3.3 Crystallization of Magma

The minerals that consist of igneous rocks crystallize in ~ a range of different temperatures. This defines why a cooling magma deserve to have part crystals in ~ it and yet remain mainly liquid. The sequence in which minerals crystallize indigenous a magma is known as the Bowen reaction series (Figure 3.3.1 and also Figure 3.3.3).

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Of the typical silicate minerals, olivine normally crystallizes first, at in between 1200° and 1300°C. Together the temperature drops, and assuming that some silica stays in the magma, the olivine crystals will react (combine) with few of the silica in the magma to kind pyroxene. As long as over there is silica remaining and also the rate of cooling is slow, this procedure continues under the discontinuous branch: olivine come pyroxene, pyroxene to amphibole, and (under the best conditions) amphibole to biotite.

At around the allude where pyroxene starts to crystallize, plagioclase feldspar additionally begins to crystallize. At that temperature, the plagioclase is calcium-rich (anorthite) (see number 2.6.1). Together the temperature drops, and providing that there is salt left in the magma, the plagioclase that forms is a an ext sodium-rich variety.

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Figure 3.3.1 The Bowen reaction series describes the process of magma crystallization.
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Figure 3.3.2 A zoned plagioclase crystal. The main part is calcium-rich and also the darker outside component is sodium-rich.

In some cases, separation, personal, instance plagioclase crystals have the right to be zoned indigenous calcium-rich in the center to more sodium-rich roughly the outside. This occurs when calcium-rich early-forming plagioclase crystals end up being coated with progressively more sodium-rich plagioclase as the magma cools. Number 3.3.2 reflects a zoned plagioclase under a microscope.

Finally, if the magma is fairly silica-rich to start with, there will certainly still be some left at about 750° come 800°C, and from this last magma, potassium feldspar, quartz, and maybe muscovite mica will certainly form.


Who was Bowen, and what is a reaction series?


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Figure 3.3.3

Norman Levi Bowen, born in Kingston Ontario, learned geology at Queen’s University and also then at MIT in Boston. In 1912, Norman Levi Bowenhe join the Carnegie institution in Washington, D.C., whereby he carried out groundbreaking speculative research right into the procedures of cooling magmas. Working largely with basaltic magmas, he established the order of crystallization that minerals as the temperature drops. The method, in brief, to be to melt the absent to a magma in a specially-made kiln, permit it come cool gradually to a specific temperature (allowing some minerals to form), and also then quench the (cool it quickly) so that no new minerals form (only glass). The results were learned under the microscope and also by chemistry analysis. This was done over and also over, each time permitting the magma come cool to a reduced temperature before quenching.

The Bowen reaction collection is among the outcomes of his work, and also even a century later, it is crucial basis because that our expertise of igneous rocks. Words reaction is critical. In the discontinuous branch, olivine is generally the first mineral to type (at just below 1300°C). As the temperature proceeds to drop, olivine becomes rough while pyroxene becomes stable. The early-forming olivine crystals react with silica in the staying liquid magma and are converted into pyroxene, something choose this:

Mg2SiO4 + SiO2 (olivine) becomes 2MgSiO3 (proxene)

This proceeds down the chain, as lengthy as there is tho silica left in the liquid.


The ingredient of the initial magma is vital to magma crystallization due to the fact that it identify how much the reaction process can proceed before all of the silica is used up. The compositions of common mafic, intermediate, and felsic magmas are presented in figure 3.3.4. Note that, unlike number 3.2.1, this compositions room expressed in terms of “oxides” (e.g., Al2O3 quite than simply Al). There are two factors for this: one is that in the early analytical procedures, the results were constantly expressed that way, and the other is that every one of these elements combine conveniently with oxygen to kind oxides.

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Figure 3.3.4 The chemical compositions of typical mafic, intermediate, and also felsic magmas and the types of rocks that form from them.

Mafic magmas have 45% to 55% SiO2, around 25% complete of FeO and MgO add to CaO, and around 5% Na2O + K2O. Felsic magmas, on the other hand, have much more SiO2 (65% come 75%) and also Na2O + K2O (around 10%) and much less FeO and MgO plus CaO (about 5%).


Exercise 3.3 determining rock types based on magma composition


The proportions of the key chemical materials of felsic, intermediate, and also mafic magmas are detailed in the table below. (The worths are comparable to those presented in figure 3.3.4.)


Table 3.1 Proportions that the main chemical components in felsic, intermediate, and mafic magmaOxideFelsic MagmaIntermediate MagmaMafic Magma
SiO265% to 75%55% come 65%45% to 55%
Al2O312% to 16%14% come 18%14% to 18%
FeO2% to 4%4% come 8%8% to 12%
CaO1% to 4%4% come 7%7% to 11%
MgO0% come 3%2% come 6%5% come 9%
Na2O2% to 6%3% come 7%1% to 3%
K2O3% come 5%2% to 4%0.5% come 3%

Chemical data for four rock samples are displayed in the adhering to table. Compare these with those in the table over to determine whether each of this samples is felsic, intermediate, or mafic.


