The Geology of Franklin Falls
Washington has more than 3000 catalogued waterfalls, so we stumble onto them regularly. Waterfalls form wherever there is a change in the lithology of the crustal rocks: along crustal faults, which naturally create vertical planes; where weaker rocks lie beneath strong ones and are reached by a downcutting river; and of course where glaciers have sculpted the land. These natural features create spectacular views as well as revealing glimpses of deep time.
Plate 1. Franklin Falls is about 70 feet high. The reddish rocks forming the cliff are Eocene sandstones and volcanic rocks (56 – 34 Ma). The park follows the South Fork of the Snoqualmie River along I-90. The parking lot is quite large and we found a parking space, even though it was great weather; more than one million people visited the site in June.
The rocks in this photo don’t reveal well-defined bedding although there is a suggestion of bedding to the right side of the image. However, I think the bedding marker is the vertical, dark streak that is slightly wavy. This curvature suggests that these rocks were deformed while ductile (buried deep enough to bend rather than break). The geologic map identifies an anticline in the area, and I think I found it.
Plate 2. Franklin Falls is located just west of Snoqualmie Pass, at an elevation of about 3000 feet. The Northern Cascades are very rugged and access is limited, especially in the winter. The topography shown in this map reveals steep slopes, especially through this pass, which I assume is the easiest route through this rugged terrain.
Plate 3. The geologic map from Rock D shows three distinct rock units, which reveal the general geological history of this area. The oldest rocks are part of the Jurassic melange belt (201 Ma), comprising metasedimentary and metavolcanic rocks that were originally deposited in deep water, possibly on a submarine fan. These sediments were buried and then scraped off the subducting ocean plate. This process deformed them as if they were put in a blender; hence the name, melange.
Almost 150 million years are missing before Eocene volcanic and sedimentary rocks were deposited in a continental setting, possibly a river or lake not too close to mountains; the rocks seen in Plate 1 don’t contain any large boulders like we see in the modern river bed below them.
Granodiorite is an intrusive igneous rock containing light-colored feldspars like albite, and slightly less quartz than granite. This batholith was emplaced during the Miocene (23-5.3 Ma) into rocks much like the Eocene rocks exposed along the river (Plate 1). However, granodiorite magma does not produce andesite, which is the most common volcanic rock in the Cascades. All those plutons being shoved into the upper crust exhibit a lot of chemical variety because the magma mixes with continental crust as it rises.
The final piece of the puzzle is the normal fault shown on the geologic map. The NW block of melange is labeled with a “U” to indicate that it moved upwards relative to the SE block of sandstone/volcanics. This displacement brought the older (Jurassic) rocks upward, eroding the rocks that were deposited above them. The fault isn’t shown as extending into the granodiorite, however; it is very difficult to identify a fault (usually by displaced stratigraphic units) in relatively uniform intrusive rocks, which contain no layering. If the field geologist saw evidence of this fault extending into the granodiorite, they would have used a dash line to indicate uncertainty. This line stops cold…
Plate 4. This photo is looking NW, across the fault, towards the melange. This peak is a block of Jurassic rock that was once tumbled in the accretionary wedge after it was scraped off the subducting ocean crust. The peak further back is Denny Mountain, Oligocene (34-23 Ma) volcaniclastic deposits. They are older than the granodiorite. We couldn’t hike around these mountains to see the rocks up close, but fortunately, gravity has made our job easier.
Plate 5. These are small boulders of the Jurassic Melange that rolled down the steep slope (i.e. face of the fault) and landed in the river bed (see Plate 1). (A) Mud deposited in the deep-sea trench was buried deeply enough to squeeze quartz and feldspar into veins (thin white lines in the image). However, it is probable that the thick layer of light-colored minerals were injected along weak points when the Miocene granodiorite was emplaced. Note that the vein becomes more orange to the right of the sample; this is probably a local concentration of K-feldspar (e.g. sanidine). (B) This sample looks like it was spun in a blender because the thin layers of light-colored minerals are twisted rather than approximately following original bedding (which has been obliterated).
Plate 6. This rounded boulder of granodiorite is about 18 inches in diameter. The large, white phenocrysts are plagioclase feldspar with a low Ca content whereas the darker ones with blurred sides are hornblende. The matrix is fine-grained plagioclase with a higher Ca content (feldspar composition varies with potassium-sodium-calcium). Calcium is associated with dark, potassium with pink, and sodium with white-colored crystals.
Summary
Today’s field trip was a unique opportunity to integrate the large scale observations (Plates 1 and 4) with hand samples (Plates 5 and 6) using a geologic map (Plate 3) as a guide to understanding the geologic history of the Cascades Range. Let’s try to create a simple, plausible geological history from what we know.
