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Igneous Geology
•Igneous geology focuses on the process and structure
(arrangement of parts) of igneous intrusions and extrusions.
•Magma composition, especially silica content, strongly
influences igneous geology. Felsic magmas are cooler, more
viscous (even to being more plastic solids than liquids), and
more prone to explosive eruptions than mafic magmas, which
generally have very gentle eruptions.
•Partial crystallization and partial melting are important. As
crystals form from a melt, the melt becomes depleted in
elements that are incorporated in those minerals and enriched in
elements that are not incorporated into those minerals. If the
crystals are removed from contact with the melt (by settling to
the bottom of a magma chamber, for example), the final melt
can have a very different composition than the initial melt.
•As a melt cools and changes in composition, components that
were once miscible can become immiscible. (Think of grease
separating from chili, or one type of grease separating from
another.) This phenomenon is called exsolution and controls
formation of some ores and whether volcanoes are explosive.
•Unlike sediments, which have very predictable distributions
due to their mode of deposition, igneous rocks are much more
chaotic and difficult to map.
v 0067 of 'Igneous Geology' by Greg Pouch at 2012-09-08 10:45:15
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Igneous Geology
3 Composition
4 Processes
5 Melting > Generation of Magma
6 Melting > Partial Melting
7 Melting > Partial Melting > Continuous
8 Melting > Partial melting > Eutectic
9 Melting > Partial Melting > Results
10 Melting > How to Melt Rock
11 Melting>Melting Temperature Varies with Pressure
12 Melt > Melting Temperature with Pressure
13 Processes > Ex-solution
14 Processes > Movement
15 Processes > Heating of Country Rock
16 Products
Extrusive
17 Products > Extrusive Features > Volcanoes
18 Products > Extrusive Features > Others
19 Flood Basalts
20 Products > Extrusive Materials
Intrusive
21 Products > Intrusions > Some Vocabulary
22 Products > Intrusions > Tabular
23 Products > Intrusions > Equant, Irregular
24 Products > Intrusions > Features > Other
25 Plate Tectonics and the Origin of Igneous Rocks
26 Plate Tectonics > Divergent Basalt
27 Plate Tectonics > Subduction Andesite
28 Plate Tectonics > Collision Granite
29 Plate Tectonics > Hotspots Basalt, maybe Granite/Rhyolite
39 Plate Tectonics > Transform Boundaries NONE
Composition
• Composition of magma is limited by source rock. The eight major
elements plus water and maybe CO2 are common in magmas and
control the properties of the melt. Sialic melts are more viscous and
less liquid-like (more polymerized) than mafic melts, which are very
liquid-like.
• In addition to the source rock and the degree of melting, magma’s
composition can change due to
–Assimilation of country rock (the surrounding, pre-existing rock),
–Segregation of early-formed minerals
–Ingress or egress of volatiles, especially water
–Mixing with another magma (rare?)
Processes
•
•
•
•
Melting
Ex-solution
Movement and emplacement
Heating of surrounding rocks
Melting > Generation of Magma
•Observations: A few compositions of igneous rock are rather common,
and most possible compositions are fairly rare (The chart and table in
the Igneous Rocks Lecture and in the book shows common igneous
rocks, not all possible igneous rocks. A more comprehensive
classification scheme is found at
http://www.geol.lsu.edu/henry/Geology3041/lectures/02IgneousClassify
/IUGS-IgneousClassFlowChart.htm ). The common igneous rocks at
the surface are
–basalt (oceans)
–granite-granodiorite (continents)
–and some andesite/diorite (volcanic chains near subduction zones)
Mantle is mostly peridotite (ol+pyr+Ca-plag). So are most meteorites.
•Conclusion: The narrow range of compositions suggests that magma
generation results in certain definite compositions, even from a wide
range of materials. Most magmas arise from partial melting of preexisting rocks.
Melting > Partial Melting
• A pure substance like quartz (SiO2) melts at one
temperature to a liquid of the same composition.
• Continuous When solid-solution minerals like
olivine (Mg,Fe)SiO4 and plagioclase (CaAl,
NaSi)AlSi2O8 melt, continuous partial melting
occurs, with melting over a range of
temperatures with smoothly-varying
composition. The first melt gets more of the lowmelting component than the parent, and the
restite more of the high-melting component than
the source. Continued melting occurs at
continuously higher temperatures and later melts
include more of the high-melting component,
until it's all melted.
