MICHAEL MANGA

In this lab we will continue to study and interpret structures in obsidian. In particular, we will focus on the interactions between various scales of microstructure. The sample we will study is from Deadman Dome, CA (picture below). This is the obsidian flow south of Obsidian Dome (see previous lab).

There is only one thin section and one hand sample for this lab.

Questions

• What is the large (>1 cm diameter) phenocryst?
• Notice that there are many scales (sizes) of crystals. Examine theses crystals and then define and subdivide them into distinct ``populations'', if possible. What criteria did you use?
• What are the compositions and properties of the crystals in each population?
• Where and under what conditions did each population of crystals form?
• Notice how the ``flow'' bands are deformed around the large phenocryst. Can you determine the type of deformation this sample experienced?

Last modified 10 February 2002
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