why are felsic magmas more viscous than mafic magmas...?

Question

explain in detail

Answer 1

Basically the atoms/molecules that for a magma fall into two groups: 1) Network formers and 2) Network modifiers.

The main network former is silica (and to a lesser exctent Al 3+ and Fe 3+). The silica foms tetrahedra (comprising one silicon atom surrounded by four oxygen atoms), and these like to link up and stick to each other.

The main network modifiers are things like iron (as Fe 2+), magnesium, water, and pretty much anything else.

Basically, if there are a large number of network modifiers in the magma, it prevents the silica tetrahedra from joining up, and reduces viscosity. If there are few network modifiers the silica tetrahedra are free to join up, and makes the magma viscous.

Mafic magmas tend to have a much higher proportion of network modifiers (particularly Fe 2+ and Mg) than felsic magmas, and so usually are more viscous.

Answer 2

Felsic magmas contain more of Sulphur mixed in them and it is this sulphur that causes the High Viscosity.Also Felsic magmas contain alot of Iron and Silica.This also contributes to its high Viscosity.On the other hand Mafic magmas contain less Sulphur and more of Magnesium and Iron.Hope this helps.