I’m going to list a series of geologic events because I don’t feel like finding/creating schematic cross-sections to pictorially demonstrate what I’m saying. I’ll start from oldest and proceed to the youngest.
- About 200 Ma: Muddy sediments were deposited in a subduction trench, which was approximately aligned N-S along the axis of the later Cascades range. Over the next 10-20 Ma these sediments were buried miles beneath younger erosional debris, until they were snagged by the overlying NA tectonic plate and deformed like putty.
- Between 120 and 60 Ma: It is unclear exactly what occurred, but subduction ceased, probably because several large crustal blocks slid to the NW along a series of transform faults. The melange we see in Plates 4 and 5 missed the boat and remained buried, slowly inching their way to the surface.
- About 60 Ma: Subduction resumed and a series of volcanoes produced lava, which mixed with sandy sediments in a continental environment. Volcanism continued for 30 Ma, creating a mixture of fresh lava and sandy sediments, which were buried while erosion continued — a delicate balance of tectonic uplift and isostatic sinking.
- Between ~30 and 23 Ma: Tectonic uplift increased, bringing the Jurassic melange and Eocene rocks (Plate 3) into the crust’s brittle fracture zone, driven by a combination of subducting plate dynamics and upwelling magma, as the ocean plate melted. This agglomeration of different rock types began to fracture along contacts, while still deforming plastically internally. The normal fault seen in Plate 3 would have occurred along such a seam during this complex exhumation process.
- A series of hot, rising plumes of magma originating at the top of the subducting ocean slab eventually reached these rocks, possibly within a few miles of the surface, between 23 and 5 Ma. The granodiorite filled every fracture and fault, creating the complex pattern seen in Plate 3. The thick veins of quartz/feldspar seen in Plate 5A would have been injected during this interval.
- Exhumation has continued, from 5 Ma to the present, modified by glacial scouring of the ancestral Snoqualmie River canyon. Today we see these rocks conveniently frozen in time, from our perspective.
Identifying the contacts between these many rock facies is a laborious task that will take decades, if not centuries, to complete. Nevertheless, it is obvious that a lot has occurred in the last few million years. The earth’s surface is a conveyor belt on which the pile of soil/rock is constantly removed by wind/rain/snow/ice…
That’s my story…
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.
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…
Cretaceous Intrusive Rocks in British Columbia
This post finds me in British Columbia (Fig. 1), where I explored fjords cut into mostly intrusive rocks that form an immense batholith composed of many overlapping plutons. The tectonic regime of this area is collision between the many small plates that comprised the Pacific Ocean as well as any island arcs and microcontinents that got in the way of N. America on its westward journey, which began about 200 million years ago (Ma). A collision between ocean crust and the N. American tectonic plate would have created a subduction zone, in which the denser ocean crust was driven beneath the continent. However, recent work suggests that this process was interrupted for uncertain intervals when islands collided with N. America; furthermore, there is evidence for a lot of strike-slip fault movement along transform faults. You can learn more about this complex history at Nick Zentner’s web page.
Figure 1. (A) Regional map showing the location of the study area in SW British Columbia, about 230 km (140 miles) north of Tacoma (starred location). (B) Detail map of the inlet (I couldn’t find a name for it), showing the location of the Sea-to-Sky gondola I rode to Squamish and Chief Viewpoint park. (C) Geologic map from RockD, showing the fairly uniform occurrence of diorite intrusive rocks intruded between 143 and 66 Ma (covering the entire Cretaceous geological period). The town of Squamish sits at the head of the fjord. Faults of unidentified type (i.e., normal, reverse, strike-slip) are common but don’t appear to control the location of major drainage basins, which have been excavated by glacial action during the Pleistocene epoch. The circle indicates the area reported on today, except for Fig. 2.
Figure 2. (A) Exposure of Late Cretaceous (100-66 Ma) sedimentary rocks at Stanley Park (bottom center of Fig. 1B). These rocks look unconsolidated, but they are actually well cemented. (B) Close-up showing lamination and sets of unidirectional cross-bedding, which indicates transport in a stream rather than a marine environment. Transport was generally to the left, but each set of cross-beds is a slice through a three-dimensional form like the bars seen in modern rivers.
Figure 3. View looking northward from Squamish and Chief Viewpoint park (circled in Fig. 1C). Besides offering a breathtaking view of the northern Rocky Mountains, this photo reveals several interesting geologic phenomena. The small peak in the foreground is solid diorite that was intruded between 143-66 Ma more than 20 km beneath the surface at that time. It has been uplifted as overlying rocks were removed by erosion. It may represent a single intrusive episode because typical plutons are 2-3 km in diameter; batholiths comprise multiple intrusions spanning millions, if not tens of millions, of years. Note the lineations running up the side. These reflect the scraping action of ice sheets during the last two million years as they flowed towards the sea. The valley in the background is the classic U-shape caused by the advance of glaciers. It has been filled in by glacial deposits very similar to those found in NW Washington. Glaciers in this area originated locally and were up to several kilometers in thickness. They flowed both towards the sea and inland to form a continental glacier.
