The pyroclastic rocks of Sunset Crater are basaltic; why did their eruption
Worksheet 4A: Use
the information gathered on the virtual field trip and the links to answer the
following questions.
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1. The
pyroclastic rocks of Sunset Crater are basaltic; why did their eruption form a
cone rather than a basalt flow like the Bonito Flow?
2. Why does the Bonito flow come out of the bottom of the
cinder cone?
3. Why are
the basalt flows so much denser than the cinders?
4. At one time scientists thought that basalt was deposited
from water. What evidence is
there that the
lava flow was once molten? What evidence is there that the squeeze-ups
were once
semi-solid when extruded?
5. The two
levels observed in the Bonito flow represent two stages of extrusion. How could
you distinguish these two stages of eruption?
Unlike the case for sedimentary rocks, in which older rocks always
underlie younger rocks, the younger lava flow is below the older. Why?
6. How did
the agglutinate mounds on top of the Bonito flow get to their present
positions? Note that there is a dip in the crater rim above the point where the
flow exits the cone. Why is it there?
7. How is a
lava tube formed?
8. How did the xenoliths get to their present location? Even though they are enclosed by
lava, they are not
melted. Why might this be the case?
