Igneous Rocks

IGNEOUS ROCKS

Categories of Igneous Rock

Volcanic (extrusive)

Plutonic (intrusive)

Skaftafjell National Park, Iceland

Fig. 4-CO, pp. 100-101

Columnar joints

from because of

cooling and

contraction of

magma.

Table 4-1, p. 103

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.

Classic Subduction Zone

Subduction melts crust

and makes mafic lava,

rich in Al, Na, K, H20

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.

Bowen’s Reaction Series

Fig. 4-9, p. 111

Table 4-2, p. 110

Mt. Rushmore National Monument, SD

Fig. 4-2a, p. 102

Carved in

Harney

Peak

Granite.

Crazy Horse Memorial, SD

Fig. 4-2b, p. 102

Carved in

Harney

Peak

Granite.

STEP 1: IDENTIFY THE

ROCKS COLOR INDEX (CI)

Classification of Igneous Rocks

Fig. 4-16, p. 117

Diagram shows the

relative proportions

of the main minerals

and textures of

common igneous

rocks.

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.

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

Mafic Igneous Rocks

Dark colored

46-85% mafic minerals

Plagioclase Feldspars,

Olivine, and Amphibole

dominate

Hornblende in rock, Iron

aluminum silicate,

Lucas County Iowa

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.

STEP 2: IDENTIFY THE MAIN

ROCK FORMING MINERALS

Quartz (gray or pink)

More info at -

http://www.geo.umn.edu/courses/1001/min

erals/quartz.shtml

Plagioclase Feldspars (white)

This white feldspar shows two

cleavages (top/bottom and sides)

plus the fracture surface (front).

Potassium Feldspar (K-Spar) (pink)

More info at -

http://www.geo.umn.edu/courses/1001/min

erals/potassium_feldspar.shtml

Muscovite (brown)

More info -

http://www.geo.umn.edu/courses/1001/minerals/muscovite.shtml

Biotite Mica (black)

More info at -

http://www.geo.umn.edu/courses/1001/minerals/biotite.shtml

Amphibole (dark gray)

More info at -

http://www.geo.umn.edu/courses/1001/min

erals/amphibole.shtml

Pyroxene (dark green)

More Info at - http://www.geo.umn.edu/courses/1001/minerals/pyroxene.shtml

Olivine (green)

More Info at - http://www.geo.umn.edu/courses/1001/minerals/olivine.shtml

STEP 3: IDENTIFY ITS

TEXTURE

Rapid cooling of extrusive lava produces an

Aphanitic Texture

Fig. 4-14ab, p. 116

Faster cooling from extrusive magma, produces

small minerals with a fine-grained texture

Slow cooling in plutons produces

Phaneritic Texture

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.

Uneven cooling produces

Porphyritic Textures

Fig. 4-14ef, p. 116

Groundmass

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.

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.

STEP 4: USE IGNEOUS

ROCK FLOW CHART

Igneous Rock Flow Cart

Andesite is a fine-

grained, extrusive

igneous rock

composed mainly of

plagioclase with

other minerals such

as hornblende,

pyroxene and biotite.

Basalt-Gabbro

Balsitic Lava: (45-52%) of silica + high

percentage of Iron (Fe); typically occur at

oceanic divergent pages

Utramafic Rock

Formed under highest temperatures

Composed largely of feerromagnesian silicates

(high in iron)

Silica content is (=<45%)

Has a very low viscosity

Periodite

Fig. 4-17, p. 118

Ultramafic rock

made up mostly of

olivine. Makes up

most of the mantel.

WHAT ARE THE LARGEST

IGNEOUS INTRUSIONS?

Plutonic Variables

Composition

Size

Depth

Block Diagram of Igneous Intrusions

Fig. 4-24, p. 123

B: > 100 km2

S: <100 km2

WHAT ARE THE MAJOR

SHEETLIKE IGNEOUS

INTRUSIONS?

Dikes and Sills

D & S: most are mafic.

Veins: most are felsic.

L: inflated sills.

Dikes in the Field

Fig. 4-25a, p. 124

Herchenberg volcano,

Eifel district, Germany

Sills in the Field

Fig. 4-25b, p. 124

Mafic sills in lighter-colored

country rock, Santa

Monica, CA