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.
Timothy R. Keen 
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