subduction has continued (at least since the Eocene), the trench moves seaward and older, previously buried, ocean sediments are pushed up over the subducting ocean plate. This motion is indicated by the white arrows on the front of the block diagram.
Figure 2. (A) Map showing Lake Crescent (outlined with a box) and Hurricane Ridge, the two locations I’ll be discussing in this post. I presented some of the geology of Victoria, British Columbia, and Vancouver Island in a previous post.
(B) Geologic map from RockD of the Lake Crescent area. Two rock types dominate the area: Eocene marine sediments and slightly younger basalt flows and volcanic breccia.
(C) Detail map of the area outlined in panel B, showing the Spruce Railroad Trail. The circle outlines the two locations discussed in the next section.
Lake Crescent
Figure 3. Lake Crescent fills a deep (~600 feet) valley created by glaciers over the last two-million years, which was blocked by a landslide several thousand years ago. The shoreline in this image is broken by a series of linear ridges. These are resistant layers of sedimentary rock that has been rotated to a high angle.
Figure 4. The entrance to McPhee tunnel has been reinforced with rock bolts, which can be seen throughout this bedding plane. The left side of the photo shows why the tunnel was dangerously unstable: the ridge through which it was cut is composed of thin layers of interbedded sand, silt and clay.
Figure 5. McPhee tunnel is only a couple hundred yards long (see Fig. 2C), but the exit reveals completely different rocks. There is a red line in the geologic map (Fig. 2B); this represents a contact between the marine sedimentary rocks (Fig. 4) and the volcanic rocks.
Figure 6. (A) Outcrop scale photo of basaltic rocks near the exit to McPhee tunnel. I have tentatively labeled one round boulder as a pillow basalt. Pillow basalts result when lava erupts on the sea floor in deep water (~1000 feet deep) because of the immense water pressure. When the rock weathers, they retain their shape because of a more homogeneous composition than the lava surrounding them.
(B) Detail showing the wide variation in clast size and shape. This mix of material, all of it basalt, is indicative of a tectonic breccia. Imagine what happens to undersea lava flows as they weather through seawater action and collapse into the trench (see Fig. 1).
Hurricane Ridge
Figure 7. This photo shows Hurricane Ridge (see Fig. 2A), which leads to Hurricane Hill at about 5500 feet above sea level. The next few photos show the kinds of rocks visible along Eagle Point trail, which leads to the summit.
Figure 8. These sedimentary rocks resemble those at McPhee tunnel’s entrance (Fig. 4). They are tilted to nearly vertical, consist of dark and bright layers (mud and sand/silt, respectively), and are fairly continuous; however, they have an additional feature that might escape one’s attention at first: some of the darker layers contain rounded forms that are brighter. If I’m right, these are boudins, concentrations of silica-rich minerals that result from the rocks being squeezed under high pressure.
Figure 9. (A) This outcrop (about 20 feet across), located a few hundred yards from Fig. 8, is dominated by sandy sediments. The surface is littered with angular blocks, which suggests that these rocks were subjected to intense deformation after they became lithified (turned to rock).
(B) Detail of area indicated by the rectangle in (A). Now we can see bedding planes, e.g. in the center of the photo. These beds are also nearly vertical. These are reminiscent of the sandstones we saw along the coast; I’ll bet they contain turbidites, layers of coarser sediment deposited by submarine landslides in deep water.
Figure 10. (A) These thin beds have been folded into a chevron. This implies extreme horizontal stresses.
(B) The bedding is almost obliterated in this exposure, where coarser sediment (lighter colored) is mixed with shale. Furthermore, the entire assemblage is fractured into small pieces like those lying on the ground.
Figure 11. This view from the top of Hurricane Hill is looking northward towards Vancouver Island, British Columbia. The main features seen here are the knobs that protrude from the summit of this adjacent ridge. Promontories like this were ubiquitous along the ridges comprising the Olympic Mountains. Compare this topography to the narrow ridge leading up to Hurricane Hill (Fig. 7). The fractured and folded rocks we saw along the trail (Figs. 9 and 10) have weathered faster than these, but what are they?
