one of the reasons why Plinian eruptions are more violent is that they release more viscous magma than Hawaiian hawaiian eruption. the thicker the magma the more violently it is released on the ground surface. Hawaiian eruption on the other hand release thin magma that flows peacefully but fast. the other reason hawaiian eruptions are less violent is the presence of many side fissures where some magma escapes through in addition to the main vent. these fissures help to reduce pressure and therefore magma flows peacefully. in plinian eruptions magma is mainly released from the main vent.