Volcano Watch — Scientists drill holes to explore volcanoes' eruptive stages

Hawaiian volcanoes pass through a sequence of eruptive stages as they grow. There are three main types of lavas in Hawai`i which erupt during the different stages. These distinct lava types have different chemical compositions and physical properties. The three types are tholeiitic basalt, alkalic basalt, and strongly alkalic basalt. The main chemical differences between these lavas have to do with the abundance of silica, relative to the alkali elements sodium and potassium.

The earliest stage is characterized by the infrequent and small-volume eruption of alkalic lavas which have moderate silica and moderate sodium and potassium contents. These lavas apparently erupt infrequently and in small volume. The only example of this eruptive stage that is known is from Lo`ihi Seamount, the small, active submarine volcano located on the southeast flank of Hawai`i. The existence of this eruptive stage for all other Hawaiian volcanoes is inferred.

The next stage is called the shield-building stage, because it is during this stage that the voluminous and frequent eruptions of tholeiitic basalt build volcanic edifices with characteristically low shield shape. Both Kīlauea and Mauna Loa Volcanoes are presently in this eruptive stage. The lavas erupted are relatively high in silica, low in sodium and potassium, and tend to be very fluid.

At the end of the shield stage, the lava chemistry changes back to alkalic basalt, and the volcano enters the post-shield stage. The lava flows tend to be smaller in volume, and eruptions become less frequent. The flows are also less fluid, and the shape of the volcano becomes steeper as short, stubby flows and fire-fountain deposits of cinder accumulate preferentially near the summit. Hualālai and Mauna Kea Volcanoes are presently in this eruptive stage.

After the post-shield stage, there is a period of erosion which carves deep valleys and high sea cliffs before the final eruptive stage occurs. This stage can follow the end of the post-shield stage by as little as a few hundred thousand years, as on Haleakalā, or by as much as several million years, as on Ko`olau Volcano on O`ahu or Ni`ihau Island. The lavas erupted are strongly alkalic, which means they have low silica and high sodium and potassium contents. These lavas tend to erupt explosively, but also form fluid flows that commonly flow down and fill valleys carved during the prolonged period of erosion. This stage is called the post-erosional, or the rejuvenated, stage. Most of the Hawaiian volcanoes pass through all these stages, but some, such as Lana`i and Ko`olau Volcano on O`ahu, never had a post-shield stage and others, such as Lana`i and West Moloka`i, never had a rejuvenated eruptive stage.

In general, it is difficult to sample the sequence of lavas erupted at a single volcano during these different stages because the islands are subsiding so rapidly that the earlier erupted lavas are carried below sea level where they cannot be sampled. The rapid subsidence is caused by the weight of the growing volcano, which flexes the underlying ocean crust downward. By the time the volcano has reached the rejuvenated stage, the total amount of subsidence is commonly 4,500 to nearly 8,000 feet.

One way to get a detailed look at the sequence of lavas erupted at a volcano that has only thin sequences exposed in erosional valleys is to drill a well and core the rocks through which the hold passes. The National Science Foundation has funded just such a project, a collaborative effort of the University of Hawai`i at Manoa, California Institute of Technology, and the University of California at Berkeley. Scientists at the U.S. Geological Survey's Hawaiian Volcano Observatory are also collaborating in this endeavor. Drilling began this past week at a site in Hilo.

The objectives of this hole are to sample the transition between the post-shield stage lavas and the underlying shield lavas on Mauna Kea Volcano, which are not exposed anywhere on the surface. The initial hole is a pilot hole that is expected to reach a depth of about 3,000 feet. If this hole proves to be technically feasible, then a deeper hole will be drilled that will penetrate somewhere between 8,000 and 15,000 feet, depending on the level of funding. A hole near the coast and far from the rift zone was selected in order to avoid the effects of hydrothermal alteration. The site began in lavas from Mauna Loa Volcano and will enter Mauna Kea lavas within the uppermost several hundred feet.

As the drilling progresses during the next five weeks, there will be periodic updates in "Volcano Watch."