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The ultimate objective of the project is to drill to a depth of about 6,000 meters, slightly more than the depth of the ocean, at a location near the city of Hilo. At near 6,000 meters depth, the drill hole would pass through the "bottom" of the volcano into the 80-million year sea floor underneath, the base upon which the volcanoes are built. Because drilling to 6,000 meters would be very expensive and drilling conditions are poorly known in the subsurface of the Hilo area, the first stage of the project is a "pilot hole," which will be drilled to a depth of about 1,000 meters. This drilling started on October 25th and is expected to continue until early December. The drilling is being done by the Tonto Drilling Company of Salt Lake City, Utah. The bright orange drill rig can be seen from the air on approach to the Hilo airport; it is located in a wooded area between the airport and the ocean.
There is great scientific interest in sampling the interior of Hawaiian volcanoes. Although the different types of lava rocks that make up the volcanic mountains look much the same to the untrained eye, there are regular changes in the type of lava that erupts from a volcano during its lifetime. These changes are subtle; they involve the chemical makeup of the lavas, the minerals that are contained in them, and the isotopic composition of the chemical elements that make up the lavas. These changes provide clues about how the deep earth produces these huge volcanic features - the Hawaiian island chain and others like it in the south Pacific ocean are major features of the earth that can be seen in photos from space. In the absence of drilling, only the final stages in the lifetime of the volcanoes can be studied. Erosion cuts canyons into the sides of the volcanoes, but the canyons cannot get very deep because the islands sink from the weight of the volcanoes themselves.
The Hawaii Scientific Drilling Project chose for study the Mauna Kea volcano on the Big Island, whose 13,762 foot high summit is the home of several astronomical observatories. The drill site is located in the city of Hilo, just to the east of the center of town and within about 100 yards of the shoreline. The drill site is some 45 km distant from the summit of the mountain. Mauna Kea was chosen for study because it is nearly extinct, so it has lived almost its full life, gone through all of its major life stages, and these stages are now preserved in the ancient lavas that lie below the surface.
The Big Island of Hawaii consists of five different volcanoes, three of which are still historically active - Kilauea, Mauna Loa, and Hualalai - and the other two - Kohala and Mauna Kea - have not been active in historical times. The lavas of the different volcanoes overlap and interleave as the volcanoes grow; older volcanoes tend to get partially covered by the lavas from younger volcanoes. Hilo is located on lava from the active Mauna Loa volcano. However, geologic models for the growth of the island indicate that the Mauna Loa lavas under Hilo should be a thin layer (about 250 meters thick), and below them lies some 6,000 meters of Mauna Kea lavas. One of the first results of the drilling project will be to test the models for the subsurface structure of the islands.
The drilling is being done using diamond-bit wireline coring techniques. The dill rig that holds and turns the drill has a derrick 38 feet high. The drill string that extends into the ground is made up of hollow steel pipe about 4 to 5 inches in diameter. At the end of the pipe is the "drill bit," which is a ring of steel that has the same outside diameter as the drill pipe but a slightly smaller inside diameter. The steel of the drill bit is impregnated with bits of diamond that wear away the rock, and therefore cut through it, as the drill pipe turns. As the steel of the drill bit wears away, new diamonds are continually exposed, so the bit cuts like it was new until it is exhausted. A single bit is usually capable of drilling through several hundred feet of rock before it is worn-out. As the drill bit cuts downward, a solid cylinder of rock about 3 inches n diameter moves into the "core barrel," a steel pipe about 3 inches in diameter that fits inside the drill pipe. When the core barrel is full (there are two sizes - 5 feet long and 10 feet long) it is pulled up to the surface inside the drill pipe, emptied, and returned to position without moving the drill pipe. When the operation is proceeding smoothly, 100 to 150 feet of core can be obtained each day. The drilling proceeds on a twenty-four hour schedule.
The rock that is retrieved from the drilling must be carefully organized, prepaed, described, and stored in order that it can be studied by scientists later. The project has several people, including students and postdoctoral fellows, on site in Hawaii to help with preparing the core.
