Answer: Silica content.
Magma is a fluid of molten rock, that is formed under pressure and heat inside the earth. It remains inside the erupting body, until eruption takes place. When it comes out as a result of volcanic eruption it is known as lava. Viscosity can be defined as the ability of a substance or material to resist it's flow. A magma's viscosity is directly related to it's silica content. Higher the silica content in the magma have higher viscosity than the magma with low silica content.
