Between 270 and 90ka, Aso volcano produced four large caldera-forming pyroclastic flows, all of which probably derived from a single zoned magma chamber, as suggested by previous geological and geochemical studies. On the other hand, after 90ka, for the post-caldera activities, it is difficult to create a wide compositional range of post-caldera magmas from a single magma chamber model, when the following observations are considered. (1) Seven distinct magma groups with distinct petrographic and compositional characteristics are found; I. 2px-rhyolite, II. bt-rhyolite, III. hb-dacite, IV. 2px-dacite, V. aphyric andesite, VI. porphyritic andesite, VII. basalt-basaltic andesite. (2) Incompatible trace element plots show that the seven magma groups were formed by at least four end-member magmas that do not show parent-daughter relationships. (3) Vents extruding basaltic magmas are centered in the caldera, whereas those extruding andesitic, dacitic, and rhyolitic magmas are distributed around them. These observations indicate that several (at least four) magma chambers are developed during the post-caldera activities, and that the compositional diversity of the magmas can be explained by magma mixing among the end-member magmas in addition to fractional crystallization.

View full abstract

Hakone Volcano is an active volcano located near the northern end of the Izu-Bonin volcanic arc. Kuno (1950) reconstructed the evolution of this volcano based on detailed study on topography and volcano stratigraphy as follows : 1) development of a large stratovolcano (OS : Old Somma), 2) collapse of the stratovolcano and formation of the first stage caldera, 3) renewed volcanism leading to the formation of a shield volcano (YS : Young Somma) inside the first caldera, 4) collapse of the shield volcano resulting to the formation of the second stage caldera, 5) growth of Central Cones (CC) inside the second caldera. Our K-Ar dating performed on several lava flows, lava domes, and dykes revealed the following interpretations : 1) Ages between parasitic volcanoes on the flank of OS and YS are overlapping. 2) Ages of 'Yugawara volcano' presumed to be pre-Hakone edifice by Kuno (1950) and that of OS are indistinguishable. 3) A lava of CC obtained from a deep well shows close age to Hk-TP eruption, later stage of which is thought to be initiation of the CC stage (Hirata, 1999). From these results and previous tephrochronological and geological studies (e.g. Machida, 1977; Takahashi et al., 1999), we propose a new classification of the edifices of the Hakone Volcano and their ages as follows : 1) formation of the Lower Older Edifice [LOE] which constitute the main part of Kuno's OS lasted until the eruption of TCu-1 tephra (0.25Ma). 2) Upper Older edifice [UOE], the parasite volcanoes on LOE, was erupted from 0.25Ma to 0.11Ma. 3) Activity of the Younger Edifice [YE; corresponding to Kuno's YS] was initiated since the emplacement of Hk-TAu12 tephra (0.13Ma) and ended at 0.08Ma. 4) CC erupted from 0.065Ma to the present.

View full abstract

The eruptive activity at Asama that started on 1 September 2004 was characterized by several vulcanian small explosions, including a continuous stromblian-explosion period. About 2×10⁶m³ of magma (DRE) was issued in this eruption, most of which filled the summit crater floor. The sequence of eruptive activity was similar to that of the 1973 eruption among previous eruptions at Asama Volcano. However, the two eruptions are different in the following points. 1) Both magnitude of seismic activity and scale of eruption products are small in the 2004 eruption; about half or less than in the 1973 eruption. 2) A-type earthquakes occurred in the later stage of the 2004 eruption, while in the early stage in 1973. 3) Partial melted country rock (rhyolite tuff) is found among the 2004 eruption products but not in the 1973 ones. As the swarm of A-type earthquakes at Asama occurred in the period of inflation detected by GPS in this decade, intrusion of magma in the deep-seated magma reservoir (or dike) was associated with A-type earthquakes. It is likely that the difference of occurrence of the A-type earthquakes in the both eruptions reflect different magma supply rates; larger in the 1973 eruption and smaller but long-lasting in the 2004 eruption. Continuous inflation and occurrence of A-type earthquakes even in 2005 suggest not declining of the eruptive activity but renewal near future.

