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Horsepen Run in December

I wrote a post about streams traversing the Potomac River flood plain in a previous post. Horsepen Run is a meandering stream that has cut down several feet across the gently undulating sediments blanketing the mile-wide flood plain at this point on the Potomac (Fig. 1).

Figure 1. Map of Horsepen Run area. The reference to Fig. 2 is from the original post.

It had rained for 24 hours prior to this field trip, and then temperatures plummeted to well below freezing. It never snowed but there was some sleet. We are going to see some interesting features that resulted from this unique event. The watershed for Horsepen Run is rocky, with bed rock never more than a few feet beneath the surface, except on the floodplain (Fig. 1).

Figure 2. Shallow ditch along the path leading to the Potomac River, just entering the black-circled area in Fig. 1.

When I first came across the curious ice structures in Fig. 2, I thought someone had ridden a bicycle along the ice for fun. I didn’t figure it out until later.

Figure 3. Meander in Horsepen Run. The surface appears to be ice. Note the ice lying along the opposite shoreline. A curious structure.

Figure 4. The surface of Horsepen Run where it empties into the Potomac is solid ice. We tossed a branch out and it broke, indicating the ice is several inches thick.

Figure 5. Close-up of Horsepen Run at the Potomac. Note the broken ice about one inch thick lying along the opposite bank.

It’s time to put it all together. The water level during the steady rain was elevated so close to the stream’s outlet. The temperature was low enough (~10 F) to freeze the surface while water continued to flow beneath the ice. Even though the temperature remained very low, when the floodwater ran out from under the ice, it cracked and collapsed like a pane of glass. The newly exposed subsurface water at the lowered level then froze. This process occurred even in a few inches of water (Fig. 2).

The ice sheets (Figs. 4 and 5) serve as a high-water marker that melted away with the next thaw, serving as markers of how much water can collect during a light rain on nonporous soils and rock.

Geology integrates rock and soil with the atmosphere and hydrosphere into a holistic system that can surprise us at every change in the weather.

Fraser Preserve

We had a chance this week to see what the Nature Conservancy does with our donations. They buy land and either maintain it or return it to the state or local government as public parks.

FIGURE 1. This post takes us back to the Potomac River, where we hiked around Fraser Preserve, a plot of land owned by the Nature Conservancy and open to the public. The photo above shows a stream flowing under an old concrete bridge as it cuts its way to the Potomac River. Downcutting here was similar to what we’ve seen elsewhere along the VA side.

FIGURE 2. The blue dot marks the gravel road that leads to the trail we took, which is indicated by the dashed line. The light-green area to the left is the schist (1000-511 Ma) we’ve seen along this stretch of the Potomac. The darker area to its right is a metagraywacke from the same era. These rocks were originally deposited in an ocean trench where ocean crust was being subducted beneath continental crust. They were buried along with the ocean crust and deformed into medium-grade metamorphic rocks.

We left the nature preserve and briefly entered Seneca Regional Park (north of the road marked DCWA in Fig. 2) to get access to the Potomac River.

FIGURE 3. A side channel of the Potomac River with a gravel bed and pristine water flowing over outcrops of schist. The elongate dark areas are lenses of schist on the river bed. Similar features were observed at River Bend park and discussed in a previous post. The banks here are gravel and would be a great place to cool off on a hot summer day.

FIGURE 4. This is a view of an abandoned channel of the Potomac River, taken from the top of a steep bank, probably 40 feet above the river. This area is about a half mile downstream of Fig. 3. The water seen through the foliage is part of a cut-off lake that is active only during high-water. Similar features have been seen further upstream but with some water flow year round, as discussed previously.

FIGURE 5. Close-up of a small exposure of metagraywacke along the access road at the blue dot in Fig. 2. Note the thin bedding and striations aligned perpendicular to the hillside. These are probably sole marks that indicate the flow direction in the original sediments. These rocks appear as lenses within the larger volume of schist, which was originally deep-sea mud. Imagine submarine flows flowing down the steep face of a submarine fan as turbidites.

This is a short post because we have seen most of these rocks and geomorphic features before. The novel feature that prompted me to write this was the wide floodplain (at least 300 yards across) and totally abandoned channel (Fig. 4). I also haven’t seen such clear water with no mud deposited at the shoreline. This location isn’t far from the narrow chasm that created Great Falls, where the river turns southward. I also noted a large number of steep gullies that appeared with no warning, indicating recent erosion from the surrounding hills, which are a couple hundred feet above river level. It seemed that some of the higher ridges were supported by cobblestones rather than bedrock, a feature we noted in Claude Moore Park that suggests ancient point bars.

