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Name: Kathryn
Status: Educator
Age: 50s
Location: N/A
Country: N/A
Date: November 27, 2004


Question:
Which is the most violent type of volcano? I thought it was the composite cone volcanoes, but outside books have referred to Krakatoa as a cinder cone volcano, and that, therefore, cinder cone volcanoes are the most violent. I just don't see how Krakatoa could be a cinder cone volcano...



Replies:
Thanks for your question, Kathryn... The violence, or rather, eruptive power of a volcanic eruption is largely determined by the chemical composition of the lava being extruded. The resulting shape and classification of the volcanic edifice, or cone, is also controlled by the composition of the extruded lava... so, in a once-removed sense, the type of cone is related to the eruptive power of a volcano. But the main issue here is the composition of the lava. In general, there are four types of volcanic (extrusive igneous) rock compositions: felsic, intermediate, mafic, and ultramafic.

The felsic rocks are abundant in silica (SiO2), and consequently, silica-rich minerals (e.g., FELdspars and quartz), and deficient in iron and magnesium rich minerals (e.g., pyroxene, hornblende, and olivine). The mafic and ultramafic rocks, as you might suspect, are deficient in silica and rich in MAgnesian and FerrIC (iron-rich) minerals. The intermediate rocks, by composition, fall somewhere between.

The key point to make concerns a process called polymerization, which is simply the process by which molecules link up to form long chains of molecules called polymers (plastics, for example, exhibit a high degree of polymerization). Silica molecules are excellent poymerizers which form very long SiO2 chains while in the liquid state of a magma or lava. When such long chains of molecules exist in a lava, the chains cause the lava to become very thick, or viscous, due to the fact that the chains prevent the liquid from flowing as easily as it might if the molecules existed as singular, non-polymerized entities.

Because high-viscosity magmas do not flow so easily, they accumulate within the magma chamber and continue to build up pressure until some critical point is reached when the pressure is released in a catastrophic eruption. Mafic magmas and lavas, by contrast, are relatively lower in viscosity and, therefore, do not resist flowing upward through the magma chamber and volcanic neck even under fairly low pressures, and consequently, do not erupt as violently.

Additionally, felsic magmas tend to be higher in volatile (gas) content (e.g., carbon dioxide, water vapor, etc.). These volatiles, which are much lower in density than the magma/lava in which they are contained, are compelled to rise up and out of the molten mass because of buoyant forces. The viscosity of the magma, however, prevents much of the gas from escaping until the very time of eruption, in which most of the gas is powerfully exhaled from the volcano, thus adding to the already potent force of the eruption.

Krakatoa, as far as I know, is a composite cone (or strato-cone), which is characteristically produced by the eruption of felsic lavas, thus making composite cones associated with the more powerful types of eruptions. Composite cones are composites of both ash/cinder material and lava flows, and build up in a layer-cake fashion (strato means "layer"). So, some parts of Krakatoa may be more "cindery" than others, but I am aware only of its classification as a composite cone. Furthermore, Krakatoa is only one example of a composite cone, and therefore, cannot be used as the measure of the eruptive power of all composite cones... so any book or author that attempts to make such an assertion is making an error in scientific logic. Hope this helps!

Scott J. Badham
Department of Geology and Geophysics
University of Wyoming



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