Figure 4. (A) The rocks within the park are fairly uniform in composition because they were intruded as part of a single pluton. This boulder shows the typical salt-and-pepper appearance of dioritic rocks. This sample is relatively fine grained, which indicates that it wasn’t emplaced more than a few tens of kilometers deep; i.e., smaller crystals indicate faster cooling which occurs nearer the surface. (B) Close-up at the highest optical magnification available on my phone, about 5X. RockD generally describes the igneous rocks from this area as granodiorite, a type of diorite that contains 20-60% quartz (labeled Q in the photo), and primarily feldspar (labeled F) that contains sodium and calcium with little potassium. The dark minerals are amphiboles (labeled A), probably hornblende. These are typical granitoid rocks from a subduction zone.
Figure 5. (A) Surface showing a vein filled with a harder mineral that is probably quartz. The softer feldspar and hornblende has weathered more easily and been removed by the scraping of ice sheets containing rocks. Note the irregular shape of the vein; minerals like quartz have the lowest temperature to solidify and fill cracks in the slowly cooling magma. (B) This vein is straighter than that in A and is filled with minerals that have weathered more than the main rock. (C) This photo shows joints (X pattern in the center of image) and lineations (top to bottom) that indicate ice moving over the surface with embedded rocks that scratch the exposed rock. (D) This image shows a finer grained material filling a void in the original magma. The pebbled appearance of the coarser material results from preferential weathering of minerals like hornblende and feldspar. These images imply that the magma had solidified sufficiently to form joints as it cooled, but was then injected with more fluid. This isn’t surprising in a region where so many plutons were being created; there was certainly a lot of overlap in their intrusion.
Figure 6. This final photo shows two pointed peaks that result from ice cutting away at any rock that protruded above the main surface. Remember that the ice was up to three kilometers thick here whereas the peaks are less than two km above sea level. It is hard to imagine that much ice.
SUMMARY
This post describes only a small part of one pluton of the hundreds exposed within the Northern Rocky Mountains in British Columbia. The scale is difficult to visualize, but they represent only a tiny fraction of the immense volume of oceanic crust that was subducted beneath the nascent west coast of N. America during the Cretaceous period. Nevertheless, what we’ve documented shows that this was a continuous process that lasted almost 70 million years. And it is continuing to this day, as evidenced by volcanism within the Cascades Range of Washington, Oregon, and N. California.
Deep igneous intrusion doesn’t necessarily create volcanoes, however; surface eruptions occur when magma is emplaced at shallow depths, which was probably not the case for these rocks from the Cretaceous. We can’t know for sure because so much rock has been removed by wind, water, and glaciers during the uplift of these intrusive rocks, from tens of kilometers beneath the surface.
Even if our picture of the earth’s history in the Pacific Northwest is incomplete and mysterious, it is awe inspiring to look back in time and deep within the crust.
And it isn’t science fiction…
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?
Cape Flattery: Conglomerates at the End of the World
Figure 1. I ran out of road and land, ending up at Cape Flattery on the Makah Reservation. This is Location D in Fig. 2. A half-mile hike down a boardwalk took me to the end of the world, where the cliffs are about 80 feet above sea level. This photo is looking northward towards Vancouver Island, Canada, in the distance. Wave action has cut a wave terrace, a seen in the center of the image, and a series of sea caves.
Figure 2. Today’s post takes us to Site D, which is shown in a geologic map from Rock D in the inset map. The rocks are a mixture of sandstone and conglomerate of Eocene age (56-34 Ma). This is the same time that the lower Cascades were being created east of Puget Sound. Note the large number of faults (thin lines) shown in the inset map. Many of these are strike-slip faults with motion of the opposing crustal block to the left, which would explain why the sedimentary layers in seen in Fig. 1 are almost horizontal, dipping slightly towards the sea.
Figure 3. The sea stacks seen further south have been replaced with rocky islands like Tatoosh Island, which has a light house. Pacific Ocean seafloor (Juan de Fuca tectonic plate) is being actively subducted along the Washington-Oregon coast at about 2 inches/year, but the plate tectonic geometry is complex in this area. This could partly explain the large number of faults seen in Fig. 2 as well as the high elevation of bedrock. Maybe.
