Reporting in the journal Geochimica et Cosmochimica Acta, researchers at Brigham Young University say that the Hawaiian Islands are slowly dissolving. Eventually, Oahu’s Koolau and Waianae mountains will dwindle to little more than a flat, low-lying island like Midway. While erosion is certainly a guilty party, researchers contend that the mountains of Oahu are, in fact, dissolving from within.
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.
“I don’t think that groundwater will win out faster,” Mr. Nelson said. “The wet parts of the island are getting flushed pretty rapidly as it is. It’s good to remember that an atoll like Midway has had 20+ million years to erode to sea level. Oahu is about 1/10th that age.”
Although the researchers focused on Oahu’s Koolau and Waianae mountains, Mr. Nelson believes that what is happening on Oahu is also happening on the other islands.
“You can already see it on the Big Island’s northwest end–the Kohala Volcano,” said Mr. Nelson. “It has thick soils and deeply carved valleys like Oahu. Kohala is extinct, so it is no longer reforming its surface via eruptions. Processes like we have observed are occurring on most volcanic ocean islands like Samoa, Tahiti, Reunion, etc.”
Mr. Nelson believes that his research will help scientists determine how different materials on the Hawaiian Islands will behave during earthquakes.
“This work has direct applications to earthquake hazards–because it allows you to characterize the stiffness of the weathered material and thereby how it will respond to earthquakes,” the geologist said, adding that, “the Big Island has a significant earthquake hazard.”
While learning more about earthquake hazards on the Hawaiian Islands is an important mission, Mr. Nelson admitted that his motivation lies elsewhere.
“My motivation […] is that you can also get at weathering rates by another means,” he said. “If you know the age of the bedrock and the thickness of the weathered horizon above it, the downward rate of advance of the weathering front is simply the thickness of the weathered horizon divided by the age of the bedrock.”
Mr. Nelson is currently working with a geophysicist colleague at BYU, John McBride. They are using high resolution seismic methods to determine the thickness of weathering horizons. They are conducting research on Oahu and may work on Kohala in the future.