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.
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.
Flaming Geyser State Park
The park is located at the upstream end of Green River, where it enters a gorge. This is one of the best places I’ve seen anywhere for taking a stroll through nature; the path is moist but not muddy, and well trimmed, with breathtaking natural scenes like this. A veritable Garden of Eden. This post is an overview of the park; I discuss the geology and flora in other posts.
With a name like that, I’m expecting a spectacular show…
I’ll summarize what you can read if you zoom in on the information presented in this billboard. An exploratory well was drilled here about 100 year ago, looking for coal–they found it, along with methane (natural gas) and other gases produced by decaying vegetation at several hundred feet below the surface. The small well produced a flammable gas that has burned intermittently since it was first lit by a geologist.
Flaming geyser wasn’t very active when we visited, but it was burning. To repeat, this is NOT a natural seep, but the result of a well drilled a century ago. Nevertheless it has become a local tradition, and it is relit whenever it goes out.
Bubbling geyser is a lot more fun. This video shows small bubbles of methane, as well as hydrogen sulfide and other, unnamed, gases. Methane is odorless, but this area smelled strongly of H2S, which is added to natural gas to help detect leaks.
This is a beautiful example of survival. The surface roots have all been exposed by erosion, but the tree is thriving; its roots have turned into separate trunks. I read somewhere that these conifers in the PNW send their roots out for hundreds of feet, far beyond their canopies.
This is where Green River Valley turns into Green River Gorge. The water was perfectly clear, with a slight green tinge, even though there is no algae visible. It reminds me of Commencement Bay, which is also devoid of aquatic plants. And it is all natural, simply a unique ecosystem.
If you wear waders, you can walk on water here.
Final Thoughts
This is a place you have to visit for yourself, especially on such a gorgeous summer day. The temperature was in the mid-seventies, the sky was clear, and people were filling the huge number of picnic areas, carrying their supplies and tubes, ready for a fun day of floating down one of the best rivers I’ve ever seen.
You’ve got to see it for yourself.
The Foothills Trail along Carbon River, Pierce County, Washington
This post is a humorous (I hope) addendum to my posts on the geology and flora of Carbon River. I have noticed a lot of quirky, regional trends, both in decoration and signage, here in the Pacific Northwest (PNW) during our nature walks. The Foothills Trail was no exception.
There are several access points to the trail, but this was the only sign. It ends with a serious warning: Use at your own risk. What lies ahead in that darkness?
A couple miles downstream from the “official” trailhead, we stopped along the Carbon River. This is a piece of ubiquitous columnar andesite that was apparently placed here as a decorative piece. Unfortunately the sign is long gone.
The trail is wide and easy to access on foot. I don’t know about a wheel chair although it is flat all the way.
We saw several of these marker stakes along the trail. I looked it up and, to the best of my knowledge, this is indicating a tree where a pair of Marbled Murrelets are nesting. They are an endangered species and extremely difficult to locate, but there is an ongoing survey program. That sounds reasonable to me; the sign gives both a distance and compass heading to the tree.
The trail intersected the Franklin Bridge, which is indefinitely closed to traffic.
We climbed a steep slope, with the help of sturdy handles attached to wire rope, and took some photos on the bridge.
After we finished on the bridge, we passed the concrete blocks obstructing the road. Several signs made it clear that the bridge is closed to cars, bicycles, and pedestrians. Apparently someone disagreed with the DOT restrictions, and modified the sign. Don’t ask me why the figure is carrying a gas can that says, “Oh ya fuck ya.” I guess they’re thumbing their nose at anyone who would dare condemn what looks like a perfectly useful bridge — at least for people and wild animals.
We walked the three miles back to our car along the abandoned road. There were several rock falls and damage to the guardrails. In this image, a large boulder smashed through the eroded roadbed. It was like a post-apocalypse movie. Pretty cool … and quiet.
These relatively new signs were posted randomly on trees on the hill of the road. Hazards. Not hazard. Maybe Western Rattlesnakes, poison ivy or … dare I say it? Bigfoot. Stay out of the woods. But we just walked three miles through the woods. Damn!
These opposite-pointing signs were on a curve. Nevertheless, I have to wonder why the guardrail stops in the middle of the curve, but reflective signs were felt necessary. Short sightedness? Budget constraints? Stupidity?
