Igneous Rocks
Magma
- is the molten, parent material of igneous rocks
magma is composed of three parts
1. liquid-the portion is called "melt", and is composed of Si, O, Al,Ca,K,Na,Fe and Mg ions
2. Solids- if present, the solid portion consists of crystallized silicate minerals
3. Volatiles- the gaseous portion; most common ar H2O, CO2 and So2
The Origin of Magma
- magma is molten material. When we see is erupting on the Hawaiian Islands, we think of it as being very hot, sticky and gooey-closer to liquid than a solid
- magma originates in the mantle, and rise toward the earth surface
- lave is magma that has erupted at the earth surface
- magma originates in the mantle whent he mantle rock called peridotite begins melting
Mechanisms for melting solid mantle rock
1. Increae the geothernal gradient, and add heat throught decompression melting.
or
2. reduce the temperature that mantle rock melts by adding volatile components
Geothermal Gradient- the rate of change of temperature, per unit depth in earth
1. Ways to Increae the Geothermal Gradient
A. Heat from the decay of radiocative elements
- this is not likely as most radioactive elements are largely concertrated within the crust
B. Heat generated by friction: in areas where rocks are sliding by one another( at the base of the lithsophere, or withing sudbduction zones)
- this likely to only cause localized heating
C. Decompression due to convcetion (decompression melting): with a decrease in confining pressure comes a decrease in melting temperature. Upwelling magma will experience decompression and being to melt at lower temperatures, thus increasing the geothermal gradient.
2. Dreceasing the melting temperature of mantle rock
fact: mixtures of multiple com ponets tend to melt at lower temperatures than the pure components alone.
fact: Volatile cause rock to melt at lower temperatures
How might volatiles be added to magma?
A. oceanic crust is covered with sediment consisting of clays (containg water) and carbonate minerals ( contiaining carbon dioxide)
B. Oceanic crust is metamorphosed as ti is subducted, going through various dehydration and decarbonation reactions
C. Oceainc crust is in contact with seawater which may occupy pore space
- addition of volatiles throught these methods could be sufficient to loaer the melting temerature of peridotite
Four Basic Types of Magma
• Ultramafic- a rock compsoed chieflu of dark, ferromagnesian minerals, trace amounts of other minerals amphiboles, pyroxenes, and plagioclase feldspar. Slica content is low +/- 40%
• Mafic – Said of a rock composed of a high percentage of dark, ferromagnesian minerals, and the remaining mineral type primarily plagioclase feldspar. Example: Gabbro (intrusive), Basalt (extrusive).
• Intermediate/Andesitic –A rock with a composition between that of mafic and felsic.
• Felsic: A rock composed almost entirely of light colored silicate minerals such as quartz, feldspar and muscovite. Silica content is highest at 70+%: Examples: Granite (intrusive), Rhyolite (extrusive).
The Evolution of Magma
Observation 1: we know there is a wide varity of ingeous rocks. it is only locigical that there must also be a wide vaarety of magma types
Observation 2: some volcanos that experience multiple eruptions may extrude levels very different compositions over a relatively short peroid of time
• rock composition is different from composition of original magma
• magma composition changes
• the magma cools as it rises
• start with magam rising form the mantle
Bowen's Reaction Series
Norman Levi Bowen (1887-1956)
- Trained in mineralogy and chemistry at queen's Uinveristy in Ontario,
- MIT class of 1912; the third PhD. granted by MIT
- conducted experiments at the Geophysical Laboratory in Washington
- taught and conducted research at the University of Chicago
- spent his professional life studying the crystallization of magma
-based on many laboratory testes and extensive field work, Bowen developed a model sequence showing the order that minerals crystallize from a magma.
two ways that minerlas crystallization can occur in a magma:
1. the Bowen's reaction. series assumes that if the solid components ( newly formed crystals) are removed from the remining melt, then one can account for many of the wide variety of ingeous rock types that otherwise would be be possible from the original parent magma.
2. if the solid compomnents arenot remvoed fromthe melt, then they will react with the cooling melt to produce the next phase in the Bowen's Reaction series, still maintaining essentially the same magma composition.
Bowen's Reaction Series in 5 Steps.
