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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.