GEOL 101 Lecture Notes - Lecture 4: Igneous Rock, Porphyritic, Aphanite
Document Summary
Reasons for crystal size: cools beneath the surface= big crystals, cools above the surface= little crystals. Fine grain crystals (tiny crystals: glassy: Zero crystals because it cooled too fast for them to form; no crystal structure: vesicular: Lots of holes (bubble chambers) containing gas; looks like a sponge: pumice: Contains so much gas that it can float on water: porphyritic: Has 2 distinctly different crystal sizes because it cools beneath the surface and then moves up. Can be applied to almost any igneous rock: pegmatitic: Huge grains; looks like many rocks put together: pyroclastic: Rocks that have curves on different sides (like when you drop a glass bottle) Contains big quarts and potassium feldspar grains; several different colors of grains: gabbro: Big crystals hard to differentiate from each other; grains include plagioclase feldspar and pyroxene: peridotite: Contains olivine (olive green color) and pyroxene.
