Petro – 03 Textures and Classification

Magma, Rocks Classification &
Textures
MAGMA
MAGMA
• Larutan silikat yang sangat panas
• Mengandung oksida, sulfida serta
volatiles (CO2, sulfur, chlorine,
fluorin, boron dll)
• Temperatur antara 600°C (magma
asam) sampai 1250°C (magma basa)
JENIS KONVERGEN
Plate Tectonic - Igneous Genesis
5
3
1
6
7
4
200 km
Continental Crust
400
Oceanic Crust
Lithospheric Mantle
?
600 km
Sub-lithospheric Mantle
Source of Melts
?
?
?
2
Environments of Magma
Formation
Environments of Magma
Formation
Stages in ascent
•
•
•
•
•
Eruption
(Fragmentation)
Vesiculation
Renewed ascent
Storage
– mixing
– assimilation
– crystallization
• Buoyant ascent
• Partial melting
The Earth’s Interior
Crust:
Crust
Depth (km)
Upper Mantle
Transition Zone
Mantle
60
220
410
660
Lower
Mantle
Granite/Andesite (felsic)
Mantle:
Peridotite (ultramafic)
Core:
2898
Outer
Core
(liquid)
Metal alloy/liquid
Core
5145
Inner
Core
(solid)
6370
Most important elements
Si
14.4%
Al Ca
S
3.0% 1.4% 1.0%
O
50.7%
Fe
15.2%
Mg
15.3%
Figure 1-5. Relative atomic abundances of the seven most common elements that
comprise 97% of the Earth's mass. An Introduction to Igneous and Metamorphic
Petrology, by John Winter , Prentice Hall.
Partial Melting: The Origin of
Basalt and Granite
Basaltic magma = 50% silica
(1100o C)
Forms the rock basalt
Melting
Asthenosphere
40% Silica
Partial Melting: The Origin of
Basalt and Granite
Granitic magma ~ 70% silica
(700-900o C)
Forms granite (a mixture of
quartz and feldspar)
Melting
Continental Crust
(Mainly low melting
point minerals such as
quartz, feldspar, mica)
Urutan pembekuan magma
• Pada pembekuan magma, pada awalnya
mineral yang terbentuk
adalah yang
anhydrous (tidak mengandung air) 
tidak mengandung gugus OH, disebut
mineral pyrogenetik.
• Cairan selanjutnya akan lebih banyak
mengandung
komponen
gas
dan
terbentuk
mineral-mineral
yang
mengandung gugusan hydroksil (OH),
disebut mineral hydratogenetik.
Diferensiasi Magma
• Proses diferensiasi meliputi semua kegiatan
yang mengakibatkan suatu jenis magma induk
yang semula relatif homogen terpecah-pecah
menjadi beberapa bagian atau fraksi dengan
komposisi yang berbeda-beda. Hal ini
disebabkan karena migrasi ion atau molekul
dalam larutan magma karena adanya
perubahan temperatur dan tekanan. Yang
pada akhirnya akan membentuk berbagai
jenis batuan beku dengan komposisi yang
berbeda-beda pula.
Bowen reaction series
DIAGRAM FASE
• Fase : padat, cair, gas
• Diagram fase : menggambarkan
kondisi magma pada kondisi P & T
tertentu
• Parameter penting dalam sistem
magma : fase, komponen, variabel
intensif
DIAGRAM fASE
• fase : padat, cair
• komponen : komponen terkecil
yang diperlukan utk pembentukan
fase-fase
•
dalam sistem (OH, H2O,
MgO, NaAlSi3O8, dll)
•
• variabel intensif : temperatur dan
tekanan, jumlah komponen
DIAGRAM FASE
•
•
•
•
•
•
Rumus fase : F = C – P + 2
F : degree of freedom :
jumlah kondisi minimum
C : jumlah komponen;
P : jumlah fase
contoh utk air – es ------ C = 1
(H2O) ; P = 2 (es dan air)
F = C – P + 2 ---- F = 1 – 2
+ 2 = 1 (unary system)
SISTEM 1
KOMPONEN
SISTEM 2 KOMPONEN (BINER) DGN TITIK
EUTEKTIK
h : titik eutektik; titik terendah fase cair ; kondisi terbentuknya 2 komponen
SISTEM 2 KOMPONEN SOLID SOLUTION
SISTEM 2 KOMPONEN INCONGRUENT
MELTING
Why storage?
stronger
crust
denser
crust
Why do some magmas stall and pond in chambers during
ascent?
Processes during storage in
magma chambers
Fractional Crystallization
http://www.geolsoc.org.uk/webdav/site/GSL/shared/images/geoscientist/Geoscientist%2019.2/7%20
Volcano%20and%20magma%20chamber%20James%20Island2resized.jpg
Processes during storage in
magma chambers
Gravity settling
http://www.geolsoc.org.uk/webdav/site/GSL/shared/images/geoscientist/Geoscientist%2019.2/7%20Volcano%20and%20magma%20cham
ber%20James%20Island2resized.jpg
Gravity settling and cumulates
http://www.geol.lsu.edu/henry/Geology3041/lectures/12LayeredMafic/Fig12-15.jpg
Buoyancy, sinking: Stoke’s
Law
2gr (r s - r l )
V=
9h
2
V = the settling velocity (cm/sec)
g = the acceleration due to gravity (980 cm/sec2)
r = the radius of a spherical particle (cm)
rs = the density of the solid spherical particle
(g/cm3)
rl = the density of the liquid (g/cm3)
h = the viscosity of the liquid (1 c/cm sec = 1
poise)
Sinking olivine in basalt
Olivine in basalt

