The Al-Du’aythah volcanic cones, Al-Madinah City: implications for volcanic hazards in northern Harrat Rahat, Kingdom of Saudi Arabia

The basaltic Al-Du’aythah volcanic cones lie in the northern part of the extensive lava field of Harrat Rahat, and only 13 km from the centre of Al-Madinah City, in the Kingdom of Saudi Arabia. Historical records indicate they may have erupted in AD 641. The four cones are formed by deposits that record a transition from phreatomagmatic to magmatic explosions followed by minor lava effusion. Three cones display elongated tuff rings at the base, and two produced late-stage lava flows. The cones themselves are symmetrical and constructed mostly by the accumulation of ballistically ejected pyroclasts. Spherical bombs and lapilli (cannonball bombs/lapilli), occasionally with country-rock fragments inside (both cored and loaded bombs/lapilli) are common within the tuff ring deposits. LiDAR data show a total volume of 1,664 × 10 −6 km 3 for the four cones (418 × 10 −6 km 3 DRE). Whole-rock chemical analyses indicate alkali-basalt compositions (SiO 2 44.7–45.9 wt%), with little compositional variation and no relationship between chemistry and eruptive styles. Small differences in composition may reflect variations in fractional crystallisation of clinopyroxene and olivine. A magnetotelluric 2D cross-section shows that the cones are located adjacent to a buried sediment-filled alluvial channel along a NNW-SSE fault dipping to the east. The Al-Du’aythah eruption was related to the ascent of magma through this structure, with the first phase of the eruption triggered by the interaction of the magma with water from the northern Harrat Rahat aquifer that exists in the Al-Madinah basin. This initial water source was rapidly exhausted, while the eruption progressed roughly from north to south and from west to east, the latter motion probably along the fault-controlled feeding dyke. Our work draws attention to the existence of recent explosive phreatomagmatic eruptions in the Al-Madinah basin, which, despite the hyperarid climate of the area, must be considered a potential future eruption hazard.