How do scientists tell whether a volcano is likely to explode or flow quietly?

Whether a volcano erupts explosively or effusively depends a lot on the magma’s viscosity and gas content. Silica-rich magma is thicker and traps gases more easily, which can build pressure until it fragments violently. Basaltic magma is usually runnier, allowing gases to escape more easily and producing lava flows rather than huge explosions. Temperature, water content, and ascent speed also matter. This is why volcanoes such as Kīlauea often produce fluid lava, while volcanoes like Mount St. Helens can be highly explosive.