Quaternary Volcanoes > Active Volcanoes > Izu-Oshima

Activities after 19th century

   After the eruption of An-ei, medium-scale activity ( amount of ejecta tens of million tons ) continued with intervals of 30 to 40 years. Smaller eruptions tend to follow medium-scale ones. The following descriptions are mostly for medium-scale eruptions.

1876 - 77: After the large-scale eruption of An-ei, there were small scale eruptions with ash falls in 1803, 1822 - 24, 1837 - 38, 1846, and 1870. In 1876, earthquakes became active from December 27th and volcanic glow was observed. From January next year, eruption started. On January 20th formation of a scoria cone as a result of Strombolian activity was confirmed within the summit crater. The activity ceased by February 5th. Since April 1887 a pit crater appeared within the summit crater.

1912 - 14: From the night of February 23rd 1912, lava started to outflow within the Mt. Miharayama crater reaching about 35 m thick by June 1st and the scoria cone reached about 100 m high. The eruption temporarily stopped by June 10th which was followed by collapse of crater bottom accompanied by a little amount of lava flow extrusion. On September 16th 1912, the bottom of the crater was filled by new lava with formation of new scoria cone. This eruption was larger than the previous one, but became inactive by October 30th. In May 1913, the part of the crater where pit crater was located subsided about 60 m, and SE half of the new scoria cone fell into this hole. From midnight of May 15th 1914, an explosive activity commenced with bombs reaching peripheral areas of Mt. Miharayama crater. Ash falls were observed in Senzu and Habuminato. The lava flow within the Mt. Miharayama crater came up within 14 m of the lowest point of crater rim. After May 26th explosion the activity became quiet accompanied with subsequent subsidence of crater floor. In 1915 and 1919 were ash and scoria eruptions, and in 1922 - 23 were small-scale lava extrusions. After the activities of 1922 - 23, pit crater was rapidly enlarged and explosive activities within the vent were repeated in 1933 - 1940, but the activity stopped thereafter.

1950 - 51 ( 25th to 26th years of Showa ): A new eruption started from pit crater at 9:15 July 16th. There were no indications of this except for a volcanic tremor felt at 9:07 on the same morning. Scoria cone was formed and lava started to flow filling pit crater and crater floor by September 13th and flowed down to the flanks of Mt. Miharayama. The activity ended on September 23rd and the lava stopped to flow on 24th. In 1951 next year, an activity with lava fragments being thrown out started on February 4th followed by lava flow on February 13th. After February 28th the lava overflowed crater rim of Mt. Miharayama down to caldera floor, but the activity ended at the beginning of April.

   Intermittent explosive activities occurred from April to June 1951. At the eruptions of June 14th and 15th, the smoke column reached 5,000 m high. On June 27th there occurred quite large explosion which brought considerable ash fall to Motomachi. The eruption ended by 28th and pit crater re-appeared on the crater floor.

   From 1953 to 56, there were Strombolian eruptions and explosions accompanied with ejections of scoria within pit crater. The eruptions became more active from August 1957 accompanied with emissions of abundant smoke and explosions. On October 13th a violent explosion occurred, which formed a new crater, causing one death and 53 injuries among sightseers and others. From 1958 to 1964, explosive eruptions continued with volcanic glows, air shocks, and ash falls. From 1964 to 1969, and in 1974, Strombolian eruptions and ash falls lasting for a few days to ten days within the bottom of the pit crater were repeated. From the last half of 1974 the activity became quiet.

1986 - 87: In July 1986, volcanic tremor was observed for the first time for 12 years and the tremor became continuous from October 29th. Rising steam was observed from the side of the summit pit crater on November 12th.

   At 17:25 on November 15th, an eruption started at the south wall of pit crater ( Crater A ): Lava fountain up to 200 - 300 m high with smoke reaching 3,000 m high. The activity of crater A changed to Strombolian after 2 - 3 days. Lava continued to flow out at a rate of 3.6 million cubic meters per day filling pit and crater of Mt. Miharayama. It overflowed the crater rim near the lookout point at about 10 a.m. on 19th and flowed down to the caldera floor ( LA lava flow ; Fig. 5 ). The eruption and volcanic tremor became quiet from about 23:00 of 19th followed by intermittent explosions afterwards.