Photographs can’t capture the complex topography of Fraser Preserve, especially with such colorful foliage interfering, so I encourage anyone who has the time to get out and see this beautiful landscape for themselves.

One final note: I support Nature Conservancy in their efforts to preserve natural lands and keep rampant development in check…

Diamond Head

Figure 1. Images of Diamond Head crater, Honolulu, Hawaii (reference). This extinct tuff cone is contemporaneous with Koko Crater. Its age is difficult to pin down but it was erupted about 50 thousand years ago. As the photos show, it emerged on the seashore, as a continuous eruption of ash that was so hot the particles stuck together to form a tuff.

After so many posts from the volcanic island of Oahu, you wouldn’t think there was much left, but I couldn’t overlook the most famous volcano of all, although technically Diamond Head (Fig. 1) is a tuff cone like Koko Crater. This brief post is going to examine the internal structure of one of its limbs, on the seaward side.

Figure 2. Road cut along the seaward margin of the crater, showing the irregular, blocky form of the tephra that was blown out of the vent over a short period. This volcanic material consisted of ash, blocks of volcanic rock, and whatever else got in the way as hot gases escaped through fissures in the overlying rock. There is a suggestion of horizontal layers, but they are discontinuous and composed of blocky and thin-bedded areas. This is a common form for pyroclastic deposits.

The lighter color of the rocks in Fig. 2, compared to what we saw at Koko Crater or elsewhere on Oahu, suggests that the underlying magma chamber was depleted of mafic minerals. Dark hues associated with basalt are caused by minerals like plagioclase feldspar, amphibole and pyroxene, and biotite mica. The lighter color of the road cut (fresh and unweathered) suggests that the magma contained felsic minerals like albite and orthoclase feldspar, quartz, and muscovite mica. I could be completely wrong about this but there is no doubt that the rocks in Fig. 2 are not dark gray or black…

My hypothesis is consistent with what is known about the crystallization sequence of minerals from a melt and the resulting viscosity of igneous rocks. Mafic minerals and the lava they form have low viscosity and flow readily, as we’ve all seen in videos of eruptions on the island of Hawaii. These magmas bubble, flow, shoot fire into the air, and release pressure easily. However, felsic minerals (especially quartz) are sticky and have high viscosity, which causes them to resist flow, contain gasses, and eventually explode spectacularly (e.g. Mt. St. Helens).

I think the Diamond Head vent (i.e. volcano) tapped a part of the magma chamber that had already lost most of its mafic minerals, but it wasn’t as explosive as Mount St. Helens.

Figure 3. The center of this image shows a volcaniclastic sedimentary deposit resting on a tongue of tephra. Note the whitish rock (weathered) angling to the upper-right (blocky) and the thin layers of convex sediment to the left. Ash mixed with water flowed down the steep slope in channels that quickly formed in the poorly consolidated ash layers.

Another surprising feature I saw along the seaward margin of the Diamond Head tuff cone was a set of vertical joints filled with reddish rock (Fig. 4).

Figure 4. This image shows the typical blocky, irregular structure of volcanic deposits, but they are dissected in three vertical joints (circled). These rocks have not been buried, deformed, or displaced. These inferred joints are not due to uplift and stress relief, but they are oriented (estimated only) north-south, which is a regional trend of fractures and fissures on Oahu. They are not filled with quartz, but rather with similar material to the host rock. They were probably secondary release fissures for material from the magma chamber, allowing highly pressurized magma to escape.

It is important to remember that the entire island of Oahu was constructed by magma escaping through innumerable fissures like those seen in Fig. 4, at first creating thick lava sequences deep beneath the Pacific Ocean’s surface, then flowing through breaks in the jumbled mass of previous flows. By the time the pile of basalt reached the water’s surface to form Oahu, the magma chamber was running out of gas (so to speak), and the lava was thicker and more viscous.

Diamond Head and Koko Craters were the result of these last gasps.