Figure 4. This photo was taken looking down towards the wave-terrace on the south side of the lookout point. For scale, the largest boulders are a couple of feet long, based on exposures at the surface of the trail. This is a textbook tectonic breccia. However, note that most of the rock fragments are rounded, so they have been transported some distance before reaching their final resting place. Some of the layers are relatively uniform with a few large boulders whereas others resemble the cobblestones we saw at Ruby Beach. Each bed might represent a single depositional event (like a landslide) or accumulation over years. These are marine rocks, deposited on a submarine fan, probably as turbidity flows. The process that created them is occurring now a few miles offshore, where the rocky outcrops we saw are eroding and supplying large and small fragments to a steep continental shelf.
Figure 5. This image looks like run-of-the-mill gravel like you see on pathways and drives, but it is Eocene rock, and I mean solid rock (Note the large, gray piece protruding from the cliff in Fig. 4). This angular slab is about a foot in length. There is no Pleistocene glacial till in this area, even if the ground looks the same. This is a picture frozen in time, where an angular piece of what looks like sandstone, but could be andesite, slid downslope along with smaller fragments.
Summary.
This is the last of my geology posts from a weekend excursion along a hundred-mile stretch of the Washington coast. This is a high wave-energy coast with a tidal range of about 10 feet, the beaches covered with sand and cobbles. Sea stacks protrude from the beach, culminating in cliffs made of 50 Ma conglomerates. Beneath our feet, ocean crust was being subducted at several inches per year, feeding a system of magma chambers that are actively venting through volcanoes like Mt. Saint Helens, Rainier, and Mt. Baker. Rainfall drops along this coast at up to 12 feet per year, feeding streams that carry immense quantities of sediment into the trench created by the subducting ocean crust.
Geology doesn’t get any realer than this…
Tectonic Breccia at Ruby Beach
Figure 1. Ruby Beach is located about 10 miles north of Kalaloch Beach (Point C in Fig. 2), but the coastal morphology has changed substantially. This photo was taken at sunrise during low tide, exposing many outcrops of basement rock, which is the same unnamed sandy rock formation we saw before, but its composition has changed.
Figure 2. Today’s post describes the coastal geology at Site C. I’ve already discussed the Quinault Rainforest and Kalaloch Beach. I will focus on what is different from the previous post. The inset geologic map from Rock D will be referred to below.
Figure 3. Basement rock of general Tertiary age (66.5-2.4 Ma) is more exposed here although it is overlain by Pleistocene glacial sediments. This photo shows a rocky island that can be reached at low tide, and a promontory. The beach is composed of rounded cobbles in a matrix of sand, silt, and some clay. The finer sediments are restricted to the lower swash zone (Fig. 1).
Figure 4. This is a sea stack, a standalone rock pillar. The beach is a thin veneer spread across a rocky basement that is more irregular than we saw at Kalaloch Beach, where the outcrops resembled a wave-cut platform more than we see here. The first thing that occurs to me is that this area hasn’t been exposed to the erosive power of waves for as long an interval. Maybe. Let’s take a look at the inset map of Fig. 2. The black lines represent geologic faults, where rocks have been displaced by tectonic activity. Note that the stretch of coast we’re concerned with is bounded by faults perpendicular to the coast. Furthermore, there is another fault separating Sites B and C. I should take a moment to point out the relationship between faults and river valleys: faults create weak zones within the crust, which are exploited by erosional forces like water and glaciers. Thus we see valleys at both B and C; however, the intervening fault is part of a complex fault system that appears to have led to less surface erosion. Faults cannot be directly dated, only indirectly by the age of the rocks they displace, and in this case those rocks are themselves difficult to date. Thus, it is possible that vertical movement, even a few hundred yards, could have made this beach more irregular than Kalaloch Beach only a few miles to the south–on the other side of a fault. Plausible, but don’t bet your retirement on my hypothesis.
Figure 5. Back to the rocks. This photo puzzled me because I’m not an experienced field geologist, despite my regular posts; however, CoPilot came to the only plausible conclusion: the primary material here is a breccia/graywacke; the smoother material is calcite filling fractures that occurred during faulting (see Fig. 2 inset). We must ALWAYS remember that a sedimentary rock’s history isn’t confined to deposition; a lot happens during burial to many miles and subsequent exhumation and deformation.
Figure 6. This photo shows a typical graywacke texture with layering, as you might expect with episodic deposition of event beds (e.g. turbidites on a submarine fan); but the upper part of the image is similar to Fig. 6. I asked CoPilot about the source of so much Calcium and I agree with its answer: Some of the Calcium came from marine invertebrates living in the area, but the bulk arrived later, when the sediments were buried deep (several miles) within the accretionary wedge where Calcium is released from minerals like feldspar. It’s really hot down there and the pressure is INTENSE.
Summary.
