Why does magma form?

Why Does Magma Form, and What Is It Made of? 

It’s Hot Inside the Earth 

Where does the heat that can cause the production of magma come from? Some of the Earth’s internal heat is a relict of the planet’s formation. In fact, during the first 700 million years or so of its existence, the Earth was very hot, and at times may even have been largely molten. But our planet has had a long time to cool since then, and probably would have become too cool to melt at all were it not for the presence of radioactive elements. Every time a radioactive element decays, it generates new heat. The Earth produces enough radioactive heat to keep its inside quite hot.

Causes of Melting 

Even though the Earth is very hot inside, the popular image that the crust floats on a sea of molten rock is wrong. The crust and the mantle of this planet are almost entirely solid. Magma forms only in special places where preexisting solid rock undergoes melting. Below, we describe conditions that lead to melting. We’ll briefly note the settings, in the context of plate tectonics, in which melting conditions develop, but will wait until the end of this chapter to characterize specific types of igneous rocks that form at these settings.

Melting due to a decrease in pressure (decompression) 

The concept of decompression melting.

Beneath typical oceanic crust, temperatures comparable to those of lava occur in the upper mantle (figure above a). But even though the upper mantle is very hot, its rock stays solid because it is also under great pressure from the weight of overlying rock, and pressure prevents atoms from breaking free of solid mineral crystals. Because pressure prevents melting, a decrease in pressure can permit melting. Thus, if the pressure affecting hot mantle rock decreases while the temperature remains unchanged, magma forms. This kind of melting, called decompression melting, occurs where hot mantle rock rises to shallower depths in the Earth. Such movement occurs in mantle plumes, beneath rifts, and beneath midocean ridges (figure above b).

Melting as a result of the addition of volatiles 

Flux melting and heat-transfer melting.

Magma also forms at locations where chemicals called volatiles mix with hot mantle rock. Volatiles, are substances such as water (H2O) and carbon dioxide (CO2) that evaporate easily and can exist in gaseous forms at the Earth’s surface. When volatiles mix with hot, dry rock, they help break chemical bonds so that the rock begins to melt (figure above a). In effect, adding volatiles decreases a rock’s melting temperature. Melting due to addition of volatiles is sometimes called flux melting.

Melting as a result of heat transfer from rising magma

When magma from the mantle rises up into the crust, it brings heat with it. This heat raises the temperature of the surrounding crustal rock and, in some cases, the rise in temperature may be sufficient for the crustal rock to begin melting. To picture the process, imagine injecting hot fudge into ice cream; the fudge transfers heat to the ice cream, raises its temperature, and causes it to melt (figure above b). We call such melting heat transfer melting, because it results from the transfer of heat from a hotter material to a cooler one. 

The Major Types of Magma 

All magmas contain silica, a compound of silicon and oxygen. But magmas also contain varying proportions of other elements such as aluminium (Al), calcium (Ca), sodium (Na), potassium (K), iron (Fe), and magnesium (Mg); each of these ions also bonds to oxygen to form a metal-oxide compound. Because magma is a liquid, its molecules do not lie in an orderly crystalline lattice but are grouped instead in clusters or short chains, relatively free to move with respect to one another.
Geologists distinguish between “dry” magmas, which contain no volatiles, and “wet” magmas, which do. In fact, wet magmas include up to 15% dissolved volatiles such as water, carbon dioxide, nitrogen (N2), hydrogen (H2), and sulphur dioxide (SO2). These volatiles come out of the Earth at volcanoes in the form of gas. Usually, water constitutes about half of the gas erupting at a volcano. Thus, magma contains not only the molecules that constitute solid minerals in rocks but also the molecules that become water or air.

The Four Categories of Magma

Magmas differ from one another in terms of the proportions of chemicals that they contain. Geologists distinguish four major compositional types depending, overall, on the proportion of silica (SiO2) relative to other metal oxides (table above). Mafic magma contains a relatively high proportion of iron oxide (FeO or Fe2O3) and magnesium oxide (MgO) relative to silica. The “ma-” in the word stands for magnesium, and the “-fic” comes from the Latin word for iron. Ultramafic magma has an even higher proportion of magnesium and iron oxides, relative to silica. Felsic magmas have a relatively high proportion of silica, relative to magnesium and iron oxides. (Occasionally, geologists use the term “silicic” interchangeably with felsic.) Intermediate magma gets its name because its composition is partway between mafic and felsic. Why are there so many kinds of magma? Several factors control magma composition, including those described below.

Phenomena that can affect the composition of magma.

• Source rock composition: The composition of a melt reflects the composition of the solid from which it was derived. Not all magmas form from the same source rock, so not all magmas have the same composition. 
• Partial melting: Under the temperature and pressure conditions that occur in the Earth, only about 2% to 30% of an original rock can melt to produce magma at a given location the temperature at sites of magma production simply never gets high enough to melt the entire original rock before the magma has a chance to migrate away from its source. Partial melting refers to the process by which only part of an original rock melts to produce magma (figure above a). Magmas formed by partial melting are more felsic than the original rock from which they were derived. For example, partial melting of an ultramafic rock produces a mafic magma. 
• Assimilation: As magma sits in a magma chamber before completely solidifying, it may incorporate chemicals dissolved from the wall rocks of the chamber or from blocks that detached from the wall and sank into the magma (figure above b). This process is called contamination or assimilation. 
• Magma mixing: Different magmas formed in different locations from different sources may enter a magma chamber. In some cases, the originally distinct magmas mix to create a new, different magma. Thoroughly mixing a felsic magma with a mafic magma in equal proportions produces an intermediate magma.

Credits: Stephen Marshak (Essentials of Geology)

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