View full abstract

The 2004 eruption of Asama volcano, central Japan, was characterized by emergence of small amount of andesite lava in the summit crater, strombolian explosions and a series of vulcanian explosions. There is a good correlation between the degree of differentiation of glass in juvenile ash and repose times between successive explosions. Glass in ash erupted after a long repose time is high in SiO₂ content and crystallinity. In contrast, glass in ash from continuous strombolian explosions is less evolved chemically and is poorer in microlites. The chemical compositions of the evolved glasses in the Qz-Ab-Or diagram suggest that the magma was degassed at lower pressures. This is supported by low H₂O contents of the least vesicular rinds on bread crust bombs. In addition, complexly deformed fragments of white-colored volcanic sediment containing high SiO₂ glass and silica minerals, which probably originated from beneath the volcano, shows substantial heating evidenced by partial melting of the sediment and further modification by strong shearing along the conduit wall during magma ascent. The eruptive sequence can be modeled, as follows; 1) magma emerged continuously in the summit crater, accompanied by strombolian explosions (lava emergence and strombolian stage). 2) Magma became stagnant and crystallized at a shallow level in the conduit (repose period and crystallizing stage). 3) Vulcanian eruption occurred when crystallizing magma was pressurized by addition of new magma and/or gas accumulation sufficient to overcome the strength of a lava plug present below the crater (vulcanian stage). The 2004 eruption was marked by repetitions of this cycle and the conduit probably was choked when magma supply stopped. The deformed nature of volcanic sediment entrapped as xenoliths suggests significant shearing between viscous magma and the conduit wall.

View full abstract

Asama volcano in central Japan became active on September 1st, 2004 with a vulcanian eruption which was the biggest eruption among a series of small eruptive events from September to December in 2004. The ejecta of the Sept. 1st eruption were mostly polygonal fresh andesitic lithic fragments. Most of them were derived from the andesite body which played the role of cap rock for the rising new magma and escaping volcanic gas. Less amount of pumice fragments were also ejected and most of them are mantled by the black and dense andesitic crust, which is broken and expanded to form breadcrust structure. These breadcrust pumices are concluded to have been the fragments of the new magma. After they were broken into pieces by the explosion, the outer margin of them rapidly consolidated, and then degassing and inflation in the inner melt caused rupturing of the outer crust. The pumices are clearly discriminated from the andesitic lithic fragments by their whole rock chemistry. The next vulcanian eruption occurred on September 23rd. Majority of the ejecta were again polygonal lithic fragments of andesite, although their chemistry corresponds not to the lithic fragments, but to the pumices of the Sept. 1st eruption. Small amount of scoria are found among the Sept. 23rd ejecta. Their whole rock chemistry and the assemblage and chemical composition of phenocrysts are quite similar to those of the Sept. 1st pumice. The black appearance of the scoria is derived from the less crystalline, therefore less differentiated groundmass glass in the scoria than in the pumice. From these petrologic evidences the following magmatic processes are deduced. Before Sept. 1st, a column of new magma had risen into the pre existing andesite body beneath the crater floor. On Sept. 1st, the built up gas pressure surpassed the tensile strength of the andesite body to result in a vulcanian eruption. This eruption provided fragments of the pre-Sept. 1st andesite as polygonal lithic fragments, along with smaller amounts of the breadcrust pumices. In the course between Sept. 1st and Sept. 23rd, the column of new magma uplifted because the cap rock had disappeared. The upper part of the column was cooler and more crystalline than the lower. Some portion of the upper part of the magma column effused and made a dome inside the crater. On Sept. 23rd, next vulcanian eruption took place. The ejecta consisted of the lithic fragments which had been upper part of the consolidated magma column, and the scoria derived from the lower level of the same magma column.

View full abstract

Continuous GPS measurements around the Asama volcano, central Japan, show a repetition of episodic inflations and deflations of the edifice. A correlation is found among plume height, seismicity and crustal deformation measured by the GPS. During the inflation period, increase of seismicity and rise of plume height were observed. A planar magmatic source situated at about 2km depth below the surface was inferred from the GPS data during both the inflating and deflating periods. It is also suggested that a buoyant bubble-rich magma surges episodically into the magma reservoir, then it loses the buoyancy through degassing process, and finally drains back to the further depth. Inflations of the edifice preceding the eruption were registered in the GPS data before both 2003 and 2004 crises, showing the usefulness of crustal deformation data for the prediction of the volcanic unrest of the Asama volcano.

