(Oct. 19, 2001)
Small eruption was observed at Idogahama (NW coast) of Iojima on the morning of Oct. 19. JMA issued the Volcano Observation Report on this volcano No. 3 in the afternoon of this day.
The Japan Maritime Self Defense Force reported grayish white-colored plume rising as high as 200-300 m for 2 to 3 minutes around 07:25 (JST) at Idogahama. The following plume rising of a similar scale was observed for 10 minutes since 08:06.
Idogahama had experienced small phreatic eruption in 1982. The latest offshore eruption just occurred in SE of the island on Sept. 21-22, 2001.
JMA reported the observation results on this eruption in the evening of this day, as follows;
"JMA inspected the eruption activity from a helicopter, being supported by both Japan Maritime Self Defense Force and Japan Air Self Defense Force. Soon after receiving the news of the first eruption at 07:25 on Oct. 19, JMA dispatched a team to Iojima, and observed the eruption condition during 16:02-17:15 and measure the temperature with infrared camera. The eruption site is located in northwestern coast of the island. The main crater formed this time was about 10-m long, and 2 to 3-m deep. Intermittent gushing of black water containing debris was observed every 10 minutes (as shown photos below). The maximum height of white plume reached 600 m during the inspection. A temperature inside the crater was 56 degrees C."
Information contact: Volcanological Division, JMA, tkanno@met.kishou.go.jp
(9/24/2001)
JMA issued Volcano Observation Report No.2 on Iojima in the September 22 afternoon, stating that the volcanic activity was going into the usual level. The number of earthquakes ranges from zero to 4 an hour after the September 22 noon. Isolated and continuous tremor events that were recorded from the September 20 night to September 22 morning had disappeared.
Photos of submarine eruptions at Iojima (September 21 and 22, 2001).
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Captions
1: Sea water ripples and breaker due to submarine (offshore) eruption, taken just after the climax eruption (1-3 p.m.), during 4-5 p.m., September 21, 2001
2: Three sites of offshore eruption can be shown, at one of which a pyroclastic cone appears on the sea surface. Taken during 4-5 p.m., September 21, 2001
3: Facing to the SW of the island. Eruptions colse to the beach of Okinahama. Taken during 4-5 p.m., September 21, 2001.
4: View of SE part of the island taken during 1-2 p.m., September 22, 2001. Arrow indicates the site of submarine eruption.
5: The top is the Okinahama beach line. White-breaker part near the beach is on pyroclastic cone. Arrows indicate the site of eruption. Taken during 1-2 p.m., September 22, 2001
(Photos #1-3 by Mr. Odai, and #4-5 by Mr. Nakahori, Volcanological Division of JMA, being supported by Japan Maritime Self-Defense Force and Japan Coast Guard)
Information contact: Volcanological Division, JMA, tkanno@met.kishou.go.jp
(09/21/2001)
JMA issued the Volcano Observation Repot No. 1 on Iojima Volcano. Submarine (offshore) eruption occurred in the SE coast of the island. This issue is first on this volcano. Maritime Self-Defense Force, stationed in this island, had observed several m-height sea-water gushing near the coast and discolored area as long as 300-400 m that started at 10:15 (JST), September 21. Around 1 p.m., water gushed several tens of meters above the normal sea level, and the accompanied steam rose up to 100-300 m. The JMA staff confirmed intermittent rising and ballooning of sea water in 2 points colored in dark gray from the air during 15:15-17:15. Water-rising (or -ballooning) points were surrounded by bubbling, white-colored swelling-water area and, furthermore, by emerald green water. The two points were 50 m away from one another, 150-200 m off the SE beach of the island (Okinahama). The temperatures of the western and eastern water-rising points were measured about 33-34 and 50 oC, respectively, by infrared themometer; the surround water temperature was 27 oC. Water-rising occurred every a few to ten minutes in the western point. Discolored zone developed from these points, about 8 km long by about 500 m wide. Though the hourly number of earthquakes was usually 1 to 2 times, it increased to be 30 times during 10-11 a.m. of this day. However, the number decreased down to 10 times by 3 p.m.
Information Contact: JMA, tkanno@met.kishou.go.jp; NIED, ukawa@geo.bosai.go.jp.
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Background
Iojima is 1250 m south of Tokyo, and a low and flat volcanic island of 8.5 km long in NE-SW and 4.5 wide, surrounded by steep cliffs under the sea. It is the summit of a large volcano with the base of 40 km across and the depth of about 2,000 m below the sea surface. Iojima itself is composed of edifice filling inside the 10-km across caldera under the sea. Rocks are andesite enriched in Na and K. Fumaloric activity is observed extensively on the island, and small phreatic eruptions often occurred. Recent eruptions took plases in 1982, 1994 and 1999. Uplifting rate of this island is very high; for example, 11 cm/year during 1911-1952 and 33 cm/year during 1952-1968.
National Research Institute for Earth Science and Disaster Prevention (NIED) keeps seismological monitoring at four stations in this island including one maintained by the Japan Defense Agency.
Though no public resident in this island, about 350 persons of the Japan Maritime and Air Self-Defense Forces station.
Photo of Iojima Island (N is right-upper). Courtesy by National Defense Academy in Japan