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Why is ignimbrite associated with only more felsic magmas?

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Felsic magmas contain much more silica (SiO2) than mafic magmas (almost by definition). The excess silica chains cause the magma to be much more viscous and therefore more prone to explosive eruptions (they trap gas and require high pressures to flow). This is why felsic magmas produce crazy explosive pyroclastic flows (forming ignimbrite) and very mafic magmas (like in Hawaii) just flow nicely downhill.

Btw, water decreases the viscosity of magmas, but the amount depends on composition as well as temperature. There is a nice transition between felsic magmas and mafic magmas showing that decrease in viscosity. Check out the source below:

Whittington et al (2009): "The viscosity of hydrous dacitic liquids: implications for the rheology of evolving silicic magmas"

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Explosive volcanism is ALWAYS associated felsic magmatism, only because of it's higher water content, whereas mafic magmas are much "drier".

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