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Batuan Piroklastik dan Volkaniklastik

SIKLUS BATUANSIKLUS BATUAN

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KlasifikasiKlasifikasi BatuanBatuan BekuBeku

Dike

Tuff

Pyroclastic breccia

Massive lava

Lava

Khuluk Tengger

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Batuan piroklastik:Terbentuk dari letusan gunung api (berasal dari pendinginan danpembekuan magma) namun seringkali bersifat klastik.

Volcanic ash plume from Cleveland Volcano, located on Chuginadak Island in the Aleutian Island Chain off Alaska. NASA image taken by Jeff Williams, Flight Engineer, from the International Space Station.

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Satellite image of an ash plume from Chaitén Volcano in southern Chile on May 3, 2008. This plume begins in Chile, crosses Argentina and extends hundreds of miles out over the Atlantic Ocean. NASA image

PIROKLASTIK PIROKLASTIK dandan TEFRATEFRA EJECTAEJECTA : : material (material (solid) yang ) yang terlontarkanterlontarkan

((ejectedejected) ) saatsaat erupsierupsi gunungapigunungapi

BatuanBatuan PIROKLASTIKPIROKLASTIK (TEFRA) (TEFRA) : : batuanbatuan yang yang terbentukterbentuk daridari akumulasiakumulasi material material ejectaejecta

UkuranUkuran material material piroklastikpiroklastik : : bomb : d > 64 mmbomb : d > 64 mm lapili : d = 2 lapili : d = 2 -- 64 mm64 mm abuabu ((ashash) : d < 2) : d < 2

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Volcanic bomb (Krakatau)

PANDANGAN GEOLOGI GUNUNG API:PANDANGAN GEOLOGI GUNUNG API:AdaAda kesinambungankesinambungan prosesproses magmatismemagmatisme, , volkanismevolkanisme dandan sedimentasisedimentasi

A

B

A: Problem Magmatisme – VolkanismeB: Problem Volkanisme-Sedimentasi

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Erupsi Mayon Volcano, FilipinaJatuhan piroklastika(Hujan abu)

Aliranpiroklastika

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PengendapanPengendapan material PIROKLASTIKmaterial PIROKLASTIK

1) Pyroclastic fall 1) Pyroclastic fall tersebartersebar meratamerata didi daerahdaerah sekitarsekitar gununggunung

apiapi, , tidaktidak dipengaruhidipengaruhi bentukbentuk topografitopografi((membentukmembentuk ““bidangbidang waktuwaktu”” untukuntuk korelasikorelasi))

distribusidistribusi material material dipengaruhidipengaruhi oleholehukuranukuran material/material/fragmenfragmen, , kekuatankekuatan dandanaraharah anginangin

seringkaliseringkali memperlihatkanmemperlihatkan hasilhasilpemilahanpemilahan ((sorting) material) material

Endapan jatuhan piroklastika di dinding kawah G. Tangkubanparahu

Aliran lava

Endapan hidroklastika/freatik/hidrotermal

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PengendapanPengendapan material PIROKLASTIKmaterial PIROKLASTIK

2) Pyroclastic flow2) Pyroclastic flow adaada bagianbagian ygyg disebutdisebut sbgsbg nunuééee ardanteardante atauatau

wedhuswedhus gembelgembel

temperature: ~700 temperature: ~700 –– 1000 1000 00CC

kecepatankecepatan bisabisa mencapaimencapai ~150 km/h~150 km/h

salahsalah batuanbatuan ygyg terbentukterbentuk ignimbriteignimbrite

Pyroclastic flow

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Rekaman korban pyroclastic-flow (Pompeii 79 AD)

Tuff

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Tuff

(Welded) Tuff

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An ignimbrite in the field, Tenerife, Canary Islands. Yellow notebook with cm gradations for scale.

Light microscope image of a welded ignimbrite

A) Section on the Santo Tomas River bridge north of San Narciso, Zambales; 32 km west-southwest of vent. Layer A is 8 mm of sand-sized ash; layer B is 4 mm of mostly fine ash. Note weak normal grading of layer C and scattered coarse clasts on surface of deposit.

