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

2. Naming Igneous Rocks Igneous rocks are classified by their: Texture
Mineral Composition
depends on: how fast/slow magma cools
depends on: chemical makeup of parent magma

4. Fig. 6.15 W. W. Norton Cooling rate in igneous rocks is determined by
which is in
turn,
determined
by proximity to
the surface.
Fig. 6.15
W. W. Norton

5. Fig. 6.16a Coarse-grained or “phaneritic” Fine-grained or “aphanitic”
0 Mm 0.5
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

7. Fig. 6.14cd
Stephen Marshak

8. Fig. 6.12
W. W. Norton

9. Fig. 6.19
Stephen Marshak

10. Mafic Felsic Naming Igneous Rocks High in Mg, Fe. Dark, dense
Basic magma types:
Mafic
Felsic
magnesium + ferrum
High in Mg, Fe. Dark, dense
feldspar + silica (quartz)
High in Si. Lighter, less dense

11. Naming igneous rocks Basalt volcanic M a f i c Gabbro plutonic
Zooming in:
Basalt
volcanic
Aphanitic
M a f i c
Gabbro
plutonic
Phaneritic

12. Fig. 6.17a
Figure 6.17
Text, page 155

13. Naming igneous rocks volcanic Andesite In t e r m e d i a t e plutonic
Zooming in
Andesite
volcanic
Aphanitic
In t e r m e d i a t e
Diorite
plutonic
Phaneritic

14. Fig. 6.17a
Figure 6.17
Text, page 155

15. Naming igneous rocks volcanic Rhyolite F e l s i c plutonic Granite
Zooming in:
Rhyolite
volcanic
Aphanitic
F e l s i c
Granite
plutonic
Phaneritic

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

17. (less than gabbro/basalt,
Naming Igneous Rocks:
Ultramafic: ~ 40% Silica
(less than gabbro/basalt,
with more Fe and Mg).
Upper mantle plutonic
igneous rocks,
“peridotite” or “dunite”.
(Near Globe, AZ)

18. Obsidian Volcanic Glass F e l s i c Volcanic “Froth” Pumice
Naming Igneous Rocks:
Glassy Volcanic Rocks
Obsidian
Volcanic Glass
F e l s i c
Volcanic “Froth”
Pumice

19. crystallize all at once!
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.

20. 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.

21. Basaltic lavas are very fluid and travel far from the vent to
produce volcanoes with low profiles.

22. Basalt eruptions on land produce flows
that travel great distances.
Fig. 6.20a
W. W. Norton
Columbia River
basalts

23. Basaltic lavas erupted under water produce “pillows”
Fig. 6.21a
W. W. Norton

24. Fig. 6.21b
Stephen Marshak

25. Fig. 6.21c
© Peter Kresan

26. Fig. 6.02 W. W. Norton Products of volcanic eruptions includes more
than
just
lava
flows!
Fig. 6.02
W. W. Norton

27. Pyroclastic Volcanic Rocks
Explosive Volcanic
Eruptions
Violently explosove volcanic
eruptions produce:
Rock fragments
(all sizes)
Finely-fragmented ash
Molten bombs
These accumulate
to form:
Pyroclastic
Volcanic
Rocks

29. Fig. 6.08a
W. W. Norton

30. Naming Igneous Rocks Pyroclastic volcanic rocks:
Composed of rock fragments ejected during eruptions. Types include:
“Tuff”: Composed of ash, finely fragmented volcanic rock. Welded tuffs form when ash is so hot, it deforms plastically.
Scoria: Red or black, frothy lava, denser than pumice.
Volcanic Breccia: Coarse, angular rock fragments, usually in an matrix of fine to coarse ash.

31. Obsidian: Volcanic glass. Formed by quenching of lava.
Pumice: Formed by
the quenching of
gas-charged lava.
Obsidian: Volcanic glass.
Formed by quenching of lava.
Volcanic ash:
Lithifies to form a
volcanic tuff.