The Harrat Rahat volcanic field, which is in the west-central part of the Kingdom of Saudi Arabia, is the largest of 15 harrats (Arabic for “volcanic field”) hosted within the Arabian plate (fig. 1). Harrat Rahat is 50 to 75 km wide (east-west) and 300 km long (north-south), covering an area of approximately 20,000 km2 and encompassing more than 900 observable vents. The map on sheet 1 shows the volcanic geology of the northern part (about 3,340 km2) of Harrat Rahat (see fig. 2 for location of the map area within Harrat Rahat), at a scale of 1: 75,000. Two additional maps highlight areas of interest at 1: 25,000 scale (sheets 2, 3). Northern Harrat Rahat is of interest owing to the location of the city of Al-Madinah Al-Munawarah (hereafter, referred to as Al-Madinah), which sits atop (and is recently expanding over) the north end of the volcanic field (Downs and others, 2018). Al-Madinah is home to more than 1.5 million residents, and the city experiences an additional influx of approximately 3 million pilgrims annually. The downtown area of Al-Madinah is less than 8 km from lava flows of the only confirmed historical eruption (unit bla; included in red-shaded area [eruptive stage 1] on fig. 3, on sheet 4), which occurred in 1256 CE (Al-Samhoody, 1486; Camp and others, 1987; Camp and Roobol, 1991; Kawabata and others, 2015; Kereszturi and others, 2016; Murcia and others, 2017; Dietterich and others, 2018; Downs and others, 2018). Mapping was undertaken by the US Geological Survey (USGS) in collaboration with the Saudi Geological Survey (SGS) as part of a project entitled “An Agreement for Implementing a Volcano and Seismic Hazard Evaluation and Mitigation in the Al-Madinah Region (Northern Harrat Rahat).” The features of primary interest within the map area are scoria cones, lava flows, lava domes, craters, and pyroclastic deposits of Quaternary age, which have compositions of tholeiitic basalt (or “transitional basalt”), alkalic basalt, hawaiite, mugearite, benmoreite, and trachyte (nomenclature from Cox and others, 1979)(figs. 4A, 5). The compositions of these Quaternary volcanic rocks are discussed in detail below, in the section entitled “Compositions and Eruptive Styles of Volcanic Rocks.” Older volcanic rocks (unrelated to volcanism at northern Harrat Rahat) have been mapped in the northernmost part of sheet 1. These include Quaternary-age (early Pleistocene) basalts (units Qbhk, Qbjs, and Qbra) erupted from the Harrat Kurama volcanic field (fig. 2), located east of Al-Madinah, and Tertiary-age basalts and hawaiites (units Tbja and Tbjj), which cap high-standing hills made up of metamorphosed Precambrian sedimentary, volcanic, and igneous rocks (unit=), located west of Al-Madinah. Precambrian rocks form broad, mountainous regions that surround northern Harrat Rahat, but they also are present as isolated, scattered kīpukas along the margins of the volcanic field. Surficial deposits include modern alluvium (unit al), which is restricted to modern drainage channels, and Quaternary alluvium, colluvium, loess, mud flat, and sabkha (Arabic for “salt flat”) deposits (together, unit Qal), which are identified throughout northern Harrat Rahat and the surrounding area.