Obsidian

OBSIDIAN IS HOT STUFF

By Jim Miller, B.Sc., M.Sc. Geology*

 

Obsidian...even the name is exotic. Ever since I had my first rock collection as a child, I've loved obsidian. Sharp and shiny, obsidian is so different from other rocks. But until a few years ago when I made my first obsidian collecting trip to Glass Buttes, Oregon, I thought obsidian was pretty much just black glass. That amazing trip really opened my eyes. The ancient volcanic hills called Glass Buttes hold a dazzling variety of gem-quality obsidian, including: mahogany, red, flame, midnight lace, jet black, pumpkin, brown, rainbow, gold sheen, silver sheen, green, lizard skin, snowflake and more. My goal in this article is to increase your awareness of some of the more fascinating aspects of this incredible stone.

 

 








Obsidian is natural glass that was originally molten magma associated with a volcano. This volcanic glass has an almost total absence of sizable mineral crystals within the glass matrix. When I say "crystals," don't visualize those beautiful pointed prisms of quartz found in geodes. All rocks consist of mixtures of various crystalline minerals. When crystallization occurs, the atoms that comprise a mineral become arranged in regular, geometric patterns that are unique to the specific mineral. Crystal faces form only where there is enough open space in the rock mass to allow the natural geometric forms of the crystals to develop as free faces. Granite is composed entirely of intergrown crystals of quartz, feldspar, mica and other minerals. These relatively large mineral crystals (easily visible to the naked eye) give granite a rough fracture surface.

 

Like all glass and some other types of naturally occurring rocks, obsidian breaks with a characteristic "conchoidal" fracture. This smooth, curved type of fracture surface occurs because of the near-absence of mineral crystals in the glass. The intersections of conchoidal fracture surfaces can be sharper than a razor. This had obvious advantages for our Stone Age ancestors, who used obsidian extensively for tool making.

 

Obsidian consists of about 70 percent or more non-crystallized silica (silicon dioxide). It is chemically similar to granite and rhyolite, which also were originally molten. Because obsidian is not comprised of mineral crystals, technically obsidian is not a true "rock." It is really a congealed liquid with minor amounts of microscopic mineral crystals and rock impurities. Obsidian is relatively soft with a typical hardness of 5 to 5.5 on the mineral hardness scale. In comparison, quartz (crystallized silicon dioxide) has a hardness of 7.0.

 

Obsidian occurs only where geologic processes create volcanoes and where the chemical composition of the magma is rich in silica. Obsidian-bearing volcanoes are typically located in or near areas of crustal instability or mountain building. In North America, obsidian is found only in localized areas of the West, where the processes of plate tectonics have created geologic conditions favorable to volcanism and the formation of obsidian. Obsidian typically forms near the end of a volcanic cycle and is often associated with domes of volcanic rock, such as the hills of Glass Buttes, Oregon.

 

If obsidian is similar in composition to granite and rhyolite, both of which were originally molten, then why is obsidian glassy? The answer relates to the original cooling rate and water content of the magma. Granite cools very slowly miles below the surface of the earth; this slow cooling over millions of years allows for the formation of sizable mineral crystals within the slowly cooling mass of molten rock. Rhyolite typically cools more rapidly near the earth's surface and contains smaller mineral crystals than granite. When rhyolite magma approaches the earth's surface and the pressure of burial decreases, most of the water in the magma is lost as steam. The resulting silica-rich magma with little remaining water becomes very viscous (thick and pasty) obsidian magma. This magma is so viscous that sizable mineral crystals cannot grow before chilling of the magma "freezes" crystal development.

 

Some obsidian is erupted as lava flows at the ground surface. These surface flows are so viscous that they flow very slowly. One article I read indicated that "an ant could probably outrun an obsidian lava flow." An excellent example of a relatively recent obsidian flow can be found at Paulina Lake (part of the Newberry Volcano), approximately 30 miles southeast of Bend, Oregon. Portions of this obsidian flow are mixed with layers of pumice, a glassy, bubble-rich, lightweight rock that develops when water vapor (steam) escapes rapidly from the molten glass at or near the ground surface.

 

Sometimes obsidian of excellent quality develops as surface lava flows. However, the best quality obsidian often forms below the ground surface around volcanic vents. Silica-rich magma squeezes into rock fractures to form layers and lenses of obsidian that are relatively free of dirt, ash and other impurities.

