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October 12, 2022  

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 2. Map showing location of this post on Koko Head. The bay just above “Koko” is shown in Fig. 1. Koko Crater is just to the west.
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…

October 12, 2022  

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…

October 11, 2022  

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.

October 11, 2022  

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.

October 11, 2022  

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…

October 9, 2022  

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…

October 8, 2022  

Back to the Beach: Sand Transport at Waikiki

Figure 1. View looking along Waikiki Beach toward Diamond Head. Everything looks wonderful from this perspective, a lovely tropical beach with gentle waves lapping at poorly sorted sand with a steep beach face. Things are not as cozy in paradise as they appear.

I have discussed beach erosion and topography in several previous posts (e.g. Australia’s east coast and The North Sea); and there is no reason paradise should be exempt from the ravages of wind and waves. Those two powerful tools of nature are on the minds of everyone who lives in Honolulu, as revealed through desperate efforts to keep these beaches from eroding.

Where is Waikiki Beach?

Figure 2. Waikiki Beach is on the island of Oahu, labeled as “Honolulu” in this image. It faces south and is subjected to southeasterly winds between April and September. Note that it is 2500 miles from Los Angeles, at a latitude of 21 degrees north. Tropical storms are rare and there are no cold fronts or other meteorological anomalies, but the wind does blow from the ESE between April and September.

What does the beach look like?

Figure 3. View looking south on Waikiki beach, showing stone retaining wall (aka groin) Note the sand collecting against the eastern face (left side of photo), flowing onto the sidewalk. This sand has been transported by wind, which blows for months at a time from the east .

The wind blows from a southeasterly direction for about six months of every year, generating moderate waves that strike Waikiki obliquely from the SE. The result is along shore drift of sand. One quickie solution to this problem is to construct resistant stone or concrete barriers perpendicular to the beach. They are called groins.

Figure 4. View looking east toward Diamond Head, showing a groin jutting out into the nearshore. Note the deep erosion on the camera side of the obstruction. This is a typical response of a nearshore wave regime to a groin. Also note the steep beach face seen in the nearer part of the image. This is not an equilibrium condition between waves and sediment type, but rather an artificial situation caused by the sidewalk (hardened back shore) not allowing the beach to reach a more appropriate angle (see Fig. 1).

So, what happens on the upflow side of a groin, like that seen in Fig. 4? We can see the erosion on the down flow side (remember the waves are coming towards the camera), but what happens when moderate waves hit a solid wall?

Figure 5. View looking seaward at the groin shown in the distance of Fig. 4. Note the rock that forms the backbone of the structure, seen to the upper right of the image. A wall similar to that seen in Fig. 3 is exposed, as is the upwind edge of a threatened walkway.

The damage moderate waves can do over years, even decades, has been demonstrated, and Figs. 4 and 5 support those results. The wind blows steadily from the south-southeast between April and October on Waikiki beach. What is the impact?

Figure 6. This rather boring image shows sand accumulating on the back shore of Waikiki, blown by the stead ESE wind and pushed back by brooms and gas-powered blowers. This process is obvious at the western end of Waikiki beach.

The westward transport of sand by the SSE wind is obvious at any obstacle to its unimpeded flow, such as sidewalks protected by low walls (Fig. 3), where sand accumulates on the windward side and spills over.

The state of Hawaii is aware of the problems identified in this post, and their solution is beach replenishment. That would explain why the Waikiki beach I visited in 1991 is no longer white, but now as dirty as the beaches of Mississippi Sound, where beach replenishment from offshore sources has been the standard for decades.

Beaches are dynamic zones, where wind, waves, land, and biology interact in a never-ending dance, which is the primary reason (in my opinion) that people are drawn to the seashore. I expected this when I lived on the Gulf coast, but I am surprised to see reality showing its ugly face in paradise…

September 25, 2022  

Claude Moore Park: How Geology Won the Civil War

That is a grandiose title and I admit it’s only part of the story. Nevertheless, the movement of General Lee’s Army of Northern Virginia was tracked from a lookout tower constructed on the ridge that bounds the northern margin of Claude Moore Park (Fig. 1), from where his troops were observed to be moving north, towards Pennsylvania. Because of this tactical intelligence, Union forces converged on Gettysburg and were able to repel the Confederate invasion. Most historians refer to Gettysburg as the high tide of the Confederacy.

Figure 1. Map of northern Virginia (left panel) and detail map (right panel) of Claude Moore Park. The labeled locations are where later photos were taken. The ridge is where the observation tower was constructed that tracked the Confederate troop movements prior to the Battle of Gettysburg.

The study area is located near the Potomac River, where it drops from the Appalachian Mountains to Chesapeake Bay (Fig. 1). The ancient rocks that underly this area have been deeply eroded by streams, producing a labyrinth of hills and valleys. This post begins at the Uplands (Fig. 1) and shows the dramatic change in both ancient and modern sedimentation that results from such a landscape.

Figure 2. (A) steep southern slope of the ridge that defines the park and the local drainage system. (B) The sediment here is Tertiary in age, and it is a typical gravel stream deposit, with rounded cobbles (up to six inches in diameter) in a matrix of sand and silt.