Not only was Ruby Beach a beautiful area on a cool winter/spring morning, it gave us more insight into the complex life of the earth’s crust. These rocks were probably deposited as poorly sorted graywacke on a steep continental margin overwhelmed by sediment eroded from the rising Cascades (more than likely the Eocene, 56-34 Ma). They were buried for a few million years, before being scraped off the subducting oceanic crust and filled with hot fluids that originated from deeper within the accretionary prism. They were never so humiliated that they became melanges but it was a pretty rough ride, even for a rock.
To put this all in perspective, dinosaurs had just gone extinct (~65 Ma) when all hell broke loose and the Cascades Subduction Zone became very active about ten-million years later; the earth became hot as hell, mammals and birds were beginning to get their legs and wings. While the ecosystem changed dramatically, these rocks were being ground up and spit out by the earth because they were stuck on the end of its tongue. We hominids didn’t come along until these rocks were exposed to the vagaries of the weather and had eroded for a VERY long time.
Try not to laugh at the punch line: Our entire history is contained in the Pleistocene glacial sediments overlying these fairly young sediments.
It boggles the mind…
Coastal Geology of Kalaloch Beach
This report is the geology supplement to my general post about the Olympic Peninsula. It focuses on the geology of the beaches because, to be honest, Quinault Rainforest was impenetrable, physically and geologically, without massive logistical planning and support. What I found was more than I expected.
Figure 1. Photo looking north along Beach 4 at Site B in Fig. 2. The tidal range is about 10 feet here and the primary sediment consists of sand with a substantial mud component. The rocks seen in this image are bedrock that protrudes above the veneer of beach sediment. The cliffs to the right comprise glacial till that is less than 2.4 my old (Ma hereafter) deposited by glaciers during the Quaternary geological period. This photo was taken near low tide. The dark color of the beach surface is caused by the relatively large clay component (i.e. mud).
Figure 2. Location map of today’s field area, which will focus on Site B.
Figure 3. Schematic of the Cascadia Subduction Zone as it appears today. The Coast Range, which includes the Olympic Mountains, consists of sediments and volcanics that have been scraped off the subducting ocean crust. The white arrows at the front of the image don’t reflect the actual geometry of these sedimentary rocks. We’ll get to that in a minute. Today’s post will focus on these sediments and discuss their original depositional environment during the Tertiary period (~65.5-2.4 Ma), as well as evidence for structural deformation.
Figure 4. This photo has been marked-up to highlight what might not be obvious to a non-geologist. First, older sedimentary rocks have been tilted to a high angle (dark blue bedding plane line) by faulting, as suggested in Fig. 3, and exposed to erosion at the surface. This erosional surface is shown by the yellow line. These Tertiary sediments were buried several miles beneath the ocean surface where the grains were cemented by heat and pressure. We don’t know exactly when this occurred or how long this process took, but they were later overlain by younger glacial sediments during the Pleistocene epoch (2.4 Ma to present). The age of sedimentary rocks can’t be pinned down unless they contain material datable by radiometric methods or, more qualitatively, by the fossils they contain. Sandy sediments don’t contain fossils very often and I didn’t find any radiometric ages in either Rock D or the USGS geologic map, so I guess the Tertiary date is the best we can do for now.
Figure 5. Close-up photo of the contact between the Tertiary rocks and the glacial till. Note that the cobbles aren’t falling out of the fine matrix sand and clay; this is because these young (>2.4 Ma) conglomerates have been partially cemented and, in fact, they were stuck to the older rocks below. This photo tells us a lot: 1) the older rocks were tilted at depth (maybe a mile or more) and then pushed upward as suggested in Fig. 3 until they were exposed to the atmosphere; 2) in a very short time they were covered by cobbles (~2 inches or less) that had already been rounded in a river, which takes several miles of transport; 3) the type of sediment varied rapidly, from coarse sand to cobbles, probably within decades if not years, due in part to periodic changes in surface drainage (e.g. the advance and retreat of glaciers); 4) this is a good example of the principle of Uniformitarianism–the present is the key to the past: we see similar processes occurring today along the many streams and rivers draining the Olympic Mountains. But what about those pesky Tertiary sandstones?
Figure 6. This photo of the Tertiary sedimentary rocks reveals packets of thin-bedded fine sandstone with intercalated dark layers of (presumably) mud. I didn’t see any cross-bedding, which would indicate deposition in a river or nearshore dominated by waves or currents. What is obvious that this ancient (~50-2 Ma) shoreline was receiving sediment from a distant source, probably the volcanic highlands of the modern Cascades (see Fig. 3). Let’s take a closer look at one of those wavy layers.
Figure 7. Close-up image of a thin layer (about 3 inches), showing what is called Flasier bedding. This is found in modern environments where high-energy (e.g. waves and tides) periods are interrupted by quiet times during which fine sediment can be deposited between ripples. It is indicative of a shallow marine environment.
