A wide variety of igneous rocks occur in the continental lithosphere, a
reflection of its heterogeneous nature compared to oceanic lithosphere.
Because the continents are not subducted and are subject to uplift and
erosion, older plutonic rocks are both preserved and accessible to study.
1. Classification of Granitic rocks based on visible pewrcentages of
"Q" quartz / "A" alkalai feldspar / "P" plagioclase feldspar
Note: true granites have between 10% and 65% of their feldspars as
plagioclase, and between 20% and 60% quartz.
All rocks will likely contain mafic minerals such as biotite, hornblende,
and perhaps pyroxenes, along with opaque oxide minerals.
"S", "I" and "A" Type Granitic Rocks
Mechanisms by which large volumes of granitic magma could be produced are:
Anatexis of metasedimentary/sedimentary rocks to form S-type
Anatexis of young crustal basic meta-igneous rocks to form I-type
Melting/Assimilation of lower crustal rocks by mantle-derived basic
Crystal fractionation/Assimilation of basaltic and andesitic magmas.
Granitization, wherein high grade metamorphism bordering on melting
converts rocks into those that appear texturally and mineralogically
similar to granitic rocks.
S-type granites are thought to originate by melting (or perhaps by
ultrametamorphism) of a pre-exiting metasedimentary or sedimentary source
These are peraluminous granites [i.e. they have molecular
Al2O3 (Na2O + K2O)].
Mineralogically this chemical condition is expressed by the presence of a
peraluminous mineral, commonly muscovite, although other minerals such as
the Al2SiO5 minerals and corundum may also occur.
Since many sedimentary rocks are enriched in Al2O3
as a result of their constituents having been exposed to chemical
weathering near the Earth's surface (particularly rocks such as shales
that contain clay minerals), melting of these rocks is a simple way of
achieving the peraluminous condition.
Many S-type granitoids are found in the deeply eroded cores of fold-thrust
mountain belts formed as a result of continent-continent collisions, such
as the Himalayas and the Appalachians, and would thus be considered
I-type granites are granites considered to have formed by melting of an
original igneous type source.
These are generally metaluminous granites, expressed mineralogically by
the absence of peraluminous minerals, and the absence of peralkaline
minerals, as discussed below.Instead these rocks contain biotite and
hornblende as the major mafic minerals.
Mesozoic or younger examples of I-type granites are found along
continental margins such as the Sierra Nevada batholith of California and
Nevada, and the Idaho batholith of Montana. In these regions the
plutonism may have been related to active subduction beneath the western
U.S. in Mesozoic times.I-type granites are also found in the Himalayas,
related to continent-continent collisions.
Plutonic suites that were emplaced in convergent continental margin
settings, show many of the same characteristics as the calc-alkaline
volcanic suite that likely erupted on the surface above. The suites
include gabbros, diorites, quartz monzonites, granodiorites, and granites.
They show only mild to no Fe-enrichment, similar to calc-alkaline
volcanic rocks, and a range of isotopic compositions similar to the
associated volcanic rocks. Nearly all are I-type granitoids.
The rocks were emplaced during the Mesozoic Era. Exposed rocks are
generally older toward the east and southeast. Kistler and Peterman
showed that the Sr isotopic ratios vary across the batholith in a
systematic way. The younger rocks in the western portion of the
batholith are mostly quartz diorites with Sr isotopic ratios less than
0.704, ratios expected from melting of the mantle or young crustal rocks.
Plutons farther east are mostly quartz monzonites and granodiorites with
ratios increasing along with age of the plutons toward the east and
southeast. One interpretation of the data is that the older rocks
contain a higher proportion of older crustal material than the younger
A-type granites are generally peralkaline in composition [molecular (Na2O
+ K2O) > Al2O3].
Minerals like the sodic amphiboles riebeckite and arfvedsonite and the
sodic pyroxene aegerine are commonly found in these rocks. In
addition, they tend to be relatively Fe-rich and thus fayalitic olivine
may sometimes occur.
These are considered anorogenic granites because they are generally found
in areas that have not undergone mountain building events.Instead, they
appear to be related to continental rifting events wherein continental
lithosphere is thinned as a result of upwelling asthenosphere.The
upwelling raises the geothermal gradient resulting in melting.Young
peralkaline granites are found in the Basin and Range Province of the
western U.S., and older examples are found throughout southeastern
Depth of Emplacement.
Because the conditions under which a magma cools can play an important
role in the texture and contact relationships observed in the final rock,
plutons can be characterized by the depth at which they were
emplaced. This is because depth, to a large extent controls the
contrast in temperature between the magma and its surroundings. Catazonal Plutons
The catazone is the deepest level of emplacement, usually considered to be
at depths greater than about 11 km. In such an environment there is
a low contrast in temperature between the magma and the surrounding
country rock. The country rock itself is generally high grade
metamorphic rock. Contacts between the plutons and the country rock
are concordant (meaning the contacts run parallel to structures such as
foliation in the surrounding country rock) and often gradational. The
plutons themselves often show a foliation that is concordant with that in
the surrounding metamorphic rocks. Migmatites (small pods of what
appears to have been melted rock surrounded by and grading into
metamorphic rocks) are common. Some catazonal plutons appear to have
formed by either melting in place or by ultrametamorphism that grades into
actually melting. Others appear to have intruded into ductile
crustal rocks. Most, but not all, Catazonal plutons are S-type
granitoids. Mesozonal Plutons
The metazone occurs at intermediate crustal depths, likely between 8 and
12 km. The plutonic rocks are more easily distinguished from the
surrounding metamorphic rocks. Contacts are both sharp and
discordant (cutting across structures in the country rock), and
gradational and concordant like in the catazone. Angular blocks of
the surrounding country rock commonly occur within the plutons near their
contacts with the country rock. The plutons generally lack foliation
and are often chemically and mineralogically zoned. Epizonal Plutons
The epizone is the shallowest zone of emplacement, probably within a few
kilometers of the surface. In such an environment there is a large
contrast between the temperatures of the magma and the country rock. The
country rock is commonly metamorphosed, but the metamorphism is contact
metamorphism produced by the heat of the intrusion. Contacts between
the plutons and surrounding country rock are sharp and discordant,
indicating intrusion into brittle and cooler crust. The margins of the
plutons often contain abundant xenoliths of the country rock. Pegmatities
Pegmatites are very coarse grained felsic rocks that occur as dikes or
pod-like segregations both within granitic plutons and intruded into the
surrounding country rock. They appear to form during the late stages
of crystallization which leaves H2O-rich fluids that readily
dissolve high concentrations of alkalies and silica. Thus, most
pegmatites are similar to granites and contain the minerals alkali
feldspar and quartz. But other chemical constituents that become
concentrated in the residual liquid, like B, Be, and Li are sometimes
enriched pegmatites. This leads to crystallization of minerals
that are somewhat more rare, such as:
Rhyolites are much more common and voluminous on the continents than in
the ocean basins. They range from small domes and lava flows to much
larger centers that have erupted volumes measured in 100s of km3
and emplaced as pyroclastic material. Most of the preserved volume
is represented as pyroclastic flow deposits, often termed "ash flow tuffs"
or "ignimbrites. Large quantities of these deposits were erupted
during the middle Tertiary in the western United States, northern Mexico,
throughout Central America, and on the western slopes of the Andes
mountains. The composition of these deposits is usually metaluminous
although peralkaline varieties are known. None are peraluminous in
composition. Although the recent examples occur near continental
margins. Most seem to be associated with episodes of continental
extension, such as in Basin and Range Province of the Western U.S. and
Continental Flood Basalts
Like the large submarine plateaus discussed in our large volumes of
basaltic magma have erupted on the continents at various times in Earth
Original Area Covered (km2)
Types of Basalts %
Karoo, S. Africa
Although each flood basalt province differs somewhat in the composition of
magmas erupted, most provinces have erupted tholeiitic basalts. With the
exception of a few early erupted picrites in some provinces, the
tholeiitic basalts tend to have lower concentrations of MgO (5 - 8%) than
would be expected from melts that have come directly from the mantle
without having suffered crystal fractionation. Thus, despite their large
volume, they are differentiated magmas that are similar in many respects
to MORBs. Still, they show incompatible trace element concentrations more
similar to EMORBs, and have 86Sr/87Sr and 143Nd/144Nd
ratios that extend from the OIB field toward and overlapping with
continental crust. This latter feature indicates that they have
likely suffered some crustal contamination.
