The transition from effusive, low mass flow rate to explosive, high mass flow rate eruptive behavior is a common aspect of the activity of calc- alkaline volcanoes. However, the process driving the shift between the two eruptive styles is at present debatable and represents a topical theme in the volcanological literature. The main challenge is to understand the mechanism that allows a high mass flow rate when eruptions of highly porphyritic (≥40 vol % crystals) and viscous magmas (>106 Pas, pascal second) occur. In this article, volcanological, compositional, and textural observations are used to demonstrate that viscous dissipation, a process that develops heating within flowing magma in a boundary layer near the conduit walls due to friction, is responsible for the long-lasting, sustained explosive phases of this eruption type, as well as for the transition from effusive to explosive behavior.