Figure 5. This image shows how close this side of the Diamond Head Crater was to the shoreline. Steep is an understatement of this slope, where the layers of ash would have been washed into the sea, as waves eroded the foundation of this young volcanic cone. The sedimentary deposit seen in Fig. 3 gives us a glimpse into how dynamic this environment was only fifty millennia ago. The tuff cone in the background is Koko Crater, which serves as a good estimate of the heterogeneity of the magma chamber.

This post concludes my visit to Oahu, an island that rose from the sea less than five million years ago, formed by a huge magma chamber that was created when the Pacific plate slid over an upper mantle hot spot so concentrated that it melted ocean crust an constructed the Hawaiian archipelago, more than 1500 miles long.

I encourage anyone reading this post to explore the amazing story of this new land as it was populated by plants and animals, culminating in the incredible story of how Polynesian culture reached this remote land…

Inside Koko Crater

Figure 1. View looking into Koko Crater from the north, where the tuff cone was breached, allowing easy access by vehicles. There is a run-down botanical garden and a trail that follows the inner walls of the volcano (lower case; actually a tuff cone).

For this post, we went inside Koko Crater (Fig. 1) on the north side (Fig. 2), where the cone was breached, allowing easy access. A road had been constructed and the interior is now filled with a botanical garden and an equestrian center.

Figure 2. Image from Wikipedia, showing Koko Crater. A previous post discussed details of the ash layers outside the crater. This post will examine the interior of the tuff cone. Note the sharp ridge in the background, all that remains of the original Ko’olau Volcano.
Figure 3. Northern end of the crater, where the low slope was breached either by volcanic processes, erosion, or machinery, to make a road into the interior.

Parking is just outside the crater and a trail leads inside (Fig. 3), where a three-mile trail goes around the periphery. We didn’t have time to complete the circuit, so we settled for entering the main crater (see Fig. 2), where the walls were visible but not accessible for close examination (Fig. 4). However, the lower parts that were visible were covered with coarse debris less than 6 inches in diameter. There were some large boulders of vesicular basalt lying around, but they were loose and could have come from anywhere.

Figure 4. View of interior, showing discoloration of the ash to produce a whitish clay mineral; note the resistant material capping the tuff cone and preventing erosion. This layer is visible from the exterior as well, but has a more-rounded edge there, which suggests (to me) that this was a lava flow that barely reached the rim before running out of pressure. This is only speculation because I didn’t climb to the top of the cone and examine these rocks; it is just as likely that the exterior limit of this layer simply eroded more from exposure to north winds.

The extreme weathering seen on the inner slope in Fig. 4 suggests that the cap rock at least has a different composition, even if it is built from layers of ash. It is important to remember that tuff cones like Koko crater don’t continually erupt for centuries or millennia; they are local phenomena that vent part of the magma chamber that underlies a truly massive volcano like Ko’olau caldera (see Fig. 2). Thus, they are only active for a while, although dating is a problem for such short time scales.

Figure 5. Close-up image of interior. The cap can be seen to have a blocky form, with what looks like voids near the bottom (the dark areas that are elongate in the upper middle of the photo). The subjacent layer is highly altered to produce a tan color rather than the original dark gray to black. Between eruptions, the material would have collapsed into the center as it cooled, and weathering would have been continuous as it erupted. The construction of the cone through multiple eruptions is evident in the layered outcrop in the center of the image (note the dark, horizontal areas which I interpret as voids). These could be either thin layers of basalt or ash beds, but a combination is likely, based on what we saw on the exterior.

It is important to note that Koko crater as we see it today has been eroded and the interior filled with breccia and ash during and between eruptions. We can’t say how much time passed between the layers seen in the middle of Fig. 5, but it could be hours to weeks. Most tuff cones are active for a couple of months, so the active period of Koko was on the order of a few years. Volcanic vents can produce a lot of ash very quickly.

Figure 6. The bottom of the crater is layered with sand and gravel, plus some clays. The soil is sufficient to support a palm exhibit (part of the botanical garden) with no planting material added.

This is my last post from Koko Crater. I didn’t have time to climb the 1048 steps to its summit, and I’m pretty sure my knees are glad.

In a nutshell, a vent formed along a fracture zone associated with the Ko’olau volcanic system and spewed ash and minimal lava flows onto the surface, where they interacted with the nearby shoreline, all of it lasting only a few decades at most.

Erosion has been minimal so we see Koko pretty much the way Pele left it….

