Geological Background

Obsidian Cliff is a thick rhyolitic lava flow of obsidian formed about 183,000 years old. The obsidian is a glass, and for it to form, the magma is rapidly cooled to below the point of crystallization, and so forms glass. Large columnar joints mark the base of the flow near the road and steep contorted layering can be seen near the top. Obsidian was deposited in layers alternating with rhyolite. Numerous spherulites are observed in the outcrop, these features are found only in acidic rocks and their origin is still unknown. They are thought to represent devitrification structures with feldspar growing outward in concentric circles, and are common in rhyolite ash and flow deposits. Spherulites are typically rounded or spherical aggregates (mm to 1-2 mm) of acicular crystals radiating from a single point. Crystals of spherulites are commonly of alkali feldspar and silica minerals (such as cristobalite, quartz). Lithophysae (lithophysa Greek for rock bubble), associated with spherulites, are generally radial or concentric cavities (up to cm in size) that is hollow, or partly to completely filled with later minerals. There is debate as to whether these features are primary or secondary. Some attribute the spherules as primary and are gas cavities, while others believe they are secondary formed as contraction cavities in cooled magma.

This glass was widely used by Native Americans for arrowheads and other cutting tools, and obsidian knives as along as 25 cm from this flow have been found in sites as far away as Ohio.