Earth Sciences 1022A/B Lecture Notes - Lecture 5: Igneous Rock, Aphanite, Intermediate Composition
27 Apr 2019
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Document Summary
Lavas: main texture is aphanitic (or glassy in extreme cases) The most common texture observed in volcanic (extrusive) igneous rocks is aphanitic (due to rapid cooling). However, keep in mind that composition can vary, producing different rocks. The three most important volcanic rocks (with aphanitic texture) are: basalt (mafic composition, andesite (intermediate composition, rhyolite (felsic composition) Surfaces of lava flows cool very quickly (are quenched ). Too fast for ions to produce mineral crystals. Very high silica content also slows down the movement of ions (further prevents crystal formation) . Ions therefore remain disordered and produce a natural glass called obsidian (with a glassy texture) Tephra is pyroclastic material that falls through the air from ash to volcanic blocks and bombs. Pyroclastic flows are clouds of hot explosive material (mostly ash) and gas which flow downhill due to gravity (too dense to maintain eruption column) Pyroclastic material settles and welds together to form a rock called tuff.
