Batuan Piroklastik
SIKLUS BATUAN
Klasifikasi Batuan Beku
Batuan piroklastik:
Terbentuk dari letusan gunung api (berasal dari pendinginan dan
pembekuan magma) namun seringkali bersifat klastik.
Eruption column of Mount St. Helens on May
18, 1980. This explosive release produced a
hot column of rising tephra, volcanic gases
and entrained air that rose to an altitude of 22
kilometers in less than ten minutes. Strong
prevailing winds carried the ash to the east at
about 100 kilometers per hour. In less than
four hours, ash was falling on the city of
Spokane about 400 kilometers away and two
weeks later the eruption cloud had encircled
the earth. USGS image by A. Post
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.
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 dan TEFRA
EJECTA : material (solid
( ) yang terlontarkan
(ejected) saat erupsi gunungapi
Batuan PIROKLASTIK (TEFRA) : batuan yang
terbentuk dari akumulasi material ejecta
Ukuran material piroklastik :
bomb : d > 64 mm
lapili : d = 2 - 64 mm
abu (ash) : d < 2
Pengendapan material PIROKLASTIK
1) Pyroclastic fall
tersebar merata di daerah sekitar gunung
api, tidak dipengaruhi bentuk topografi
(membentuk “bidang waktu” untuk korelasi)
distribusi material dipengaruhi oleh
ukuran material/fragmen, kekuatan dan
arah angin
seringkali memperlihatkan hasil
pemilahan (sorting) material
Pengendapan material PIROKLASTIK
2) Pyroclastic flow
disebut juga sbg nuée ardante atau
wedhus gembel
temperature: ~700 – 1000 0C
kecepatan bisa mencapai ~150 km/h
batuan yg terbentuk disebut sbg ignimbrite
WEDHUS-GEMBEL
(pyroclastic
WEDUS clouds)
GEMBEL
pyroclastic clouds
(pyroclastic clouds)
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.
Klasifikasi
Classification of the pyroclastic rocks. a) Based on type of mat erial. 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.
http://www.earth.ox.ac.uk/~oesis/micro/medium/tuff1_pm15 -14.jpg
http://www.earth.ox.ac.uk/~oesis/micro/medium/kimberlite_pm21
http://www.earth.ox.ac.uk/~oesis/micro/medium/tuff1_pm15 -14.jpg
-27.jpg
http://f10.aaa.livedoor.jp/~dunite/3nenjpeg/v7 -1_bubble1_5.JPG
http://www.earth.ox.ac.uk/~oesis/micro/medium/kimberlite_pm21
http://www.earth.ox.ac.uk/~oesis/micro/medium/tuff1_pm15 -14.jpg
-27.jpg
http://f10.aaa.livedoor.jp/~dunite/3nenjpeg/v7 -1_bubble2_5.JPG
http://www.earth.ox.ac.uk/~oesis/micro/medium/kimberlite_pm21
http://www.earth.ox.ac.uk/~oesis/micro/medium/tuff1_pm15 -14.jpg
-27.jpg
Lightly-compacted tuff In this tuff, the irregularly-shaped glass shards are still relatively
undeformed. Also note the phenocryst of quartz (clear) and biotite (dark red) in this slide.
http://www.geolab.unc.edu/Petunia/IgMetAtlas/volcanic -micro/tuff1.UX.html
Poorly-welded tuff In this sample, the glass shards are fused together in a swirly
mass, and the large pumice fragment at center right is flattened. In contrast, quartz
phenocrysts are relatively undeformed.
http://www.geolab.unc.edu/Petunia/IgMetAtlas/volcanic -micro/tuff2.UX.html
Welded tuff In this sample, the glass shards are starting to get deformed. N ote the
phenocrysts of quartz (clear) and biotite (dark red) in this rock.
http://www.geolab.unc.edu/Petunia/IgMetAtlas/volcanic -micro/tuff3.UX.html
Pumice fragments flattened Heterogeneous assemblage of
around rock and crystal fragments rock and crystal fragments
(PPL; fov 6.25 mm) (PPL; fov 6.25 mm)
