Two factors that determine the viscosity of lava

Viscosity is the ability of a substance (in this case, lava) to resist flow. First of all, the temperature of the lava affects the viscosity. Lava with a higher temperature has lower viscosity than cooler lava (higher viscosity) since cooler lava is starting to solidify thereby increases its resistance to flow. The composition of the lava is also a determinant of viscosity such as the silicon content of the lava. The higher the silicon content of the lava, the higher the internal friction of the lava, thereby higher resistance to flow. 

Argon, carbon-14, and carbon-12

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