Continental Rift Valleys
Continental Rift valleys are linear zones of extension within continental
crust.Some of these extensional zones may be eventually become zones along
which the continents break apart to form a new ocean basin, however, there
are many examples where such break-ups have failed.
The East African Rift which extends from Syria in the north to Mozambique
in the south has been active throughout the Cenozoic. During the
initial stages of rifting fissure eruptions produced large volumes of
basalt and siliceous ignimbrites. During the late Miocene and
Pliocene these eruptions became more focused, and produced shield
volcanoes consisting of basanites, rhyolites and phonolites. In
Plio-Pleistocene times rhyolites were erupted along the main axis of the
rift, while basalts continued to be erupted on the plateaus adjacent to
the rift. Quaternary volcanoes along the axis of the central rift
zones, in Kenya and Tanzania consist of phonolite, trachyte, or
peralkaline rhyolite. This province illustrates the wide variety of
unusual rock types found in continental rifting settings. Note,
however, that parts of the rift along the Red Sea and Gulf of Aden have
evolved to oceanic ridges and produce MORBs to form new seafloor.
The convergent plate margins are the most intense areas of active
magmatism above sea level at the present time. Most of world's violent
volcanic activity occurs along these zones. In addition, much magmatism
also has resulted in (and probably is resulting at present) significant
additions to the crust in the form of plutonic igneous rocks.
The "Pacific Ring of Fire" surrounds the Pacific Ocean basin and extends
into the Indian Ocean and Caribbean Sea. Active subduction is taking
place, along these convergent plate boundaries, as evidenced by the zone
of earthquakes, called a Benioff Zone, that begins near the oceanic
trenches and extends to deeper levels in the direction of plate motion.
Earthquake focal depths reach a maximum of about 700 km in some areas.
Volcanism occurs on the upper plate about 100 to 200 km above the Benioff
Zone. For this reason, volcanism in these areas is often referred to as
Two situations occur.
In areas where oceanic lithosphere is subducted beneath oceanic
lithosphere the volcanism is expressed on the surface as chains
of islands referred to as island arcs. These include the
Caribbean Arc, the Aleutian Arc, the Kurile Kamachatka Arc, Japan, the
Philippines, the South Sandwich Arc, The Indonesian Arc, the Marianas,
Fiji, and Solomon Islands.
In areas where oceanic lithosphere is subducted beneath
continental lithosphere volcanism occurs as chains of volcanoes
near the continental margin, referred to as a continental margin
arc. These include the Andes Mountains, Central American Volcanic
Belt, Mexican Volcanic Belt, the Cascades, the part of the Aleutian
arc on Continental crust, and the North Island of New Zealand.
Within these volcanic arcs the most imposing, and therefore most
recognized by early workers, features of the landscape are large
stratovolcanoes. These usually consist of predominantly
andesitic lava flows and interbedded pyroclastic material. But, in
the late stages of volcanism more silicic lavas and pyroclastics like
dacites and rhyolites are common.
Many of these stratovolcanoes pass through a stage where their upper
portions collapse downward to form a caldera.
These caldera forming events are usually associated with explosive
eruptions that emit silicic pyroclastic material in large-volume
It is the sudden evacuation of underlying magma chambers that appears to
result in the collapse of the volcanoes to form the calderas.
The imposing presence of these large mostly andesitic stratovolcanoes led
to an early widespread perception among petrologists that basalts were
rare or absent in these environments.In recent years, however, it has
become more evident that basalts are widespread, but do not commonly erupt
from the stratovolcanoes.
Instead, they are found in areas surrounding the stratovolcanoes where
they erupt to form cinder cones and associated lava flows.
One explanation for this distribution is that the magma chambers
underlying the stratovolcanoes intercept the basaltic magmas before they
reach the surface and allow the basalts to differentiate to more siliceous
compositions before they are erupted.
Basaltic magmas that are not intercepted by the magma chambers can make it
to the surface to erupt in the surrounding areas.
Probably the most distinguishing feature of subduction-related volcanic
rocks is their usually porphyritic nature, usually showing
glomeroporphyritic clusters of phenocrysts. (A textural term used to
describe igneous rocks that contain clusters of phenocrysts, which are
large crystals in a finer-grained matrix or groundmass)
Basalts commonly contain phenocrysts of olivine, augite, and plagioclase.
Andesites and dacites commonly have phenocrysts of plagioclase, augite,
and hypersthene, and some contain hornblende.
The most characteristic feature of the andesites and dacites is the
predominance of fairly calcic plagioclase phenocrysts that show complex
Such zoning has been ascribed to various factors, including:
Kinetic factors during crystal growth. As one zone of the
crystal is precipitated the liquid immediately surrounding the crystal
becomes depleted in the components necessary for further growth of the
same composition. So, a new composition is precipitated until
diffusion has had time to renourish the surrounding liquid in the
components necessary for the equilibrium composition to form
Cycling through a chemically zoned magma chamber during convection.
As crystals grow, they are carried in convection cells to warmer and
cooler parts of the magma chamber. Some zones are partially
dissolved and new compositions are precipitated that are more in
equilibrium with the chemical compositions, pressures, and
temperatures present in the part of the magma chamber into which the
crystal is transported.
Magma mixing As magmas mix the chemical compositions of liquids and
temperatures change during the mixing process. This could result
in dissolution of some zones, and precipitation of zones with varying
Rhyolites occur as both obsidians and as porphyritic lavas and
pyroclastics. Phenocrysts present in rhyolites include plagioclase,
sanidine, quartz, orthopyroxene, hornblende, and biotite.
In addition to these features, petrographic evidence for magma mixing is
sometimes present in the rocks, including disequilibrium mineral
assemblages, reversed zoning etc.
Xenoliths of crustal rocks are also sometimes found, particularly in
continental margin arcs, suggesting that assimilation or partial
assimilation of the crust could be an important process in this
Possible explanation Calc-alkaline Suite of rocks that are commonly
associated with subduction...
Subduction carries oceanic crust and sediment to depth. As the
pressure and temperature rise, the MORB crust and sediment undergo
metamorphism that releases hydrous fluids.
These hydrous fluids carry with them high concentrations of LILE and
REE, but leave behind the relatively insoluble HFSE. They also
carry the isotopic signature of the basaltic crust sediment mixture
that released the fluids, and thus have higher 87Sr/86Sr
ratios and lower 143Nd/144Nd ratios, reflecting
the isotopic composition of the subducted material. If the
sediments are young, they may contribute 10Be to these
The fluids act to metasomatize the overlying mantle wedge, enriching
it in LILE, REE, B, 87Sr/86Sr, and possibly 10Be
and lowering the 143Nd/144Nd ratio of this
Adding H2O to the mantle wedge lowers the solidus
temperature allowing for partial melting of this metasomatized mantle
and generating hydrous basaltic magmas.
