This lecture began with a reiteration of the material from Lecture 17 on the effects of water on the style of volcanic eruptions in volcanic arcs. The lecture then moved on to consider the concept of bimodal volcanism. These two types of rocks are related because the basalts, which leave the mantle at temperatures between 1200 and 1300 C, provide the heat to partially melt the crust and generate the more silicic crustal melts. Because these two compositionally distinct magma types have significantly different viscosities, they tend to erupt with very different styles.
Basaltic lavas have low viscosities.
The escape of volatiles reduces the violence of eruptions of basaltic lava, although volatile escape can create spectacular lava fountains up to 200 meters high. Consequently, volcanos in volcanic arcs are often composites made up of interlayered basalts flows and layers of volcanic ash. Such volcanos are called composite volcanos and typically have relatively steep conical forms. One of the best examples of the form of a composite volcano is Mt. Fuji in Japan, shown in the picture at right. Not all composite volcanos have such perfect conical shapes as Mt. Fuji.