Figure 8. These holes were evident throughout the exposure. They are problematic and probably originated in several ways; however, one plausible explanation is that soft-bodied animals like worms burrowed in the sediments and their burrows filled with sediment with a different composition (e.g. fecal pellets). These channels then preferentially eroded when subjected to the harsh nearshore environment in which they are found. The presence of such holes at the unconformity (see Fig. 5) proves that they predate deposition of the overlying glacial till. Unfortunately, such trace fossils don’t tell us anything about geologic age, only that this seashore was teeming with life.
Figure 9. I stopped at another accessible beach a couple of miles south, but still indicated as Site B in Fig. 2, where the rocks were sandstone but with very little clay. The beds seen in this photo were thick (1-3 feet). They were not deposited in a high-energy, environment like those we saw before; they were probably deposited on the shoreface below fair-weather wave base–possibly part of a nearshore bar or barrier island. After deposition and burial, they were tilted less steeply than those seen further north (Fig. 6). The unconformity is seen in the upper part of this image, but it doesn’t seem to have as many rounded cobbles as before (Fig. 5). I have no way of knowing the relative age of these two exposures; this Tertiary rock unit is approximately 3000 feet thick and represents millions of years. All we can say for certain is that this shoreline wasn’t that different from what we observe today. Uniformitarianism in action.
Figure 10. This exposed outcrop within the tidal zone reveals what are probably concretions of iron oxides, formed during early diagenesis. They superficially resemble fossils, but that is not the case. As the sediment was squeezed during burial, incompatible elements like iron formed irregular blobs in an otherwise uniform quartz sand matrix.
Summary
This coastline was part of the Cascadia subduction zone during most of the Tertiary period. Sediment, including a lot of sand, was eroded from rocks being uplifted further inland where volcanism was active, especially during the Eocene (55-35 Ma). Beaches like those we see toady were common, as well as the various depositional environments found along the Olympic peninsula, including fluvial and submarine fans, beaches, and cliffs. But no glacial sediment.
These sediments were buried as more material was removed from the rising orogenic belt, which included granitic rocks with lots of quartz. Eventually they were caught by the complex trench uplift (see Fig. 3) and scraped onto the edge of N. America. This involved faulting primarily, suggesting this was a fairly shallow process (folds occur at depth where rocks are ductile). The tilted rocks were eroded for millions of years; then, about 2.4 Ma glaciers (nothing to do with subduction) covered the region and deposited all kind of sediment: rivers, lakes, undifferentiated till, moraines, etc.
Just as the East Coast of America has been a passive margin for more than 200 my, the West Coast has been a convergent margin. Instead of being steadily worn away by wind and water, these rocks are rising out of the sea at about 2 inches per year. And what a ride it is…
Active Delta Accretion at Dash Point State Park
INTRODUCTION
Puget Sound is a complex water body with many channels and bays, all of them within a couple of hours drive from my home in Tacoma. I haven’t posted much about this because I’ve been overwhelmed by so many active sedimentological processes that are neither marine nor fluvial. But I had to say something about what we discovered on this short trip, less than 30 minutes from our house. Because of the bluffs (~300 feet) surrounding Puget Sound, streams draining into the various inlets and bays are short and relatively steep. There are exceptions of course, such as the Nisqually River, which flows from Mt. Rainier to the southern end of Puget Sound; however, short streams are very common although many of them have been incorporated into municipal storm drainage systems. Today, we found one that was in an almost natural state.
Figure 1. This photo, looking landward from Dash Point beach, says it all: A small stream enters between the two bluffs, each about 200 feet in height, and dumps copious amounts of sand, silt and clay onto a mesotidal shoreline (the tidal range is about 7 feet), where the volume of sediment input overwhelms the nearshore wave and tidal regime to create an expanding delta.
Figure 2. (A) Regional map showing the complex, glacially sculpted, morphology of Puget Sound. The star is approximately where my house is. (B) Google Earth image from about 7000 feet showing Dash Point and the extensive delta being constructed by sediment delivered by a small stream.
Figure 3. The small stream feeding the delta has been confined to a stone-lined channel. This is the entire inflow. The water looks pretty clear, but appearances can be deceiving in the context of sediment transport. This is a bedload-dominated stream, which means that it is mostly transporting sand and silt, and minimal clay particles. The entire area surrounding Puget Sound is constructed of glacial till, which is mostly gravel and sand. A closer examination will confirm this inference.
Figure 4. This is the delta seen in Fig. 2B during an ebbing tide that is near its minimum. The expanse of the delta, at approximately the same tidal height, is seen here at ground level, revealing tidal channels and swales; but the surface is sand and silt with minimal clay restricted to the swales, like those seen in the middle of this photo.