This hydrous basaltic magmas become saturated with water at crustal
depths and differentiate by crystal fractionation, possibly
accompanied by contamination of crustal material, to generate the
andesites, dacites, and rhyolites of the calc-alkaline suite.
The ocean basins cover the largest area of the Earth's surface.Because of
plate tectonics, however, most oceanic lithosphere eventually is subducted
and thus the only existing oceanic lithosphere is younger than about
Jurassic in age and occurs at locations farthest from the oceanic
spreading centers.Except in areas where magmatism is intense enough to
build volcanic structures above sea level, most of the oceanic magmatism
is difficult to access.Samples of rocks can be obtained from drilling,
dredging, and expeditions of small submarines to the ocean floor.Numerous
samples have been recovered and studied using these methods.Most of the
magmatism is basaltic. Still, few drilling expeditions have penetrated
through the sediment cover and into the oceanic lithosphere. Nevertheless,
we have a fairly good understanding of the structure of the oceanic
lithosphere from seismic studies and ophiolites.
Here we will first look at ophiolites, then discuss basaltic magmatism in
general, and then discuss the various oceanic environments where magmatic
activity has occurred. Ophiolites
An ophiolite is a sequence of rocks that appears to represent a setion
through oceanic crust. Ophiolites occur in areas where obduction (the
opposite of subduction) has pushed a section of oceanic lithosphere onto
continental crust. During this process, most ophiolite sequences have been
highly deformed and hydrothermally altered. Nevertheless, it is often
possible to look through the deformation and alteration and learn
something of the structure of oceanic lithosphere.
An idealized ophiolite sequence shows an upper layer consisting of deep
sea sediments (limestones, cherts, and shales), overlying a layer of
pillow basalts. Pillow basalts have a structure consisting of
overlapping pillow-shaped pods of basalt. Such pillow structure is
typical of lavas erupted under water. The pillow basalts overly a layer
consisting of numerous dikes, some of which were feeder dikes for the
overlying basalts. Beneath the sheeted dike complex are gabbros that
likely represent the magma chambers for the basalts. The upper
gabbros are massive while the lower gabbros show layering that might have
resulted from crystal settling.
At the base of the layered gabbros, there is a sharp increase in the
density of the rocks, and the composition changes to ultramafic rocks.
This sharp change in density is correlated with what would be expected at
the base of the crust, and is thus referred to as the petrologic moho. At
the top of the ultramafic sequence the rock type is harzburgite (Ol +
Opx), a rock type expected to be the residual left from partially melting
peridotite. The base of the ultramafic layer is composed of peridotite.
Because most ophiolites have been hydrothermally altered, most of the
mafic rocks have been altered to serpentinite. Note that ophiolite means
"snake rock" Volcanic Settings
Volcanism occurs at three different settings on the ocean floor.
Oceanic Ridges - these are the oceanic spreading centers
where a relatively small range of chemical compositions of basalts are
erupted to form the basaltic layer of the oceanic crust. This chemical
type of basalt is referred to as Mid Ocean Ridge Basalts (MORBs).In
some areas, particularly Iceland, where there has been a large
outpouring of basalts on the oceanic ridge, basalts called Enriched
Mid Ocean Ridge Basalts (EMORBs)have been erupted.
Oceanic Islands - these are islands in the ocean basins that
generally occur away from plate boundaries, and are often associated
with hot spots, as discussed previously. A wide variety of rocks
occur in these islands, not all are basaltic, but those that aren't
appear to be related to the basaltic magmas. In general these
rocks are referred to as Oceanic Island Basalts (OIBs).
Large Igneous Provinces (LIPs) - these are massive
outpourings of mostly basaltic lavas that have built large submarine
plateaus. Most are mid-Cretaceous in age. They are not well
studied, but most have compositions similar to OIBs, and some may have
once had oceanic islands on top, but most of these have since been
removed by erosion.
Thus, most oceanic magmatism is basaltic, so we will first discuss
basaltic magmas in general.
On a chemical basis, basalts can be classified into three broad groups
based on the degree of silica saturation. This is best seen by first
casting the analyses into molecular CIPW norms (the same thing as CIPW
norms except the results are converted to mole % rather than weight %). On
this basis, most basalts consist predominantly of the normative minerals -
Olivine, Clinopyroxene, Plagioclase, and Quartz or Nepheline. These
minerals are in the 4 component normative system Ol-Ne-Cpx-Qtz, shown here
as a tetrahedron.In the tetrahedron, plagioclase plots between Ne and Qtz,
and Opx plots between Ol and Qtz. The basalt tetrahedron can be divided
The plane Cpx-Plag-Opx is the critical plane of silica
saturation.Compositions that contain Qtz in their norms plot in the
volume Cpx-Plag- Opx-Qtz, and would be considered silica
oversaturated.Basalts that plot in this volume are called Quartz
The plane Ol - Plag - Cpx is the critical plane of silica
undersaturation. Normative compositions in the volume between the
critical planes of silica undersaturation and silica saturation are
silica saturated compositions (the volume Ol - Plag - Cpx -
Opx). Silica saturated basalts are called Olivine Tholeiites
Normative compositions that contain no Qtz or Opx, but contain Ne
are silica undersaturated (the volume Ne-Plag-Cpx-Ol). Alkali
Basalts, Basanites, Nephelinites, and other silica
undersaturated compositions lie in the silica undersaturated volume.
Note that tholeiitic basalts are basalts that show a reaction relationship
of olivine to liquid which produces a low-Ca pyroxene like pigeonite or
Opx.Both olivine tholeiites and quartz tholeiites would show such a
relationship and would eventually precipitate either Opx or pigeonite.
The critical plane of silica undersaturation appears to be a thermal
divide at low pressure.This means that compositions on either side of the
plane cannot produce liquids on the other side of the plane by crystal
fractionation.To see this, look at the front two faces of the basalt
tetrahedron. These are in the three component systems Ol-Cpx-Qtz and
Mid Ocean Ridge Basalts (MORBs)
The Oceanic Ridges are probably the largest producers of magma on Earth.
Yet, much of this magmatism goes unnoticed because, with the exception of
Iceland, it all takes place below the oceans. This magmatism is
responsible for producing oceanic crust at divergent plate boundaries.
Magma is both erupted and intruded near the central depressions that form
the oceanic ridges. Thus, both basalts and gabbros are
produced. But, little is known of the gabbros since they are rarely
exposed and most oceanic lithosphere eventually is subducted.The main
melting mechanism is likely decompression melting as rising convection
cells move upward through the mantle beneath the ridges. At most
oceanic ridges the basalts that are erupted are tholeiitic basalts
sometimes referred to as NMORBs (normal MORBs)
At the oceanic ridges, the basalts erupted range in composition from
Olivine tholeiites to Quartz tholeiites.The compositions are by and large
restricted to basalt, i.e. less than about 52% SiO2. The
diagram shown here is called an AFM diagram. It is a triangular
variation diagram that plots total alkalies at the A corner, total iron at
the F corner, and MgO at the M corner.MORBs show a restricted range of
compositions that fall along a linear trend extending away from the
compositions of Mg-rich pyroxenes and olivines.
his is the trend that would expected from fractional crystallization
involving the removal of early crystallizing olivines and pyroxenes from a
tholeiitic basaltic liquid. Note that the trend is often referred to
as an Fe-enrichment trend.
