什么是 Pyroclastic density currents？

A pyroclastic density current is a ground-hugging mixture of hot gas and pyroclasts that flows downhill or outwards because it is denser than the air. A pyroclastic flow is a concentrated current of rock fragments, ash and gas, generally at $200\text{ to }700\,{}^{\circ}\mathrm{C}$, that moves at tens of metres per second; a pyroclastic surge is a more dilute cloud of ash, rock and gas, of several hundred degrees, that moves at the speed of a hurricane wind. An ignimbrite is the rock deposited by pyroclastic density currents, which may be welded by its own heat.