Lahars...

A lahar is a deadly kind of pyroclastic mudflow that flows down from a volcano after an eruption into a river valley. The word lahar comes from an Indonesian term that describes a mixture of hot or cold water and rock fragments flowing down the slopes of volcanoes and (or) river valleys.
There are many possible ways that Lahars can form, but one of the most common reason is because of a volcanic eruption. A volcanic eruption can cause more that one Lahar at once. The extreme heat coming from the volcano can rapidly melt snow and ice on the top of a volcano, rejecting water from a crater lake. Most of the time, lahars are formed because of intense rainfall after an eruption. Rainwater can easily erode loose volcanic rock/soil from hillsides and in river valleys. Many of the largest lahars begin as landslides of saturated and hydrothermaly altered rock on nearby and adjacent hillsides of a volcano.
Lahars almost always appear near stratovolcanoes. These volcanoes usually erupt explosively and their tall, steep cones are covered with snow or topped with a crater lake. Which is a place on the volcano where it is constructed of weakly consolidated rock that is easily eroded or internally weakened. Lahars are also common around that snow and ice covered shield volcanoes where eruptions blast fluid lava frequently on top of glaciers. One example of this would be the volcano at Vatnajokull, Iceland.
Lahar is made of debris from avalanches, pyroclastic flows that dilute themselves in river water as they travel down slopes, natural dam failure and rain fall on loose material such as ash. Lahars that contain twenty to sixty percent sediment are usually very turbulent and con carry large boulders up to tem meters in diameter. Lahars that contain more that eighty percent sediment usually flow smoother and much faster than their turbulent partner; these lahars can pick up rock to the size of ten meters in diameter.
An example of a deadly lahar is the Nevado del Ruiz stratovolcano, which is near the Andes Mountains in South America. Over the years, this volcano has produced many lahars and has killed over twenty thousand people. On November 13, 1985, this volcano violently erupted and formed a large lahar in the Guali river valley. Many towns were destroyed in this lahar and more that 23,000 people were killed.
Lahars are very deadly and there is not a way to prevent them. lahars are very deadly and many people die each year.

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