Viscosity is a measure
of the internal resistance of a liquid body to flow. It is the inverse of mobility
or fluidity. Water is highly fluid and has a very low viscosity, whereas honey,
or heavy machine oil is much more viscous. Viscosity varies with temperature
and composition of liquid.
Viscosity increases with an increase in SiO2 in a magma. As shown in the figure above, rhyolitic magma has a higher viscosity than basaltic magma due to the higher SiO2 content of rhyolite.
| Density is the mass per unit volume of a substance. We are used to thinking of density (specific gravity, really) as it applies to a single mineral, but magmas are mixtures of substances. The only information we have is from whole rock chemistry. So, we calculate the density of a rock as the sum of the masses for all components of a substance divided by the sum of all their volumes. It can be shown by the equation shown on the left. |
| As can be seen from this figure, density varies with temperature. An increase in the temperature of a melt results in a decrease in its density. More importantly, density varies with composition. Mafic melts such as basalts have a higher density relative to felsic melts. Increasing water content, regardless of composition, will lower the density of the melt.
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Example
