The Cenozoic volcanic rocks of Saudi Arabia cover about 90,000 kin2, one of the largest areas of alkali olivine basalt in the world. These volcanic rocks are in 13 separate fields near the eastern coast of the Red Sea and in the western Arabian Peninsula highlands from Syria southward to the Yemen Arab Republic.
The initial phase of rifting of the Arabian Plate from the African Plate began as a wide zone of continental-crust extension manifested by basin and range topography. Freshwater lakes, northwest-trending marine gulfs, and alkali olivine basalt flows occupied these basins. Extensive dike swarms intruded parallel to the proto-Red Sea and marked the first phase of new mafic crust formed by volcanic processes. After a hiatus in volcanic activity, counterclockwise rotation of the Arabian Plate during middle Miocene time changed the stress pattern in the plate and a second phase of extrusion of alkali olivine basalt commenced along north-trending fractures. This stress pattern continues to
influence Holocene volcanism.
The earliest (pre-uplift) basalts to erupt on the Arabian Plate were predominantly under saturated picrite and ankaramite, whereas those to erupt near the axis of the proto-Red Sea rift zone were tholeiite. The within-plate volcanic rocks evolved from picrite-ankaramite to alkali olivine basalt with minor volumes of fractionated, under-saturated felsic rocks. Continued crustal thinning and dike intrusion along the proto-Red Sea were accompanied by melting of the continental crust to produce silicic magma as part of a bimodal volcanic suite (tholeiite-rhyolite). These magmas were emplaced as dikes, sills, layered bodies, and flows that mark the early construction of the Red Sea crust. Second-phase lavas are predominantly fractionated hawaiites and alkali olivine basalts. Because under saturated and oversaturated silicic magmas represent the second phase of activity, both fractional crystallization of the basaltic magma and melting of the crust are believed to have occurred.
The historical record of volcanic activity in Saudi Arabia suggests that volcanism is dormant. The harrats should be evaluated for their potential as volcanic hazards and as sources of geothermal energy. The volcanic rocks are natural traps for groundwater ; thus water resources for agriculture may be significant and should be investigated.