Ounces (volcano)

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Ounces
Directions to the ounce-dake

Directions to the ounce-dake

height 1486  m
location Nagasaki Prefecture , Japan
Coordinates 32 ° 45 '24 "  N , 130 ° 17' 40"  O Coordinates: 32 ° 45 '24 "  N , 130 ° 17' 40"  E
Unzen (volcano) (Nagasaki Prefecture)
Ounces (volcano)
Type active complex volcano
Last eruption 1995
Satellite image of the Shimabara Peninsula with the ounce

The ounce ( Japanese 雲 仙岳 ounce-dake ) is a volcanic complex in Japan . It consists of several peaks of stratovolcanoes and craters and is located near the city of Shimabara in Nagasaki Prefecture on the Shimabara Peninsula of Kyushu Island . Its volcanoes are among the most dangerous in the country.

Currently, the Fugen-dake ( 普賢 岳 ) with 1359 meters and the Heisei-Shinzan ( 平 成 新 山 ) with 1486 meters are the highest peaks. The latter got its name after the time it was created in the early 1990s. According to the Japanese calendar, this was the early Heisei period , which began in 1989 with the accession of Akihito - tennō .

The ounce was best known for its most recent phase of activity from 1989 to 1995, which ended a 197-year dormant period and in which it produced thousands of pyroclastic flows that reshaped the landscape. Nearly 50 people were killed in the outbreaks.

Topographical and geological classification

The ounce is located in the very south-west of Japan . On the island of Kyushu in the west is the Shimabara Peninsula, in the middle of which is the volcano. The city center of Shimabara, the main town on the peninsula, is 6.7 kilometers east of the highest peak.

The ounce was formed as a result of tectonic activity from three lithospheric plates that meet off Japan. In connection with the volcano, the subduction of the Philippine plate is particularly relevant. This heavy oceanic plate dips below the lighter continental Eurasian plate in the southern part of the Japan Trench . The wedge above the plate kink is then melted due to fluids that escape from the crust portion of this plate into the mantle. The resulting magma has a lower density than the surrounding mantle rock and rises vertically due to the buoyancy. It looks for a way to the surface of the earth.

However, the volcano is located within an east-west trench system and thus lies exactly between a basaltic volcanic province in northwestern Kyūshū, which has been active for around 15,000,000 years, and the south-west Japanese volcanic belt near Shibahiki, a good 70 kilometers south of the subduction . This trench system expands by around 14 millimeters per year, while at the same time it sinks by two millimeters. Volcanologists see this trench as an extension of the Okinawa Backarc Basin , which extends southwest along the Ryūkyū Islands into the East China Sea . It was created by the so-called "slab roll-back effect". This is characterized by the fact that the angle of immersion of the subducted plate becomes steeper. This shifts the subduction zone to the east ("rolls" back) and the Eurasian plate above it is pulled and stretched. This expansion ultimately leads to the creation of faults and ditches and to the thinning of the crust. The underlying jacket area is relieved of pressure, which in turn causes the formation and rise of melts. The area of ​​the trenches and faults is referred to as the backarc basin (i.e. as located behind the arch of the island ), which explains the name of the basin.

Because of its location, the ounce is part of the Pacific Ring of Fire , a volcanic belt that stretches around almost the entire Pacific Ocean .

The lavas of the ounce are very thick and known under the name Dazitlava . They usually have a silicon dioxide content of almost 65 percent and are rich in volatile elements. Since they do not drain away quickly, they form an above-average number of lava domes in the summit area .

Geological evolution

Geological formations

The Shimabara Peninsula has been shaped by volcanism for millions of years . The oldest volcanic deposits in the region are around 6 million years old. Extensive eruptions occurred over the entire area of ​​the peninsula 2,500,000 to 500,000 years ago.

At that time, the earth's crust was folded into a trench , parts of the peninsula sank up to 1,000 meters deep in the sea. The rift formation could be the cause that the volcanic activity concentrated in one place and the ounce began to form in this rift. Dazite lava eruptions began at a location a little south of today's ounce and shifted northward over time.