Some thoughts on the origin of MORBs.
Melting is likely caused by decompression of the mantle as it rises
beneath the oceanic ridges as a result of convection.
Primary MORB magmas appear to be produced by partial melting of the
mantle at pressures between 15 and 20 kb.
Most MORBs erupted are not primary melts of the mantle, but instead
appear to have suffered olivine fractionation.
The small range in composition of MORBs can be explained by crystal
fractionation of Olivine + Plagioclase + Cpx at low pressures near the
MORBs appear to be the result of melting of an incompatible element
depleted mantle, both in terms of their incompatible trace element
compositions and isotopic ratios of Sr and Nd.
Ocean Island Basalts (OIBs)
Oceanic islands are, in general, islands that do not occur along the
divergent or convergent plate boundaries in the ocean basins.
Nevertheless, EMORBs, such as those that occur in Iceland, as well as the
Alkalic basalts of Iceland have much in common with magmas erupted in the
oceanic islands. In the Atlantic Ocean, which is a slow-spreading oceanic
basin, as well as in the Galapagos Islands of the eastern Pacific Ocean,
some of the islands occur close to oceanic ridge spreading centers.
In all cases we must keep in mind that the parts of these islands that are
accessible for sampling represent only a fraction of the mass of the
volcanic structures which rise from the ocean floor at depths up to 10,000
m. Thus, as with the ocean ridge volcanic rocks, there is a potential
Here we discuss not only the magmatism that has occurred recently at
Oceanic Islands, but also the magmatism that produced massive submarine
plateaus on the sea floor during the Cretaceous. The latter are often
referred to as Large Igneous Provinces (LIPs). Oceanic Islands
Most oceanic islands appear to be related to ascending plumes of hot
mantle. These plumes must be relative narrow features because they appear
to operate independent of the main convection cells that ascend beneath
the oceanic ridges and descend at subduction zones. Still, in places like
Iceland on the ocean ridge, magma production rates are high, and
compositions of rocks are similar to those found in oceanic islands. So
Iceland could also be considered an oceanic island.
If these rising plumes of hot mantle remain stationary in their positions
in the mantle, they produce hot spots, as discussed previously. Hot spots
are most recognizable when they occur beneath plates that move with higher
velocities. Beneath faster moving plates, like the Pacific Plate, this
results in linear chains of islands.
At the position directly over the hotspot, rising mantle melts to
produce magma that erupts on the seafloor, eventually building a volcanic
island directly over the hot spot. As the lithospheric plate moves
over the hot spot the volcano eventually is cut off from its source of
magma, and becomes extinct, and a new volcano forms on the plate at the
location directly above the hot spot.
The volcanoes that have moved away from the hot spot eventually begin to
erode until their elevations are reduced below sea level. At this point
they are called seamounts.
Such linear chains of islands and sea mounts are most evident in the
Pacific ocean. The largest of these is the Hawaiian - Emperor chain. The
hot spot that produced this chain is currently located under the position
of the big island of Hawaii, which has the only currently active volcanoes
in the chain. The bend in the Hawaiian-Emperor chain must have resulted
from a change in the direction of plate motion. Volcanic rocks dredged
from the sea floor at the location of this bend are about 40 million years
old. Thus, prior to 40 million years ago the Pacific Plate was moving in a
more northerly direction. The most northerly seamount is dated at about 60
million years.Seamounts older than 60 million have apparently been
The reason such island/seamount chains are not as evident in the other
oceans is because the plate velocity is lower and volcanoes tend to remain
over the hot spots for longer periods of time, building elongated groups
of islands rather than linear chains. Large Igneous Provinces (LIP)s
Large igneous provinces are areas where large volumes of magma have been
added to the Earth's crust over relatively short periods of time. Although
here we discuss these in terms of the ocean basins, it should be noted
that they also include the continental areas where large volumes of magma
have been erupted as flood basalts. Eruption of large amounts of magma on
the surface of the Earth can have drastic consequences.
The magmas may release large amounts of CO2 gas into the
atmosphere and force global warming.
Large amounts of magma erupted on the seafloor change the volume of
the ocean basins causing higher sea level and flooding large areas of
The mid Cretaceous Period was a time of higher than normal global
temperatures and high stands of the oceans.Eruption of magma on the ocean
floor at this time might have been the cause of these conditions.
Evidence is preserved on the sea floor in the form of large submarine
plateaus that were emplaced during this time period.
In the Pacific Ocean, much of the oceanic lithosphere east of the position
of the ridge 80 million years ago has been subducted.Thus, if the
submarine plateaus formed at the ridge, then it would be expected that
half of each plateau became separated at the ridge and have since been
subducted.These probable plateaus are shown in the map, and if they were
present would double the original size of the plateau.Thus, for example
the largest of the plateaus is the Ontong-Java Plateau, now located in the
southeastern Pacific, with a volume of about 50 million km3.But
if the other half had been present, the total volume of magma erupted over
a 25 million year period would have been over 100 million km3The
few studies that have looked at the rocks in these submarine plateaus
suggest that their compositions are similar to EMORBs and OIBs.
Unlike the ocean ridges, which have a rather limited range of rock
compositions, the oceanic islands have produced a broader
range. Basalts are still predominant, but other compositions are part
of the series, and the types of rocks produced are variable from one
island to the next. The table below shows that variety of rock types found
at different oceanic islands. Some produce tholeiitic rocks similar to
EMORBs and others produce alkalic basalts that are saturated to
undersaturated with respect to silica.
Melting is caused by decompression of the mantle as it rises in
narrow plumes beneath the oceanic islands.
Melting probably takes place at relatively low pressure to produce
silica saturated tholeiitic magmas, but at high pressure to produce
the silica undersaturated varieties such alkali basalts, basanites and
These basic magmas evolve by crystal fractionation to produce a wide
variety of other rock types including hawaiites, mugearites,
trachytes, and rhyolites. The more silica undersaturated
basanites would fractionate to produce nephelinites and phonolites.
Lower degrees of melting produce the most incompatible element rich
magmas, like basanites, while successively higher degrees of melting
could produce the alkali basalts and tholeiitic basalts.
OIBs (including EMORBs) appear to be the result of melting of an
incompatible element enriched mantle, both in terms of their
incompatible trace element compositions and isotopic ratios of Sr and
Based on the IUGS Subcommission on the Systematics of Igneous Rocks
The main QAPF classification for plutonic and volcanic rocks which
is based on the modal mineral proportions of quartz (Q), alkali feldspar
(A) and plagioclase (P) or of alkali feldspar (A), plagioclase (P) and
feldspathoids (F). Rocks with mafic content >90% have their own
This is followed by the classification separates and individually
classifies the pyroclastic, carbonatitic, melititic, lamprophyric and
charnockitic rocks before entering If the mineral mode cannot be
determined as is often the case for volcanic rocks, then a chemical
classification of total alkalis versus silica (TAS) is used. The
nomenclature for these classifiations necessitates only 297 rock names out
of the about 1500 that exist
Ten principles of IUGS Igneous Rock
Magmas must require special circumstances in order to form and do not form
just anywhere in the crust.
The Different Layers of the Earth Have Differing Chemical Compositions
Crust - variable thickness and composition
Continental 10 - 70 km thick, underlies all continental areas, has
an average composition that is andesitic.
