Answer:

Since volcanic eruptions are caused by magma (a mixture of liquid rock, crystals, and dissolved gas) expelled onto the Earth's surface, we'll first review the characteristics of magma that we covered previously.

Three basic types of magma:

Mafic or Basaltic-- SiO2 45-55 wt%, high in Fe, Mg, Ca, low in K, Na

Intermediate or Andesitic-- SiO2 55-65 wt%, intermediate. in Fe, Mg, Ca, Na, K

Felsic or Rhyolitic-- SiO2 65-75%, low in Fe, Mg, Ca, high in K, Na.

Gases - At depth in the Earth nearly all magmas contain gas. Gas gives magmas their explosive character, because the gas expands as pressure is reduced.

Mostly H2O with some CO2

Minor amounts of Sulfur, Cl , and F

Felsic magmas usually have higher gas contents than mafic magmas.

Temperature of Magmas

Mafic/Basaltic - 1000-1200oC

Intermediate/Andesitic - 800-1000oC

Felsic/Rhyolitic - 650-800oC.

Viscosity of Magmas

Viscosity is the resistance to flow (opposite of fluidity). Depends on composition, temperature, & gas content.

Higher SiO2 content magmas have higher viscosity than lower SiO2 content magmas

Lower Temperature magmas have higher viscosity than higher temperature magmas.

Thus, basaltic magmas tend to be fairly fluid (low viscosity), but their viscosity is still 10,000 to 100,0000 times more viscous than water. Rhyolitic magmas tend to have even higher viscosity, ranging between 1 million and 100 million times more viscous than water. (Note that solids, even though they appear solid have a viscosity, but it very high, measured as trillions time the viscosity of water). Viscosity is an important property in determining the eruptive behavior of magmas.

Summary Table

Magma Type Solidified Volcanic Rock Solidified Plutonic Rock Chemical Composition Temperature Viscosity Gas Content

Mafic or Basaltic Basalt Gabbro 45-55 SiO2 %, high in Fe, Mg, Ca, low in K, Na 1000 - 1200 oC Low Low

Intermediate

or Andesitic

Andesite Diorite 55-65 SiO2 %, intermediate in Fe, Mg, Ca, Na, K 800 - 1000 oC Intermediate Intermediate

Felsic or Rhyolitic Rhyolite Granite 65-75 SiO2 %, low in Fe, Mg, Ca, high in K, Na 650 - 800 oC High High

Explanation:

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