Explosive volcanic eruptions are known to dramatically impact life on Earth and makes the understanding of volcanic phenomena important for society today. The style of volcanic eruptions of silicic magmas ranging from slow effusive outpourings of lava to violent explosive Plinian eruptions is explored in this thesis. During magma rise to the Earth's surface bubble networks can form inducing gas escape from the magma. When this process of magma outgassing is efficient, eruptions lose their energy and may effuse as lava flows or as domes. When exsolved gas cannot escape easily, the magma foam may fragment and the flow style abruptly shifts to a turbulent gas flow carrying magma fragments. Magma outgassing and fragmentation govern the effusive-explosive transition and are investigated by means of studying the pumices formed by the Kos Plateau Tuff eruption in combination with computational fluid dynamics of conduit flow.