Classification Of Igneous Rocks Flashcards
molten rock material generated by partial melting of Earth ’ s mantle and crust
Magma
At temperatures over ____ above the crystallization temperature of most minerals, magma tends to be enriched in liquids and dissolved gases.
1200 ° C
Gas solubility in liquids is related to pressure. At high pressures in the lower crust and mantle, gases are readily dissolved in liquid magma. As magma rises towards Earth ’ s surface, decompression causes gases to segregate from the melt as a separate phase.
exsolution
Magma that solidifies within Earth produces _ or_ rocks.
intrusive or plutonic
are magma chambers of various sizes, shapes and depths that store magma within Earth
Plutons
Magma that rises and erupts onto the surface of Earth is called
Lava
rocks form by solidifi cation of lava and volcanic debris on Earth ’ s surface, producing rocks with small crystals and/or non - crystalline particles of various sizes.
Volcanic or extrusive
How d o w e c lassify i gneous r ocks?
Igneous rocks are classifi ed according to composition and texture.
enriched in the elements silicon and oxygen which bond together to form the silica tetrahedron.
silicate magmas
Silicate magmas contain anywhere from? (%)
40% to over 75%
magma and igneous rocks are classified as _, _, _, _based upon percent SiO 2
ultrabasic , basic , intermediate and acidic
Acidic rocks are also referred to as _ , based on their high SiO 2 content.
silicic
Dark - colored minerals are generally enriched in the elements _, _and are referred to as _, _
iron and magnesium, ferromagnesian or mafic minerals
Light colored felsic minerals are depleted in ferromagnesian elements and are generally enriched in elements such as_, _, _, _
silicon, oxygen, potassium and sodium.
is a very dark - colored (ultramaf i c) rock, depleted in SiO 2 (ultrabasic) and commonly enriched in the minerals pyroxene, olivine, amphibole and plagioclase. Ultramafi c plutonic rocks occur in Earth ’ s mantle
Peridotite
_, _are dark - colored (mafi c), SiO 2 - poor (basic) rocks rich in plagioclase, pyroxene and olivine. Basalt is a very common volcanic rock – encompassing the upper few kilometers of ocean
Basalt and gabbro
Classification of ig rock: Dark or greenish rocks rich in olivine; may also contain pyroxene or amphibole
Ultramafic
Dark - colored rocks containing pyroxene, amphibole ± olivine ± biotite
Mafic
Classification of ig rock Grayish to salt and pepper colored rocks rich in plagioclase, amphibole ± biotite ± quartz
Intermediate
Light - colored or red rocks rich in potassium feldspar, quartz ± biotite or muscovite
Felsic
Ultramafi rocks_, _(coarse and fine equivalent)
PERIDOTITE, Komatiite
Mafic _, _ (coarse and fine grained equivalent)
Gabbro Basalt
Intermediate _, _(coarse and fine grained equivalent)
Diorite Dacite
Felsic _, _
_, _
(coarse and fine grained equivalent)
Granodiorite Dacite
Granite Rhyolite
Light-colored, lightweight rock rich in gas holes (vesicles)
Pumice
Dark-colored, lightweight rock rich in gas holes (vesicles)
Scoria
Rock composed of fine-grained ash- to sand-sized volcanic rock fragments
Volcanic tuff
Rock composed of coarse-grained gravel and larger sized volcanic rock fragments
Volcanic breccia
are gray - colored (intermediate) to salt and pepper - colored rocks rich in hornblende, pyroxene and plagioclase. _, _contain more than half to almost two - thirds SiO 2 . _, _is a common volcanic rock around the Pacifi c Ring of Fire. _ volcanoes overlie diorite plutons.
Andesite and diorite
_, & are light - colored (felsic) rocks, containing approximately two - thirds SiO 2 , rich in plagioclase, alkali feldspar and quartz and also containing small amounts of hornblende and biotite. _ is a volcanic rock that, like andesite, occurs around the Pacifi c rim. _ is a plutonic rock that underlies andesite – dacite volcanoes.
Dacite and granodiorite
_, _ are light - colored (felsic) rocks containing more than two thirds SiO 2 (silicic or acidic) and rich in quartz, alkali feldspar with small percentages of plagioclase and biotite. _ is a volcanic rock that usually erupts in thick, continental crust. _ plutons also occur in continental crust.
