What is pyroclastic

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Pyroclastic currents, glowing clouds and glowing avalanches

Many manifestations of volcanism hold great potential for danger. Pyroclastic currents are particularly destructive and dangerous. They often occur suddenly and suddenly, have a long range, reach high speeds and are almost silent.
Pyroclastic currents are a phenomenon that predominantly occurs in subduction zone volcanoes. These volcanoes produce a particularly viscous lava from which gases are difficult to escape. The lava gushes out of the extraction chute and pushes itself up around it, so that a lava dome grows.

3 types of pyroclastic currents

Pyroclastic currents can arise in 3 ways and are also referred to in the specialist literature by different names (pyroclastic current, glowing cloud and glowing avalanche). In principle, however, they describe the same natural phenomenon: in a suspension of super-hot gas and volcanic ash, lava rocks of various sizes are transported down the slope. Ash and gas form a cloud that can rise several kilometers high. Near the ground, the gas cushion is so hot that the volcanic ash glows. At night you can very nicely see a trail of glowing ash that marks the path of the pyroclastic flow. Hence the name Ember Cloud.

Images of pyroclastic currents

A trail of embers from a pyroclastic flow.

Pyroclastic flow at Soufriere Hills.

Pyroclastic flow at Soufriere Hills.

Pyroclastic current at the Sinabung.

Pyroclastic current at the Sinabung.

Pyroclastic flow with volcanic lightning.


Pyroclastic currents form when large packets of rock break off from a lava dome. The gas dissolved in the lava is released explosively and the lava rock is fragmented. This process continues while the first chunks of lava rush down into the valley. Therefore, the pyroclastic current grows as it descends on the volcanic slope. For the first few meters, small pyroclastic currents can hardly be distinguished from falling rocks. A rockfall or the discharge of a small pyroclastic flow can destabilize the lava dome to such an extent that a large pyroclastic flow is created, or even the entire lava dome collapses and then explodes.
The larger and steeper the lava dome, the more likely it is that pyroclastic flows will develop.
In recent years I have also been able to observe pyroclastic flows that emanated from the front of a very viscous lava flow, or that emerged when a new lava flow burns its way through the volcanic slope.
Fire avalanches (base surges) are formed by the collapse of large eruption clouds, preferably during Plinian eruptions. If the gas burst in an eruption column, the pyroclastics expelled can suddenly sink to the ground and race down the volcanic slope in the form of a glowing avalanche. Fire avalanches contain more ash and gas and fewer large blocks than pyroclastic flows.
Glowing clouds (nuées ardentes) arise from pelean eruptions. A sideways explosion intensifies the glowing cloud. More large blocks of lava are being transported.

The force behind the pyroclastic currents

The driving force behind the pyroclastic currents is gravity. Similar to avalanches, the speed of a pyroclastic flow primarily depends on the slope of the volcano. In contrast to rockfall avalanches, a hot gas cushion at the base of the pyroclastic flow reduces its friction with the ground. The glowing cloud floats a few centimeters above the ground like a hovercraft and can thus reach speeds of more than 400 km / h. Usually, however, they travel at speeds between 50 and 80 km / h.
Typical ranges of pyroclastic flows are between a few hundred meters and up to 7 kilometers. However, particularly large pyroclastic currents can also flow up to 60 kilometers and cover long distances over water.

High destructive potential of catastrophic pyroclastic flows

Their destructive potential is based on the fact that temperatures between 300 and 800 degrees Celsius prevail inside the pyroclastic flows and lava rocks weighing tons are transported. Their effect on buildings can be compared to the force of a tanker truck that thunders silently into a house and explodes.

The largest pyroclastic currents observed destroyed entire places. In 1883, a pyroclastic current created by the Krakatau explosion covered a distance of more than 50 kilometers and destroyed several villages on Sumatra.
In 1902 the Dom des Montagne Pelée exploded on the Caribbean island of Martinique and destroyed the city of Saint-Pierre. 29,000 people died. The only survivor was a prisoner who was protected by the prison's thick walls.
One of the most powerful sideways eruptions occurred on Mount St. Helens in 1980. A pressure wave with subsequent pyroclastic currents destroyed an area of ​​600 square kilometers.
In 1996, pyroclastic flows from the Soufrière Hills volcano destroyed the island's capital, Plymouth, on Montserrat. The city was evacuated a few months earlier.

Protection against pyroclastic currents

There is no really reliable protection against a pyroclastic current! Only a few people have survived direct contact so far. I visited victims of a pyroclastic flow of the Merapis in Yogjakarta hospital. They were on the edge of the glowing cloud and suffered severe burns. These were especially bad where the skin was bare. Fire-resistant long clothing made of wool, or Nomex, may provide a certain level of protection if you are only exposed to the radiant heat in the edge area. This edge zone outside the actual glowing cloud can extend over several hundred meters. On the Merapi, houses were still burning a good 150 m outside the pyroclastic flow. In no case should you breathe in while you are exposed to the heat: it will cause severe burns to your lungs. When I film pyroclastic currents, I always wear a Nomex balaclava and have a gas mask and gloves close at hand (see protective equipment).

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