8.5: Activity 8D - Magma Viscosity

The following questions address what factors control how fast a melt can flow. The viscosity, a material’s resistance to flow, depends primarily on the melt composition, but can also be affected by temperature and gas release.

1. Intermediate lavas can flow ??? than mafic lavas, due to the ??? viscosity.

   1. slower, higher

   2. slower, lower

   3. faster, higher

   4. faster, lower

2. Consider (or actually try) the following, if we compare how honey (or dark corn syrup) and water flow when poured from a container,

   1. then the honey (corn syrup) represents felsic magma, and the water represents mafic magma.

   2. then the honey (corn syrup) represents mafic magma, and the water represents mafic magma.

3. Imagine putting the honey (corn syrup) in the refrigerator overnight; will its viscosity be affected?

   1. yes, the viscosity will increase

   2. yes, the viscosity will decrease

   3. no, there will be no change as the composition stays the same

4. Keep the honey (corn syrup) in mind while you answer this question. When first erupted, basaltic lava typically erupts at around 2100°F. After flowing away from the vent, the temperature falls. How will the viscosity change as the lava flows away from the vent?

   1. Viscosity will increase.

   2. Viscosity will decrease.

   3. Viscosity will stay the same.

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