The Flora of Carbon River, Pierce County, Washington
Plate 1. Our first stop was at Carbonado, where the Carbon River exits the canyon that has constrained it from its origin on Mt. Rainier. It immediately forms multiple channels as it cuts through glacial till on its circuitous path to Commencement Bay.
Plate 2. This lovely flower is red clover (Trifolium pratense), a common wildflower along rivers.
Plate 3. CoPilot and I argued too long about this one, but, after some back and forth, I decided it might be a genus of Spiraea. They are another common shrub in this ecosystem, but they aren’t in flower in July. Note the shriveled flowers in the right image. Too late to be sure. That’s why I’ve always ignored these bushes.
Plate 4. Cow parsnip (Heracleum maximus) is a common wildflower in meadows and roadsides in Washington.
Plate 5. Tansy (Tanacetum vulgare) is a common wildflower along the Carbon River corridor.
Plate 6. I thought, when putting this post together, that this was red clover (Plate 2), but CoPilot identified it as Canada thistle (Cirsium arvense). It loves disturbed areas like river edges. I did detect some spines when I zoomed in on the second photo. Why am I not surprised? If it’s pretty it will have spikes ready to draw blood from any inquisitive passers by.
Plate 7. This view is looking downriver from Franklin bridge, which is permanently closed. I’ll now describe some of the flora we encountered as we followed the river along a steep slope.
Plate 8. I (kinda) got this one right. This is a wild rose, Nootka rose (Rosa nutkana). This is another plant that waits for an opening in the forest canopy to grow, and then it colonizes like crazy — until the forest canopy catches up with it. Moist but well-drained soil pretty much sums up Carbon River canyon.
Plate 9. This familiar flower is foxglove (Digitalis purpurea), introduced from Europe; but it is so naturalized that it behaves like a native plant, similar to Cascade penstemon (see Plate 11), thriving in disturbed ground at the edge of conifer forests and in temporarily sunny areas.
Plate 10. Besides white flowers, I am also fixated on thorns; however, I don’t know anything about thorns except that roses have them, as well as many of the shrubs I’ve encountered on my nature walks. This is probably Himalayan blackberry (Rubus armeniacus), an invasive species to Pierce County. I guess that’s why it isn’t called Washington blackberry, or some such. I checked a previous post from Point Defiance, and there it was! Invasion of the forest snatchers.
Plate 11. This purple wildflower is probably Cascade penstemon (Penstemon serrulatus), a native to the PNW. It is what CoPilot calls an ecotone plant — one that thrives where a conifer forest is encroaching on a prairie environment. We encountered it where the conifers hadn’t yet blocked out the sun along the trail.
Plate 12. I’m easily fooled by white flowers. This non-woody shrub is False Solomon’s seal (Maianthemum racemosum). It grows in the understory of a conifer forest, which is a good description of Carbon River canyon.
Plate 13. This is the first triplet I’ve seen from a nurse log. These young hemlocks will certainly compete for sun and nutrients. There can be only one survivor.
Plate 14. I thought this was cow parsnip, but CoPilot identified it as ocean spray (Holodiscus discolor). The white flowers were all I needed to see, but this tall, woody shrub (6-12 feet) is completely different from the weed in Plate 4.
Final Thoughts
The only thing I can add to the photos is that Carbon River is part of a transitional ecosystem, not unlike Mima Mounds. I Imagine this borderland environment is also alternating between a lowland/prairie and an upland/conifer forest, depending on conditions. However, much to my disappointment, the plants here were different from those we saw before. Furthermore, there were far more species along the Carbon River corridor. We were fortunate to visit when many of the flowers were in bloom.
I enjoy taking pictures of flora and learning about the ecosystem, but this is too complex for me. I’m only a simple geologist.
Geological Survey of the Columbia River Gorge
The popular route east from Portland, Oregon, is I84 following the Columbia River, which cuts across the Cascades range. There are plenty of scenic views and geology to examine, but few safe places to stop. Thus we followed the Washington shoreline along state route 14.
