Pyroclastic Rocks

Study Questions for Final Exam

Fall 2003

[20 points] Two quite different processes have been proposed for the emplacement of ash flow tuffs. One process assumes en masse deposition and mobility controlled by fluidization. In contrast, the other process involves grain-by-grain deposition and mobility controlled by the viscous medium. Choose one of these hypotheses and defend it against the other.

[20 points] Draw a cross section through a volcanic arc to illustrate the principal geotectonic elements and processes in operation. Specifically show how tectonics affects the origin of magmas in the back arc basis and how that differs from the processes in the main volcanic arc. Show how magmas that originate from the subducted slab would differ in origin and composition from those originating in the mantle wedge.

[20 points] Given a topographic map of a Cascade volcano, draw a hazard map that considers the most common and dangerous types of volcanic events typically of this area.

[20 points] Lahars flow by different mechanisms than pyroclastic flows and avalanches, however their general deposit morphology is superficially similar. Explain the physics of lahar flow using your preferred model. How does the physical model explain the deposit characteristics?

[15 points] Three active or potentially active volcanoes in Mexico include Colima, Popocatepetl, and Pico de Orizaba. Describe the potential risk that each volcano presents to the surrounding population and give some estimate of the annual probability that renewed activity will effect populated areas for each volcano.

[15 points] Set up a table that shows 5 distinct contrasting features that distinguishes pyroclastic flow deposits, pyroclastic fall deposits and pyroclastic surge deposits

[15 points] Given a topographic profile of a volcano, construct a specified energy line. Determine the maximum runout or the maximum velocity predicted by the energy line. For what sorts of pyroclastic phenomena are energy lines best? Worst?

[15 points] Describe the process of welding in tuffs; be as quantitative as you can. How do welding and the associated process of secondary crystallization change the physical appearance and properties of a tuff? Why is welding more common in rhyolites than in andesites?

[10 points] What are the similarities and differences of the FLOW3D, Titan2D and LAHARZ models? What are their valid applications and major limitations?

[10 points] List the common features of volcanic debris avalanche deposits. How do Bezymianny, Bandai, and Unzen type debris avalanches differ? Give an additional example for each of these types.

[10 points] How can a plinian fall deposit be used to estimate the height of its eruptive column? What are some pitfalls in this method?

[10 points] On a probability vs. size graph draw three lines, one for a Plinian fall, one for a pyroclastic flow and one for a pyroclastic surge. What are the mean and standard deviation values for each curve?