Effusive vs. Explosive​

Kilauea​

Mt. St. Helens​

Viscosity=resistance to flow

​ 1. Temperature

2. Amount of Silica

3. ​Dissolved Gases

​High viscosity=more explosive

low viscosity=more diffuse/less explosive​

Temperature​

​High temperature=less viscosity

Low temperature=more viscosity​

Amount of

Silica​

​High silica=high viscosity

Low silica=less viscosity​

Amount of

Silica​

​High silica=high viscosity

Low silica=less viscosity​

• ​Igneous rocks are composed mainly of silicate minerals (silica). Magma contains small amount of other elements. As magma solidifies, these elements combine to form two major groups of silicate minerals, dark silicates(low in silica) and light silicates(high in silica).

• Igneous rocks can have the combination of light and dark silicates.

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Amount of

Silica​

​High silica=high viscosity

Low silica=less viscosity​

Basaltic (Mafic)-Magnesium and Ferrum

Andesitic (Intermediate)-Igneous rocks can have the combination of light and dark silicates.

Rhyolitic (Felsic)-Feldspar and Silica(Quartz)

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Dissolved

Gases

​High dissolved gases=explosive

Low dissolved gases=effusive

​Gas gives magmas their explosive character, because volume of gas expands as pressure is reduced. The composition of the gases in magma are:

1. Mostly H2O (water vapor) & some CO2 (carbon dioxide)

2. Minor amounts of Sulfur, Chlorine, and Fluorine gases

The amount of gas in a magma is also related to the chemical composition of the magma.

Rhyolitic magmas usually have higher gas contents than basaltic magmas.

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