Ecology Note: Point Defiance Park
This Douglas Fir survived the onslaught of loggers and is now approximately 450 years old–middle aged for this species.
From Dalco Passage Point, we saw a whale hanging around; the only signs were occasional water spouts and a brief glimpse of a pair of flippers. It was pretty small from 200 feet above sea level and several hundred yards distance. I was too slow to get a video. Maybe next time.
I spotted this glacial erratic (boulder transported by a glacier) and took a look. It looks like a granitic rock, probably granodiorite. It was transported, over possibly a million years, from the Northern Cascades. We saw a lot of it on my last post.
You can’t walk Five-Mile Drive without stopping for a photo of the Tacoma Narrows bridge.
Salal (Gaultheria shallon) dominates the conifer understory in places. It is a native evergreen shrub.
It was a beautiful fall day to stroll around the park, and I’ll try to get a video of some whales next time.
Ecological Note from Ross Lake National Recreation Area
We recently followed the North Cascades Loop along State Route 20 through Washington Pass, followed the Columbia River, and then recrossed the Northern Cascades along US 2. I reported on some of the sights in a previous blog. The geology along our path was discussed in a series of posts that started just west of today’s location.
The North Cascades Visitor Center was closed for the season, so we stopped at Newhalem Creek where we followed a trail that led to a small hydroelectric generation plant on the Skagit river. I noticed several interesting ecological points of interest along the way.
Nurse logs are a common sight in the Pacific Northwest. Usually they comprise young trees growing out of stumps; I’ve even see them where the stump was gone, leaving the new tree standing on legs. There was a sign discussing this phenomenon, but this caught my eye. Note the hollow interior; this was not a fully grown tree, but the original nurse stump is completely gone.
I haven’t seen an example of a nurse log as good as this before. Trees are growing out of this decaying log; its top is covered by moss and lichens; and there was a poster explaining that the trees within the park often grew in linear sets of 3-6 trees. Some of these were large trees. Now I know why forest services, from county to federal, leave logs lying where they fell.
Geologic Survey of the Northern Cascades: 4. Thrust Faulting within a Subduction Zone
Figure 1. This is my last post from our Cascades Loop drive. The geologic map shows a wide variety of rock types west of the Columbia River, some of which we’ve seen before. But we also see two unfamiliar lithologies: Tertiary basalts that erupted throughout eastern Washington between 17 and 6 million-years ago; and mixed Paleozoic/Mesozoic metamorphic rocks (539-66 ma), which appear as a sliver of gray pointing like an arrow towards Site 12. These older rocks have been thrust over younger Mesozoic metamorphic rocks along a thrust fault (dashed line with arrow indicating direction of relative movement).
Figure 2. Stop 11 is located along the Columbia River, where steep slopes conceal high-grade, metamorphic rocks generally assigned Mesozoic ages. This image shows a knob dissected by numerous intersecting dikes. Ages for metamorphic rocks indicate the time when they were transformed into gneiss, schist, etc. The description from the NGMDB map describes a jumble of rocks in complex interrelationships.
Figure 3. This outcrop is about 10 feet across (the tree is young); it shows what the geologic map’s metadata are saying. There is no continuity visible, yet thin layers of sandstone/mudstone are visible, adjacent to blocks of buff material in the lower left; meanwhile, the right side reveals quite different layering.
Figure 4. This image reveals an intrusive, igneous texture that has been modified into what I would call an orthogneiss. Large grains of feldspar have formed layers because dark minerals like amphibole and biotite have been remineralized into the dark laminae. None of these minerals (except perhaps an occasional zircon) is original. The entire outcrop appears to have been folded into a three-dimensional, origami form. The gray area exposes the core of this spheroidal structure.
Figure 5. These two photos from Stop 12 are so speculative that they could be considered as schematic. I included them to demonstrate the change in geomorphology when crossing something as subtle as a thrust fault. These images are from the intersection of US-2 and US-97; I don’t need GPS to know where I am. The NGMDB map shows a thrust fault crossing the Columbia River at this spot.
