What is magma and how is magma formed?

There is a super-hot mixture of molten and semi-molten rock that flows deep beneath the Earth’s surface and it is called magma. But what exactly is magma and how and where does it come from? In this post, we’ll learn what magma is, how magma is formed, what types of magma there are, and we'll also see what is the difference between magma and lava.

What is magma, and what is the chemical composition of magma?

Magma is a hot viscous material that consists of inorganic components such as minerals and rocks as well as dissolved gasses, all embedded in the melt, which is the hot liquid base of the magma.

The composition of the magma depends on the type of magma we’re talking about.

Depending on the chemical composition of the magma, we can classify magma in the following three different categories: basaltic magma, andesitic magma, and rhyolitic magma

Types of magma

Basaltic magma

Basaltic magma has the lowest concentration of silica, about 45-55 wt%, and it is high in Fe, Mg and Ca, and low in K and Na. Basaltic magma also has the lowest gas content and the lowest viscosity of the three types of magma. It’s also the magma with the highest temperatures.

Andesitic magma

Andesitic magma has a higher weight percentage of silica than the basaltic magma, about 55-65 wt%, and it has moderate amounts of Fe, Mg, Ca, K, and Na, as well as moderate amounts of gasses. Andesitic magma has a lower temperature and a higher viscosity than basaltic magma.

Rhyolitic magma

Rhyolitic magma has the highest weight percentage of silica, 65-75 wt%, and it is low in Fe, Mg, and Ca, and high in K and Na, and it also has a higher gas content. Rhyolitic magma has the lowest temperature of all the different magma types and it has the highest viscosity.

How come magma stays fluid?

To find out the answer to this question, let’s first have a look at the Earth’s layers. In brief, Earth consists of four main layers: the inner core, the outer core, the mantle, and the crust.

Magma originates deep beneath the Earth’s surface, in the lower crust and the upper mantle. At those depths, the magma experiences high pressures and temperatures, and it is fluid because of the different combinations of temperature, pressure, and some other factors we’ll discuss in the next section where we learn how magma is formed.

How does magma form?

There are three main mechanisms through which magma is formed. Thus, magma can form through partial melting, decompression melting, or flux-induced melting.

Partial melting

Magma forms through the heat-induced melting of the rocks that are present in the upper mantle or the crust. In the case of partial melting, only some of the minerals from the rock’s composition are melting.

But why is it that only some of the minerals melt?

Rocks contain different minerals in their compositions, each made up of various combinations of chemical elements. These minerals have different melting temperatures, and those with higher melting temperatures will remain as solid rock, while those with lower temperatures will start melting.

Magma moves upward from the mantle, and can go all the way through the crust. On its way up, as it encounters other rocks though the mantle and crust layers, it transfers some of its heat to those rocks. Those rock components that have a lower melting temperature than the temperature of the magma will also start melting and join the magma flow.

Decompression Melting

To understand decompression melting, we first have to look at phase diagrams. A phase diagram is a graphical representation of the physical state of a material as a function of different temperature and pressure conditions.

This phase diagram example shows the temperature and pressure ranges where a hypothetical material is a solid, a liquid, or a gas. At those pressure and temperature conditions indicated by the blue star, the material is a solid. If we kept the pressure constant, as we increase the temperature, the solid melts, shown here by the red star.

Another way for the solid to melt is to decrease the pressure that’s acting on that solid. The hollow red star shows that even though the temperature hasn’t changed, we can move from a solid state to a liquid state by reducing the pressure. This is what happens in decompression melting. The temperature in the Earth’s mantle is very high, but so is the pressure. And under those high pressures, the mantle rocks are solid. But if the rocks move upward where the pressure is lower, or in the regions where the crust is thinner, such as in rift zones, those rocks will experience a smaller pressure, and this reduction in pressure will lead to a decompression melting of the rocks.

Flux-Induced Melting

When a rock is hot, close to its melting point, but not quite there yet, there are certain factors that could lower its melting point. This is the basis of flux-induced melting. A flux that can lower the melting temperature of the rocks is water, but carbon dioxide and other gases can have the same effect. When water reaches the rock, it lowers its melting temperature, and once the rock starts melting, we’re back to the first process, and from here, the partial melting takes over.

What is the difference between magma and lava?

Sometimes the magma breaks through the earth’s surface, either during a volcano eruption or through another vent. The magma that’s ejected and reaches the surface is now called lava. And when this lava then cools, it forms volcanic rocks and volcanic glass.

Magma originating in deep volcanic eruptions that happened billions of years ago is also what transports diamonds to the Earth's surface.