The ridge probably follows the erosion-resistant channel of a river, with coarser sediment forming uplands because it is resistant to erosion. It is like a topographic inversion; what was once low (the bed of a gravel river) is now high because cobbles are difficult to move by water that is no longer focused in a river channel.

Figure 3. (A) Typical lowland view of young trees and muddy areas filled with grass. (B) The sediment here is silt and clay. See Fig. 1 for location.

The slope reduces southward and drains into a pond (Fig. 1), which is simply the lowest topography and thus wet year round. However, the entire area labeled as “Wetland” in Fig. 1 is probably periodically inundated during wet times.

Figure 4. (A) Incised stream in the wetland, indicating continuing uplift during the Holocene. Note the thick underbrush in this intermittently submerged area. (B) Fine-grained sediment is dominant but small boulders (this one is about 12 inches in length) are distributed, left over from a previous high-flow environment during the Tertiary.

This post has shown how sedimentation can change over short spatial distances, and topography can become inverted. The wetland area was once the flood plain of a gravel river (e.g. Fig. 2), but the silt and clay that collects on a flood plain is easily eroded and weathered when it is exposed to the elements, creating local wetlands surrounded by ancient river beds.

This interpretation is speculative because bed rock is never far beneath the surface in northern Virginia. We saw no outcrops on the ridge, however, which doesn’t mean that there is no resistant rock beneath the gravel-covered slopes, but it is consistent (geologists love that word) with my interpretation. Further support for the ancient stream-bed hypothesis comes from the orientation of the ridge, which doesn’t follow the regional structural trend of NE-SW for basement rocks. Note that the Appalachian Mountains define this trend (see Fig. 1), as have all of the Paleozoic and Mesozoic rocks we have encountered in the area. The uplands (see Fig. 1) ridge is oriented east-to-west, which goes against the grain, geologically speaking.

Thanks to an ancient gravel stream, the Union army was alerted to the movements of the Confederate invasion and stopped them at Gettysburg. Geology saved the day again…

August 26, 2022  

The Forest and the Trees: Glacial Topography of the Central German Plain

Figure 1. Typical scene driving between Usedom and Goseck (See Fig. 2 for approximate location). The hills are glacial moraines as we saw at Usedom in the last post.

The wind blows pretty steady over the North German Plain and Central Uplands, so there are wind turbines everywhere, scattered among the hay fields (Fig. 1).

Figure 2. Map of central Germany, showing stops discussed in this and the next post. The red line is the (very) approximate path taken for this post.

The topography of the Central Germany Plain is very similar to Nebraska, Kansas, and the Dakotas, because they were all created by the advance and retreat of multiple glaciers during the Pleistocene Ice Age. As we saw in the last post, advancing ice sheets as thick as a mile push rock and soil in front of them, before melting back for a few millennia, leaving piles of soil and boulders behind. They are like gigantic bull dozers.

Figure 3. Gentle slope north of Berlin. This is a ground moraine, pulverized rock forming a thin layer of rich soil just waiting for the plows of modern farmers.
Figure 4. The hills in the background are either terminal moraines or eskers (another German word). See text for explanation.

As ice slides over the landscape, scraping off whatever gets in its way, the ice at its base can melt from the friction, creating streams that transport already ground-up rocks. The usual rules of sedimentology apply to these ice-encased streams. They can deposit their sediment load as eskers, which identify these sub-glacier streams, or as piles of scraped-off soil (terminal moraines). I don’t know which I’m looking at in Fig. 4, but this image gives a good impression of their impact on the landscape. Keep in mind that the ice was approximately ONE-MILE thick above the landscape shown in Fig. 4…

Figure 5. Entering the Saale River valley near Goseck.

As I alluded to in the previous post, geology isn’t a sequence of static processes; there is always more than one cause of what we see today, and none of them are stationary. Thus, the landscape produced by the continental glaciers that advanced over the Central German Plain during the Pleistocene were constantly in competition with alpine glaciers created in the valleys and peaks of the Alps. The huge ice sheets had the power to overcome any obstacle…but they couldn’t surmount the steep slopes of the Alps.

The glaciers that originated in the Alps waited until the last retreat of the great ice sheets that originated in Scandinavia, before they could make their play in the Holocene. Vast quantities of easily weathered feldspar were washed down their steep slopes into a panoply of rivers, which cut through the moraines left behind during the retreat of the continental ice sheets, creating broad river valleys like that of the Saale River (Fig. 5). Germany’s central plain and uplands were cut to ribbons by these growing streams, resulting in one of the most water-navigable regions in the world.

You always have to watch your back…

August 26, 2022  

An Erratic Path: Glacial Geology in Usedom, Germany

This post finds me on the Baltic Sea, although I never actually saw it first hand. But this report isn’t about coastal geology; instead, I will be talking about an unusual feature of glacial terrains.

Figure 1. View of the lagoon that separates the island of Usedom from the mainland. An elongate lagoon is surrounded by marsh grass and is connected to the Baltic by narrow channels.
Figure 2. Map of northern Germany. Usedom is the island indicated by the pin. Figure 1 was taken on the west side, where the island is accessed via a short drawbridge.

