There should not be any changes in the gene pool.
In Hardy-Weinberg equilibrium, the gene pool should have a constant rate. The equilibrium can be achieved if there is no factor that favors one gene more than the others. Since the genes have the equal chance to survive and mating, the ratio of the gene pool should not change dramatically.
That is a subduction or convergent boundary that can form volcanoes when two plates collide against each other.
I believe the correct answer is B. Detrital