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…

Trackbacks / Pingbacks

  1. Volcanism at Koko Crater | Timothy R. Keen - October 11, 2022
  2. Ko’olau Volcanic Rocks | Timothy R. Keen - October 11, 2022

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