Igneous Rocks
(Revised 2/3/04)
Origin of Magmas
- Igneous rocks form from molten rock
- Magma - molten rock below surface
- Lava - molten rock at surface
- Gases can escape lava more easily than magma
- Crystallization vs. precipitation
- Crystallization - crystals forming from magma or lava; occurs
with cooling ("freezing")
- Precipitation - crystals forming from aqueous (water) solution;
due to supersaturation of ions
Melting
- Melting
- Different minerals have different melting temperatures
- Silica-rich minerals first to melt (lower melting T); melt first,
crystallize last
- Mg- and Fe-rich, SiO2-poor minerals last to melt (first
to crystallize, last to melt)
Partial Melting
- Partial melting
- Only some of the minerals of a rock may melt
- Minerals with lowest melting T melt first
- Partial melt rich in silica
- Magma of partial melt may migrate to magma chamber
- Ocean crust (basaltic ) forms from partial melting of
Fe-Mg rich, silica-poor mantle rock (Fig. 3.20)
Why and where do melts form?
- Role of Heat
- T increases with depth: Geothermal gradient
- Igneous minerals melt at T’s of 700° to 1100°C
- Subducting slab gets heated and minerals begin to melt
- Ex: Convergent boundary (Fig. 1.16)
- Role of Pressure
- Melting T of minerals increases with increasing P
- Hot, rising material melts as pressure decreases
- Ex: Divergent boundary (ocean crust) and hot spots
- Role of Volatiles (especially water)
- Volatiles--compounds or elements readily
vaporized to gas
- Cause explosive volcanic eruptions
- Easily lost when magma is at surface
- Addition of water lowers melting temperatures--promotes melting
(Fig. 3.19)
- Subducted crust carries water which enhances melting of minerals
- Ex: Convergent boundaries
Crystallization and Textures of Igneous Rocks
- Individual minerals crystallize from the magma
- Minerals with higher melting temperatures crystallize first (e.g.,
olivine at mid-ocean ridges, plagioclase at subduction zones)
- The slower the cooling, the larger the crystals
Igneous Textures
- Crystal size and shape are characteristics of the texture of igneous
rocks
- Igneous textures provide evidence for the origin of the igneous rock
- Well-formed crystals crystallized early
- Coarse crystals - crystallized within the earth -Intrusive or Plutonic
- Fine crystals - crystallized at the earth’s surface -Extrusive or
Volcanic
Studying Igneous Rocks
- Petrography--description and classification of rocks
- Thin-section--thin slice of rock (~0.03 mm thick) polished for viewing
under petrographic microscope (p. 72)
Figure of hand specimen and thin-sections
Figure of plagioclase
- Note coarse plagioclase crystals in finer-grained matrix
- Ca-rich plagioclase crystallizes first (underground)
- Fine-grained crystals crystallized later, after eruption (Extrusive
rock)
Igneous Textures
- Phaneritic - matrix grains visible with naked eye
- Aphanitic - matrix grains not visible with naked eye
- Porphyritic - large crystals (phenocrysts) in a fine-grained
matrix (noun: porphyry)
More about Texture
- Quick cooling results in a more random arrangement of atoms--Glass
- Is glass made up of minerals?
