Lahars

Lahars are rapidly flowing mixtures of rock debris and water that originate on the slopes of  a volcano.Explosive eruptions can deposit huge amounts ash and other volcanic debris on a volcano’s slopes. Lahars form when water from intense rainfall, melting snow and ice, or the sudden failure of a natural dam, mixes with this loose volcanic material, creating mudflows that can be particularly dangerous and destructive.Volcanic eruptions may directly trigger one or more lahars by quickly melting snow and ice on a volcano or ejecting water from a crater lake.Although lahars contain at least 40% (by weight) volcanic ash and rock fragments—making them dense and viscous like wet concrete—they actually flow faster than clear-water streams.
As it rushes downstream, its size, speed, and the amount of water and rock debris it carries are constantly changing.Lahars can fall under three categories of classification, syn-eruptive, post-eruptive or
non-eruptive.Primary (syneruptive) lahars can occur when an eruptions
melts ice or a crater lake is breached.Secondary lahars can result
from debris avalanches (landslides) or from erosion of fresh, unconsolidated pyroclastic material.

Lahars Deposits
Because of bed friction, bottom layers move more slowly
-Consequently, larger particles work toward the front and margins
- Lower concentration flows can be normally graded as coarse material settles first
- Inverse grading is a consequence of “kinetic sieving”

Lahars have been the cause of major damage and loss of life throughout history. They have buried entire towns, and destroyed cities beyond repair.Their extreme danger makes them an important topic of research throughout the geosciences in the hopes of mitigating future disaster.
Lahars have the strength to rip huge boulders, trees, and man-made
structures from the ground, carrying them for great distances.