viscosity_with_volcanoes

Viscosity with Volcanoes

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 Hawaii 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) that do the Cascade volcanoes, which erupt a lava called andesite.

Viscosity Calculations:

Jefferys (1925) derived a formula to calculate the viscosity of a fluid based on its physical properties and flow characteristics. The formula is:

V = g h² Ó sin A
3 É

where V is the mean velocity of the flow, g is the coefficient of gravity (9.807 m/s2), A is the angle of the slope, h is the depth of the flowing liquid, Ó is the density of the liquid (cool, basaltic rock has a density of 2.65 g/cm³ so the hot, basaltic lava must be less than this ‑ MacDonald used a value of 2 g/cm³), and É is the coefficient of viscosity (É is the Greek letter eta).

MacDonald (1954) calculated the viscosity of lava during Mauna Loa eruptions he observed in 1940, 1942, 1949, and 1950. Geologists realized they could apply this equation to flowing lava to estimate its viscosity. In the field he measured the angle of the slope, depth of lava, and velocity of flow. Angle of slope can be measured from a compass or calculated from a topographic map. The depth of the lava channel is used as an approximation of the depth of the lava. Velocity is calculated by throwing a stick on the surface of a flow and measuring the amount of time it travels a premeasured distance. The acceleration of gravity and density of the liquid are constants.

Use MacDonald's estimates and the equation to calculate the viscosity. The units for viscosity used by MacDonald are poises. A poise is 1 g/an s. Therefore, to make the calculations, all measurements must be converted from meters to centimeters.

Date	Location	Speed (m/s)	Lava depth (m)	Slope angle	Viscosity (poise)
4/ 12/40	Side of cone at vent	6.7	1	20
5/4/42	Edge of core at vent	8.3	2	6
1/20/49	cascade 0.6 km from vent	13.3	2	15
6/1/50	Entire Honokua flow	2.7	2	10.5
6/2/50	Cascade in Kaapuna flow 20 km from vent	11.1	2	17.5
6/2/50	Lava river in Kaapuna flow 20.5 km from vent	6.9	3	8
6/7/50	cascade cose to vent	13.9	1.5	25

Based on your calculations, answer the following questions:

1. Which eruptions had the lowest viscosities?

2. Why are the viscosities low for these eruptions?

3. Which eruptions had the highest viscosities?.

4. Why is the viscosity high for these eruptions?

5. In general, do the estimates show that viscosity increases with distance from the vent?