View of Anak Krakatau from the W (May 2008). The 2007 crater is seen degassing on the left, the principle cone is in the center, the rim of pre-1960 cone is slightly to the right.
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| Location of Anak Krakatau in the Krakatau archipelago (N at top) |
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| Rough sketch of Anak Krakatau in 2008 |
Following the 1883 eruption, the former cones of Danan and Perboewatan, as well as most of Rakata disappeared into the sea. Only the S flank of Rakata remained aswell as the older islands of Sertung and Panjang. Between these a nearly 300m deep depression (the 1883 caldera) was located. No significant eruptive activity was reported between 1883 and 1927. However, in December 1927, surtseyan submarine activity was reported in an area on the NE flank of the depression which had a depth of about 180m following the 1883 eruption. The activity was initially attributed to a series of vents along a 500m fissure trending along the same NNW-SSE fault line as the former craters of Perboewatan, Danan and Rakata. Activity
progressively become focussed at a single vent. The crater rim of Anak Krakatau first emerged on 28.01.1928, only to be washed away and rebuilt before eventually establishing itself as a sickle-shaped island with a height of 38m and a length of 275m by 1929 (Stehn 1929. Proc. 4th Pacific Sci. Congress, p.1-55). The island continued to grow and the N-NW rim had reached a height of 125m by 1940, whilst the crater remained open to the sea on the S-SW side (Seibold and Seibold 1996. Geol. Rundschau 85, p.615-618). The shape of the island, which to the present day remains slightly steeper on the SW side, may be explained by its position on the flank of the submarine 1883 caldera. This provided more support for the NW portion of the island during its evolution. Eruptions of Anak Krakatau generally involve materials of basaltic or basic andesite composition. This suggests that their source is already slightly differentiated, suggesting that residual magma from the 1883 eruption may be involved or that unusually rapid differentiation of basaltic magmas from a deeper source has occurred. The upper magma chamber is located at a depth of about 9km based on S wave attenuation studies. This is thought to have been sufficiently large to account for differentiation of the 1883 eruptates. A further larger chamber is thought to lie at a depth of 22km and beyond (Harjono et al., 1989. J. Volc. Geotherm. Res.39, p.335-348). Interestingly, the center of seismic activity below the edifice appears to rise and fall vertically from a depth of 100km to the surface. It has been suggested that it does this with a cycle length of about 30 years, corresponding to the 30 year intervals between recent peaks in activity (1927-30, 1958-63 and 1988 -93). Seismic monitoring over a longer period is necessary to confirm this hypothesis (Spicak et al., 2002. Stud. Geophys. Geod. 46, p.545-565).
By 1959, the crater rim had reached a height of 152m, whilst the crater lake remained. The morphology of Anak Krakatau was significantly changed by the series of eruptions occurring in 1959-1963. These led to the emplacement of a new cone structure which was nested in the former crater and displaced the crater lake by 1960. Eruption sequences occurred on average every 3-4 years and typically lasting for about 6 months during the period from 1965 to 1988, building the new cone to a height of about 159m. During a phase of vulcanian activity in 1981, dacite bombs were erupted (62-63% SiO2), although this was unusual and did not mark the transition of the volcano into a new highly explosive dacitic phase of its long-term eruption cycle. The dacite eruption may have been the result of a residual pocket of differentiated lava from the 1883 eruption or a tiny pocket of freshly differentiated dacite at the top of the magma chamber. Anak Krakatau was particularly active in Nov. 1992 with the emplacement of two lava flows, one of which reached the sea to the NE, and between 1000 and 4000 strombolian explosions per day. Explosions continued and further lava flows (53.5% SiO2 - low Si basic andesite) were emplaced in Feb. and Apr. 1993 (Smithsonian BGVN 18:05). Several tourists were injured and one killed on the old crater rim by volcanic bomb impact on 13.06.93. By this time the new cone had reached a height of nearly 300m. Phases of minor strombolian activity were observed for much of the 1990s and up until 2002, when Anak Krakatau entered a rest phase with only sporadic discharges of gas.
