3.2 Magma and Magma Formation

Magmas deserve to vary widely in composition, however in basic they are comprised of only eight elements; in order of importance: oxygen, silicon, aluminum, iron, calcium, sodium, magnesium, and potassium (Figure 3.6). Oxygen, the many abundant aspect in magma, comprises a small less than fifty percent the total, complied with by silicon at just over one-quarter. The remaining elements make increase the various other one-quarter. Magmas derived from crustal material are overcame by oxygen, silicon, aluminum, sodium, and also potassium.

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The ingredient of magma depends on the rock it was formed from (by melting), and the conditions of that melting. Magmas acquired from the mantle have higher levels the iron, magnesium, and also calcium, yet they space still likely to be dominated by oxygen and also silicon. All magmas have actually varying proportions of aspects such as hydrogen, carbon, and also sulphur, which room converted into gases like water vapour, carbon dioxide, and hydrogen sulphide together the magma cools.

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Figure 3.6 typical elemental proportions in earth crust, which is close come the average composition that magmas in ~ the tardy

Virtually every one of the igneous rocks that we check out on earth are obtained from magmas that created from partial melting of currently rock, one of two people in the top mantle or the crust. Partial melt is what happens when only some parts of a rock melt; the takes place since rocks space not pure materials. Most rocks are consisted of of several minerals, each of which has a various melting temperature. The wax in a candle is a pure material. If you placed some wax into a warm oven (50°C will do as the melting temperature of many wax is about 40°C) and leave the there for a while, that will shortly start come melt. That’s finish melting, no partial melting. If rather you took a mixture of wax, plastic, aluminum, and glass and put it right into the same warm oven, the wax would shortly start come melt, yet the plastic, aluminum, and glass would not melt (Figure 3.7a). It is partial melting and the an outcome would it is in solid plastic, aluminum, and also glass surrounded by liquid wax (Figure 3.7b). If we warm the stove up to roughly 120°C, the plastic would melt too and also mix with the fluid wax, but the aluminum and glass would stay solid (Figure 3.7c). Again this is partial melting. If us separated the wax/plastic “magma” indigenous the other components and also let the cool, the would eventually harden. As you deserve to see from figure 3.7d, the fluid wax and also plastic have mixed, and on cooling, have created what looks prefer a solitary solid substance. The is most likely that this is a very fine-grained mixture of heavy wax and solid plastic, however it could additionally be some other substance the has formed from the combination of the two.

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Figure 3.7 Partial melting of “pretend rock”: (a) the original materials of white candle wax, black color plastic pipe, environment-friendly beach glass, and aluminum wire, (b) after heater to 50˚C because that 30 minutes just the wax has melted, (c) after heater to 120˚C because that 60 minutes much of the plastic has actually melted and also the two liquids have mixed, (d) the liquid has actually been separated native the solids and permitted to cool to make a “pretend rock” through a different in its entirety composition.

In this example, we partially melted part pretend rock to create some ~ pretend magma. Us then be separated the magma indigenous the source and allowed it to cool to do a new pretend rock v a composition quite different from the original material (it lacks glass and also aluminum).

Of course partial melting in the real people isn’t exactly the same as in our pretend-rock example. The main differences are that rocks are lot more facility than the four-component mechanism we used, and also the mineral components of many rocks have much more similar melt temperatures, so 2 or an ext minerals are most likely to melt at the very same time to differing degrees. An additional important difference is that when rocks melt, the process takes thousands to numerous years, no the 90 minute it took in the pretend-rock example.

Contrary to what one could expect, and contrary come what we did to make our ~ pretend rock, most partial melt of real rock does not involve heater the absent up. The two main mechanisms with which rocks melt are decompression melting and also flux melting. Decompression melting takes place within earth when a human body of rock is hosted at about the exact same temperature yet the push is reduced. This happens due to the fact that the absent is being relocated toward the surface, one of two people at a mantle plume (a.k.a., hot spot), or in the upwelling component of a mantle convection cell.<1> The device of decompression melting is presented in figure 3.8a. If a rock that is hot sufficient to be close to its melting point is relocated toward the surface, the push is reduced, and the rock deserve to pass to the liquid side that its melt curve. At this point, partial melting starts to take it place. The process of flux melt is shown in figure 3.8b. If a rock is close come its melting allude and part water (a flux the promotes melting) is added to the rock, the melt temperature is diminished (solid heat versus dotted line), and partial melting starts.

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Figure 3.8 Mechanisms for (a) decompression melting (the rock is relocated toward the surface) and (b) flux melt (water is added to the rock) and also the melting curve is displaced.

The partial melt of rock happens in a wide variety of situations, most of i m sorry are related to plate tectonics. The much more important of this are displayed in figure 3.9. In ~ both mantle plumes and also in the upward parts of convection systems, rock is being moved toward the surface, the pressure is dropping, and at part point, the rock crosses to the fluid side the its melt curve. In ~ subduction zones, water from the wet, subducting oceanic late is transferred right into the overlying warm mantle. This offers the flux required to lower the melt temperature. In both of this cases, just partial melting takes ar — frequently only about 10% the the absent melts — and also it is always the most silica-rich materials of the rock the melt, creating a magma that is more silica-rich than the absent from which that is derived. (By analogy, the melt from ours pretend absent is richer in wax and plastic than the “rock” native which it to be derived.) The magma produced, being less dense than the surrounding rock, move up through the mantle, and eventually into the crust.

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Figure 3.9 common sites the magma development in the top mantle. The black color circles are areas of partial melting. The blue arrows represent water being moved from the subducting plates into the overlying mantle.

As it moves toward the surface, and especially when it move from the mantle into the reduced crust, the warm magma interacts with the bordering rock. This generally leads to partial melt of the neighboring rock because most such magmas room hotter 보다 the melting temperature the crustal rock. (In this case, melting is resulted in by an increase in temperature.) Again, the much more silica-rich parts of the bordering rock are preferentially melted, and this contributes to an increase in the silica content of the magma.

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At really high temperatures (over 1300°C), many magma is totally liquid because there is also much power for the atom to shortcut together. As the temperature drops, usually because the magma is progressively moving upward, things start to change. Silicon and oxygen incorporate to form silica tetrahedra, and also then, together cooling continues, the tetrahedra begin to attach together to make chains (polymerize). This silica chains have the important impact of making the magma an ext viscous (less runny), and also as we’ll check out in thing 4, magma viscosity has significant implications because that volcanic eruptions. As the magma continues to cool, crystals begin to form.