15 June 2011

Obsidian hunting in the Jemez Mountains

Posted by Jessica Ball

This past weekend I took a day to go revisit the Valles Caldera. While I was wandering the trails, I decided to do a little “rockhounding”. Geologists are often notorious for their rock collections, and I felt like mine was missing something that no volcanologist should be without: Obsidian.

Obsidian is basically volcanic glass – the result of lava cooling so quickly that it doesn’t have time to develop crystals. Pure glass is basically SiO2 (or silicon dioxide), and obsidian tends to form from rhyolitic magmas (which already have a high percentage of SiO2, typically greater than 68%). “Heavy” minerals in the obsidian (such as those containing iron and magnesium) make the glass dark, and (depending on their oxidation state) can give it reddish or greenish hues.

Heavy minerals give obsidian its dark colors.

Because obsidian has no crystal structure, it breaks like glass, with what’s called conchoidal fracture. This produces curved rather than straight surfaces, and is one of the reasons that obsidian was (and still is) desirable as a tool. Ancient cultures chipped obsidian into extremely sharp cutting tools and weapon points, and obsidian scalpels – which have much sharper edges than steel ones – are used today in delicate surgeries, including eye surgery.

An edge on an obsidian fragment formed by the intersection of two conchoidal fracture surfaces.

Obsidian is pretty common in the Jemez Mountains, particularly in the Banco Bonito rhyolite. Once you leave the Valles Caldera (a National Preserve), you’re in the Santa Fe National Forest; casual rock collecting is allowed, as long as you’re not carting off massive amounts of material (no more than a bucket per day). The best place to find bits you can legally collect is in the East Fork of the Jemez River, which you can get to using the Las Conchas trail.

An outcrop of the Banco Bonito rhyolite (the dark rocks shining in the sunlight).

East Fork of the Jemez River on the Las Conchas trail. The scenery alone is worth the trip!

Like sea glass, obsidian that ends up in a riverbed gets worn down (as if it was in a rock tumbler), so your best bet is to look for dark pebbles and cobbles that have chips out of them – this is where obsidian’s conchoidal fracture helps you out. The chunks are usually very rough and won’t be translucent at first sight, but you can crack them open to reveal the glassy interior.

"Tumbled" obsidian, straight from the riverbed. Some pieces have been chipped and reveal the tell-tale conchoidal fracture.

The interior of an obsidian cobble.

As an archaeology minor as well as an avid collector, I find obsidian fascinating for its historical as well as volcanic significance. And it’s pretty! (A word of warning, however: If you’re going to break your obsidian pieces, be really careful and wear eye protection – this stuff is sharp, too!)