Due December 1, 1997  

1. In lab, you had the opportunity to study a number of common igneous rocks, and to observe their diversity of composition and texture.  One particularly important textural type is known as porphyritic.

A.  Describe this texture, and explain its origin.

B.  The larger minerals in porphyritic rocks are often plagioclase feldspar.  These individual plagioclase crystals are typically chemically zoned with calcium-rich cores and sodium-rich rims.  Explain how such chemically zoned crystals are produced in igneous rocks.

2. List and briefly explain five different geological techniques that might be useful in predicting volcanic eruptions.