October 16, 2022  

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….

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