GL101 Lecture Notes - Lecture 8: Amphibole, Sediment Transport, Uniformitarianism
Document Summary
Igneous textures: size, shape and arrangement of crystals, environment in which the rock was formed, make inferences about rock"s origin. 3 factors affecting crystal size: rate of cooling. Very fast: glass (unordered ions: amount of silica (sio2) present, amount of volatiles (dissolved gases, types of igneous textures, aphanitic. Rapid rate of cooling of lava or magma. May contain vesicles (holes from gas bubbles: phaneritic texture (coarse-grained) Crystals can be identified without a microscope: porphyritic texture (porphyry rock) Minerals form at different temps and rates. Large crystals (phenocrysts) embedded in a matrix of smaller crystals (groundmass: glassy texture. Volcanic glass: obsidian: pyroclastic texture (fragmental) Textures often appear similar to sedimentary rocks. Welded tuff common pyroclastic rock: pegmatitic texture (pegmatites) Common crystals of quartz, feldspar and muscovite are abnormally large. Naming igneous rocks: classified by texture and mineral composition, textures result from different cooling histories, similar compositions but different textures different names, example: granite rhyolite. Figure 3. 12: felsic (granitic) rocks, granite.
