The Flow of Pyroclastic Materials

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  • 0:08 Proclastic Flow
  • 0:59 Dangers of a Pyroclastic Flow
  • 2:22 Volcanic Particles and Lapilli
  • 3:30 Nuees Ardentes and Ignimbrites
  • 4:36 Pyroclastic Surge
  • 5:28 Lesson Summary
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Lesson Transcript
Instructor: Rebecca Gillaspy

Dr. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.

A pyroclastic flow is the flow of volcanic ash, gases and fragments following a volcanic eruption. Learn about the types of pyroclastic flows and the materials that make them up and contribute to their fluid nature.

Pyroclastic Flow

Volcanoes produce wondrous displays of fire and ash that explode into the atmosphere from below the surface of the earth. We have all seen the spectacular pictures of red-hot lava oozing down the sides of a volcanic mountain. But, would you believe that lava flows are rarely deadly? Lava eruptions are fairly easy to predict, and lava typically flows slow enough that people have time to get out of the way.

So, what is the most dangerous flow that comes from a volcano? The answer is the pyroclastic flow, which is a dense collection of fragments and gases from a volcanic eruption that flows down the slope of a volcano. In this lesson, you will learn about pyroclastic flows that come from explosive volcanic eruptions and why they are the most dangerous and deadly phenomenon associated with a volcano.

Dangers of a Pyroclastic Flow

The term 'pyroclastic' does a good job describing this volcanic activity. The prefix 'pyro' means 'fire,' and the suffix 'clastic' means 'fragments.' When we combine 'pyroclastic' with the word 'flow,' we see that the term literally means the 'flow of fiery fragments.'

The pyroclastic flow is very dangerous in part because of the toxic gases and fiery fragments it contains. But, it's also deadly because of the speed at which it travels, which can be more than 100 miles per hour. Considering that the fastest land animal - a cheetah - runs at a top speed of 75 miles per hour, your chances of outrunning the flow, along with the cheetah's chances, are not good.

And, when the flow overtakes a living organism, the extreme temperatures and suffocating gases leave little chance of survival. The pyroclastic flow that occurred when Mount St. Helens erupted in Washington State in 1980 had temperatures of 350 degrees Celsius, and while this is so hot that anything in its path would instantly be turned to charcoal, it's not the hottest temperature recorded. In fact, the pyroclastic flow from Mount Pelee in 1902 is reported to have reached temperatures as high as 1,075 degrees Celsius!

Volcanic Particles and Lapilli

To an observer watching a pyroclastic flow from a distance, it looks similar to an avalanche of dirty snow. And, it behaves somewhat like an avalanche near the ground surface, as the biggest pieces of rock and debris are carried by gravity down the slope. But, the snowy, flowing appearance is due to the upper layer of the pyroclastic flow that is a dust storm of suspended particles and gases.

The hot, expanding gases within the pyroclastic flow suspend small fragments of ash and lapilli. Lapilli is Latin for 'little stones,' and these small volcanic pieces of rock typically range in size from 2-64 millimeters in diameter, which would be about the size of a pea for the smaller fragments, up to the size of a walnut.

The suspension of fine fragments along with the gases provides a fluid medium, which in turn carries larger fragments within the mixture. This is almost like a beach ball that is suspended above a crowd of concertgoers. The beach ball is batted by hands and stays suspended above the crowd as it moves along the arena.

Nuée Ardentes and Ignimbrites

Some pyroclastic flows are very dense. Nuée ardentes are a type of pyroclastic flow that contains dense lava fragments. A French geologist first described nuée ardentes, which is why it has such an interesting name. However, if you translate this French term into English, it means 'burning cloud.' And, this is easy to recall because at night these pyroclastic flows glow as incandescent clouds of volcanic ash, gases and blocky fragments.

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