VORISA SCIENTIFIC MEETING NOVEMBER 17 – 18, 2013 19 Mechanisms of the Largest Explosive Trachytic Eruptions of Harrat Rahat Shane J. Cronin 1 , Mohammed Rashad H. Moufti 3 , Ian E.M. Smith 2 , Marco Brenna 1 , Karoly Nemeth 1 1 Massey University, New Zealand; 2 The University of Auckland, New Zealand; 3 King Abdulaziz University, Kingdom of Saudi Arabia Introduction Trachytic magmas are the most evolved products observed in the Harrat Rahat, Kingdom of Saudi Arabia (Camp and Roobol, 1989). Seventeen centres erupting such evolved magmas have been identified and studied in detail. All of these eruptions began with the shallow intrusion of highly crystalline magmas to very shallow depths. In one case the intrusion stalled as a cryptodome, in several others simple, through to complex multi- stage domes were extruded. On at least eight occasions, highly explosive eruptions resulted, some producing small tuff rings, while others went on to produce large, widespread pyroclastic flows and falls. Here we discuss the mechanisms of trachytic eruptions of Harrat Rahat and attempt to understand the magmatic and environmental factors that lead to the most violent and hazardous outcomes in this volcanic field. Background Evolved magmas in all geologic settings produce the most violently explosive eruptions known. This is due to a combination of increasing dissolved volatile contents (mainly H 2 O) as magmas evolve via fractional crystallisation, and increases in the viscosity of evolving magmas, caused by decreasing temperatures and increasingly coordinated silica-rich melts. If evolved magmas retain high volatile contents by the time they erupt, explosive eruptions eventuate. If intrusion is slow at shallow levels and associated with open degassing, highly crystalline, high viscosity magmas extrude to form domes and stubby lava flows. “Second-boiling” often induces explosive eruptions, where crystallisation under closed-system conditions leads to oversaturation of volatiles in the residual melts. Long-resident magmas may also be destabilised by mafic intrusions, causing rapid rise and eruption. A typical feature of trachytic and phonolitic eruptions is the eruption of a variety of compositions, which often reflects a zoned magma reservoir (Wo฀rner and Schmincke, 1984; Panter et al., 1997). The range of compositions with different temperatures, viscosities and gas contents leads to a variety of eruption mechanisms and hazards during an event. A further key feature in Harrat Rahat is the presence of shallow groundwater aquifers, particularly in the higher central portions of the Harrat. If volatile-rich magma occurs below a cooler crystal-rich cap, phreatomagmatic explosions may cause a rapid unroofing, generating rapid gas-expansion, vesicle growth and rapidly accelerating mass ejection rates. Sampling and Analytical methods Trachytic volcanic centres were mapped with stratigraphic sections described in detail. Domes were traversed with samples taken from all lithological domains found. Volcanic centres with pyroclastic sequences were described and mapped to construct a full-composite stratigraphy. Based on this, and in all main deposition sectors around each volcano, sets of stratigraphically controlled samples were collected for geochemical, petrological and textural studies. Geochemical samples were crushed and milled in tungsten equipment. Major element concentrations were measured by X-ray fluorescence (XRF; Siemens SR3000 spectrometer) at The University of Auckland. Theoretical detection limit is 1-2 ppm and reproducibility is <5% (2σ). Trace elements were measured by laser-ablation inductively-coupled-plasma mass-spectrometry (LA-ICP-MS, except for Pb) at the Research School of Earth Sciences, Australian National University, using an Excimer LPX120 laser (193 nm) and Agilent 7500-series mass spectrometer. Precision is <4% (RSD) and accuracy better than 5% at the 95% confidence level for most elements. Analytical conditions are described within Brenna et al. (2012). Results The overall geochemical variations observed in the trachytic suite from Harrat Rahat relate to shallow level (mid-crust) crystal fractionation. In most cases the fractionation of plagioclase feldspar plays the prominent role. Individual volcanic centres are distinguishable via their Fe contents and most display a large range in silica content (Fig. 1). The largest-volume eruptions (Al Elfairia and Gura 2) show the greatest chemical variations. In