Pyroclastic flow facts for kids
A pyroclastic flow is a very fast-moving wave of hot gas and volcanic rock. It flows along the ground away from a volcano. These flows are one of the most dangerous results of a volcanic eruption. They usually move at speeds of about 100 km/h (62 mph), but they can reach incredible speeds of up to 700 km/h (430 mph). The mixture of gas and rock inside can be extremely hot, reaching temperatures of about 1,000 °C (1,830 °F).
Because they are so fast and hot, pyroclastic flows destroy almost everything in their path. They normally travel downhill, following the shape of the land.
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What does the name mean?
The word pyroclast comes from the Greek words pyr (meaning "fire") and klastós (meaning "broken in pieces").
Sometimes, these flows glow red in the dark. When this happens, they are called by the French name nuée ardente, which means "burning cloud". This name was famously used to describe the eruption of Mount Pelée in the Caribbean in 1902.
Some flows have a lot more gas than rock. These are called pyroclastic surges. Because they are lighter and less dense, they can sometimes flow over hills or even across water.
How do they form?
There are several ways a volcano can create a pyroclastic flow:
- Column collapse: In a large explosive eruption, the volcano shoots a column of ash and rock high into the sky. If this column gets too heavy or cools down, it cannot stay up. It collapses and falls back to Earth, rushing down the sides of the volcano. This is what happened when Mount Vesuvius destroyed the Roman cities of Pompeii and Herculaneum.
- Lava dome collapse: Some volcanoes have thick, sticky lava that piles up near the vent to form a "dome". If this dome becomes unstable, it can break apart. The hot rocks tumble down the mountain, creating a flow. The Soufrière Hills volcano on Montserrat is known for this type of flow.
- Explosive blast: Sometimes, a volcano explodes from the side rather than the top. This creates a powerful blast of gas and rock that shoots out sideways. This occurred during the famous 1980 eruption of Mount St. Helens.
Size and destructive power
Pyroclastic flows can be small or massive. Most flows contain between 1 to 10 cubic kilometres (0.24 to 2.40 cu mi) of material and travel for several kilometres.
A flow usually has two parts:
- A basal flow (bottom part) that hugs the ground. This contains heavy rocks and boulders.
- An ash plume (top part) that rises above the flow. This is a cloud of hot ash and smoke caused by the turbulent air.
The energy of a moving pyroclastic flow is huge. It can flatten trees and knock down buildings. The hot gases are very dangerous to living things.
Famous historical examples
- Pompeii (79 AD): The ancient Roman cities of Pompeii and Herculaneum were buried by pyroclastic surges from Mount Vesuvius. The ash preserved the shapes of the city and its people for centuries.
- Mount Pelée (1902): The town of St. Pierre was destroyed by a massive flow. Despite warnings, many people remained in the city. The flow moved so fast that nearly all of the 30,000 residents lost their lives.
- Mount Unzen (1991): A flow in Japan tragically killed famous volcanologists Katia and Maurice Krafft and Harry Glicken. They were observing the volcano to learn more about how to keep people safe.
- Montserrat (1997): A flow traveled down a valley called Mosquito Ghaut. It spilled out of the valley and reached a village that had been evacuated, though sadly some people who had returned were harmed.
Can they move over water?
Yes, pyroclastic flows can travel across water. Evidence from the 1883 eruption of Krakatoa showed that flows reached the coast of Sumatra, about 48 km (30 mi) away.
Experiments by scientists have shown how this works. When the hot flow hits the water, the water on the surface instantly turns into steam. The flow then slides across the water on this cushion of steam, moving very fast. This was also seen at the Soufrière Hills volcano, where flows built new land (a delta) into the sea.
On other planets
Pyroclastic flows are not just found on Earth.
- The Moon: Astronomers believe that long ago, volcanic eruptions on the Moon created flows that carved winding valleys, known as rilles.
- Mars: Some volcanoes on Mars have deposits of rock that look like they were placed there by pyroclastic flows.
See also
In Spanish: Flujo piroclástico para niños
- Pyroclastic fall
- Pyroclastic rock
- Welded tuff
 | Shirley Ann Jackson |
 | Garett Morgan |
 | J. Ernest Wilkins Jr. |
 | Elijah McCoy |
