What is the single, main characteristic that determines whether or not a plate will subduct?

In what ways did the compromise of 1850 and the kansas-nebraska act of 1854 differ from the missouri compromise? both compromises used the established 36°30′ parallel to determine if territories would become free or slave states. both compromises used the mason-dixon line to determine if the territories would become free or slave states. both compromises let territories use popular sovereignty to determine if they would become free or slave states. both compromises only granted congress the power to determine if territories would become free or slave states.