Olivine (rs = 3.3 g/cm3, r = 0.1 cm)

Basaltic liquid (rl = 2.65 g/cm3, h = 1000 poise)

V = 2·980·0.12 (3.3-2.65)/9·1000 = 0.0013
cm/sec

that’s ~1m per day
Sinking x’tal in rhyolite
Rhyolitic melt
h = 107 poise and rl = 2.3 g/cm3
3
 hornblende crystal (rs = 3.2 g/cm , r = 0.1 cm)
-7 cm/sec, or 6 cm/year
 V = 2 x 10
 feldspars (rl = 2.7 g/cm3)
 V = 2 cm/year
4
 = 200 m in the 10 years that a stock might cool
 If 0.5 cm in radius (1 cm diameter) settle at 0.65
meters/year, or 6.5 km in 104 year cooling of stock

IGNEOUS ROCKS
CLASSIFICATION
Ternary diagrams
Classification of Igneous
Rocks
X
10
10
20
%Z
30
20 %Y
A
Inc
20
10
Z
%Z
r
Inc
r%
Y
Incr %X
%X
30
30
30
20 10
%Z
Y
Figure 2-1a. Method #1 for plotting a point with the components: 70% X, 20% Y, and 10% Z on
triangular diagrams. An Introduction to Igneous and Metamorphic Petrology, John Winter, Prentice Hall.
Know how to classify a rock
Volcanic rocks: aphanitic
Ultra-mafic rocks & felsic vs. mafic
Classification of Igneous Rocks
Plagioclase
Anorthosite
Figure 2-2. A classification of the phaneritic
igneous rocks. b. Gabbroic rocks. c. Ultramafic
rocks. After IUGS.
lite
cto
Tro
Ga
bb
ro
90
Olivine
gabbro
Olivine
Dunite
90
Peridotites
Plagioclase-bearing ultramafic rocks
Pyroxene
Lherzolite
Olivine
(b)
40
Pyroxenites
Olivine Websterite
Orthopyroxenite
10
(c)
10
Orthopyroxene
Websterite
Clinopyroxenite
Clinopyroxene
Classification of Igneous Rocks
Phonolite
13
Tephriphonolite
Wt.% Na2O+K2O
11
9
Phonotephrite
(Foid)ite
Trachyte
Trachy- Trachydacite
andesite
Rhyolite
Basaltic
trachyTephrite
Basanite Trachy- andesite
7
basalt
5
Dacite
3
Basalt
Basaltic
Andesite
Andesite
Picrobasalt
1
37
41
ULTRABASIC
45
45
49
BASIC
53
57
61
52 INTERMEDIATE
65
63
69
73
77
ACIDIC
wt% SiO2
Figure 2-4. A chemical classification of volcanics based on total alkalis vs. silica. After Le Bas et al.
(1986) J. Petrol., 27, 745-750. Oxford University Press.
Classification of Igneous Rocks
Ash (< 2 mm)
Glass
Lapilli (2-64 mm
Tuff
Lapillistone
Lapilli
Tuff
30
30
Vitric
Tuff
Lapilli -Tuff
Breccia
70
Lithic
Tuff
Crystals
Rock Fragments
(a)
70
Pyroclastic
Breccia or
Agglomerate
Crystal
Tuff
Blocks and Bombs
(> 64 mm)
(b)
Figure 2-5. 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.
TEXTURES
IN IGNEOUS ROCKS
Textures: result of nucleation+growth
Grain size
Fast growth
a
Ocean Drilling Program
Crystal zoning
Crystal shape
Growth order
Quartz - feldspar intergrowth
Remelting
Matrix texture
Twinning
Replacements
North Carolina
State University
Smith College