VISCOSITY LAB
Name ________________________
Hour _________ Date ___________
All liquids have the property of viscosity, or resistance to flow. Water’s
viscosity is very low so it is highly fluid and flows readily. For other liquids,
viscosity is so high that they flow much more slowly. Good examples are
cold motor oil and syrup, both of which are quite viscous and thus flow
only with difficulty. But when these liquids are heated, their viscosity is
much lower and they flow more easily; that is, they become more fluid
with increasing temperature. Accordingly, you might suspect that
temperature controls the viscosity of magma and lava, and this inference is
partly correct. We can generalize and say that hot magma or lava moves
more readily than cooler magma or lava, but we must qualify this
statement by noting that temperature is not the only control of viscosity.
Silica content strongly controls magma and lava viscosity. With increasing
silica content, numerous networks of silica tetrahedral form and slow down
flow. Mafic magma and lava with 45-52% silica have fewer silica
tetrahedral networks and, as a result, are more mobile than felsic magma and lava flows. Felsic magma, in
contrast, because of its higher viscosity, does not reach the surface as often as mafic magma. And when felsic
lava flows do occur, they tend to be slow moving and thick, and move only short distances.
Temperature and silica content are important controls on the viscosity of magma and lava, but other factors
include gases, mostly CO2, as well as the presence of mineral crystals and friction from the surface over which
lava flows. Lava with a high content of dissolved gases flows more readily than one with a lesser amount of
gases, whereas lava with many crystals or that flows over a rough surface tends to be more viscous.
Viscosity is important in volcanology. The more fluid a magma, the more likely it is to erupt. On the other
hand, when more viscous (higher viscosity) lavas do erupt, they usually do so explosively. Viscosity also affects
the shapes of lava flows and the mountains they erupt from. The more viscous the magma, the fatter the lava
flow. Also, the more viscous the magmas a volcano erupts, the steeper the volcano. Thus, shield volcanoes like
we have in Hawai'i have gentle slopes (less than 10 degrees), while stratovolcanoes like the Cascades in the
northwestern mainland are much steeper (roughly 25 degrees). As expected, hawaiian volcanoes erupt more
fluid lavas (called basalt) than do the Cascade volcanoes, which erupt a lava called andesite.
_____ 1) Which statement is true about magma and volcanoes?
A. A high viscosity magma will flow more readily and easily than a low viscosity magma.
Therefore, the eruption will be fluid and passive.
B. A high viscosity magma will flow more readily and easily than a low viscosity magma.
Therefore, the eruption will be explosive.
C. A low viscosity magma will flow more readily and easily than a magma with high viscosity.
Therefore, the eruption will be explosive.
D. A low viscosity magma will flow more readily and easily than a magma with high viscosity.
Therefore, the eruption will be fluid and passive.
_____ 2) The primary factor(s) that determine whether a volcano extrudes magma violently or gently is(are):
A. Composition
B. Amount of Dissolved Gases
C. Temperature
D. Temperature, Composition, and Amount of Dissolved Gases
One way to test the viscosity of a liquid is to drop something into it and see how long it takes to sink.
Go to google.com and type in ‘viscosity explorer’ into the search engine. Select the first
link: SEED – Viscosity Explorer. You’ll have to click on the virtual explorer (picture of
the lab) to get the activity on your screen so that you can manipulate different variables.
Next, you’ll play around with different substances and temperatures to see what impact they have on the
liquids’ viscosity. In doing so, fill out the table below:
In each box, fill in the TIME it takes, in SECONDS, for the steel ball to sink through the liquid.
15°C
25°C
35°C
45°C
55°C
65°C
Olive
Oil
Ethanol
Corn
Syrup
Honey
Which falls faster (predict the winner, then test):
Your Prediction (before testing)
Honey 50°C vs. Water 20°C
Honey 50°C vs. Ethanol 0°C
Corn Syrup 40°C vs. Ethanol 20°C
Olive Oil 70°C vs. Corn Syrup 70°C
(pick 2 variables) List Below:
(pick 2 variables) List Below:
Actual Result (after testing)