Figure 12. (A) Closer view of the left side of Fig. 11, showing several interesting features: a series of knobs to the left, then a deep ravine cutting across the ridge, then massif to the right. These are basalt that has not been as deformed as the sedimentary rocks we saw lower down the trail. Seafloor basalts erupt into layers of marine sediments, but they are harder and more resistant to both chemical and physical alteration during burial and deformation. These layers are unusually strong layers.
(B) Close-up of a knob on Hurricane Hill. I didn’t approach closer because signs asked visitors to remain on the trail, and it was a pretty steep slope into a deep canyon. I don’t see any possible basalt pillows (see Fig. 6A); instead, it seems to be fractured into rectangular blocks. However, some of the rounded boulders seen in the foreground look like lava pillows. RockD calls this formation, Crescent Formation, pillowed…
Final Thoughts
The Coastal Range extends from Alaska to Mexico but not as a continuous mountain range; instead, it comprises a series of mountain chains like the Olympic Mountains. These individual mountain chains are lumped together because they all formed during subduction of multiple tectonic plates underlying the Pacific Ocean during the last 200 million years. The Olympic Mountains are simply the most volcanically active area in recent times.
The subduction zone trench (see Fig. 1) is continuously filled with sediment eroded from the adjacent highlands in a process that doesn’t cease until plate convergence ends. Sediment is carried into the trench, buried, then exhumed ; there is simply more volume of sediment/rock than the trench can contain. Today the trench is approximately 100 miles west of Seattle whereas it was located within the Olympic Mountains between 55 and 34 million-years ago, a difference of about 70 miles; in other words, the subduction zone has moved about 30 miles westward in less than 60 million years.
I am bothered by one discrepancy, as I understand the picture of marine sedimentation and eruption of seafloor basalt in the westward-propagating trench: Where is the modern analog of the thick sequences of basalt that occur within the Olympic Mountains?
Things to consider (I don’t have the answers):
- Volcanism within the Cascades Range (see Fig. 1) is overwhelmingly andesitic, which results from mixing of the sinking ocean plate (gabbro and basalt) with continental crust (granitic) as magma rises.
- Massive outpourings of basalt occurred in eastern Washington between 17 and 5 Ma, known as the Columbia River Basalt Group.
- The North American plate has been moving over a hot spot in the upper mantle currently located beneath Yellowstone National Park.
- Pillow basalt is a definitive indicator of submarine eruption, as seen occurring today in Hawaii. Thus, these features are consistent with either (a) eruption of flood basalt in the trench or (b) collision of a sea mount with North America.
Every subduction zone is unique and overprinted with whatever else is going on in the earth’s crust and upper mantle. I wonder if the Olympic Mountains basalts are the result of mixing of marine sediments with flood basalts, and possibly incorporation of islands or seamounts.
Food for thought…
Addendum
After thinking about it a little longer, I want to add that the basalt breccias we saw (Figs 6 and 12) probably originated from the subducting ocean crust. It would have been covered by deep-sea sediments, including muds and limestone (from ocean phytoplankton). These would have been scraped off and mixed into sediments with a terrestrial source. That would explain layers of basalt intercalated with marine sediment; however, the question of pillow lavas remains controversial because they aren’t found at mid-ocean ridges, where ocean crust is created.
So, no flood basalts but probably a seamount is mixed into the Olympic Mountains.
The Olympic Mountains
The Olympic Peninsula is home to the last old-growth temperate rainforest in N. America. It is also dominated by the Olympic Mountains, a wilderness area only accessible on foot for the most part. However, the north side, facing Juan de Fuca Strait and Canada, is only a few miles from US 101 and Port Angeles; and less than three-hours from Tacoma.
It’s late September and the tourist season is about to end in Olympic National Park, so we fit in a weekend visit to the Log Cabin Resort on Lake Crescent.
The highest peak in the Olympic Mountains is Mount Olympia, standing at 7980 feet, and there are many peaks over 6000 feet. None of them are volcanoes, however; they were formed by collision of N. America with the tectonic plate underlying the Pacific Ocean. I’ll discuss that in another post.
