Restate how the viscosity of magma is related to its explosivity.

Viscosity of magma depends mainly on composition, temperature, and gas ...

The arrival on the surface of magma that has lost its gas ... It is a 0-to-8 index of increasing explosivity.

Increased viscosity of magma results in increased explosivity.

Explanation:

The explosivity of magma is directly proportional to the viscosity of magma in straight terms. Viscous magma takes longer to reach the surface of the Earth and gives time for the development of more exhaust gases as more material is burnt and melted on its way upwards. This ultimately adds to the potential energy of the active magma. The increased levels of gases finally culminate in tremendous explosions.