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32 Igneous Rocks

321 Origin of Igneous rocks

An igneous rock is any crystalline or glassy rock that forms from cooling of magmaMagma consists mostly of liquid rock matter but may contain crystals of various minerals and may contain a gas phase that may be dissolved in the liquid or may be present as a separate gas phase

Magma can cool to form an igneous rock either on the surface of the Earth - in which case it produces a volcanic or extrusive igneous rock or beneath the surface of the Earth in which case it produces a plutonic or intrusive igneous rock At depth in the Earth nearly all magmas contain gas dissolved in the liquid but the gas forms a separate vapor phase when pressure is decreased as magma rises toward the surface This is similar to carbonated beverages which are bottled at high pressure The high pressure keeps the gas in solution in the liquid but when pressure is decreased like when you open the can or bottle the gas comes out of solution and forms a separate gas phase that you see as bubbles Gas gives magmas their explosive character because volume of gas expands as pressure is reduced The composition of the gases in magma is

Mostly H2O (water vapor) with some CO2 (carbon dioxide) Minor amounts of Sulfur Chlorine and Fluorine gases

The amount of gas in magma is also related to the chemical composition of the magmaRhyolitic magmas usually have higher dissolved gas contents than basaltic magmas

Types of Magma

Types of magma are determined by chemical composition of the magma Three general types are recognized but we will look at other types later in the course1 Basaltic magma (1000-1200oC) -- SiO2 45-55 wt high in Fe Mg Ca low in K Na2 Andesitic magma (800-1000oC) -- SiO2 55-65 wt intermediate in Fe Mg Ca Na K3 Rhyolitic or Granitic magma (650-800oC) -- SiO2 65-75 low in Fe Mg Ca high in K Na

Viscosity of Magmas

Viscosity is the resistance to flow (opposite of fluidity) Depends on composition temperature amp gas content Higher SiO2 content magmas have higher viscosity than

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lower SiO2 content magmas Lower Temperature magmas have higher viscosity than higher temperature magmas

Summary TableMagma Type

Solidified Volcanic Rock

Solidified Plutonic Rock

Chemical Composition

Temperature

Viscosity Gas Content

Basaltic Basalt Gabbro 45-55 SiO2 high in Fe Mg Ca low in K Na

1000 - 1200 oC

Low Low

Andesitic Andesite

Diorite 55-65 SiO2 intermediate in Fe Mg Ca Na K

800 - 1000 oC

Intermediate

Intermediate

Rhyolitic Rhyolite Granite 65-75 SiO2 low in Fe Mg Ca high in K Na

650 - 800 oC

High High

Origin of Magma

In order for magmas to form some part of the Earth must get hot enough to melt the rocks present Under normal conditions the geothermal gradient is not high enough to melt rocks and thus with the exception of the outer core most of the Earth is solid Thus magmas form only under special circumstances To understand this we must first look at how rocks and mineral melt As pressure increases in the Earth the melting temperature changes as well For pure minerals there are two general cases For a pure dry (no H2O or CO2 present) mineral the melting temperate increases with increasing pressure For a mineral with H2O or CO2 present the melting temperature first decreases with increasing pressure Since rocks mixtures of minerals they behave somewhat differently Unlike minerals rocks do not melt at a single temperature but instead melt over a range of temperatures Thus it is possible to have partial melts from which the liquid portion might be extracted to form magmaThe two general cases are

Melting of dry rocks is similar to melting of dry minerals melting temperatures increase with increasing pressure except there is a range of temperature over which there exists a partial melt The degree of partial melting can range from 0 to 100

Melting of rocks containing water or carbon dioxide is similar to melting of wet minerals melting temperatures initially decrease with increasing pressure except there is a range of temperature over which there exists a partial melt

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Origin of Basaltic Magma

Much evidence suggests that Basaltic magmas result from dry partial melting of mantleBasalts make up most of oceanic crust and only mantle underlies crust Basalts contain minerals like olivine pyroxene and plagioclase none of which contain water Basalts erupt non-explosively indicating a low gas content and therefore low water content

The Mantle is made of garnet peridotite (a rock made up of olivine pyroxene and garnet) Evidence comes from pieces brought up by erupting volcanoes In the laboratory we can determine the melting behavior of garnet peridotite Under normal conditions the temperature in the Earth shown by the geothermal gradient is lower than the beginning of melting of the mantle Thus in order for the mantle to melt there has to be a mechanism to raise the geothermal gradient Once such mechanism is convection wherein hot mantle material rises to lower pressure or depth carrying its heat with it If the raised geothermal gradient becomes higher than the initial melting temperature at any pressure then a partial melt will form Liquid from this partial melt can be separated from the remaining crystals because in general liquids have a lower density than solidsBasaltic or gabbroic magmas appear to originate in this way

Origin of Granitic or Rhyolitic Magma

Most Granitic or Rhyolitic magma appears to result from wet melting of continental crust The evidence for this is