B) Tephra-fall deposits on unimproved road along the Marella River 10.5 km southwest of vent. Layer A, about 4 cm thick, consists of coarse ash and fine lapilli; layer B consists of several thin layers of ash; layer C is 33 cm thick and is the thickest section of the climactic pumice-fall deposit yet found. Note normal grading overall, but 2-cm pumice lapillus in upper left. Layer D consists of two 3- to 4-cm-thick beds of fine ash separated by a bed of water-reworked pumiceous ash.

C) Tephra deposits on unimproved road about 9 km southeast of vent, north side of Gumain River. Layer B is 23 cm thick and consists of numerous graded ash beds; layer C is 31 cm thick and has two zones in lower part with minor fine ash coatings.

D) Section at mouth of Pasig River canyon about 15 km east of vent. Layer B is 10 cm thick and layer C is about 18 cm thick; note ash-rich zones that stand out owing to increased cohesiveness. USGS Images by W.E. Scott and J.J. Major.

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Paleosol/paleosoil(tanah purba)

Hydrotermal mud/phreatic deposit

Phreatomagmaticdeposit

Classification of the pyroclastic rocks. a) Based on type of material. After Pettijohn (1975) Sedimentary Rocks, Harper & Row, and Schmid (1981) Geology, 9, 40-43. b) Based on the size of the material. After Fisher (1966) Earth Sci. Rev., 1, 287-298.

KlasifikasiKlasifikasi

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SCORIASCORIA :: dark brownish black or red vesicular ejecta of dark brownish black or red vesicular ejecta of basaltic and basaltic and

andesitic magmasandesitic magmas (mafic version of pumice)(mafic version of pumice) forming from dissolved gases rich magma richforming from dissolved gases rich magma rich composed of volcanic glass fragments and few mineral composed of volcanic glass fragments and few mineral

crystalscrystals its specific gravity ranges from 0.8 to 2.1its specific gravity ranges from 0.8 to 2.1

PUMICEPUMICE :: vesicular ejecta of intermediate to vesicular ejecta of intermediate to siliceous magmassiliceous magmas

(including rhyolite, trachyte and phonolite)(including rhyolite, trachyte and phonolite) usually light in colour ranging from white, yellowish, gray, usually light in colour ranging from white, yellowish, gray,

gray brown, and a dull red, gray brown, and a dull red, average porosity of 90%average porosity of 90%

SKORIASKORIA ((SCORIASCORIA))KomposisiKomposisi BASABASA

BATUAPUNGBATUAPUNG (PUMICE)(PUMICE)KomposisiKomposisi ASAMASAM

BATUAN PIROKLASTIKA

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OBSIDIAN :OBSIDIAN : produced when a produced when a felsic lava cools rapidlyfelsic lava cools rapidly and freezes without sufficient and freezes without sufficient

time for crystal growth, composed of glasstime for crystal growth, composed of glass because of the lack of crystal structure, obsidian blade edges cbecause of the lack of crystal structure, obsidian blade edges can reach an reach

almost molecular thinness (use as arrowheads and surgical scalpealmost molecular thinness (use as arrowheads and surgical scalpel blades)l blades)

BATUAN PIROKLASTIKA

BREKSI VOLKANIK BREKSI VOLKANIK ((VOLCANIC BRECCIAVOLCANIC BRECCIA))

BOM VOLKANIK BOM VOLKANIK ((VOLCANIC BOMBVOLCANIC BOMB))

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BATUAN PIROKLASTIKA

TUFATUFA

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Batuan Volkaniklastik(lahar, landslide deposit, mudflow)

Aliran Laharaliran lumpur, campuran air denganmaterial hasil letusangunung api yang tertumpuk di badangunung dan kemudianmengalir ke arah kaki gunung sebagaibanjir bandang

Lahar Panas/Erupsi(terjadi bersamaan denganletusan pada gunung api yang mempunyai danau kawah)

dan

Lahar Dingin/Hujan(longsor dari gunung saat hujanlebat)

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LETUSAN ST. HELENS (1980)

BENDUNG TERTIMBUNBENDUNG TERTIMBUNRUMAH RUMAH TERTIMBUNTERTIMBUN

LAHAR = LARUTAN PEKATBERUPA PIROKLASTIKA / DEBU VOLKANIK YANG TERBAWA OLEH AIR

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GUNUNG RINJANI & SEGARA ANAKAN, LOMBOK