(A) This mountain forms steep, rock faces. It is composed of Mesozoic metamorphic and igneous rocks. (B) This mountain, only a few miles north, is predominantly covered with loose soil and has a few outcrops on the upper slope.
There is an identified thrust fault between them, pushing the northern rocks over the younger rocks to the south (some time in the past). At first glance, the two look alike but, on closer examination, we can identify differences even from a mile away. I should add that a thrust fault is not cut with a knife; instead, it is a general zone of brittle failure within colliding rock masses. The result is seldom obvious.
Figure 6. Stop 13 in East Wenatchee revealed deformed rocks similar to those at Chelan (Stop 11), which isn’t surprising; however, this road-cut revealed larger exposures that aren’t as criss-crossed by veins. The lighter colors are the result of illusive changes in chemistry that interact with the environment, but they seem to reflect subtle differences in lithology. Note the massive, layered section at the bottom that seems to have been cut off; perhaps this is an outcrop-scale expression of regional compression and thrust faulting.
Figure 7. This photo is difficult to explain. Thin layers of what was once an intrusive rock abut an intrusive rock that hasn’t been as altered. This is an opportunity to point out the fallacy of jumping to geological conclusions: this might look like a sedimentary rock laid down on a granite boulder, which once formed a shoreline–it is NOT.
Final Thoughts
These four posts have explored a subduction-zone orogen, from youngest to oldest. The rocks got progressively more complex during our transit. They tried to fool us repeatedly by revealing patterns that, at a glance, seemed familiar and could be mistaken for textbook schematic concepts.
The rocks are never wrong, but they are known to have a bit of fun at our expense.
Geologic Survey of the Northern Cascades: 3. Metamorphic and Igneous Rocks East of the Orogenic Core
Today’s post is a continuation of the last one, which explored the central core of the Northern Cascades. We followed the Methow River south until it joined the Columbia River through what is generally a region of marine sediments that have been dissected by faults (Fig. 1), and resemble the metasedimentary and metavolcanic rocks from the western region.
Figure 1. The Methow River follows the axis of a broad valley underlain by marine sediments (green area). We didn’t follow any of the side roads, so I can’t say anything about those rocks. We didn’t see any good outcrops until Site 7, so the story will pick up there. The eastern region is highly faulted (thin black lines on the geologic map), many of which are inferred to be thrust faults. One such prominent fault is outlined with a dashed yellow line because it delineates an area of anomalous high-grade metamorphic and igneous rocks similar to those within the central highlands. The curved regional contact with the Igneous province to the east reflects the resulting uncertainty.
Figure 2. This is a typical exposure along the Methow River, Site 7 near Carlton. There are several horizontal lines visible in the outcrop, but it is impossible to say if this is an example of metasedimentary rocks or jointed metamorphic rocks, both of which are present in this complex geological environment.
Figure 3. There were a lot of good exposures along State Route 153 so we stopped a lot. (A) This photo was taken a few miles SE of Fig. 2, where the geologic map indicates a sliver of Mesozoic (252-66 Ma) metamorphic and intrusive rocks. Of course, this boulder was in an old flood plain of the Methow River; the rounded nature of the sample (~two-feet across) suggests it was transported several miles from its origin.
(B) This boulder has a similar appearance but it was located on the western side of the Cascades, and is tens-of-millions of years older. Both samples are granitic in composition. It appears to be more weathered and beaten up by Skagit River, which is steeper than the Methow. I included it as an example of the continuation of several geological processes in the Northern Cascades for more than 100 Ma.
After a lengthy conversation with ChatGPT, the best I can say about the curious juxtaposition of different surface characteristics within these samples is that they may have resulted from differential weathering within fracture zones in the original igneous rock. The darker material is not a dike, but instead a zone of increased stress (possibly associated with faulting) where water preferentially altered some minerals into weaker products like clays.
Sometimes there isn’t a direct answer to a geological question.
