A Working Paper-2010
Merapi, History
and Future Changes - Merapi Volcano, in the central
part of Java, is regarded as the most active and most dangerous volcano in
Indonesia. Merapi is a young stratovolcano with a total volume estimated
between about 100 and 150 km3, according to the importance given to the
Pre-Merapi (Berthommier, 1990). A strong uncertainty remains concerning the
beginning of its activity. Information on Merapi eruptive activity is
scattered. A concise and well-documented summary of this activity has been long
needed to assist researchers and hazard-mitigation efforts. The present
effusion rate is about 105 m3/month (Siswowidjoyo et al., 1995); if we assume a
constant rate since the beginning, Mount Merapi could be between 8300 and 125
000 years old. On the basis of field studies and geochronological data, its
history is divided into four Periods: Ancient, Middle, Recent and Modern
Merapi. The Ancient Period may have begun around 40 000 y BP and lasted until
14 000 y BP when the Middle Period begun. The Recent Period begun around 2200 y
BP and was replaced by the Modern Period after the eruption of 1786.
Hartmann (1935a) believed that
the Merapi eruptions seemed to evolve according to several general patterns.
Hartmann classified activity intofour groups, which he inferred to be related to
the gas content of the erupting magma. The classes, A, B, C, and D, were
arranged in order of increasing explosivity. Hartmann’s classification has been
used by many researchers at Merapi (e.g. Van Bemmelen, 1949, pp. 199–200),
although a few authors have reported problems in application (e.g. Ratdomopurbo
and Poupinet, 2000).
Class A activity is associated
with gas-poor magma which rises through the vent and spreads itself into a dome
or a tongue-like coule ´e. It may extrude through B. Voight et al. / Journal of
Volcanology and Geothermal Research 100 (2000) 69–138 74a pre-existing
solidified dome structure. Small initial explosions commonly accompany eruption
onset, and dome growth may produce dome-collapse nue ´es ardentes. Based on his
limited observations, Kemmerling (1921) expected nue ´es from dome collapse to
remain small. However, events since the 1920s have proved that moderate-sized
nue ´es, with relatively long runouts, sometimes occur with this class of
activity. Because of the low gas pressure of the magma, large explosive
outbursts do not occur. Examples of periods of class A activity at Merapi
include 1883–1885, 1909–1918, 1939–1941 (Hartmann, 1935a; Van Bemmelen, 1949),
and perhaps 1992–1993.
Class B activity is associated
with magma higher in gas content. As it rises in the vent, relatively small
explosions blow out the material plugging the orifice, allowing viscous magma to
flow out. Because new magma presumably is more gas-rich than class A magma
(there are little data on gas content; see Hammer et al., 2000), subsequent,
more energetic explosions can produce fountain-collapse nue ´es artentes. These
small vulcanian eruptions can destroy parts of the dome or edifice, and
dome-collapse nue ´es are not precluded and can occur, especially in the final
phase when a viscous gas-poor lava effuses from the vent. Thus, ultimately, two
types of nue ´es ardentes may form, and the order of occurrence can vary. The
small initial explosions marking the beginning of this eruptive activity
(Hartmann’s fore-phase) can perhaps provide warning and allow time for
evacuation and other mitigation measures before onset of the more destructive
phase. Examples of periods of time when class B activity has occurred at Merapi
include 1862–1869, 1887–1889, 1891–1894, 1902–1908, 1920–1922 (Hartmann,
1935a); also, 1942–1945, 1953–1956, 1961, 1967–1969, 1972–1974, 1976–1979,
1980–1984, 1994–1998. The activity of 1930–1931 is commonly listed as class B
by Hartmann, but events of this period were complex and partly atypical of this
class.
Class C activity involves a moderately gas-rich magma, which causes explosions large enough to pulverize the magma into a full range of possible sizes with (Hartmann assumed) no initial small explosions to serve as a warning of possible larger explosions. Thus class C eruptions are inherently more dangerous. The summit is usually partially destroyed, forming a new explosion crater, with the duration of the explosions generally brief. Fountain-collapse nue ´es ardentes are formed. After the explosions, the degassed magma commonly forms a lava dome or tongue. Examples of periods of time where class C activity has been reported at Merapi include 1832–1836, 1837–1838, 1846–1847, possibly 1878 and 1879, 1897, and 1933–1935 (Hartmann, 1935a).
Van Bemmelen (1949) also noted that
1897, and possibly 1878 and 1879, belong to this category, but we suggest that
these events, and also 1837–1938, might better be graded as B activity. In a
number of cases, the distinction between B and C activity seems poorly defined,
and thus we suspect that some of the events previously listed as C might
deserve a lower rank. A main distinction with class B appears to be its high
explosivity near the onset of activity. As a generalization, fountain-collapse
nue ´es ardentes of class C should be volumetrically larger and affect larger
areas than those of class B, although Hartmann appeared to emphasize the
several successive phases of activity comprising an eruption, and not just
size. In our view, class C climax eruptions can generally be thought of as moderate
to moderately large vulcanian explosions (VEI 2–3), and those of class B as
small to moderate vulcanian explosions (VEI 1–2).
Class D represents the eruption of a highly gas-saturated magma, usually initiating with fountain-collapse nue ´e activity that clears the upper part of the orifice. The escaping gases ream the vent, and lower the fragmentation surface on the depressurized magma column, leading to a culminating “intermediate gas phase”. Eruptions of this class commonly destroy the top of the volcano, and are accompanied by abundant and voluminous nue ´es ardentes, as in 1849 and 1872. Vent collapse can follow the “main phase”. An “after-phase” can occur with effusion of gas-poor viscous magma as in 1822–1823. We presume the eruption style at climax to be moderately large to large vulcanian to sub-plinian (VEI 3–4, and more rarely VEI 5). This is the most dangerous class of activity at Merapi.
Generally saying, that a major
difference in eruption style exists between the twentieth and nineteenth
centuries, although the periodicity between larger events seems about the same.
During the twentieth century, activity has comprised mainly the effusive growth
of viscous lava domes and lava tongues, with occasional gravitational collapses
of parts of oversteepened domes to produce the nue ´es ardentes—commonly defined
as “Merapi-type”. In the 1800s, however, explosive eruptions of relatively
large size occurred (to VEI 4), and some associated “fountain-collapse” nue ´es
ardentes were larger and farther reaching than any produced in the twentieth
century. These events may also be regarded as typical eruptions for Merapi. The
nineteenth century activity is consistent with the long-term pattern of one
relatively large event every one or two centuries, based on the long-term
eruptive record deduced by others from volcanic stratigraphy.
*Geoinformation for Spatial Planning and Risk Management - Batch 6/2010 - Gadjah Mada University*
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