 




Changes in magma composition and water content often occur during the eruption and subsurface emplacement of obsidian flows. The high viscosity of the molten obsidian prevents effective mixing of these magmas, resulting in obsidian that is "streaked" with different layers or colors. Each of these streaks or lines may represent a distinct pulse of an obsidian eruption. You can visualize the process that results in streaked obsidian if you consider two blobs of green and red taffy (a viscous candy) that are mixed together. Distinct streaks of red and green taffy result as the blobs are mixed. In the case of obsidian, the slow flow of stiff, viscous magma away from the source vent provides the mixing needed to create the layered or streaked varieties of obsidian. The "midnight lace" variety of obsidian often has incredibly contorted streaking, apparently formed as the obsidian layers were stretched and rolled with slow movement of the magma.

 

The various colors of obsidian are a result of several factors. Clear varieties of obsidian contain very few opaque impurities or microscopic mineral crystals. Red or brown obsidian generally results from tiny crystals or inclusions of hematite or limonite (iron oxide). Abundant, microscopic crystals of minerals like magnetite, hornblende, pyroxene, plagioclase and biotite, combined with tiny fragments of rock, likely produce the jet-black varieties of obsidian. Microscopic crystals of various types of feldspars may yield the unique blue, green, purple or bronze colors associated with rainbow obsidian. The reflectance of rainbow obsidian is likely attributed to a preferred orientation of microscopic crystals of feldspar or mica oriented along flow layers.

 

A certain amount of water always is present in obsidian. Very small inclusions of water vapor in the form of bubbles often are trapped in the glass. Tiny gas bubbles that have been stretched nearly flat along the flow layers in obsidian generally cause the reflectance of gold sheen and silver sheen obsidian. Some of these bubbles are visible to the naked eye. The bubbles can be seen readily with a strong magnifying glass or a microscope.

 

Obsidian is relatively unstable from a geologic perspective. It is rare to find obsidian older than about 20 million years, which is very youthful in comparison to most continental rocks that form the Earth's crust. Over a long period of time, obsidian gradually changes from glass to rock in a process known as "devitrification." In this process, the silica molecules within the glass slowly rearrange into organized crystal patterns. The "snowflakes" in snowflake obsidian are quartz crystals that have formed through devitrification of the original obsidian. The crystals that develop through devitrification cause obsidian to lose its conchoidal fracture and glassy texture.

 

Native Americans discovered almost all of the obsidian locations in North America. Each obsidian source area has a unique assemblage of trace elements, allowing identification of the original source locality for the obsidian used in prehistoric artifacts. Trace element analyses have shown that native peoples traded this valuable commodity many hundreds of miles from the volcanic source areas along numerous trade routes. The fact that obsidian was transported great distances attests to the mystique of this unique material.

 

My favorite place to collect obsidian is Glass Buttes in central Oregon. A trip to Glass Buttes is well worth the effort for the natural scenery, abundant and beautiful gem-quality obsidian, and exploration of a fascinating geological area. These sagebrush- and juniper-covered hills are remote but accessible by automobile. Located in central Oregon south of Highway 20 and about 80 miles east of Bend, a visitor will be rewarded with many different varieties of obsidian through exploration of the area. Fist-sized pieces are abundant and can be collected with no digging. For large pieces of high quality obsidian (ranging up to several hundred pounds), considerable effort with a shovel and pry bar are necessary. Further information on Glass Buttes can be found in several rockhound guidebooks that include the central Oregon area.




 

 

* Jim Miller is a practicing geologist and an avid flint knapper residing in Bothell, Washington. Examples of obsidian from Glass Buttes and stone tools made from those materials by the author are included with this article.




Several varieties of obsidian from Glass Buttes, Oregon and tools made by the author from those materials. From left to right across the top, the obsidian varieties are silver sheen, midnight lace, pumpkin and red mahogany.





Source material and stone tools of red mahogany obsidian.





Source material and stone tools of red mahogany obsidian. Pencil gives scale.





Source material and stone tool of silver sheen obsidian.





Source material and stone tools of pumpkin obsidian. Pencil gives scale.





Multi-colored streaks oriented along flow lines in midnight lace obsidian.





Contorted streaks within midnight lace obsidian.

