3.4.4 Igneous Textures: Glassy Texture Rocks, Obsidian from different magma compositions Granite and Basalt

Glassy texture. During some volcanic eruptions, molten rock is ejected into the atmosphere where it is quenched quickly. Rapid cooling of this type may generate rocks having a glassy texture. Glass results when unordered ions are frozen before they are able to unite into an orderly crystalline structure.

Obsidian, a common type of natural glass, is similar in appearance to a dark chunk of manufactured glass. Because of its excellent conchoidal fracture and ability to hold a sharp, hard edge, obsidian was a prized material from which Native Americans chipped arrowheads and cutting tools. Today, scalpels made from obsidian are being used for delicate plastic surgery because they leave less scarring than those made of steel.

Playlist of Geology and Earth Science Videos from Snow Mountain
https://www.youtube.com/watch?v=owlPSr39Nn8&list=PL6taRb3I0WLhYc8g1cuBk5crqUtLs3oXZ

Lava flows composed of obsidian a few hundred feet thick occur in some places. Thus, rapid cooling is not the only mechanism by which a glassy texture can form. As a general rule, magmas with a high silica content tend to form chain-like structures before crystallization is complete. These structures in turn impede ionic transport and increase the magma viscosity. Viscosity is a measure of a fluid’s resistance to flow.

Granitic magma, which is rich in silica, may be extruded as an extremely viscous mass that eventually solidifies to form obsidian. By contrast, basaltic magma, which is low in silica, forms very fluid lavas that upon cooling usually generate fine-grained crystalline rocks. However, the surface of basaltic lava may be quenched rapidly enough to form a thin glassy skin. Hawaiian volcanoes sometimes generate lava fountains, which spray basaltic lava tens of meters into the air. Such activity can produce strands of volcanic glass called Pele’s hair, after the Hawaiian goddess of volcanoes.