Which term matches the definition: fast moving avalanches of hot gas, ash, and rock?

• A. Pyroclastic Flow
• B. Ash Fall
• C. Lava Flow
• D. Extinct

Answer: (A)

This item tests understanding of a fast, deadly volcanic hazard that travels as a hot, gas-rich mixture hugging the ground. Pyroclastic flows are gravity-driven currents produced by explosive eruptions, carrying hot volcanic gases, ash, pumice, and rock fragments that race downslope. They are much more dangerous and destructive than ordinary falling ash or slow lava flows because of their extreme speed, heat, and the way they surge along the terrain. Ash fall, by contrast, involves fine tephra settling from the atmosphere and does not move as a ground-hugging, high-speed surge. Lava flow is molten rock that moves downslope but lacks the hot gas-dense mixture that characterizes a pyroclastic flow. Extinct refers to a volcano’s status, not a type of flow.