Volcano Watch — Scientific drill plumbs isle's depths

About nine months ago I wrote about a scientific drill hole in Hilo that was funded by the National Science Foundation 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. Much of the laboratory work on therecovered core is now completed, and some of the results will be described today. A more complete discussion of the results will take place in early December at a special session of the American Geophysical Union meeting in San Francisco.

The project was proposed and managed by a consortia of scientists from the University of California at Berkeley, the California Institute of Technology, and the University of Hawaii at Manoa. Scientists from the U.S. Geological Survey, including several from the Hawaiian Volcano Observatory, were involved in the planning, proposal preparation, core handling, core description, and analysis of samples.

The drill site was located in Hilo between the airport and the harbor. 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. The drilling started in Mauna Loa flows and then progressed downward into Mauna Kea lavas. The hole was drilled to 3,464 feet below the surface or 3,448 feet below sea level, and was cored continuously with about 90% recovery.

The upper 920 feet of the core consists mainly of lava flows from Mauna Loa Volcano whereas the lower 2,544 feet consists entirely of lava flows from Mauna Kea Volcano. All the flows erupted and were emplaced above sea level, indicating that Hilo has subsided more than 3,348 feet since the oldest flows erupted, probably about 450,000 years ago. We have done most of our work on the upper Mauna Loa section because understanding the history of Mauna Loa Volcano allows us to better evaluate lava flow hazards.

The uppermost 100 feet of core are three lobes of the Panaewa flow, aroughly 1,400-year-old Mauna Loa flow that forms the coastline east from Hilo Bay, around Leleiwi Point to Haena. Beneath this flow is about 85 feet ofcarbonate sand and coral fragments. We have dated the top of this unit at 1,700 years old and the bottom at about 9,700 years old. No lava flows entered the Hilo Bay area for more than 8,000 years and the Hilo area was blessed with an extensive white sand beach during this time period.

The rate of upward growth by the addition of lava flows and the rate of subsidence were roughly the same from perhaps 55,000 years ago until the present. This is seen in the repeated appearance of beach sands, shallow water fragmental rocks formed where lava entered the sea, and an ash layer that was deposited in a freshwater bog near the shoreline. The shallowest black-sand beach deposit in the core is from 550-564 feet; the glass fragments are compositionally similar to Mauna Loa lavas, and they formed when a Mauna Loa flow entered the ocean in the vicinity of Hilo Bay, perhaps 37,000 years ago.

The ash unit, present from about 595 to 600 feet, has black organic-rich zones at the tops of two sections that are separated by a thin Mauna Loa flow. Radiometric carbon ages on this organic material suggest that the ash was deposited 39,000 years ago. This ash may be the Pahala Ash, which erupted from Kīlauea Volcano.

The age and depth of the ash can be used to calculate a subsidence rate for the Hilo area for the last 39,000 years, after we correct for changes in absolute sea level. The subsidence rate determined is 2.2 millimeters per year, which is nearly the same as the 2.3 millimeters per year estimated from tide-gauge records for the last 43 years.

Another black-sand beach deposit was recovered from 754 to 784 feet. This is one of the more interesting units in the section because the glass fragments are compositionally similar, not to Mauna Loa lavas, but to Kīlauea lavas. In addition, many of the sand grains apparently erupted in shallow water because they have not degassed sulfur, as occurs with lava erupts above sea level. We think this black sand formed near the shoreline of the east rift zone of Kīlauea Volcano near Cape Kumukahi about 55,000 years ago. The sand was transported the roughly 21 miles to Hilo Bayby north-northwesterly longshore currents.

Below this lower black-sand beach deposit, but above 850-feet in the hole, there are two more fragmental deposits that formed when Mauna Loa lava flows entered the ocean. Similar deposits form where the present day Kīlauea flows enter the ocean at Laeapuki. With time, such coarse deposits become finer and finer as the fragments break apart. Such deposits are preserved where lava flows quickly cover them. This is probably what happened in Hilo Bay as well.

In a future column I will summarize the results from the lower Mauna Kea section of the drill hole.