Abstract

Distributed volcanic fields are found in continental settings throughout the world and in some cases pose serious hazards to local populations and infrastructure. Saudi Arabia hosts numerous dominantly basaltic distributed volcanic fields that have erupted into the recent geologic past. One of the largest is the Harrat Rahat volcanic field that covers ~20,000 km² and spans ~310 km from near Makkah at its southern end to Al Madinah at its north end. Harrat Rahat commenced eruptions ~10 Ma, with its most recent activity being a historic eruption of basalt in 1256 CE that effused a 23-km-long lava flow into what is now eastern Al Madinah, highlighting the hazard this volcanic field presents. To assess its eruptive history and volcanic hazards, the U.S. Geological Survey and Saudi Geological Survey conducted a 5-year collaborative investigation of Harrat Rahat’s northern ~70 km. Results include a detailed geologic map that delimits individual eruption products supported by publications reporting and interpreting extensive high-precision geochronologic (⁴⁰Ar/³⁹Ar, ³⁶Cl), geochemical, geophysical, and paleomagnetic datasets. Building on prior studies of Harrat Rahat, these results document spatial and compositional changes in volcanism through time, delimit the rates of magma differentiation, and characterize the volcanic recurrence rate and volume output. Findings include that multiple lava flows near Al Madinah, previously interpreted as post-Neolithic but pre-historical, are actually late Pleistocene; that a minimally documented eruption in 641 CE probably occurred at Harrat Khaybar, ~150 km north of Al Madinah, rather than from northern Harrat Rahat; and that an explosive and dome-forming trachyte eruption took place in northern Harrat Rahat in the Holocene.  Insights learned from our study have important implications for future volcanic activity in the northern Harrat Rahat, for hazards to Al Madinah, and for volcanism in the Arabian Peninsula more broadly. Furthermore, distributed volcanic fields in Saudia Arabia represent an important, and well characterized, reference point for ongoing studies of distributed volcanic fields in other continental regions.

Biography

Mark Stelten received his BS in geology from the University of North Carolina, Chapell Hill in 2008. He then received his MS in geology from the University of California, Davis in 2010, where he studied volcanism at South Sister volcano (Oregon, USA) using U-series disequilibrium dating techniques. Mark then received his Ph.D. in geology from the University of California, Davis in 2014 where he integrated ²³⁸U-²³⁰Th crystallization ages, trace-element concentrations, and isotopic compositions of minerals and glass from volcanic rocks at Yellowstone caldera to investigate the origin and evolution of the magmatic system and constrain timescales of magma generation. Mark then conducted his postdoctoral research with the U.S. Geological Survey from 2014 to 2018, where he studied the eruptive history of northern Harrat Rahat volcanic field as part of a joint USGS-Saudi Geological Survey. Mark is now the Deputy Scientist-in-Charge of the Yellowstone Volcano Observatory for the USGS, leads efforts into studying distributed volcanism in the western U.S. for the USGS, and runs the ⁴⁰Ar/³⁹Ar geochronology lab based out of Moffett Field, CA.