GL23C IGNEOUS & METAMORPHIC PETROLOGY LAB 5

STUDENT NAME ____________________________________________________________

Basalt-Andesite-Rhyolite Association

Describe thoroughly the thin sections provided by your instructor, noting texture, mineralogy, mineral size and shape, and any other special features. Determine the crystallization history on the basis of the inter-relationships of minerals in the rock. Sketch the thin section and label diagram.

In your petrographic descriptions, you must identify and describe in detail the optical properties of the different minerals present in the rock and estimate the modal mineralogy for each. In the case of porphyritic rocks estimate the percentage of phenocrysts to the percentage of groundmass. If the rock has a crystalline groundmass, then apply the IUGS QAPF classification.

List of Volcanic Rocks to be Examined in Lab 5

1. Obsidian
2. Dacite
3. Andesite
4. Basalt

Read ATLAS OF IGNEOUS ROCKS AND THEIR TEXTURES by MacKenzie, Donaldson & Guildford.