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Physical Properties of Silica Melts - Coggle Diagram
Physical Properties of Silica Melts
Temperature
Typical melt temperatures.
Basalt 1000-1200°C
Andesite 950-1200°C
Dacite 800-1100°C
Rhyolite 700-900°C
Temperature will affect melt density. hot liquids are far more buoyant than cold solids, allowing magma to travel up through rocks. This density will decrease with increasing silica content.
Viscosity
Viscosity is the measure of stress to strain rate.
Silicate melts are Bingham fluids i.e. they only flow when a force is applied at the critical yield strength (the point at which they can no longer receive any stress without deformation).
Viscosity is measured in Pa/s
An increased crystal or phenocryst content of a magma makes the magma more 'solid' and so increases it's viscosity.
Gas Content
Some important volatiles include: H2O, CO2 , SO2 , S2 , H2S, Cl2 , F2 , HCl, HF, N2.
Once lava's are cooled they often contain vesicles- pockets of empty space where the lava's used to contain gas. Lava's that contain lots of vesicles will have had a higher volatile content and therefore erupted explosively.
In basalts there is a minor impact of bubble content on the magma and viscosity is only reduced slightly. In rhyolitic lavas bubbles will increase the viscosity of lava.
Pumice= a highly vesiculated lava with an acidic composition (and light colour). Scoria= a highly vesiculated lava with a basic composition (and dark colour).
Composition
Magma's are silicate minerals based on linked tetrahedra, this polymer chain in retained in liquid form. The lengths of these chains increase with decreasing temperatures but also depend on mineral composition.
Classifications based on Silica content.
Ultrabasic: <46% Silica Weight
Basic: 46-52% Silica Weight
Intermediate: 52-66% Silica Weight
Acid: >66% Silica Weight
Composition affects viscosity: Si, Al and K contribute to high viscosities where as Na, Ca, Fe and Mg contribute to low viscosities.