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Monday Musings: How Does Magma Form and What is it Made Of?

The inside of our planet is full of left over heat from when it was created plus new heat from the decay of radioactive elements. However, the crust of the earth does not float on a sea of molten rock as some models predict. Molten rock, or magma, only forms in certain places under special conditions.

The upper mantle gets to temperatures comparable to those of lava, however due to the great pressure it is under, the rock remains solid. So, how can melt rock if pressure prevents melting? Lessen the pressure but keep the temperature unchanged. We use geothermal gradients to determine just where that point is.

The geotherm is the temperature as a function of depth. The solidus represents conditions at which rock starts to melt and the liquidus represents the conditions at which rock completely melts. What this graph shows from point A to point B, the pressure decreases a lot but only cools a little. We call this decompression melting.

Another way rock can melt is through the addition of volatiles. Volatiles are substances that evaporate easily and can exist as gases at Earth surface pressures. Typically, these are water or carbon dioxide. This will drop the melting temperature of the hot, dry rock. We sometimes call this flux melting.

The third way to melt rock is to inject hot magma into the crust. If it has enough heat, it will raise the temperature of the surrounding rock which may be just enough to melt part of it. Thing hot fudge in ice cream. We call this heat-transfer melting.

Now that we have the how, let's learn about the composition. What is magma made out of? Lots of elements actually. Silica is a big portion. Silica is made of silicon-oxygen tetrahedrons.

There are also other elements such as aluminum, calcium, sodium, magnesium, and iron. If it is a "wet magma" it will also contain elements that form volatiles such as water, carbon dioxide, sulfur dioxide, nitrogen, and hydrogen. "Dry magmas" do not contain volatiles.

There are four major types of magma based on the amount of silica present. These are ultramafic, mafic, intermediate, and felsic.

Ultramafic magmas have almost no silica in them. They are primarily made of olivine and pyroxenes, ferromagnesium minerals. Some will have a little bit of calcium-rich plagioclase.

Mafic rocks start to lose the high olivine content and add other minerals like amphiboles and biotite as well as a mixture of calcium and sodium-rich plag. Ultramafic and mafic rocks tend to be very dark in color. This magma has a low viscosity and flows easily.

Intermediate igneous rocks typically contain between 52% and 63% silica content. Here we lose olivine completely and add quartz and k-spar as well as increasing the amount of sodium-rich plag, biotite, and amphiboles while decreasing the amount of pyroxene. Here we start to add muscovite as well. The color also tends to be a mix of light and dark minerals.

Finally, we have felsic rocks which contain more than 65% silica content. They are primarily made of quartz, k-spar, and sodium-rich plag. There are also smaller amounts of mica (muscovite and biotite) and amphibole. These igneous rocks are light in color. They are also highly viscous and resistant to flow.