Rhyolite and granite
_ those characterized by the absence of crystals, include frothy, vesicular rocks such as pumice (light colored) and scoria (dark colored). Other non - crystalline rocks include those with glassy textures such as obsidian or those enriched in rock fragments. Fragmental, also known as _, volcanic rocks include tuff (volcanic ash to gravel size) and breccia (larger than gravel size).
Non crystalline, pyroclastic
The degree of crystallization can be classifi ed into _, _, _
holocrystalline , hypocrystalline and holohyaline
_ minerals contain complete crystal faces that are not impinged upon by other crystals
Euhedral
_ crystal faces contain partially complete crystal forms in which at least one of the crystal faces is impinged upon by adjacent rock material
Subhedral
In subhedral textures, crystal growth may be aborted due to:
Contact against previously formed minerals.
• Nucleation on pre - existing surfaces such as early formed crystals or the margins of the magma chamber.
• Resorption in which pre - existing euhedral crystals are partially remelted.
• Other secondary alteration processes that destroy pre - existing euhedral faces.
_ crystals lack any observable crystal faces.
Anhedral
remaining voids between existing crystal forms are referred to as _
interstitial space.
Textures: plutonic rocks include _, _, _
coarse - grained pegmatitic, phaneritic, phaneritic – porphyritic textures.
Textures: fine grained _, _
fine - grained aphanitic or aphanitic – porphyritic crystalline textures.
texture is characterized by large crystals averaging more than 30 mm in diameter.
Pegmatitic
Pegmatitic textures develop most commonly in?
granitic plutons with high volatile contents.
Phaneritic texture implies crystal diameters ranging from
1 to 30 mm.
Phaneritic textures may be subdivided into fine (_ in diameter), medium ( _mm) or coarse ( _mm) grained. Fine - grained phaneritic textures commonly develop in shallow plutonic structures such as _, _
1-3mm
3 – 10
10 – 30
dikes and sills.
textures contain small crystals less than 1 mm in diameter that are not generally discernible to the eye.
Aphanitic
textures contain microlite crystals large enough to be identifi ed with a petrographic microscope
Microcrystalline
> 50% of the rock this is a_ . If phenocrysts encompass < 50% of the rock volume, the term _
granite porphyry
porphyritic granite
Rocks with _ textures consist of two distinctly different size crystals
porphyritic
Large crystals are referred to as _ ; finer grained material constitutes the _ .
phenocrysts
groundmass
In _ textures, all crystals are visible to the eye, but the phenocrysts are distinctly larger than the groundmass crystals
porphyritic – phaneritic
In rocks with _ textures, the larger phenocrysts are embedded in an aphanitic groundmass composed largely of microcrystalline, cryptocrystalline or glassy material
porphyritic – aphanitic
The _ represents the initial crystallization temperature below which crystals and liquids coexist
liquidus line
Below the liquidus temperature, crystals nucleate and continue to grow at _
subliquidus temperatures.
The _ separates a higher temperature f i eld containing liquids and solid crystals from a lower temperature fi eld in which only solid crystalline material exists
solidus line
Temperatures below the solidus line are referred to as _.
subsolidus temperatures
involves the formation of new crystals, called nuclei or “ seed ” crystals, large enough to persist and grow into larger crystals.
Crystal nucleation
The crystal nucleation rate , the number of new seed crystals that develop per volume per time unit, is commonly expressed as
nuclei/cm 3 /s
occurs when liquids are cooled to temperatures below the liquidus line.
Undercooling
represents undercooling of the magma.
Subliquidus crystal nucleation
a measure of the increase in crystal radius over time.
crystal growth rate
, the rate at which elements migrate through magma, depends primarily on the viscosity of the melt.
Diffusion
are elements that tend to increase molecular linkage, thereby increasing viscosity.
Network formers
Network formers in silicate melts include _, _, _
silicon, oxygen and aluminum.
are elements that decrease molecular linkage. I. E Mg and Fe
Network modifiers
occurs when melts of any composition come into contact with liquid water or air
Quenching
_contain recognizable phenocrysts in a glassy groundmass and are said to have a _ .
