Viscosity is an internal property of a fluid that offers resistance to flow, the nature of volcanic eruptions is highly dependent on magma viscosity and also on dissolved gas content (dissolved gases become trapped by thick magma and the magma chamber begins to build pressure).

The viscosity of magma is controlled by the size of the molecules that comprise it.

<em>Felsic lava contains elements of small size (sodium and aluminum), these elements form polymers and large crystalline macromolecules like silica, the more extensive silicate chain molecules the more viscous the magma becomes, this happens because larger molecules have stronger intermolecular bonds preventing movement and spreading. </em>

Mafic and ultramafic lava, on the other hand, have lower viscosities, also contain lower quantities of silica and aluminum and are rich in magnesium oxide, which does not form large molecular structures, therefore lava flows smoothly, unrestricted by intermolecular forces.

I hope you find this information useful and interesting! Good luck!

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3 years ago