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What Is a Pyroclastic Flow?

Angie Bates
Angie Bates

A pyroclastic flow is a dense combination of extremely hot gases and solid matter that issues from a volcano during an eruption or collapse. These flows are the most dangerous aspect of volcanic activity since they travel at high speeds, can cover miles of ground, are extremely hot, and contain toxic gases. Although they are composed of both gases and solid fragments, pyroclastic flows behave closer to a liquid than a solid or a gas. When this phenomenon occurs at low density, it is called a pyroclastic surge.

As a whole, a pyroclastic flow behaves much like a liquid, except, unlike a fluid, the density of the flow changes as it travels. The flow appears to be a thick billowing cloud but contains both solid and gaseous materials. When a flow finally stops, deposits, sometimes over 328 feet (100 m) thick, are created.

Volcanic eruptions can produce pyroclastic flow, a combination of very hot gases and solid matter.
Volcanic eruptions can produce pyroclastic flow, a combination of very hot gases and solid matter.

There are two general classifications of pyroclastic flows. A nuée ardente flow occurs when part of the lava dome, or a volcanic vent, collapses. A pyroclastic flow of this nature contains solid lava fragments and gases. The first recorded instance of a nuée ardente flow was in 1902 during the eruption of Mount Pelée.

Pumice flows occur when the eruption column collapses. The resulting solid material is vesiculated, or covered in tiny holes, and is therefore less dense than the fragments in a nuee ardente flow. These lighter fragments are called pumice.

Pumice igneous rocks are created from once melted rock pumice.
Pumice igneous rocks are created from once melted rock pumice.

Since it is so dense, a pyroclastic flow follows the ground terrain, usually spilling down the volcano and into valleys. When it hits water, it will sink below water level and usually continue moving forward. These flows may stretch as far as 124 miles (200 km), and can travel over 62 miles per hour (100 km/hour) covering as many as 7,722 square miles (20,000 km2) before they stop. Extremely hot, pyroclastic flows can reach temperatures of 1,967&degF; (1,075&degC;).

There are two main sections to a pyroclastic flow. The heavier solid particles travel near the ground, creating the basal flow. Above the basal flow are lighter particles and gases which create the ash cloud. Pumice flows have an additional component called a ground surge, which consists of clouds of ash that jump ahead of the main flow. The ash from ground surges actually glows from the heat produced.

When a pyroclastic flow is made up of dilute materials, it is called a pyroclastic surge. Pyroclastic surges are much lighter than flows, though they still contain both gases and solid particles. Since they are low-density, they span out in the air above the ground rather than following the terrain.

Frequently Asked Questions

What exactly is a pyroclastic flow?

A pyroclastic flow is a fast-moving current of hot gas and volcanic materials that are expelled during an explosive volcanic eruption. These flows can reach speeds of up to 700 km/h (approximately 435 mph) and temperatures of about 1,000°C (around 1,832°F). They are capable of destroying nearly everything in their path due to their immense density and heat.

How does a pyroclastic flow form?

Pyroclastic flows form during explosive volcanic eruptions when the magma chamber collapses or when the eruption column collapses. This results in a mixture of ash, pumice, rock fragments, and volcanic gas that rushes down the slopes of the volcano at high speeds, following the contours of the terrain.

What are the dangers of a pyroclastic flow?

Pyroclastic flows are extremely dangerous due to their high speed, high temperature, and the suffocating gases they contain. They can obliterate structures, incinerate vegetation, and are lethal to any living thing in their path. Survival is nearly impossible for those caught in a pyroclastic flow, making them one of the most feared volcanic hazards.

Can we predict when a pyroclastic flow will occur?

Predicting the exact timing of a pyroclastic flow is challenging, but volcanologists can often provide warnings based on seismic activity, gas emissions, and changes in the volcano's shape. Monitoring these signs helps to forecast potential eruptions and associated pyroclastic flows, allowing for timely evacuations.

Has a pyroclastic flow ever caused significant human fatalities?

Yes, pyroclastic flows have been responsible for numerous fatalities throughout history. One of the most notorious events was the eruption of Mount Vesuvius in 79 AD, which destroyed the Roman cities of Pompeii and Herculaneum, killing thousands. More recently, the 1991 eruption of Mount Pinatubo in the Philippines resulted in hundreds of deaths, many due to pyroclastic flows.

What can be done to protect communities from pyroclastic flows?

To protect communities, authorities implement hazard zoning, evacuation planning, and public education on volcanic risks. Early warning systems and continuous monitoring of volcanic activity are crucial. Engineering measures like diversion barriers can sometimes be used, but the best protection is often to avoid settlement in high-risk areas.

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    • Volcanic eruptions can produce pyroclastic flow, a combination of very hot gases and solid matter.
      By: R. Sueswit Apriliant
      Volcanic eruptions can produce pyroclastic flow, a combination of very hot gases and solid matter.
    • Pumice igneous rocks are created from once melted rock pumice.
      By: Patricia Hofmeester
      Pumice igneous rocks are created from once melted rock pumice.