Rebirth of Ko’olau Volcano

Figure 1. Photo of a young tuff cone created within the last 100 ky during the rejuvenation phase of Ko’olau Volcano, contemporaneous with Koko Crater and Diamond Head.

These volcanic rocks were erupted more than 1.7 million years after the devastating collapse of the Ko’olau Caldera. It was probably a last gasp to release pressure within the magma chamber. Exact dating of these younger tuff cones is problematic, but they were all created within about a 50 ky window.

Figure 3. Two islands, one constructed of lighter volcanic material (background), and one of darker in the foreground. I wonder if the larger is a remnant of the original Ko’olau caldera, a conjecture I can’t address with the data I have. The lighter color could be due to a different degree of alteration.
Figure 4. Blocks taken from the road cut seen in Fig. 1, showing similar vesicular basalt as observed at Koko Crater. However, there seems to be a lot less ash at this location, possibly because these rocks originated from lower in the volcanic cone.

The kinds of volcanic tephra produced by the original Ko’olau volcano and the younger tuff cones shows a tendency towards more ash and less basalt. Certainly, Koko, Diamond Head, and this unnamed crater were part of monogenetic fields. These cones degassed a part of the magma chamber then became dormant; others appeared to perform the same function in another part of the chamber, part of the overall development of the Ko’olau volcano.

With the extinction of the fires here and at Koko Crater, it is safe to say that Ko’olau is dead and the goddess Pele has moved to her new home in the Kilauea volcano, on Hawaii…

Beach Erosion at Kailua

Figure 1. Map of SE peninsula, showing Kailua with the push pin.

This is a quick post to summarize some effects on the beaches of the windward side of Oahu, where basalt rocks of the Ko’olau volcano don’t protect the coast. Kailua isn’t far from Honolulu (Fig. 1) and the climate is similar. The wetter coastline, such as at Nu’uanu park, doesn’t extend this far. Kailua is a broad flat area, unlike the deep valleys of the north shore. The caldera is set back much further from the coast and the basalt is buried.

Figure 2. View looking inland at Kailua beach. The stream that passes through the break in the sand dune is typical for Oahu, in that it doesn’t reach the sea during the dry season. Note the erosional scarp at the top of the sand dune, which may be a seasonal feature that is healed during the rainy and stormy season. If I had to guess, however, I’d say it is a long-term feature.
Figure 3. Photo of dune face, showing consistent scarp. Note also the erosion around the life guard station. This doesn’t look like a case of summer/winter beach profiles. For one thing, this photo was taken in early October, the end of the summer season. Persistent erosion such as this indicates a lack of either sediment (most likely cause) or waves and wind to return it to the beach face. This is what is supposed to (theoretically) happen in the summer.
Figure 4. Close-up of typical sediment at Kailua beach (5x magnification). This is a poorly mixed assortment of calcite from the offshore coral reef and related organisms. If the beach is sediment starved, it could be a lack of growth or erosion from the reef. I can’t tell but (again, if I had to guess), I bet the reef is stressed and not as productive as it once was.

There is nothing surprising about what we saw at Kailua beach. Beach erosion is ubiquitous around the world; for example, it takes years for scarps like that seen in Fig. 3 to recover from a tropical storm in the Atlantic Basin. Recent studies suggest that in general, sandy beaches are being eroded; the proximate cause is a lack of sediment or increased wave energy, but the root cause most-often blamed is climate change and sea level rise.

We need to stop blaming the climate and reconsider all of the dams and diversions we’ve constructed on rivers that feed the world’s beaches, and ill-considered engineering projects completed in coastal zones.

The beach isn’t a play pool…

Remnants of Ko’olau Caldera

Figure 1. View looking NW along what’s left of the Ko’olau caldera, after it exploded and was blown into the Pacific Ocean about 1.7 million years ago (my). The cliffs in the distance indicate how elongate it was (see Fig. 2).
Figure 2. Today’s post is. taken from the lower-left part of panel C, northwest of the Koko Crater. The red line is the general outline of the steep NE side of the Ko’olau Range (see Fig. 1). The original size of the Ko’lolau volcano is indicated by the green line in panel A, and the approximate area of the debris avalanche is shown by the dashed line (Nu’uanu Debris Avalanche).

Today, we can see some of these rocks up close. Also, these are not as weathered as we saw previously. The lava flows are well preserved at Nu’uanu Pali park (Fig. 4).