Oceanic 8 - 10 km thick, underlies all ocean basins, has an
average composition that is basaltic.
Mantle - 3488 km thick, made up of a rock called peridotite
(Olivine + Opx + Cpx). Evidence comes from Seismic wave velocities,
experiments, and peridotite xenoliths (foreign rocks) brought to the
surface by magmas. Experimental evidence suggests that the mineralogy of
peridotite changes with depth (or pressure) in the Earth. At low
pressure, the mineral assemblage is Olivine + Cpx + Opx + Plagioclase
(plagioclase peridotite). At higher pressure the assemblage
changes to Olivine + Cpx + Opx + Spinel [(Mg,Fe+2) (Cr, Al,
Fe+3)2O4] (spinel peridotite). At
pressures above about 30 kilobars, the assemblage changes to Olivine +
Cpx + Opx + garnet (garnet peridotite). This occurs because Al
changes its coordination with increasing pressure, and thus new minerals
must form to accommodate the Al.
At greater depths, such as the 400 km discontinuity and the 670 km
discontinuity, olivine and pyroxene likely change to high pressure
polymorphs. Despite these changes in mineral assemblage, the
chemical composition of the mantle does not appear to change much in
terms of its major element composition.
Core - 2883 km radius, made up of Iron (Fe) and small
amount of Nickel (Ni). Evidence comes from seismic wave velocities,
experiments, and the composition of iron meteorites, thought to
be remnants of other differentiated planets that were broken apart due
to collisions. Lithosphere - about 100 km thick (up to 200 km thick beneath
continents), very brittle, easily fractures at low temperature. Note
that the lithosphere is comprised of both crust and part of the upper
mantle. The plates that we talk about in plate tectonics are made
up of the lithosphere, and appear to float on the underlying
asthenosphere. Asthenosphere - about 250 km thick - solid rock, but
soft and flows easily (ductile). The top of the asthenosphere is
called the Low Velocity Zone (LVZ) because the velocities of
both P- and S-waves are lower than the in the lithosphere above.
But, not that neither P- nor S-wave velocities go to zero, so the LVZ is
not completely liquid. Mesosphere - about 2500 km thick, solid rock, but still
capable of flowing. Outer Core - 2250 km thick - liquid. We know
this because S-wave velocities are zero in the outer core.
If Vs = 0, this implies m = 0, and this implies that
the material is in a liquid state. Inner core- 1230 km radius,solid
Magmas are not likely to come from the only part of the Earth that is in
a liquid state, the outer core, because it does not have the right
chemical composition. The outer core is made mostly of Fe with
some Ni, magmas are silicate liquids.
In the ocean basins, magmas are not likely to come from melting of the
oceanic crust, since most magmas erupted in the ocean basins are
basaltic. To produce basaltic magmas by melting of the basaltic
oceanic crust would require nearly 100% melting, which is not likely.
In the continents, both basaltic and rhyolitic magmas are erupted and
intruded. Basaltic magmas are not likely to have come from the
continental crust, since the average composition is more siliceous, but
more siliceous magmas (andesitic - rhyolitic) could come from melting of
the continental crust.Basaltic magmas must come from the underlying
Thus, with the exception of the continents, magmas are most likely to
originate in the mantle from melting of mantle peridotite.
Origin of Magmas
Temperature varies with depth or pressure in the Earth along the
geothermal gradient. The normal geothermal gradient is somewhat
higher beneath the oceans than beneath the continents, at least at
shallow levels.If we compare the normal geothermal gradients with the
experimentally determined phase diagram for peridotite containing little
water or carbon dioxide, we find that the peridotite solidus temperature
is everywhere higher than the normal geothermal gradients.Thus, under
normal conditions the mantle is solid, as we would suspect from the
Thus, in order to generate a melt, either we must find a way to increase
the geothermal gradient so that it is above the peridotite solidus or
reduce the temperature of the peridotite solidus. In either case
note that all we have to do is get the temperature in some part of the
Earth, as expressed by the geothermal gradient, into the field of
partial melt. Partial melting is the most likely case because it
requires less of an increase in temperature or less of a decrease in the
peridotite solidus. Once a partial melt has formed, the liquid
portion can be easily separated from the remaining solids since liquids
are more mobile and, in general, have a lower density than solids.
Raising the Geothermal Gradient
Radioactive Heat - Elements like U, Th, K, and Rb have radioactive
isotopes. During radioactive decay, sub-atomic particles are
released by the decaying isotope and move outward until they collide
with other atomic particles. Upon collision, the kinetic energy
of the moving particles is converted to heat. If this heat cannot be
conducted away, then the temperature will rise. Most the heat
within the Earth is generated by radioactive decay, and this is the
general reason why temperature increases with depth in the Earth. But
most the radioactive isotopes are concentrated in the crust.
Although there are areas in the continental crust where high
concentrations of radioactive elements have locally raised the
temperature, at least high enough to cause metamorphism, this is a rare
occurrence. It is even more unlikely that areas of high concentration
develop within the mantle. Thus, concentrations of radioactive
elements is not likely to cause melting.
In areas where rocks slide past one another, such as at the base of the
lithosphere, on at subduction zones, heat could be generated by
friction.If this heat cannot be conducted away fast enough, then it may
cause a localized rise in temperature within the zone where the sliding
or shearing is taking place.This could cause a localized spike on the
geothermal gradient that could cause local temperatures to rise above
Decompression due to Convection
Convection is a form of heat transfer wherein the heat moves with the
material.Convection can be induced if the temperature gradient is high
enough that material at depth expands so that its density is lower than
the material above it. This is an unstable situation and the hotter,
lower density material will rise to be replaced by descending cooler
material in a convection cell.
The rate of convection depends on both on the temperature gradient and
the viscosity of the material (note that solids convect, but the rate is
lower than in liquids because solids have higher viscosity). In the
Earth, temperature gradients appear to be high enough and viscosity low
enough for convection to occur. Plate tectonics appears to be
driven by convection in some form.
Anywhere there is a rising convection current, hotter material at depth
will rise carrying its heat with it. As it rises to lower pressure
(decompression) it will cool somewhat, but will still have a temperature
higher than its surroundings. Thus, decompression will result in raising
the local geothermal gradient. If this new geothermal gradient reaches
temperatures greater than the peridotite solidus, partial melting and
the generation of magma can occur. This mechanism is referred to as
Lowering the Solidus Temperature
Mixtures of components begin melting at a lower temperature than the
pure components.In a two component system addition of a third component
reduces both the solidus and liquidus temperatures.This suggests that if
something can be added to the mantle, it could cause the solidus and
liquidus temperatures to be lowered to the extent that the solidus could
become lower than the geothermal gradient and result in partial melting,
without having to raise the geothermal gradient. Such a melting
mechanism is referred to as flux melting
It's difficult to imagine how solid components could be added to the
mantle.But volatile components, for example H2O and CO2,
because of the high mobility, could be added to the mantle, particularly
at subduction zones.
Oceanic crust is in contact with sea water, thus water could be in
oceanic crust both due to weathering, which produces hydrous
minerals like clay minerals, and could be in the pore spaces in the
Oceanic sediments eventually cover the basaltic oceanic crust
produced at oceanic ridges. Much of this sediment consists of
clay minerals (which contain water) and carbonate minerals (which
contain carbon dioxide).
