C. TEXTURE - size & shape of constituents (see p. 47)

1. Glassy = volcanic glass. Cooling is so rapid that individual mineral crystals don’t have time to form. Atoms/ions are trapped in an irregular arrangement. Obsidian = natural volcanic glass. 47D

2. Aphanitic = fine grained. Slightly less rapid cooling allows small mineral crystals to form (<1 mm across). Many extrusive igneous rocks have both glass and small crystals; easily studied with a microscope or a hand lens. 47A

3. Phaneritic = coarse grained. Slower cooling, deep in the crust, allows larger crystals to form; visible without a microscope. Results in intrusive igneous rocks, which are only exposed on the Earth’s surface after overlying rocks are eroded away. 47B

4. Porphyritic - often, a few crystals grow to a larger size than others; these are called phenocrysts. The fine-grained stuff is called matrix or groundmass. 47C

5. Pyroclastic texture - formed when solid ash or rock fragments are ejected during a violent eruption.

6. Vesicular - as gas evolves from magma or lave, it forms bubbles. These bubbles can become trapped during solidification = vesicles. 48 Frozen volcanic froth > pumice. 55

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