Figure 3. Poster giving some topographic and geological information for the Nu’uanu Pali area.
Figure 4. Approximately 100 feet of lava flows are exposed at the park. This photo shows the excellent preservation of these rocks and the irregular layering typical of basalt flows.
Figure 5. Detail of lava flow. Note the irregular structure reminiscent of pillow lava, vesicles in lower part, and curved and pinched bed with a brighter color in the middle of the photo. (Image is about 12 feet high.)
Figure 6. Road cut in cliff. The road was the original path used until construction of the modern highway. Note the unaltered dark basalt exposed.
Figure 7. Ulupau Crater was created during the rejuvenation phase of Ko’olau volcano, and is contemporaneous with Koko and Diamondhead tuff cones.

This post visited the remains of the Ko’olau volcano, which exploded/collapsed/disappeared into the Pacific Ocean about 1.7 my, leaving a sharp mountain range that was the edge of the central caldera. A much younger tuff cone erupted about 100 thousand-years ago as part of the rejuvenation of the magma chamber. Its last gasp as it were.

Ko’olau Volcanic Rocks

This is going to be a relatively short post about some of the older volcanic rocks on Oahu, basalts from the Ko’olau volcano, with an age between 2.8 and 1.7 million years ago (my). I introduced these in a previous post. This post examines some of these rocks up close.

Figure 1. Photo of Waimea Bay, showing the general topography of Oahu’s north shore. The coastline is broken by many small promontories and peninsulas, small streams, bays filled with rocky islands and floored by wave-cut platforms.
Figure 2. Pillow lava forming the coast and nearshore bottom. This structure is associated with submarine eruption, usually thought to occur at great depth, but it can also occur in shallow water.
Figure 3. Photo looking eastward, showing basalt and sandy beach. The volcanics form a ledge that is emergent to the west but buried beneath sandy, calcareous sediment to the east. Note the small, rocky island in the upper center of the image. The flat surface suggests that this is a wave platform, cut when sea level was about 10 feet higher.
Figure 4. Sediment at 5x magnification. The vast majority of the grains are broken shells from an offshore reef. Very few particles originate from the volcanics, although they do supply some clay-sized alteration products, giving the water and beach a slightly tan appearance.

Low rainfall along this coast has brought erosion in the mountains to a standstill. None of the streams that penetrate the Ko’olau Range reach the ocean during the summer and fall. There probably are episodic floods that deliver find-grained sediment from the highly weathered volcanic rocks.

Figure 5. Abraded pillow structures at the water line, showing the onion skin texture that is suggestive of shallow-water eruption.
Figure 6. Rocky promontory that is a relict wave-cut platform. Note the erosion at current sea level, dropping the rocks into the sea one block at a time.
Figure 7. Top of rocky promontory seen in Fig. 6, showing the highly altered basalt, forming clay minerals in situ. Basalt weathers to produce a consistent array of minerals, including kaolinite.
Figure 8. Image of resistant dike in the weathered relict wave platform.

This post has shown some of the characteristics of the Ko’olau Volcano and its associated basalts. The original lava has been chemically altered to produce clay minerals, which are easily transported. There are no mud flats because of the slow weathering and delivery to the coast, although soils can be very rich in the inland parts of Oahu. A previous high-stand of sea level created a bench that is found throughout the island, as noted in a previous post.

We’ll look at what the Ko’olau Volcano looks like today in a later post.

Volcanism at Koko Crater

Figure 1. The study area at the SE tip of Oahu (beneath arrow in inset map) is indicated by the white ellipse. Note the sharp topographic scarp indicated by red shading in the inset; this is the SW rim of the Ko’olau volcano, which split in two. Hanauma Bay and Koko Crater are dormant tuff cones, where ash and some lava erupted as part of the larger Ko’olau volcano. They were active sometime between ~200 and 50 Ky (thousand years).

This was a great opportunity to examine geologically recent (less than 1 my old), large-scale volcanism up close. I’ve seen small cinder cones in northern Arizona that are part of the San Francisco volcanic field, but they were small enough to be surmounted in a couple of minutes; other volcanoes I have seen were either huge (entire mountains) or so old that they were unrecognizable. Koko Crater is a perfect fit. It is 1200 feet high and young enough to preserve a range of volcaniclastic textures, yet easily accessible from a safe parking area.

