Which factor activates a convection current, starting the flow of a fluid?

temperatures differences in the mantle

Explanation:

It's the temperature differences in the mantle. Warmer magma near the core rises because it becomes less dense. It reaches the surface, pushes against the tectonic plates, then cools and falls back down to be replaced by new warm magma.

These happens in volcanic eruption and leads to the formation of volcanic rocks.