Typical lava flows from Hawaiian volcanoes are about 10 to 15 feet thick. If the project is successful and a depth of 1,000 meters (3,280 feet) is reached, we will have obtained core from 250 to 300 different lava flows. The total amount of rock recovered will be about 1.5 tons. The lava present at the surface is about 1,200 years old. We expect the lava at 1,000 meters depth to be about 250,000 years old. The expected average recurrence rate of lava flows at the site (one or two flows every thousand years) is what is thought to be typical for Hawaiian volcanoes.
Based on studies of the lavas accessible at the surface of Hawaiian volcanoes, ach volcano is presumed to go through several stages of development. In the first stage the volcano starts to grow up from the ocean floor and it is entirely under water. An underwater volcano is called a "seamount." Eventually the volcano breaches the ocean surface and forms an island. As the volcano grows in elevation, the area of the island increases proportionately. However, the weight of the volcano also depresses the sea floor below it, so the island is sinking. During most of the life of a volcano, lava is erupting at a sufficiently fast rate so that the growth of the volcano is faster than the rate of sinking (subsidence). However, as the volcano's lifetime comes to an end, it starts to sink more rapidly than it grows, and the island's area starts to decrease again. As each volcano goes extinct, a new volcano starts to form on its southeastern flank. The next Hawaiian volcano (called "Loihi") has already started to form underwatr southeast of the Big Island.
The type of lava that erupts from Hawaiian volcanoes is also thought to change ystematically through the lifetime of each volcano. There are basically two common types of basalt - "tholeiitic" basalt and "alkalic" basalt - and both types erupt from Hawaiian volcanoes. However, the "standard model" says that alkalic basalt comes out during the early and late stages of a volcano's lifetime, and tholeiitic basalt comes out during the "main stage" in the middle, when most of the volcano's growth occurs. This model has never really been tested, however, because the "main stage" is mostly buried and inaccessible in the interior of the volcano. One of the aims of the drilling project is to test this model by observing the interior of a volcano that has presumably gone through most of this developmental sequence.
The chain of volcanoes that makes up the Hawaiian islands is believed to form fom melting of a hot stream of material in the mantle that is rising up toward the surface from great depth in a so-called a mantle "plume." Some scientists think that these plumes originate from 3,000 km down at the boundary between the earth's iron core and the rocky mantle. Geologists do not agree on the details of the processes that result in the formation of lava from this hot mantle rock. One good way to study the process is by measuring in detail how the composition of lava coming from a volcano changes with time. The succession of lavas encountered in the drill core will be an exceptionally good venue for this type of "time series" analysis. The lava compositions will also tell whether the deep mantle, where the mantle plumes may originate, is different in composition from the shallow mantle, which acts as the source of lava for most other types of volcanoes.
An interesting aspect of the pilot hole project concerns the rise and fall of sa level that has occurred in the geologic past because of ice ages. It is usually possible to determine from the texture of the lavas whether they formed underwater (submarine) or above sea level (subaerial). The subsidence of the volcanoes, coupled with the accumulation of new lavas and the rise and fall of sea level, means that the shoreline moves back and forth with time. This complex situaion is reflected in the drill core by alternating submarine and subaerial geologic units. The first surprise of the drilling is that underneath the lava flow that makes up the land surface of Hilo is a thick deposit of coral and lagoon sediment. Coral reefs are not supposed to form on the flanks of Hawaiian volcanoes until they are extinct, and Mauna Loa is far from extinct. The surficial lava flow is very thick (about 100 feet) and is known to be about 1,200 years old. Below the coral layer is another thick lava flow; from the geologic relations the lower lava flow must be at least 12,000 years old. This means that only once in the last 12,000 years has a lava from Mauna Loa reached the east side of Hilo, an observation that should warm the hearts of Hilo residents.
As of November 7th, the drilling depth had reached 800 ft (250 meters).