Figure 2. Summary of volcanism on Oahu. Koko Crater is indicated by the black circle in panel C; it was created during the rejuvenation stage of the Ko’olau volcano (outlined in green in plate A). We visited the central caldera of this massive complex in a previous post, and we’ll see it again later.
Figure 3. Photo looking north at the base of Koko crater (see Fig. 1). The ash, tephra, and some lava met the Pacific Ocean here and formed a lunar landscape consisting of wave cut platforms (flat areas in mid-image), boulders of every size, and sediment ranging from sand to clay.
Figure 4. View looking NW, showing the steep slope of the cone, capped with what looks like a lava flow. Note the tumultuous appearance of the exposed rock in the middle of the photo. Ash and lava (all volcaniclastic sediments) fill low spots, like streams and valleys, before forming flat layers. This results in a chaotic assemblage of intercalated ash, large chunks of lava, and lava flows.
Figure 5. This photo shows horizontally layered ash beds to the left, truncated by what may have been a valley, that filled with younger ash which weathered much faster (the reddish appearance is caused by the oxidation of iron). Another factor is the cumulative effect of water running down any channel, enlarging it and focusing its powerful chemical impacts. Every fracture or weak point in the tephra layers would have been taken advantage of by water, the best acid nature ever invented.

Field Relations

Figures 3 through 5 show us the lay of the land, but we have to get a bigger picture to understand what we’re looking at; and that is difficult in such an immense landscape. I will try.

Figure 6. View towards the west at the base of the Koko Crater (see Fig. 1 for location). The capping lava? flow seen in Fig. 4 is visible in the upper left; note the steep slop upon which it erupted. The scale of this image is shown by the people circled in blue. The lines, labeled “1”, “2”, and “3” are representative of structural trends (i.e. strike and dip) seen in the volcanic layers comprising the tuff cone.

Structural Trend 1 appears to be close to its original orientation because it conforms to the steep sides of the crater. This alignment is seen on other “ribs” projecting from the crater’s rim. Ash at a temperature of 800 to 1500 F would have stuck wherever it landed and formed horizontally uniform laminae (less than 1/16 inch) and thin beds. This was primary depositional orientation of the ash layers. Of course, ash will fall into steep valleys and fill low spots, just like snow, which is also sticky.

Trend 2 cuts across Structure 1 at more than 30 degrees, but it appears to be local .

Trend 3 dips about 20 degrees towards the crater but its primary orientation is towards the canyon bisected by other ridges seen in Fig. 6. This is a post-eruption feature, as is Trend 2, both of them created as thick layers of ash consolidated, settling more in preexisting valleys than hills. My interpretation is that the valley cutting across (left to right) in Fig. 6 was the edge of the crater when Koko was active. Tephra and basalt collected there to form a plateau (indicated by white ellipse in Fig. 1), which was subsequently eroded, before a few tendrils of basalt flowed out of the crater (e.g. upper parts of Figs. 4 and 6), forming resistant ridges. This post will focus on the stratigraphy of one of these ridges.

Figure 7. Speculative structural analysis of a ridge ~200 yards west of the study area (purple ellipse in Fig. 6). The steeply dipping unit (indicated by “3?”) assumed to be the same as Structural Trend 3 (Fig. 6). If this bedding plane was originally similar to Trend 3 (dotted line), then there is a fault (indicated by the black, dashed line) with unknown slip. The cyan line delineates the estimated base of an hypothesized stratigraphic unit (labeled “4…5?”). Note that the structural trends labeled in Fig. 6 are not identical with their use in this figure, because specific volcanic beds are identified here. In other words, beds “3” and “3?” are assumed to be the same, which suggests that the seaward extension of this ridge dropped almost 100 feet seaward and rotated as much as 45 degrees. Layer “4…5?” is younger because it appears to cover Layers “3” and “3?” equally while not being disturbed.

Figure 7 reveals at least two unconformities; the fault that dropped the seaward block down a few feet as it rotated, and what appears to be an erosional surface (the cyan line delineating layer “4…5?”). The complex pattern of episodic eruption, erosion, and deformation on the eastern margin of Koko crater is not unusual for a monogenetic volcanic cone, which remains active for a short while, until the magma chamber decompresses (so to speak).

