Ask any rockhound where to find obsidian in Oregon and they’ll most likely tell you Glass Buttes. Situated about 70 miles east of Bend, Oregon on Highway 20, Glass Buttes lies in the High Lava Plains of central Oregon and has been a favored site of rockhounds for decades. The area consists of rhyolitic domes containing large amounts of obsidian that have been extruded to the surface. Administered by the Bureau of Land Management (BLM), the area has 6000 acres that have been designated as a Free Use Area for the public. Rockhounds and knappers from around the state make annual trips to collect the many varieties of obsidian that are found there.
Geological and Mineralogical Setting
Glass Buttes is a bimodal (basalt/rhyolite) volcanic complex, which erupted about 5.5 m.y.a. Sitting upon a flood basalt plateau, the area consists of numerous domes, vents and flows made mostly of rhyolite and obsidian. The two most prominent features are Glass Butte (6300 ft.) and Little Glass Butte (6200 ft.). The entire area lies within the Brothers Fault Zone (BFZ), which extends, northwest from Harney Basin to Newberry Caldera. In addition to the abundant obsidian found there, Glass Buttes also contains a variety of minerals. Hematite, plagioclase, hornblende, quartz and biotite can be found in the underlying basalt and rhyolite. Cinnabar is present in opalite deposits on the eastern side of Little Glass Butte, where it was mined for mercury back in the 1960s. Today, that area is under lease by Magma Energy Corp. for early stage geothermal exploration.
How Obsidian is Formed
Obsidian is an igneous rock that has no crystalline structure. It is produced from rhyolite that is high in silica and low in water. When it reaches a temperature of about 1,200° C, its physical composition changes to a polymerized magma. As it erupts, its temperature quickly drops below the temperature to create mineral crystals and a natural silica glass is formed. Black is the most abundant color in obsidian, and it is derived from oxidized magnetite contained within the silica. The presence of Hematite creates a red color, which often flows in bands to form mahogany obsidian. Patterns of gas bubbles can create sheens within the glass. When they are combined with oxidized chemical elements, they can produce an internal gloss, such as silver obsidian or an iridescence like rainbow obsidian. In addition to black obsidian, Glass Buttes boasts at least eight other varieties.
Rules and Recommendations
Much of the rock at Glass Buttes can be found lying on the surface. They range from small flakes and gravels to moderate sized pebbles and cobbles. Larger specimens (boulders) have to be dug up. Since this is public land, there are certain rules and courtesies expected from everyone. The high desert ecosystem needs to be respected.
- Digging is by hand only and it is recommended that you dig in established pits. There is at least one pit where every variety is found and they still contain plenty of quality rock. Undermining a pit is prohibited. It not only destroys the root structure of the surrounding flora, but it is dangerous as well.
- The Juniper trees are an old growth ecosystem. It is prohibited to cut them.
- Glass Buttes is a primitive area. Whatever you bring in, take it back out. If you see trash left by someone else. Please take it with you when you leave.
- Camping is allowed and there are a number of spots where you can pitch a tent or park a travel trailer.
- Fires are allowed in the cooler seasons if you bring your own wood, but not during the summer.
- There are private lease areas within the region. Respect them and don’t trespass.
- Drive only on established roads. If it's a trail, park your vehicle and hike.
Tips and Tools
Navigating Glass Buttes is relatively easy. Most pits can be seen from the road, but portions of the road may be inaccessible without a four-wheel drive. A map of some of the sites can be found using Google Earth and in Garret Romaine’s updated 2008 edition of “Gem Trails of Oregon”. If your goal is surface collection, then at a minimum, you’ll need a geology pick or rock hammer, safety glasses or goggles, a good pair of boots, gloves, a small backpack and lots of water. If you plan to dig, you’ll need all of the above, plus a sturdy pick and shovel. In either case, you’ll leave with some nice specimens.
Sources:
Johnson, J. Michael, “Geology, Alteration and Mineralization of a Silicic Volcanic Center, Glass Buttes, Oregon”, Department of Geology, Portland State University, (May 1984)
Roche, L. Richard, “Stratigraphic and Geochemical Evolution of the Glass Buttes Complex, Oregon”, Department of Geology, Portland State University, (Jun 1987)
Berri, A. Dulcy, “Geology and Hydrothermal Alteration, Glass Buttes, Southeast Oregon”, Department of Geology, Portland State University, (May 1985)
Romane, Garret, Gem Trails of Oregon, Gem Guides Book Co., Baldwin Park, 2008
 |
 |
 |
 |
 |
