![]() ![]() It is suggested that viscosity is roughly measured by how long it takes the treacle to flow from one end of a boiling tube to the other. The "Treacle Test" experiment is designed to enable students to investigate how temperature, volatiles and crystals may affect viscosity. A “crystal mush” will clearly have increased viscosity. Again, this applies particularly to the cooler, more viscous magmas typical of destructive plate margins. Some magmas have already begun to crystallise by the time they reach the surface. Once the value of K is known, the viscosity can be determined. where K K is the value of a liquid with known viscosity and density such as water. The viscosity is calculated with Equation 1 1. The release of gas during eruption is particularly likely to be explosive if the magma is both viscous (as gas is released, so viscosity is immediately increased) and volatile rich. The liquid is then drawn through the side 2 of the U-tube by use of suction and lastly, the flow is time between marks C and B. A high volatile content decreases viscosity (like adding water to treacle), and is probably the main factor in enabling some highly viscous (but also volatile-rich) melts to reach the surface at all. Magmas formed by melting of mantle rocks have generally low volatile contents, but those formed by partial melting of crustal rocks are often volatile-rich. Magma invariably contains small amounts of dissolved gas (water, CO 2 etc) which is released as pressure is removed. ![]() Rhyolites may reach the surface at temperatures of less than 900 oC, and so have much higher viscosity. Thus, water is thin, having a low viscosity, while vegetable oil is thick having a high viscosity. Does water have a high viscosity Viscosity describes a fluid’s internal resistance to flow and may be thought of as a measure of fluid friction. ![]() Not surprisingly, magmas formed by partial melting of mantle rocks are much hotter – well over 1200 oC for some Hawaiian basalts – than is the case for crustally derived melts. Mercury is a substance has greatest viscosity. Magma temperatures reflect the melting points of their mineral components. These magmas erupt as basalts or intrude as gabbro, and are far less viscous. Low-silica magmas are typically formed by partial melting of mantle rocks beneath mid-ccean ridges or at “hot spots” like Hawaii. The more extensive silicate chain molecules render these magmas highly viscous, so when eruption occurs it is usually explosive (e.g. Such magmas erupt as andesites and rhyolites or are intruded as granite masses. Silicate-rich magmas are typically formed at destructive plate boundaries, by partial melting and/or assimilation of crustal rocks (which are richer in silica than the rock of the mantle). The nature of volcanic eruptions is highly dependent on magma viscosity and also on dissolved gas content. ![]()
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