The inset map shows the distribution of volcanic rocks within Washington and Oregon. The oldest are predominantly andesites erupted from volcanoes (triangles) within the Cascades between about fifty and five million years ago (Ma), shown in light brown. The bright green represents the Columbia River Basalt Group, which flowed from fissures between seventeen and five Ma. The youngest rocks are primarily andesite erupted from volcanoes within the last million years (e.g. Rainier, St. Helens, Hood, Baker). The rectangle shows the area we are traversing, which contains a mixture of these rocks.
We stopped frequently, but I’ve lumped the photographs into four areas: 1) Beacon Rock is near the beginning of Columbia River Gorge; 2) Lake Bonneville and 3) Hood River give a good picture of the central canyon; and 4) Columbia Hills is where the river enters the gorge before cutting through the thickest section of volcanic rocks.
1. Beacon Rock
This photograph looks east towards Beacon Rock, which has an interesting origin. It was originally injected into a cinder cone volcano about 60 thousand years ago (Ka). Subsequent, multiple glacial floods eroded the loose material away, leaving the core, which is called a neck. This region was never covered by continental glaciers, although there is evidence of alpine glaciers like those still existing on the high volcanoes (e.g. Rainier or Hood). During numerous advances and retreats of continental glaciers into Canada, large lakes formed and periodically drained catastrophically. These floods, which were as deep as 1000 feet, naturally followed the Columbia River to the Pacific Ocean.
This low road cut reveals a thick layer of volcanic rock (basalt, according to Wikipedia) overlain by volcaniclastic rocks, which are loosely cemented. That’s why the DOT placed netting over the friable layer. These are sedimentary rocks consisting of volcanic ejecta as well as material transported by water.
According to Wikipedia, Beacon Rock is 848 feet tall and there is a trail to the top that is popular with hikers. It doesn’t look that high from the bottom, but I’m glad I didn’t trust my first impression and climb it; as stubborn as I am, I would have made it–and wished I hadn’t for the next week. It looks a little pale to be basalt, including the boulder visible at the bottom of the image; in a terrain with continuous volcanism, spanning the gamut from rhyolite to basalt, for 50 Ma, you just can’t tell from surface features. Some basalt is a little lighter colored and some andesite is darker–it’s a spectrum based on mineralogy, not color.
This eroded slope got my attention because it reveals an interesting juxtaposition of an exposed basalt outcrop that is rounded (unlike the earlier exposures we saw) and light-colored boulders of much smaller size (less than three feet). These rocks are too uniformly light in color to be weathering of basalt or andesite. There is some rhyolite (a leucocratic extrusive rock found within the Cascades) in the region, but an alternative explanation is that these are flood deposits from the aforementioned glacial lakes. There are many deposits from these mega floods within the gorge, but I couldn’t (easily) find a map of them. Anyway, this is what I would expect to find in such a sedimentary deposit–mixed rock types that are rounded by transport tens, if not hundreds, of miles during flooding episode. The bedrock would be rounded by collisions with these boulders. If the shoe fits…
2. Lake Bonneville
The central part of Columbia River Gorge is characterized by several broad valleys with sediments filling the margins of the canyon. This is a typical exposure from this area. The rock looks like basalt to me; the map (see first plate) shows a mingling of volcanic rocks along the river, which would have been a low point for lava to flow towards. However, this is not a volcaniclastic deposit as we saw before; instead, there are several, heavily weathered (i.e. smooth) flows of lava (3-10 feet thick). The lowest layer seems to be dipping towards the camera as if flowing down a steep slope. Maybe…
3. Hood River
This location is close to the eastern entrance to Columbia River Gorge, where flood basalts erupted from multiple fissures in the crust. In other words, there are no nearby volcanoes and steep slopes; thus, the basalt flowed over a relatively flat landscape, forming rolling hills. This photo reveals basalt flows that gently slope to the left, as seen in the middle-right and background of the image. These massive flows partially blocked the river many times–long before glaciers dominated the landscape. The island in the center of the channel is a remnant of one. I haven’t heard of any glacial lakes in this area, however, so the blockage must have been partial–these thick sequences of basalt didn’t occur at one time, but over millions of years, giving the ancient Columbia River time to erode passages through them.
4. Columbia Hills
Columbia Hills is the eastern end of the gorge, where the Columbia River ends its meandering path to the Pacific. The rocks are basalts erupted from many fissures between 17 and 5 Ma. According to the latest interpretation, these rocks were ejected from the same mantle plume that now underlies the Yellowstone caldera in NW Wyoming. They have nothing to do with subduction or the Cascades volcanic belt, even though the much younger Mt. Hood (in the background) towers over them.