 

Hotter Side of Obsidian

Formation of That Black Glass Obsidian

 Volcanic glasses such as obsidian form when some physical property of lava restricts ion mobility preventing an ordered crystalline pattern to develop, and for obsidian it is the viscosity that has the greatest control on the ordered crystalline pattern, the measure of viscosity is dependent on the temperature, crystal content and chemical composition . Viscosity is a measure on the ability of substance to flow, high viscosity means poor ability to flow and low viscosity means good ability to flow, an example of magma with a low viscosity is basalt and magma with high viscosity is rhyolite.  

For obsidian to form, magma is trapped below the eutectic, point of crystallization, by loss of heat. Therefore leaving a magma that is unable to crystallize will form (glass) obsidian. For this process to occur during a lava flow the lava is caught just below crystallization temperature, thus forming a glass due to the inability to form a crystalline solid. The formation of obsidian could also be the melt, liquid remaining from a magma after crystallization, of a rhyolite magma that has been erupted before any crystals can form as stated earlier. The gas content of obsidian is very low so for this to occur the gas has to be released in some way before the eruption of the obsidian.

Obsidian occurs as a flow, not as an explosive eruption in contrast to a vesiculated rhoylite pumice or dacite. This difference is due to the difference in composition, specifically volatile, gas, content.  Volatiles within highly viscous magmas can produce eruptive events due to the inability for the volatiles to escape easily so as they rupture, burst, they release an enormous amount of pressure producing an eruption such as the Mount. St. Helens eruption on May 18th 1980. For these explosive types of eruptions not occur in the emplacement of obsidian the volatile content for the obsidian must be low. The average water content of obsidian is (0.3 wt %) where as crystalline rhyolite is <2.0 wt % water (Bakken Barbara., 1977). The low amount of water in comparison with rhyolite pumice indicates that the flow of obsidian must take place at the end stage of the explosive eruption phase of rhyolite magma (Bakken Barbara., 1977) end stage indicating after the vent of the volcano has released a large amount of gas through the explosive eruption stage. When it comes down to it the temperature ot the obsidian magma is the may property controling the viscosity over any of the other properties.

The viscosity of obsidian must be lower than rhyolite so it can flow; difference in eruption temperature is the greatest control over this difference. Initially the magma erupts at a temperature around 900 degrees C however this first eruption is rich in volatiles producing pumice, so this stage still has a greater viscosity than obsidian. As volatile as pumice is released and the obsidian melt from the rhyolite is built up at a temperature around 900-700 degrees C is than released as the obsidian at a low viscosity of a magnitude of 10^8 Pa s, which is an order of 3 magnitudes less than that of the original rhyolite pumice eruption, this therefore indicates that the eruption of the obsidian has a similar temperature of eruption as the initial pumice only difference is in the magma's gas content. After obsidian has been erupted what to follow is typically a rhyolite dome rich in crystals due to slow cooling of the magma; this event makes it clear that the reason for lack of crystals in the obsidian are due to being erupted at a temperature greater than the final rhyolite magma. This information comes to the conclusion that the flow of obsidian is dependent on the time for the obsidian magma melt to cool, and that amount of time for cooling is very short because if allowed to cool to a temperature lower than initial temp it will not flow.

  Bibliography:

  • Bakken, B., (1977), Obsidian and Its Formation. North West Geology. 6-2, (88-92)

Obsidian and its Groovy Flow Emplacement

 A model for Obsidian Flow emplacement

As a magma body of rhyolite composition approaches the surface volatiles exsolve, separate, from the magma and rise to the plumes top. The decrease in the confining pressure leads to frothing of the volatile rich top section of the plume as ascension of the stratified mass occurs. This process of vesiculation leads to some number of explosive eruptions depositing tephra. After these eruptions the remaining volatile rich magma reaches the surface as highly inflated pumiceous lava. The transition from the explosive eruption to the pumiceous lava occurs once gas pressure in vesicles can no longer exceed  strength of magma.

Later in the eruption following the pumiceous lava flow the event of the emplacement of obsidian occurs, this process happens with the bubble free obsidian flowing out over the earlier emplaced coarsely vesicular pumice (highly inflated pumiceous lava). The Obsidian is slightly less viscous but more dense than the coarse pumice.  There remains a gradual transition from the coarse pumice to obisidian due to volatile decrease with the progression from pumice phase to obsidian phase. Essentially while obsidian is erupted vesicular material continues to be extruded.