- In addition to slow cooling, presence of water in a magma promotes
growth of large crystals
- Pegmatites--extremely coarse-grained igneous rock (crystals can
have cm to m lengths)
Evolution of Magmas
- If early-formed crystals are separated from magma, overall composition
of melt changes
- Process called magmatic differentiation (or fractional crystallization)
(Fig. 3.24)
Evolution of Magmas
Magmatic Differentiation
- Magmas can change in overall composition
during the course of crystallization--producing magmas (and rocks) different
in overall composition from the original magma (and rocks)
- Results in change in mineral composition and rock type with evolution
of magma
- N.L. Bowen made igneous rocks from basaltic magma in his laboratory
- Provided general understanding of the order of mineral crystallization
from a melt
Bowen’s Reaction Series (p. 82-84)
- Continuous reaction series
- Ca-rich plagioclase (feldspar) crystallizes
first
- As T decreases, more substitution of Na for Ca is possible
- No structural change
- Discontinuous reaction series
- Start with basaltic magma
- Mineral structure changes at each step - "discontinuous"
Magmatic Differentiation
- Change in composition of magma may lead to zoned crystals --composition
of mineral changes from mineral’s core to exterior
- Common with plagioclase (Continuous series)
- Core of crystal removed from contact with magma
- Mineral continues to grow with slightly different composition
Zoning in Plagioclase (figure)
Origin of Magmas
- Reequilibration
- If crystals are NOT removed, magma (melt) and crystals reequilibrate
at lower T--new mineral replaces old one (e.g., pyroxene replaces olivine)
- Overall composition of magma does not change
- Other factors
- Partial melting (discussed earlier)
- Assimilation (melting) of country rock
- Subduction zones, hot spots under continent (
Fig. 3.21)
- Mixing of magmas (Fig. 3.25)
Rock Classification
- Classified by origin
- Igneous, metamorphic, sedimentary
- Plutonic versus volcanic (igneous)
- Classified by mineralogy (composition)
- Quartzite versus marble (metamorphic)
- Granite versus gabbro (igneous)
- Classified by texture (size, shape, arrangements of grains)
- Sandstone versus siltstone (sedimentary)
- Phaneritic versus aphanitic (igneous)
Classifying Igneous Rocks
Origins
- Intrusive
- Rock crystallizes from magma within the crust
- Also called Plutonic rocks
- Phaneritic texture
- Extrusive
- Rock crystallizes from lava at Earth’s surface
- Also called Volcanic rocks
- Aphanitic texture
Chemistry and Mineralogy of Igneous Rocks (see Figs.
3.8)
- Silica (SiO2) makes up 40 - 70%
- Felsic - rich in silica (feldspar abundant )
- Also called silicic (silica-rich)
- Ex: granite, rhyolite, granodiorite
Granite/Rhyolite (Felsic) (figure)
- Granite/Rhyolite - abundant quartz and patassium
feldspar (K-spar)
Enchanted Rock
Granodiorite/Dacite (Felsic) (figure)
- Granodiorite/ Dacite - qtz; plag > K-spar
- Granodiorite - most common intrusive rock on continen
t
Half Dome
Chemistry and Mineralogy of Igneous Rocks (cont.)
- Intermediate - intermediate in silica, Mg, and Fe compared with felsic
and mafic rocks
Diorite/Andesite (Intermediate) (figure)
- Diorite/ Andesite -plag, amph; minor pyx; little
or no qtz
Stratovolcano - Mt. Jackson, OR
Chemistry and Mineralogy of Igneous Rocks (cont.)
- Mafic - Magnesium and Fe rich; silica poor
Gabbro/Basalt (Mafic) (figure)
- gabbro/ basalt -pyx, plag; olivine; no qtz
- Basalt is the most common extrusive rock
Ultramafic Rocks
- Ultramafic - more Mg and Fe rich, and more silica
poor, than mafic rocks
Ex: peridotite (pyroxene and olivine rock)
Igneous Rock Types - Summary
Intrusive/ Extrusive (Fig. 3.8)
- granite/ rhyolite -abundant qtz and K-spar
- granodiorite/ dacite -qtz; plag>K-spar
- diorite/ andesite -plag, amph; minor pyx; little or no qtz
- gabbro/ basalt - mostly pyx & plag; ol; no qtz
- peridotite/ komatiite (rare) - mostly pyx & ol
- Decreasing silica, increasing Mg downward; becoming darker downward
Other Extrusive Rocks
- Obsidian - glass (dark brown or black but silica rich)
- Pumice - glass with abundant vesicles ("frozen" bubbles; froth)
- Pyroclastic Rocks ("fire particles")
- Tuff - forms from volcanic ash
- welded tuff - ash fused by heat
- Volcanic breccia - forms from larger pyroclasts (broken fragments
of rock)
Magma and Plate Tectonics
- Divergent boundary--simple
- Basaltic magma from partial melting of mantle
- Convergent boundary--complex
- Basaltic magma from downgoing slab of mantle and ocean crust
- Granitic magma from melting of sediments; assimilation of SiO
2-rich country rock; magmatic differentiation of andesitic magma
- Intermediate from assimilation, mixing, partial melting of basaltic
rocks, magmatic differentiation of basaltic magma, etc.
Rock Cycle (Fig. 1.12)
Useful Terms
- Bedrock - solid rock underlying soil or unconsolidated
sediments
- Basement or basement rock - the igneous and/or
metamorphic rock underlying sediment and sedimentary cover
- Parent rock - original rock before it its changed
by metamorphism or other process
- Country rock - rock enclosing an igneous intrusion
or vein