Figure 4. This is a high-resolution image (~10x magnification) of the dioritic igneous rock found in some areas along the Methow river. Grayish blobs are quartz whereas the visible white grains are feldspar; it is impossible to tell if they are Na, K, or Ca feldspar without a microscope. The dark areas are amphibole.
Figure 5. This excellent exposure along the Methow River (to the left) was visible at Site 8. The appearance of this rock is determined by a strong joint set. There are two candidates for preliminary guesses about its lithology from the geologic map descriptions: 1) Mesozoic orthogneiss (high-grade metamorphic rock derived from granitic rocks); and 2) Tertiary intrusive rocks of granitic type. There is a difference in their appearance.
Figure 6. This outcrop looks like a gneiss to me. Note the near-horizontal layering (i.e. foliation); at high pressure, minerals become oriented parallel to the direction of least stress (i.e. they flatten). There are several large crystals protruding from the surface, however; these are porphyroblasts, which form when incompatible elements are squeezed out of the matrix.
Figure 7. This highly fractured outcrop at Site 9, near Pateros, caught my eye because the rocks didn’t form a vertical wall like we saw in Fig. 5. According to the map this is another example of undifferentiated Mesozoic orthogneiss, which occurs in small blocks throughout the region (purple areas in Fig. 1).
Figure 8. This face presents a pretty striking facade. The streaks of white against the dark surface look like sedimentary bedding; however, they are not bedding and never were. What I don’t see in this exposure are folds, where foliation has been warped into oscillating patterns that often reverse direction at this scale.
Figure 9. (A) Site 10 is located within a mile of the Columbia River. This road-cut is at least 60 feet high. This looks like more Mesozoic orthogneiss. All of those lineations are metamorphic foliations. Slight variations in local mineralogy create a complex pattern, especially on the surface.
(B) This close-up (~10x magnification) shows that some part of this mass of rock is undeformed diorite–that is, it hasn’t been smashed into orthogneiss.
(C) The edge of this sample was recently broken, revealing granitic rock beneath the surface. The dark lines in (A) are, in many cases, no more than surface weathering phenomena. We saw this a lot during this post. I can’t explain it, but it no-doubt entails surface chemistry, probably involving air temperature/humidity and imperceptible alterations in mineralogy during burial and exhumation over tens of millions of years.
Final Thoughts
We didn’t see any metasedimentary rocks on our trip along the Methow River, despite the preponderance of these rocks on the geologic map (green areas in Fig. 1). What we saw were a lot of slivers of granitic rocks and their metamorphic products. That’s probably because these rocks are very durable whereas lightly altered metasedimentary and metavolcanic rocks weather relatively quickly.
Glance back at Fig. 1 now that you’ve seen the rocks, noting the large number of faults (thin black lines), and ask yourself how Mesozoic rocks that were once buried miles beneath the surface pop up like wack-a-moles in this gently undulating terrain.
Maybe the Cooperative National Geologic Map is right to list every possible rock type in the geological metadata.
What a mess!
Geologic Survey of the Northern Cascades: 2. The Central Orogen
Figure 1. This magnificent vista of Washington Pass, at an elevation of 5477 feet, is representative of the core of the Northern Cascades. Highway 20 has to change direction to get out of this dead end, created by glaciers among massive mountains of high-grade metamorphic rocks, some of which have been exhumed from depths greater than twenty miles. Some of the intrusive rocks were emplaced at much shallower depths. Remember that it took tens of millions of years for this striking rock-scape to evolve on the surface of a dynamic planet.
Figure 2. I have annotated this piece of the USGS national geologic map to give an overview of the terrain we traversed. Today’s post will only be discussing Sites 4-6, which fall within the core of the orogen. We saw low-grade metamorphic rocks in my last post. The bright-colored area represents rocks that are classified as orthogneiss–a mixture of non-plutonic rocks that were subjected to high pressure and heat. The dark-purple shades are Mesozoic intrusive rocks and the lighter hues are Tertiary. Site 6 is approximately where Fig. 1 was photographed.