Lake Crescent was created when a landslide blocked the outlet of a valley cut by alpine glaciers over the last couple million years. Its maximum depth is 600 feet, and it contains exceptionally clear water because of a low nitrogen content (no pollution) and thus no algal growth. It is twelve miles long and thus popular with sport boaters. I didn’t see anyone fishing, so maybe no algae means no fish.
We followed the Spruce Railroad Trail 1.7 miles to this tunnel, which is huge when you pass the smaller entrance; however, it is only a couple hundred yards long. Nevertheless, as the sign says, it is very dark inside, especially later in the day. It leads to a favorite site for visitors.
The Spruce Railroad Trail circumnavigates Lake Crescent (about 11 miles), but we didn’t go that far. Devils Punch Bowl is a deep hole with steep cliffs and a walkway. Adventurous young people were leaping into the cold water and sunning on the nearby rocks, but I didn’t see anyone jumping from the top of the cliff more than 60 feet above us.
This is the view from our back yard. I took some photos of the half-moon when it was still light, but this is my favorite because of the moon’s reflection on the still water of Lake Crescent.
We spent a very quiet night in our log cabin, which had a modern heating system and was very cozy, then drove to the Hurricane Ridge trail head. The trail was asphalt paved and climbed about 1000 feet in (supposedly) 1.8 miles. It was steep near the top but wide enough for wheelchairs to use easily. This is the view from the top, which was at an elevation of about 5500 feet; that’s Mount Olympia across a deep valley.
This is the view looking north. That’s Vancouver Island, British Columbia, across the cloud-covered Juan de Fuca Strait. The brighter area above the two peaks is Victoria. We were there last month.
This photo shows the trail we followed along a ridge like those seen in the background. The many sharp peaks are reminiscent of the Cascades Mountains, but these were formed by alpine glaciers in combination with a high uplift rate, rather than volcanic processes.
Final Thoughts
The weather in the Olympic Mountains was exceptionally nice this weekend. The drive was easy and traffic low because it was the last day of the season. Overall, I can’t think of a better place to spend a couple of days.
Try to make it if you’re ever in the Pacific Northwest…between April and September.
Port of Tacoma Tour
Every year, the Port of Tacoma and Pearce County operate a series of one-hour tours on a cruise ship. There is no charge and passengers have a timed ticket, so there are no long lines. It turned out to be a rainy day, but that didn’t dampen the enthusiasm of the Tacoma’s residents.
The Spirit of Seattle usually runs various tours around Seattle, operated by Argosy Cruises. It was only half full for the port tours so there was plenty of space to move around on the deck.
The first facility we saw is the tank farm, which temporarily stores refined petroleum products for distribution by tanker trucks. There is also a small refinery on the port, but the guide didn’t say anything about that.
This power plant was used to generate electricity for the port, but it is no longer operational. In fact, it is for sale.
This is s roll-on-roll-off ship that, as its name suggests, is used to transport vehicles that require no special handling. However, it also transports heavy equipment, some of which requires partial disassembly to fit into the ship’s hold.
These are the main cranes for handling container ships. The one in the foreground is unloading a ship (One Strength, registered in Singapore) that can carry about 14,000 TEUs–twenty foot equivalent cargo container–or about 7000 40-foot shipping containers. The largest container ships can carry 24,000 TEUs.
Along with storage facilities for cargo, an aluminum plant, and a couple of gypsum plants, there is a repair facility. It is currently preparing an ocean-going fishing ship for the upcoming season.
The Port of Tacoma also handles grains and cereals through a modern facility that can load these bulk ships in all weather. The corn, soy beans, etc arrives by trains that arrive practically year round, mostly from central and eastern Washington.
Final Thoughts
Dreary weather didn’t hamper my interest is seeing the Port of Tacoma up close. This place can handle everything a nation needs to import or export. Places like this are the real backbone of our economy, not computer chips and programmers.
But running a complex facility like this isn’t easy. The smelter that used to operate along Commencement Bay produced so much pollution that my back yard was part of an EPA Superfund site, and there is a current site along the eastern shore where an aluminum plant operated for decades. We have a long way to go, but the waters surrounding Tacoma are the cleanest I’ve ever seen in my life.
