Factors Affecting Viscosity
The effect of temperature on viscosity is easily visualized. Just as heating syrup makes it more fluid (less viscous), the mobility of lava is strongly influenced by temperature changes. As a lava flow cools and begins to congeal, its mobility decreases and eventually the flow halts.
But more significant to volcanic behaviour is the chemical composition of magmas. A major difference among various igneous rocks is their silica (SiO2) content. The same is true of the magmas from which rocks form. Magmas that produce basaltic rocks contain about 50 percent silica, whereas magmas that produce granitic rocks contain over 70 percent silica.
Variations in properties among magmas of differing compostions
Note that a magma's viscosity is directly related to its silica content. In general, the more silica in magma, the greater is its viscosity. The flow of magma is impeded because silicate structures link together into long chains, even before crystallization begins. Consequently, because of high silica content, granitic lavas are very viscous and tend to form comparatively short, thick flows. By contrast, basaltic lavas, which contain less silica, tend to be more fluid and have been known to travel distances of 150 km ( 90miles) or more before congealing.
In Hawaiian eruptions, the magmas are hot and basaltic, so they are extruded with ease. By contrast, highly viscous granitic magmas are more difficult to force through a vent. On occasion, the vent may become plugged with viscous magma, which results in a buildup of gases and a great pressure increase, so a potentially explosive eruption may result. However, a viscous magma is not explosive by itself. It is the gas content that puts the "bang" into a violent eruption.
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