C.
R. Carrigan defines the concept of viscosity in a nice and sussinct
way: Viscosity is "the resistance of a fluid to flowing in response to
applied pressure forces. The higher the viscosity, the more resistant a
fluid is to flowing, (2000)." Water has a very low viscosity rating
(think of rivers of water and how fast they can flow, Mississippi River
anyone?) whereas toothpaste has a higher viscosity rating (it does not
flow as easy as water.)
Many different parameters can effect the viscosity of lava flows. Chief
among these parameters
include, among others: composition and temperature. What follows
represents a brief rundown of each parameter:
- Composition: By
far the most important parameter in determining the viscosity of lava
flows is the composition thereof. The oxide that contributes most to
the viscosity of lava flows is silica oxide, also known as silica, (SiO2).
Because silica tetrahedrans can arrange themselves in many ways, their
arrangement can affect the viscosity of the lava flow. Silica arranged
as individual tetrahedra favor lower viscosity values whereas
interconnected tetrahedra favor greater viscosity levels.
- Temperature:
Basalts erupt within a narrow temperature range of roughly 1000 to 1200
degrees Celsius, (Bloom 2004). As the temperature of basaltic lava
cools, the associated viscosity increases. Amazingly, viscosity has
been shown by Ryan and Blevens to increase by a factor of 10 upon
just 100 degrees of cooling, (1987). The reason for the astonishing
increase of viscosity is that many of the constituent
minerals in a basalt (eg. olivines and pyroxenes) have
crystallizing temperatures that hover around the 1000 degree
temperature range. Once crystallization of solid minerals from lava
flows
commence, the viscosity of any lava flow increases with an
astonishingly rapid pace.
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