I haven’t explicitly described glacial terrains in previous posts, and this is not going to be a summary. As always, I’m only going to discuss what I saw with my own eyes. The flat, poorly drained topography of glacial areas (Fig. 1) is often interrupted by linear mounds of loose gravel, sand and silt. These features are called moraines and they are ever-present in northern Germany, especially in Usedom (Fig. 3).

Figure 3. A glacial moraine is seen in this image, in the background, expressed as a low hill, but it is entirely composed of unconsolidated soil.

The primary glacial feature I encountered on this trip is the titular Erratic–large, rounded boulders scattered around a featureless landscape (Fig. 4).

Figure 4. Several glacial erratics have been collected and used as curb markers in this rest stop in Usedom. These boulders have been transported hundreds of miles and have no local source, thus the name erratic.

Let’s look at a couple of examples and see what they tell us about their source.

Figure 5. Photos of a boulder from Fig. 4. The left image shows the appearance of the stone in the field, and the right photo is a close-up (~4X magnification). See text for discussion.

The first erratic boulder I found (Fig. 5) contains no more than 5% quartz (left photo caption), and is dominated by K-feldspar, which is unusual. The magma from which this rock formed (deep beneath the surface where it cooled for millions of years) didn’t contain very much water, which is indicated by the small amount of quartz. The chemistry of the magma is quasi-frozen in the minerals, the second-most-abundant of which is Na Feldspar. The feldspars form a continuum that depends on the relative abundance of potassium (K), sodium (Na), and calcium (Ca); K and Na both form lighter-colored minerals whereas Ca forms dark feldspar minerals. Based on the mineralogical composition of this rock (inset in left image of Fig. 5), this would be classified as a syenite (middle left side of Fig. 6).

Figure 6. Classification of granites based on Na-K feldspar (A point), Ca feldspar (P corner), and quartz (labelled Q). We don’t need to worry about the bottom half of the plot because those are very rare minerals.

Syenites are formed in thick, continental crust. An example today would be the Alps (far beneath them) or the Himalayas, where subduction of denser oceanic crust is not occurring. In other words, the rock shown in Fig. 5 was created deep beneath the surface (~30 miles) when continents collided.

Figure 7. Photos of another erratic from the same location as Fig. 4. Note that this rock (left photo) has a less-reddish hue, an impression supported by the mineralogy (right image). K-feldspar comprises ~15% of the minerals, as opposed to 60% in Fig. 5. The increased quartz content (inset in right image) would classify this as a quartz-syenite; more water was contained in the original magma, but not enough to form significant quantities of quartz.

The boulders seen in Figs. 5 and 7 could have come from the same magma chamber because, as you would expect, there would be variations in local chemistry in a magma chamber tens of miles in diameter, and slow rates of convection wouldn’t mix the magma to a uniform consistency, even over millions of years. Magma, even when heated to 2000 F and buried tens of miles beneath the surface, is still thicker than molasses; it doesn’t mix well.

Figure 8. Photos of another erratic found along the road (Fig. 4), revealing a very different texture from Figs. 5 and 7. This sample contains large (~1/2 inch) irregular clumps of what looks like K-feldspar (note the reddish hue). The overall mineralogical composition (inset of left image) suggests that this rock came from the same magma chamber as the quartz-syenite in Fig. 7.

Phenocrysts like those seen in Fig. 8 are created in intrusive igneous rocks when they go through a multistep cooling process; for example, magma near the edge of the magma chamber loses heat to the surrounding rock and forms crystals like those seen in Fig. 8. These phenocrysts are then captured by the still-molten components of the magma and dragged along for probably hundreds-of-thousands of years (at a very slow speed, like inches per thousand years).

When the magma finally cools enough to become solid rock, it is uplifted as overlying rocks (of all kinds) are eroded by wind and water, not to mention ice. They are finally exposed in great mountain ranges like the Himalayas, where the rock breaks into smaller-and-smaller pieces along joints. When these pieces become small enough to be transported at the base of glaciers (you’ve heard the phrase glacially slow), they are dragged along, scraping over more rocks, sand, and gravel, which leaves evidence of their precarious journey (Fig. 9).

Figure 9. Photos of a glacial erratic from Usedom (not Fig. 4), showing striations that indicate it was dragged across a (rock) hard substrate.
Figure 10. A glacial erratic similar to that shown in Fig.9, but with striations (formed by movement at the base of glaciers) that flow into a joint (circled), suggesting that it was sand and not bedrock, over which this boulder traveled during the most arduous part of its journey. (Sand is mostly quartz, which is much harder than steel or window glass.)

This post has been erratic, starting out looking at a glacial terrain (Figs. 1-4), then taking a detour into igneous petrology, the chemistry of magmas, and mineralogy, with a little plate tectonics thrown in. That’s how geology is; everything is an ongoing process that never quite reaches equilibrium (e.g. the phenocrysts in Fig. 8), and the journey is unending.

I didn’t investigate the origin of the syenite boulders examined in this post, but (if memory serves) they match the mineralogy of intrusive rocks from Sweden, which is a long way from Usedom.

Stockholm is about 500 miles north of Usedom…

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