Since volcanic eruptions are caused by magma (a mixture of liquid rock, crystals, and dissolved gas) expelled onto the Earth's surface, we must first discuss the characteristics of magma and how magmas form in the Earth.
Characteristics of Magma
Types of Magma
Types of magma are determined by chemical composition of the magma. Three general types are recognized:
Basaltic magma -- SiO2 45-55 wt%, high in Fe, Mg, Ca, low in K, Na
Andesitic magma -- SiO2 55-65 wt%, intermediate. in Fe, Mg, Ca, Na, K
Rhyolitic magma -- SiO2 65-75%, low in Fe, Mg, Ca, high in K, Na
Gases in Magmas
At depth in the Earth nearly all magmas contain gas dissolved in the liquid, but the gas forms a separate vapor phase when pressure is decreased as magma rises toward the surface of the Earth. This is similar to carbonated beverages which are bottled at high pressure. The high pressure keeps the gas in solution in the liquid, but when pressure is decreased, like when you open the can or bottle, the gas comes out of solution and forms a separate gas phase that you see as bubbles. Gas gives magmas their explosive character, because volume of gas expands as pressure is reduced. The composition of the gases in magma are:
Mostly H2O (water vapor) & some CO2 (carbon dioxide)
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.
Temperature of Magmas
Temperature of magmas is difficult to measure (due to the danger involved), but laboratory measurement and limited field observation indicate that the eruption temperature of various magmas is as follows:
Basaltic magma - 1000 to 1200oC
Andesitic magma - 800 to 1000oC
Rhyolitic magma - 650 to 800oC.
Viscosity of Magmas
Viscosity is the resistance to flow (opposite of fluidity). Viscosity depends on primarily on the composition of the magma, and temperature.
Higher SiO2 (silica) content magmas have higher viscosity than lower SiO2 content magmas (viscosity increases with increasing SiO2 concentration in the magma).
Lower temperature magmas have higher viscosity than higher temperature magmas (viscosity decreases with increasing temperature of the magma).
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 times the viscosity of water). Viscosity is an important property in determining the eruptive behavior of magmas.
Summary Table
Magma Type Solidified Rock Chemical Composition Temperature Viscosity Gas Content
Basaltic Basalt 45-55 SiO2 %, high in Fe, Mg, Ca, low in K, Na 1000 - 1200 oC Low Low
Andesitic Andesite 55-65 SiO2 %, intermediate in Fe, Mg, Ca, Na, K 800 - 1000 oC Intermediate Intermediate
Rhyolitic Rhyolite 65-75 SiO2 %, low in Fe, Mg, Ca, high in K, Na. 650 - 800 oC High High
How Magmas Form in the Earth
As we have seen the only part of the earth that is liquid is the outer core. But the core is not likely to be the source of magmas because it does not have the right chemical composition. The outer core is mostly Iron, but magmas are silicate liquids. Thus, magmas DO NOT COME FROM THE MOLTEN OUTER CORE OF THE EARTH. Since the rest of the earth is solid, in order for magmas to form, some part of the earth must get hot enough to melt the rocks present.
We know that temperature increases with depth in the earth along the geothermal gradient. The earth is hot inside due to heat left over from the original accretion process, due to heat released by sinking of materials to form the core, and due to heat released by the decay of radioactive elements in the earth. Under normal conditions, the geothermal gradient is not high enough to melt rocks, and thus with the exception of the outer core, most of the Earth is solid. Thus, magmas form only under special circumstances, and thus, volcanoes are only found on the Earth's surface in areas above where these special circumstances occur. (Volcanoes don't just occur anywhere, as we shall soon see). To understand this we must first look at how rocks and mineral melt. To understand this we must first look at how minerals and rocks melt.
As pressure increases in the Earth, the melting temperature changes as well. For pure minerals, there are two general cases.
Answers & Comments
Answer:
Since volcanic eruptions are caused by magma (a mixture of liquid rock, crystals, and dissolved gas) expelled onto the Earth's surface, we must first discuss the characteristics of magma and how magmas form in the Earth.
Characteristics of Magma
Types of Magma
Types of magma are determined by chemical composition of the magma. Three general types are recognized:
Basaltic magma -- SiO2 45-55 wt%, high in Fe, Mg, Ca, low in K, Na
Andesitic magma -- SiO2 55-65 wt%, intermediate. in Fe, Mg, Ca, Na, K
Rhyolitic magma -- SiO2 65-75%, low in Fe, Mg, Ca, high in K, Na
Gases in Magmas
At depth in the Earth nearly all magmas contain gas dissolved in the liquid, but the gas forms a separate vapor phase when pressure is decreased as magma rises toward the surface of the Earth. This is similar to carbonated beverages which are bottled at high pressure. The high pressure keeps the gas in solution in the liquid, but when pressure is decreased, like when you open the can or bottle, the gas comes out of solution and forms a separate gas phase that you see as bubbles. Gas gives magmas their explosive character, because volume of gas expands as pressure is reduced. The composition of the gases in magma are:
Mostly H2O (water vapor) & some CO2 (carbon dioxide)
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.
Temperature of Magmas
Temperature of magmas is difficult to measure (due to the danger involved), but laboratory measurement and limited field observation indicate that the eruption temperature of various magmas is as follows:
Basaltic magma - 1000 to 1200oC
Andesitic magma - 800 to 1000oC
Rhyolitic magma - 650 to 800oC.
Viscosity of Magmas
Viscosity is the resistance to flow (opposite of fluidity). Viscosity depends on primarily on the composition of the magma, and temperature.
Higher SiO2 (silica) content magmas have higher viscosity than lower SiO2 content magmas (viscosity increases with increasing SiO2 concentration in the magma).
Lower temperature magmas have higher viscosity than higher temperature magmas (viscosity decreases with increasing temperature of the magma).
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 times the viscosity of water). Viscosity is an important property in determining the eruptive behavior of magmas.
Summary Table
Magma Type Solidified Rock Chemical Composition Temperature Viscosity Gas Content
Basaltic Basalt 45-55 SiO2 %, high in Fe, Mg, Ca, low in K, Na 1000 - 1200 oC Low Low
Andesitic Andesite 55-65 SiO2 %, intermediate in Fe, Mg, Ca, Na, K 800 - 1000 oC Intermediate Intermediate
Rhyolitic Rhyolite 65-75 SiO2 %, low in Fe, Mg, Ca, high in K, Na. 650 - 800 oC High High
How Magmas Form in the Earth
As we have seen the only part of the earth that is liquid is the outer core. But the core is not likely to be the source of magmas because it does not have the right chemical composition. The outer core is mostly Iron, but magmas are silicate liquids. Thus, magmas DO NOT COME FROM THE MOLTEN OUTER CORE OF THE EARTH. Since the rest of the earth is solid, in order for magmas to form, some part of the earth must get hot enough to melt the rocks present.
We know that temperature increases with depth in the earth along the geothermal gradient. The earth is hot inside due to heat left over from the original accretion process, due to heat released by sinking of materials to form the core, and due to heat released by the decay of radioactive elements in the earth. Under normal conditions, the geothermal gradient is not high enough to melt rocks, and thus with the exception of the outer core, most of the Earth is solid. Thus, magmas form only under special circumstances, and thus, volcanoes are only found on the Earth's surface in areas above where these special circumstances occur. (Volcanoes don't just occur anywhere, as we shall soon see). To understand this we must first look at how rocks and mineral melt. To understand this we must first look at how minerals and rocks melt.
As pressure increases in the Earth, the melting temperature changes as well. For pure minerals, there are two general cases.
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