Figure 5. The flattened sea grass serves as a current indicator that perfectly matches the ripple orientation. Note that the tops of the ripples are flattened by the high velocity of the ebbing tide. Waves weren’t very large on this particular day.
Figure 6. Sand bars are constantly being created and destroyed at the fringe of the delta, probably in seasonal cycles. I don’t know where a late February date fits into this cycle. Perhaps I’ll need to return in the summer.
Figure 7. The delta was less symmetric on this February day than the undated image in Fig. 2B. This image looks to the west. It reveals multiple shore-perpendicular sand bars, until the delta is interrupted by shoreline development. Nothing like this was present on the eastern side of the delta on this day. The sediment supplied by the stream in Fig. 3 is being transported westward in this shallow water, but not by waves like we saw on this day, which are perpendicular to the shore.
SUMMARY
Dash Point beach provided a great opportunity to study nearshore sedimentology. Just look at those ripples in Fig. 7! Despite being created and destroyed twice a day, we find rocks preserving similar ripples throughout geologic time.
With so much gravel in the Puget Sound area eroded from the glacial till (gravel and sand), it was exciting to see finer grained sedimentation occurring in real time. Gravel beaches are uncommon in the geologic record whereas rippled, sandy beaches are very common; after all, the previous source of this sand and silt was a glacial till, itself the product of erosion by ice and fluvial transport during the previous twenty-thousand years.
Material is recycled by our tectonically active planet.
Earth abides…
Pinnacle Peak Park: Miocene Andesitic Volcanism
Introduction.
This post examines some of the rocks we discovered while climbing Pinnacle Peak (Fig. 1), a small shield volcano created during the Miocene epoch, between 23 and 5.3 Ma (millions of years ago determined by radiometric dating). The wide range of dates suggests that this volcano remained active for a very long time, producing volcanic material of different types as it released pressure from the magma chamber feeding it. I can’t pin down the dates any better, although they are better known within the geological literature. Thus, my discussion and the model that follows are meant to apply to the entire period of activity.
Figure 1. Pinnacle Peak is about 1000 feet in height. This photo reveals the characteristic low profile of a shield volcano, making it appear smaller because the summit is more than a mile distant; however, note that it is slightly asymmetrical because the southern (right side) slope is lower than the north side.
Figure 2. (A) The Pacific Northwest (PNW) base map I will refer to in this series of posts. Tacoma is indicated by a star and Pinnacle Peak park by the circle. It was less than an hour drive on a weekend at 0800; traffic picked up substantially and, by the time we left at about 1130, there was a traffic jam throughout the area because the access roads are small. Go early! (B) The geological map from RockD shows several volcanoes of similar Miocene age standing out in an ocean of glacial till. Note the presence of a deposit of till on the southern slope of the mountain. This was difficult to identify because the main trails followed old logging roads that had been covered with gravel, probably from this till and the White River flood plain below, in which there was active quarrying on the day of our visit. Note the blue arrows at the summit because they will be referred to below.
Observations.
We walked up the Pinnacle Peak Loop Trail counterclockwise, taking photos and noting the geology along the path. However, this is not a geological report but only a casual observation. All dates and rock types are from RockD, a compilation of geological maps produced by uncountable numbers of geologists engaged in active geological field and laboratory research. Thank them for the accessibility of their data, and blame me for any errors in interpreting it.
Figure 3. This photo was taken about half-way up the volcano where a road-cut revealed volcaniclastic sediments like we saw at Snoqualmie Falls. This nice exposure reveals a fine matrix of dark material including irregular, but rounded, boulders up to a few feet in diameter. These are volcanic bombs, semi-molten lave blown out of the vent by gas pressure which landed hundreds of feet from the summit.
Figure 4. (A) This boulder of volcanic breccia was placed in the parking lot, probably as an example for public viewing. It is about 3 feet in diameter and contains numerous volcanic bombs, as well as fine lamination near what was its original bottom, as labeled in the figure. These would have been layers of ash with alternating chemical properties that gave them different colors. (B) Close-up showing the contrast between the lighter colored ash matrix and an andesite bomb. Imagine this semi-molten, andesite bomb flying hundreds of yards and landing in still-hot ash. The number of bombs visible in (A) suggest that this was part of a very explosive event.
Figure 5. (A) Close-up of a boulder within a semi-hidden exposure of andesite near where Fig. 4 was taken. (B) This close-up reveals phenocrysts (solid mineral crystals) and vesicles (pockets left by escaping volcanic gases). I cannot identify the minerals comprising the phenocrysts, but common ones within andesite are pyroxenes, plagioclase (high-calcium feldspar), hornblende, and biotite. These are all dark minerals that crystallize at higher temperatures than high-silica (Si02) minerals. The specific mineralogy tells volcanologists about the chemistry of the magma chamber feeding the volcano. The vesicles of gas (e.g. CO2, CO, water vapor, sulfur compounds, H2S) are a crude indication of how explosive the magma was; more vesicles implies more explosive potential, especially in andesites, which are more viscous than basalt.