• Eutectic When minerals don't form a solid
solution series (separate crystals in the solid
phase), like albite+quartz or water+halite, each
acts to depress the freezing point of the other,
and ANY mixture of the crystals start melting at
the same temperature to produce melt of the
same composition (eutectic point). The eutectic
melting temperature is often MUCH lower than
the melting temperature of either component.
For iron-carbon (steel):
Pure iron melts at 1535C,
Pure carbon melts at 4200C, and a
Eutectic iron-carbon mixture (a cast iron) melts at about 1154C
and 4.3%wt Carbon, which is more than 400C colder than iron.
Melting > Partial• Some
Melting
>
Continuous
minerals can accommodate different ions at a particular lattice site
in their crystal structure. (For example, minerals containing Mg+2 can
hold Fe+2 in that same site.) As the composition varies, so do the
properties, like melting temperature.
• A solid-solution mineral melts over a range of temperatures, with melt
and restite composition and relative amounts depending on the initial
composition and the temperature. (and pressure, water content, …)
At 1 atm pressure: Olivine, the magnesian endmember Mg2SiO4 Forsterite (Fo) melts around
1890°C whereas the Fe endmember Fe2SiO4 Fayallite (Fa) melts at 1205°C.
Plagioclase, the sodic endmember Albite (Ab) NaSiAlSi3O8=NaAlSi3O8 melts at 1118°C, whereas
the calic endmember Anorthite CaAlAlSi3O8=CaAl2Si2O8 (An) melts at 1500°C.
• In the phase diagrams, the white area at high temperature is liquid, the
green, banana-shaped area is slush (liquid+crystals), and the light gray
area is solid crystals. The blue, upper boundary of the slush region is
called the liquidus, and the red, lower boundary is the solidus. At any
composition, melting starts when the mixture is heated to the solidus and
finishes when it gets to the liquidus (freezing starts at the liquidus and
finishes at the solidus). At any temperature, the melt's composition lies
on the liquidus and the crystals' composition on the solidus.
• To find the first melt, go straight up from the parent composition to the
solidus to get the temperature; go straight over from there to the liquidus
to get the first melt's composition. In the 50%wt Fa mixture shown on
the olivine diagram, the first melt happens around 1450°C with an
80%wt Fa melt. The last solid would have a 20% Fa composition.
• In ferromagnesian minerals the iron-rich endmember has a lower
melting temperature than the magnesian endmember but higher density
when solid. In Ca-Na minerals, the sodic endmember has a lower
melting temperature.
Melting > Partial melting > Eutectic
• Eutectic The melting temperature of a mixture of minerals that don't form a solid-solution but are miscible
as liquids is lower than either of their melting temperatures (in CHEM, this is called freezing-point
depression, and is what road-salt and antifreeze are for)
• If you make a mixture of a pair minerals that don’t form a solid solution, like albite+quartz or
water_ice+salt, and start heating it, regardless of the composition of the mixture, it will start melting at the
same temperature (lower than either separately) and same composition (for that combination), called the
eutectic point (which will vary with pressure etc.). Continued heating of the mixture doesn't warm it, it
melts more of the eutectic mixture leaving behind one component (unless your mixture happened to have
the eutectic composition), until you have only A or B in the solid, at which point the leftovers start melting,
at a range of temperatures and altering the composition of the melt, until it matches the starting mixture.
• For water+NaCl at 1atm pressure, the eutectic point is 23.3%wt and -21.1°C. Below this temperature, water
salt combinations are frozen. If you start at -30°C with 90% water and 10% salt and heat it, you won't see
any melt until you get it to -21.1°C, at which point you get 23.3% salt+76.7% water mixture, which depletes
the mixture in salt (enriches it in water), giving eutectic mixture+water_ice. For a while, continued addition
of heat doesn't change the temperature until you run out of salt in the solid, at which point the water_ice
starts melting and the temperature starts going up again, until your final solution/melt matches your initial
mixture. If you started with 30% salt, you would still get melting starting at -21.1°C, but continued melting
would deplete the solution in water, until you started raising temperatures and dissolving/melting the salt.
• The eutectic melting temperature is often MUCH lower than the melting temperature of either component.
Melting > Partial Melting > Results
• Partial melting is common. It results in a magma (the part that
melts) and a restite (the part that rests, or stays behind). The
magma includes the more easily melted components.
• Partial melting is a refining process, in that elements end up
getting separated.
• Compared to the source, partial melting gives melts usually
enriched in
–Fe relative to Mg
–Na relative to Ca
–K relative to Na
–Si relative to Al
• Eutectic partial melting gives rise to melts of constant
composition from a wide variety of sources, and at much lower
temperature.
