What is Lahar

Interestingly, the term lahar means lava. Geologists and volcanologists have adopted this word to refer to the flow that, on certain occasions, descends the slopes of volcanoes and valleys. The truth is that, although lahars do not seem to be as dangerous as lava flows, in reality they are one of the greatest dangers associated with volcanoes.

What is a lahar and its causes

The term volcanic lahar refers to the flow, made up of sediments of different sizes and water , that moves along the slopes of volcanoes or valleys. The speed with which lahars travel is closely related to the flow rate, the density of the flow and the slope of the slope. When these three factors are enhanced, lahars can reach speeds of up to tens of kilometers per hour.

Once we understand what a volcano lahar is, we must know that they constitute a huge risk for the surrounding communities. Although when referring to lahars as flows we can think that their effect is similar to flooding, the truth is that lahars erode everything in their path, devastating and dragging buildings from their foundations. In addition, its danger increases depending on whether it is cold lahars or hot lahars .

Now, volcanic lahars can originate for different reasons, but in a general way we can include their causes in two groups:

Primary lahars or volcanic lahars

  • Caused by the melting of glaciers in the upper parts of a volcano, during the volcanic eruption. In this way, the water that made up the glacier slides downhill dragging all kinds of particles.
  • Caused by the expulsion of the water present in the lakes found in the craters of volcanoes, during volcanic eruptions. As in the previous case, the water of the lake slides down the slope taking with it a huge variety of particles.

Lahares secundarios o lahares posteruptivos

  • Caused by heavy rainfall . As a result, unconsolidated pyroclastic material, produced by past volcanic eruptions, is washed down the slopes.
  • Caused by lake overflows or dam ruptures . In this way, the volcanic particles found on the slopes end up forming part of the lahar.

How a lahar is formed

As we have already seen in the previous section, lahars can be formed for various reasons. Regardless of its causes, for the formation of lahars to occur, 4 key factors are needed:

  • Water source : lahars have a large percentage of water. This water can come from water tables, glaciers and melted ice, lakes in volcano craters, or lakes adjacent to volcanoes. It can also be water from the hydrothermal or phreatic system of a volcano.
  • Large amounts of unconsolidated particles : These particles are rock fragments of various sizes. They arise from volcanic activity and remain as easily removable deposits. You can also find out more about Volcanic rocks: types and characteristics , here.
  • Steep slopes : for the lahar flow to flow, there must be a slope that, by gravity, causes the flow to decrease. The volcanoes that stand out for having steep slopes are those of the stratovolcano type, which are characterized by their great height and their conical shape. If you want to know all the Parts of a volcano , take a look at this Green Ecology article.
  • Triggering Mechanism : Here an agent that causes the flow displacement comes into play. Some of them are: volcanic eruptions, earthquakes, heavy rains and collapse of volcano slopes, mountains and valleys. All of them can evolve and trigger lahars. Here you can discover more information about Types of volcanic eruptions .

The greatest danger posed by lahars is that it is not easy to predict when it will happen and, even less, its magnitude. Depending on the size of the particles they transport and the speed the lahar reaches, its erosive power can be very high.

On the other hand, the lahars that have occurred throughout history have shown that the immediate dangers are reflected rather in the infrastructure, since the fluid destroys everything in its path. However, the dangers that prevail after the disaster are associated with periods of famine and shortages and the presence of disease.

Fortunately, early warning systems are increasingly being implemented . These systems warn about the formation of volcanic lahars from the beginning. We could say that they are very important systems, since they allow taking measures that contribute to reducing the risks associated with lahars. One of the simplest, but effective measures is evacuating the area. In turn, there are mitigation measures that are carried out prior to the occurrence of a volcanic lahar. An example of this are the canalization works and dams, as well as the extraction, in key areas, of rock fragments that increase the danger of the lahar.

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