Vitrophyres, vitrophyric texture
Growth commonly occurs outward from existing crystal nuclei to produce rounded masses of radiating crystals called _
spherulites
,glassy SiO 2 - rich volcanic rocks with higher water contents than obsidian
Perlites
characterized by a cloudy appearance and curved or subspherical cooling cracks called perlitic crack
perlitic texture
is encountered when bubbles constitute 70 – 80% of the magma volume
fragmentation surface
defi ned as containing > 30% vesicles by volume, include pumice and scoria
Vesicular rocks
Rocks that contain smaller amounts (-%) of vesicles are named using a modifi er such as vesicular basalt or vesicular andesite
5 – 30%
rocks with just a few vesicles ( _%) are given names such as vesicle - bearing basalt and andesite.
< 5
Volcanic eruptions eject broken rock particles of varying sizes, known as _ (which means fi ery fragment)
pyroclasts
Pyroclasts may be ejected into the atmosphere as airborne _ or transported along Earth ’ s surface as pyroclastic fl ows.
tephra
Pyroclasts consist of several different types of materials:
_: pyroclasts contain fragments such as basalt, andesite or other rocks.
_:composed of glassy fragments, most commonly pumice or scoria shard
_:pyroclasts contain minerals.
Lithic
Vitric
Crystal
Ash consists of sand - sized and fi ner sized pyroclasts ( _ mm diameter) which can be subdivided into coarse ash (_ mm) and fi ne ash ( _ mm) or dust
< 2
0.0625 – 2
< 0.0625
display random shard orientations and spherical to ellipsoidal pumice vesicles
Unwelded tuffs
_ results as fragments become progressively fused together as porosity decreases during compaction.
Welding
have concentrations greater than 1 wt % in Earth ’ s crust.
Major elements
_and _ are the third and fourth most abundant elements by weight in Earth ’ s crust.
Aluminum, iron
minor oxide compounds such as _, _, _
titanium oxide, manganese oxide and phosphate.
rhyolite contains _ SiO 2
dacite _% SiO 2
andesite _% SiO 2
basalt _% SiO 2 .
> 66%
63 – 66%
52 – 63
45 – 52
Note the chemical trends depicted: as SiO 2 concentration _, FeO, MgO, CaO and TiO 2 _ while Na 2 O and K 2 O _.
increases, decrease, increase
_ consist of those elements that commonly occur in concentrations of 0.1 1.0%
Minor elements
(4)are among the most common minor elements
Chromium, manganese, phosphorous, hydrogen and titanium
Igneous rocks with SiO 2 concentrations in excess of 66% tend to be enriched in minor elements such as (3)
Li, Be and Ba.
occur in crustal rock in concentrations of < 0.1% by weight and are typically measured in parts per million ( < 1000 ppm).
Trace elements
is a measure of the ease with which an element fi ts into a crystal structure, and is analogous to the description of a compatible or incompatible person
Compatibility
Compatible elements tend to form long - lasting bonds and incorporate into crystal structures. Compatible elements are immobile in that they do not readily migrate from the crystal structure. True or false
T
The restite is enriched in compatible elements and depleted in incompatible elements. Conversely, magmas derived from partial melting are enriched in incompatible elements and depleted in compatible elements. T or f
T
With small degrees of partial melting, incompatible elements such as (4)are depleted in the restite and enriched in magmas that migrate upward towards Earth ’ s surface.
K, Rb, Sr and Ba
The restite rock, from which the partial melts have been removed, tends to be enriched in compatible elements such as (9)and depleted in incompatible elements
Fe, Mn, Zn, Ti, V, Cr, Co, Ni and Cu,
Trace elements include rare (3)
Earth elements, high fi eld strength elements and large ion lithophile
This process standardizes the differences between even and odd atomic numbered REE concentrations and makes data presentation graphs easier to plot and comprehend. The net result typically produces a relatively fl at REE concentration pattern. This process is referred to as a _ (Hess, 1989 )
chondrite - normalized pattern
Light rare Earth elements (LREE) such as lanthanum (La), cesium (Ce), praseodymium (Pr), neodymium (Nd) and samarium (Sm) are situated on the _ end of the periodic table
Left
all REE are incompatible. T or f
T
LREE elements are more incompatible than HREE. T or f
T
elements are characterized as having a relatively high ionic charge (+3 or +4) for a given radius
High fi eld strength (HFS)
Ti, Ni, Cr, V, Zr, Hf, Nb, Ta and Y
HFS
_ , situated at the right end of the periodic table, include europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu).