   In the morning of November 21st, explosions resumed and some of the explosions were accompanied with rising black smoke and visible shock waves. Air shocks were felt in northern Kanto and southern Tohoku regions. From about 14:00, a swarm of earthquakes was observed on the north of the caldera, and near the rim of caldera the earthquakes were felt by people. At 16:15, a fissure with NW-SE direction opened up on the NW caldera floor and eruption from it commenced ( Fissure B ). At 16:44, crater A also resumed eruption. Fissure B continued large-scale lava fountain activity extruding lava to the north and NE directions ( LBI and LBIII lava flows ). The eruption column reached 16,000 m high and blown to the east by prevailing wind, and scoria and ash fell to the eastern parts of the island ( Fig. 5 ). At night on the same day ash fall was observed at Tateyama, Boso Peninsula, Honshu. At 17:47, from the extension of Fissure B on a slope outside of the caldera a new fissure eruption started ( Fissure C ), with lava flows coming down from them. The lava flows flowed down along a valley to Motomachi reached 70 m from Motomachi crematorium ( LC lava flow ). Combined Action Headquarter issued an order that every resident be evacuated from the island.

   Between midnight of 21st and early morning of 22nd the eruptions from Crater A and Fissure C stopped. Fine-grained ash eruptions from Fissure B near Kengamine continued on until the morning of 23rd. In the afternoon of 23rd, a small lava flow about 300 m was discovered on the NE side of Fissure B. On the NW side of the island from 21st and on the SE side from 22nd, frequent earthquakes were felt. Many open cracks were formed mainly in the NW and SE parts of the island, most of them were in the same direction with main fissure craters. Leveling survey carried out in December 1986 revealed that NW to SE sides of the island had subsided with the eruptions. In the morning of December 17th, volcanic tremors were observed and at 17:23 of 18th a Strombolian eruption started from Crater A and continued until 19:30 when it became quiet. After the eruptions of 1986 new hot springs and fumaroles appeared in several places such as Koshimizu, Motomachi.

Ejecta: Total amount of ejecta from November 15th to 19th was about 29,300,000 tons, and that for November 21st was 29,000,0000 tons. Ejecta from the summit crater of Mt. Miharayama was pyroxene basalt at SiO2=52.5-53.2 wt% with visible plagioclase phenocrysts, while those from fissure craters ( Fissure B and C ) were aphyric andesite to dacite showing wide range of compositions with SiO2=54.5-67 %. Ejecta from crater A show compositional gap with those from fissure B and C indicating that magma reservoirs for crater A ejecta and those from fissure B and C were different and independent to each other.

Activities after the eruption of November 16th 1987: Since about July 1987 earthquakes with their foci located at the summit increased, and fumarolic activity aligned in the shape of a ring around old pit crater intensified. At 10:47 on November 16th a violent explosion with a big bang occurred and lava which filled the pit crater was scattered in fragments causing about 30 m sudden collapse of the crater. Explosion was repeated again on 18th and about 150 m deep pit crater with diameter of about 350 to 400 m was formed. On January 25th and 27th of the next year small eruptions followed. Sulfurous acid gas emitted from the summit crater caused damage to agricultural crops by this activity.

   After April 1990, the activities of volcanic smoke, earthquakes, and tremors became weak. However, from mid August earthquakes at the summit area became active again and in the early morning of October 4th a small eruption occurred. On the western and northeastern foot of Mt. Miharayama a little ash falls was observed. Fist-sized cinders were ejected for a couple of hundred meters away at the summit crater of Mt. Miharayama. At the northern part within the pit crater, a collapse crater about 100 m across was formed. Materials ejected by this eruption were preexisting rocks around the vent without any new magma fragments being involved.

   After this eruption, not much new activities have been observed except for intermittent scattered earthquakes and volcanic tremors. On tiltmeter and distance survey data, the mountain is on inflating trend suggesting that magma has been supplied continuously beneath the mountain.

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