GLY431/531                                         Volcanology                                                   fall 2001

Study Questions for Exam I

10-point question

1)  Given the following information, calculate the rate (in m/s) of settling of an olivine crystal through a basaltic magma chamber.  Then calculate how long it would take the crystal to settle from the top to the bottom of a 6-km-diameter magma chamber.  Give your results in seconds and years. You must show your work to get any credit for this problem.

a)  density of olivine crystal = 3300 kg/m3
b)  density of basaltic magma = 2700 kg/m3
c)  radius of olivine crystal = 10-4 m
d)  magma viscosity = 25 Pa s
2)  Do the following factors cause magma and lava viscosity to increase or decrease?
a)  increasing silica content
b)  increasing temperature
c)  increasing volatile content
d)  decreasing vesicle (bubble) content
e)  decreasing crystal content
3)  Describe the chemical and physical characteristics of the lithosphere.  Consider its global distribution in your answer.

4)  How does the orientation of the three principal stresses different in a compressional regime from that of an extensional regime?  Use a diagram to illustrate the orientation of the principal stresses and show the most favorable plane of magma intrusion.

15-point questions

5)  Three common gas species in volcanic plumes are: H2, S2, and H2S.

a)  Listing H2S as a product, write a balanced equation showing the relationship between these gases.
b)  Give the mathematical relationship for their equilibrium constant, K.
6)  What is meant by a melt being in equilibrium with water?
a)  How does this differ from a melt being saturated with water?
b)  Suppose magma contained 1% water.  After the crystallization of 10% olivine and 30% plagioclase, how much water would the melt contain? (Note: exam question would have different proportions of crystals).
7)  Explain how the process of crystal fractionation can generate magma with a rhyolitic composition from magma with a basaltic composition.

8)  What are the 3 main ways a chunk of mantle can be melted to produce magma?  Which of these generates the greatest amount of melt? Which generates the least?
 

9)  Suppose that a 1 mm gas bubble is within a 20 m diameter magma conduit at depth of 4 km.  What will the diameter of the bubble be at the surface if it rises with the same velocity as the surrounding magma?  Assume an ideal gas and that all of the pressure on the bubble is due to the "hydrostatic" load of the magma in the column.  For the calculation disregard the growth of the bubble by diffusion of gas and the effect of surface tension on the bubble size.
 

10)  SO2 gas

a)  How does the emission of SO2 from a volcano help us to forecast an eruption?
b)  Where does the SO2 in the volcanic plume come from (sources)?
c)  Where does the SO2 go (sinks)?
d)  Give some quantitative evidence to show a case where SO2 was helpful in forecasting an eruption.
e)  What are the limitations or uncertainties in using SO2 for forecasting eruption?
11)  Describe the different magma source areas in subduction zones.
20-point questions

12)  Answer the following questions based on the water phase diagram.

a)  What does the critical point on this diagram indicate?
b)  At 100 degrees and 100 bars, what phase exists?
c)  At what temperature would water at 50 degrees boil?
d)  What would be the approximate change in volume from water to steam at 50 bars?
e)  At what temperature would water at 300 bars boil?
13)  Discuss the composition of Earth's mantle, and address the following questions.
a)  How do we know the mantle composition?
b) What is the "bulk composition" of the mantle?
c)  Is the mantle heterogeneous?  If so, what are the main "end-member" compositions?
d)   How does the mantle composition differ from MOR basalts in mineral content (type and amount)?
e) How does the mantle composition differ from MOR basalts in bulk chemical composition?