Most granites and rhyolites are found in areas of continental crust When granitic magma erupts from volcanoes it does so very explosively

indicating high gas content Solidified granite or rhyolite contains quartz feldspar hornblende biotite and

muscovite The latter minerals contain water indicating high water content

Origin of Andesitic Magma

Average composition of continental crust is andesitic but if andesite magma is produced by melting of continental crust then it requires complete melting of crust Temperatures in crust unlikely to get high enough Andesitic magmas erupt in areas above subduction zones suggests relation between production of andesite and subduction One theory involves wet partial melting of subducted oceanic crust But newer theories suggest wet partial melting of mantle

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Magmatic Differentiation

When magma solidifies to form a rock it does so over a range of temperature Each mineral begins to crystallize at a different temperature and if these minerals are somehow removed from the liquid the liquid composition will change Depending on how many minerals are lost in this fashion a wide range of compositions can be made The process is called magmatic differentiation by crystal fractionation Crystals can be removed by a variety of processes If the crystals are denser than the liquid they may sink If they are less dense than the liquid they will float If liquid is squeezed out by pressure then crystals will be left behind Removal of crystals can thus change the composition of the liquid portion of the magma Over the years various processes have been suggested to explain the variation of magma compositions observed within small regions Among the processes are

1 Distinct melting events from distinct sources2 Various degrees of partial melting from the same source3 Crystal fractionation4 Mixing of 2 or more magmas5 Assimilationcontamination of magmas by crustal rocks6 Liquid Immiscibility7 Combined process (a combination of one of these)

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 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

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liquid composition changes Thus a wide variety of liquid compositions could be made by various degrees of partial melting of the same source rock

Crystal Fractionation

Liquid compositions can change as a result of removing crystals from the liquid as they form In all cases 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

Bowens 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 Bowens Reaction Series (Fig 37)

Figure 37 Bowenrsquos Reaction Series

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Bowen suggested that the common minerals that crystallize from magmas could be divided into a continuous reaction series and a discontinuous reaction series

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 SettlingFloating - In general crystals forming from magma will have different densities than the liquid

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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 then be expected 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

Magma Mixing

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 each other to produce compositions intermediate between the end members If the compositions of the magmas are greatly different (ie basalt and rhyolite) there are several factors that would tend to inhibit mixing

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Temperature contrast - basaltic and rhyolitic magmas have very different temperatures If they come in contact with one another the basaltic magma would tend to cool or even crystallize and the rhyolitic magma 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 kgm3 whereas rhyolitic magmas have densities of 2300 to 2500 kgm3 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

Crustal AssimilationContamination

Because the composition of the crust is generally different from the composition of magmas which must pass through the crust to reach the surface there 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 processes would produce a change in the chemical composition of the magma unless the material being added has the same chemical composition as the magmaIn 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

Liquid Immiscibility

Liquid immiscibility is where liquids do not mix with each other We are all familiar with this phenomenon in the case of oil and watervinegar in salad dressing We have also discussed immiscibility in solids for example in the alkali feldspar system Just like in the alkali feldspar system immiscibility is temperature dependentTwo important properties of immiscible liquids

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1 If immiscible liquids are in equilibrium with solids both liquids must be in equilibrium with the same solid compositions2 Extreme compositions of the two the liquids will exist at the same temperature

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 systemsThere are however three exceptions where liquid immiscibility may play a role1 Sulfide liquids may separate from mafic silicate magmas2 Highly alkaline magmas rich in CO2 may separate into two liquids one rich in carbonate and the other rich in silica and alkalies This process may be responsible for forming the rare carbonatite magmas3 Very Fe-rich basaltic magmas may form two separate liquids - one felsic and rich in SiO2 and the other mafic and rich in FeO

Combined Processes

As pointed out previously if any of these processes 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 accompany 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

322 Mode of occurrence of igneous bodies

Eruption of Magma

When magmas reach the surface of the Earth they erupt from a vent They may erupt explosively or non-explosively Non-explosive eruptions are favored by low gas content and low viscosity magmas (basaltic to andesitic magmas) Usually begin with fire fountains due to release of dissolved gases Produce lava flows on surface and produce Pillow lavas if erupted beneath water

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Figure 38 Types of lava flow (a) Ropy surface of a pahoehoe flow (b) aa flow the left side on the photo is a pahoehoe flow

Explosive eruptions are favored by high gas content and high viscosity (andesitic to rhyolitic magmas) Expansion of gas bubbles is resisted by high viscosity of magma which results in building of pressure High pressure in gas bubbles causes the bubbles to burst when reaching the low pressure at the Earths surface Bursting of bubbles fragments the magma into pyroclasts and tephra (ash) Cloud of gas and tephra rises above volcano to produce an eruption column that can rise up to 45 km into the atmosphere

Figure 39 Explosive eruptions producing tephra fall (ash) deposit (a) if eruption column collapses a pyroclastic flow (b) may occur wherein gas and tephra rush down the flanks of the volcano at high speed This is the most dangerous type of volcanic eruption The deposits that are produced are called ignimbrites

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