​​Volcanism in Saudi Arabia
 
In north-western and central western Arabia crustal extension has resulted in significant Cenozoic volcanism. More than 80 percent of the volcanism has resulted in shield volcanoes, with fairly flat slopes (2o to 6o), due to thin fluid basalt lava flows and with clearly-marked craters, and cinder (scoria) and spatter cones marking degassing points along fissures.  Ash cones also occur, such as in Harrats Lunayyir and Kishb.

The first phase of the volcanism 20 to 30 million years ago was associated with the opening of the Red Sea. These older lava fields are so eroded that no morphological volcanoes remain on the surface. The more recent basaltic lava fields and volcanoes date from 10 million years ago up to the historic eruptions. They lie along a 900 km line that extends from the Great Nafud Desert, through the cities of Al Madinah and Makkah, and then as far south along the coastal plain as Al Qunfudah. The northernmost 600 km of this trend forms a north-south graben structure named the Makkah-Madinah-Nafud (MMN) volcanic line, which includes Harrats Rahat, Khaybar and Ithnayn and forms the axis of uplift in western Saudi Arabia.  Harrat Rahat, between Makkah and Madinah, covers about 20,000 km2, and has 644 scoria cones, 36 shield volcanoes and 24 domes.  Most of the volcanism in western Saudi Arabia is probably due to a northward flow in the asthenosphere from the Afar triple junction at the southern end of the Red Sea, where the East African rift joins the spreading centres of the Red Sea and the Gulf of Aden. The Afar junction is probably underlain by an upwelling mantle plume, a convection phenomenon from deep within the Earth’s mantle. The northward flow then provides the material that generates the surface volcanism by upwelling along the MMN axis and rifts related to seafloor spreading and regional tension within the Arabian plate.  Low level geothermal activity and seismicity indicate that the MMN trend remains active.
 
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A:  Main Cenozoic lava fields in Saudi Arabia showing
     the MMN volcanic line.
B:  The three-armed rift of the Red Sea – Gulf of Aden
     – East African Rift zone.  An inferred mantle plume
     is below the Afar triangle. 
 
Harrat Lunayyir (Al-Shaqah)
Harrat Lunayyir (Al-Shaqah) is a late Cenozoic to Holocene basaltic lava field north of Yanbu, in which one of the volcanic cones may have erupted around the 10th century AD.  Since 2007 a swarm of more than 30,000 earthquakes has occurred beneath Harrat Lunayyir, including a M5.4 event on 19 May, 2009, which caused minor damage to structures in the town of Al Ays (40 km to the SE).  An 8-km-long surface rupture across the northern part of the volcanic field that occurred during this event has been modelled from Interferometric Synthetic Aperture Radar (InSAR) data as being due to an intrusion of a 2 m thick magmatic dike of about 0.13 km3 volume, with its top at less than 2 km depth.  Although the magma did not reach the surface, SGS is continuing to monitor the area for any signs of renewed activity and possible surface eruptions of lava from the dike. 
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Aerial view of vent zone of historic (1256 AD) volcanic eruption in northern Harrat Rahat.
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The Wahbar crater in Harrat Kishb, about 200 m deep and 2 km wide, is due to a phreatic steam explosion at the contact between magma and ground water.
Historic volcanic activity

Although most harrats are inactive, the volcanic lava field of Harrat Rahat between Makkah and Al Madinah has experienced volcanism in historic times. The oldest lavas near Madinah are only about 2 million years old, and the youngest lavas (less than 6000 years old) resulted from 11 eruptions, with 2 historic eruptions in AD 641 and AD 1256. The 641 AD eruption resulted in a small line of cinder cones to the southwest of the city. The last well-documented eruption in Saudi Arabia occurred in the northern end of Harrat Rahat near Al Madinah in 1256 AD/ 654 AH, and was preceded by significant earthquake activity for several days.  Fountains of basalt lava occurred 19 km to the southeast of the city, and lava advanced toward the city. The eruption continued for 52 days, and the lava flow reached to within 12 km of the city.  About half a cubic kilometer of alkali olivine basalt was extruded from a 2.25 km-long fissure during this eruption. Three large scoria cones and three spatter cones were produced at the vent zone, and the lava flowed a maximum distance of 23 km. The city is now expanding into the area of the flow, and SGS maintains a local seismograph network around this end of the harrat  to warn of any impending risk from an eruption, although there is a very low probability of this happening. An eruption was reported in Harrat Khaybar to the north of Harrat Rahat in the 7th century AD, and there was a possible eruption in about 640 AD at Harrat Uwayrid.

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The May 19, 2009, fissure in soft sediments in Harrat Lunayyir .
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Cenozoic lava flows filling valleys between the hills (Precambrian basement) in Harrat Lunayyir.
Hot springs and fumaroles

Geothermal phenomena occur as shallow water wells with elevated temperatures, fumaroles and hot springs.   In some places along the harrats steam emerges from the ground, such as in Harrats Ithnayn and Khaybar, although the steam temperature is less than 50oC and can only be observed during the colder months.  The main geothermal springs are in the foothills of the Precambrian shield adjacent to the Red Sea, and temperatures up to 100oC have been reported for a hot spring at Al Lith, with similar temperatures near Jizan.  These thermal springs result from heating of rain water that has infiltrated through faults.  SGS continues to monitor the temperatures of the hot springs.