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.
22. Classification of Igneous Rocks
Diagram shows the
relative proportions
of the main minerals
and textures of
common igneous
rocks.
Fig. 4-16, p. 117
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.
37. Rapid cooling of extrusive lava produces an
Aphanitic Texture
Faster cooling from extrusive magma, produces
small minerals with a fine-grained texture
Fig. 4-14ab, p. 116
38. Slow cooling in plutons produces
Phaneritic Texture
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.
Fig. 4-14cd, p. 116
40. Other Textures
Glassy texture Gasses expand Fragmental texture
because magma and leave a formed by explosive
cooled extremely vesicular texture. eruptions.
quickly.
Fig. 4-14g-i, p. 116
41. Volcanic Breccia
Consists of poorly
sorted mixture of
fine grains and
larger angular
fragments produced
by especially violent
eruptions, volcanic
landslides and
mudflows near
volcanoes.
Fig. 4-15, p. 117
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
Ultramafic rock
made up mostly of
olivine. Makes up
most of the mantel.
Fig. 4-17, p. 118
