Origin of Magmas
Lecture Outline
- Origin of magmas
- Crystallization and igneous textures
- Evolution of magma
- Rock Classification
- Magma composition and Plate Tectonics
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
Types of Igneous Rocks
- Extrusive or Volcanic rocks (GEODe 360-365)
>> Rock crystallizes from lava at Earth’s surface
- Intrusive or Plutonic rocks (GEODe 366-370)
>> Rock crystallizes from magma within the crust
Intrusive Rocks (Yosemite)
http://www.stanford.edu/%7Ewgupta/images/yosemite%20-%20yosemite%20falls%20from%20halfway%20up.jpg
Why do rocks melt?
- Role of Heat
>> T increases with depth - Geothermal gradient: 30°C per kilometer
Temperatures of Local Groundwaters
Local geothermal gradient
Why and where do melts form?
- Role of Heat
>> T increases with depth: Geothermal gradient
30°C per kilometer
>> Near the Earth’s surface, igneous minerals melt at T’s of 700°
to 1100°C
>> At what depth does melting begin?
Convergent Plate Boundary
- If minerals melt at 700° to 1100°C, why isn’t the lower mantle
molten?
Why and where do melts form?
- Role of Pressure (P)
>> Melting T of minerals increases with increasing P
>> Hot, rising material melts as pressure decreases
(decompression melting)
>> Example: Divergent boundary (ocean crust) and hot spots
- Mid-ocean ridge (MOR)
>> Rising mantle rock undergoes decompression melting
>> Less dense magma rises and collects in magma chamber
Why and where do melts form (cont.)?
- Role of Volatiles (especially water)
>> Volatiles – compounds or elements readily vaporized to gas
Ex: H2O and CO2
- Cause explosive volcanic eruptions
- Easily lost when magma is at surface
>> Addition of water lowers melting temperatures – promotes
melting (Fig. 4.19)
>> Subducted crust carries water which enhances melting of minerals
>> Ex: Convergent boundaries
Origin of Magmas
- Melting (not so simple)
>> Different minerals have different melting temperatures
- Silica-rich minerals melt first (lower melting T),
crystallize last
- Which silicate mineral is silica rich?
Origin of Magmas
Partial melting
>> Only some of the minerals of a rock may melt
- Minerals with lowest melting T melt first (i.e., silica rich
minerals)
- Partial melt rich in silica
>> Magma of partial melt migrate to magma chamber
>> Ocean crust (basalt) forms from partial melting of Fe-Mg rich,
silica-poor mantle rock (Fig. 4.20)
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, shape, and arrangement are characteristics of the
texture of igneous rocks
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. 118)
Plagioclase (Albite)
http://www.pslc.ucla.edu/pet/mineral_html/
Igneous Textures
- Phaneritic - matrix grains visible with naked eye
>> Slow cooling
- Aphanitic - matrix grains not visible with naked eye
>> Fast cooling
- Porphyritic - large crystals (phenocrysts) in a
fine-grained matrix (noun: porphyry)
>> Phenocrysts formed early
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)
Crystal Settling
Palisades Sill, NJ
*http://www.bedford.k12.ny.us/flhs/science/stevek/palisadessill.html*
Evolution of Magmas
Magmatic Differentiation (cont.)
- 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
Magmatic Differentiation
Bowen’s Reaction Series (p. 128-129)
- Continuous reaction series
>> Ca-rich plagioclase (feldspar) crystallizes first
>> As T decreases, more substitution of Na for Ca is possible
>> No structural change
Magmatic Differentiation
Bowen’s Reaction Series (cont.)
- Discontinuous reaction series
>> Start with basaltic magma
>> Mineral structure changes at each step - “discontinuous”
Magmatic Differentiation
Bowen’s Reaction Series
Bowen Reaction Series
Bowen Reaction Series mnemonic
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
Magmatic Differentiation (cont.)
- Other factors
>> Partial melting (discussed earlier)
>> Assimilation (melting) of country rock (rock enclosing an
igneous intrusion or vein)
Subduction zones, hot spots under continent (Fig. 4.21)
>> Mixing of magmas (Fig. 4.25)
<http://faculty.evansville.edu/rl29/art105/img/incan-macchu1.jpg>http://faculty.evansville.edu/rl29/art105/img/incan-macchu1.jpg
Rock Classification
- Rocks are classified by
>> Origin (where and how they form)
>> Mineralogy (composition)
>> Texture (size, shape, arrangements of grains)
Origins of Igneous Rocks
- 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. 4.7 and 4.8)
- Silica (SiO2) makes up 40-70%
- Felsic - rich in silica (feldspar abundant)
>> Also called silicic (silica-rich)
>> Ex: granite, rhyolite, granodiorite
- Intermediate - intermediate in silica, Mg, and Fe compared
with felsic and mafic rocks
>> Ex: andesite, diorite
- Mafic - Magnesium and Fe rich; silica poor
>> Ex: basalt, gabbro
Granite/ Rhyolite (Felsic) (Fig. 4.9)
Enchanted Rock, Hill Country, Texas http://www.tpwd.state.tx.us/spdest/findadest/parks/enchanted_rock/
Granodiorite/Dacite (Felsic)
- Granodiorite / Dacite - qtz; plag > K-spar
- Granodiorite - most common intrusive rock on continent *http://geology.about.com/library/bl/peaks/blhalfdome.htm*
Diorite/ Andesite (Intermediate) (Fig. 4.14)
Stratovolcano (My. Jackson, Oregon)
*http://seis.natsci.csulb.edu/basicgeo/ANDESITE/ANDESITE.html*
Chemistry and Mineralogy of Igneous Rocks (cont.)
>> Andesite
Gabbro /Basalt (Mafic) (Fig. 4.15)
Ultramafic Rocks
>> Ultramafic - more Mg and Fe rich, and more silica poor, than
mafic rocks
Ex: peridotite (pyroxene and olivine rock)
Other Extrusive Rocks (Fig. 4.11)
- 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 Composition 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
SiO2-rich country rock; magmatic differentiation of andesitic magma
>> Intermediate from assimilation, mixing, partial melting of
basaltic rocks, magmatic differentiation of basaltic magma, etc.