Figure 3. Steep peaks like this one (near Site 4 in Fig. 2) replaced the ridges seen in the sedimentary prism rocks to the west. When you see spires and domes, it is safe to assume that you are looking at high-grade metamorphic/intrusive igneous rocks.
Figure 4. The Skagit River canyon between Sites 4 and 5 is filled with boulders and gravel, but there are no exposures. This boulder is interesting because it is composed of two lithologies with different properties. I think this is a piece from miles upstream; the irregular part looks like a coarse-grained intrusive rock like granite whereas the smoother component is probably an intrusive rock that filled cracks in the host rock.
Extreme physical erosion in the river has emphasized several sets of intersecting dikes that were filled at different times in this rock’s history.
Figure 5. (A) Photo looking north from Site 6 in Fig. 2. According to the geologic map, this mountain comprises Tertiary (~66-2.3 Ma) intrusive rocks that don’t contain a lot of quartz (e.g. diorite).
(B) This is a closer view of the rectangle in A, showing vertical lineations that form ridges in the steep face of the mountain. At first glance these structures remind me of columnar jointing in basalts, but similarity in appearance doesn’t indicate a common origin. These lineations are irregular and wavy in some places. They probably followed jointing that developed as the massive rock cooled…under a continuously changing stress environment; e.g., uplift from erosion of overburden, sliding along a transform fault just to the west of here, and a jumble of motion along numerous tensional and compressive faults.
Figure 6. This view is looking southward towards a mountain comprising Paleozoic-Mesozoic (~250-66 Ma) intrusive rocks. The pluton in Fig. 5 was injected into this mass of far-older rock with a similar composition. This difference is striking at this distance: these older rocks present a curvilinear pattern of joints and fractures. I could speculate about the complex, almost turbulent, flow suggested by this view–but I won’t.
These rocks are so much older than those in Fig. 5 that they experienced several lifetimes before the tectonic stress field of the Tertiary (~66-2.3 Ma) developed; nevertheless, they reveal the cumulative effects of continuous deformation associated with ocean crust subduction.
Figure 7. I included this high-resolution (~20x) photo as a baseline for some rocks we’ll see later. This is the Tertiary intrusive rock, but the fabric (i.e. mineralogy and structure) has been modified from the original magma. Most of the minerals are feldspar, but only chemical and microscopic analysis can differentiate potassium and sodium end members. The anomalously large, dark crystal is probably the result of solid-state diffusion after the magma cooled. There is quartz, identifiable as the amorphous gray areas, throughout but not at high concentrations.
The description in the geologic map covers a wide range of quartz-poor plutonic rocks, so I won’t try to name it. What matters is that this is not a rock that simply cooled and was frozen in time.
Final Thoughts
Driving through batholiths, which are built up over millions of years from multiple plutons, gives plenty of opportunities for panoramic photography, but the real geological story is buried within the minerals themselves.
Washington Pass has exposed the beating heart of the Northern Cascades. We’ll have to look at the periphery to learn more.
Geologic Survey of the Northern Cascades: Deformed Trench Sediments
Figure 1. The ridges in the background of this photo represent the western foothills of the Cascades Range (Site 1 in Fig. 2). Today’s post will follow Highway 20 as it winds its way into the heart of the orogen, exploring sedimentary rocks that were deposited when Washington’s coast was located east of Puget Sound.
Figure 2. (A) We drove a loop around the Northern Cascades and stopped at the numbered locations. Today’s post will report on sites 1-3, which form the western flank of the central peaks. (B) The Northern Cascades are not primarily volcanic, although there are active volcanoes in the region, e.g. Mt. Baker. The box outlines the larger map area. Note that these mountains are not primarily composed of intrusive rocks; and they are northwest of the Columbia River volcanics.
Figure 3. This geologic map is derived from the Cooperative National Geologic Map (https://ngmdb.usgs.gov/nationalgeology). The sediment prism represents lithified sediments that collected within the subduction trench. These are called metasedimentary and metavolcanic rocks because they are only slightly modified by heat and pressure.
