Following the destructive, deadly series of volcanic eruptions at Unzen from 1990 – 1995, the Japanese needed to understand the eruption mechanisms better so as to better predict its future behavior. They started the Unzen Scientific Drilling Project (USDP) in April 1999. This project proceeded in two drilling phases completed in 2003 and 2004 respectively.
The first phase defined the well design and targets of the drilling.
The second phase penetrated the magma conduit feeding the current dome at the summit of the volcano.
The scientists wanted to characterize where the magma was effectively degassing, as they believe that this is the major factor that controls the eruption styles. Styles are dependent on pressure dependent solubility of volatiles, mostly water, which accelerates frothing of the magma as it approaches the surface. The degassing creates earthquakes which in turn define the place where water depressurizes sufficiently to flash to steam and drive explosive eruptions.
The project was the first on site observation of Unzen’s conduit and surrounding rock walls following a recent eruption.
Participants and Partners
The project was funded by ICDP Germany – the German Science Foundation, International Continental Scientific Drilling Program, and the Japanese Government. Principal investigators included four Japanese scientists and a professor from USGS in Fairbanks. Partners included the University of Tokyo, National institute of Advanced Industrial Science and Technology, University of Alaska, German Research Centre for Geosciences, Japan Metals and Chemicals Co., Ltd, and Kyushu University.
As of today, drilling operations are finished, the well capped, and scientific evaluation complete.
The majority of what follows is paraphrased from “Scientific Results of Conduit Drilling in the Unzen Scientific Drilling Project” by Setsuya Makada, Kozo Uto, John C. Eichelberger and Hiroshi Shimizu dated Jan. 3, 2005.
Geothermal modeling before drilling began suggested a temperature over 600 C in the center of the conduit if it cooled by conduction only. Drilling target was the center of the region of tremors that occurred prior to magma extrusion in 1991. Drilling began some 840 m above sea level and 1000 m north of the summit of the Fugen dome on Unzen. Drilling was planned directionally with the initial run vertical with a turn toward the summit as the intersection depth was below sea level. Precision direction of a drill is a common practice in oil and natural gas drilling and requires what is nowadays common equipment and expertise.
This required preparatory work on the volcano itself included a new road up the mountain, water wells and a water pipeline. As the drilling operation was water-cooled, a significant amount of water was required for active drilling.
The drilling team expected problems with trajectory control in loose volcanic formations during the large diameter phase and drilling, sampling and logging efforts within the anticipated high temperature formations. What they found were cavities in the formation that made it difficult to meet their drilling schedule. They initially were sited in a small basin associated with an active fault on the flank. They lost drilling mud circulation, suffered frequent wall collapse and had deviations in trajectory. These were solved by changing to an aerated drilling mud with more water and a top drive system. These increased their ability to control direction and vector of the drill. The paper also mentions a safety and oversight committee for conduit drilling that improved positioning.
Drilling resumed in 2004 with the target changed by 250 m East based on reanalysis of seismic data taken during phase 1. The second phase went faster than planned because the middle and deeper parts of the volcano were more stable. Well casing was used to control wall collapse.
Formations around the conduit did not show circulation or drilling fluids loss which was interpreted as few cracks in the targeted rock. Spot coring and logging were done as the drill approached the conduit which was penetrated near sea level, about 2,000 m downhole or 1,300 m below the level of the crater.
Drilling was terminated in July 2004 with the well cased and plugged in preparation for the next (if desired) drilling attempt.
Drilled formations in the volcano were found to be mainly homogeneous volcanic breccias. They were very dry, with few fractures and low gaseous permeability. There are seven dikes in the conduit. Thicknesses of the dikes range from 7 – 40 m. The dikes have multiple pyroclastic veins to tens of cm thick. Most of the dikes are thought to be magma conduits of older eruptions. They are not cylindrical. They are plate-like in shape. The dikes and veins are nearly vertical stretching in an east – west direction. They are stacked from north to south in a 0.5 km zone which defines the magma conduit of Unzen.
Composition of the dikes are dacite with differing chemical compositions. Magmas of each eruption of Unzen vary chemically and are individually unique. Dike magmas are denser and lower in porosity than the surrounding breccias of the greater mountain.
One surprise was the newest dike. It was expected to still be hot, somewhere in the vicinity of 600 C. Instead it was only 180 C. It is thought that an active and robust hydrothermal system in Unzen accelerated cooling and chemical alteration of the newest dike. Most importantly, the hydrothermal system did its work quickly. Samples at the boundary of the most recent dike and surrounding breccias were not taken due to operational limitations. Isotopic analysis of the newest dike magma cores are identical to that of the most recent emplaced dome.
This sort of rapid cooling requires a small conduit and a highly permeable mountain body.
There are few gaseous bubbles in conduit lavas. Conduits of different eruption events are not bundled into a single conduit. Rather, each eruption event created its own unique conduit. Normal repose periods are much longer than cooling times, so there is no thermal influence from a previous dike to a new one. Degassing of the ascending magma occurred along cracks ahead of the growing dike. Formation of the cracks and gas migration appear to drive the volcanic tremor events during the eruption.
Analysis of dike conduit microlites suggest that magma ascended at slower rates as it reached a shallower depth, essentially stalling out as it reached the surface. Most of the microlite growth took place during the latter portion of dewatering and degassing.
Perhaps the most important conclusion from the drilling program, this single data point, is that volcanic systems can cool down quickly over the over the course of less than a decade even if the eruption lasted for years. Because of this and the crystallization due to decompression and cooling there is speculation that silicic magmas like erupted form Unzen can not be stored in a magmatic state (hot, pressurized, eruptible, crystal mush) at the base of a volcanic edifice. That depth is simply too shallow and neither sufficiently pressurized or heated.
Previous to this data point, it was thought that eruptible magma could be stored for an extended period quite close to the surface at the base of an edifice. Now that we have a single observation that demonstrates it did not happen at Unzen, how to apply that observation elsewhere? What is the shallow depth limit for active silicic magma storage above which the magma simply solidifies and below which it remains active? There is a transition depth. We don’t know what it is yet. An application of this concept would be renewed activity at Augustine and St. Helens where early seismic activity began at very shallow depth.
Finally, doing this sort of drilling on volcanoes with shorter repose times and more explosive eruptions will give different data points that can be used to compare the various systems and perhaps start modeling them for predicting future eruptions. https://www.uaf.edu/files/ail/AlmbergEtalJVGR08.pdf