• Usually, both continuous and eutectic partial melting occur
simultaneously.
Melting > How to Melt Rock
•Heat It is possible to generate a magma by applying heat to a rock.
This probably happened early in earth history (first billion or two years)
but is rare now, except where material is pushed down into hotter
regions in subduction zones.
•Making it want to melt
How many psychologists does it take to change a light bulb?
One, but only if the light bulb really wants to change.
•Similarly, you can melt a rock by making it want to melt by changing its
melting temperature: by altering its pressure, or by introducing volatiles
(This is mainly how it happens.)
Melting>Melting Temperature Varies with Pressure
•Depending on the composition of the
melt and its water content, its melting
temperature can increase or decrease
with pressure. See diagram. This is
why granite (not rhyolite) and basalt
(not gabbro) are common.
•Granite, especially wet granite, has a
melting temperature that increases as
depth decreases, so granite freezes up
as it ascends. Granite mainly comes
from compressing wet sediments,
andesite, etc.
•Basalt has a melting temperature that
decreases as pressure decreases, so as
basalt rises, it gets further above its
melting temperature. Basalt mainly
comes from decreasing pressure on
nearly-melting mantle peridotite
Melt > Melting Temperature with Pressure
• Magma of basaltic composition (mafic magma) has a PT melting curve that causes more melting to occur as
pressure decreases, like most materials. A mafic magma gets further above its melting range as it ascends,
so mafic magmas usually erupt, at temperatures well above melting.
• Granites have a PT melting curve that causes them to freeze as pressure decreases, like water. As a felsic
melt ascends, it usually goes below its melting temperature and crystallizes at depth. If a felsic magma
erupts, it's usually at or well-below its melting temperature.
• Rhyolites are usually associated with large granitic intrusions; basaltic volcanoes often occur without
plutons or with only small intrusives.
Processes > Ex-solution
As minerals crystallize from a melt, the melt can become depleted in elements that are incorporated in those
minerals and enriched in elements that are not incorporated into those minerals (incompatible elements).
Magmas contain volatiles (gases and liquids) in solution at high temperature, but the combination might be
unstable at low temperature. Amongst others, water concentrates into the melt, as do lots of rare elements
like Ag and Pb.
As a melt cools and changes in composition, components that were once miscible can become immiscible, like
grease separating from broth, separating into two fluids. This phenomenon is called ex-solution.
The combined volume of the two fluids is often greater than the volume of the single fluid. At the surface, this
can be explosive. Below the surface, this can result in a fracture network and extensive metasomatic activity.
Intermediate and felsic magmas often ex-solve into a water-rich phase and a silica-rich phase. Ex-solution can
generate porphyries by changing the melting temperature of the siliceous liquid which is suddenly NOT as
water-rich, thus suddenly below its freezing temperature, and so freezes, becoming the groundmass.
The water-rich phase contains lots of elements like Au and Cu and Cl that didn't go into early-formed
minerals like plagioclase and can lead to cool ore deposits and pegmatites.
Processes > Movement
Magmas can move upward in two main ways.
•Fluid flowing in cracks (basalts and metasomatic fluids).
–This requires that the country rock be brittle to sustain cracks.
–Where a fluid magma encounters plastic rocks, the magma can rise
only if it is of lower density. If it is of higher density, it gets stopped
below the plastic rocks (underplating). This heats and might melt
the overlying rocks. Likely source of many granites is basalts
underplating continental crust and partially melting crustal rocks.
•Plastic oozing upward (granites).
–Requires that the surrounding rocks be able to move out of the way,
by flowing, by oozing, or by falling through the magma (stoping)
Processes > Heating of Country Rock
•Heating As hot magmas passes through colder country rock, the
magma cools and the country rock heats. This can result in contact
metamorphism and melting of the country rock or making it plastic. The
magma might develop chilled margins from the rapid cooling at the
edges.
Products
• Igneous Rocks (discussed elsewhere)
• Extrusives
• Intrusives
Products > Extrusive Features > Volcanoes
Extrusives are igneous rocks that erupt onto the earth’s surface (are extruded from the earth)
• Volcanoes are mounds of extrusive igneous rock built up by successive eruptions. The style of eruption
(runny or viscous) determines the type of volcano that forms.
