Lava is molten rock that flows onto the Earth's surface. Lava flows move downslope away from a vent and bury or burn everything in their paths. Lava domes form when lava piles up over a vent.
Pyroclastic flows are high-speed avalanches of hot rock, gas, and ash that are formed by the collapse of lava domes or eruption columns. They can move up to 100 miles per hour and have temperatures to 1500 degrees F. They are lethal, burning, burying, or asphyxiating all in their paths.
Explosive eruptions blast lava fragments (tephra) and gas into the air. Tephra can also be carried aloft in billowing ash clouds above pyroclastic flows. Large fragments fall to the ground close to the volcano, but smaller fragments (ash) can travel hundreds to thousands of miles downwind.
Debris avalanches are rapid landslides of rock, soil and overlying vegetation, snow, or ice. They can bury or smash objects in their path. All or some portion of debris avalanches can transform into lahars. Lahars are fast-moving slurries of rock, mud, and water that move down river valleys. They can bury, move, or smash objects in their path. Lahars form when pyroclastic flows melt snow or ice, or by the transformation of debris avalanches, or by mobilization of loose debris on the flanks of volcanoes.