We are now in Eastern Washington, a climatic zone with completely different characteristics than west of the Cascades. This volcanic range creates a rain shadow and resulting precipitation is less than 20 inches here; and it shows in the scrubland ecosystem. These extensive basalt flows are no longer covered by younger andesites from the high Cascades (the young volcanoes like Mt. Hood).
The volcanic layers are thin and extensive (see the map at the beginning of this post). They include columnar joints as I described in a previous post. The textures seen in this photo reveal the variability of lava coming from a single source; for example, individual, blocky layers cap this exposure whereas the rock presents a ropy texture lower down (middle-right of the photo).
Summary
The Pacific Northwest (PNW) didn’t exist before the Tertiary period, which began at 65.5 Ma. However, Pangea began to split apart at about 200 Ma, which should have created plate collisions here because the N American plate would have necessarily overrun the plates comprising the ancient Pacific Ocean. The west coast of N America was located approximately at the WA-ID boundary. So why don’t we see Jurassic and Cretaceous volcanoes and their associated volcanic deposits in the PNW?
This question has perplexed geologists for decades. After carefully collecting data from far and wide, a still-controversial theory has evolved: For more than 100 million years, this tectonic collision was accommodated by transform faults (e.g. the San Andreas fault system in California). A tectonic plate collision is not a conveyer belt as shown in schematic representations.
This schematic profile of the PNW shows several transform faults, which misalign the Pacific mid-ocean ridge (note the misalignment of the dark, Juan De Fuca Ridge. This tectonic scenario didn’t develop until those transform faults, which were not perpendicular to the mid-ocean ridge, could no longer accommodate the displacement of these microplates with N America. That apparently happened about sixty-million years ago. Some of these slivers of volcanic terrain have probably become exotic terranes that are now part of Alaska.
That is probably why we didn’t encounter any Mesozoic ((251-65.5 Ma) volcanic rocks within the Columbia River Gorge. Instead, we discovered a Tertiary volcanic landscape dominated by andesite/basalt lava flows, preserved because the transform faults had stopped absorbing the collisional, crustal tectonics. A real subduction zone emerged from this chaos and created the Cacades.
Superimposed on this was the unexpected (tectonically speaking) effusion of basalts as the westward-propagating N American plate rode over a mantle plume, which buried the evidence for this slipping history beneath miles of volcanic rocks. I can’t say anything else about this because I’m not actively researching the PNW’s geologic history.
My last word is that I can’t wait to see what new discoveries the PNW holds for me.
A Day Trip to Yakima
We wanted to see what was on the other side of the mountain, so we headed east from Tacoma and crested the Cascades at Chinook Pass. It was snowing at the pass and fog/clouds obscured what were probably majestic views. We pushed on to Yakima and found the Cowiche Canyon recreation and conservancy area.
This is a typical view from the trail, which follows an old rail line that was used for seventy years to haul apples out of the area. I have discussed the columnar basalts and vegetation in previous posts. It was late spring and all of the plants were showing their color while water rushed past in the creek at the bottom of the canyon.
This undulating columnar basalt caught my eye because of the color and its wavy appearance.
The Burlington Northern railroad wasn’t afraid to use explosives to create a path through what was described at the time as, “A dry, rocky canyon good for nothing except a railroad.” The rails have been removed, but the eleven bridges required to construct a rail line to cover the 3 miles of the trail system are mostly still in place.
Here’s one that didn’t last. It was replaced by a pedestrian bridge constructed by the Cowiche Canyon Conservancy and the Bureau of Land Management.
We took a more circuitous route back to Tacoma using US12 through White Pass. It didn’t snow, and we got a look of one of the many water falls in Washington.
This was the best angle I could get, and I still couldn’t see the bottom! It turns out that Washington has more than 3000 catalogued water falls, more than any other U.S. state. Clear Creek is rather small at only 300 feet, so I guess there is some more air down there.
It was a beautiful day in the Pacific Northwest, including the snow flurries. A ten-hour day trip took us from coastal Washington, over a 6000 foot pass, into dry eastern Washington, where we hiked through a canyon filled with native plants and rocks (hahaha), and back over the Cascades past a magnificent water fall in Wenatchee National Forest.
