Why a Pyroclastic Flow is so Dangerous

Volcanic eruptions of any magnitude are an amazing spectacle and truly one of the more impressive displays of raw power occurring within nature. Throughout history, some of the worst disasters ever recorded came as the end result of volcanic eruptions. While the eruption itself is impressive, the truly impressive part is the destructive force of the pyroclastic flow that ensues.

Pyroclastic flow can best be described as an admixture of volcanic ash, pulverized rock, debris, and extremely hot toxic gases. It moves down the slope of the volcano at speeds that can approach 450 miles per hour. The temperature of a pyroclastic flow can approach 750 degrees Fahrenheit. When you consider the speed, temperature, and composition of a pyroclastic flow, it is easy to appreciate why geologists consider it the most dangerous and destructive part of a volcanic eruptive event.

The first danger to consider in a pyroclastic flow is the admixture of toxic gases. While the vast majority of the gas released is water vapor, volcanoes also release carbon dioxide (CO2), hydrogen sulfide (H2S), sulfur dioxide (SO2), and fluorine gas (F2). The hydrogen sulfide and sulfur dioxide both combine with the water vapor in the air to form poisonous acidic gases. The hydrogen sulfide combines with water to become hydrosulfuric acid. The sulfur dioxide combines with water to form sulfuric acid.  The fluorine gas has a companion acid present, hydrogen fluoride. Hydrogen fluoride is extremely corrosive and causes substantial internal burns and attacks calcium in the bones. Even if you survive initially, the gases from the pyroclastic flow can continue to wreak havoc upon your body.

The second danger to consider in a pyroclastic flow is the speed at which it is flowing down slope. When you consider that this is a combination of gases, liquids, semi-solids, and solids moving at speeds faster than any land based vehicle has ever been engineered to be capable of, you don’t want to take either a direct or indirect hit. Pyroclastic flows contain rock fragments ranging in size from ash to boulders. As it runs downslope, picking up speed, it will shatter, bury, knock over, or carry away anything it doesn’t burn down. Virtually no naturally occuring objects and certainly no man-made structures can withstand that kind of force.

The third and most obvious danger is the temperature of the pyroclastic flow. Temperatures either estimated or recorded from previous volcanic eruptive events and their attendant pyroclastic flows have reached many times the boiling point of water. Pyroclastic flows incinerate everything that they don’t bulldoze down. To make it even more dangerous, the extreme temperatures can excite the gases present into a volatile state that can cause mid-air explosions over the top of acids, liquids, semi-solids, and solids at temperatures that are multiples of the boiling point of water!