3.3 Crystallization of Magma

The minerals that comprise igneous rocks crystallize in ~ a selection of different temperatures. This explains why a cooling magma have the right to have some crystals in ~ it and yet remain predominantly liquid. The succession in which minerals crystallize from a magma is recognized as the Bowen reaction series (Figure 3.10 and also Who was Bowen).

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Of the usual silicate minerals, olivine normally crystallizes first, at between 1200° and also 1300°C. As the temperature drops, and assuming that part silica stays 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 the price of cooling is slow, this process continues down the discontinuous branch: olivine come pyroxene, pyroxene come amphibole, and also (under the ideal conditions) amphibole come biotite.

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

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

Norman Levi Bowen, born in Kingston Ontario, learned geology at Queen’s University and also then in ~ MIT in Boston. In 1912, he joined the Carnegie college in Washington, D.C., whereby he carried out groundbreaking speculative research right into the procedures of cooling magmas. Working greatly with basaltic magmas, he figured out the stimulate of crystallization that minerals as 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 slowly to a details temperature (allowing part minerals to form), and also then quench it (cool that quickly) so that no brand-new minerals form (only glass). The results were studied under the microscope and by chemistry analysis. This to be done over and over, each time allowing the magma to cool to a lower temperature prior to quenching.


The Bowen reaction series is one of the outcomes of his work, and also even a century later, that is crucial basis for our expertise of igneous rocks. The word reaction is critical. In the discontinuous branch, olivine is frequently the first mineral to form (at just listed below 1300°C). Together the temperature continues to drop, olivine becomes turbulent while pyroxene i do not care stable. The early-forming olivine crystals react through silica in the remaining liquid magma and are converted right into pyroxene, something choose this:

Mg2SiO4 + SiO2 ——> 2MgSiO3

olivine pyroxene

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

In cases where cooling happens reasonably quickly, individual plagioclase crystals deserve to be zoned indigenous calcium-rich in the center to an ext sodium-rich roughly the outside. This occurs when calcium-rich early-forming plagioclase crystals come to be coated with progressively more sodium-rich plagioclase together the magma cools. Number 3.11 reflects a zoned plagioclase under a microscope.

Figure 3.11 A zoned plagioclase crystal. The main 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° to 800°C, and from this critical magma, potassium feldspar, quartz, and maybe muscovite mica will form.

The composition of the initial magma is an important to magma crystallization since it identify how much the reaction process can proceed before every one of the silica is provided up. The compositions of typical mafic, intermediate, and felsic magmas are displayed in figure 3.12. Keep in mind that, unlike number 3.6, these compositions room expressed in terms of “oxides” (e.g., Al2O3 quite than simply Al). There room two reasons for this: one is the in the early on analytical procedures, the results were always expressed that way, and the various other is that all of these elements combine conveniently with oxygen to type oxides.

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

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

Exercise 3.3 Rock types Based top top Magma Composition

The proportions that the main chemical materials of felsic, intermediate, and mafic magmas are noted in the table below. (The values are similar to those presented in figure 3.12.)

OxideFelsic MagmaIntermediate MagmaMafic Magma

Chemical data for four rock samples are shown in the following table. To compare these with those in the table over to determine whether each of this samples is felsic, intermediate, or mafic.

As a mafic magma starts come cool, some of the silica combines v iron and magnesium to make olivine. As it cools further, lot of the staying silica goes into calcium-rich plagioclase, and also any silica left may be supplied to convert some that the olivine come pyroxene. Soon after that, every one of the magma is used up and also no further alters takes place. The minerals current will it is in olivine, pyroxene, and also calcium-rich plagioclase. If the magma cools gradually underground, the product will certainly be gabbro; if that cools easily at the surface, the product will be basalt (Figure 3.13).

Felsic magmas have tendency to it is in cooler than mafic magmas as soon as crystallization begins (because lock don’t have to be as hot to continue to be liquid), and so they may start the end crystallizing pyroxene (not olivine) and also plagioclase. Together cooling continues, the assorted reactions ~ above the discontinuous branch will certainly proceed since silica is abundant, the plagioclase will become increasingly sodium-rich, and eventually potassium feldspar and also quartz will certainly form. Generally even an extremely felsic rocks will not have actually biotite or muscovite due to the fact that they may not have enough aluminum or sufficient hydrogen to do the oh complexes that are necessary for mica minerals. Typical felsic rocks room granite and also rhyolite (Figure 3.13).

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

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

A number of processes the take ar within a magma room can influence the species of rocks developed in the end. If the magma has a low viscosity (i.e., it’s runny) — which is most likely if that is mafic — the crystals that form early, such together olivine (Figure 3.14a), may gradually settle toward the bottom of the magma chamber (Figure 3.14b). The means that the all at once composition of the magma near the height of the magma chamber will become an ext felsic, as it is shedding some iron- and magnesium-rich components. This procedure is well-known as fractional crystallization. The crystals the settle could either form an olivine-rich layer close to the bottom the the magma chamber, or they could remelt because the lower component is most likely to be hotter 보다 the upper part (remember, from chapter 1, the temperatures boost steadily v depth in Earth since of the geothermal gradient). If any melting bring away place, decision settling will make the magma at the bottom of the chamber an ext mafic than it to be to start with (Figure 3.14c).

Figure 3.14 an instance of decision settling and also the development of a zoned magma room

If decision settling does no take place, due to the fact that the magma is as well viscous, climate the process of cooling will proceed as suspect by the Bowen reaction series. In part cases, however, partially cooled however still fluid magma, v crystals in it, will certainly either move farther up right into a cooler component of the crust, or all the method to the surface throughout a volcanic eruption. In either of this situations, the magma that has actually moved toward the surface is most likely to cool much quicker than the did within the magma chamber, and also the remainder of the rock will have actually a finer crystalline texture. One igneous rock 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 the magma is then associated in a volcanic eruption, the rest of the liquid will certainly cool conveniently to form a porphyritic texture. The absent will have actually some relatively big crystals (phenocrysts) of the minerals that crystallized early, and also the rest will be an extremely fine grained or also glassy. Using the diagram displayed here, predict what phenocrysts might be present where the magma cooled as much as line a in one case, and also line b in another.