volcano innate power question

explosive eruption quesion.

if i want reach last effect of explosive eruption, i destroy 10 presence at once?

if i destroy 4 presence at second turn, and third turn i destroy 6 presence. then i can reach last effect at third turn?

Unfortunately, no.

The voluntary presence destruction is part of using Explosive Eruption, and the thresholds only care about the amount of presence destroyed on that activation of the power.