Volcanic rock with basalt rock fragments from ounce

For the first 200,000 years, the volcano grew rapidly and formed a large cone. Later eruptions over the next 150,000 years filled a large part of the rift. Initially, the activity was characterized by andesitic , blocky lava and pyroclastic surges , but 500,000 to 400,000 years ago the eruptions changed to focus on Dacite lavas, pyroclastic flows and debris falling from the air. In the period from 400,000 to 300,000 years ago, pyroclastic currents and lahars left behind the deposits that form the main part of the volcanic plastic fan that surrounds the volcano. 300,000 to 150,000 years ago, mighty phreato-magmatic deposits were deposited, which are formed by contact of hot melt with water, which indicates a rapid filling of the trench by pyroclastics of the volcano at this time.

Activity for the past 150,000 years has taken place in a number of different locations within the volcanic complex. This created several peaks (in the order of height,  map with all coordinates: OSM | WikiMap ): f1Georeferencing

Most of the eruptions over the past 20,000 years have occurred at Fugen-dake, only six kilometers from the center of Shimabara City.

Activities until 1990

The first documented and observed eruption of the Unzen dates from 1657 and lasted with short interruptions until 1663. The volcano experienced its most violent eruption in historical times on May 21, 1792 with a large Dazzite discharge from the Fugen-dake. After the eruption appeared to be over, the eastern flank of the Mayuyama collapsed unexpectedly as a result of an earthquake. The resulting avalanche slid into the ocean at up to 200 km / h and triggered a 20 meter high tsunami that almost completely destroyed the city of Shimabara and killed more than 15,000 people. This was the most momentous geological eruption in Japan to date.

Activity phase from 1990 to 1995

After the great eruption of 1792, the ounce was considered temporarily inactive. In November 1989, seismic activity in the vicinity of the volcano began again when weak swarms of earthquakes were registered at a depth of 20 kilometers under the Bay of Chijiwa - about ten kilometers west of the mountain . Their hypocenter migrated eastward over time until it was under the fugue-dake in July 1990. Immediately afterwards, the measuring stations recorded the first volcanic tremor . Tremors are regular tremors that are generated by the rise of magma or the oscillation of gases in the volcanic vents. In the months between August and November 1990 both the earthquakes and the number of tremors intensified and increased. The Japanese volcanologists on site expected an eruption.

The ounce from a slightly southeastern viewing direction. Extensive, cooled pyroclastic currents and lahar deposits can be seen.

The first phreatomagmatic eruption occurred on November 17th. In the course of this activity, two small temporary craters formed . After the brief eruption ended, a committee was formed to organize surveillance and measures to protect the population. While hardly any activity was detected in December, it increased again from January 1991. In the afternoon of February 12, there was another, this time stronger, phreatomagmatic eruption from a newly formed third crater, after new fumarole fields had already formed on the slope in the morning . Alerted by the increasing activity of the ounce, the authorities developed a plan to evacuate 16,000 people living in the danger zone (within ten kilometers of the mountain) of possible pyroclastic eruptions.

The eruptions lasted until March 29th and did not reach the strength of February for the time being. This changed at the turn of March / April when they strengthened again. In April the ounce's seismic activity remained virtually unchanged. By mid-May, however, scientists measured an increase in earthquakes and tremors under the mountain. On May 15, a first lahar - triggered by rains that had combined with the thin ash deposits - poured into the valley of the Mizunashi River (more mud rivers formed in the next few weeks). Then began the evacuation of the population. Five days later, on May 20th, a first lava dome rose up in the shallow Jigokuato crater.

Many more were to follow him. The dome grew rapidly accompanied by tremors and soon reached the crater rim, beyond which it grew. Soon the entire top of the ounce was covered with it. The destruction of the cathedral began on May 26th, when a small piece of it broke off and the previously cohesive bubble-filled lava broke into many small hot melt fragments, which created a primary pyroclastic current on the mountain flank , which reached a length of 2.5 kilometers.

Towards the end of the month, 35 pyroclastic currents formed on the volcano in just one day. Although most of them were almost insignificantly small, the unusual accumulation of this otherwise rather rare phenomenon has now made more and more European volcanologists aware of the activities of the Japanese mountain. Many arrived only a few days later and set up camp together with journalists and photographers at the foot of the mountain in the evacuated zone.

Overview map of the pyroclastic flow of the Unzen from June 3, 1991.

