No abstract

The scientific evaluation of hazards and risks remains a primary concern in poorly known volcanic regions. The use of such information to develop an effective risk management structure and risk reduction actions however also poses important challenges. We here present the results of a series of focus group discussions (FGDs) organised with city councillors from three municipalities around Mt Cameroon volcano, Cameroon. The Mt Cameroon area is a volcanically and tectonically active region regularly affected in the historical past by lava flows, landslides and earthquake swarms, and has a potential for crater lake outgassing. The lower flanks of the volcano are densely populated and the site of intense economic development. The FGDs were aimed at the elicitation of (1) the knowledge and perception of geological hazards, (2) the state of preparedness and the implementation of mitigation and prevention actions by the municipalities, (3) the evaluation of the effectiveness of the structure of communication channels established to respond to emergency situations, and (4) the recovery from an emergency. In all three municipalities stakeholders had good knowledge of the risks, except for processes never experienced in the region. They generally grasped the causes of landslides or floods but were less familiar with volcano-tectonic processes. Stakeholders identified the lack of strategic planning to monitor hazards and mitigate their impacts as a major weakness, requesting additional education and scientific support. Response to natural hazards is mostly based on informal communication channels and is supported by a high level of trust between local scientists, decision makers and the population. Actions are taken to raise awareness and implement basic mitigation and prevention actions, based on the willingness of local political leaders. The strong centralisation of the risk management process at the national level and the lack of political and financial means at the local level are major limitations in the implementation of an effective risk management strategy adapted to local risk conditions. Our case study highlights the need for earth and social scientists to actively work together with national and local authorities to translate the findings of scientific hazard and risk assessment into improved risk management practices.

This chapter provides a synthesis of the geological history, chronology and magmatic evolution of Merapi. Stratigraphic field and geochronological data are used to divide Merapi into three main evolutionary stages and associated volcanic edifices (Proto-, Old and New Merapi) and eight broad volcanostratigraphic units to characterise the eruptive activity and structural evolution of the volcano through time. Complementary petrological, geochemical and isotopic data are used to characterise the eruptive products of Merapi and shed light on the geochemical evolution and petrogenetic processes. The data indicate that the eruptive products of Merapi are mainly basaltic andesite of both medium-K and high-K type and support a two-stage petrogenetic model, where primary magmas are derived from a heterogenous, Indian Ocean MORB-like mantle source metasomatised by slab-derived components. Subsequently, these magmas are modified during transfer through the crust by complex magmatic differentiation processes, including contamination by carbonate rocks of the local upper crust. The available data indicate that, since * 1900 14 C y BP, the lavas and pyroclastic rocks of Merapi are essentially of the high-K type and that regular geochemical variations with systematic shifts in whole rock SiO 2 content occurred since at least the Late Holocene, although erupted magma R. Gertisser (&) School of Geography, Geology and the