Igneous Rocks

1. IGNEOUS ROCKS

2. Categories of Igneous Rock
Volcanic (extrusive)
Plutonic (intrusive)

3. Skaftafjell National Park, Iceland
Fig. 4-CO, pp. 100-101
Columnar joints
from because of
cooling and
contraction of
magma.

4. Table 4-1, p. 103

5. Magma Composition
Felsic Lava: high percentage (>63%) of silica, and
trapped gasses; highest viscosity, lowest
temperatures; rich in iron (Fe) and (Mg)
Intermediate Lava: (52-63%) of silica
Mafic Lava: (45-52%) of silica + high percentage of
Magnesium (Mg); typically occur at subduction zones;
rich in aluminum (Al), sodium (Na), potassium (K) and
water.

6. Classic Subduction Zone
Subduction melts crust
and makes mafic lava,
rich in Al, Na, K, H20

7. Volcanic Monitoring, Hawaii
Fig. 4-1, p. 102
Temp: Lava
can range
from 1000
to 1200 C.
Viscosity:
controlling
factors include
temp., silica
content,
volatile
content, shear
stress, and
crystallinity.

8. Bowen’s Reaction Series
Fig. 4-9, p. 111

9. Table 4-2, p. 110

19. Mt. Rushmore National Monument, SD
Fig. 4-2a, p. 102
Carved in
Harney
Peak
Granite.

20. Crazy Horse Memorial, SD
Fig. 4-2b, p. 102
Carved in
Harney
Peak
Granite.

21. STEP 1: IDENTIFY THE
ROCKS COLOR INDEX (CI)

22. Classification of Igneous Rocks
Fig. 4-16, p. 117
Diagram shows the
relative proportions
of the main minerals
and textures of
common igneous
rocks.

23. Felsic Igneous Rocks
Light colored
0-15% mafic minerals
Quartz and Potassium
Feldspar dominant
Oriskany Sandstone from Hancock, West
Virginia: also known as “glass sand” contains
light gray quartz crystals.

24. Intermediate
Light colored to gray
16-45% mafic crystals
Plagioclase Feldspars
dominate
Feldspars can be split into two main
groups, the Alkali Feldspars and the
Plagioclase feldspars

25. Mafic Igneous Rocks
Dark colored
46-85% mafic minerals
Plagioclase Feldspars,
Olivine, and Amphibole
dominate
Hornblende in rock, Iron
aluminum silicate,
Lucas County Iowa

26. Ultramafic Igneous Rocks
Very dark in color.
86-100% mafic
minerals.
Olivine and pyroxene
are dominant crystals
This is a rock called
peridotite (= olivine and
pyroxene), which forms
much of the upper mantle.

27. STEP 2: IDENTIFY THE MAIN
ROCK FORMING MINERALS

28. Quartz (gray or pink)
More info at -
http://www.geo.umn.edu/courses/1001/min
erals/quartz.shtml

29. Plagioclase Feldspars (white)
This white feldspar shows two
cleavages (top/bottom and sides)
plus the fracture surface (front).

30. Potassium Feldspar (K-Spar) (pink)
More info at -
http://www.geo.umn.edu/courses/1001/min
erals/potassium_feldspar.shtml

31. Muscovite (brown)
More info -
http://www.geo.umn.edu/courses/1001/minerals/muscovite.shtml

32. Biotite Mica (black)
More info at -
http://www.geo.umn.edu/courses/1001/minerals/biotite.shtml

33. Amphibole (dark gray)
More info at -
http://www.geo.umn.edu/courses/1001/min
erals/amphibole.shtml

34. Pyroxene (dark green)
More Info at -http://www.geo.umn.edu/courses/1001/minerals/pyroxene.shtml

35. Olivine (green)
More Info at -http://www.geo.umn.edu/courses/1001/minerals/olivine.shtml

36. STEP 3: IDENTIFY ITS
TEXTURE

37. Rapid cooling of extrusive lava produces an
AphaniticTexture
Fig. 4-14ab, p. 116
Faster cooling from extrusive magma, produces
small minerals with a fine-grained texture

38. Slow cooling in plutonsproduces
PhaneriticTexture
Fig. 4-14cd, p. 116
Slower cooling from plutons where the rate of
growth exceeds the rate of nuclei formation forms
relatively large mineral grains that can be seen.
These visible course-grained minerals have a
phaneritic texture.

39. Uneven cooling produces
PorphyriticTextures
Fig. 4-14ef, p. 116
Groundmass

40. Other Textures
Fig. 4-14g-i, p. 116
Glassy texture
because magma
cooled extremely
quickly.
Gasses expand
and leave a
vesicular texture.
Fragmental texture
formed by explosive
eruptions.

41. Volcanic Breccia
Fig. 4-15, p. 117
Consists of poorly
sorted mixture of
fine grains and
larger angular
fragments produced
by especially violent
eruptions, volcanic
landslides and
mudflows near
volcanoes.

42. STEP 4: USE IGNEOUS
ROCK FLOW CHART

43. Igneous Rock Flow Cart

44. Andesite is a fine-
grained, extrusive
igneous rock
composed mainly of
plagioclase with
other minerals such
as hornblende,
pyroxene and biotite.

45. Basalt-Gabbro
Balsitic Lava: (45-52%) of silica + high
percentage of Iron (Fe); typically occur at
oceanic divergent pages

46. Utramafic Rock
Formed under highest temperatures
Composed largely of feerromagnesian silicates
(high in iron)
Silica content is (=<45%)
Has a very low viscosity

47. Periodite
Fig. 4-17, p. 118
Ultramafic rock
made up mostly of
olivine. Makes up
most of the mantel.

48. WHAT ARE THE LARGEST
IGNEOUS INTRUSIONS?

49. Plutonic Variables
Composition
Size
Depth

50. Block Diagram of Igneous Intrusions
Fig. 4-24, p. 123
B: > 100 km2
S: <100 km2

51. WHAT ARE THE MAJOR
SHEETLIKEIGNEOUS
INTRUSIONS?

52. Dikes and Sills
D & S: most are mafic.
Veins: most are felsic.
L: inflated sills.

53. Dikes in the Field
Fig. 4-25a, p. 124
Herchenberg volcano,
Eifel district, Germany

54. Sills in the Field
Fig. 4-25b, p. 124
Mafic sills in lighter-colored
country rock, Santa
Monica, CA