On June 3, the long-awaited breakout of the ounce began. This was possibly the result of a pressure relief of the magma column after a landslide in the crater. The volcano ejected clouds of ash . A pyroclastic flow was formed, the extent not previously recorded or observed at the ounce. He used the Mizunashi river valley for his course. But since this was almost filled up by the lahars that had already passed in May, the current jumped over to neighboring valleys. 43 scientists and journalists who drove cars from their camp into what they believed to be safe valleys died. Among the victims were the most famous volcanologists at the time: the French couple Katia and Maurice Krafft and the American Harry Glicken . Your colleagues were able to follow the course of the pyroclastic flow from the base before it came to a standstill after 4.5 kilometers.

The evacuation of local residents, which took place in several stages, was completed on June 7th without incident. On the following day, another pyroclastic current that had separated from the lava dome exceeded the range of the current on June 3 by one kilometer. Like all previous ones, he rolled down the eastern slope of the Unzen. On June 9, the local geologists noticed that a second lava dome had formed on the edge of the first one. From this new pyroclastic currents arose in the following months. On June 11th, part of this new cathedral erupted. This explosion hurled pumice stones into the nearby coastal town of Shimabara . The local people were advised to stay in their homes. On June 17, after another strong eruption, an ash shower fell over the city of Obama (today: Unzen ) located 9.7 kilometers southwest of the mountain on the other side of the Shimabara Peninsula on the coast .

For three years, the lava continuously filled the summit craters and by 1994 eleven more lava domes were observed, which during this time were responsible for at least 10,000 small to tiny pyroclastic currents that now slid down the east, north and south flanks.

A house destroyed by the 1991 eruption

From 1993 the lava output gradually decreased and in the middle of 1994 the bulging of the eleventh lava dome slowed down significantly. A year later, in the summer of 1995, the only isolated eruptions subsided and finally subsided completely. The surface activity of the volcano became Solfatarian . Despite the calm, several dozen earthquakes and a few tremors were still registered each month.

A good four and a half years had passed between the first and the last volcanic activity visible on the surface during this eruption phase. Around 2,000 houses were destroyed during this time. Apart from the scientists who died in 1991, the outbreak did not claim any further victims thanks to good protection and evacuation planning. Since the end of the eruptions, heavy rains have regularly set pyroclastic material in motion and triggered new, albeit smaller, mudslides.

The ounce itself has changed its face seriously. Before 1990 it was an overgrown green mountain, but today its summit region is bare and desolate due to the numerous pyroclastic flows and lahars that have passed away. The eastern flank is particularly badly affected. There, the vegetation-free zone stretches down into the valleys, in which volcanic deposits that have hardened meters high are still partly to this day and no plants grow. Many square kilometers of arable land around the volcano will be unusable for years and several thousand people have had to be relocated. Some have not been able to return to their homeland to this day.

Exploration and monitoring of the volcano

The photo from the beginning of 2007 shows a concrete drainage channel with the Fugen-dake in the background and the nearby Shimabara in the back. The direction of view is to the west.

The Unzen is considered to be one of the best explored and monitored volcanoes in the world. A network of seismographs and other geophysical measuring stations has stretched across it, not just since the most recent eruptions . There is also a volcano observatory on its base .

The volcano was declared one of the 16 volcanoes of the decade as early as 1991 as part of the United Nations ' International Decade to Combat Natural Disasters . This was justified with its history of violent eruptions and its location in a densely populated area.

To protect Shimabara and the smaller villages near the mountain from further pyroclastic currents, after the eruption, concrete drainage channels began to be built on the eastern flank. These attach to the ends of the old pyroclastic deposits and are supposed to direct new currents in a controlled manner in the event of a renewed eruption. The two main channels meet in a basin almost 600 meters wide. From there the drain continues through Shimabara and into the sea. However, not all of the stretches to the reservoir have been excavated. In some cases, only high concrete walls serve as protection, which are supposed to keep the currents in the train. The longest canal has a length of 5.8 kilometers from the slope to the ocean.

