Download - 0307 syssttem 1 Google mobii hf xbx
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STUDI BATUAN GUNUNG API PUMIS:
MENGUNGKAP ASAL MULA BREGADA GUNUNG
API PURBA DI PEGUNUNGAN SELATAN,
YOGYAKARTA
Yogyakarta, 27-29 Nov 2007
Oleh:
Gendoet HartonoStaf pengajar di Jurusan Teknik Geologi STTNAS, Yogyakarta
E-mail: [email protected]
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Latar belakang
Masih adanya perbedaan pandangan diantara para
ahli geologi tentang keberadaan batuan gunung api
pumis.
permasalahan
Apakah kelimpahan batuan gunung
api pumis berasosiasi dengan
letusan gunung api yang diikuti oleh
pembentukan Bregada atau Kaldera
Genesis dan kelimpahan batuan gunung
api pumis belum diungkap secara
komprehensif.
Petrologi & tataan tektonik
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…..like this ?
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…they can also be..?
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Maksud & tujuan
Metode penelitian
Mempelajari karakter fisik, mikroskopis &
kimiawi batuan gunung api pumis.
Mengetahui hubungan karakter pumis dengan
tipe letusan yang menghasilkan Bregada
Melakukan studi petrologi & volkanologi
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Lokasi yang
menjadi topik
bahasan
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Pembentukan batuan gunung api
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Mate
rial gunung a
pi pro
duk
letu
san (
vid
eC
om
potn
, 1985)
<2mm
2-16mm
>16mm
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Igneous Rock Chart
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Bregada BaturagungBregada Gajahmungkur
SRTM Bregada Baturagung & Gajahmungkur
(Hartono, 2000; Hartono, et al., 2000; Hartono & Syafri, 2007)
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Pelamparan Formasi Semilir
(Rahardjo, et al., 1977; Surono, et al., 1992)
Tms
TmsTms
Tms
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Volkanisme konstruktif & destruktif di Pegunungan Selatan,
Yogyakarta pada Jaman Tersier
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Breksi pumis tuf, Formasi Semilir
Warna putih terang – keabuan, bertekstur gelas,
Ø 1-10 cm dan mencapai Ø 15 cm, struktur
internal berserat atau pumisius, berongga atau
berlubang (Ø bervariasi), komposisi mineral
felsik (kuarsa, feldspar) dominan dan mineral
mafik (< 2%), dan ciri fisik yang lain adalah
tajam, ringan dan mengapung di air, walaupun
sering juga dijumpai yang tenggelam di air.
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Kenampakan megaskopis pumis
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Kenampakan mikroskopis pumis
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Perajahan batuan gunung api pada diagram Peccerillo & Taylor, 1976; Irvine & Baragar, 1971
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Perajahan batuan gunung api pada diagram Pearce, et al., 1977
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Penampang Bregada Baturagung & Bregada Gajahmungkur
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Newhall & Self, 1982
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Hubungan tingkat letusan (VEI) dengan masa istirahat gunung api (Simkin, 1993)
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• Batuan pumis yang tersingkap di Pegunungan Selatan, Yogyakarta dan sekitar berkomposisi andesit – riolit (SiO2 = 57,12-75,87 %berat), dan berafinitas kapur-alkali (K2O= 0,3-4,19 %berat).
• Batuan pumis berasosiasi dengan letusan gunung api paroksimal (VEI = 5 – 8) membentuk Bregada Baturagung dan Bregada Gajahmungkur.
KESIMPULAN
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Shape of pumices
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Composite Volcano - Mt St Helens
Images from http://vulcan.wr.usgs.gov/Volcanoes/MSH/SlideSet/ljt_slideset.html
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The major
element
chemistry of
MORBs
Table 13-2 . Average analyses and Norm s of MORB
glasses (BVTP Table 1.2.5.2).
Oxide Wt% All MAR EPR IOR
SiO2 50.53 50.68 50.19 50.93
TiO2 1.56 1.49 1.77 1.19
Al2O3 15.27 15.60 14.86 15.15
FeO* 10.46 9.85 11.33 10.32
MgO 7.47 7.69 7.10 7.69
CaO 11.49 11.44 11.44 11.84
Na2O 2.62 2.66 2.66 2.32
K2O 0.16 0.17 0.16 0.14
P2O5 0.13 0.12 0.14 0.10
Total 99.69 99.7 99.65 99.68
Mg# (m ol) a
56 58 53 57
Norm
Q 0.94 0.76 0.93 1.60
or 0.95 1.00 0.95 0.83
ab 22.17 22.51 22.51 19.64
an 29.44 30.13 28.14 30.53
di 21.62 20.84 22.50 22.38
hy 17.19 17.32 16.53 18.62
m t 4.44 4.34 4.74 3.90
il 2.96 2.83 3.36 2.26
ap 0.30 0.28 0.32 0.23
All: Ave f rom Atlantic, Pacif ic and Indian Ocean ridges.
MAR: Ave. f rom Mid-Atlantic Ridge. EPR: Ave. f rom EPR.
IOR: Ave. Indian Ocean. a Min. value since all Fe as FeO
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