The mountains on Oahu in the Hawaiian Islands are dissolving from within under the slow but inexorable onslaught of groundwater, researchers say.

Sometime in the distant future the mountainous tropical island will be reduced to a flat, low-lying island similar to Midway, scientists at Brigham Young University reported Friday.

While external erosion is the primary force that wears down mountains, that's not the case on Oahu, they said.

"We tried to figure out how fast the island is going away and what the influence of climate is on that rate," BYU geologist Steve Nelson said. "More material is dissolving from those islands than what is being carried off through erosion."

That was the result from comparing groundwater against stream water to see which removed more of the total quantity of mineral material mass that disappeared from the island each year, the researchers said.

"All of the Hawaiian Islands are made of just one kind of rock," Nelson said, referring to the volcanic composition of the islands. "The weathering rates are variable, too, because rainfall is so variable, so it's a great natural laboratory."

While Oahu is at present still growing, after about 1.5 million years the force of groundwater will eventually triumph and the island will begin its descent to a low-lying topography, the researchers said.