Proceeding International Geomorphologist Seminar Yogyakarta, Indonesia, 2008
1 Department of Environmental Geography, Gadjah Mada University, email : [email protected]. 1
The Truth about Paroxysmal Merapi Volcano’s Eruption from
Geomorphology of Southwestern Merapi’s Fluviovolcanic Plain
Alva Kurniawan1
Abstract
The Bemmelen’s statement about Merapi’s volcano paroxysmal eruption at 1006 A.D.
was a polemic till right now. Bemmelen stated that Merapi Volcano erupted explosively at
1006 A.D. There are many Indonesian scientists belief to Bemmelen’s statement because it
based on long geologic survey and geomorphologic analysis. The further study about this
topic needed due to make a right conclusion about any study about Merapi Volcano and
Geomorphology of southwestern Merapi Volcano’s slope.
The research objects are anticlines in southwestern of Merapi’s fluviovolcanic plain
such as Gendol hills, Gunungsari hill, and Gunungpring hill. The methods of the research are
based on geomorphological and cultural approach. The objects located in Muntilan District,
Salam District, Ngluwar District, Magelang Regency, Central Java, Indonesia. Analysis from
geomorphological approach was done from map interpretation of RBI Map 1:25.000,
Muntilan sheet, with number sheet 1408-243; map interpretation of Geologic Map 1:100.000,
Yogyakarta sheet, with number sheet 1408-2 and 1407-5; field geomorphologic survey, and
also studying journals and reference books.
Bemmelen’s statement about Merapi’s Volcano paroxysmal eruption in 1006 A.D.
couldn’t be proved. Based on the analysis result, the eruption never happened in 1006 A.D.
even the Merapi’s Volcano never erupts in paroxysmal level. The conclusion from
geomorphologic analysis is also wrong, Bemmelen explained that the hills in southwestern
Merapi Volcano are an anticline that formed by southwestern Merapi Volcano’s slope slid
but unfortunately the dip and strike of the hills are not match with the hill’s forming process.
Further study about this topic is still needed to calibrate the result of this research.
Keywords : Bemmelen’s statement, Merapi Volcano, paroxysmal, eruption, geomorphologic,
southwestern slope of Merapi Volcano.
Proceeding International Geomorphologist Seminar Yogyakarta, Indonesia, 2008
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1. Introduction
The year of 2006 A.D. was
commemorated as the first millennium of
paroxysmal Merapi Volcano’s eruption. In
the year of 2006 A.D., Merapi Volcano
showed its activities after hibernated for
some years. Most of volcanologists
worried about the Merapi Volcano’s
activity. They thought that Merapi
Volcano could erupt explosively after
hibernated for some years. Their way of
think based on Bemmelen’s statement
about the paroxysmal Merapi eruption that
wrote in a famous reference book, The
Geology of Indonesia. Bemmelen stated
that proves of the paroxysmal eruption was
lied in the anticlines that formed at the
southwestern of Merapi’s fluviovulcanic
plain. Bemmelen stated that the forming
process of the anticlines had a strong
relation with geomorphologic change of
Merapi Volcano. Unfortunately the prove
that Bemmelen told not match with
geomorphologic survey.
1.2. Study Scope
Regional study scope concentrates
in southwestern anticlines of Merapi
Volcano’s fluviofulcanic plain such as
Gendol hills, Gunungsari hill, and
Gunungpring hill. Administratively the
regional study scope is located at Central
Java Province, Magelang Regency,
Muntilan, and Salam. Object study scopes
include Bemmelen’s Statement about
paroxysmal Merapi Volcano’s Eruption in
1006 A.D., and southwestern anticlines
physiographic of Merapi Volcano’s
fluviovulcanic plain.
1.3. Purposes
The purposes of why this topic was
chosen and why this paper was wrote such
as:
a. it’s important to correct a wrong
statement and give the true fact
about Merapi Volcano’s eruption
characteristics in order to make the
next research about Merapi
Volcano not wrong;
b. this paper could give some
recommendation ideas in disaster
mitigation of the eruption of
Merapi Volcano;
c. this paper told facts about
paleogeomorphic process that
happen in southwestern
Figure 1. Gendols Hills from
Gunungsari Anticline.
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fluviovolcanic plain of Merapi
Volcano;
d. this paper could be a reference to
the next study about Merapi
Volcano’s eruption characteristics;
2. Methods
The methods in writing this paper
are map interpretation of RBI Map
1:25.000, Muntilan sheet, with number
sheet 1408-243; map interpretation of
Geologic Map 1:100.000, Yogyakarta
sheet, with number sheet 1408-2 and 1407-
5; field geomorphologic survey, and also
studying journals and reference books
about Merapi Volcano. Interpretation of
both of Geologic and RBI maps were done
to know about the geomorphologic
condition of Gendol hills, Gunungsari hill,
and Gunungpring hill.
Field geomorphologic surveys
were done in all anticlines to extract data
about geologic structural in each anticline.
