North Shore NewsChapter one; The Geology of O`ahu The Waialua Library presented the first of a series of talks titled “The Saga of the North Shore” on March 10. This series of talks by Captain Richard W. Rogers will continue on the second Saturday of each month, beginning at 11:00. 60 people filled the library to hear the first talk. A summary follows;
There is a “Hotspot” of molten lava that rises from the earth’s core, some 1800 miles beneath the Island of Hawai`i. The “Pacific Plate”, a large slab of the Earth’s crust, has been riding over that hotspot, at a rate of 3.4 inches pr. year, for 90-million years. This has produced a series of islands that have eroded and sunk to atolls and seamounts, going all the way to the Aleutian Islands.
3.7 million Years ago the Waianae Mountain broke the surface, followed about 10-million years later by the Ko`olau Mountain. They both grew to over 9,000’ tall with masses of each eroding away and sliding into the ocean. The major growth of O`ahu stopped about 1.7 Million years ago. Things were seismically quite for about 1-million years. Then another, deeper vent arose from the core. In time, 36 volcanoes of the “Honolulu Series” erupted on O`ahu. These included Diamond Head, Punchbowl, Salt Lake and Koko Head. Is it over? Possibly not. Tantalus has hot springs and there was an under-water eruption between O`ahu and Kaua`i on May 26, 1956.
As the islands were developing, sea-level rose and fell as the Earth’s glaciers froze and melted. During the high stands of the sea, reefs formed and sand dunes built up on areas that are inland today. Two such stands are actually named after North Shore landmarks; The “Kaena Stand” (320,000 years ago) left deposits that can be found there some 95’ above sea-level. Later, (120,000 years ago) the “Kahuku Stand” deposited a thick reef and layer of calcified dunes about 45’ above today’s sea level. Those white rocks you see at Kahuku Point are remnants of this stand. The cliffs running behind the Sunset Beach area and into the mouth of Waimea Valley were caused by wave action cutting away at the base of the mountain.
Meanwhile, indigenous flora and fauna was making a foothold. From the sweet succulent a~ali`i and hao along the shore-line, to the ilima, hapu`u ferns and Loulu palms of the central plane, to the koa, ohia lehua and sandalwood mauka, O`ahu was forested. Many types of birds flew in or evolved. Some were forest song birds. One was a large flightless bird. Gobbles and shrimp evolved in the fresh-water streams and specialized insects and spiders became unique to each valley. Singing land-snails crawled across the island. The only mammals to become established were bats and seals.
Then, about a thousand years ago, a canoe beached itself. We will pick up the saga there at 11 AM on Saturday, April 14, at the Waialua Library.