As the oceanic lithosphere descends into the mantle at a
subduction zone, it will be taken to increasingly higher
temperatures as it gets deeper. This will result in
metamorphism of both the basalt and the sediment. As we will
see later in our discussion of metamorphism, metamorphism is
essentially a series of dehydration and decarbonation reactions,
i.e. chemical reactions that transform hydrous and carbonate
minerals into nonhydrous minerals and give up H2O and CO2
as a fluid phase.
Addition of this fluid phase, either to the subducted lithosphere
or the mantle overlying the subducted lithosphere could lower the
solidus and liquidus temperatures enough to cause partial melting.
In the continental crust, it is not expected that the normal geothermal
gradient will be high enough to cause melting despite the fact that
hydrous and carbonate minerals occur in many continental
rocks.Furthermore, because continental rocks are at low temperature and
have a very high viscosity, convective decompression is not likely to
occur.Yet, as we will see, there is evidence that continental crustal
rocks sometimes melt. This is called crustal anatexis. The
following scenario is one mechanism by which crustal anatexis could
Basaltic magmas, generated in the mantle, by flux melting,
decompression melting or frictional heat, rise into the crust,
carrying heat with them.
Because basaltic liquids have a higher density than crust, they
may not make it all the way to the surface, but instead intrude and
cool slowly at depth.
Upon cooling the basaltic magmas release heat into the crust,
raising the geothermal gradient (increasing the local temperature).
Successive intrusions of mantle-derived mantle into the same area
of the crust may cause further increases in temperature, and
eventually cause the geothermal gradient to become higher than the
wet solidus of the crustal material, resulting in a partial melt of
Magmatism and Plate Tectonics
From the discussion above it should be obvious that magmatism is closely
related to plate tectonics.The diagram below summarizes melting
mechanisms that occur as a result of plate tectonics and may be
responsible for the generation of magmas in a variety of plate tectonic
settings, such as oceanic ridges, near subduction zones, and at rift
Diverging Plate Boundaries
Diverging plate boundaries are where plates move away from each other.
These include oceanic ridges or spreading centers, and rift
Oceanic Ridges are areas where mantle appears to ascend due to rising
convection currents. Decompression melting could result,
generating magmas that intrude and erupt at the oceanic ridges to create
new oceanic crust.
Iceland is one of the few areas where the resulting magmatism has been
voluminous enough to built the oceanic ridge above sea level.
Continental Rift Valleys or Extensional Zones are areas, usually located
in continental crust where extensional deformation is occurring.
These areas may be incipient spreading centers and may eventually evolve
into oceanic ridges, such as has occurred in the Red Sea region.
Whether or not they develop into spreading centers, they are likely
caused by mantle upwelling below the zone of extension.Mantle upwelling
may result in decompression melting of the mantle, and could induce
crustal anatexis. A good example of a continental rift valley is the
East African Rift Valley. Another example is the Rio Grande Rift in
Colorado and New Mexico, which is part of a larger region of extension
that includes much of the western U.S. and is called the Basin and Range
Province (Eastern California] Converging Plate Boundaries
Converging plate boundaries are where plates run into each other.The
most common type are where oceanic lithosphere subducts. Several
mechanisms could contribute to the generation of magmas in this
environment (see diagram at top of this section).
Frictional heating is likely to occur along the boundary between
the subducted plate and the overlying mantle wedge.
Flux melting of either the subducted lithosphere or the overlying
mantle wedge could occur as a result of the release of volatiles as
the subducted plate heats and metamorphoses producing water and/or
carbon dioxide fluids.
The process of subduction may drag the overlying mantle wedge down
with it. In order to replace the mantle dragged down in this
process, part of the mantle wedge will have to rise.This upwelling
of the mantle could result in decompression melting.
If an oceanic lithospheric plate subducts beneath another oceanic
lithospheric plate, we find island arcs on the surface above the
If an oceanic plate subducts beneath a plate composed of continental
lithosphere, we find continental margin arcs If magma generated near the
subduction zone intrudes and cools in the crust, it could induce crustal
In areas where two continental lithospheric plates converge fold-thrust
mountain ranges develop as the result of compression. If
water-bearing crustal rocks are pushed to deeper levels where
temperatures are higher, crustal anatexis may result.
Intraplate Magmatism & Hot Spots
There are a few areas where magmatism does not appear to be related to
converging or diverging plate boundaries.These areas occur in the middle
of plates, usually far from the plate boundaries.This phenomenon is
referred to as intraplate magmatism. Intraplate magmatism is thought to
be caused by hot spots formed when thin plumes of mantle material rise
along narrow zones from deep within the mantle.The hot spot remains
stationary in the mantle while the plate moves over the hot spot.
Decompression melting caused by the upwelling plume produces magmas that
form a volcano on the sea floor above the hot spot.The volcano remains
active while it is over the vicinity of the hot spot, but eventually
plate motion results in the volcano moving away from the plume and the
volcano becomes extinct and begins to erode.
Because the Pacific Plate is one of the faster moving plates, this type
of volcanism produces linear chains of islands and seamounts, such as
the Hawaiian - Emperor chain, the Line Islands, the Marshall-Ellice
Islands, and the Austral seamount chain.
Rocks emplaced in any given restricted area during a short amount of
geologic time were likely related to the same magmatic event.
Evidence for some kind of relationship between the rocks, and therefore
between the magmas that cooled to form the rocks came from plotting
A variation diagram is a plot showing how each oxide component in a rock
varies with some other oxide component. Because SiO2
usually shows the most variation in any given suite of rocks, most
variation diagrams plot the other oxides against SiO2 as
shown in the diagram to right, although any other oxide could be chosen
for plotting on the x-axis.Plots that show relatively smooth trends of
variation of the components suggested that the rocks might be related to
one another through some process. Of course, in order for the
magmas to be related to one another, they must also have been intruded
or erupted within a reasonable range of time. Plotting rocks of
Precambrian age along with those of Tertiary age may show smooth
variation, but it is unlikely that the magmas were related to one
another.If magmas are related to each other by some processes, that
process would have to be one that causes magma composition to
Any process that causes magma composition to change is called magmatic
differentiation. Over the years, various process have been suggested to
explain the variation of magma compositions observed within small
regions.Among the processes are:
Distinct melting events from distinct sources.
Various degrees of partial melting from the same source.
Mixing of 2 or more magmas.
Assimilation/contamination of magmas by crustal rocks.
Initially, researchers attempted to show that one or the other of these
process acted exclusively to cause magmatic differentiation. With
historical perspective, we now realize that if any of them are possible,
then any or all of these processes could act at the same time to produce
chemical change, and thus combinations of these processes are
possible. Still, we will look at each one in turn in the following
discussion. Distinct Melting Events
One possibility that always exists is that the magmas are not related
except by some heating event that caused melting.In such a case each
magma might represent melting of a different source rock at different
times during the heating event. If this were the case, we might
not expect the chemical analyses of the rocks produced to show smooth
trends on variation diagrams. But, because variation diagrams
based on a closed set of numbers (chemical analyses add up to 100%), if
the weight% of one component increases, then the weight percent of some
other component must decrease. Thus, even in the event that the
magmas are not related SiO2 could increase and MgO could
decrease to produce a trend. The possibility of distinct melting
events is not easy to prove or disprove. Various Degrees of Partial Melting
When a multicomponent rock system melts, unless it has the composition
of the eutectic, it melts over a range of temperatures at any given
pressure and during this melting the liquid composition changes.
Thus, a wide variety of liquid compositions could be made by various
degrees of partial melting of the same source rock.
