Two distinct phases of continental magmatism are evident in western Arabia. The first, from about 30 to 20 Ma, produced tholeiitic‐to‐transitional lavas emplaced along NW trends. The second, from about 12 Ma to Recent, produced transitional‐to‐strongly‐alkalic lavas emplaced along N‐S trends. The older phase is attributed to passive‐mantle upwelling during extension of the Red Sea Basin, whereas the younger phase is attributed to active‐mantle upwelling but was facilitated by minor continental extension perpendicular to plate collision. The younger magmatic phase is largely contemporaneous with a major period of crustal uplift to produce the West Arabian Swell after about 14 Ma. A variety of evidence suggests that the West Arabian Swell is thermally supported by hot, upwelling asthenosphere. In contrast to the distinct asymmetry of uplift and magmatism on opposing sides of the Red Sea Basin, these processes were symmetric across a N‐S line marking the central axis of the West Arabian Swell. This axis coincides with two fundamental features: the Ha'il‐Rutbah Arch in the north, and the Makkah‐Madinah‐Nafud (MMN) volcanic line in the south. The symmetry of magmatism is demonstrated by petrochemical evidence that the MMN harrats were derived by greater degrees of partial melting, at shallower depths, than those harrats lying to the west and east of the MMN line. The potential temperature of the asthenosphere is estimated to be about 1436°C beneath the MMN line and about 1354°C beneath the flanking, more undersaturated harrats. The source of upwelling is either a mantle plume centrally located beneath the West Arabian Swell or an elongated and extended lobe of hot asthenosphere emanating from the Ethiopian mantle plume. Convective flow may have been channelled along a preexisting, regional flexure in the continental lithosphere which concentrated hot asthenosphere beneath the central axis of the AfroArabian Dome. This crest of mantle upwelling underlies the MMN line in the north and the Danakil Depression and Ethiopian rift system in the south. It also passes through the Red Sea Basin at the midpoint of an unusual, doubly propagating rift system where axial seafloor spreading began 4–5 m.y. ago. The NW trend of the Red Sea Basin was well established by crustal attenuation during the older magmatic phase. The Pliocene invasion of this basin by a N‐S zone of mantle upwelling has resulted in seafloor spreading parallel to the preexisting structure along a rift system that has continued to propagate away from its eccentric mantle source in two opposing directions.

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Abstract. The stratigraphy of the Soufriere Hills volcano is dominated by dense-andesite block and ash flow deposits. Forty-one radiocarbon ages permit a reconstruction of the eruptive history over the past 31,000 years. A single stratigraphic member yielding radiocarbon ages of 22,000 to 20,000 years B.P. is composed of block and ash flow deposits with semi-vesiculated clasts concentrated in the upper third of each deposit. The present eruption is also producing block and ash flow deposits with both dense and semi-vesiculated clasts. It marks a return to a more explosive driven type of activity as occurred 22,000 to 20,000 years ago. Deposits of this member have a broad sheet-like form and volumes in excess of any produced so far by the present eruption.

Abstract. During a 52-day eruption in 1256 A.D., 0.5 km 3 of alkali-olivine basalt was extruded from a 2.25-km-long fissure at the north end of the Harrat Rahat lava field, Saudi Arabia. The eruption produced 6 scoria cones and a lava flow 23 km long that approached the ancient and holy city of Madinah to within 8 km. Three chemical types of basalt are defined by data point clusters on variation diagrams, i.e. the low-K, high-K, and hybrid types. All three erupted simultaneously. Their distribution is delineated in both scoria cones and lava flow units from detailed mapping and a petrochemical study of 135 samples. Six flow units, defined by distinct flow fronts, represent extrusive pulses. The high-K type erupted during all six pulses, the low-K type during the first three, and the hybrid type during the first two.Three

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