Komori-san on Mt. Hossho
Crunch…crunch…crunch…
Gravel crunches under my feet as I make my way up the long, rocky slope. This used to be a road, but I only catch occasional glimpses of asphalt; it’s mostly buried under tons of rockfall. The early afternoon sun is burning down on me, and the sweat is rolling off, dripping in my eyes, making them sting. My hard-hat doesn’t offer any protection from the sun (I have to wear it, though–it’s the rules), and, although it’s not very heavy, my packboard is digging into my back. These packboards were made to haul geophysical equipment, and comfort obviously didn’t figure into the design. I look back over my shoulder, but I don’t see anyone behind me. I can’t help wondering if they sent me ahead because they figured I’d need extra time, or because they thought I might not make it at all, and nobody wanted to be around when I collapsed?
I’m high up on the eastern slope of Mt. Hossho, hauling equipment for an EDM (Electronic Distance Measurement) survey. Mt. Hossho is one of about 20 lava domes and cinder cones that make up the Kuju volcanic complex, an active volcano north and a little east of Aso that is part of the Aso/Kuju National Park– one of Japan’s oldest national parks. They’ve been doing these surveys ever since the eruptions of 1995. Immediately after the eruptions they surveyed it every month or so, but the last few times it’s only been once per year. Looking around, I get the feeling they might survey it even less frequently in the future…
When we left from the Aso Volcanological Laboratory early this morning we were all under the impression that we could drive to the survey stations, because a high-altitude road ran from the town to the old sulfur works where the stations are located. This naive idea began to tarnish as soon as we came to the first river crossing. The water in the stream was shallow enough, but moving the rocks out of the roadbed by hand was more work than anyone expected. Just after lunch we gave up trying to drive, put everything on the packboards, and started up the hill.
The "road" up Mt. Hossho. What remains of the road is visible on the lower right-hand side of the photo.
Eventually, I turn the corner on the road and start up the valley that leads to the survey points. As I do, I catch my first glimpse of the fumarole area. At the sight of the white plumes of vapor streaming out in the wind, I forget all about the packboard, the sun, and my aching feet.
Fumaroles on the eastern slope of Mt. Hossho, Kuju complex.
Kuju was dormant for hundreds of years until 11 October, 1995, when a phreatic eruption blew 20,000 cubic meters of ash into the air, and opened new lines of vents on the eastern slope of Mt. Hossho. Another eruption followed in December 1995, and many thousands of earthquakes under the Kuju volcanic complex have been recorded since. Most of the seismic activity is centered at a depth between 800 meters above sea level and 1000 meters below sea level. A strong high-frequency attenuation of the seismic waves indicates there is probably a zone of fluids (steam, water, and gas) underneath the volcanic complex. Those fluids are believed to be the reservoir that feeds the fumaroles on the eastern slopes of Mt. Hossho.
EDM (or GPS) surveys on active volcanos are a common activity. The reason is that volcanos often swell up before they erupt. No, I’m not kidding. As magma and volatiles (like water, steam, and carbon dioxide) flow into the shallower reaches of a volcano preparatory to an eruption the ground actually begins to inflate; individual points can gain on the order of a meter (3 feet) of elevation. Often, this deformation can be detected by a distance or GPS survey.
Unfortunately, no one thought to do an EDM survey on Kuju BEFORE the eruption. To be fair, the thing had been dormant for hundreds of years! But now that it’s given a little burp, someone decided maybe they’d better keep an eye on it. The last ten years of surveys have all shown the same thing: a decreasing trend in volume. Most likely this is because the mountain has been venting gas through the fumaroles; like slowly letting the air out of a balloon, Kuju is deflating.
Inoue-san setting up the EDM
By the time everyone catches up with me I’m already at the first base station for the survey. They don’t really need me, other than to get the gear up here; after all, I’m a hydrogeologist, not a geophysicist. So, while they’re setting up, I decide to take a walk over to the fumarolic area.
The abandoned sulfur works on Mt. Hossho. Some fumarole activity is visible at the upper right.
This used to be a sulfur mine, but it’s been abandoned since the eruption. I wander around below the fumaroles, unwilling to go up into the cloud of vapor without a gas mask. The air is thick with fumes, in spite of the breeze. There are chunks of native sulfur scattered around everywhere; I pick one up and put it in my pocket “for luck.”
A native sulfur sample. This sample came from Mt. Hossho, Kuju complex. (Scale about 25mm/1 inch.)
Volcanologists used to think that volcanos swelled before an eruption because magma was being injected into the magma chamber, and lots of modeling was done to estimate the magma volumes on the basis of how much swelling was observed. Recently, however, it has come to people’s attention that some percentage of the swelling is not due to magma at all, but to the volatile components that come with the magma. So now we have to re-think all those calculations of magma volumes. I say, “good!” More work for hydrogeologists.
By the time I get back from the fumaroles everyone is ready to move to another base station, and an hour or two later the EDM survey is finished. As we pack up to head back to the car, someone mentions how strong the smell of sulfur is. It’s true. The air reeks of it, and I know the odor will cling to us for the rest of the day. “Smell of freedom” I say. Everyone stops packing and looks at me.
“Well, when I’m with my students at hot springs they sometimes complain about the rotten egg smell. I always say ‘it’s the smell of freedom,’ because, you know, instead of being in an office or a classroom or somewhere, we’re outside in the middle of all this…” I wave my hand around vaguely at the scenery.
Everyone smiles and shrugs. I can’t tell if they know what I’m talking about or not, so I just slip on my packboard and pick up an extra tripod that’s laying on the ground near my feet. It’s all downhill to the car, there’s a cool breeze blowing, and I’m up on a volcano in Japan. Suddenly, I don’t care anymore about the sun, or the rock slide across the road, or the packboard digging into my back–I’m just happy. After all, this is one of the reasons I wanted to be a geologist in the first place.
The author, a very happy man, on Mt. Hossho. Photo by S. Komori.