Table 3.2 chemical Data for 4 Unidentified rock SamplesRock SampleSiO2Al2O3FeOCaOMgONa2OK2OWhat kind of magma is it?Rock 1Rock 2Rock 3Rock 4
 
55%17%5%6%3%4%3%
74%14%3%3%0.5%5%4%
47%14%8%10%8%1%2%
65%14%4%5%4%3%3%

See attachment 3 for exercise 3.3 answers.


As a mafic magma starts to cool, several of the silica combines through iron and also magnesium to make olivine. As it cools further, lot of the remaining silica goes right into calcium-rich plagioclase, and any silica left might be used to transform some the the olivine to pyroxene. Quickly after that, all of the magma is offered up and also no further alters takes place. The minerals existing will it is in olivine, pyroxene, and also calcium-rich plagioclase. If the magma cools gradually underground, the product will certainly be gabbro; if the cools conveniently at the surface, the product will be basalt (Figure 3.3.5).

Felsic magmas often tend to it is in cooler 보다 mafic magmas as soon as crystallization starts (because lock don’t have to be as hot to continue to be liquid), and so they might start the end crystallizing pyroxene (not olivine) and plagioclase. As cooling continues, the various reactions ~ above the discontinuous branch will certainly proceed since silica is abundant, the plagioclase will come to be increasingly sodium-rich, and eventually potassium feldspar and quartz will certainly form. Typically even an extremely felsic rocks will not have biotite or muscovite because they might not have sufficient aluminum or enough hydrogen to make the five complexes the are essential for mica minerals. Common felsic rocks room granite and also rhyolite (Figure 3.3.5).

The cooling behaviour of intermediary magmas lie somewhere in between those the mafic and also felsic magmas. Common mafic rocks room gabbro (intrusive) and basalt (extrusive). Typical intermediate rocks room diorite and also andesite. Typical felsic rocks space granite and also rhyolite (Figure 3.3.5).

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Figure 3.3.5 instances of the igneous rocks that type from mafic, intermediate, and felsic magmas.

A variety of processes the take place within a magma chamber can affect the varieties of rocks developed in the end. If the magma has a low viscosity (i.e., that runny)—which is most likely if that is mafic—the crystals that type early, such together olivine (Figure 3.3.6a), may slowly settle toward the bottom of the magma chamber (Figure 3.3.6b). The means that the as whole composition the the magma near the peak of the magma chamber will certainly become an ext felsic, together it is shedding some iron- and also magnesium-rich components. This procedure is well-known as fractional crystallization. The crystals the settle can either type an olivine-rich layer near the bottom the the magma chamber, or they can remelt since the lower part is likely to it is in hotter than the upper part (remember, from thing 1, the temperatures increase steadily v depth in Earth because of the geothermal gradient). If any kind of melting take away place, crystal settling will certainly make the magma in ~ the bottom that the chamber more mafic 보다 it was to start with (Figure 3.3.6c).

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Figure 3.3.6 an instance of crystal settling and also the formation of a zoned magma chamber.

If decision settling does no take place, because the magma is as well viscous, climate the procedure of cooling will continue as predicted by the Bowen reaction series. In some cases, however, partly cooled yet still fluid magma, v crystals in it, will certainly either relocate farther up right into a cooler part of the crust, or every the means to the surface during a volcanic eruption. In one of two people of this situations, the magma that has actually moved toward the surface is most likely to cool much faster than it did in ~ the magma chamber, and the remainder of the absent will have actually a finer crystalline texture. One igneous absent with huge crystals installed in a procession of much finer crystals is indicative the a two-stage cooling process, and also the structure is porphyritic (Figure 3.3.7). For the rock to be dubbed “porphyritic” there needs to be a far-ranging difference in decision size, whereby the bigger crystals room at the very least 10 times larger than the median size of the smaller crystals.

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Figure 3.3.7 Porphyritic textures, left: 1.3 cm lengthy amphibole crystals in an intrusive igneous absent in which most of the crystals are less than 1 mm, right: 1 come 2 mm lengthy feldspar crystals and 1 mm long amphibole crystals in a volcanic absent where most of the crystals are much less than 0.1 mm.

As a magma cools below 1300°C, minerals start to crystallize within it. If the magma is then involved in a volcanic eruption, the remainder of the liquid will cool conveniently to kind a porphyritic texture. The rock will have actually some relatively large crystals (phenocrysts) that the minerals the crystallized early, and the rest will be an extremely fine grained or even glassy. Using number 3.3.8, guess what phenocrysts might be present where the magma cooled as far as line a in one case, and line b in another.

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Figure 3.3.8 Bowen reaction series. Heat a – in ~ high temperature – intersects olivine, line b – at a reduced temperature – intersects pryroxene and amphibole ~ above the left, and plagioclase feldspar ~ above the right

See appendix 3 for practice 3.4 answers.