There are many names used to refer to pyroclastic flows: fiery cloud, pyroclastic flow, pyroclastic density current, and more. All these terms refer to the same thing, to the powerful mass of gases and particles that is expelled from the crater of a volcano and travels at great speed. However, pyroclastic flows are not, popularly, one of the best known parts of the volcano and, in fact, their presence can bring many unwanted consequences.
What is pyroclastic flow
It is a mixture that arises during volcanic eruptions and is made up of gases and solid particles at high temperatures . Specifically, the temperature of the pyroclastic flow is between 300 to 800 ° C. Once the pyroclastic flow is expelled from the erupting volcano and reaches the earth’s surface, it moves along the ground at a speed that varies in the range of ten to hundreds of meters per second.
As we mentioned in the previous paragraph, the pyroclastic flow is composed, among other things, by solid particles. These solid particles are called pyroclasts or tephras , and they are nothing more than fragments of solidified magma that have been expelled from the volcano. Depending on the size of the fragments, the pyroclast can be classified into:
- Ashes: particles smaller than 2 millimeters in diameter.
- Lapilli: particles whose diameter varies between 2 and 64 millimeters in diameter.
- Bombs or blocks: fragments of more than 64 millimeters in diameter.
For its part, the size of the particles determines the speed and extent of the pyroclastic flow. Those streams made up of blocks have low mobility and are generally restricted to a few tens of kilometers from the emission center. While those flows formed by ash and lapilli can reach a radius of 200 kilometers from their emission center.
It is important to mention that the pyroclastic flow represents one of the greatest dangers of an erupting volcano because, due to the high speed with which the flow advances, it can affect a huge area of land in a short term. In addition, it not only affects human lives and infrastructure, but always causes long-term adverse effects on the climate, soil and water of the area.
In the previous section we learned that pyroclastic flows arise from volcanoes that are erupting. However, not all volcanoes cause pyroclastic flows during their eruption, but the pyroclastic flow is formed only in those volcanoes with moderate to high explosive eruptions , such as in volcanoes whose eruption is of the Strombolian, Plinian or Vulcanian type, among others. . Here you can learn more about the Types of volcanic eruptions .
Pyroclastic flows can be formed in different ways and here we mention two of them:
- By gravitational collapse of the high-altitude eruptive columns . Collapse occurs when the density of the column is greater than the density of the surrounding atmosphere.
- By collapse of a lava dome , which is a bulge that occurs when lava is so viscous that it does not flow easily. Collapse occurs when the lava dome is so large that it becomes unstable and culminates in an explosion.
Pyroclastic flows can be classified according to their composition, according to the deposits they originate, according to how they originated and more. For example, according to its density, that is, the gas / solid particle ratio it has and the deposits it forms, we can find:
They are characterized by being not very dense (since they present few concentrations of solid particles), energetic and turbulent . The waves, in turn, can be classified into hot waves and cold waves . Their temperature can be lower than the boiling point of water, as is the case with cold waves, or it can reach temperatures above 1000 ° C, as is the case with hot waves. Pyroclastic wave deposits are characterized by being abundant in lapillis and lithics (rock fragments that were in a solid state at the time of the eruption). However, it is worth clarifying that waves are often not considered a type of pyroclastic flow.
They are a type of flow produced mainly by Plinian-type eruptions and present a higher density compared to pyroclastic waves. The deposits that the flows form are difficult to study, since they do not present a marked internal stratification, but, in general, their deposits are called ignimbrites, and they are composed of particles of all types of size: from ashes to blocks .