Oahu Dissolving; Groundwater Eroding Mountains From Inside Out
Oahu is dissolving, according to a study from Brigham Young University. But it's not traditional erosion that's wearing away on the Koolau and Waianae mountains, it's the slow and steady trickle of groundwater inside the mountains that is the reason Oahu is dissolving.
"We tried to figure out how fast the island is going away and what the influence of climate is on that rate," Brigham Young University geologist Steve Nelson said in a news release. "More material is dissolving from those islands than what is being carried off through erosion.
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Nelson and his team spent two months studying Oahu dissolving. They examined groundwater and stream water to see which was removing more minerals from the island, and combined that data with reports from the U.S. Geological Survey that measured water levels on Oahu to determine how much mass was rinsed away each year.
"Someday, Oahu's Koolau and Waianae mountains will be reduced to nothing more than a flat, low-lying island like Midway," said the news release.
That 'someday' is a long, long time from now. Oahu is dissolving, but it will take 1.5 million years of groundwater erosion before the mountains start to level off. In addition, Oahu is being pushed upward by plate tectonics, meaning the overall elevation will rise even though the mountain ranges on Oahu are dissolving. Nelson claims the geological profile of Oahu made for an intriguing geological study.
"All of the Hawaiian Islands are made of just one kind of rock," Nelson said. "The weathering rates are variable, too, because rainfall is so variable, so it's a great natural laboratory."
That rock, basalt, is the most common bedrock mineral on Earth but is rarely seen above ground because basalt is only found in areas that used to be (or still are) underwater. Groundwater is part of the erosion process that is causing Oahu to dissolve, but another culprit might also lend a hand: fungus. As fungus grows on basaltic rocks it leaves small cracks and pits that trap rainwater. As the rainwater flows out, greater erosion occurs.
Oahu is dissolving, and rising, very slowly. The study did not address how Oahu would be impacted by any change in sea levels, however. During the last ice age sea levels were so low that Hawaiian Islands Maui, Molokai, Lanai and Kahoolawe were connected. If sea levels rise, this could also have an impact on how quickly Oahu dissolves.
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