This is undoubtedly the most beautiful place I have ever lived…
Volcanic Rocks at Cowiche Canyon
Introduction
Burlington-Northern Railroad built a line through Cowiche Canyon in 1913 to transport apples, but it was abandoned in 1984 and the land was acquired by the Cowiche Canyon Conservancy for a non-motorized vehicle trail system. The main trail extends 2.9 miles along the South Fork Cowiche Creek, crossing the 11 bridges constructed for the railroad line.
The left panel shows the distribution of Columbia River flood basalts, deposited between 16 and 6 Ma. Yakima and Cowiche Canyon are outlined by a rectangle. These volcanic rocks were erupted in overlapping flows with erosion and landslides occurring between individual layers, which are irregular and not shown in this map. The ages from the USGS national geologic map are Tertiary (66-2.6 Ma). Tacoma is marked by a smiley face.
The right panel shows the Cowiche Canyon trail system and the specific area discussed in this post. The stream itself hosts a riparian habitat whereas the uplands comprise a shrub-steppe environment.
Observations
The canyon walls consist of a series of ledges like this with eroded slopes between them. The ledges are erosional margins of basalt flows and the slopes consist of talus and fine sediments weathered from the mafic rocks.
As we traveled west up the canyon, columnar-jointed basalt began to appear in the ledges overlooking the trail. Several pieces had rolled down the slope into the stream bed; these were about three-feet in diameter. Columnar jointing results from slow cooling of a uniform basalt flow, which causes joints to form hexagonal blocks like these because of thermo-mechanical failure during a decrease in volume.
These semi-circular blocks got my attention because, if you look closely, they appear to be eroded hexagons I estimate to be more than six feet in diameter. These are very large basalt columns that have either toppled or…
The top of this photo shows a birds-eye view of columnar basalt blocks because of their horizontal position. The size varies from smallest on the left to the largest blocks on the right. I reported on similar, horizontal columnar joints in a previous post and proposed that the lava flowed down a slope before solidifying.
The lower-right part reveals columnar basalt in a vertical position. This juxtaposition suggests (if my model is correct) that the lava from multiple flows covered an irregular landscape–sometimes flowing into canyons like Cowiche Canyon, and somtimes over fairly level ground.
This remarkable set of columnar joints got my attention because of their undulating form. I’ve never seen anything like this before. This style of jointing (supposedly) results from uneven cooling and weathering; for example, a heavy load on the layer during cooling leads to pinching and swelling at fairly uniform spacing. That sounds reasonable to me.
Another weathering feature of these rocks is the fissile structure revealed in this image. More solid blocks are interspersed with flaky layers, possibly (I’m speculating here) associated with necked and wider segments of an individual column. For example, the wider sections might undergo shear during stretching, resulting in microscopic shear layers within the minerals comprising the original lava. These weak layers would permit water to penetrate and weather the mafic minerals of which basalt is made.
These highly weathered columns are more than six-feet in height. They suggests an alternative mechanism, shear from flowing as the basalt cooled; this might disrupt the microscopic structure without interrupting the macroscopic jointing process. Maybe…
This photo really got my attention. It reveals horizontal columnar joints abutting vertical ones in the upper-center of the image. There’s a lot going on as hot lava flows over an irregular landscape, but I think this is a vent where more magma flowed out; not a large eruption, but enough to have a separate cooling history from the rock it penetrated. I should note that the Columbia River basalts flowed from fissures rather than point sources like volcanoes. The entire area shown in the map above was cut by fissures that led to a shallow magma chamber, which is still down there although it has probably solidified by now. Or not…
I like this picture because it reveals how much weathering can change the appearance of what was once molten lava in only a few million years. Note the layer of angular blocks sandwiched between weathered columns.
Conclusions
This is a typical basalt column that isn’t as weathered as some of the others. All those shards I’m standing on resulted from the breakdown of the rock by water seeping into its internal structure, where it altered the mafic minerals (e.g. pyroxene, plagioclase feldspar, biotite), which are susceptible to chemical weathering. This is where all the mud in the world comes from.
