Whose work first explained why volcanoes can erupt explosively instead of quietly?

Explosive eruptions happen when magma contains lots of dissolved gases and is too viscous to let them escape easily. In the 20th century, volcanologists such as George P. L. Walker helped classify eruption styles and understand the physical conditions behind them. Their work showed why basaltic lava flows can be relatively calm, while silica-rich magma can produce violent explosions, ash columns, and pyroclastic flows. This distinction is crucial for hazard forecasting. It also explains why different volcanoes pose very different risks.