A Multidisciplinary Approach to Unraveling the Spatio-Temporal Relationships of Basalt Flows within Northermost Harrat Rahat, Kingdom of Saudi Arabia

Tuesday, 15 December 2015
Poster Hall (Moscone South)
Drew Tyler Downs1, Mark E Stelten2, Duane E Champion3,4, David R Sherrod1,2, Joel E Robinson3,4, Khalid H. Hassan5, Fawaz A. Muquyyim6 and Mahmod S. Ashur5, (1)USGS Western Regional Offices Menlo Park, Menlo Park, CA, United States, (2)USGS Alaska Science Center, Anchorage, AK, United States, (3)USGS, Menlo Park, CA, United States, (4)California Volcano Observatory, USGS, Menlo Park, CA, United States, (5)Saudi Geological Survey, Jeddah, Saudi Arabia, (6)Saudi Geological Survey, Al-Madinah, Saudi Arabia
The Harrat Rahat volcanic field in the Kingdom of Saudi Arabia covers ~20,000 km2 and has a volume of ~2,000 km3, making it the largest intraplate volcanic field in that country. The vast majority of this field is rural and uninhabited land. However, sitting on basalt flows at the northernmost limit of this volcanic field is a city that is home to >1.5 million people and ~3 million pilgrims annually. Over the centuries, population growth and city expansion have led to levelling of and construction atop the surrounding lava flows. Although mapped previously, none of these basalt flows have been dated, and hence, it is unknown how many times the area encompassed by this city has been inundated by these lava flows. As part of an ongoing collaboration between the USGS and the Saudi Geological Survey, which allowed access throughout, we have conducted new field mapping, petrography, 40Ar-39Ar dating, paleomagnetism, and geochemistry studies to define the number, age, and areal extent of basaltic lava flows that have reached areas that are now populated. More than 50 samples have been collected from within the city and analyzed. New ages on five of the flows range from ~340 to 100 ka, a sharp contrast with previous age estimates that extended as far back as 2.5 Ma. In addition, paleomagnetic, petrographic, and geochemical analyses indicate that four basalt flows within the western half of the city are from individual eruptions. At least five flows underlie the eastern half of the city, and several of these flows may be from a single eruptive episode. The youngest flow (part of the 1256 AD eruption) came within ~5 km of the city center. Our results indicate that pre-1256 AD basalt flows inundated the populated areas more frequently, and are younger than previously thought. This new temporal and geochemical record will help improve the accuracy and precision of future modeling and quantitative hazard assessments.