The wide range of ages results from the difficulty of applying radiometric dating to such deformed rocks, as well as the assortment of material that is mixed together within the trench. For example, the maroon area north of Site 2 is described as melange–a mixture of different rock types that have been folded and faulted as if put in a blender.
The dashed line is thought to be a strike-slip fault along which the eastern block has moved southwards; we don’t know which side moved, of course, without studying the regional distribution of rocks. That is beyond the sope of this post. The rocks to the east side of this fault are the core of the Northern Cascades: metamorphic rocks that have lost all evidence of their origins, and younger igneous rocks that we’ll see in the next post.
Figure 4. This is a typical exposure of these deformed sediments from Site 2 (see Fig. 3 for location). These are sedimentary rocks yet no bedding can be seen from this distance.
Figure 5. This photo from Site 2 was taken in a small quarry, which permitted examination of a fresh surface on a boulder that was removed from somewhere on the hillside. Note the absence of bedding. I am pointing to the next photo.
Figure 6. Close-up of the boulder in Fig. 5, showing foliation in lieu of bedding (top to bottom in photo). There is a thin quartz vein running from left-to-right near the top of the image, and several micro-veins throughout the sample. The original sediment was heated sufficiently, under pressure, for quartz to become mobile, but there wasn’t a lot of quartz in the original sediment. It may have been either mud or possibly volcanic layers.
Figure 7. This road cut near Site 3 is representative of the melange found in the western foothills of the Northern Cascades. There is a suggestion of bedding but nothing is persistent, and there are no straight lines. This is typical for deformed sedimentary prism sediments.
Figure 8. This close-up of the rocks in Fig. 7 is about 12 inches across. It reveals quartz filling a cavity within a larger mass of breccia. Quartz is very mobile under moderate heat and pressure.
Figure 9. This close-up is a few inches across. It reveals a mixture of mineral crystals (seen as rectangles and shiny grey areas throughout the image) that are feldspar (another mobile mineral) and quartz, but no lineation. Deformation was pervasive, from the outcrop scale (Fig. 7) to the microscopic scale. These rocks behaved like mush on geological time scales (e.g. millions of years).
Final Thoughts
This schematic block diagram of a subduction zone has to be advanced a few million years to explain the rocks we discussed in this post. The Trench (orange area with white arrows on the front of the diagram) in this photo represents the last few million years whereas the rocks we saw are located closer to the Cascade Volcanoes today.
The rocks haven’t moved that far from where they were deposited as sediment and lava in the submarine trench; instead the trench has moved westward as the thickening continental crust (right side of diagram) forced the ocean plate to sink further to the west. It’s a simple question of conservation of mass and energy; as powerful as plate spreading is, it has limits on geologic time and space scales. In other words, N. America was bigger and heavier.
The melange being created in the current trench will be exposed in tens of millions of years, and we probably won’t be around to observe it.
Ecology Notes from Tacoma Nature Center
Today’s field trip took us to the center of Tacoma, where the city operates a nature center focused on education and outreach programs.
We weren’t looking for birds, but this display showcases some of the birdhouses constructed within the area.
We walked the entire trail system, which is almost three miles, and noticed several signs indicating stops on different self-guided tours. Unfortunately we didn’t have a guidebook. This post is about learning from experience. (LOL!)
The trail climbed about 100 feet to a ridge that gave a great view of Mt. Tahoma (aka Mt. Rainier) on a beautiful day. Note the cloud trailing off the top of the mountain in a northerly wind.
Snake Lake was dry and the deciduous trees in the wetland were losing their leaves, so I didn’t take much interest in them. It wasn’t until we followed the Hilltop Trail that I began to notice the different ecosystems juxtaposed in this seventy-acre site. The rest of this post, therefore, will focus on trying to salvage something intellectual from what started as simply a walk in the woods.