As the obsidian rides over the coarse pumice, volatiles continue to evolve from the cooling upper flow surface of the obsidian flow. This process of the volatiles exsolving from the obsidian flow forms a finely vesicular layer that keeps the flow interior of the obsidian insulated and warm; this contact is also gradational, just as the coarse pumice to the obsidian was.

Within the volcanic vent that the obsidian flowed from the remaining magma from within cools and crystallizes and then erupts as a crystal rich rhyolite flow capping the vent mouth as a rhyolite dome.

The Big Obsidian Flow of Newberry Volcano, Central Oregon is very similar in the process of emplacement. Firstly the eruption began with explosive eruptions depositing pumice and lava blacks of 1 meter (3 feet) in diameter, after these eruptions pyroclastic flow occurred essentially taking the place of the pumiceous lava flow, however there remain deposits of course pumice within the region of Big Obsidian Flow which many indicate a short event of pumiceous flow.  The final stage was the emplacement of Big Obsidian Flow that moved very slowly and according to Sherrod et al (1997) this lava flow probably moved a few meters to tens of meters a day. From observations in the field of the Big Obsidian Flow the presence of a thin visculated layer coats some of the surface of the obsidian flow. The final stage of the emplacement of a rhyolite dome is not present the vent is actually plugged by the obsidian flow indicating that the rhyolite stage was not present in the eruption or possibly the rhyolite magma was already to cool and harden to come to the surface of the flow. 

CVO Big Obsidian flow; Excerpt from: Sherrod, et.al., 1999

This is an image of the Big Obsidian Flow of Newberry Volcano,a USGS Photograph taken in October 1987 by Willie Scott. The structure of the deposit indicates that the obsidian flowed over being erupted explosively.

Fink (1983) provided most of the information regarding the flow emplacement model of obsidian as well as the figures displayed.

 

 

 

steps to obsidian flows

Obsidian occurs as a flow, not as an explosive eruption in contrast to a vesiculated rhoylite pumice or dacite. This difference is due to the difference in composition, specifically gas content.  Gases within highly viscous magmas can produce eruptive events due to the inability for the volatiles to escape easily from the magma so as they rupture they release an enormous amount of pressure producing an eruption such as the Mount. St. Helens eruption on May 18th 1980. For these explosive types of eruptions not occur in the emplacement of obsidian the gas content of the obsidian must be low. The average water content of obsidian is (0.3 wt %) where as crystalline rhyolite is <2.0 wt % water (Bakken Barbara., 1977). The low amount of water in comparison with rhyolite pumice indicates that the flow of obsidian must take place at the end stage of the explosive eruption phase of rhyolite magma (Bakken Barbara., 1977) end stage indicating after the vent of the volcano has released a large amount of gas through the explosive eruption stage. When it comes down to it the temperature ot the obsidian magma is the  property controling the viscosity over any of the other properties, higher the temperature the lower the viscosity. 

The viscosity of obsidian must be lower than rhyolite so it can flow; difference in eruption temperature is the greatest control over this difference. Initially the magma erupts at a temperature around 900 degrees C however this first eruption is rich in volatiles producing pumice, so this stage still has a greater viscosity than obsidian. As volatile rich pumice is released and the obsidian melt from the rhyolite is built up at a temperature around 900-700 degrees C the obsidian melt can be erupted at a low viscosity of a magnitude of 10^8 Pa s, which is an order of 3 magnitudes less than that of the original rhyolite pumice eruption, this therefore indicates that the eruption of the obsidian has a similar temperature of eruption as the initial pumice only difference is in the magma's gas content. After obsidian has been erupted what to follow is typically a rhyolite dome rich in crystals due to slow cooling of the magma; this event makes it clear that the reason for lack of crystals in the obsidian are due to being erupted at a temperature greater than the final rhyolite magma. This information comes to the conclusion that the flow of obsidian is dependent on the time the obsidian is erupted during the eruption, the temperature of the obsidian and the gas content. 

Bibliography:

  • Fink, J.H., (1983), Structure and emplacement of a Rhyolite Obsidian Flow: Little Glass Mountain, Medicine Lake Highland, Northern California. Geological Society of America Bulletin. 94 (362-380)
  • Sherrod., Mastin., Scott., Schilling. (1997). Newberry caldera, Oregon Big Obsidian Flow - Latest Eruptive Event Excerpt from: Sherrod, Mastin, Scott, and Schilling, 1997, Volcano Hazards at Newberry Volcano, Oregon. U.S. Geological Survey Open-File Report, 97(513)

The Fly Style of Obsidian

Flow Textures and Features

Obsidian flows usually consist of fold surfaces, explosion craters, flow banding, and cavities underling the craters.

