Image credit: Scott Brande
A glassy texture results from a very rapid loss of heat and also a loss of dissolved gas (such as water vapor). These conditions prevent the crystallization process from proceeding very far, and as a result, no or few visible crystals are formed. The solid largely lacks crystalline structures and long-range structural disorder among molecules prevails.
The non-crystalline solid is termed a glass, and the primary igneous rock of this type is obsidian - volcanic glass.
A rapid chilling that results in the formation of obsidian may result from a volcanic eruption on land, or underwater on the surface of the seafloor. Pillow lava that forms underwater is skinned by obsidian because the cool seawater extracts heat from the surface of the lava almost instantaneously.
A pattern of breakage of glass differs from patterns of breakage of other igneous rock. A particularly distinctive breakage pattern of obsidian is conchoidal fracture.
In our simplified classification of the common igneous rocks, we will recognize obsidian as the only rock of glassy texture.
Watch this short video to learn how to observe obsidian and to recognize its distinctive pattern of breakage, conchoidal fracture.