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Lahar's might be considered the most dangerous and life-threatening component of volcanic activity in today's world. In our technologically advanced society, volcanology and the seismographic monitoring stations located on most non-dormant volcanoes can provide sufficient warnings, if heeded, of most aspects of volcanic activity; enabling appropriately timed evacuations. Lahars, however, can occur at any time, even when the volcano is quiet, and it is this element of surprise that can make them deadly.

The word "lahar" comes to English from Bahasa Indonesia, a language derived from Malay and one of the four main Malayo-Polynesian languages in its own right. Spoken by some 26 million citizens of Indonesia, it is commonly referred to elsewhere simply as Indonesian. In that language, lahar usually means lava, but it is also used to refer to a mudflow of volcanic origin. It is in that second definition that lahar has come to be used in English.

Mud means water, something that might not first come to mind when thinking about volcanic eruptions. We tend to think of fire and molten rock first; after all, another name for a volcano is a fire mountain. But once a volcano has first risen and erupted, it may lie dormant for long periods, essentially as a mountain with a depression or crater at the top. Depending on the volcano's height and the local weather patterns, it may trap rain or snowfall in the crater forming a crater lake and/or develop a mantle of thick snow on its shoulders. Although very picturesque and sometimes providing excellent skiing fields while the volcano is dormant, these visions of natural beauty can be exceedingly dangerous when the sleeping giant starts to wake.

Large volume crater lakes in particular can be deadly early on. Magma swelling within the volcano prior to eruption can modify the external terrain morphology and result in tremors. These can result in the unanticipated collapse of some part of the crater wall, unleashing a devastating flood of water thickened by stirred up sediment from the lake floor down the side of the volcano, following the path of least resistance and maximum gradient. Snow and rocks from the upper slopes will be joined by trees, bushes and even the very soil, stripped from further down. This thick, muddy soup will crush and destroy anything it meets until it gradually slows and settles in the lowest levels of the local topography.

On Christmas Eve, 1953, an unpredicted lahar resulted from the collapse of the crater

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