It was a great day to drive over the Cascades at Chinook Pass, where it snowed on us (in June), and explore the Columbia plateau. I’ve never seen so much variability in basalts before. The magma chamber underlying central Washington was a giant chemical reactor that released pressure by erupting a mix of fluids that cooled to form minerals and then these magnificent rocks. These rocks tell us how the magma chamber evolved over several million years; and once they were exposed to the atmosphere, they began to record the slow process of being reduced back to their basic constituents (fine-grained minerals like clay), which can remain suspended in water and begin their long and perilous journey to their final resting place–sometimes a lake but, ultimately, the ocean.
Everything eventually returns to the sea…
Mount St Helens After Forty-Five Years
Introduction
I was a geology student at Arizona State University in May, 1980, when Mount St. Helens made the headlines. It is the largest volcanic eruption in North America, and when one-cubic-mile of mountain collapsed, it became, and remains, the largest landslide in human history. I followed the progress of geological investigation into the eruption with interest as I pursued my education, but progress was slow. It isn’t easy to reconstruct an event that occurred in a few minutes. The area was too dangerous to approach for more than a year because of gas explosions from within the pile of debris, which reached 600 feet in thickness.
Mount St. Helens faded from memory for decades, eventually becoming just another geologic event in a long chain of cataclysms covering billions of years. I never thought about it until I found myself living less than 100 miles from ground zero. I had to check it out. This post is a brief summary of what I found when I visited Mount St. Helens National Volcanic Monument. I hope I can convey some of the excitement I felt at stepping on ground that was literally on fire less than fifty years ago.
Figure 1. I arrived at about 10:30 in the morning and got this image before low clouds settled in, accompanied by fog later in the day. The characteristic volcanic cone is missing; the top of the mountain slants slightly upward to the right in the center of this image. That isn’t a lake in front of St. Helens, just fog collecting in the valley that feeds the Toutle River.
Figure 2. Mount St. Helens is about 2.5 hours from Tacoma. The closest you can drive is Hummocks Trail, but a four-mile hike will take you to the rim of the caldera. Maybe another time.
Older Volcaniclastic Rocks
Figure 3. Mount St. Helens is less than forty-thousand years old, but it is constructed on a thick sequence (~2 miles thick) of volcanic rocks as old as 300-thousand years. Beneath these Pleistocene volcaniclastic rocks mixed rocks of uncertain age that comprise “bed rock” in this area; however, these igneous and sedimentary rocks are much older–spanning the Oligocene epoch (~34-23 Ma). I stopped to look at several road cuts along the new Highway 504 (the original is buried under debris).
A. Exposure of andesite volcanic rocks showing a complex eruption history, which includes ash layers and what looks like tuff (ash so hot it melted together to form a glass-like volcanic rock).
B. Close-up from the left side of (A) showing a layer of volcanic breccia that is now vertical. Individual clasts are visible but there is very little matrix. The dashed line indicates the approximate bedding plane of the layer. The rounded block labeled with ?? is about 12 feet in diameter. This is a puzzling structure. My guess is that there was a collapse of one or more volcaniclastic layers into a ravine after deposition. A jumble of material.
C. Photo of a complicated structure separating the left side of the exposure (tilted beds and blocks) from the right side (horizontal ash layers). The dash line is meant as a reference to the vertical beds in (B), but it was difficult to determine orientation. However, a clear change in texture suggests a depression, which may have been a conduit for volcanic material to erupt. The BLOCK/PLUG label reflects this interpretation. Within this “BLOCK” there is a discrete region of thin, irregular bedding that I’ve labeled (for convenience) as a CHANNEL? The question mark reflects my doubts about this identifier. Nevertheless, this road cut exposes a sequence of events: lava and ash being erupted onto an irregular volcanic landscape; probable surface erosion for some period of time; physical disruption and collapse, probably while still hot, of some part of later volcanic material. I don’t have enough experience with volcaniclastic sediments to say anymore than that.
D. The pushpin shows the location of this road cut. The blue ribbon is Coldwater Lake, where Hummocks Trail is located (see Fig. 2); this exposure is more than ten miles from the caldera, and hundreds of feet above the valley floor.
