More igneous rocks

In Lecture Today:
1. Geology in the news presentation by Michael
Zara.
2. Classification of igneous rocks.
3. Nature and classification of igneous plutons.
4. Origins of magma and relationship to plate
tectonic setting.


Naming Igneous Rocks

Igneous rocks are classified by their:
Texture
depends on: how fast/slow magma cools

Mineral Composition
depends on: chemical makeup of parent magma



Cooling rate
in igneous
rocks
is determined
by

Fig. 6.15

cooling rate,
which is in
turn,

determined
by proximity to
the surface.

W. W. Norton


Coarse-grained

Fine-grained

or “phaneritic”

or “aphanitic”

Fig. 6.16a

0 Mm 0.5

Grain size in igneous rocks
is determined by cooling rate.
Photod by Dr. Kent Ratajeski, Dept. Geology and Geophysics, University of Wisconsin, Madison

0 Mm 0.5



Fig. 6.14cd
Stephen Marshak

Fig. 6.12
W. W. Norton


Fig. 6.19
Stephen Marshak


Naming Igneous Rocks

Basic magma types:

Mafic

magnesium + ferrum

High in Mg, Fe. Dark, dense

Felsic

feldspar + silica (quartz)

High in Si. Lighter, less dense


Naming igneous rocks

Mafic

Aphanitic

Gabbro
Phaneritic

plutonic

Basalt

volcanic

Zooming in:


Fig. 6.17a

Figure 6.17
Text, page 155


Naming igneous rocks

Aphanitic

Diorite
Phaneritic

volcanic

Andesite

plutonic

In t e r m e d i a t e

Zooming in


Fig. 6.17a

Figure 6.17
Text, page 155


Naming igneous rocks

Aphanitic

Granite
Phaneritic

plutonic

Felsic

Rhyolite

volcanic

Zooming in:

Fig. 6.17a
W. W. Norton. Mineral proportions
after Hamblin and Howard.


Naming Igneous Rocks:
Upper mantle plutonic
igneous rocks,
“peridotite” or “dunite”.

(Near Globe, AZ)

Ultramafic: ~ 40% Silica
(less than gabbro/basalt,
with more Fe and Mg).


Naming Igneous Rocks:
Glassy Volcanic Rocks

Felsic

Obsidian

Pumice

Volcanic Glass

Volcanic “Froth”


Bowen’s Reaction Series
Magmas don’t
crystallize all at once!
Minerals with the highest
melting temperatures come
out first, followed by
minerals with successively
lower melting temperatures.
The sequence of silicate
mineral crystallization in
magmas was first studied by
University of Chicago
geologist, N.L Bowen, in the
1920’s.


Bowen’s Reaction Series
Box 6.2 Text, page 145.

Fig. 6.06
W. W. Norton
This diagram shows the order in which silicate
minerals crystallize from magma and how that
Relates to mineral content and rock type.


Basaltic lavas are very fluid and travel far from the vent to

produce volcanoes with low profiles.


Basalt eruptions on land produce flows
that travel great distances.

Fig. 6.20a
W. W. Norton
Columbia River
basalts


Basaltic lavas erupted under water produce “pillows”

Fig. 6.21a
W. W. Norton


Fig. 6.21b
Stephen Marshak


Fig. 6.21c
© Peter Kresan