For example...A simple in a three component system containing natural
minerals, the system Fo - Di - SiO2, shown in simplified form
here. A proxy for mantle peridotite, being a mixture of Ol, Cpx, and Opx
would plot as shown in the diagram. This rock would begin to melt
at the peritectic point, where Di, En, Ol, and Liquid are in
equilibrium. The composition of the liquid would remain at the
peritectic point (labeled 0% melting) until all of the diopside melted.
This would occur after about 23% melting. The liquid would then
take a path shown by the dark curve, first moving along the En - Ol
boundary curve, until the enstatite was completely absorbed, then moving
in a direct path toward the peridotite composition. Fractional Melting
Note that above some of the liquid must be left behind.If all of the
liquid is removed, then we have the case of fractional melting, which is
In fractional melting some all of the liquid is removed at each stage of
the process. Let's imagine that we melt the same peridotite again,
removing liquids as they form. The first melt to form again will
have a composition of the peritectic, labeled "Melt 1" in the
diagram. Liquids of composition - Melt 1 can be produced and
extracted until all of the Diopside is used up. At this point,
there is no liquid, since it has been removed or fractionated, so the
remaining solid consists only of Enstatite and Forsterite with
composition "Solid 2". This is a two component system. Thus
further melting cannot take place until the temperature is raised to the
peritectic temperature in the two component system Fo- SiO2.
In our discussion of phase diagrams we saw how liquid compositions can
change as a result of removing crystals from the liquid as they
form. In all cases, except a eutectic composition, crystallization
results in a change in the composition of the liquid, and if the
crystals are removed by some process, then different magma compositions
can be generated from the initial parent liquid. If minerals that
later react to form a new mineral or solid solution minerals are
removed, then crystal fractionation can produce liquid compositions that
would not otherwise have been attained by normal crystallization of the
parent liquid. Bowen's Reaction Series
Norman L. Bowen, an experimental petrologist in the early 1900s,
realized this from his determinations of simple 2- and 3-component phase
diagrams, and proposed that if an initial basaltic magma had crystals
removed before they could react with the liquid, that the common suite
of rocks from basalt to rhyolite could be produced.This is summarized as
Bowen's Reaction Series
Bowen suggested that the common minerals that crystallize from magmas
could be divided into a continuous reaction series and a discontinuous
The continuous reaction series is composed of the plagioclase
feldspar solid solution series. A basaltic magma would
initially crystallize a Ca- rich plagioclase and upon cooling
continually react with the liquid to produce more Na-rich
plagioclase. If the early forming plagioclase were removed,
then liquid compositions could eventually evolve to those that would
crystallize a Na-rich plagioclase, such as a rhyolite liquid.
The discontinuous reaction series consists of minerals that upon
cooling eventually react with the liquid to produce a new
phase. Thus, as we have seen, crystallization of olivine from
a basaltic liquid would eventually reach a point where olivine would
react with the liquid to produce orthopyroxene. Bowen
postulated that with further cooling pyroxene would react with the
liquid, which by this time had become more enriched in H2O,
to produce hornblende. The hornblende would eventually react
with the liquid to produce biotite. If the earlier
crystallizing phases are removed before the reaction can take place,
then increasingly more siliceous liquids would be produced.
This generalized idea is consistent with the temperatures observed in
magmas and with the mineral assemblages we find in the various
rocks. We would expect that with increasing SiO2 oxides
like MgO, and CaO should decrease with higher degrees of crystal
fractionation because they enter early crystallizing phases, like
olivines and pyroxenes. Oxides like H2O, K2O and
Na2O should increase with increasing crystal fractionation
because they do not enter early crystallizing phases. Furthermore,
we would expect incompatible trace element concentrations to increase
with fractionation, and compatible trace element concentrations to
decrease. This is generally what is observed in igneous rock
suites. Because of this, and the fact that crystal fractionation
is easy to envision and somewhat easy to test, crystal fraction is often
implicitly assumed to be the dominant process of magmatic
differentiation. Mechanisms of Crystal Fractionation
In order for crystal fractionation to operate their must be a natural
mechanism that can remove crystals from the magma or at least separate
the crystals so that they can no longer react with the liquid.Several
mechanisms could operate in nature.
Crystal Settling/Floating - In general, crystals forming from a
magma will have different densities than the liquid.
If the crystals have a higher density than the liquid, they
will tend to sink or settle to the floor of the magma body. The
first layer that settles will still be in contact with the
magma, but will later become buried by later settling crystals
so that they are effectively removed from the liquid.
If the crystals have a lower density in the magma, they will
tend to float or rise upward through the magma. Again the
first layer that accumulates at the top of the magma body will
initially be in contact with the liquid, but as more crystals
float to the top and accumulate, the earlier formed layers will
be effectively removed from contact with the liquid.
Inward Crystallization - Because a magma body is hot and the
country rock which surrounds it is expected to be much cooler, heat
will move outward away from the magma. Thus, the walls of the
magma body will be coolest, and crystallization would be expected to
take place first in this cooler portion of the magma near the
walls. The magma would be expected then to crystallize from
the walls inward. Just like in the example above, the first
layer of crystals precipitated will still be in contact with the
liquid, but will eventually become buried by later crystals and
effectively be removed from contact with the liquid.
Filter pressing - this mechanism has been proposed as a way to
separate a liquid from a crystal-liquid mush. In such a
situation where there is a high concentration of crystals the liquid
could be forced out of the spaces between crystals by some kind of
tectonic squeezing that moves the liquid into a fracture or other
free space, leaving the crystals behind. It would be kind of like
squeezing the water out of a sponge. This mechanism is
difficult to envision taking place in nature because (1) unlike a
sponge the matrix of crystals is brittle and will not deform easily
to squeeze the liquid out, and (2) the fractures required for the
liquid to move into are generally formed by extensional forces and
the mechanism to get the liquid into the fractures involves
compressional forces. Filter pressing is a common method used
to separate crystals from liquid in industrial processes, but has
not been shown to have occurred in nature.
If two or more magmas with different chemical compositions come in
contact with one another beneath the surface of the Earth, then it is
possible that they could mix with to produce compositions intermediate
between the end members. If the composition of the magmas is
greatly different (i.e. basalt and rhyolite), there are several factors
that would tend to inhibit mixing.
Temperature contrast - basalts and rhyolites have very different
temperatures. If they come in contact with one another the
basaltic magma would tend to cool or even crystallize and the
rhyolite would tend to heat up and begin to dissolve any crystals
that it had precipitated.
Density Contrast- basaltic magmas have densities on the order of
2600 to 2700 kg/m3, whereas rhyolitic magmas have
densities of 2300 to 2500 kg/m3. This contrast in
density would mean that the lighter rhyolitic magmas would tend to
float on the heavier basaltic magma and inhibit mixing.
Viscosity Contrast- basaltic magmas and rhyolitic magmas would
have very different viscosities. Thus, some kind of vigorous
stirring would be necessary to get the magmas to mix.
Despite these inhibiting factors, there is evidence in rocks that magmas
do sometimes mix. The smaller the difference in chemical
composition between two magmas, the smaller will be the contrasts in
temperature, density, and viscosity.
If magmas of contrasting composition come in contact and begin to mix
some kind of stirring mechanism would first be necessary.Such stirring
could be provided by convection, Evidence for Mixing
Mingling of magmas
If,in the initially stages of such mixing, the magma were erupted,
then we might expect to find rocks that show a "marble cake"
appearance, with dark colored mafic rock intermingled with lighter
colored rhyolitic rock.This, however, is mingling of magmas.Note
that differences in color are not always due to differences in
composition, so even in rocks that show this banding, mingling of
magmas may not have occurred.
