text: volcano’s history, using all historical data supplemented and extended back by stratigraphic studies. The products of each eruption should be identified, their areal distribution and volume measured, and the type of eruption established. It is also worthwhile monitoring the chemical composition of the materials emitted during the course of a prolonged eruption, because system- atic change in composition can in some cases be correlated with the type and violence of volcanic activity. Such zoning maps show the nature and frequency of specific volcanic hazards, and hence the risk to life and property. These maps are assential when planning action to minimize risk if and when an eruption happens. 23 7.3 Risk assessment and mitigation Where the type of volcanic activity is capable of causing total devastation, the vulnerability is 100 per cent and the risk is directly proportional to the hazard. A numerical assessment of the hazard should be made by vulcanologists after they have carried out a systematic and comprehen- sive study of relevant historical precedents. The nature and violence of most volcanic phenomena make it practically impossible to reduce the vulnerability of human life and property to below 100 per cent. The only way to mitigate risk is therefore to reduce the elements at risk either, on a long-term basis, by restricting human settlement and investment in hazardous zones or, on a shor%term basis, by evacuating populations and movable goods from such zones during periods of increased hazard (i.e. periods of actual OK predicted eruptive activity). There nevertheless remain certain possibilities of reducing vulnerability to some volcanic phenomena such as ash falls, lapilli showers, etc. Sloping roofs are less liable than flat roofs to collapse under layers of ash; windows of houses may be boarded up to reduce the risk of fires started by incandescent lava bombs, etc; some agricultural crops are less vulnerable to ash falls than others. 
analysis: The text appears to focus on the scientific study of volcanoes and the mitigation of volcanic hazards, specifically in terms of risk assessment and reduction. It discusses the importance of monitoring volcanic activity, identifying potential hazards, and developing strategies to minimize risk to human life and property. The text also touches on the idea of vulnerability and the need to reduce the elements at risk through evacuation or restricting human settlement.