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Gunung Kelud (Kelut)

The Chemical Composition of Mt. Kelud's Eruptive Materials!
Mt. Kelud
Crater Lake
Chemical Composition
A Quick History Lesson
Cool Facts!

As we've already learned in Geology 1020, anytime magma contains a high concentration of SiO2, it is much more viscous than mafic (basaltic) magmas, which have a lower content of silica. This explains why the lavas that flow from Mt. Kelud are so explosive!

Silicic magmas are formed at convergent plate boundaries (and composite volcanoes, in particular are formed at a subduction zone (Montgomery, 2000)), so the magma from a stratovolcano is very viscous and explosive.

Figure 5. Source: Bernard, 2000.

One thing that's particularly interesting about the magmatic composition of a stratovolcano is the fact that the lava is very high in SiO2, but it also has a lot of basalt in it since it's produced at a subduction zone. We know this because there is a high basaltic content in oceanic crust.
As we learnt in Geology 1020, mafic magmas contain about 50% SiO2, while silicic magmas contain from 60-75% SiO2.
Incidentally, while the magma from the 1990 eruption was slightly low in its silica content, it's still considered to be a granitic magma. However, the magma from a stratovolcano still has a relatively high water content (roughly 5%)(USGS, 2003). 
Consequently, the water content makes the magma more fluid because it breaks up its Silica-Oxygen bonds, and also makes the magma erupt more violently because it expands gas bubbles.

by Victoria Gauthier (0330250)
Geology 1020
January 2008