1 Convecting, upwelling magma at a divergent plate boundary experiences a decrease inpressure and beings top melt.
2 as the magma nears the suface, the internal temperature of the magma begins to decrease
Now we being Bowen's Reaction Series
3 Ultramafic minerals begin to crystallize within the magma, the first minerals to cyrstallize are rich in iorn (Fe), magnesium (Mg), and calcium (Ca)
4. If Fe/Mg/Ca are "removed" from the magma ( locked up in the new minerals stucture and somehow separated fromt he melt), the magma is best defined as mafic, slowly becoming richer in silica ( Si), sodium (Na), and potassium (K).
5. If new crystals are removed from the magma, the magma composition continues to change and becomes felsic ( Si, Na, K-rich) crystallizing minerals sucha K- feldspar and quartz
Magmatic Differentiation
any process that causes magma composition to change is calld magmatic differentiation
- three processes that induce magma composition change:
1 Crystal settling- crystals settle to the bottom of the magma reservoir as magma cools.
2 Assimilation - "country rock" becomes incorporated into the magma
3. Magma mixing- one magma intrides another magma
Partial Melting
partial melting- is a process in which a rock does not completely melt
- if a rock doesn't melt completley, using what we know fron Bowen's Reaction Series:
- the liquid portion will consist largely of the melted minerals seen at the bottom/cool end of the Bowen's Reaction Series
- th soild portion will consist of higher temperature minerals.
- if both portions are separated at this point, the melt will be of a more felsic composition.
Extrusive and Intrusive magma
extrusive- magma that erupts thorught the Earth's surface.
Intrusive- magma that solidifies before it reaches the earth's surface
Examples
Intrusive Extrusive
Mafic Gabbro Basalt
Intermediate/ Andesitic Diorite Andesite
Felsic Granite Rhyolite
Ultramafic Magma
- is very rich in Fe and Mg
- Typically contians < 40 to 45% silica
- Consits priamrilyof periodotite
- may also be referred to as " ultrabasic", which may include rock types like carbonatite and ultrapotassic ingeous rocks
- the earth's mantle is composed of ultramafic magma.
Basaltic Magma
basaltic magma is mafic
- most magma that erupsts is basaltic, with eruption temperatures of 1,000 to1,200 C. This suggests partial or complete melting of a mantle source for the following reasons:
1 Rocks from the lower crust are typically < 700 C
2. Basaltic magam often contians unmelted peridotite ( mantle rock) fragments
- " Primitve" basalti magmas are derived from the direct melting of mantle rocks. these magmas havenot "evolved"
Rhyolitic / Andestic Magma
Andesitic magma - intermeditate
Rhyolitic magma- felsic(Si-rich)
these magmas are typically erupted along contiental margins.
Andesitic magma- forms as risign basaltic magma ponds beneath Si-rich crustal rocks and slowly cools. Magmatic differentiation results in a more Si-rich melt
Rhyolitic magam- felsic magma, though not very common, may form as
1. Basaltic magma migrates upward and ponds beneath crustal rocks and experineces magmatic differentiation
2. Andesitic magma rises throught crustal rocks and is magmatically differentaited
Intrusive Igneous Bodies
Granitic / Andesitic / Basaltic Magma
- basaltic magma that cools before reaching the surface is called Gabbro
-Andesitic magma- Intermediate
- Granitic magma - Felsic (Si-rich)
these magmas are typically erupted along continental margins
- Andesitic magma- form as rising basaltic magma ponds beneath Si-rich crustal rocks and slowly cools. Magmatic differentiation results in a more Si- rich melt
-Granitic magma-felsic magma, though no very common, may form as:
1 basaltic magma migrates upward and ponds beneath curstal rocks ans experiences magmatic differentiation
2 Andesitic magma rises through crustal rocks and is magmatically differentiated
Intrusive Igneous Bodies
- ignwous intrusions, called plutons, may be descirbed as massive, tabular, discordant ( cutting across existing stuctures) nor concordant.
Tabular Varieties
-Sill- Concordant with the surroundign rock stucture.
-Dike - Discordant, often forming in swarms
- Phacolith- Minor concordant intrusive in either the crest of an anticline or trough of a syncline
- Laccolith- concordant, causing doming of overlying country rock, is relatively thin, and has a known floor and likely feeder
Massive Varieties
- Batholith-typically felsic or intermediate, discordant intrusive with a surface exposure greater than 40 square miles and no known floor
- Stock- not always discordant, and with less than 40 mi2 surface exposure
Minerals in Igneous Rocks
Igneous rocks are composed mainly of silicate minerals.
