Hotter Side of Obsidian

(By K. Weldon, 2010)

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).