Eventually, the pressure of the collected gases rises so high that they blow through the magma like a cork out of a champagne bottle. The result is an explosion of gas, ash, and fiery fragments of volcanic rock. Most of the damage in stratovolcano eruptions comes not from lava flow but from a phenomenon known as pyroclastic flow. A pyroclastic flow is an avalanche of ground-hugging hot rock accompanied by a cloud of ash and gas that races down the slope of a volcano. The flow can reach speeds of up to 60 miles per hour, and temperatures of nearly 1,300 degrees Fahrenheit.
Pyroclastic flows cause more death and destruction than any other volcanic hazard.
Pyroclastic flow and lava aren't the only hazards created by volcanic eruptions. Ash clouds are a particular problem for aircraft. They can cause engine failure, damage electrical systems, scratch the outer surface of a plane, and contaminate its interior. The effects of a volcanic eruption can also be felt over the long term. The sulfur mixes with water vapor in the atmosphere to form clouds of sulfuric acid.