Figure 4. This post-eruption road cut is a mile or more further from the caldera. The solid line was a striking lineation that could be a fracture or possibly a contact between eruption beds. The dash-dot lines are apparent bedding planes between andesite flows. There is no ash present at this location. The dash lines delineate what I’m calling a BRECCIA because there is no evidence of bedding and the overall appearance is irregular; also, some large blocks were evident, although they could be a result of differential weathering. Lava flows are notoriously difficult to trace any distance laterally. Nevertheless, this exposure is similar in appearance to Fig. 3 with respect to the discontinuity between identifiable volcaniclastic deposits.
Figure 5. This road cut is located (see inset map) in an area the geologic map (Rock D) identifies as volcaniclastic rocks of Oligocene age (33.9-23.04 Ma). This is a smaller exposure than seen in Figs. 3 and 4, but it is also very different in appearance. That could be an optical illusion; close examination of the photo (I took it from my vehicle stopped in the middle of the road) suggests to me that this is ash that was so hot it formed what is called a welded tuff when it fell, after being blown into the atmosphere by an eruption. The light color suggests a magmatic composition more like granite than gabbro, or even diorite. That wouldn’t be surprising in the complex magmatic environment of a subduction zone, where partially melted, oceanic crust (e.g. gabbro and basalt) chemically mixes with continental crust (e.g. granite and diorite).
This completes my survey of older volcanic rocks near Mount St. Helens. There are no rocks exposed that explain the apparent hiatus in volcanism between about 23 and 2 million years ago. That is a story for another day…
Volcaniclastic Deposits from May 1980 Eruption
Figure 6. Photo A was taken a few days before the eruption in May, 1980. Image B was taken a few days afterward. The pre-eruption volcano was 9677 feet tall, but the obliterated peak is only 8365 feet. The total volume of material displaced exceeded one cubic mile, most of which was rock that collapsed during the rock slide preceding the actual eruption. The flat area fronting the volcano in (B) is a large fraction of the previous peak, which filled in several channels originating at the volcano. Note the holes in (B), which are probably blow-outs of gases trapped in the debris.
Figure 7. (A) This plaque was located along Hummocks Trail. I’ve supplemented it with the map of volcaniclastic deposits shown in (B). I will focus on the three major volcaniclastic deposits I encountered at Hummocks Trail (blue circle in B), which are numbered 1-3 in both figures.
The first stage was collapse of the north flank of Mount St. Helens (brown in A and cross-hatched in B). This was a run-of-the-mill massive landslide that followed existing drainage, until stage two occurred.
When the rock containing the highly pressurized magma was removed. The resulting release of pressure created an explosion equivalent to 10-50 megatons of TNT; although a volcanic eruption is not analogous to a nuclear explosion, the energy released was roughly 1600 times the energy of the Hiroshima atomic bomb. It moved a lot of rock. This blast occurred seconds after stage 1 began; the hot gases overtook the rock slide, driving rock and debris up the sides of the valley, removing everything, including top soil, within a few miles of the volcano. The orange area in B shows how widespread this explosion was.
Stage 3 was a pyroclastic flow (red in B), which was limited to the immediate vicinity of the caldera; however, what goes up must come down, so several inches of ash were deposited as far as Spokane, Washington–400 miles distant.
Video 1. This video shows debris from the landslide (Stage 1) that was pushed at least 300 feet from the original valley floor to the ridge by the unimaginable blast of Stage 2. The main slide followed Toutle River (North Fork), but with this impetus, debris was launched over the canyon walls. That’s what this video shows. It’s like the heavy stuff collected at the top of the ridge whereas the sand/gravel-sized debris was blown miles further.
Figure 8. This photo is looking south towards Mount St. Helens from Hummocks Trail (see Figs. 2 and 7B for location) along the Toutle River (North Fork), which is more than a hundred feet lower, even after being filled with debris from the 1980 eruption. Note the difference between this scene and the deposit from Video 1. It’s difficult to comprehend the dynamics of a blast (Stage 2) capable of pushing rock (Stage 1) up this slope; when the explosion lost energy it simply dumped its load, creating a surreal landscape. The debris has very little clay and is thus non cohesive; thus slumps like this are common. Note the small hill and “ridge” in the middle of the photo. This terrain makes no sense, other than erosion has been etching it for 40 years.