A Quick Visit to Portland
Mount Saint Helens is only 25 miles as the crow flies from Portland, so we stopped by our southern neighbor for a visit. It was raining, so we opted for indoor activities.
World Forestry Center
The World Forestry Center is next to the Oregon Zoo, and it was a dry place to wait for our next stop to open. It turns out that it is a children’s museum. A new exhibit had just opened so we toured Siberia by train, China by boat, the Amazon by a tree-top crane, and…
South Africa in a 1960s International Harvester Scout. I was ready to go, but the engine wouldn’t start.
Someone left a 22 caliber rifle in this tree back in the 60s, and it became one with nature.
This fake evergreen tree extends to the ceiling in the middle of the World Forestry Center. There were lots of activities for children of all ages, so this is probably a good place to visit with the family.
Japanese-American Museum of Oregon
After discovering forestry around the world, we went to the museum.
JAMO is small, but very interesting.
The early Japanese immigrants lived much like the Chinese, whose accommodations we saw in a previous post. By the 1930s they had established themselves throughout the West Coast, especially the PNW. This is a representative display of a general store.
There were dentists and all the trades because, despite being legal residents–many of them American citizens–they were prevented from fully participating in the American dream.
They lived in nice homes, owned businesses, spoke English better than most of their fellow citizens, but…
There weren’t enough of them to overwhelm the government’s demographers (unlike the German and Italian Americans), so they were rounded up in a show of national security; they were shipped to places where, if they were lucky, they shared a room like this with their entire family.
The JAMO has a lot of documents to read, but I focused on physical objects because I forgot my “museum glasses”, which focus at six feet. You’ll need to visit the museum yourself to learn the true history of the Japanese-Americans.
One brave activist violated curfew after release from a detention center, and found himself in jail for nine months. He eventually won in the courts, but at a high personal cost.
This terrible injustice wasn’t as horrific as what was done to the Native Americans, but it was pretty bad. The U.S. government eventually apologized, in the 1990s, but that was little consolation to those Americans who lost everything during a period of national paranoia. Some of them actually received a few thousand dollars in reparations (I guess they used 1940s dollars, not adjusted for inflation).
Mount Saint Helens Revisited: Ape Cave
We made it to our destination (Fig. 2) and explored a rare lava tube at Mount Saint Helens. This lava was erupted 1900 years ago as part of the same sequence that destroyed the forest a short distance away.
A full discussion of this fascinating natural feature is available from Washington Department of Natural Resources.
Figure 1. This lava tube is a lot larger than the tree casts we described in the last post. This immense cavern, which is more than 2.5 miles long, was created when a single basaltic lava finger flowed over the landscape. The outer material cooled in the air and insulated the interior, like an oven, allowing the remaining lava to exit the end of the tube. This left a void to be filled by subsequent lava flows. According to the information displayed at the site, the entire eruption lasted about 100 days.
Figure 2. Ape Cave is presumably named after the first eruption sequence of Mount Saint Helens, dated about 40 Ka. I didn’t find an explanation for this confusion, but it might have something to do with the Bigfoot legend associated with Ape Canyon, not far away. At any rate, the lava tube is indicated by the square in (B).
This video captures the scale of one of the many chambers better than any image. My narration isn’t very technical.
Figure 3. These parallel ridges running along the bottom of the cave near its terminus are the remnant of the last flow to use this conduit to reach open air.
Figure 4. A block of basalt transported through this narrow passage became stuck where the walls of a previous flow almost meet, leaving it stranded. It looks like it partially melted into the walls, however.
Figure 5. This view of the ceiling reveals a joint pattern caused by cooling of the outer layer of lava. Several generations of fractures occur throughout the cave, created as individual flows cooled differentially.
Figure 6. This is the end of the tube as it exists today. Apparently, there is no modern opening; it probably collapsed during the last 1900 years.