-there are two major groupings of silicate minerals within igneous rocks:
- Ferromagnesian- typically dark colored, rich in iron and/or magnesium, relatively depleted in silica examples: Olivine, pyroxene, amphiole, biotite mica
- Non- ferromagnesian- typically light colored, contain more potssium, sodium, and calcium than iron magnesium examples: quartz, muscoite mica, feldspar
Types of Ingeous Rocks
-the texture of an igneous rock reveals much about the enironment that the rock formed in.
For example:
1. Magma that cools very slowly will tend to result in rock that has larger crystal
2. Magma that cools quickly will tend to result in a rock that has fine-grained/ small crystal
3. A magma that been quenched quickly will result in a rock/mineral that deos not have a crystalline stucture -- it is a glass
Exampels
- aphanitic - fine-grained; often seen in extrusive rocks
- phaneritic- coarse-grained; common to intrusive igneous rocks
-porphyritic- a texure in which there are large crystals ( phenocrysts) in matrix of smaller minerals.
- matrix- usually fine-grained material; also called groundmass
- pyroclastic- a rock texture resulting from a violent eruption causing roc fragments (clasts) to be caught in a finer matrix (often an ash). Rocks exhibiting pyroclastic texture appear similar to some sedimentary rocks
-glassy- a texture produced by rapid cooling or quenching
- pegmatitc- extremely coarse-grained rock formed of interlocking crystals, each larger than 1 centimeters (cm) in diameter
Igneous Rock Classification
Igneous rocks are usually identified by describing composition and texture. example: basalt porphyry
-In this case, we know that the rock is basaltic ( based on mineralogy),
and that there are large crystals ( most likely plagioclase feldspar) scattered throughtout a fine-grained groundmass
Classification: Felsic & Phaneritic
example:
-Granite - a plutonic rock; often phaneritic
Quartz= 10 -50%
K-spar/ total Feldspar ratio= 65 50 90%
Dark Minerals <10%
Other minerals: Muscovite, Biotite, amphibole
-Rhyolite- extrusive equivalent of granite; typically aphanitic, though may contain small phenocrysts
- Granite Porphyry- a granite containing phenocrysts of mica, amphibole, pyroxene, or feldspar in a fine-grained matrix
Classification: Intermediate & Phaneritic
- Diorite- a plutonic, intermediate (between felsic and mafic) rock typically composed of hornblende, oligoclase or andesine, pyroxene, and sometimes a litttle quartz; diorite is the pproximate intrusive equivalent of andesite.
- andesite- an dark colored, aphanitic extrusive rock ( equivalent of the intrusive diorite). Named in 1826, based on rock type from the Andes Mountians.
- Andesite Porphyry- same composition as andesite; phenocrysts typically plagioclase, biotite, horneblende, or pyroxene.
Classification: Mafic & Phanertic
Gabbro - Dark colored intrusive rock typically compsed of plagioclase feldspar, augite ( an amphibole) and sometimes olivine and /or orthopyroxene. It is the approximate intrusive equivalent of basalt
Basalt- extrusive compositional equivalent of gabbro
Basalt prophyry- a basalt containing phenocrysts of plagioclase feldspar
Rock Type: Pyroclastic
-Tuff - a genral term for all consolidated pyroclastic rocks, but usually used to describe a rock composed of ash-size fragments. A welded tuff is glass-rich pryroclastic rock where ting glass shards have been welded together due to heat, weight, of overlying materials and hot gases. A vitric tuff is composed chiefly of fragment of glass
- Volcanic breccia- a pyroclastic rock composed of paricles larger than ash-size in a fine matrix. Fragments are typically either angular volcaniic fragment in volcanic matrix, or else non-volcanic fragments in a volcanic matrix.
- Volcanic bomb- a blob of lava that was ejected while viscous and received a rounded shape while in filght- typically hardening prior to landing
Pyroclastic Volcanism
- explosive, pyroclastic volcanic situatiuons develop as volcanic gase build up inside a volcano
- gas pressures within magmas typically increase with increasing SiO2 content
- as SiO2 content increases, the magma becomes thicker and sticky
Rhyolitic magma 65-75% SiO2 extremely violent eruption
Andesitic magma 55- 65%
Basaltic Magma 45-55% Ongoing, flowing eruptions