–Shield volcanoes (a.k.a. domes) are broad, gently sloped volcanoes produced by runny (non-viscous)
lava. Side slopes are usually less than 10º. They are often very big, but don't look very conspicuous
because of the gentle slopes. Shield volcanoes are usually not dangerous. Hawaii
–Domes are usually rhyolitic (granitic, felsic) and more oozed than erupted.
–Cinder consist of loose material, most of which has been airborne. They usually have steep slopes at the
angles of repose for loose pyroclastic material, which has been ejected from the vent. Very difficult to
climb. Cinder cones are usually small, but conspicuously volcanoey-looking. Mexico
–Stratovolcanoes/Composite volcanoes Andesite can be fluid or plastic depending on the volatiles of a
particular eruption. Stratovolcanoes consist of strata of both cinders and flows, and have a characteristic
shape (very much like a bell-shaped curve). A stratovolcano can be thought of as a cinder cone
superimposed on a shield volcano. Japan
Products > Extrusive Features > Others
•Floods (book calls these plateau basalts) are extensive layers of extrusive igneous rock that moved liquidly
and are almost always basaltic.
–The lava usually comes out of fissures which are fed by dikes. Areas like the Columbia River Plateau
are covered by hundreds of 3-100 meters thick basalt flows, each covering hundreds to tens of thousands
of square kilometers. Basalt flows like this frequently show columnar jointing. (see text).
–There are not any currently active regions of flood basalts.
•Falls are extensive layers of ash and other debris, usually transported by air and are often very violent.
Typical of granite/rhyolite. They can include nuée ardente (glowing mix of pyroclasts and hot gases).
•Pillow basalts are extruded below water. In a pillow basalt, lava breaks through a hole in the already-frozenpart-of-the-flow and flows out there, resulting in a tube of hardened rock. In cross section, they look like a
stack of pillows, with the outer edge showing evidence of quenching (Good video on the CD)
•Plugs and Domes are the volcanic equivalent of toothpaste being squeezed out of a tube. Rhyolitic.
Flood Basalts
•
From http://www.geolsoc.org.uk/template.cfm?name=fbasalts
Products > Extrusive Materials
Extruded rocks cool quickly, and are fine-grained (aphanitic).
• Basaltic
–Pahoehoe is smooth and ropy.
–Aa is jagged and sharp.
–Pillows form under water.
• Pyroclastic (granitic and andesitic) materials are hot airborne fragments, and include dust, ash , cinders,
lapilli, and bombs/blocks
• Bubbles can be frozen into a rock, resulting in vesicular (scattered bubbles) to scoriaceous to pumaceous
(mainly bubbles) textures.
• If a lava doesn't crystallize, but instead just gets cold and very viscous, you get volcanic glass obsidian
• Porphyries are common, due to ex-solution at shallow depths or magma loitering in a magma chamber
before eruption.
Products > Intrusions > Some Vocabulary
• Intrusives are igneous rocks that were emplaced into solid rock (intruded into rock), also called plutonic
rocks. The body of rock is called a pluton or an intrusion.
–Large deep intrusions cool at depth, so cool more slowly and often have large crystals (phaneritic texture).
–Thin intrusions. especially shallow ones, can cool quickly and often have small crystals (aphanitic).
• Country rock is the rock that was there before the intrusion.
• Xenoliths are fragments of some foreign rock (often country rock, sometimes from the source region) in an
igneous body.
Products > Intrusions > Tabular
• Sheet intrusions are common at shallow depths and with fluid magmas, and tend to be
basaltic. Sheet intrusions imply runny, flowing magma.
–Sills are parallel to layering in country rock (concordant). They often occur below beds
that flow plastically, like shale.
–Dikes cut across country rock (discordant). If “cut across country rock” is undefined, it’s a
dike.
Products > Intrusions > Equant, Irregular
• Rather than being sheet-like, plutons can be equant (similar sizes in all directions), amoebaelike in shape, and are often granitic, granodioritic or dioritic. They often occur in swarms.
–Batholiths are over 100 km2 The word batholith can refer to an individual intrusion or to a
set of merged plutons.
–Stocks are under 100 km2
Products > Intrusions > Features > Other
• Other shapes
–Laccoliths (sometimes domes) are “hemi-spherical” with the convex side up. They are usually granitic.
–Lopoliths are “hemi-spherical” with the convex side down. They are usually basaltic.
–Pipes a.k.a. necks are “circular” and “vertical” and often feed volcanoes
–Veins are irregular and are filled with material you might count as igneous or metamorphic.
Plate Tectonics and the Origin of Igneous Rocks
• Plate tectonics explains current igneous activity fairly well. Most igneous intrusions are associated with
plate boundaries. There is also igneous activity associated with hot-spots.
