It can be absorbed by the ash particles, which later fall to the ground, poisoning livestock living on ash-coated grass and contaminating water supplies. Carbon dioxide, which is heavier than air, becomes collects and becomes concentrated in depressions where it can cause suffocation of people and animals. A pyroclastic flow is a hot, chaotic avalanche of pumice, ash and gasses. They result from the collapse of the vertical eruption column of tephra. Their distribution is strongly controlled by topography, they tend to follow valleys and other depressions.
Pyroclastic flows are extremely hazardous due to their high speeds and temperatures.
Objects and structures in their path generally get destroyed or swept away by the impact of debris. Click here to see a picture of a pyroclastic flow from the eruption of Mt. St. Helens in 1980. Pyroclastic Surge Pyroclastic surges are turbulent, low-density clouds of volcanic ash and hot gases, which hug the ground as they travel at speeds often exceeding 100km per hour. Pyroclastic surges are more energetic and dilute than pyroclastic flows, they are one of the major hazards for people living at or near the base of the volcano. The high temperatures of pyroclastic surges can start fires and kill or burn people and animals.