The translation into English of Natural History written by Pliny the Elder of Rome shows a few sentences on the subject of a volcanic glass called obsidian (lapis obsidianus), discovered in Ethiopia by Obsidius, a Roman explorer. Crystalline rocks with obsidian's composition include granite and rhyolite.Obsidian is mineral-like, but not a true mineral because as a glass it is not crystalline; in addition, its composition is too variable to be classified as a mineral. Because obsidian is metastable at the Earth's surface (over time the glass becomes fine-grained mineral crystals), no obsidian has been found that is older than Cretaceous age.Geological analysis relies mainly on chemistry, physics and mathematics. a'a' lava - basaltic lava characterized by a rough or rubbly surface composed of broken lava blocks called clinker.
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Pure obsidian is usually dark in appearance, though the color varies depending on the presence of impurities.
Iron and other transition elements may give the obsidian a dark brown to black color. In some stones, the inclusion of small, white, radially clustered crystals of cristobalite in the black glass produce a blotchy or snowflake pattern (snowflake obsidian).
This breakdown of obsidian is accelerated by the presence of water.
Having a low water content when newly formed, typically less than 1% water by weight, obsidian becomes progressively hydrated when exposed to groundwater, forming perlite.
Douglass, Richard Ciolek-Torrello, Sarah Van Galder, Benjamin R. The Coleville and Bodie Hills NRCS Soil Inventory, Walker and Bridgeport, California: A Reevaluation of the Bodie Hills Obsidian Source (CA-MNO-4527) and Its Spatial and Chronological Use. Spatial and Temporal Distributions of Archaeological Heated-Rock Cooking Structures in the Transverse Mountain Ranges: Proposed Markers of Land-Use Shifts since the Early Holocene [SCA Proceedings - PDF].
Grenda, Jeffrey Homburg, Manuel Palacios-Fest, Steven Shelley, Angela Keller and Davis Maxwell Spatial and Temporal Distributions of Archaeological Heated-Rock Cooking Structures in the Transverse Mountain Ranges: Proposed Markers of Land-Use Shifts since the Early Holocene [SCA Proceedings - PDF].(Hydraulicity is actually defined as the ability to set under water, without any carbon dioxide.) The current debate is focused on the blending of non-hydraulic lime putty and hydraulic hydrate.It is the author’s view that gauging with hydraulic hydrate is poor practice.Although it is important to make clear that the principal proponents of this practice can provide many examples of the successful use of these ‘complex mixes’, long-term durability of these mortars has yet to be assessed.The combination of ordinary lime and hydraulic lime in the same mortar causes concern for the following reasons: Lime was produced by burning locally available limestone in a coal or wood fired kiln at a temperature rarely in excess of 900 degrees C.abyssal plain - The ocean floor offshore from the continental margin, usually very flat with a slight slope.