Which term describes fast moving avalanches of hot gas, ash, and rock?

• A. Lava Flow
• B. Mudflows
• C. Pyroclastic Flow
• D. Quiet Eruptions

Answer: (C)

Pyroclastic flows are fast-moving avalanches of hot gas, ash, and rock that race down the sides of volcanoes. This happens during explosive eruptions when a dense mixture of volcanic fragments and high-temperature gases surges along the ground, propelled by gravity and gas pressure. The description in the question matches this exactly—the rapid movement and the combination of hot gas with ash and rock.

Lava flows are streams of molten rock that ooze or surge more slowly from a vent, not the hot, gas-rich mixture moving along the ground. Mudflows, or lahars, are heavy, water-soaked debris flows that behave like mud, not hot gas and ash. Quiet eruptions tend to be effusive, producing lava flows rather than the energetic, gas-rich flows described. So the term that best fits is pyroclastic flow.