the truth about paroxysmal merapi volcano's eruption · pdf filethe truth about...

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.


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.

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.

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.

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.

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

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.

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.

Volcano International Gathering,

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 :

Direktorat Vulkanologi.

Lipman, Peter W. 1981. The 1980

Eruption of Mount St. Hellens,

Washington. Washington DC : United

States Government Printing Office.

Mason, Ben G. 2004. The Size and

Frequency of The Largest Explosive

Eruptions on Earth. Bulletin of

Volcanology.

Mulyaningsih, Sri. 2006. Very Old and

Younger Temple Discoveries in

Yogyakarta Area: Based on Volcano-

Stratigraphic Study. Volcano

International Gathering, Yogyakarta.

Newhall, Christopher G. 1982. The

Volcanic Explosivity Index (VEI): An

Estimate of Explosive Magnitude for

Historical Volcanism. Journal of

Geophysical Research.

Ollier, Cliff. 1969. Volanoes.

Massachusetts : The MIT Press.

Ratmodipurbo, A. 2000. Evolusi 100

Tahun Morfologi Gunung Merapi.

BPPTK

Siefferman, R.G. 1990. An Ecosystem

Under Acid Rain at Merapi Volcano in

Central Java, Indonesia.

Van Bemmelen, R.W. 1970. The Geology

of Indonesia (Second Edition). The

Hague : Martinus Nijhoff.

Voight, B. 2000. Journal of Vulcanology

and Geothermal Research, Special

Issue Merapi Volcano.

Zen, M.T. 2006. Merapi : Dishtung und

Wahreit. Volcano International

Gathering, Yogyakarta.

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