Volcanic Eruption: Mount Merapi in Indonesia

I was going to write more today about high altitude, but with the current eruption of Mount Merapi in Indonesia and resultant tragic deaths, I have been asked a lot of questions about the dangers associated with volcanic eruptions, and in particular, "what is a pyroclastic flow?"

Since recorded time, pyroclastic flows (which I will describe below) have killed the most people, while flying solid objects (called "tephra" by vulcanologists) thrown out by the volcano are the most common killers. Usually, when an explosive volcano erupts, a cloud of gas, ash, and lava fragments rises up into the atmosphere. If the eruption cloud becomes more dense than the surrounding air, it may fall back towards the ground, propelled by the forces of gravity.

This can lead to formation of an incredibly hot (more than 1500 degrees F), high-speed (faster than 180 miles per hour) avalanche of ash, volcanic gases, lava fragments and heated air, which is called a pyroclastic flow. The extreme temperatures may also generate gale-force winds. Because of the forces and temperatures involved, no living being caught in this infernal river can survive. Most human victims die from suffocation, exposure to the heat, or burial under the volcanic debris.

Anyone who travels in the vicinity of an active volcano should be aware of the risks and have a well thought out, realistic evacuation plan