Final answer:
Surges and pyroclastic flows are differentiated by their density, with surges being less dense but still capable of high velocities, often due to a higher concentration of gases like steam.
Step-by-step explanation:
The primary difference between surges and pyroclastic flows lies in their density. Surges are less dense than pyroclastic flows. Pyroclastic flows consist of a high-density mix of hot lava blocks, pumice, ash, and volcanic gas. They move at high speeds down volcanic slopes, driven by the force of the erupted materials together with the gravitational pull.
On the other hand, surges are low-density, fast-moving flows, which are typically richer in gas content, especially steam, and can be generated when the eruptive material interacts violently with water, among other mechanisms. This lower density does not necessarily mean they are slower; in fact, surges can travel at high velocities, similar to pyroclastic flows, but they are not as laden with pyroclastic material.