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O’ahu Geological History

Wai’anae Shield

O’ahu began as a small, active volcano about 4 million years ago.  The young island grew rapidly for about a million years as the lava flows built the huge Wai’anae shield volcano. The eroded remains of this volcano are now the Wai’anae mountain range, which includes the island’s highest point, Mt. Ka’ala, at 4,003 feet.

Like many other Hawaiian volcanoes, the Wai’anae shield volcano had a large caldera at the summit, estimated to have been 4.5 miles across.  Most of the lava coming from this volcano consisted of the thin, pāhoehoe lava, the oldest of which is between 3 and 4 million years old.

As the Wai’anae volcano completed its shield, eruptions became concentrated at the summit.  However, as the mountain was still growing, its western slope began to slowly slump into the Hawaiian Deep.  Finally, most of the shield’s western flank actually slid into the ocean, spreading volcanic debris as much as fifty miles from the island!  The mountain’s northern flank also became unstable, and when it collapsed, causing the “Ka’ena Slide,” debris traveled almost 70 miles across the deep ocean floor, forming an underwater escarpment parallel to what is now the North Shore.  Just a History major, I really have trouble imagining a landslide that goes for 70 miles, especially underwater.  But this really seems to be the case; one can only imagine the volume of material involved and the speed at which it must have been traveling, to go that far underwater.  Imagine the tsunami(s) it must have generated!

Ko’olau

But we’re only half done talking about BIG volcanoes on O’ahu.  The other huge volcano was to the east, on the eastern edge of the island at what is now known as Ko’olau.  The Ko’olau volcano is considerably younger than its Wai’anae sister, having broken through sea level “only” 2.7 million years ago.  The first stage of the mountain’s development was the rapid eruption of pāhoehoe basalt flows.  the result was a caldera 8 miles long and 4 miles wide.  Imagine that: a volcanic caldera with an area of some 32 square miles.  That staggers my imagination ubt it certainly explains how such a large volume of magma could emerge from the earth.

It is possible the the Wai’anae and Ko’olau volcanoes may actually have been two separate islands at one time.  The broad plain between them, called the Leilehua Plateau, consists largely of alluvial deposits shed from the Wai’anae volcano and lava flows from the Ko’olau mountain.

While the Leilehua Plateau helped brace its western flank, the Ko’olau volcano’s eastern flank sloped rather steeply into the ocean.  This steep flank grew continued to swell as more and more magma rose into the mountain, then deflated as it erupted.  Those continued movements of the mountain may well have contributed to causing the Nu’uanu Slide, one of the largest landslides ever on earth.

Just like western flank of the Wai’anane volcano, the eastern flank of the Ko’olau volcano carried away one day, causing a slide 20 miles wide and one hundred and twenty miles long.  This is absolutely staggering in scope.  I do not understand how rubble can “flow” for 120 miles under water, but it did.  One fo the blocks of rubble is a mile thick and 18 miles long.  That’s big rubble!  Scientists believe this great slide occurred a little over 1.8 million years ago.

Shortly after that, the Ko’olau volcano entered its last period of activity.  For the next 800,000 years eroded and sank thousands of feet into the ocean as it drifted to the northwest, away from the hot spot.  Then, about 850,000 years ago another period of volcanism occurred near the Mōkapu peninsula, on the northern edge of the Ko’olau caldera. Since then there have been approximately 40 eruptions.  The largest group of the “Honolulu volcanoes” tends south from the Mōkapu peninsula and includes Punchbowl, Pu’u Kahea (Sugarloaf), Pu’u ‘ ōhia (Tantalus Peak), Pu’u ‘Ualaka’a (Roundtop) and perhaps the most famous volcano in Hawai’i, Lē’ahi, known by some as “Diamond Head.”

A lesser cluster of vents rises from the coastal plain between Honolulu and Pearl Harbor; it includes the Salt Lake Crater, Makalapa, and Alaiamanu cones.

And lastly, an even younger set of pyroclastic cones emerged between 32,000 – 6,000 years ago, extending to the northwest, from Makapu’u Point and Koko Head.

Take a look at the accompanying picture; it will help you visualize the massiveness of the two volcanoes that built this beautiful island.  Remember now, the Wai’anae and Ko’olau mountains shown in the image are just the remaining sections of the walls that formed the two immense calderas mentioned above. Incredible!  Like many (perhaps most?) people, when I got to O’ahu I pretty much assumed that Diamond Head was the source of the lava that made the island (hey, it made sense: it was the only really obvious volcano on the island, seemed pretty big and therefore capable of producing the lava flows required to build the island, and there was no other obvious choice.  Boy, was I wrong!.  In fact, the (remains) of the two mountains that make up the island were so huge as to be “hiding in plain sight.”  I just had no idea of the scale of the two original mountains!




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