Tacoma Nature Center supplies a catalogue of the flora and fauna located within its limits. I took a couple of photos and, when I got home, discovered the lists they supplied on their web page. Thus, instead of guessing what I’d seen, it was more like a homework assignment with a limited number of choices.
Trees
This is a list of the trees known to exist at Tacoma Nature Center. The ones I identified are in bold typeface.
- Sitka Alder
- Oregon Ash
- Cascara Buckthorn
- Black Cottonwood
- Douglas Fir
- Pacific Madrone
- Scouler Willow
- Garry Oak
(A) This young tree still had all of its leaves, so I used my copy of Pacific Coast Tree Finder to identify it. (B) The leaf shape and vein structure was critical. If I hadn’t already had a list of possible choices, however, this would have been a lot harder; in fact, my answer may well be incorrect.
I’ve seen enough Douglas Firs to guess these conifers, especially since it is the only native conifer found within the nature center; all of the old-growth forest was clear cut and this species planted because it grows fast and is a good source of lumber.
Shrubs
This is where my homework got very difficult. So I asked for some assistance from ChatGPT, but I fact-checked everything it said and, after a long discussion, we agreed on the final identifications. It may have read everything ever written, but it can’t see what isn’t visible in a photograph, and it is prone to making things up–just like people.
As with the trees, I’ve emphasized my answers in bold type.
- Baldhip Rose
- Beaked Hazelnut
- Black Twinberry
- Bracken Fern
- Clustered Wildrose
- Common Snowberry
- Creeping Snowberry
- Douglas Spirea
- Dwarf Oregon Grape
- Evergreen Huckberry
- False Lily-of-the-valley
- Indian Plum
- Oceanspray
- Orange Honeysuckle
- Red Elderberry
- Red Huckleberry
- Salal
- Sword Fern
- Tall Oregon Grape
- Trailing Blackberry
- Western Trillium
This bramble of shrubs contains five separate species of shrubs or ground cover from the list. I’ve circled examples of each and discussed them below. I supplied ChatGPT with close-up images to get its input. It was very useful, but not infallible. In fact, I identified (A) and (D) on my own, after taking a closer look at the photo–and using Wikipedia as a reference.
(A) Bracken Fern (Pteridium sp., family Dennstaeditaceae) is the oldest known fern, with fossils from the Eocene epoch (~55 my). If you zoom in on this area, you’ll notice that each branch has perpendicular stems, which then have perpendicular leafs. The entire plant reminds me of what I’ve heard about fractals in nature; a simple pattern repeating over and over.
(B) Baldhip Rose (Rosa gymnocarpa, family Rosaceae). There are two roses on the list, but their leaves are very different.
(C) Dwarf Oregon Grape (Mahonia nervosa, family Berberidaceae). Wikipedia redirected my reality check of ChatGPT’s identification to Berberis nervosa. The explanation was that Mahonia is an alternative scientific name to the accepted one. Not being a botanist, I don’t care about such subtleties. The name, Berberis, certainly fits the family lineage better. I’ll leave the name as is and include these notes.
(D) Western Sword Fern (Polystichum munitum, family: Dryopteridaceae). This more modern cousin (note the different family) of the Bracken Rose has a simpler leaf design: it has one broad leaf growing perpendicular to the main stalk. I didn’t notice such a blatant difference when the two plants were right in front of my eyes!
(E) Creeping Snowberry (Symphoricarpos mollis, family Caprifoliaceae) is my best estimate after a long conversation with ChatGPT, examining photos of several species, and comparing their growth habits. This example is more like ground cover than an erect, woody plant. I wouldn’t bother with the species name except that the catalogue lists two species. I’ll look for the other one next time I get to the nature center.
Final Thoughts
I hope that everyone who reads this takes more notice of the plants they encounter wherever they go. And you can always take a picture.
And don’t be afraid to ask a Large Language Model (e.g. ChatGPT, Claude, Gemini) for help identifying whatever you find. They are knowledgeable, unassuming, and fun to work with.
And they will readily admit when they are wrong…
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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