Volcanoes Sink Under Water When They Stop Erupting Scientists Say

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Scientists who have discovered ancient underwater volcanoes say they are gradually sinking deeper and deeper into the sea. The revelation that they sink under water when they stop erupting follows the discovery of a volcano in Hawaii that was once 3,300 feet above the level of the sea.

According to a research study published in the May 2, 2014 online issue of The Geological Society of America Bulletin, the Island of O’ahu was once elevated above the ocean, with three volcanic peaks. Previously it was thought that there were only two volcanoes on the island, one in the west known as Wai’anae, and another in the east, Ko’olau. The newly discovered volcano, Ka’ena, is believed to have formed in a deep channel under water, south of Kauai around 5 million years ago. Wai’anae is believed to have formed next, about 3.9 million years ago, and then Ko’olau, 3 million years ago.

According to geologist John Sinton, lead study author from the University of Hawaii, Ka’ena only ever reached a height of 3,300 feet above sea level. Even though it is older in terms of origin than the other two volcanoes, Ka’ena only broke the surface of the ocean about 3.5 million years ago, some 400,000 years after Wai’anae had surfaced. Ko’olau was the third volcano to surface, about 500,000 years after Ka’ena.

Today O’ahu, which is home to Honolulu, Hawaii’s largest city and capital, features just two submarine mountain ranges, Wai’anae and Ko’olau. The discovery that a third submerged volcano also exists has come as a great surprise. According to Sinton, once volcanoes stop erupting, “they continue to sink” under the water, and Ka’ena has receded right back under the surface of the Pacific Ocean. Even though it is now deep in the sea, scientists say they know it was once an island peak that was capped with lava following underwater explorations with a remote-controlled device. This lava could only have formed in the air, and not under water.

We just don’t know when Ka’ena went under water. John Sinton, geologist

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Rock samples collected with a remote controlled device show that Ka’ena is an older volcano than the other two on the island, with its own chemical makeup

According to Sinton, the discovery of Ka’ena was made while the team of geologists was investigating an area that lies off the tip of O’ahu in the west, once considered a submarine – or underwater – extension of the well-known Wai’anae mountain range. This, he said, explained “a lot of things about O’ahu.”

In the abstract of the research report, the scientists state that “volcanic structure, geochemistry, and age all indicate a precursor volcano” on O’ahu Island. Samples taken from the volcano’s “landslide deposit” were not consistent with samples from the other volcanoes. For instance there was a “newly recognized lava flow field” on the southern side of the Ka’ena Ridge. This was previously reputed to have been from volcanic activity that occurred in 1956. But current researchers say that it is unlikely that this activity formed the pumice that has been found on the shores of O’ahu.

The volcanoes that were once a landmark of the Island of O’ahu are said to have disappeared about 2 million years ago, sinking slowly but progressively under the water. The process has been described as something like the pressure applied by a person standing or jumping on a trampoline, that causes the once springy surface area to sag. And this only happens after the volcanoes have stopped erupting, scientists say.

By Penny Swift

Sources
GSA
New Discovery
Reuters

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