3.3 Crystallization the Magma

The minerals that consist of igneous rocks crystallize at a variety of different temperatures. This defines why a cooling magma can have some crystals in ~ it and also yet remain primarily liquid. The succession in which mineral crystallize indigenous a magma is well-known as the Bowen reaction series (Figure 3.10 and also Who was Bowen).

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Of the usual silicate minerals, olivine generally crystallizes first, at in between 1200° and 1300°C. Together the temperature drops, and also assuming that part silica stays in the magma, the olivine crystals react (combine) with several of the silica in the magma (see box 3.1) to kind pyroxene. As lengthy as over there is silica remaining and also the price of cooling is slow, this process continues down the discontinuous branch: olivine come pyroxene, pyroxene come amphibole, and (under the best conditions) amphibole come biotite.

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

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

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

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The Bowen reaction collection is among the outcomes of his work, and even a century later, the is an important basis because that our understanding 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 stormy while pyroxene becomes stable. The early-forming olivine crystals react v silica in the staying liquid magma and also are converted into pyroxene, something choose this:

Mg2SiO4 + SiO2 ——> 2MgSiO3

olivine pyroxene

This proceeds down the chain, as long as there is quiet silica left in the liquid. >

In situations where cooling happens reasonably quickly, separation, personal, instance plagioclase crystals deserve to be zoned indigenous calcium-rich in the centre to much more sodium-rich about the outside. This occurs when calcium-rich early-forming plagioclase crystals come to be coated v progressively much more sodium-rich plagioclase together the magma cools. Number 3.11 mirrors a zoned plagioclase under a microscope.

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

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

The composition of the original magma is crucial to magma crystallization because it identify how much the reaction procedure can proceed before all of the silica is provided up. The compositions of usual mafic, intermediate, and also felsic magmas are presented in figure 3.12. Keep in mind that, unlike number 3.6, these compositions room expressed in regards to “oxides” (e.g., Al2O3 rather than just Al). There are two reasons for this: one is the in the at an early stage analytical procedures, the outcomes were constantly expressed that way, and the other is that all of these elements combine conveniently with oxygen to type oxides.

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Figure 3.12 The chemical compositions of typical mafic, intermediate, and also felsic magmas and the species of rocks that type from them.

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


Exercise 3.3 Rock varieties Based on Magma Composition

The proportions that the key chemical contents of felsic, intermediate, and mafic magmas are detailed in the table below. (The values are comparable to those shown in figure 3.12.)


OxideFelsic MagmaIntermediate MagmaMafic Magma
SiO265-75%55-65%45-55%
Al2O312-16%14-18%14-18%
FeO2-4%4-8%8-12%
CaO1-4%4-7%7-11%
MgO0-3%2-6%5-9%
Na2O2-6%3-7%1-3%
K2O3-5%2-4%0.5-3%

Chemical data for four rock samples are shown in the following table. To compare these with those in the table above to recognize whether every of these samples is felsic, intermediate, or mafic.


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

Felsic magmas have tendency to it is in cooler than mafic magmas once crystallization starts (because they don’t need to be as warm to continue to be liquid), and also so they might start out crystallizing pyroxene (not olivine) and also plagioclase. Together cooling continues, the various reactions on the discontinuous branch will proceed because silica is abundant, the plagioclase will become increasingly sodium-rich, and eventually potassium feldspar and quartz will form. Commonly even very felsic rocks will not have actually biotite or muscovite since they may not have sufficient aluminum or enough hydrogen to do the five complexes the are vital for mica minerals. Common felsic rocks are granite and also rhyolite (Figure 3.13).

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

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

A number of processes that take ar within a magma room can impact the varieties of rocks produced in the end. If the magma has a short viscosity (i.e., that runny) — i beg your pardon is likely if that is mafic — the crystals that form early, such together olivine (Figure 3.14a), may progressively settle towards the bottom the the magma chamber (Figure 3.14b). The method that the overall composition of the magma near the optimal of the magma chamber will become much more felsic, as it is losing some iron- and also magnesium-rich components. This procedure is well-known as fractional crystallization. The crystals that settle can either type an olivine-rich layer near the bottom that the magma chamber, or they can remelt since the lower part is most likely to it is in hotter 보다 the upper component (remember, from chapter 1, that temperatures boost steadily v depth in Earth since of the geothermal gradient). If any kind of melting take away place, decision settling will certainly make the magma at the bottom of the chamber more mafic 보다 it to be to start with (Figure 3.14c).

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Figure 3.14 an example of crystal settling and also the development of a zoned magma chamber

If decision settling does not take place, because the magma is as well viscous, climate the procedure of cooling will proceed as suspect by the Bowen reaction series. In some cases, however, partially cooled however still liquid magma, through crystals in it, will either relocate farther up into a cooler part of the crust, or all the means to the surface throughout a volcano eruption. In one of two people of these situations, the magma that has actually moved toward the surface ar is likely to cool much quicker than it did in ~ the magma chamber, and also the rest of the rock will have actually a finer crystalline texture. An igneous absent with large crystals installed in a matrix 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: volcanic 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 within it. If that magma is then affiliated in a volcanic eruption, the rest of the liquid will certainly cool conveniently to type a porphyritic texture. The absent will have some relatively huge crystals (phenocrysts) the the minerals that crystallized early, and also the rest will be really fine grained or even glassy. Making use of the diagram displayed here, guess what phenocrysts could be present where the magma cooled as far as line a in one case, and also line b in another.