Volcanic emergencies differ from other types of large-scale emergency such as earthquakes and hurricanes in that it is possible to delineate very specific and relatively small danger areas of generally less than 100 km2 where devastation may be nearly total. In a situation where an entption threatens to become violent, evacuation becomes a logical and necessary step, Precursors to a possible violent eruption of a known volcano may develop over a period as long as many months before the eruptive climax, and this poses the problem of deciding upon the point at which the evacuation of population becomes necessary and also the point at which the evacu- ation should be ended, In other regions where volcanic activity occurs at locations over a wide area without clear correlation with previous craters, the interpretation of possible precursors is more difficult. Decisions involving mitigation of risk after the outbreak of an eruption should, however, be based on experience of the character and course of previous eruptions at the better-known volcanoes in the region. There are numerous different types of volcanic activity which present substantially different hazards. For example, glowing avalanches may descend the flanks of a volcano at speeds in excess of 100 km/h, whilst lava flows generally advance at no more than a few tens of metres per hour, A detailed review of the different types of volcanic activity, their physical consequences and the appropriate protective measures to be applied is given in the UNDRO publication entitled “Disaster prevention and mitigation - Volume 7 : Volcanological Aspects”. 7.2 Hazard zoning Information on volcanic hazard needed by civil defence authorities in volcanic areas is best presented in the form of hazard zoning maps. Such maps must be based on the records of each 
**Analysis**
The text discusses volcanic emergencies and the measures to be taken to mitigate their effects. It highlights the specific challenges posed by volcanic eruptions, such as the need to evacuate populations and the difficulty in predicting the timing and severity of an eruption. The text also mentions the importance of experience in understanding the character and course of previous eruptions. These themes suggest that the text is primarily focused on disaster management and relief, rather than economic development or other topics.

**Category**
Humanitarian aid and relief, Science and technology