The Owyhee Obsidian Source

Eastern flank of Silver City Range as seen from Snake River Plain. Toy Pass on horizon at center. Brown's Creek drainage in foreground.

Almost 2000 years ago the Roman naturalist, Pliny the Elder, wrote that obsidian was named after Opsuis, a traveler who had seen it in Ethiopia. This dark. glassy rock was often used in the manufacture of tools by stone-age societies because of its predictable flaking qualities and sharpness. The edges of freshly broken obsidian are sharper than the best steel blades.

Obsidian usually occurs in rhyolite lava flows. When lava cools slowly, large crystals are developed and give the rock a grainy texture. On the other hand, rapid cooling inhibits crystal growth and results in the formation of more glassy rocks. So it has long been known that obsidian formed under conditions of  rapid cooling. However, that still failed to explain why it sometimes occurs in thick domes. The thickness of the domes should have retained enough heat to allow the growth of larger crystals as they slowly cooled, and thus formed into rocks other than obsidian. The answer was found in the fact that great quantities of water are lost  in steam when rhyolite lava are erupted from volcanos. The resulting lava are drier, which causes them to be thick and  pasty. They then form into finer grained rocks even if they cool rather slowly, because the elements cannot move freely enough within the pasty lava to gather into larger crystals. Some of the driest portions of rhyolite develop into obsidian. It is now understood that obsidian can be formed either by rapid cooling or by drier conditions in lava.

Before erupting onto the earth’s surface and producing obsidian, a large mass of molten rhyolitic magma is contained in a deep underground chamber. At this stage, its elements become thoroughly mixed, giving the rhyolite a particular chemical composition. lava from different volcanos have their own ratios of elements, making each deposit chemically distinct from others. As part of the rhyolite, the obsidian in each flow likewise has its own  chemical “fingerprint”. This fingerprint can be characterized through a process known as x-ray fluorescence analysis. 

During this procedure, obsidian is bombarded with x-rays, and certain trace elements (barium, rubidium, titanium, etc) are detected and measured even though they are present in only parts per million. Researchers are thus able to distinguish one obsidian deposit from another and trace the raw material of artifacts back to their sources. It seems remarkable that the chemical compositions of the various obsidians are so consistent that such an analysis will work: and yet, researchers routinely source artifacts by this method.

Obsidian is not stable. Having formed under unusually dry conditions, it absorbs water from its surroundings with the passing of time. This characteristic allows obsidian artifacts to be dated. Fresh surfaces are, of  course, created when an artifact is made. These services begin to absorb water molecules at a certain rate. The age of an artifact can thus be estimated by measuring the depth of water penetration. Measurements range from 1 micron for early historic artifacts to almost 30 microns for the ancient African tools which may be as much as 180,000 years old. If obsidian absorbs enough water through millions of years, it changes into a brittle piece of rock known as perlite. Because of this, little obsidian is left from the more ancient geological periods.

The Owyhee obsidian source is located in northern Owyhee County. Idaho. It is also known in literature as Toy Pass, Oreana, and Brown’s Castle. It had received some attention in past decades, but a more comprehensive study has recently been completed. The boundaries of the source area have  been further defined, and numerous obsidian specimens have been analyzed.

A new obsidian source was discovered during the course of the study. It has been named Sinker Canyon and is located in the northwestern portion of the Owyhee source. Nodules from both sources mingle together in this area. Sinker Canyon material is much less abundant than Owyhee. Owyhee and Sinker Canyon obsidian nodules in ash deposits, at the base of the rhyolites, which make up the eastern flank of the Silver City Range. These rhyolites are up to 800 ck and nave been dated at 16 million years old.

Four Owyhee ash outcrops have been found in the vicinity of Toy Pass near the eastern end of the rhyolites. It is very likely that more outcrops occur west of the pass. but that area is rather inaccessible and has not been thoroughly explored. The ash is lightly consolidated in some places, and obsidian can easily be removed with a pick and shovel. It was welded into solid rock at other location, and a bar and hammer are needed to obtain the nodules. In addition to obsidian, the ash contains blocks of rhyolite and zones of perlite. Remnants of obsidian are often found within the perlitic zones. Owyhee perlite is usually a silvery gray color. 

Only one Sinker Canyon ash outcrop has been discovered. Although Sinker Canyon surface nodules are of good quality, the obsidian in the ash deposit is well on its way to becoming perlite and is not knappable.

Owyhee obsidian is mostly black, translucent, and banded. A small percentage grades into shades of gray. Much of the world’s obsidian has the same appearance, and it is usually impossible to identify where a piece of obsidian originated just by looking at it. However, some researchers are able to visually identify some colors from certain sources.

Owyhee obsidian nodules in volcanic ash. Note silvery-gray perlite about obsidian

The darker Owyhee gray obsidian is an off-black, charcoal color and is sufficiently distinctive that it could probably be visually sourced. Surface nodules in the vicinity of the ash deposits sometimes have a thin coating of perlite adhering to their surfaces and look as though they had been sprayed with silver paint. This coating has weathered off the nodules, which are miles away from the ash. 

Also, the nodules in the ash have smooth surfaces, but those which are miles away have rough, textured surfaces from the effects of weathering. The majority of Owyhee obsidian is of high quality. It is very glassy and has excellent knapping characteristics. However, some has an uneven, granular composition and does not knap as well.