Figure 7. As a retired sedimentologist, I immediately noticed the layer outlined by the dash lines. This looks like cross-bedding; the layer fills a depression below the line, and is truncated by an overlying flow. This is interesting because cross-bedding is associated with particles being transported by wind or water; how it appears in a lava flow is an intriguing problem–perhaps heavy minerals like pyroxenes and lighter plagioclase feldspar?
Figure 8. This is a good time to briefly explain how the ledges seen in Figs 1, 4, and 6, and the video, were formed.
Once the lava tube was created by cooling of the surface of the original flow, subsequent flows ran through, but didn’t fill, an insulated tube. Their surface was usually below the original ceiling. The coolest parts of this conduit would have been the sides (heat dissipates more quickly through older, cooler basalt than the superheated air above the lava river). Thus, ridges like these formed where the molten basalt hardened. Of course, these subsequent flows were of different volumes and the rules of stratigraphy cannot be applied. Geologists have worked out these relationships but that is beyond the scope of this report.
Let’s just say that many rivers of magma flowed through this channel, like stormwater through a sewer pipe–some larger than others.
Figure 9. This is an enigmatic photograph because I don’t remember which wall it was on. I didn’t notice the detail when I took the photo, and now I can’t say if this is an excellent example of cross-bedding (always tilts downstream) as in Fig. 7. or a total mystery. My only excuse is that it was TOTALLY BLACK in the cave and I was disoriented the entire time.
Figure 10. We made it out without spraining an ankle or hitting our heads. This cave-in is the only entrance/exit for the downstream part of the tube. There is another one 1.5 miles upstream, but that route is difficult (according to the park information).
Final Thoughts
The Ape Cave lava tube, and the missing forest from my previous post, are part of the Spirit Lake eruption phase of Mount Saint Helens, which began about 1900 years ago and continues to this day. The magma chamber didn’t erupt at the center of the stratovolcano, but instead found an opening along one of the many cracks in the thick pile of volcanic rock that had accumulated during at least 50 million years.
Lava tubes present a unique eruption environment. Ground water that finds its way into the magma chamber cannot escape, thus pressurizing the conduit and altering the lava’s behavior. Imagine lava as thin as water surging through this passage, sloshing around, encountering obstacles, rushing headlong downhill towards what apparently was a small opening, if Fig. 6 is any indication of the narrowing channel.
It was a great drive through a wild terrain to reach Mount Saint Helens, and the destination was worth the effort.
Mount Saint Helens Revisited: A Missing Forest
My last post reported on Miocene (23-5.3 Ma) volcanic rocks on the eastern flank of Mount Saint Helens. Those rocks predate the creation of Mt St Helens, however; we reached our destination and, before exploring a lava tube called Ape Cave, we examined some interesting geology that is part of the most recent eruptive stage of this active stratovolcano. The Spirit Lake stage began about 4500 years ago. There were three earlier stages, beginning about 40,000 years ago; each lasted less than 5000 years.
Figure 1. About 1900 years ago, a large amount of basalt lava flowed from a fissure on the south flank of the volcano (indicated by a square in the inset map of Fig. 2). The lava was hot (~1200 C or 2200 F) and set the forest on fire; when the wood had been turned to charcoal, the lava cooled to leave casts of the trees. My wife is taking a photo of a small-diameter tube we’ll examine it more closely below.
Figure 2. Mount Saint Helens is about 70 miles south of Tacoma (Home in the right map), but the route we followed, around the north and east flanks, takes 3.5 hours to drive. The yellow areas in the inset map are mostly andesite lava flows older than 5 Ma. The circle is where we looked at them in the last post.
Figure 3. These holes were everywhere, forming a hazard for people walking, so a walkway was constructed over the area where the tree holes are densest. They haven’t been cleaned out and most are 3-4 feet deep.
Figure 4. This is the cast of a fallen log. The entrance is shown in Fig. 1; visitors are encouraged to crawl through it, about 40 feet, but it was too tight for us. However, several children I met there had braved the dark, tight tunnel.
Figure 5. This photo was taken a couple-hundred yards from the missing forest. The background reveals the leading edge of a lava flow that’s about ten-feet high. The foreground shows a stream being incised into the landscape, probably created along a collapsed lava tube. Regrowth of the forest is slow in the lava surface; however, there was no mention of whether or not this area was logged in the last century–it probably was, given the documented greed of the forest industry when it came to cutting down trees. However, I didn’t see any large stumps, so maybe it wasn’t worth the effort.
