I am an Associate Professor at the School of Information Studies at the University of Ottawa. This blog contains some informal musings about my work.
The photograph below, which I use as the banner for this blog, was taken by Jim Elder. It is of a 1cm-thick slab cut from a soccer-ball-size piece of obsidian from Glass Buttes, Oregon, USA. The photo spans about 10cm of the slab.
Obsidian is naturally-occurring glass often produced by volcanoes with silica-rich magma. This piece of obsidian is mostly translucent (there is a brown background in the photo), with swirls of black and red caused by minerals mixed into the glass as the magma folded during flow.
Obsidian can be used for dating because water slowly affects its newly-exposed surfaces (eg., when fractured to make stone-age tools) and it is unstable — over millions of years it gradually crystallizes (devitrifies) into minerals (eg., quartz and feldspar) — so it is rare to find obsidian older than 20 million years.