RESEARCH ARTICLE Vents to events: determining an eruption event record from volcanic vent structures for the Harrat Rahat, Saudi Arabia Melody G. Runge & Mark S. Bebbington & Shane J. Cronin & Jan M. Lindsay & Catherine L. Kenedi & Mohammed Rashad H. Moufti Received: 2 September 2013 /Accepted: 1 February 2014 /Published online: 25 February 2014 # Springer-Verlag Berlin Heidelberg 2014 Abstract Distributed “monogenetic” volcanic eruptions com- monly occur in continental settings without obvious structural alignments or rifting/extensional structures. Nevertheless, these may develop as fissures, representing the surface expression of dykes with a range of orientations, especially when stress regimes vary over time and/or older crustal features and faults are exploited by rising magmas. Dykes reaching the surface as fissures can last hours to months and produce groups of closely aligned vents, hiding the true extent of the source fissure. Grouped or aligned vents in a distributed volcanic environment add complexity to hazard modelling where the majority of eruptions are single-vent, point-source features, represented by cones, craters or domes; i.e. vent groups may represent fissure events, or single eruptions coincidently located but erupted hundreds to tens of thousands of years apart. It is common practice in hazard estimation for intraplate monoge- netic volcanism to assume that a single eruption cone or crater represents an individual eruptive event, but this could lead to a significant overestimate of temporal recurrence rates if multiple-site and fissure eruptions were common. For accurate recurrence rate estimates and hazard-event scenarios, a fissure eruption, with its multiple cones, must be considered as a single multi-dimensional eruptive event alongside the single-vent eruptions. We present a statistical method to objectively deter- mine eruptive events from visible vents, and illustrate this using the 968 vents of the 10 Ma to 0.6 ka volcanic field of Harrat Rahat, Saudi Arabia. A further method is presented to estimate the number of hidden vents in a thick volcanic pile. By com- bining these two methods for Harrat Rahat, we determined an updated spatial recurrence rate estimate, and an average tem- poral recurrence rate of 7.5×10 -5 events/year. This new analy- sis highlights more concentrated regions of higher temporal hazard in parts of Harrat Rahat, which has significant implica- tions for the city of Al-Madinah and surroundings. Keywords Monogenetic volcanic fields . Hazard analysis . Spatio-temporal recurrence rate . Harrat Rahat Introduction Volcanic eruptions in rifting, extensional and volcanic-flank settings can develop as fissures, representing the surface ex- pression of dykes. Post-eruption, a single fissure may be pre- served in the geologic and geomorphic record as a group of aligned or sub-aligned vents, most commonly spatter or scoria cones. Examples have been noted in incipient rifts (e.g. Tavenui, Fiji; Cronin et al. 2001), and the large volcanic prov- inces of the Arabian Peninsula (e.g. Harrat Rahat, Saudi Arabia; Moufti et al. 2010). In many intraplate continental “monoge- netic” volcanic fields without strong extension-related struc- tures, aligned or grouped vents also occur e.g. Auckland Volcanic Field (Bebbington and Cronin 2011), Yucca Mountain, Nevada (Connor et al. 2000), Newer Volcanic Region, Victoria, Australia (Lesti et al. 2008) and the Michoacan-Guanajuato Volcanic Field, Mexico (Wadge and Editorial responsibility: A. Gudmundsson Electronic supplementary material The online version of this article (doi:10.1007/s00445-014-0804-z) contains supplementary material, which is available to authorized users. M. G. Runge (*) : J. M. Lindsay : C. L. Kenedi School of Environment, University of Auckland, 23 Symonds Street, Private Bag 92019 Auckland, New Zealand e-mail: mhum400@aucklanduni.ac.nz M. S. Bebbington : S. J. Cronin Volcanic Risk Solutions, Institute of Agriculture and Environment, Massey University, Private Bay 11222, Palmerston North, New Zealand M. R. H. Moufti Faculty of Earth Sciences, King Abdulaziz University, P.O. Box 80200, Jeddah 21589, Kingdom of Saudi Arabia Bull Volcanol (2014) 76:804 DOI 10.1007/s00445-014-0804-z