Figure 9. This layer of ash I encountered along the Hummocks Trail is a remnant of a vast sheet originally deposited during Stages 2/3–breaking such a continuous eruption sequence into stages is useful but not particularly elucidating. It all happened too fast to comprehend. You might call this an “over-bank” deposit, like when a river tops its natural levee and deposits fine-grained sediment on its floodplain.
Figure 10. Hummocks Trail followed a small stream through a bizarre landscape comprising multiple small ponds like this one. A dam constructed of eruption debris (left-to-right in the center of the photo) has blocked the stream flow, creating a meandering channel during low-water conditions; but I can imagine all those dormant grasses erupting later this spring. I encountered several of these marshes, each one defined by water trying to find a way out of this labyrinth. According to Fig. 2, this is the outflow from Coldwater lake; the eruption partially blocked its path but nature is finding a way through this discombobulated landscape.
Figure 11. This photo perfectly captures the topography of Hummocks Trail. This is the northern end of the trail, which is a little higher; the Toutle River (North Fork) is more distant and the cliffs we saw in Fig. 8 have receded, replaced by moderate slope; however, note the fine-scale nature of the hummocks here. They are ubiquitous but contain only a few scattered boulders.
Figure 12. This is a view of Coldwater Lake from the visitor center, looking south towards Mount St. Helens. There are two things to note from this image: 1) the Toutle River valley is a broad, flat plain because it was filled with debris from Stage1; 2) the overflow deposits (e.g. Video 1) have blocked the lake and raised its level. The source of water we saw in Fig. 10 is evident in the center of the photo. This isn’t a sedimentary dam, created by flood deposits, or even a glacial dam produced from the debris scraped up by thick ice sheets. The shoreline was modified when Mount St. Helens collapsed and then exploded with unbelievable violence.
Final Thoughts
The eruption of Mount St. Helens in 1980 gave geologists a rare opportunity to see how Earth produces sausage. Let me explain my metaphor.
Ocean crust is denser than continental crust it encounters at a convergent plate boundary, and at a lower elevation. It is thus overridden by the continent, forcing it to dive into denser and hotter rocks at depths of hundreds of miles. This obviously displaces hot and ductile rocks, heating the subducting rocks even more; the sausage-making operation has begun. Still pushed from behind by the conveyor belt of oceanic crust being subducted, this basalt/gabbro mixture heats enough to boil off the water contained in its sedimentary cover as well as any constituents with a reduced melting point (e.g. silicon-rich minerals). This superheated material rises through tens-to-hundreds of miles of crust, heating it and producing a mixture of oceanic and continental crust.
Plate tectonics keeps turning the crank on the sausage machine for tens (even hundreds) of millions of years. The molten mass of magma keeps rising because it is less dense than the rocks through which it is passing, until the pressure has decreased enough for it to find cracks in the solid crust. The magma fills these cracks, which often lead to fissures in the surface; the magma erupts. The magma cools and begins to solidify, forming a pluton at depths of several miles to tens of miles.
This process keeps operating, in fits and starts, until the tectonic plates change direction. In the meantime, the upper crust has been filled with multiple plutons (solidified magma chambers), sometimes squeezed into the volcanic rocks from previous eruptions. Individual magma chambers/plutons are about one mile in diameter, although they take many shapes, and remain active for less than one-hundred-thousand years (often much less). These are injections of fluid rising through weak points in the crust, driven by pressure paths–not missiles launched from the mantle. They are like the sweat on your brow after a hard workout.
Mount St. Helens has released a lot of pressure, although it is still very dangerous at human scales. Chances are that this magma chamber will solidify within the next hundred-thousand years; but the unstoppable sausage machine will continue cranking out more mixed material in a molten form until it has filled every nook and cranny in the upper crust. That’s how mountain ranges like the Sierra Nevada were formed. It takes a really long time.
Mount St Helens probably won’t erupt again at the scale of the 1980 event, but that doesn’t mean much if you happen to be camping near the caldera when a smaller eruption occurs. After all, the magma chamber is still extruding lava in an unsteady process that is currently pretty slow. However, we humans can’t see fractures in bed rock; thus, the next and the next, etc, magma chamber could appear anywhere from Northern California to British Columbia. There are plenty of young volcanoes in the Cascades that haven’t blown their tops yet.
Who’s next?
Recent Comments