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Naturally Speaking:

The Dynamic Planet Pt. 4

Community contributed by G.T. Larson

Along the coast of southeast Alaska is a small narrow inlet approximately seven miles long and less than a mile wide called Lituya Bay. On July 9, 1958, an 8.3 earthquake triggered a massive rock fall from a steep mountainside at the head of the bay. A resulting wave immediately swept the opposite side of the bay, flattening an entire forest 1,720 feet up an adjacent steep valley wall.

To this day, this is the largest tsunami, more accurately called mega-tsunami, recorded in modern history. A mega-tsunami, also sometimes referred to as a iminami meaning “purification wave,” is a wave caused by a large scale landslide or impact event, such as a catastrophic meteorite impact. These waves can be hundreds to even thousands of feet high. Fortunately, these are extremely rare events, but they are a part of Hawaii’s geologic past.

Many residents and visitors alike have gone up to the summit of Haleakala on
Maui and even a few have traveled to the peaks of Mauna Kea and Mauna Loa on the Big Island. But in reality, all of us here are on mountain tops. The Hawaiian Islands are all of volcanic origin, but these volcanoes began on the seafloor, 15,000 to 20,000 feet below the ocean’s surface. In actuality the highest mountain on Earth is not Mt. Everest in the Himalayas, but Mauna Kea, which is several thousand feet higher from base to summit than Everest – though only 13,796 feet of Mauna Kea is above sea-level. The combination of the steepness of the Hawaiian Islands, rising so quickly from such great depths, and the inherent instability of volcanic slopes have led to mega-tsunamis that have affected and been generated in Hawaii, including Molokai.

The timing of the deposition of marine fossils on Mt. Kohala on the Big Island coincide with a massive landslide from Mauna Loa, leading scientists to conclude that a mega-tsunami deposited the fossils on Mt. Kohala – not that the summit was below sea level and acquired the marine life. Mt. Kohala would have still been in its building stage with near continuous volcanic activity during that period. Coral debris has been found over 560 feet above sea level on Lanai.

Here on Molokai, coral and shell debris can be found inland of the south shore of La`au Point, and in Ranch Camp among other locations; all of these being indicative of massive ocean run ups, more so than would be expected from a typical tsunami. Geologically, these events pale in comparison to the Nuuanu Slide on windward Oahu and the Wailau Slide on Molokaiʼs north shore. In our next installment we will look at how these events greatly changed the shape and size of our island home. Aloha Ke Akua.

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