View full abstract

A geodetic observation network with GPS and EDM at Asama volcano detected pre-and co-eruptive crustal deformation. Observed horizontal displacements by GPS before the 2004 eruption indicated radial pattern outward from the active summit crater. An east-west baseline indicated secular extension with 7mm/yr from 2002 to the occurrence of the 2004 eruption. Rapid extension of 10mm was detected at the baseline near the summit, which may suggest magma discharge into the shallower chamber. We estimated the magma supply system from the observed geodetic data assuming buried two pressure sources. One is at a height of 2200m above sea level beneath the summit crater (Source A) and another is 1.5km east of the summit at sea level (Source B). Our data indicated that before the first eruption discharged magma from the deeper to source B was 200×10³m³, and from source B to source A was 14×10³m³. Total amount of increase in volume of source A was estimated to be more than 50×10³m³ by EDM and GPS data by the end of the eruptive activity. These small but significant signals could be observed only in our network near the crater. Our results indicated that dense geodetic observation near the active crater or vent could give important and interesting data to clarify and monitor the active volcano.

View full abstract

Asama volcano, one of the most active andesitic volcanoes in central Japan, started a series of eruptions on September 1, 2004 and the eruptive activity lasted about three months. We have carried out microgravity measurements at the volcano three times; one year before, immediately after the eruption and one and a half months later. “Hybrid measurement”, combining relative measurements over an area and an absolute measurement at a reference station, was employed to observe temporal changes in absolute gravity value. The data obtained before and after the eruption shows that the gravity changes preceding the eruption are from -6 to +9 microgals, which are of the nearly same value as that of the accuracy of observations. The gravity changes predicted from the tensile fault models and Mogi models proposed for the ground deformations are always less than 1 microgal at any gravity station. The observational fact that gravity changes did not exceed 10 microgals gives some constraints on the magma accumulation in the conduit. A numerical examination suggests that the volume of the magma accumulated in one year preceding the eruption may be less than 2×10⁷m³.

View full abstract

We analyze five explosion earthquakes observed at Asama volcano in 2004. The main phase consisting of Rayleigh wave is well explained by a vertical downward single force with peak amplitude of 10¹⁰-10¹¹ N and pulse width of 5-6s. These source parameters are in the range expected from a scaling relation presented by Nishimura and Hamaguchi (1993), which suggests that the 2004 explosions of Asama volcano are typical Vulcanian eruptions. The internal pressure built up beneath the crater is estimated to be 0.2-1.5MPa, which tends to become large after the formation of lava dome in the crater.

View full abstract

On September 1, 2004, a middle-scale eruption occurred at Mt. Asama. Before the eruption, long-period volcanic earthquakes were observed with the broadband seismographs located at the summit of Mt. Asama since October 17, 2003. The signals are so feeble that we can hardly recognize them even at the second nearest station from the summit crater. The long-period earthquakes had been occurring at least before September 5, 2002. The earthquakes have very unique waveforms and can be categorized in 3 types (type 1, type 2, type 3). The sources of these long-period events are located relatively shallow at a depth of a few hundreds meters, just beneath the crater. The depth of the type 1 event is just beneath the vent, the types 2 and 3 being 100～200 meters deep. The activity of these long-period events had been synchronized with the activity of volcanic earthquakes until the last ten days of July 2004. However, the activity had dropped gradually, and no event has been observed since August 24, 2004, just 8 days before the eruption. An earthquake swarm started on August 31, lasting just before the eruption. The hypocenters of swarm, which are determined using the Double Difference method, lie just beneath the vent ranging from 300m to 800m deep, suggesting that the shallowest part of the vent is broken during the swarm activity.

View full abstract

The Asama volcano (elevation 2568m), central Japan, erupted on September 1, 2004. Since then thick volcanic fumes prevented monitoring of the topography on the summit crater floor by standard optical methods. To detect geomorphic changes of the volcano, Airborne Synthetic Aperture Radar (AirSAR) measurements including interferometry were repeatedly carried out on September 16, October 22, December 15, 2004 and March 10, 2005. The comparison of AirSAR result in September with a digital elevation model (DEM) by laser profiling measurements in October 2003 revealed that a pancake shape lava mound (volume=0.9×10⁶m³) was formed on the floor. The subsequent AirSAR measurement on October 22 indicated that the pancake shape mound had disappeared and a small pit hole was formed. The volume of ejecta amounted to 2.1×10⁶m³. The AirSAR results in December 2004 and March 2005 suggested that the shape of the crater floor stayed generally unchanged during that time window. From these, we infer that lava extrusion during the 2004 Asama eruption occurred within a short period around September and October.

View full abstract