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Volcano Classification
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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