The surveys were done by strike
measurement with Geologic Compass, and
rocks description in each anticline. The
surveys are important to know about the
strike of the anticlines and to know about
the anticlines composite materials which
have been generalized in the geologic map.
Studying references and journals that tell
about Merapi Volcano is important to
know about its eruption characteristic.
Knowledge in Merapi volcano’s eruption
characteristic will be correlated with
Bemmelen’s statement about the
paroxysmal Merapi Volcano’s eruption.
3. Geomorphology of Southwestern of
Merapi’s Fluviovolcanic Plain
Southwestern of Merapi’s
Fluviovolcanic Plain consist of some
different landforms. Within the main
landform of fluviovolcanic plain there are
landforms that formed by different
processes. There are Kaliputih River and
Figure 2. Gendols Hills in Geologic
Map Yogyakarta Sheet 1995.
Figure 3. Gendols Hills in RBI Map
Muntilan Sheet 2001.
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Blongkeng River that formed natural
levee, flood plain, depositional floor, back
swamp, cut bank, point bar deposit, bar
deposit, depositional terrace. Kaliputih and
Blongkeng River are cooperating with
Merapi’s volcanic activity that provides
avalanche materials, forming Merapi’s
Southwestern Fluviovolcanic Plain.
There are strange landforms in the
fluviovolcanic plain. A sequence of
anticlines presents in the fluviovolcanic
plain. Their genesis till now is still
mysterious things. None of scientist can
say for a sure, what kind of process that
formed them. The anticlines formed an arc
from the southeast to the northwest. The
anticlines also formed a strange formation.
At the most southeast antcline that locally
named Gunungwukir, it separated from
other anticlines. The anticlines near
Gunungwukir are connected each other
and they locally named Gendol hills,
which consist of Gununglempong
Anticline, Gununggendol Anticline,
Gunungdepok Anticline, Gunungpuguhan
Anticline, Gunungtegal Anticline, and
Gunungtugel Anticline. On the northwest
of them there are Gunungsari Anticline
and Gunungpring Anticline that far
separated each other by Blongkeng River
and Kaliputih River. The famous scientist
that explained about their genesis is
Bemmelen. Bemmelen stated that the
anticline formed by paroxysmal eruption
of Merapi’s Volcano or local scientist said
a horseshoe eruption of Merapi Volcano.
The eruption was greatly made a quake on
the southwestern slope of Merapi’s
Volcano, then it result in a slid of the
southwestern slope of Merapi Volcano.
The materials that slid to the southwestern
were hit the basement rock of Menoreh
hills. It resulted in a fold in the contact
zone between Merapi’s slope and the
Menoreh hills basement rock. Bemmelen
also stated that it happen in around 1006
A.D., when ancient Mataram Kingdom’s
citizen still exist in there. Bemmelen also
related that the movement of ancient
Mataram Kingdom’s citizen due to the
paroxysmal eruption of Merapi Volcano.
The composite materials of the
landforms in the fluviovulcanic plain are
almost same except the composite
materials that form the anticlines. The
materials that formed most of landforms in
the fluviovolcanic plain mainly consist of
Figure 4. Folded Breccias in
Gunungsari Anticlines.
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Quaternary alluvium sediment of young
Merapi Volcano.
Differentially materials that formed
the anticlines consist of breccias,
agglomerate, tephra and lappilus deposit,
lava avalanche, andesite, and olivine
basalt. They are Quaternary older than the
materials around. In some place there are
some outcrops that shown the geologic
structure of the anticlines and also the
rocks that consisted anticlines. In one of
the outcrops, the soil has red color like a
soil from the result of coaling process.
The red soil can’t be said as coaling soil
because the rocks within the soil didn’t
appear to be morphed. The minerals of the
rock sample within the soil seem like
never heating since they froze firstly. In
one of the outcrops there is a folded
breccias formation. The measurement in
folded breccias formation result that it has
strike about N 60˚ E, and dip about 40˚.
4. Result and Discussion
Merapi Volcano didn’t erupt
explosively in 1006 A.D. even it never
erupts explosively since thousand years
ago. That statement has one big evidence
which clearly visible. The evidence is
there no trace of Merapi Volcano’s
paroxysmal eruption left. Every great
volcanic explosion in this world left a clear
trace although it was happened thousand or
even million years ago. There are some
volcanoes in this world that ever erupted
greatly like Toba Volcano, Krakatoa
Figure 5. Tephra and lappilus deposit in
Gendol Anticline.
Figure 7. Dip and strike measurement in
Gunungsari Anticline
Figure 6. Red soil in Gunungsari
Anticline.