On 23 Oct. 2007, grey eruption plumes were observed rising from a vent to the SW of the central crater of Anak Krakatau. These marked the onset of a new phase of activity which climaxed in mid-late November with powerful strombolian (and possibly vulcanian) eruptions being reported. Activity paused in early 2008, yet by April the eruption had resumed again. The photos in this section are from a visit from 19.-23. May 2008. A lava flow had been emplaced on the SW flank about 10 days prior to this date. The crater is notably larger than it was in November and partial collapses of the wall separating it from the main crater could be observed. It is likely that the two craters will eventually merge if activity continues.
progressively become focussed at a single vent. The crater rim of Anak Krakatau first emerged on 28.01.1928, only to be washed away and rebuilt before eventually establishing itself as a sickle-shaped island with a height of 38m and a length of 275m by 1929 (Stehn 1929. Proc. 4th Pacific Sci. Congress, p.1-55). The island continued to grow and the N-NW rim had reached a height of 125m by 1940, whilst the crater remained open to the sea on the S-SW side (Seibold and Seibold 1996. Geol. Rundschau 85, p.615-618). The shape of the island, which to the present day remains slightly steeper on the SW side, may be explained by its position on the flank of the submarine 1883 caldera. This provided more support for the NW portion of the island during its evolution. Eruptions of Anak Krakatau generally involve materials of basaltic or basic andesite composition. This suggests that their source is already slightly differentiated, suggesting that residual magma from the 1883 eruption may be involved or that unusually rapid differentiation of basaltic magmas from a deeper source has occurred. The upper magma chamber is located at a depth of about 9km based on S wave attenuation studies. This is thought to have been sufficiently large to account for differentiation of the 1883 eruptates. A further larger chamber is thought to lie at a depth of 22km and beyond (Harjono et al., 1989. J. Volc. Geotherm. Res.39, p.335-348). Interestingly, the center of seismic activity below the edifice appears to rise and fall vertically from a depth of 100km to the surface. It has been suggested that it does this with a cycle length of about 30 years, corresponding to the 30 year intervals between recent peaks in activity (1927-30, 1958-63 and 1988 -93). Seismic monitoring over a longer period is necessary to confirm this hypothesis (Spicak et al., 2002. Stud. Geophys. Geod. 46, p.545-565).
By 1959, the crater rim had reached a height of 152m, whilst the crater lake remained. The morphology of Anak Krakatau was significantly changed by the series of eruptions occurring in 1959-1963. These led to the emplacement of a new cone structure which was nested in the former crater and displaced the crater lake by 1960. Eruption sequences occurred on average every 3-4 years and typically lasting for about 6 months during the period from 1965 to 1988, building the new cone to a height of about 159m. During a phase of vulcanian activity in 1981, dacite bombs were erupted (62-63% SiO2), although this was unusual and did not mark the transition of the volcano into a new highly explosive dacitic phase of its long-term eruption cycle. The dacite eruption may have been the result of a residual pocket of differentiated lava from the 1883 eruption or a tiny pocket of freshly differentiated dacite at the top of the magma chamber. Anak Krakatau was particularly active in Nov. 1992 with the emplacement of two lava flows, one of which reached the sea to the NE, and between 1000 and 4000 strombolian explosions per day. Explosions continued and further lava flows (53.5% SiO2 - low Si basic andesite) were emplaced in Feb. and Apr. 1993 (Smithsonian BGVN 18:05). Several tourists were injured and one killed on the old crater rim by volcanic bomb impact on 13.06.93. By this time the new cone had reached a height of nearly 300m. Phases of minor strombolian activity were observed for much of the 1990s and up until 2002, when Anak Krakatau entered a rest phase with only sporadic discharges of gas.
On 23 Oct. 2007, grey eruption plumes were observed rising from a vent to the SW of the central crater of Anak Krakatau. These marked the onset of a new phase of activity which climaxed in mid-late November with powerful strombolian (and possibly vulcanian) eruptions being reported. Activity paused in early 2008, yet by April the eruption had resumed again. The photos in this section are from a visit from 19.-23. May 2008. A lava flow had been emplaced on the SW flank about 10 days prior to this date. The crater is notably larger than it was in November and partial collapses of the wall separating it from the main crater could be observed. It is likely that the two craters will eventually merge if activity continues.
2 komentar:
kraktau volcano still active
veri nice :)
krakatao volcano is most active volcano in indonesia
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