Volcano Watch — The Big Isle's rapidly sinking into the sea, researchers say

The National Science Foundation recently supported a scientific drill hole to examine the long-term growth of a Hawaiian volcano. The objective identified by the principal investigators was to drill through as much as possible of Mauna Kea Volcano and determine the variations in the compositions of the lavas over time.

The project was proposed and managed by a consortia of scientists at the University of California at Berkeley, California Institute of Technology, and the University of Hawai`i at Manoa. The U.S. Geological Survey, including several scientists located at the Hawaiian Volcano Observatory, was involved in the planning, proposal preparation, core handling, and core description.

The drill site selected was located in Hilo between the airport and the harbor. The site was selected to start drilling in Mauna Loa flows and then progress downward into Mauna Kea lavas. The location was chosen to accommodate drilling logistics and to avoid the rift zone and summit region of Mauna Kea where hydrothermal circulation might alter the lava flows. Drilling was completed last weekend, and the hole reached to 3,464 feet below the surface or 3,448 feet below sea level. The hole was cored continuously, and about 90% was recovered. Most of the scientific results from this drill core will not be known until detailed chemical analyses and radiometric dating of samples are completed, probably in about one year.

One striking finding, based on visual examination of the core, is that Hawai`i is sinking rapidly into the sea. The observation is simply that all the recovered lava flows were emplaced above sea level, where they were altered in air and formed bright orange soils. Since the deepest of these subaerial flows is at the bottom of the hole, we have direct evidence that the Hilo area has subsided at least 3,448 feet. The part we do not know yet is how long it took for the island to sink by this amount. Detailed laboratory studies of the recovered samples should provide the time control. Our initial guess is that the deepest recovered lava flows erupted about 400,000 years ago.

The idea that the island sinks is hardly new, but it has usually been difficult to separate changes in worldwide sea level with subsidence of the island. Worldwide sea level changes due to growth or shrinkage of the polar ice caps, which remove or add volume to the oceans. These changes are directly related to global climatic change so that when the planet is colder, the polar ice caps expand and thicken, thereby removing water from the oceans and lowering sea level. On the other hand, when the planet is colder, the polar ice caps melt and add water to the oceans, thereby raising sea level. The history of these global sea level changes is fairly well known for the last million years or so. The last two times that sea level changed from falling to rising occurred about 18,000 and 125,000 years ago.

An interesting aspect of changing sea level is that the growth of coral reefs around Hawai`i is controlled more by changes in relative sea level than by ocean temperature. During periods of cold global climate, both global sea level and the island are sinking together, the shoreline is relatively stable, and coral reefs flourish. During periods of warm global climate, global sea level rises, and the island sinks. The sum of the rise in sea level and the sinking of the island means that the corals would have to grow very rapidly to keep their heads at sea level, so to speak. In most places, the corals cannot keep up with the rate of relative sea level rise and they drown. This results in counterintuitive in that coral reefs grow best in Hawai`i when global climate is cold and die when global climate is warm.

The reason why coral reefs are more abundant around the older islands in the chain is that the islands do not sink at a constant rate. Instead, the rapidly growing island of Hawai`i also sinks rapidly and each progressively older island sinks more and more slowly. At O`ahu, the rate of sinking is very small compared to that of Hawai`i; thus, the corals have to grow only as fast as sea level rises.

At the end of global cold periods 18,000 and 125,000 years ago Hawai`i was nearly surrounded by coral reefs and extensive white sand beaches. These coral reefs and beach and lagoonal sands are now located offshore at depths of about 450 and 1,280 feet for the 18,000 and 125,000 year old reefs, respectively. The coral reef that drowned 125,000 years ago can be traced from Hilo Bay around the north side of the island and as far south as Kealakekua Bay. The 18,000-year-old reef has much of the same extent, and a small section to the northeast of South Point.

The drill hole in Hilo also recovered a section of coral white sand and mud just below the uppermost lava flows. Thus, Hilo was graced with white sand beaches until they were buried by the Pana`ewa flows from Mauna Loa Volcano about 1,400 years ago.

Global sea level in no longer rising very rapidly, so in the future, Hawai`i may once again be partly surrounded by extensive coral reefs and white sand beaches. However, do not hold your breath as it may be several thousands of years before this occurs.