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Volcano, Tambora Volcano, Vesuvius
Volcano, Mount St. Hellens Volcano,
Yellow Stone Volcano, etc. Toba Volcano
left evidence for its great eruption in Toba
Lake. Toba Lake was formed from
collapsed batholiths after Toba Volcano
throw up all of the magma inside its
batholiths. Krakatoa Volcano and Tambora
Volcano left evidence for their great
eruption in a caldera. Unlike some volcano
mentioned before, Mount St. Hellens
Volcano and Vesuvius Volcano left
evidence for their great eruption in very
thick of volcanic material sediment that
deposited around them. Yellow Stone
Volcano is a volcano that supposed ever
erupt in highest power eruption in the
world that left a hot spring known as
Yellow Stone National Park. Merapi
Volcano didn’t left any clear trace about
its great eruption. There is no thick
volcanic material deposit around Merapi
Volcano. In some places around Merapi
Volcano moderately thick volcanic
materials buried some temple around
Merapi Volcano’s slope but from radio
dating that was done by Mulyaningsih
(2006), the materials did not originate
from a single eruption but from multiple
eruption which occurred in different years.
Zen also stated that Indonesian Volcano’s
great eruption often left a clear trace like
volcano tectonic depression that contains
highly rhyolite ash flow and pumestone-
tuff (Zen, 2006; Kurniawan, 2007).
Bemmelen’s statement about
anticlines genesis was so reasonable
because the anticlines formation like an arc
that curved to the southwestern slope of
Merapi Volcano direction. Bemmelen’s
statement was also reasonable because
mainly composite materials of the
anticlines and the older volcanic cone of
Merapi Volcano were same. There is a
strange thing when looking to the
anticlines formation. There are some
anticlines that were connected each other
and also there are some anticlines that
were separated each other. If the
Bemmelen’s statement true, the anticlines
should be connected each other and if
there any separated anticline, it must not
be separated far away. By looking for
geomorphology of the southwestern slope
of Merapi Volcano, it seems that
Blongkeng River can’t flow through
Gunungsari Anticline and just flow around
it. That thing can be concluded as the river
flow can’t flow through barrier.
Gunungsari Anticline could become a
water flow barrier if there are a massively
rock structure that impermeable with
water. Back to the Bemmelen’s statement
that the anticline formed by folding of
Merapi Volcano’s southwestern slope
materials, Bemmelen indirectly said that
the materials that formed the anticlines is
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not a massive materials but a mixture of
consolidated and unconsolidated materials.
Logically those materials can be eroded by
stream flow easily especially when the
stream on flood condition, but as the fact
Gunungsari Anticline can’t be eroded by
Blongkeng River flow.
Field geomorphologic survey
which was done on the anticlines; give
different truth about their genesis. The
anticlines have strike about N 60˚ E, and
dip about 40˚. Bemmelen explanation
about their genesis should show strike
about N 315˚ E not N 60˚ E. The strike
about N 60˚ E gives an imagination that
the anticline folded up by a power from N
150˚ E direction, and that direction is not
in line with Merapi Volcano’s
southwestern slope that has direction from
the anticline about N 45˚ E.
Further research about this topic is
needed in order to calibrate the result of
this research. There are still many
mysteries in this research that need to be
discovered such as the power that formed
the anticlines, what kind of processes that
actually build the anticlines, and when the
anticlines were built.
5. Conclusions
Merapi Volcano never erupts
explosively because there is no trace that
proofs that event. Bemmelen’s statement
about the eruption related with anticlines
on southwestern fluviovolcanic plain of
Merapi Volcano is not match with the
strike direction of the anticlines rock
structure. The geomorphology of Merapi
Volcano’s southwestern fluviovolcanic
plain can be correlated with Merapi
Volcano’s activity except the anticlines
formation. The anticlines formation must
be formed by a geologic event that happen
Figure 8. Bemmelen’s strike direction
estimation on the anticline.
Figure 9. True strike direction based on
geological survey 2007-2008 on the
anticline.
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a long time ago and can’t be correlated
with Merapi Volcano’s activity.
6. References
Andreastuti, S.D. 1999. Stratigraphy and
Geochemistry of Merapi Volcano,
Central Java, Indonesia.
Implication for Assessment of Volcanic
Hazards.
Andreastuti, S.D. 2006. Did A Large
Eruption of Merapi Occure in 1006 AD
?. Volcano International Gathering,
Yogyakarta.
Brotopuspito, Kurbani Sri. 2006. Merapi
Volcano Inspires Scientific Curiosity.
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Yogyakarta.
Kurniawan, Alva. 2007. Erupsi
Paroxysmal Gunungapi Merapi Tahun
1006 Masehi. Yogyakarta : Research
and Development of Geoscience and
Environmental Matter.
Kusumadinata, K. 1979. Data Dasar
Gunungapi Indonesia. Jakarta :
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Lipman, Peter W. 1981. The 1980
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Mason, Ben G. 2004. The Size and
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Mulyaningsih, Sri. 2006. Very Old and
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Newhall, Christopher G. 1982. The
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Ollier, Cliff. 1969. Volanoes.
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Ratmodipurbo, A. 2000. Evolusi 100
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Central Java, Indonesia.
Van Bemmelen, R.W. 1970. The Geology
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Voight, B. 2000. Journal of Vulcanology
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Zen, M.T. 2006. Merapi : Dishtung und
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Proceeding International Geomorphologist Seminar Yogyakarta, Indonesia, 2008
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