Figure 6. View from near the summit looking south toward the White River. This is a very different perspective of Pinnacle Peak than Fig. 1. The river valley is filled with glacial till (< 1 million years old) that covered all of the volcanic material from the volcano, which stands out today because it was never covered by ice.
Figure 7. Photo taken on the SE side at the top of Pinnacle Peak (blue arrow in Fig. 2A). These horizontal rocks are not boulders, but instead the ends of columnar joint blocks of andesite. They are less than one foot in diameter; most of them are hexagonal, but irregular shapes are also present. They form when lava cools slowly enough to allow this kind of crystallization to occur, but rapidly enough that the lava doesn’t form massive beds.
Figure 8. Photo of columnar andesite near the summit on the NNW side. These are much larger in diameter than those seen in Fig. 7. Columnar jointing is usually oriented with the ends vertical, but this isn’t necessarily required because gravity is not the dominant force; chemical bonds between the individual minerals determines the development and size of columnar joints. As long as the “top” of the lava flow (facing the camera in this photo) is exposed to the atmosphere, they can form. I reported on incredible examples of this in my post on Organ Pipes National Park near Melbourne, Australia.
Figure 9. This exposure, from further down the south slope of Pinnacle Peak, reveals blocky lava that seems to be bedded, with bedding planes dipping away and to the right of the camera. No columnar jointing is visible, and the rocks are solid. This could be an outcrop of sandstone if it weren’t for the grayish color. Maybe limestone?
Figure 10. (A) Exposure of andesitic lava that looks much the worse for wear than anything I’ve seen before on this field trip. (B) Close-up of the exposure outlined in (A) that reveals a jumbled mass of lava that reveals several secondary textures: contamination and preferential erosion along “bedding” planes (fissility); irregular and tilted bedding planes; fine-scale fracturing and weathering (fractured); and blocks with a hint of hexagonal form (columnar joints). This was near the bottom of the volcano and the rocks are presumably older than those seen in Figs. 3, 7, 8, and 9; in other words this photo implies that eruption style was not uniform throughout this volcano’s lifetime. I further suggest that, over the lifetime of this volcano’s active period, the magma chamber became more stable, and thus the eruptions more predictable.
Figure 11. All of the textures, mineralogy, and fabric of the rocks comprising Pinnacle Peak fall within the expected composition of a shield volcano, except for the horizontal columnar jointing seen in Figs. 7 and 8, which point in opposite directions (NW and SE) even though the lava erupted directly from a rather small outlet (~200 feet in diameter). I didn’t understand what I saw at Organ Pipes National Park, but I let it go; now that I’ve encountered similar textures again, I want to have at least a naive understanding of how lava can form structures similar to a wilted plant. This is my model. The purple represents thousands of lava flows, volcaniclastic deposits, etc; the red is the lava just before the eruption(s) represented by Figs. 7 and 8; and the yellow is a thin layer of viscous lava that flowed out over a slightly older deposit. This last eruption wasn’t immediately covered by more lava and it cooled according to the laws of thermodynamics. In other words, it formed columnar joints, which are represented by the squiggly lines. The red arrows point to toward the “top” of the flow at every point; as you can see, the “top” isn’t always pointing towards the sky. This model is based on simple thermodynamics and mineralogy–I assume that the lava is a homogeneous mixture of the components of andesite (e.g. quartz, feldspar, pyroxene), which are cooled only by their exposure to the atmosphere. The different diameters in the columns (compare Figs. 7 and 8) suggests that this is an oversimplification. Nevertheless, it makes sense to me.
SUMMARY.
I had a great time climbing Pinnacle Peak and I learned something new from the rocks that surround and support us. A large mountain range like the Cascades doesn’t appear overnight. Subduction along the NW coast of N. America continued, interrupted by collisions with offshore continents, from 200 Ma to the present, creating multiple mountain ranges which created the Pacific Northwest. A small volcano like Pinnacle Peak would have burned out in a few million years. The range in age is based on stratigraphy, and the absence of funding/geologists to date rocks from every minor volcano in a major subduction zone, leaves me to apply common sense.
Pinnacle Peak erupted about 5 Ma, based on the physical status of the summit. It sputtered for a few hundred-thousand years as the magmatic system decompressed; eventually the magma chamber, or a subsystem of pathways, gave a last gasp. The columnar jointing we saw today suggests that this final eruption consisted of some spitting and then the appearance of timid lava, flowing a few-hundred yards from the summit (Fig. 11). That is what we saw on today’s field trip.
That’s my story…
Timothy R. Keen 
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