Lahars are still a very dangerous and life-threatening component of volcanic activity in today's world. While other aspects can be more physically dangerous, in our technologically advanced society, the seismographic monitoring stations located on most non-dormant volcanoes provide sufficient warnings, if heeded, of these aspects of volcanic activity; enabling appropriately timed evacuations. Lahars, however, can occur at any time, with little if any warning, even when the volcano is quiet. 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 wall at 8pm on Mount Ruapehu, in New Zealand's North Island; a volcano that had been dormant since 1945. Approximately 360,000 cubic meters of lake water joined the headwaters of the Whangaehu river, easily overflowing the normal river banks. Two hours later the lahar demolished the supports of the main trunk railway bridge at Tangawai, causing its collapse. Despite the best efforts of a passing motorist in flagging the train driver and the driver's attempt to stop, the engine and the first four carriages of the eight carriage Auckland bound express plunged into the torrent; the fifth carriage dangled briefly before tipping in as well. Of the 285 people on board, 151 lost their lives, with bodies strewn over 60 kilometers of river-way and twenty bodies never found, presumed washed out to sea. One of the carriages was carried two kilometers downstream.
Lahars are dangerous because they can be unpredictable and are fast. A lahar from Mount Rainier in Pierce County, Washington, would reach the town of Orting in about 30 minutes with a wall of mud 15 to 30 feet high. Erupting volcanoes with a crater lake or heavy mantles of snow will produce lahars, if they provide advanced warning it's all to the good, but lahar warning systems must be very quick-response if they are going to save lives.