Summit acid crater lakes and flank instability in composite volcanoes

Volcanic landslides, including flank and sector collapses, constitute a major hazard in many parts of the world. While composite volcanoes are innately unstable, the presence of a hydrothermal system maintained by a magmatic source at depth is recognized as a key factor increasing the risk of failure. This relates to the formation of hydrothermally altered rock masses within the core and upper flanks of the volcano which leads to heterogeneous distribution of rock strength properties and pore fluid pressures. Here an emphasis is placed on acid crater lakes perched high on active volcanoes. By acting as a trap for magmatic heat and gas flows, these lakes localize extreme acid attack on their surrounds, thereby creating a source of instability. We outline how acid crater lakes form in relation to magmatic hydrothermal systems hosted within composite volcanoes, and describe the associated hydrothermal alteration and its relationships to flank instability. The sustainability of a volcanic slope is partly governed by the degree of rock alteration, which in turn reflects the time-integrated flux of acidic gases (SO2 and HCl) released from the subsurface magmatic source. Transient or longer-term changes in pore fluid pressure linked to hydrothermal system activity also readily affect the slope stability of composite volcanoes. Such fluctuations can be initiated by both magmatic and external non-magmatic processes such as major rainfall events and regional seismicity. Kawah Ijen hyper-acid crater lake, Indonesia, is used as a case study to illustrate the cascade of effects that may ensue following slope rupture linked to hydrothermal alteration

Challenges and Benefits of Standardising Early Warning Systems: A Case Study of New Zealand’s Volcanic Alert Level System

Volcano early warning systems are used globally to communicate volcano-related information to diverse stakeholders ranging from specific user groups to the general public, or both. Within the framework of a volcano early warning system, Volcano Alert Level (VAL) systems are commonly used as a simple communication tool to inform society about the status of activity at a specific volcano. Establishing a VAL system that is effective for multiple volcanoes can be challenging, given that each volcano has specific behavioural characteristics. New Zealand has a wide range of volcano types and geological settings, including rhyolitic calderas capable of very large eruptions (\u3e500 km 3 ) and frequent unrest episodes, explosive andesitic stratovolcanoes, and effusive basaltic eruptions at both caldera and volcanic field settings. There is also a range in eruption frequency, requiring the VAL system to be used for both frequently active ‘open-vent’ volcanoes, and reawakening ‘closed-vent’ volcanoes. Furthermore, New Zealand’s volcanoes are situated in a variety of risk settings ranging from the Auckland Volcanic Field, which lies beneath a city of 1.4 million people; to Mt. Ruapehu, the location of popular ski fields that are occasionally impacted by ballistics and lahars, and produces tephra that falls in distant cities. These wide-ranging characteristics and their impact on society provide opportunities to learn from New Zealand’s experience with VAL systems, and the adoption of a standardised single VAL system for all of New Zealand’s volcanoes following a review in 2014. This chapter outlines the results of qualitative research conducted in 2010–2014 with key stakeholders and scientists, including from the volcano observatory at GNS Science, to ensure that the resulting standardised VAL system is an effective communication tool. A number of difficulties were faced in revising the VAL system so that it remains effective for all of the volcanic settings that exist in New Zealand. If warning products are standardised too much, end-user decision making and action can be limited when unusual situations occur, e.g., there may be loss of specific relevance in the alert message. Specific decision-making should be based on more specific parameters than the VAL alone, however wider VAL system standardisation can increase credibility, a known requirement for effective warning, by ensuring that warning sources are clear, trusted and widely understood. With a credible source, user groups are less likely to look for alternatives or confirmation, leading to faster action. Here we consider volcanic warnings within the wider concept of end-to-end multi-hazard early warning systems including detection, evaluation, notification, decision-making and action elements (based on Carsell et al. 2004)