• Older igneous activity, especially more than 2.5 Ga old (Archean), has a different style, suggesting that
modern plate tectonics was not dominant.
Plate Tectonics > Divergent
Basalt
• Oceanic basalts are derived by decompressive partial melting of
mantle material, and occur extensively at mid-ocean ridges and some
oceanic hotspots. Most volcanic activity occurs as pillow basalts at
divergent plate boundaries. Early in the rifting apart of a continent, bimodal volcanism (rhyolites and basalts) occurs.
Plate Tectonics > Subduction Andesite
• At ocean-subducting convergent
boundaries, wet basalt is heated as it
subducts, resulting in partial melting
of basalt to produce andesite.
• Subduction-zone andesites are
derived by water- and pressureinduced partial melting of basalt and
sediments that are being subducted.
There may also be partial melting of
mantle peridotite leading to basaltic
magma. Sub-equal volcanic and
intrusive activity occur in a continentocean collision.
• The constancy of composition of
andesite in collision zones suggests it’s
a partial melt, rather than a mixing
phenomenon.
Plate Tectonics > Collision
• At continent-continent convergent
boundaries (collision zones), wet
andesitic, granodioritic, and
sediments and metamorphic rocks
are compressed and yield granitic
magma which freezes as it ascends.
• Granitic magmas appear to have
several sources.
–Most are due to compressive
melting of water-rich sediments,
as occurs in deep burial or
continent-continent collisions.
–Another is secondary melting,
due to underplating by basaltic or
andesitic magmas and heat
transfer.
• Andesitic magmas might
incorporate sediments and move
towards a granitic composition.
Continent-continent collisions
mainly result in intrusive granites.
Granite
Plate Tectonics > Hot Spots
• Hot spots are tracks of volcanism (age increases away from current activity) that are not associated with
plate boundaries (and earthquakes and structural deformation). The Hawaiian islands are one in an ocean
(an aseismic ridge), Yellowstone lies on another (look at the geologic map of North America in your text). A
lot of Archean volcanism looks like hotspot tracks on continental crust.
• We think hot spots are where a plume of hot material has welled up from the mantle (sort of like a
thunderhead). Under oceans, they almost always erupt as flood basalts or shield volcanoes. Under
continents, they might erupt as flood basalts or shield volcanoes, or they might heat the continental material
enough to make it plastic and block further mafic eruptions, but trigger secondary underplating granites and
rhyolites.
Plate Tectonics > Transform
NONE
• Transform boundaries really don't seem to have igneous activity at all.
Igneous Geology
• Partial Melting is a refining process. Certain elements go into the melt,
others stay in the restite. Crystallization can be a refining process
• Origin of Magmas
–As mafic magma ascends, its melting temperature drops, so basalt usually
erupts. Basaltic magmas form by decompressive partial melting of mantle
peridotite.
–As wet felsic magma ascends, its melting temperature increases, so it
usually freezes out at depth as a granitic pluton. Felsic magmas mostly
arise from compressive melting of andesites, granites, and sediments, and
underplating (=>heating) by basalts.
–Intermediate melts form in subduction zones, where wet basalt is heated
as it is carried into the mantle, resulting in andesitic volcanoes and
dioritic plutons.
• Igneous style depends on silica and water content.
–Silica makes magma more viscous, so felsic and intermediate magmas
can explode violently, while mafic magmas erupt gently.
–Concentration of incompatible elements into the melt results in a melt
that differs markedly from the original melt. Felsic and intermediate
magmas often ex-solve into a water-rich phase and a silica-rich phase,
forming porphyries, pegmatites, and hydrothermal ores.
• Plate tectonics provides a good framework for understanding modern
igneous activity, but there are some problems, especially with Archean rocks
and anorthosites.
This line is
what you
would get if
you have as
much water
as needed.
Melt > Melting Temperature with Pressure
•
Magma of basaltic composition has a PT melting curve that causes more melting to occur as pressure
decreases. Granites have a PT melting curve that causes them to freeze as pressure decreases.
• Basaltic magmas are well above their melting point if they erupt. Granites/rhyolites are often at or wellbelow their melting point when they erupt. Rhyolites are usually associated with large granitic
intrusions; basaltic volcanoes often occur without plutons or with only small intrusives.
Figures are from Petrology by Ehlers & Blatt p97. Y-axis is pressure in kilobars. Multiply by 3 to get depth in kilometers of rock.