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.
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.
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 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).
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.
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.
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).
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…
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