Mud flows are amongst the most dangerous of volcanic phenomena. They took the lives of 25,000 inhabitants of the town of Armero, Colombia, during the 1985 eruption of Nevado del Ruiz. Some mudflows are caused by the breaching of crater lakes (Kelut 1966); others by melting of snow and ice by an eruption (Nevado del Ruiz 1985) and still others by rain falling on fresh, unconsolidated ash (Pinatubo 1991). This miniature Chilean example illustrates many of the main features. It formed after a rare rainstorm in 1969. Broad levées 30 cm high and a sinuous central channel are well displayed. After emplacement, the mud set hard. (Fig. 13.18).

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