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Oahu peaks shrinking mainly from within, scientists find

By Star-Advertiser staff

LAST UPDATED: 6:06 a.m. HST, Dec 23, 2012

Oahu's Koolau and Wai­anae mountain ranges will one day be reduced to naught, scientists affirm, but not quite in the way lay people might assume.

According to a new study by Brigham Young University researchers, Oahu's mountains are losing more mass to internal dissolution than external erosion.

The study, co-authored by BYU geologists Steve Nelson and David Tingey and undergraduate researcher Brian Selck, appears in the current edition of the journal Geochimica et Cosmochimica Acta.

Nelson and other researchers spent two months sampling groundwater and stream water to see which removed more mineral material. They also used ground and surface water estimates from the U.S. Geological Survey to help calculate the total quantity of mass that disappeared from the island each year.

The result?

"More material is dissolving from those islands than what's being carried off through erosion," Nelson said in a statement Friday.

Which isn't to say that the effects will be noticeable anytime soon.

Plate tectonics activity has been moving Oahu steadily northwest, causing the island to rise in elevation at a slow, steady rate, in essence offsetting the mountains' loss of elevation due to dissolution and erosion.

The researchers estimate that Oahu will continue to rise for up to another 1.5 million years before groundwater eventually carries away so much material from the mountains that they begin to flatten.

The study doesn't address potential sea level changes. During the last ice age, when sea levels were lower because so much water was trapped in ice caps and glaciers, Maui, Molokai, Lanai and Kahoolawe were all part of one island, which geologists call Maui Nui.

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