Thursday, June 28, 2012

Obsidian - Part I

Sometime between 77-79 CE, the Roman naturalist Pliny the Elder (Gaius Plinius Secundus, 23-79 CE), published Natural History (Naturalis Historia) - a massive tome which attempted to cover the entire field of ancient knowledge.  It was, in essence, the first encyclopedia.

One of the sections in Natural History dealt with glass and Pliny mentioned the following:

"Among the various kinds of glass, we may also reckon Obsian glass, a substance very similar to the stone which Obsius discovered in Æthiopia. The stone is of a very dark colour, and sometimes transparent..."

The term obsianus lapis "stone of Obsius" eventually morphed into our English word obsidian and refers to naturally-formed volcanic glass - a type of igneous rock.

While obsidian wasn't written about until Pliny, it was known to humans and quite possibly by our hominid ancestors like Homo habilis as well. This was due to a property known as conchoidal fracture - from the Greek κογχε (konkhē) which means "mussel" after the ridges on a mussel shell.  Obsidian, being a glass, is amorphous which means it doesn't have an underlying crystalline structure and will typically break along a curved, ridged fracture surface (similar to other natural materials such as chert or flint).  The edges of these fractures can be sharper than surgical steel scalpels.

Obsidian is widespread and associated with certain types of volcanic eruptions where it forms by the fast cooling of silica-rich (rhyolitic) magma.

One model for the emplacement of obsidian by Fink, J.H. (1983) GSA Bulletin 94 (362-380)

Obsidian is found in many areas around the world including most of the Western U.S. states.

Obsidian Cliffs in Yellowstone National Park, Wyoming

One interesting fact about obsidian is that each eruptive occurrence is slightly different with regard to trace amounts of various elements in the source magma. This provides obsidian from a specific locale with a unique chemical signature and allows archaeologists to trace obsidian tools to specific quarries which, in many cases, reveals ancient trade routes.

Obsidian is typically 70% or more amorphous SiO2 (silicon dioxide) with smaller amounts of other elements.  These elements form microscopic minerals wihich color the obsidian black but there are variations of brown, gray, green, gold, and even reddish (mahogany obsidian).  This leads to another interesting property of obsidian in that it breaks down over time by a process called devitrification which is caused by hydration - the absorption of atmospheric water vapor. 

Mahogany obsidian from Mexico

Because obsidian alters over time (dependent on temperature and water available), this process can be used to roughly date obsidian samples and artifacts.  It also means that it's rare to find obsidian over 20 million years or so ago and there is no obsidian in the geologic record older than the Cretaceous Period (65-145 million years ago).

Tomorrow I'll talk about what forms from devitrified obsidian.

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