Researchers spent several months collecting samples of groundwater and stream water to determine which source removed more mineral material. They also put to use surface water estimates from the U.S. Geological Survey to calculate the quantity of mass that vanished from the island each year. Researchers point out that Oahu is actually rising in elevation at a slow but steady rate due to plate tectonics.
“The Big Island is so large that it actually depresses the ocean crust–kind of like a dimple on a golf ball,” BYU geologist Steve Nelson told The Science Recorder via email. “Oahu is close enough to the Big Island such that as the plate drifts to the northwest, Oahu is moving up and out of the side of the dimple. Kauai is far enough away (and older) that it has moved out of the dimple. The estimate of the duration of rising is based on the time it will take for Oahu to drift to where Kauai is now.”
Mr. Nelson and colleagues believe that Oahu will continue to grow for as long as 1.5 million years. Beyond that, the force of groundwater will eventually win and Oahu will begin its transformation to a flat, low-lying island like Midway. Researchers are confident that it will be a very long time before Oahu begins its descent.