Figure 6. This is a mound of lava projecting up to 8 feet above the surrounding landscape. Several features are worth mentioning: 1) it is circular in plan and about 30 feet in diameter; 2) the upper-center of the image shows concentric bands of ropy lava known as pahoehoe, from its common occurrence in Hawaii; and 3) there is a small cave beneath it where lava ran out of the hardened surface. We’ll see a lot more of that in my next post.
Final Thoughts
Our journey along the backroads took us through a vast terrane of andesitic volcanism created more than five-million years ago. Then, about forty-thousand years ago, a new phase of volcanism began and Mount Saint Helens was created from this rugged ground.
Stratovolcanoes don’t just erupt at their summit, even though that is exactly what Mt St Helens did in 1980; more often they leak lava from the myriad faults and fissures surrounding them, especially in an active subduction zone where the earth’s crust is being compressed, twisted, and warped by tectonic forces.
Life is difficult in such an environment and we are fortunate to have a moment in time frozen for us to contemplate how immense the forces are that constantly reform the surface. Eruptions like the one that destroyed this forest are unlikely to kill many people, but they are crucial in reforming the environment. A new forest grows, possibly inhabited by different species of fauna and flora, and life goes on.
My next post will examine this same eruption event, but at a scale that dwarfs what we saw today.
Mount St. Helens Revisited: Miocene Volcanic Rocks
I couldn’t stay away from this fascinating active volcano. I reported on the recovery of the north flank, after the 1980 eruption, in a previous post. Today I visited the eastern and southern flanks, which were spared the devastation of an enormous explosion.
Figure 1. Mount Saint Helens National Volcanic Monument is about three-hours from Tacoma (Home in the left plate). We took back roads and circumnavigated the volcano today, following NF 25 (seen in the right plate), which took us through a volcanic terrain constructed during the last 23 million years. The circled area is the focus of this post. The rock types vary widely, but they are predominantly andesitic flows, ash layers, and breccias. The usual mixed bag of volcaniclastic rocks we expect in a subduction zone plate margin.
Figure 2. This photo reveals a series of thin beds that suddenly dip towards the road. They are highly fractured to the right of the image. Obviously, these were not recently erupted; they have been deformed in the last twenty-million years.
Figure 3. This image shows layers of volcanic ash that are weathering to reveal how thin the laminae are.
Figure 4. This photo dramatically reveals how much can happen in 10-20 my (million years). It was taken a few hundred yards from the previous photo. To the left are layers of ash that dip away from the camera–juxtaposed with steeply dipping layers that curve to the right and become almost horizontal (in the plane of the image) in the upper-right image quadrant.
Figure 5. This image, taken less than a mile from Fig. 4, reveals thick layers of ash and flows. I didn’t examine them closely, but the middle-right of the photo shows a substantial change in bedding.
Final Thoughts
Mount Saint Helens is part of the Cascades range, an elongate series of volcanoes that have been active for the last 60 my because oceanic crust is being subducted beneath the North American tectonic plate. The rocks we saw today are from an earlier phase of volcanism; they were buried to depths of several miles, deep enough to become cemented, but not deep enough to become ductile. Thus, they failed in brittle fracture as they were compressed by the huge pile of volcaniclastic rocks created along the ocean margin.
What comes up must go down. This simple phrase means that the earth’s upper mantle (including the subducting ocean plate) melted and produced a lot of magma, which rose because of its lower density and high pressure, filling every fracture, expanding them into magma chambers. The mantle collapsed because of these voids and the weight of the recently extruded magma. A balloon being repeatedly inflated and deflated.
Meantime, the compression was inexorable. The recently buried volcanic rocks broke along faults, creating the juxtaposition of volcaniclastic material we see in Fig. 4.
These aren’t geologically old rocks. This is a continuous process that we are able to witness in real time throughout the world.
We’ll see this continuous process in closer detail with my next post.
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