A pyroclastic surge contains hot ash and gases, but not the large fragments of rock seen in a pyroclastic flow. Surges are usually secondary phenomena, generated around the margins of a pyroclastic flow. They can be more dangerous as they are not restricted by topography such as valleys and can be carried by the wind. In practice, it can be difficult to distinguish between them, and scientists often prefer to use the term "Pyroclastic Density Current" to describe both.

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