Making Magma

Types of Magma

By Restie Carullo

Effusive vs. Explosive​

Kilauea​

Mt. St. Helens​

Multiple Choice

What determines the manner in which volcanoes erupt?

• mantle

• location of the volcano

• magma

• altitude

Multiple Choice

What are the primary factors that affect the behavior of magma?

• amount of dissolved gases

• silica content

• temperature

• all of the choices

Multiple Choice

Why is it that some volcanic eruptions are explosive and others are quiescent?

• mineral composition of rocks

• viscosity differences of magma

• melting point differences

• none of the choices

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.

​

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)

​

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.

​

Multiple Choice

What magma makes up the shield volcano?

• Basaltic (Mafic)

• Andesitic (Intermediate)

• Rhyolitic (Felsic)

Multiple Choice

Cinder cones are typically made of andesitic magma.

• True

• False

Multiple Choice

Composite cones could start from basaltic composition then increase viscosity as it erupts.

• True

• False