Volcaniclastic Examples

It is time to speak to the rocks, hear their side of the story, and listen to them if we can. All wisdom comes from the earth, and thus it behooves us to hear with our eyes and our imagination (rocks don’t talk), the story that has been written in … stone.

Figure 8. A volcanic bomb (~1 foot across) depressing preexisting layers of ash. Considering the weight of the class, the ash must have been partly consolidated when the bomb struck. Note that the ash layers may represent multiple eruptions over a period of days to years.
Figure 9. Resistant ash layer forming the ridge seen in Fig. 6, containing irregular pieces of vesicular and massive basalt contained in a matrix of ash, which weathers to a rust color. These clasts are actually cemented to the ash, similar to the relationship seen in Fig. 8. Note the layering of the ash at the lower-right part of the image.
Figure 10. Irregular basalt clast in an ash matrix. There are two interesting features of this photo: (1) the bomb is highly irregular, as if it was blown out of the vent, ripped from older volcanics; and (2) the circular line around the object is the projection in map view (horizontal) of the depression seen in cross-section in Fig. 8. When these objects hit sticky ash, they form hemispherical depressions that look like bowls.
Figure 11. Vesicular basalt bomb in ash matrix. Note the rounded form, which suggests transport some distance by water. The bomb next to it is not vesicular and it is less rounded, even though exposed to the same weathering forces since exposure in the ash flow. (Believe it or not, these two boulders are absolutely stuck to the muddy looking rock below them.)
Figure 12. Large boulder (~3 feet across) of volcanic diamictite containing gravel and small boulder-sized particles. Even the ones that look like they’re lying on the boulder are stuck hard (they have been exposed to ocean waves for millennia).

Sedimentary Examples

Figures 8 through 12 unambiguously show that volcanic tephra of every size and shape were thrown into the air and landed in hot and sticky ash, where these bombs became clasts, forming volcaniclastic rocks. Now we will look at some examples of rocks that are just as hard, but that reveal sediment transport by wave action.

Figure 13. Photo of cross-bedded layer from within the purple ellipse in Fig. 6. Note the coarse layer tilted to the right just above the water bottle, which also indicates a convex-down depression in the subjacent layer, suggesting erosion by running water. The cross-bedding is unidirectional, which suggests reworking of tephra and ash by waves, like in Fig. 3. The overlying layer shows no sign of disturbance, the ash laminae being horizontal.
Figure 14. In-place volcaniclastic rock at current sea level, showing overlapping coarse and fine layers similar to what is seen in the swash zone of any beach with such different sediments available. The large voids are probably caused by dissolution of carbonate minerals that formed during shallow burial (less than a hundred of feet of lava and ash), rather than vesicular lava. The image is approximately 6 feet across.
Figure 15. Layer of unidirectional cross-bedded layer, approximately 18 inches thick. Gravel collected on the slip face of sand waves near the beach, scattered among sand-sized particles. The wave direction would have been to the right (approximately NE), very similar to today’s environment.

Summary

I’ve presented a lot of data on volcaniclastic rocks at Koko Crater. Before I summarize I’d like to present one last annotated photo, which shows the stratigraphy as well as I can tell from my limited access to good cross-sections (aka road cuts).

Figure 16. View looking downdip, showing the approximate stratigraphy of the rocks exposed within the purple ellipse in Fig. 6. Ash collected, without being disturbed, in the oldest part of the section, and was replaced by basalt with features that suggest eruption beneath the ocean, usually pretty deep, but shallow-water occurrence has been observed. A sequence of ash then collected quietly (Ash Laminae) before being replaced by grains of lava and other rock fragments (Ash/Sand/Gravel), culminating in Cross-bedded Ash and gravel (see Fig. 13). I would estimate the thickness in this figure to be about 30 feet, which would have taken thousands of years to collect, because of the implied change in water depth. Of course, the “Pillow” structure may be misinterpreted, which would change the estimate considerably.

I would like to add a few points to Fig. 16. First, older sediments were seen at the current beach (Figs. 14 and 15) which clearly indicate a surf zone environment. These rocks may be from as much as 100 feet down-section (older), which is consistent with changing relative sea level caused by global sea level fluctuations during the last 100 Ky, and uplift and settling of the Koko crater itself, as magma pushed up and relaxed.

