3.3 Crystallization of Magma

The minerals that make up igneous rocks crystallize in ~ a range of different temperatures. This explains why a cooling magma deserve to have some crystals in ~ it and yet remain predominantly liquid. The sequence in which minerals crystallize indigenous a magma is recognized as the Bowen reaction series (Figure 3.10 and also Who to be Bowen).

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Of the usual silicate minerals, olivine typically crystallizes first, at between 1200° and 1300°C. As the temperature drops, and also assuming that some silica remains in the magma, the olivine crystals reaction (combine) with few of the silica in the magma (see crate 3.1) to kind pyroxene. As lengthy as over there is silica remaining and also the price of cooling is slow, this procedure continues down the discontinuous branch: olivine come pyroxene, pyroxene come amphibole, and (under the ideal conditions) amphibole to biotite.

At around the point where pyroxene starts to crystallize, plagioclase feldspar likewise begins come crystallize. At the temperature, the plagioclase is calcium-rich (anorthite) (see number 2.15). As the temperature drops, and providing the there is salt left in the magma, the plagioclase that creates is a more sodium-rich variety.

Figure 3.10 The Bowen reaction series describes the process of magma crystallization Who was Bowen, and also what’s a reaction series?

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


The Bowen reaction series is among the results of his work, and also even a century later, that is crucial basis because that our expertise of igneous rocks. Words reaction is critical. In the discontinuous branch, olivine is generally the an initial mineral to kind (at just below 1300°C). Together the temperature continues to drop, olivine becomes turbulent while pyroxene becomes stable. The early-forming olivine crystals react with silica in the continuing to be liquid magma and also are converted into pyroxene, something like this:

Mg2SiO4 + SiO2 ——> 2MgSiO3

olivine pyroxene

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

In instances where cooling happens reasonably quickly, individual plagioclase crystals can be zoned indigenous calcium-rich in the centre to more sodium-rich roughly the outside. This occurs once calcium-rich early-forming plagioclase crystals become coated v progressively much more sodium-rich plagioclase together the magma cools. Figure 3.11 shows a zoned plagioclase under a microscope.

Figure 3.11 A zoned plagioclase crystal. The central part is calcium-rich and also the outside part is sodium-rich:

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

The composition of the initial magma is an essential to magma crystallization due to the fact that it identify how far the reaction procedure can proceed before every one of the silica is offered up. The compositions of usual mafic, intermediate, and also felsic magmas are shown in number 3.12. Note that, unlike figure 3.6, this compositions space expressed in terms of “oxides” (e.g., Al2O3 quite than just Al). There are two factors for this: one is the in the early analytical procedures, the outcomes were constantly expressed the way, and the other is that all of these elements combine conveniently with oxygen to form oxides.

Figure 3.12 The chemical compositions of usual mafic, intermediate, and felsic magmas and the types of rocks that form from them.

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

Exercise 3.3 Rock varieties Based ~ above Magma Composition

The proportions the the main chemical contents of felsic, intermediate, and mafic magmas are listed in the table below. (The worths are similar to those shown in figure 3.12.)

OxideFelsic MagmaIntermediate MagmaMafic Magma

Chemical data for 4 rock samples are shown in the complying with table. Compare these v those in the table above to determine whether each of these samples is felsic, intermediate, or mafic.

As a mafic magma starts come cool, few of the silica combines v iron and also magnesium to do olivine. As it cools further, lot of the continuing to be silica goes into calcium-rich plagioclase, and any silica left might be provided to convert some that the olivine come pyroxene. Shortly after that, every one of the magma is provided up and also no further alters takes place. The minerals existing will be olivine, pyroxene, and calcium-rich plagioclase. If the magma cools progressively underground, the product will certainly be gabbro; if the cools quickly at the surface, the product will certainly be basalt (Figure 3.13).

Felsic magmas have tendency to be cooler than mafic magmas when crystallization starts (because castle don’t have to be as hot to continue to be liquid), and so they might start out crystallizing pyroxene (not olivine) and plagioclase. Together cooling continues, the miscellaneous reactions top top the discontinuous branch will certainly proceed since silica is abundant, the plagioclase will end up being increasingly sodium-rich, and eventually potassium feldspar and quartz will certainly form. Commonly even very felsic rocks will certainly not have biotite or muscovite due to the fact that they may not have enough aluminum or enough hydrogen to do the five complexes the are important for mica minerals. Typical felsic rocks space granite and rhyolite (Figure 3.13).

The cooling action of intermediary magmas lied somewhere in between those of mafic and also felsic magmas. Usual intermediate rocks space diorite and andesite (Figure 3.13).

Figure 3.13 instances of the igneous rocks that kind from mafic, intermediate, and also felsic magmas.

A number of processes that take place within a magma room can affect the types of rocks produced in the end. If the magma has a low viscosity (i.e., that runny) — i m sorry is likely if that is mafic — the crystals that form early, such as olivine (Figure 3.14a), may slowly settle towards the bottom of the magma room (Figure 3.14b). The means that the all at once composition of the magma near the optimal of the magma chamber will become more felsic, together it is shedding some iron- and magnesium-rich components. This procedure is known as fractional crystallization. The crystals that settle might either type an olivine-rich layer close to the bottom that the magma chamber, or they can remelt due to the fact that the lower component is most likely to be hotter 보다 the upper part (remember, from chapter 1, the temperatures rise steadily through depth in Earth due to the fact that of the geothermal gradient). If any kind of melting bring away place, crystal settling will make the magma in ~ the bottom of the chamber an ext mafic 보다 it was to begin with (Figure 3.14c).

Figure 3.14 an instance of crystal settling and also the formation of a zoned magma chamber

If crystal settling does not take place, since the magma is as well viscous, climate the procedure of cooling will proceed as predicted by the Bowen reaction series. In some cases, however, partly cooled but still liquid magma, with crystals in it, will either move farther up right into a cooler component of the crust, or all the method to the surface during a volcano eruption. In either of this situations, the magma that has actually moved toward the surface ar is most likely to cool much much faster than that did in ~ the magma chamber, and also the rest of the absent will have a finer crystalline texture. One igneous absent with big crystals embedded in a procession of finer crystals is indicative that a two-stage cooling process, and the texture is porphyritic (Figure 3.15).

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Figure 3.15 Porphyritic textures: volcano porphyry (left – olivine crystals in Hawaiian basalt) and intrusive porphyry (right)

Exercise 3.4 Porphyritic Minerals

As a magma cools listed below 1300°C, minerals start to crystallize in ~ it. If that magma is then associated in a volcano eruption, the remainder of the liquid will certainly cool quickly 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 very fine grained or also glassy. Utilizing the diagram presented here, predict what phenocrysts can be existing where the magma cooled as far as line a in one case, and also line b in another.