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. 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 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. The average water content of obsidian is (0.3 wt %) where as crystalline rhyolite is <2.0 wt % water (Bakken Barbara., 1977). For these explosive types of eruptions not occur in the emplacement of obsidian the volatile content for the obsidian must be low. 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. This difference is due to the difference in composition, specifically volatile, gas, content. Obsidian occurs as a flow, not as an explosive eruption in contrast to a vesiculated rhoylite pumice or dacite. 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. 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. 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. Therefore leaving a magma that is unable to crystallize will form (glass) obsidian.
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.įor obsidian to form, magma is trapped below the eutectic, point of crystallization, by loss of heat. This upward movement is why the caldera is called resurgent, which means “risen again.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. Over time, the refilling of the magma chamber pushes up the caldera floor.
#Volcano viscosity definition series#
These broad, vast calderas result when very large magma chambers empty quite forcefully, causing a series of pyroclastic flows. The Kilauea caldera on Kilauea, one of the volcanoes that make up Hawai’i, is one example.Īnother type of caldera is a resurgent caldera. Over time, this creates a series of nested depressions. Lava flows from shield volcanoes more slowly and often at regular intervals. This often occurs with shield volcanoes, which are typically flatter and more gradually sloped. One of these eruptions was so large it formed an island in Crater Lake named Wizard Island.Ĭalderas such as Crater Lake and those in Yellowstone National Park result from dramatic eruptions, but slower eruptions can also create calderas. For several thousand years after this eruption, smaller volcanic eruptions continued inside the caldera. This caldera formed about 7,000 years ago when a stratovolcano, Mt. A famous example is Crater Lake, in Oregon.
Some calderas form a lake as the bowl-shaped depression fills with water. Calderas vary in size from one to 100 kilometers (0.62 to 62 miles) in diameter. As a result, the sides and top of the volcano collapse inward. When the magma chamber empties, the support that the magma had provided inside the chamber disappears. During a volcanic eruption, magma present in the magma chamber underneath the volcano is expelled, often forcefully. A caldera is a large depression formed when a volcano erupts and collapses.
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