Rocks from~50 feet further up section (younger) demonstrate that the ash was deposited at or slightly above sea level (Figs. 8-12). The total section has a thickness of approximately 100 feet (guesstimate not based on careful measurements, but eyeballed), which must have been deposited within a few thousand years because of the young age of Koko Crater (~50 Ky).

Finally, Koko crater is a tuff cone, which means that it mostly created hot ash as a magma chamber locally depressurized. This kind of eruption doesn’t reoccur once pressure has dropped below the strength of the overlying rocks. Tuff cones are part of larger volcanoes, in this case the Ko’olau volcano, one of three that created Oahu and dominated volcanism for the last four-million years (Fig. 2).

One last word–I have never seen photographs like those I took for this post, not in a text book, Wikipedia, or anywhere else. I don’t know why that is, maybe academic geologists are too busy teaching geology to stop and smell the roses…

A Quick Visit to the Ko’olau Volcanic System

Figure 1. Photo looking NW from the Byodo-In Temple (see Fig. 2 for location). The jagged peaks are the remnants of the Ko’olau volcano, which constructed the eastern half of Oahu between about 2.8 to 1.7 my (million years ago). More than half of the caldera collapsed into the Pacific Ocean, dissecting a series of vents (small volcanos) and leading to extreme erosion.
Figure 2. (A) Map of Pacific Ocean including Oahu (shaded in green), showing three Pleistocene calderas: Ka’ena to the NW (outlined in blue, age older than 4 my); Wai’anae outlined in black (age ~4-2 my); Ko’olau shown in green (age ~2.8-1.7 my). Panel (B) is a schematic cross-section from NW to SE, showing the structural relationship between the three volcanic systems (Ko’olau is mostly hidden beneath panel C). Panel (C) is a map of the SE part of Oahu; the subject of today’s post (Byodo-In Temple in the Ko’olau Caldera) is circled and marked with a balloon, and tomorrow’s (Koko Crater) is circled. The solid red line outlines the SW flank of the original Ko’okau caldera. The NE part of the island slid into the Pacific to form the Nu’uamu Debris Avalanche indicated in panel (A). The dashed lines show the approximate orientation of ridges that define the original slope of the Ko’olau caldera.

Let’s look at some of the rocks from the Ko’olau caldera, originally erupted in shallow water between 2.8 and 1.7 million-years ago, from a fissure that was below, or not far above, sea level.

Figure 3. Panel A shows the steep cliffs that define the NE side of the Ko’olau range, which approximately aligns with the SW margins of a series of volcanoes that comprised the caldera. These rocks are highly altered. (B) Close-up of boulder used as landscaping on the temple grounds; vesicular texture is labeled “V” and fine-grained areas are labeled “F” (image is about three feet across). These areas seem to have some kind of stratigraphic relationship, rather than the vesicular lava being clasts of random shapes contained in an ash matrix.

These rocks have been highly altered because of interaction with water when erupted, but the structures seen in Figs. 3B and 3C were preserved. Magma chambers contain a lot of gas, e.g., carbon dioxide and hydrogen sulfide, which expands when the magma moves towards the surface where pressure is lower. Small cavities (vesicles) remain when the gases escape (see Fig. 3C). The volcanic gases are not uniformly mixed within the magma however, so some of the erupted material will have vesicles whereas some will not (see Fig. 3B). Nevertheless, well-defined contacts between gas-rich (vesicular) and gas-poor (solid) magma when it flows onto the surface cannot be easily explained. The “V” and “F” layers in Fig. 3B were not necessarily flowing, although it is likely that they were moving if they were erupted onto a sloping surface. The contacts (white lines in Fig. 3B) could simply be the result of very hot lava flowing out from a common source, the differences that existed within the magma chamber mirroring fine-scale variations of the its chemical characteristics.

The Ko’olau volcanic system would have been venting along fractures, thus creating many, often overlapping, volcanos on the flanks of the main “super” volcano. Such an immense volcanic system couldn’t remain stable for long (i.e. millions of years) and it collapsed into a huge debris field (refer to Fig. 2A), leaving the fragile remnants of its glorious past naked to face the extreme weather that came from the arctic, creating the jagged peaks of the Ko’olau range (Figs. 1 and 3A).

The collapse of the Ko’olau caldera occurred sometime between 1.7 my ago and when volcanism began at several fissures to the southeast about 500 thousand-years ago, adding more land to the island of Oahu.

We’ll see what that was like in my next post…