In 1999 plans were developed to drill a hole into the interior of the volcano. The aim was to advance into the chimney and collect magma from the eruption four years ago. In addition, some basic questions of volcanology should be clarified, such as

  • why the magma repeatedly uses the same channels, although at the end of each eruption these are clogged by cold rock and
  • how the magma can lose enough gas on its ascent to freeze as it flows out rather than explode.

The researchers were also interested in the fact that hardly any gases were found in the ounce lava during the eruption from 1990 to 1995. It was assumed that these remained in the chimney during the ascent.

However, the project, which was carried out under the direction of the geologist Setsuya Nakada of the University of Tokyo , harbored numerous risks. An active volcano had never been drilled into and the scientists could not be sure that they would not encounter a gas bubble and thus provoke another eruption.

Temperatures of up to 600 ° C were assumed in the chimney. In order to prevent the drill rods from melting, they were permanently cooled with cooling liquid which was pumped down into the depths by pumps driven by diesel engines. First, test drilling began to check the feasibility of a deep borehole. Two holes 750 meters and 1,500 meters deep were drilled. The recovered drill cores were used to better understand the history of the ounce eruption. Another hole 350 meters deep was used to test methods for the actual deep drilling project.

The actual main drilling, for which 60 drill rods each 30 meters long, began in February 2003 on the northern flank of the volcano. The bore had a diameter of 44.5 centimeters and initially led into the volcano at an angle of 25 ° from the vertical. It was soon discovered that the inside of the mountain must be very rugged, because the cooling water from the drill seeped away after a while. This circumstance finally led to a four-month interruption of the project because those responsible had run out of money. After drilling was resumed, the drilling channel was swiveled at greater depths in the direction of the eruption channel and, at a depth of 800 meters, an inclination to the vertical of 75 ° was achieved. When the planned drilling depth of 1,800 meters had been reached, the vent had not yet been hit. This was only reached in July 2004 with a borehole length of 1,995 meters (other sources speak of 1,550 meters). The depth below the summit was 1,500 meters at that time. The drilling was thus completed, but the investigations were only just beginning.

Amazingly, the temperature in the chimney was only around 155 ° C. The geologists cited strong ramifications in the outbreak channel as the reason for this, so that the smaller amounts of magma could have cooled down more quickly. As expected, a lot of gas was found in the chimney, both in the form of inclusions and gas bubbles. However, these did not pose a threat to the drilling. The researchers also suspect that the porous rock accelerates the degassing of the magma into the open air. This would explain the low gas content of the magma.

The drill cores extracted from inside the ounce were examined and evaluated in a laboratory in Japan. Rock samples are now to be sent to volcanologists around the world.

Religious and touristic importance

The first traces of settlement in the area around the Unzen date from the year 701. At this time the famous Buddhist monk Gyōki founded the temple Manmyō-ji ( 満 明 寺 ) at the foot of the mountain . The temple's sphere of influence expanded steadily and soon it was known as "Koya-san in the West" (Koya-san is a temple and monastery complex south of Osaka ). At times, over 1,000 monks lived in asceticism within the walls. The Manmyō-ji fell victim to an arson during the Shimabara uprising of 1637, but was able to be rebuilt two years later and still stands on the slopes of the Ounce today.

In ancient times, according to the Hizen no Kuni Fudoki , the mountain was called Takaku-mine ( 高 来 峰 ). In the early days of the temple, at least since the early 8th century, it was referred to as ounce-zan, but in the spelling 温泉 山 . This means "mountain of hot springs ".

religion

This name is derived from the Unzen jigoku , the ounce hell, which lies below the crater of the Unzen on the flanks of the mountain. This is a wide field of fumaroles, thermal springs and hot mud pots, some of which reach temperatures of well over 100 ° C and emit sulphurous vapors. Over the centuries the landscape there has taken on a chalk-white color due to the mineral deposits, reminiscent of the cooled lavas of Ol Doinyo Lengai in Tanzania . The area, in which there is hardly any vegetation other than pine trees , is a preferred habitat for ravens.

The ounce jigoku (the ounce hell)

In the 16th century, the ounce jigoku was the scene of numerous tortures of Christian believers and criminals. After the ban on Christianity in Japan , around 30 Christians are said to have been burned in particularly hot springs. To commemorate these cruel deeds, a simple wooden cross was placed on a rock.

