Most of the volcanic Harrats were formed during the first phase of volcanism, which coincided with the rifting of the Red Sea. The recent volcanic Harrats that were formed during the second phase of volcanism in the area, extending from the north of Mecca to the south of Madina, are considered active. The area hosts historical volcanoes that were formed during the past 6,000 years, and were concentrated in the northern part of the Harrat Rahat. Eleven historical eruptions had occurred in this area, the last of which was the eruption in the northern Harrat Rahat in 654 H (1256 AD), which formed a number of scoria volcanoes, accompanied by a flow of basaltic lava that came close to 12 km from Madina. That volcanic eruption was well documented in some surviving reports and history books. A few days before the volcanic eruption, several strong earthquakes took place, beginning on the 1st of Jumada Al-Akhira, 654 H (1256 AD). The earthquakes were weak at the beginning and were not felt by a large part of the population. The tremors, however, intensified during the following days, and totaled to 18 earthquakes. The main earthquake was accompanied by clouds of black smoke, and fire covered the horizon in the east of Madina. This volcanic activity continued for 52 days.
Geological evidence that also include the scoria volcanoes and the recent basaltic flows in a number of Harrats, including the Harrat Al-Shaqa, indicates that the volcanoes were formed probably a thousand to two thousand years ago. The volcanic Harrats cover vast areas of the Kingdom and belong to the Cenozoic volcanic rocks, and their formation is associated with the rifting of the Red Sea. These volcanic fields are located in the western side of the Arabian Plate, which was separated from the African Plate along the Red Sea rift. Within the Arabian Plate and the Red Sea, approximately 21 historical volcanic eruptions occurred in the Arabian Peninsula, including the eruption in Jabal Al-Tair in the southern Red Sea in 2007.
The Kingdom’s Harrats are generally described as a result of a volcanic activity, known as “monogenetic,” which means that the volcanic eruptions, especially scoria volcanoes, occur just once in a certain period in a particular place, and then, the eruptions cease and do not recur, similar to the historic eruption in the Harrat Rahat in Madina in 654H. Any future activity will occur in another place, with new magma sources and new formation of volcanoes, and so on. In certain Harrats, there are other but few volcanic forms, such as the tephra rings, maar craters, and volcanic domes, whose rock composition ranges from those of basalts to those of the trachytes and rhyolites.
The volcanic Harrats are mostly composed of sequences of basaltic lava flows, compacted on top of one another, forming the distinctive geomorphologic shape of the Harrats, which appear as topographic volcanic hills. The most dominant volcanic forms are basaltic lava, which were formed by flows along distances of up to 25 km from the source, and the scoria volcanoes, which are densely spread in the volcanic Harrats with elevations of up to 250 m. They were formed by Strombolian eruptions, named after the Stromboli volcano in Italy. This type of eruption the least violent, compared to other volcanic eruptions with the height of their tephra clouds reaching 2 km.
A satellite image of Madina in the northwest and the basaltic flows in the black areas.
A multi-crater volcano in the Harrat Rahat near Madina.
Volcanic cones of the historic volcanoes that was formed in 654 H (1256 AD) volcano.
Basaltic flows of the historic volcano in the northern Harrat Rahat.
Fissures associated with volcanism in the northern Harrat Rahat.
A large fissure associated with the volcanic activities in the northern Harrat Rahat.
A lava tube that was formed during the historic volcano eruption