Volcanism

It is now established that the Arabian plate is drifting away from the African plate at a rate of 2 cm per year. The crustal extension in the northwestern and central western parts of the Kingdom has resulted to Cenozoic volcanism. There are two common types of volcanoes, representing 80% of volcanic formations, namely, the shield volcanoes and the scoria cones. These volcanoes are clustered along fissures that are predominantly aligned along a north-south or northwest-southeast trend.

These volcanic Harrats were formed in two phases of volcanism, the first of which began about 30 million years ago and coincided with the opening of the Red Sea and the attendant extrusion of basaltic lavas. The second phase was dated to have occurred 10 million years ago until the last historic volcanic eruption south of Madina in 1256 AD, which was concentrated along an active volcanic range, called the Mecca-Madina-Nafud (MMN) extending for 900 km from Al-Qunfudah in the south through Mecca and Madina, then to the Nafud Desert in the north, including the Harrats Rahat, Khaybar, and Ithnayn (Fig. 1). The MMN volcanic line is a rift, which spreads at an annual rate of 0.054 mm and is manifested by crustal lifting, doming, and extension due to the rise and retention of magma inside the magma chambers at great depths from the Earth’s surface. Thermal and seismic activities along this MMN volcanic range are observed, which signifies that it is still active

The Harrat Rahat is one of the largest Harrats in the Kingdom (Fig. 2). It extends from the north of Mecca to the south of Madina and covers an area of about 20,000 km2. It hosts numerous volcanic forms: 644 scoria cones, 36 shield volcanoes, and 24 trachyte domes (Figs. 3-6). These Harrats are believed to have formed as a result of the magmatic flow into the upper mantle layer, the asthenosphere, extending from the Afar Triangle, where the African Rift joins the Red Sea and the Gulf of Aden rifts, to the north of the Arabian Shield. The Afar Triangle is underlain by hotspots with rising convection currents that generate tensions on both sides of the rift, which cause the rifting. As a result, the magma rises, forming basaltic flows that cover large areas along the MMN active volcanic zone.

Volcanic cones in northern Harrat Rahat.

The Wahbar crater in Harrat Kishb, one of the distinctive volcanic forms, scattered throughout the Harrats, 200 m deep and 2 km wide, is the product of a volcanic eruption, where the rising magma comes into contact with the groundwater.

The Dabaa Hill, a volcanic dome in the Harrat Rahat

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