Hugo Murcia

Geosite of a steep lava spatter cone of the 1256 AD, Al Madinah eruption, Kingdom of Saudi Arabia

UNESCO promotes geoconservation through various programs intended to establish an inventory of geologically and geomorphologically significant features worldwide that can serve as an important database to understand the Earth’s global geoheritage. An ultimate goal of such projects globally is to establish geoparks that represent an integrated network of knowledge transfer opportunities, based on a specific array of geological and geomorphological sites able to graphically demonstrate how the Earth works to the general public. In these complex geoconservation and geoeducational programs, the identification of significant geological and geomorphological features is very important. These are commonly referred to as ‘geosites’ or ‘geomorphosites’, depending on whether the feature or processes the site demonstrates is more geological or geomorphological, respectively. The Kingdom of Saudi Arabia is an extraordinary place due to its arid climate and therefore perfect exposures of rock formations. The Kingdom is also home to extensive volcanic fields, named “harrats” in Arabic, referring particularly to the black, basaltic lava fields that dominate the desert landscape. Current efforts to increase awareness of the importance of these volcanic fields in the geological landscape of Arabia culminated in the first proposal to incorporate the superbly exposed volcanic features into an integrated geoconservation and geoeducation program that will hopefully lead to the development of a geopark named, “The Harrat Al Madinah Volcanic Geopark” [1]. Here we describe one of the extraordinary features of the proposed Harrat Al Madinah Volcanic Geopark, namely a steep lava spatter cone formed during a historical eruption in 1256 AD.

The 1256 AD Al Madinah historic eruption geosite as the youngest volcanic chain in the Kingdom of Saudi Arabia

Volcanic geosites, geomorphosites and geotops are the smallest ‘‘units’’ of intact geological features that are identifiable through their uniqueness, or because they are graphic examples of specific volcanic phenomena, or form a vital landscape representative of a specific volcanic processes. Here, we identify significant volcanic features that bear not only regional, but global, volcanic value in a confined area that could be organized and promoted as the first volcanic geopark in the Kingdom of Saudi Arabia: the Harrat Al Madinah Volcanic Geopark (HAMVG). Harrat Al Madinah (‘‘harrat’’ in Arabic means lava field) is among many intraplate basaltic volcanic fields (Camp and Roobol 1989; Camp et al. 1991, 1992) that are located in the western margin of the Arabian Peninsula forming a broad zone sub-parallel to the Red Sea Rift (Fig. 1), which has been active over the last 30 Ma. These harrat fields are relatively thin (typically \300 m) but cover vast areas, the largest of which is *60,000 km. The proposed HAMVG has many spectacular volcanic geosites including the last historically erupted volcanoes in the Kingdom of Saudi Arabia (Camp et al. 1987; Moufti et al. 2012). The 1256 AD eruption site is located near to the culturally significant Al Madinah city (*1.5 million population), which is one of the holiest places to Muslims. A major geotop tentatively named as ‘‘The 1256AD Al Madinah Historic Eruption Site’’ with distinct individual geosites/ geomorphosites has been selected to demonstrate the diversity of volcanic phenomena associated with intraplate volcanism of the Al Madinah Volcanic Field. Hawaiian to Strombolian type eruptions created lava spatter and scoria cones visible from major highways, allowing visitors to stop near the 1256 AD historic eruption site just 10 km SE of Al Madinah (Fig. 1). The historically documented eruption lasted for 52 days and formed a *2.25 km long chain of NW–SE-aligned scoria and lava spatter cones (Fig. 2), producing alkali-olivine basalt (*0.5 km) a0a and pahoehoe lava flows (Camp et al. 1987). At least seven cones have been identified. Most of them are nested lava spatter and scoria cones such the largest cone in the northern edge of the fissure shown on Fig. 2. A combination of energetic explosive eruptive episodes (violent Strombolian-style eruptions) interrupted by less explosive lava fountaining, lava flow outbreaks and corresponding volcanic cone rafting formed a moon-like

Geology and geochemistry of Late Quaternary volcanism in northern Harrat Rahat, Kingdom of Saudi Arabia: implications for eruption dynamics, regional stratigraphy and magma evolution

Abstract Harrat Rahat (<10 Ma) is one of the largest volcanic fields on western Arabia. In the north of the field, some of the youngest volcanic centres evolved through either point-like, complex or multiple aligned vents (i.e. along fissures), and have pyroclastic cones, lapilli fall deposits and/or lava flows associated with them. The products reflect dominantly Hawaiian eruptions, and only one centre experienced phreatomagmatism. Results from new 3He surface-exposure dating provide constraints on stratigraphy of the youngest (<0.3 Ma) products. The rocks are compositionally alkali-basalt and hawaiite, with intra-plate basalt (prevalent mantle (PREMA)) affinity. Each eruption displays a distinct whole-rock composition in an overall linear trend. We suggest that the magma source for each centre is similar, and that composition of the products is different due to different degrees of fractionation. In a single eruption, the magma that reaches the surface first is the least evolved, with the most evolved magma erupting last. We also found that the most primitive magmas erupt less explosively. We think that the degree of magma evolution might correlate with ascent times, assuming that the more evolved magma spent more time en route. We suggest that magma ascent time is likely to be longer than that of other more primitive intra-plate basalts. Supplementary material: Whole-rock chemistry results, mineral chemistry results and fractional crystallization modeling data are available at https://doi.org/10.6084/m9.figshare.c.3488988