Lava Flow, Pyroclastic Flow, Pyroclastic Surge & Tephra: Preventing Volcanic Hazards

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  • 0:06 Volcanoes
  • 0:45 Volcanic Hazards & Lava Flows
  • 2:21 Pyroclastic Flow
  • 3:26 Pyroclastic Surge
  • 4:23 Tephra
  • 5:44 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.

Volcanic hazards are any type of volcanic process that leads to destruction or threatens life. Learn about specific volcanic hazards including lava flows, pyroclastic flows, pyroclastic surges and tephra and measures used to prevent or reduce damage


If you had an opportunity to live next to a volcano, would you? Your gut reaction might be that you would want to live as far away from a volcano as possible. Yet, volcanoes provide many benefits to the areas they surround. For instance, they're a source of valuable minerals and geothermal heat, and when volcanic ash mixes into the earth, it greatly improves the fertility of the soil.

Having said that, there's no overlooking the fact that volcanoes present many hazards to the areas surrounding them. In this lesson, you will learn about different types of volcanic hazards and ways to prevent harm to humans living in areas prone to volcanic activity.

Volcanic Hazards and Lava Flows

A volcanic hazard is any volcanic process that threatens life or destroys land or infrastructure. One type of volcanic hazard that most people think of first is lava flow, which is the flow of hot molten rock. Yet a lava flow is one of the least deadly of all of the volcanic processes. This is partly because lava flows do not, for the most part, move very fast. Even the more runny lava flows typically only travel a few miles per hour.

Lava Flow
Flow Lava

Therefore, people have a chance to get out of the way of a lava flow. Having said that, I wouldn't recommend walking up to a lava flow because the temperatures can reach as high as 1,400 degrees Celsius. This is definitely hot enough to singe your eyebrows or cause you more serious bodily harm. Lava flows can also kill vegetation and destroy property by burning homes and destroying infrastructure, such as bridges and roads. The lava can also melt snow and ice, which leads to flooding.

So, how do you prevent a lava flow from creating damage? That's a good question, and the answer is not always easy. Some success has come from diverting the flow by cutting channels for the lava to follow, bombing the lava channel with dynamite to dissipate the flow and its energy or spraying the lava with copious amounts of water to cool the lava to a point where it will solidify. These methods may not always work, so if all other avenues prove ineffective, you could always do what the ancient Hawaiians did and appeal to the volcano goddess, Pele.

Pyroclastic Flow

There is another type of flow that comes from a volcanic eruption that is much more dangerous than a lava flow, and that is a pyroclastic flow. A pyroclastic flow is a dense collection of fragments and gases from a volcanic eruption that flows down the slope of a volcano. You can recall this term by remembering that the prefix 'pyro' means 'fire,' and the suffix 'clastic' means 'fragments.' So a pyroclastic flow is literally the 'flow of fiery fragments.'

Pyroclastic Flow
Flow Pyroclastic

Whereas a lava flow is something that might inch towards you giving you time to flee, a pyroclastic flow is something that races down the side of a volcano, leaving little time to react. The high speeds at which a pyroclastic flow travels, which can be more than 100 miles per hour, make this volcanic hazard very dangerous. And, not only are pyroclastic flows dangerous because of their speeds, but also because they are very hot and contain toxic gases. Any living organism unlucky enough to be caught in a pyroclastic flow will find it impossible to breathe, and most likely succumb to the heat and burn.

Pyroclastic Surge

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 surges of material contain a lot of toxic gases, they can asphyxiate anything in their path and their flow is more turbulent. Like the pyroclastic flow, a pyroclastic surge moves very fast.

Pyroclastic Surge
Surge Pyroclastic

To keep the two hazards straight, you might want to think of a more dense pyroclastic flow as a rocky soup flowing downhill and a pyroclastic surge as a turbulent cloud of gray ash and gas billowing down the slope. There is little that can be done to prevent a pyroclastic flow or surge from a volcano, so if a volcano is showing signs that it could erupt or a volcano does erupt, the best way to prevent the loss of life is for anyone in the area to evacuate.

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