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2. There are three main factors that determine the nature of volcanic eruptions. Each factor has defined characteristics that can determine the effects of the volcano. The factors are the magmas composition, temperature and the amount of dissolved gases. Each of these affects the magmas viscosity, which is directly related to the nature of the eruption. The amount of silica content is directly related to the viscosity, as well.
The composition of Mafic magma is about 50 % silica, which makes it the least viscous of the three composition types. It also contains the least amount of dissolved gas (1 - 2 %). This type of volcano is found on the island of Kilauea, in Hawaii. Intermediate or Andesitic magma contains approximately 60 % silica and 3 - 4 % of dissolved gas.
This type of volcano forms a composite cone, such as Mt. Rainier in Washington. The third form of magma composition is Felsic. It contains the most of amount of silica (approximately 70 %) and dissolved gas (4 - 6 %), which defines the reason for it having the highest viscosity, as well. Mt.
St. Helens is an example of this type of eruption. The temperature of the magma correlates to the viscosity, as well. Magmas that have a higher temperature are going to be more fluid and flow easier, more likely for a longer period of time and a greater distance. The lava flows are slow, as well.
Highly viscous and cooler magmas are going to have a more difficult time flowing. This can block a vent in the volcano, which causes a buildup of gases. At high temperatures and low surface pressures, the gases will expand and build up pressure against the ground. The pressure of the buildup of gases is what causes the great explosions. It is the viscosity of the magma, combined with the quantity of dissolved gases, which determines the nature of volcanic eruptions. Bibliography:
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