A good.And, when the flow overtakes a living

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.

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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.

Nu;e Ardentes and Ignimbrites

Some pyroclastic flows are very dense. Nu;e ardentes are a type of pyroclastic flow that contains dense lava fragments.

A French geologist first described nu;e 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.Ignimbrites are a type of pyroclastic flow that contains light materials. These pyroclastic flow deposits contain glass shards and pumice.

Pumice is an extremely porous and lightweight rock that forms during a volcanic eruption. This volcanic rock contains pores because gas bubbles get trapped in the rock as the rock rapidly cools. You might even have one of these volcanic rocks in your bathroom drawer. A pumice stone is a common and effective tool to remove rough, dry skin from the heels of your feet.

Pyroclastic Surge

A pyroclastic surge is similar to a pyroclastic flow, but it’s a low-density flow of volcanic material with a higher proportion of gas to rock.

Because these flows are mostly gas, their flow is more turbulent, and they have the ability to travel up and over hills and ridges, as opposed to the more dense pyroclastic flows that are pulled down a slope by gravity. The toxic gases in pyroclastic surges are dangerous and can kill.In fact, a husband and wife team of volcano scientists, named Katia and Maurice Krafft were killed along with 41 other people by a pyroclastic surge that occurred in Japan in 1991. While filming the volcanic eruption, a pyroclastic surge unexpectedly climbed up and out of a channel and onto a ridge where the couple were standing.

Lesson Summary

Let’s review. Explosive volcanic eruptions can produce a pyroclastic flow, which is a dense collection of fragments and gases from a volcanic eruption that flows down the slope of a volcano. Pyroclastic flows are very dangerous because they contain toxic gases and fiery fragments, and they are capable of traveling at speeds of more than 100 miles per hour.

The hot, expanding gases within a pyroclastic flow suspend small fragments of ash and lapilli. Lapilli is Latin for ‘little stones.’ They range from 2-64 millimeters in diameter. The suspension of fine fragments within the gases provides a fluid medium that carries larger fragments and contributes to the fluid nature of a pyroclastic flow.Nu;e ardentes are a type of pyroclastic flow that contains dense lava fragments.

This term means ‘burning cloud,’ and they have an incandescent glow that can be seen at night. Ignimbrites are a type of pyroclastic flow that contains light materials. These pyroclastic flow deposits contain glass shards and pumice, which is an extremely porous and lightweight rock that forms during a volcanic eruption. A pumice stone is a common tool for smoothing rough skin on your feet.

A pyroclastic surge is similar to a pyroclastic flow, but it is a low-density flow of volcanic material with a higher proportion of gas to rock. Because these flows are mostly gas, their flow is more turbulent and they have the ability to travel up and over hills and ridges.

Learning Outcomes

Watch this video lesson so that you can:

  • Understand what a pyroclastic flow is and why it is dangerous
  • Know the meaning of lapilli
  • Note the characteristics of the two types of pyroclastic flow: nu;e ardentes and ignimbrites
  • Compare a pyroclastic surge to a pyroclastic flow
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