Drilling and coring a 3.5‐km‐deep hole into Mauna Kea Volcano, the Hawaii Scientific Drilling Project (HSDP) obtained a virtually unweathered core with lava flows, hyaloclastite, minor intrusives and sediment and a 250-meter and a 280-meter section of the Mauna Loa volcano. This international project obtained the deepest core samples from a Hawaiian volcano, and the recovered rock samples were perhaps the longest continuous stratigraphic record from any ocean island volcano. HSDP used a custom-made large top-drive rig, which set records for deep continuous coring.
HSDP revealed the geochemical and geophysical processes within mantle plumes that form “mid-plate” volcanoes (e.g. volcanoes in Hawai’i, the Galapagos, and the Yellowstone volcanic system). The HSDP rock core provides the first systematic cross-sectional sampling of a deep mantle plume. For 15 years, an international team of 40 scientists characterized the core sample’s geochemistry. From a 3,260-meter section of Mauna Kea, this core covered an age range from 200,000 to 700,000 years and yielded geological data and results:
- Lava stratigraphy and volcano evolution
- age, growth rates, and thermal history of Hawaiian volcanoes
- structural evolution of Hawaiian volcanoes
- the deepest parts of the Earth’s mantle
- the deep mantle origin of the lava that built the islands
- the magma’s interactions as it rises to the surface
- formation of hotspot magma in the subsurface
- Geochronology and petrology
- Geochemical structure of the Hawaiian mantle plume
- chemical evolution of Hawaiian volcanoes
- alteration geochemistry
The Mauna Loa core extended the continuous lava stratigraphy of that volcano to 100,000 years and revealed major changes in lava geochemistry over that time period. The Mauna Kea core provided a 700,000-year record of the lava output from a single volcano. With these cores, HSDP confirmed the following:
- high magma supply rates
- an eruption of tholeiites and picrites between 600,000 and 400,000 years ago
- a subsequent decline in eruption rates between 300,000 and 400,000 years ago
- an onset of post-shield volcanism
- an eruption of alkalic basalts
The volcano drifted some 60 to 80 kilometers across the melting region of the Hawaiian mantle plume.
The hole shows a complex subsurface hydrological regime that differs greatly from the generalized view of ocean island hydrology. HSDP2’s first phase revealed unanticipated hydrological findings:
- several deep pressurized aquifers with varying salt content
- cold seawater circulating through the volcanic pile
- The large volumes of seawater refrigerate the entire section below 700-meter depth to temperatures about 25 degrees below a normal geothermal gradient
The project results answer important questions about oceanic volcanoes, mantle plumes, and ocean island water resources, but raise many more. They might be answered with further moderate-depth drilling in other Hawaiian volcanoes.
Beginning – 1986: A proposal to drill the Moho under Hilo was submitted to the National Science Foundation.
HSDP1 – 1993-1996: The HSDP1 pilot hole was located 2 kilometers to the NNW, north of the airport, within fifty meters of the shoreline of Hilo Bay. In 1993, the project cored a pilot hole to 1,052 meters below sea level. In 1995, the project submitted a proposal for HSDP2.
HSDP2 – First Phase – 1999: An abandoned quarry at Hilo International Airport was the site for HSDP2. From March 13, 1999, to September 27, 1999, using a commercial drilling rig and a hybrid coring system, the project core-drilled a hole to 3,098 meters below sea level (total depth of 3,110 meters) with 98 percent recovery.
HSDP2 – Second Phase – 2003-2007: In 2003, due to variable formation fluid pressures, at the hole, HSDP performed complex re-opening, casing, and cementing operations. From April 27, 2003, to February 24, 2007, HSDP deepened the hole to a final depth of 3,508 meters below sea level (total depth of 3,520 meters).
At the conclusion of drilling, the following was done:
- the 5-inch casing was perforated
- cement was pumped into the annulus at depth
- at 2,031 meters below sea level, the casing was cut and the top section was removed from the hole
The final depth of the HSDP core hole was about 914 meters less than was originally planned in 1996, but was still nearly twice as deep as the next deepest core hole drilled in Hawaii (as compared with SOH-2 at 2073 meters on the Kilauea East Rift Zone).
Scientific Importance of Core Drilling
Intraplate or “hot spot” volcanic island chains, such as the Hawaiian islands, have revealed plate tectonics. However, unlike volcanism at convergent and divergent margins, the plate tectonic paradigm does not explain the origin of intraplate volcanoes. As the most widely held view of the origin of island chains, in the mantle’s thermal boundary layers, decompression melting of localized, buoyant upwellings, or “plumes,” generates magma, which forms island chains.
The study of hot spot volcanoes is limited because most of each volcano is inaccessible to sampling. Erosion usually exposes only a few hundred meters of the volcano’s interior out of a total thickness of 6 to 20 kilometers. Continuous core drilling through a sequence of lavas on an oceanic volcano’s flank is the only way to sample its long-term history. From a sequence of core samples, scientists can study the following:
- variations in the petrology
- geochemistry of erupted lava
- changes in volcanic processes and structures over the one million year history of an oceanic volcano
Hence, drilling can uncover valuable information on mantle composition, processes, and structure.
Project Reports (external links)
- Operational Report: Deep Drilling into a Mantle Plume Volcano: The Hawaii Scientific Drilling Project
- Final environmental assessment and finding of no significant impact : South Hilo District, Hawaii Island, State of Hawaii.
- 1993 Project Report