The explosion craters are commonly located on the surface of the flow and can have diameters of 10-25m (36-91ft), at 5-15m (15-55ft) deep; the accompanying cavities can be 5-15m in length. The cavities form near the surface by a buckling mechanism. Buckling occurs by contrast of stiff upper layer relative to the near liquid flow interior, which forces the upper layer to make an almost triangular shape at the surface, separating it from the lower layer. These cavities can collect magmatic/meteoric water*, and as vapor pressure builds up and exceeds the strength of the surface crust of cavity an explosion results producing the explosion caters.

*Meteoric water: rain water/ground water, not from the magma.

buckling

Image depics the event of cavities forming by buckling and exploding due to gas pressure exceeding the strength of the rock. This illustration comes from Castro's  Structural origin of large gas cavities in the Big Obsidian Flow, Newberry Volcano.

Flow banding in obsidian is a distinctive characteristic and the abundance of the flow banding. Even with the abundance of these flow bands within obsidian little is known about the origins of these features, but what is known is that these freatures derive from both crystallization and deformation processes. According to a paper by J.M. Castro the flow bands develope from the deformation of microlites (very small phenocryst) by shear strains during the flow of the obsidian. The picture below shows the flow banding as pointed out by the arrow.

Bibliography

  • Castro, J., Cashman, K., Jaslin, N., Olmsted, B., (2002) Structural origin of large gas cavities in the Big Obsidian Flow, Newberry Volcano. Journal of Volcanology and Geothermal Research. 114 (3-4), (313-330) 
  • Castro, J. (2005). New insignts on the origin of flow bands in obsidian. Special Paper - Geological Society of America, 39655-65. doi:10.1130/2005.2396(05).
  • Water Plus Obsidian

    Hydration of Obsidian

    After an obsidian flow has been emplaced it is subject to the atmosphere that causes weathering to the obsidian. One specific type of weathering done on to obsidian is called hydration, which occurs by the water within the atmosphere being absorbed by the obsidian thus increasing the water content within the rock. The product of a hydrated obsidian is called devitrified obsidian, (de- to remove) (vitrify- glassy), so essentially the removal of the glassy property of the obsidian. The products of devitrified obsidian can produce secondary fiberous mineral crystals that can form ball like shapes called spherulites that are embedded within the obsidian, these secondary minerlas can develope into what is called snowflake obsidian as well, which makes the obsidian look as if it is decorated with snowflackes due to the snowflake shapes the secondary minerals produce. The snowflake obsidian can be used as jewlery and many rockhounds search for it, Yellowstone caldera is home to snowflake obsidian. 

     

     

    snowflake obsidian

     This is an image of snowflake obsidian, as you can see the seconday minerals group together forming snowflake like shapes scattered across the rock surface. This image is from  Rockhoundblog.com.

    spherulites

       This is an image of what a spherulite looks like due to secondary mineralization from devitrification of obsidian.
    Photo by (c) 2008 Andrew Alden, licensed to About.com

     

    New Research concerning Hydration of Obsidian

     Some researchers have used the hydration rate of obsidian to acquire specific dates of the obsidian, which is based on the idea that a freshly broken obsidian surface begins to absorb water from its environment almost immediately. Absorption continues with time, generating a hydrated layer whose thickness is proportional to the time the glass surface was exposed  this according to Anovitz et al (2006). This technique of dating can be unreliable due to the many factors influencing rate of hydration. Just recently Anovitz et al discovered that the natural hydration of obsidian can be used as a tool to provide paleoclimatic reconstructions. Anovitz et al (2006) article in GSA’s Geology Journal, “Obsidian hydration: A new paleothermometer,” the first successful application of this idea was done on samples from the the Chalco site in the Basin of Mexico successfully obtaining the temperature change within the region of study, for further information refer to the Anovitz (2006) article.

    Bibliography:

    • Anovitz, L. (2006). Obsidian hydration; a new paleothermometer. Geology [Boulder], 34(7), 517-520. doi:10.1130/G22326.1.

    • Francis, Peter., and Oppenheimer, Clive. (2004), Volcanoes, Second Edition. New York: Oxford University Press Inc., New York: (162-164).