Magma Chemistry and Physical Properties

Dr. G. Kent Colbath

Silica in Magma

The two most abundant elements in earth's crust and mantle are oxygen and silicon.  These two elements combine to form the molecule called silica.  (Note that silicon is an element, while silica is a molecule).  The formula for silica is:

SiO₂

Most magmas range in silica content from about 50% silica to a little over 70%.  The three major types of magma are as follows:

• Basaltic Magma -- 50% silica
• Andesitic Magma -- 60% silica
• Granitic Magma -- 70% silica

Dissolved Gasses

Magma also contains dissolved gas.  If magma erupts on the earth's surface, some or all of the gasses escape, and the magma either becomes lava, or it explodes to produce pyroclastic material.  The gas content ranges from about 0.5% to 6% of the magma.  Basaltic magma is usually low in dissolved gas, while andesitic and granitic magmas tend to have much higher gas contents.

The most abundant gas dissolved in magma is water vapor (H₂O).  The second most abundant is carbon dioxide (CO₂).  Various chlorine and sulfur compounds, along with a trace of argon make up the rest of the gasses.

Physical Properties of Magma

We've already noted that liquid magma is less dense than solid rock, and thus when melting occurs the magma will tend to rise upward within the earth.  There are two additional properties of magma that are important.

    Temperature

Temperatures of magmas vary widely in nature, ranging from about 700°C to 1350°C (Kilauea is the world's hottest volcano, with a temperature of 1350°C).  Magmas that are highest in silica have the lowest temperatures, and magmas that are lowest in silica have the highest temperatures.

    Viscosity

Viscosity is defined as the resistance to flow exhibited by a fluid.

Water (on the left) has a lower viscosity than honey (on the right)

The higher the temperature of a magma, the lower the viscosity will be.

The higher the silica content of a magma, the higher the viscosity will be.

Lets put all of this information together for the three types of magma:

• Basaltic magma:  High temperature, low in silica, with low gas content.  This type of magma is most likely to erupt on the surface and produce fluid lava flows with relatively little explosive activity.
• Andesitic magma:  Intermediate temperature, intermediate silica content, with abundant dissolved gasses.  This magma will be more viscous than basaltic magma, and is more likely to explode when it reaches the surface (it's much harder for the gas bubbles to escape from a viscous magma than from one lower in viscosity).
• Granitic magma:  Low temperature, high silica, with high gas content.  These magmas are usually so viscous that they get stuck before they reach the surface.  Granitic magmas are much more likely to cool deep below the ground and become plutonic igneous rocks.  Granitic plutons are an important component of most batholiths.

Silicate Minerals in Igneous Rocks

Silicate minerals are made of silica combined with various proportions of other elements.  The minerals that you get when the magma cools depend on how much silica is in the magma.  We can thus study ancient magmas by studying the igneous rocks that they formed.

Magmas that are high in silica will form minerals high in silica (quartz is pure silica, for example, while muscovite and potassium feldspar are high in silica), while magmas that are low in silica will form minerals low in silica (olivine has the lowest silica content of the silicate minerals, and augite is also very low).  Magmas intermediate in silica content will tend to form minerals like hornblende and calcium feldspar, which are intermediate in silica content.

"Copyright 2004 by Andrew Alden, geology.about.com, reproduced under educational fair use."

This photograph of a piece of basalt includes both vesicles (bubble holes) and a few relatively large crystals of olivine.  You can look at this rock and tell that the magma erupted on the earth's surface (because of the vesicles) and that the magma was relatively low in silica (because of the olivine).

"Copyright 2004 by Andrew Alden, geology.about.com, reproduced under educational fair use."

In this photograph of a piece of granite the crystals of clear quartz and pink potassium feldspar are clearly visible to the naked eye.  This magma cooled slowly deep below the ground (thus the phaneritic texture), and was high in silica (thus the abundant quartz crystals).

The table below summarizes the way igneous rocks are named based on their silica content and cooling history:

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