Lava flows and lava domes form from molten rock that erupts and cools on or near the Earth’s surface. A lava flow is an outpouring of fluid, relatively low-viscosity molten rock, whereas a lava dome is a pile of relatively viscous lava that cannot flow far from the vent (cf. Dietterich et al., 2025; Harnett et al., 2025), the collapse of which may be hazardous.
Tephra is a collective term for volcanic fragments (pyroclasts) generated by the fragmentation of fresh magma and old (i.e., pre- existing) rocks ejected into the atmosphere during an explosive eruption, irrespective of size, composition and shape. The term 'volcanic ash' refers to the finest particles of tephra (<2 mm in diameter)(cf. Pistolesi et al. 2025).
Pyroclastic density currents are hot, fast-moving mixtures of volcanic particles and gas that flow according to their density relative to the surrounding medium and the Earth's gravity. They typically originate from the gravitational collapse of explosive eruption columns, lava domes or lava-flow fronts, and explosive lateral blasts (cf. Charbonnier et al., 2025).
Lahars are discrete, rapid, gravity-driven, water-saturated flows containing water and solid particles of volcanic rock, sediment, ice, wood, and other debris that originate at volcanoes (Gudmundsson, 2015; Vallance and Iverson, 2015).
Volcanic gas includes any gas-phase substance that is emitted by volcanic or volcanic-geothermal activity. Volcanic aerosols include liquid or solid particles that are small enough to be suspended in the air, and that are emitted by volcanic or volcanic-geothermal activity (adapted from Baxter and Horwell, 2015, Fischer and Chiodini, 2015, and Williams-Jones and Rymer ,2015).