Explain why a rhyolitic lava flow is more viscous than a basaltic ...

[OSWALD]

Explain why a rhyolitic lava flow is more viscous than a basaltic lava flow.
 
  What will be an ideal response?

Question 2

You are a field geologist, dropped off in a new area. You examine the rocks of this area and find rhyolite, welded tuff, and scattered pumice.
 
  All are very young. You begin mapping the area, and soon have a rough sense of the distribution of rock units, as shown here:
 
  Note that the welded tuff varies systematically in its thickness, and that a lake is present near the center of your mapping area. How do you interpret this pattern?
  What will be an ideal response?

[duke02]

Answer to Question 1

Answer: Viscosity is dependent primarily on temperature and silica content. The more silica in the magma, the greater the viscosity. Since rhyolitic flows are high in potassium, sodium, and high silica content (70) they have a much higher resistance to flow. Basaltic flows have a much lower silica content (50). Applying heat to a fluid makes the substance more fluid thus increasing its mobility.

Answer to Question 2

Answer: This looks like an explosive volcanic center, with a caldera in the middle, a spot where the rhyolite flows emerged. The pyroclastic flows that deposited the tuff emerged there, too, as evidenced by their thickening closer to the source. Apparently, these pyroclastic flows flowed out in all directions, but in particular to the west, as the oval shape of the tuff deposits extend furthest from the vent in that direction. Because these features appear to be relatively undisturbed, the area is potentially at risk of another eruption. Seismic and gas monitoring should be started immediately.

[OSWALD]

Gracias!

[raili21]

Excellent