Geothermal Anomalies Along the MMN Volcanic Line Including the Cities of Al Madinah al Munawwarah and Makkah al Mukarramah (A Reprint of Dmmr Madinah-CR-15-2)

SGS-TR-2007-6 M. John Roobol, Khalid Bankher, and Sameer Bamufleh
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A north-south line extending for 900 km from the southern edge of the Great Nafud Desert, south along Harrats Ithnayn and Khaybar to the city of Al Madinah al Munawwarah*, south along Harrat Rahat to the city of Makkah al Mukarramah*, and south along the Red Sea coastal plain as far as the city of Al Qunfudah, shows geothermal phenomena. These take the form of shallow waterwells with elevated temperatures, fumaroles and hot springs with visible steam columns on winter mornings situated in the volcanoes of the harrats, in fissures cutting flank lava flows, in the Precambrian basement between the harrats, and on the Red Sea coastal plain. Above the escarpment (where the crust is around 40 km thick) these have temperatures up to 53°C., and on the Red Sea coastal plain (where the crust is 15 km or less thick) temperatures up to 130°C. Commercial hot mineral baths have been developed at at least four sites along the north-south line. Ground water wells in and around the cities of Madinah and Makkah show eleveated temperatures.  Monitoring of the Madinah area for the past decade identified a 3.2 km long line of weak fumaroles adjacent to the 1256 A.D. historic lava flow and only 4 km from apartment buildings of the modern city. A DMMR local seismic network recorded microseismic activity and positioned clusters of epicenters along this line. Both the geothermal and associated microseismic activity appear to have peaked between May 1986 and June 1988 and have since waned and ceased a this site, although continue in the region.  From late 1984 to the end of 1991 the water well monitoring revealed that about half of the wells measured in the Madinah area were slowly increasing in temperature with about a quarter decreasing.  One shallow well (No. 250 - 30 m deep with water level at 16 m) had a temperature of 48°C in early 1984, 53°C in late 1989 and 47.7°C in late 1994. Another well (N0. 60A) had a temperature of 42.5°C on late 1991 and 39.6°C in late 1994, Isotopic compositions of groundwater near Madinah and from hot springs on the Rd Sea coastal plain reveal only a meteoric component without indication of a magmatic source.  The northern most 600 km length of this geothermally anomalous line is marked by Harrats Ithnayn, Khaybar and Rahat, and has been named the Makkah-Madinah-Nafud (MMN) Volcanic Line.  This marks the axis of the Hail Arch or western part of the Afro-Arabian dome named the West Arabian Swell (WAS). The WAS, lava fields and geothermal phenomena are interpreted to be the surface expression of an axis of upwelling mantle (asthenosphere) – i.e. the WAS is thermally supported by upwelling mantle. Along the axis (MMN Volcanic Line) there is a weakly propagating rift zone where crustal extension has averaged about 0.054 mm per year over the past 10 million years. This is distinct from the Red Sea Rift which is believed to have formed 28° obliquely to the main line of upwelling asthenosphere due to rifting along pre-existing geological structures in the Arabian-Nubian Shield.  The geothermal phenomena, the associated microseismic activity and the historic basaltic volcanism are all interpreted as resulting from the weakly propagating rift zone of the MMN Line along the axis of the thermally-supported crustal dome. Although the local activity in the vicinity of Madinah has waned and the 3 km line of weak fumaroles ceased to operate, other fumaroles and the presence of hot springs and elevated groundwater along the MMN Volcanic Line and its southern extension onto the Red Sea coastal plain indicate that this weakly propagating rift remains active. Continued monitoring of the cities of the MMN Line is recommended with the objectives of defining risk and formation of adequate building codes. The city of Makkah lies not only on the MMN Volcanic Line but also within the coastal belt of Cenozoic rotational normal faults and close to the axial trough of the Red Sea rift



Roobol, M.J., Bankher, K., and Bamufleh, S. 2007, Geothermal anomalies along the MMN volcanic line including the cities of Al Madinah al Munawwarah and Makkah al Mukarramah: Saudi Geological Survey Technical Report SGS-TR-2007-6, 87 p. 21 figs, 10 tables, 3 apps, 9 plates (A Reprint of DMMR-Madinah-CR-15-2)