The locals still consider the Unzen to be a sacred mountain. For this reason, they were initially very critical of the drilling project, since they considered it wrong to disturb the volcano in its recently regained calm. Therefore a Shinto priest had to be called in. He gave the ounce his blessing and then consented to the drilling.

Tourism and nature protection

In 1693 the Shimabara feudal lord Tadafusa Matsudaira ordered that the environment in the vicinity of the volcano had to be protected. He banned the killing of both birds and animals living in the ground in the area, as well as the picking of wild azaleas . More than two centuries later, the area around the Unzen was declared Unzen Amakusa National Park on March 16, 1934 . This first national park in Japan also includes some of the Amakusa Islands with an area of 28,289 hectares .

As early as 1653, Zenzaemon Kato opened the Enreki-yu, the first spa in the Unzen-jigoku. It offered hot steam baths for the sick. Today around 30 thermal springs and fumaroles are used commercially in the Unzen-jigoku and are grouped under the name of Unzen-Thermen. Hiking trails lead through the steamy landscape.

Although the tourist industry had to cope with several setbacks as a result of the eruptions from 1990 to 1995 - such as the destruction of the infrastructure, massive changes to the landscape and impairment of guided tours - the area around the volcano is now as well opened up for tourism as before . The most popular offer for travelers is the drive up to Fugen-dake crater. This can be reached after a three-minute ride on a cable car from the Nitta Pass. You can also hike to the summit of the same name. From there, on a clear day, you can see the Aso volcano, 76 kilometers away .

literature

  • H. Hoshizumi, K. Uto, A. Matsumoto: Core stratigraphy of the Unzen Scientific Drilling: Volcanic History of the Unzen Volcano, Kyushu, SW Japan . American Geophysical Union, Fall Meeting 2001.
  • H. Hoshizumi, K. Uto, A. Matsumoto, A. Kurihara: Growth History Of Unzen Volcano, Kyushu, Japan . American Geophysical Union, Fall Meeting 2004.
  • S. Sakuma, S. Nakada, K. Uto: Unzen Scientific Drilling Project: Challenging drilling operation into the magmatic conduit shortly after eruption . American Geophysical Union, Fall Meeting 2004.
  • K. Uto, H. Hoshizumi, A. Matsumoto, K. Oguri, H. Nguyen: Volcanotectonic history of Shimabara Peninsula and the evolution of Unzen volcano in Southwest Japan . American Geophysical Union, Fall Meeting 2001.
  • K. Uto, S. Nakada, H. Shimizu, S. Sakuma, H. Hoshizumi: Overview and the achievement of the Unzen Scientific Drilling Project . American Geophysical Union, Fall Meeting 2004.

Web links

Commons : Unzen (Vulkan)  - Collection of images, videos, and audio files

Individual evidence

  1. ↑ 3 主 な 山岳 . In: 第 53 版 （平 成 18 年） 長崎 県 統計 年鑑 ("53rd (2006) Statistical Yearbook of Nagasaki Prefecture"). Nagasaki Prefecture, 2006, accessed February 10, 2014 (Japanese).
  2. ↑ Harro Hess : Haack TaschenAtlas volcanoes and earthquakes . Klett PERTHES , 2003, ISBN 3-623-00020-5 , p. 88.
  3. ↑ Ursula Schümer (Ed.): Vulkane . Könemann, 2000, ISBN 3-8290-5671-0 , p. 143.
  4. ↑ Edward Bryant: Tsunami. ISBN 978-0-521-77599-1 , p. 299. (English), accessed on May 20, 2011.
  5. ^ Before Extrusion of Lava. Tōkyō University , archived from the original on February 6, 2009 ; accessed on September 1, 2016 .
  6. ↑ Axel Bojanowski : Daring Project: Researchers drill open the volcanic gorge. In: Spiegel Online . December 21, 2004.
  7. ^ History. Unzen Tourism Association, accessed on May 17, 2008 (English).
  8. ↑ a b 雲仙 の 歴 史. Unzen Tourism Association, accessed September 1, 2016 (Japanese).
  9. ↑ ounces. ( Memento of October 14, 2009 in the Internet Archive ) at: frommers.com
This version was added to the list of articles worth reading on March 27, 2008 .