Disequilibrium Mineral Assemblages
If convective stirring progresses beyond the point of mingling, some
evidence might still be preserved if the crystals present in one of
the magmas does not completely dissolve or react. This might
leave disequilibrium mineral assemblages.For example, if a basaltic
magma containing Mg-rich olivine mixed with a rhyolite magma
containing quartz and the magma was erupted before the quartz or
olivine could be redissolved or made over into another mineral, then
we would produce a rock containing mineral that are out of
Reverse Zoning in Minerals
When a mineral is placed in an environment different than the one in
which it originally formed it will tend to react to retain
equilibrium. Instead of dissolving completely or remaking
their entire composition, solid solution minerals may just start
precipitating a new composition that is stable in the new chemical
environment or at the new temperature. This can result in
zoned crystals that show reversals of the zoning trends. for
Mg-Fe solid solution minerals normally zone from Mg-rich cores to
Fe-rich rims. If a Fe-rich olivine or pyroxene is mixed into a
Mg-rich magma that is precipitating Mg-rich olivine or pyroxene, it
may precipitate the more Mg-rich composition on the rims of the
added crystals.Analyses of such crystals would reveal a reversal in
zoning.Similarly, if a Na-rich plag. originally crystallizing from a
rhyolitic magma were mixed into a basaltic magma precipitating a
Ca-rich plag., a Ca-rich rim may be added to the Na-rich
Crystal growth from liquids is sometimes not perfect. Sometimes
the crystal grows incompletely trapping liquid inside. If that
liquid is quenched on the surface and a thin section is cut through the
crystal this trapped liquid will be revealed as glass inclusions in the
Since the glass inclusions should represent the composition of the magma
that precipitated the crystal, chemical analysis of glass inclusions
give us the composition of the liquid in which the crystal formed.
The groundmass may also contain glass representing the composition of
the liquid in which the crystal resided at eruption. If the composition
of glass inclusions is different from glass in the groundmass, and if
the groundmass composition is not what is expected from normal
crystallization of the minerals present, this provides evidence of magma
If the mixing process proceeds to the point where other evidence
is erased, evidence for mixing will still be preserved in the
composition of the mixed magmas. On oxide-oxide variation diagrams
mixtures will lie along a straight line. Thus if diagrams show a
group of rocks that lie along the same straight line, and the
proportional distances are the same on all diagrams, one could
hypothesize that the chemical variation resulted from magma mixing.
Because the composition of the crust is generally different from the
composition of magmas which must pass through the crust to reach the
surface, their is always the possibility that reactions between the
crust and the magma could take place.If crustal rocks are picked up,
incorporated into the magma, and dissolved to become part of the magma,
we say that the crustal rocks have been assimilated by the magma.If the
magma absorbs part of the rock through which it passes we say that the
magma has become contaminated by the crust.Either of these process would
produce a change in the chemical composition of the magma unless the
material being added has the same chemical composition as the magma.
In a sense, bulk assimilation would produce some of the same effects as
mixing, but it is more complicated than mixing because of the heat
balance involved. In order to assimilate the country rock enough
heat must be provided to first raise the country rock to its solidus
temperature where it will begin to melt and then further heat must be
added to change from the solid state to the liquid state. The only
source of this heat, of course, is the magma itself.
Evidence for Assimilation/Contamination
As magma passes upward through the crust pieces of the country rock
through which it passes may be broken off and assimilated by the
magma.Just as in magma mixing, various stages of this process may be
preserved in the magma and rock that results. Xenoliths (meaning foreign
rock) are pieces of rock sometimes found as inclusions in other
rocks.The presence of xenoliths does not always indicate that
assimilation has taken place, but if the xenoliths show evidence of
having been disaggregated with their minerals distributed thought the
rest of the rock it is likely that some contamination of the magma has
taken place.This may result in disequilibrium mineral assemblages and
reversely zoned minerals, just as in the case of magma mixing.
And, if the assimilation goes to completion, with all of the xenoliths
being dissolved in the magma, the only evidence left may be chemical,
and again similar to the straight line mixing patterns produced by
Perhaps the best evidence of assimilation/contamination comes from
studies of radiogenic isotopes. Here we give an example using the
systematics of the Rb - Sr system.
87Rb is a radioactive isotope that decays to 87Sr
with a half life of billion years.
Because Rb is an incompatible element, it has been extracted from
the mantle by magmas and added to the crust. Thus the
concentration of Rb in the crust (avg. about 100 ppm) is much higher
than it is in the mantle (avg. about 4 ppm).
86Sr is a stable, non radiogenic isotope whose
concentration does not change with time.
Because 87Rb decays to produce 87Sr and
because there is more Rb in the crust than in the mantle, the 87Sr/
87Sr of the crust has, over time changed to much higher
values than the 87Sr/87Sr ratio in the
The 87Sr/87Sr ratio of the mantle is
generally in the range between 0.702 - 0.705. Thus rocks
derived from melting of the mantle should have 87Sr/87Sr
ratios in this range.
87Sr/87Sr ratios of crustal rocks will
depend on their age and concentration of Rb. Older crustal
rocks will have high values of 87Sr/87Sr in
the range 0.705 - 0.720, younger crustal rocks having been recently
derived from the mantle will 87Sr/87Sr ratios
more similar to the mantle.
If mantle derived magmas assimilate or are contaminated by older
crustal rocks, then we would expect to find ratios of 87Sr/87Sr
in these contaminated rocks that are higher than those found in the
mantle and extend up to values found in older crustal rocks.
Liquid immiscibility is where liquids do not mix with each other. We're
all familiar with this phenomenon in the case of oil and water/vinegar
in salad dressing.We've also discussed immiscibility in solids, for
example in the alkali feldspar system.Just like in the alkali feldspar
system, immiscibility is temperature dependent.
For example, in a two component system if there is a field of
immiscibility it would appear as in the diagram to shown here.Cooling of
a liquid with a composition of 25%B & 75%A would eventually result
in the liquid separating into two different compositions.Further with
further cooling these liquids one liquid would become more enriched in A
and the other more enriched in B.Eventually both liquids would reach a
temperature where crystals of A would start to form.Note that both
liquids would be in equilibrium with crystals of A at the same
temperature. Further cooling would result in the disappearance of
the A-rich liquid.
This points out two important properties of immiscible liquids.
If immiscible liquids are in equilibrium with solids, both liquids
must be in equilibrium with the same solid compositions.
Extreme compositions of the two the liquids will exist at the same
Liquid immiscibility was once thought to be a mechanism to explain all
magmatic differentiation. If so, requirement 2, above, would
require that siliceous liquids and mafic liquids should form at the same
temperature. Since basaltic magmas are generally much hotter than
rhyolitic magmas, liquid immiscibility is not looked upon favorably as
an explanation for wide diversity of magmatic compositions. Still,
liquid immiscibility is observed in experiments conducted on simple rock
systems. Combined Processes
As pointed out previously, if any of these process are possible, then a
combination of the process could act to produce chemical change in
magmas. Thus, although crystal fractionation seems to be the
dominant process affecting magmatic differentiation, it may not be the
only processes. As we have seen, assimilation is likely to accompanied
by crystallization of magmas in order to provide the heat necessary for
assimilation.If this occurs then a combination of crystal fraction and
assimilation could occur. Similarly, magmas could mix and
crystallize at the same time resulting in a combination of magma mixing
and crystal fractionation.In nature, things could be quite complicated.