Heavy rare Earth elements (HREE)
Trace elements containing an ionic radius : valence charge ratio of greater than 0.2 are referred to as
large ion lithophile (LIL) elements
elements include Cs, Ba, Rb, Sr, U, Pb, K, Zr, Th and Ta
LIL
_ elements tend to be mobile in partial melts and depleted in the restite
LIL
useful in determining the role of hydrous fl uid interaction and the parental source of the partial melt
LIL
lithospheric magma source is indicated if an igneous rock is _ in Sr and Nd (LREE)
Enriched or depleted?
enriched
asthenospheric magma source is suggested if Sr and Nd are _ in an igneous rock.
Enriched or depleted?
depleted
Rb, Ba and Th are among the (less, moderate, most) incompatible elements
Most
Sm and Hf are)(most, moderately, less) incompatible.
Moderately
Sr is more compatible than Rb; Sm is more compatible than Nd and Lu is more compatible than Hf. T of f
T
Eu is incompatible, except in the presence of _?
plagioclase
occur in concentrations of less than ∼ 5%
Accessory minerals
The _, also called the _, is the proportion of mafi c minerals in the total population of felsic and mafi c minerals given as:
color index (CI), mafi c index
< 40% DCM: _
Felsic
40 – 70% DCM:
intermediate
70 – 90% DCM:
Mafic
> 90% DCM:
ultramafi c
IUGS color index rock modifier terms
< 35% DCM:
leucocratic
IUGS color index rock modifi er terms
35 – 65% DCM:
mesocratic
IUGS color index rock modifi er terms
> 65% DCM:
melanocratic
Accurate point count analysis requires moving a thin section incrementally on a grid system such that at least mineral points are tabulated for each thin section.
400
The fi rst norm classifi cation
CIPW norm
Quartz ± feldspars and/or magnesium orthopyroxene
SiO 2 oversaturated
Feldspars and/or magnesium orthopyroxene only
Saturated sio2
Forsterite olivine, nepheline, leucite and other feldspathoids ± feldspars and/or orthopyroxene minerals. Excludes quartz
Sio2 undersaturated
(2) – indicators of SiO 2 undersaturation
magnesium olivine or feldspathoids
Al 2 O 3 > CaO + Na 2 O + K 2 O
Peraluminous
Muscovite, corundum, topaz, garnet, tourmaline, cordierite, andalusite, biotite
Peraluminous
Hornblende, epidote, melilite, biotite, pyroxene
Al abundance?
Metaluminous
Na 2 O + K 2 O < Al 2 O 3 < CaO + Na 2 O + K 2 O
Metaluminous
Al 2 O 3 = Na 2 O + K 2 O
Subaluminous
Olivine, orthopyroxene, clinopyroxene
Subaluminous
Al 2 O 3 < Na 2 O + K 2 O
Peralkaline
Aegerine, riebeckite, arfvedsonite, aenigmatite, astrophyllite, columbite, pyrochlore
Peralkaline
Altered plag
Scapolite
Q in qapf
Quartz, tridymite, cristobalite
A in qapf includes 5 moles of anorthite. T or f
T
Feldspathoids include the minerals nepheline, sodalite, cancrinite, leucite, analcite, nosean, hauyne and kalsilite. T or f
T
> 40% olivine rock
Peridotite
<40% olivine
Pyroxenite
applies for igneous rocks with > 10% felsic minerals and < 90% mafi c mineral (M) content by volume
QAPF modal classification
used for ultramafi c plutonic rocks containing > 90% dark - colored minerals
Modal classifi cation of ultramafi c plutonic rocks
classifications are suitable for use with extremely fine grained or glassy volcanic rocks that are relatively unaltered and contain less than 2% H20 and 0.5% CO2
total alkali to silica (TAS)
