Continued intro & classification
Phonolite
13
Tephriphonolite
Wt.% Na 2O+K 2O
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
wt% SiO2
65
63
69
ACIDIC
73
77
Updates
•Lab starts this week
•Monday 9.30-12.20 lab,
•Wednesday 2.30-5.30 lab
Today’s topics:
1a. Heat, pressure and the geotherm
1b. Geotherm and melting
1c. Different ways to melt
2. Classification of igneous rocks
The Pressure
Gradient
0
Pressure (GPa)
30
10
20
1000
• P increases = rgh
• Nearly linear through mantle
– ~ 30 MPa/km
  1 GPa at base of ave
crust
• Core: r incr. more rapidly
since alloy more dense
Figure 1-8. Pressure variation with depth.
From Dziewonski and Anderson (1981). Phys.
Earth Planet. Int., 25, 297-356. © Elsevier Science.
Mantle
2000
3000
4000
Core
5000
6000
40
Heat
Heat Sources
in the Earth
The Geothermal Gradient
Crust
Temperature(K)
0
5
Lithosphere
10
Asthenosphere
Mantle
Depth (km)
1000
S waves
Mesosphere
2000
P waves
3000
Outer
Core
4000
Liquid
5000
Inner
Core
S waves
6000
Solid
Rocks at different depth:
change in P & T: Phase
change
T
T
P
P
Atoms have a “choice” as to
how they want to arrange
themselves in a phase
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
Geotherm and solidus: how to melt
Processes That Form Magma
Processes that form magma
and rock
Environments of Magma
Formation
Environments of Magma
Formation
Environments of Magma
Formation
Environments of Magma
Formation
Environments of Magma
Formation
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)
Classification of Igneous
Rocks
X
10
10
20
%Z
30
20 %Y
A
Inc
%Z
r
Inc
r%
Y
10
Z
Incr %X
%X
30
20
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.
Classification of Igneous
Rocks
X
100
70
A
%X
0
Z
67
Y
Figure 2-1b. Method #2 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.
Classification of Igneous Rocks
Plagioclase
Anorthosite
Figure 2-2. A classification of the phaneritic
igneous rocks. b. Gabbroic rocks. c. Ultramafic
rocks. After IUGS.
l ite
cto
Tro
Ga
bb
ro
90
Olivine
gabbro
Olivine
Dunite
90
Peridotites
Lherzolite
Plagioclase-bearing ultramafic rocks
Pyroxene
Olivine
40
(b)
Pyroxenites
Olivine Websterite
Orthopyroxenite
10
(c)
10
Orthopyroxene
Websterite
Clinopyroxenite
Clinopyroxene
Classification of Igneous Rocks
Phonolite
13
Tephriphonolite
Wt.% Na 2O+K 2O
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
Crystal
Tuff
Lithic
Tuff
Crystals
Rock Fragments
(a)
70
Pyroclastic
Breccia or
Agglomerate
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.