Hydrogeophysical Study of Sub-Basaltic Alluvial Aquifer in the Southern Part of Al-Madinah Al-Munawarah, Saudi Arabia

Groundwater is extremely important in a water-scarce country such as Saudi Arabia, where permanent surface water resources are absent. Sustainable and future developments plans are essentially relying on the clear understanding of water resources. To evaluate the water resources in arid countries, the groundwater should be quantified through either traditional or scientifically advanced techniques. Aquifer characteristics, particularly the hydraulic conductivity and transmissivity, are essential for the evaluation the aquifer properties as well as the management and development of groundwater modelling for specific aquifers. The present study aims to evaluate the sub-basaltic alluvial aquifer in the northern part of Harrat Rahat, south of Al-Madinah city, and then estimates the principal aquifer’s hydraulic parameters based on the interpreted 1D resistivity-depth models along the study area. For that, 49 Vertical Electrical Soundings (VES’s) utilizing a Schlumberger electrode array were performed along the southern part of Al-Madinah city. The resistivity of the water-bearing formation, thickness, porosity, hydraulic conductivity, and transmissivity parameters were calculated along the measured longitudinal profile from the interpreted VES data. The estimated porosity, hydraulic conductivity, and transmissivity were achieved along the whole profile with average values of 0.2, 3.5 m/day, and 369.6 m2/day, respectively. The resulting transmissivity values from the VES models were compared with those of previous pumping test measurements carried out in the area and a reasonable correlation between the two data sets was observed. These results indicate that surface geoelectrical resistivity techniques may provide an alternative, rapid, and cost-effective method of estimating the aquifer hydraulic parameters where pumping data is rare or unavailable.

Mechanical Performance of High-Strength Sustainable Concrete under Fire Incorporating Locally Available Volcanic Ash in Central Harrat Rahat, Saudi Arabia

This study investigated the effect of elevated temperatures on the mechanical properties of high-strength sustainable concrete incorporating volcanic ash (VA). For comparison, control and reference concrete specimens with fly ash (FA) were also cast along with additional specimens of VA and FA containing electric arc furnace slag (EAFS). Before thermal exposure, initial tests were performed to evaluate the mechanical properties (compressive strength, tensile strength, and elastic modulus) of cylindrical concrete specimens with aging. Additionally, 91 day moist-cured concrete specimens, after measuring their initial weight and ultrasonic pulse velocity (UPV), were exposed up to 800 °C and cooled to air temperature. Subsequently, the weight loss, residual UPV, and mechanical properties of concrete were measured with respect to exposure temperature. For all concrete specimens, test results demonstrated a higher loss of weight, UPV, and other mechanical properties under exposure to higher elevated temperature. Moreover, all the results of concrete specimens incorporating VA were observed before and after exposure to elevated temperature as either comparable to or slightly better than those of control and reference concrete with FA. According to the experimental results, a correlation was developed between residual UPV and residual compressive strength (RCS), which can be used to assess the RCS of fire-damaged concrete (up to 800 °C) incorporating VA and EAFS.

Intra-eruptive trachyte-phonolite transition: Natural evidence and experimental constraints on the role of crystal mushes

The generation of silica undersaturated phonolite from silica saturated trachytes is uncommon, as it implies the crossing of the thermal barrier and critical plane of silica undersaturation. Nevertheless, a co-genetic suite displaying compositional transition from benmoreite-trachyte to phonolite has been observed within the Al Shaatha pyroclastic sequence in the Harrat Rahat Volcanic Field (Kingdom of Saudi Arabia). We performed crystallization experiments on benmoreite and trachyte starting compositions to simulate the pressure-temperature-volatile conditions that generated the observed liquid line of descent. The experimental conditions were 200–500 MPa, 850–1150 °C, 0–10 wt% H2O, 0.0–0.5 wt% CO2, and NNO+2 oxygen buffer. The experimental mineral assemblage consists of clinopyroxene, feldspar, and titanomagnetite, as well as glass in variable proportions. The degree of crystallinity of hydrous runs is lower than that of anhydrous ones at analogous pressure and temperature conditions. Clinopyroxene crystallizes with compositions diopside-augite and augite-hedenbergite, respectively, at 500 and 200 MPa. The saturation of feldspar is primarily controlled by temperature and volatile content, with the more potassic composition equilibrating at low temperature (850–900 °C) and anhydrous (for benmoreite) or hydrous (for trachyte) conditions. At low pressure (200 MPa), temperatures below 850 °C, and anhydrous conditions, the degree of crystallization is extremely high (>90%), and the residual glass obtained from trachyte experiments is characterized by peralkaline and sodic affinity. This finding is consistent with natural eruptive products containing interstitial phonolitic glass within an anorthoclase framework. The shift from trachyte to phonolite is therefore interpreted as the result of open system interaction between trachytic magma and intercumulus phonolitic melt, as well as of dissolution of anorthoclase from a crystal mush.

The timing and compositional evolution of volcanism within northern Harrat Rahat, Kingdom of Saudi Arabia

Harrat Rahat, one of several large, basalt-dominated volcanic fields in western Saudi Arabia, is a prime example of continental, intraplate volcanism. Excellent exposure makes this an outstanding site to investigate changing volcanic flux and composition through time. We present 93 40Ar/39Ar ages and six 36Cl surface-exposure ages for volcanic deposits throughout northern Harrat Rahat that, when integrated with a new geologic map, define 12 eruptive stages. Exposed volcanic deposits in the study area erupted <1.2 Ma, and 214 of 234 eruptions occurred <570 ka. Two eruptions occurred in the Holocene, including a historically described basalt eruption in 1256 C.E. and a trachyte eruption newly recognized as Holocene (4.2 ± 5.2 ka). An estimated ∼82 km3 (dense rock equivalent) of volcanic product have erupted since 1.2 Ma, though this is a lower limit due to concealment of deposits >570 ka. Over the past 570 k.y., the average eruption rate was 0.14 km3/k.y., but volcanism was episodic with periods alternating between low (0.04–0.06 km3/k.y.) and high (0.1–0.3 km3/k.y.) effusion rates. Before 180 ka, eruptions vented from the volcanic field’s dominant eastern vent axis and from a subsidiary, diffuse, western vent axis. After 180 ka, volcanism focused along the eastern vent axis, and the composition of volcanism varied systematically along its length from basalt dominated in the north to trachyte dominated in the south. We hypothesize that these compositional variations <180 ka reflect the growth of a mafic intrusive complex beneath the southern portion of the vent axis, which led to the development of evolved magmas.

Crustal imaging of northern Harrat Rahat, Saudi Arabia, from ambient noise tomography

SUMMARY Harrat Rahat is a volcanic field located in west-central Saudi Arabia and is the site of the most recent eruption in the country (1256 CE). An earthquake swarm at a nearby volcanic field in 2009 prompted the need for new hazard models for this region, which includes the holy city of Medina. Tomography studies can be used to infer material properties of the subsurface such as partial melt, and are instrumental for volcanic hazard assessment. Regional earthquakes have been used to determine mantle structure, but such crustal models are often hindered by an insufficient number of earthquakes in the plate interior. We use ambient seismic noise to compute Rayleigh and Love surface-wave dispersion maps between 5 and 12 s for northern Harrat Rahat. The surface-wave maps are inverted to produce shear-wave velocities using a neighbourhood algorithm and interpolated into a pseudo-3-D model. The distributions of surface-wave and shear-wave velocities are heterogenous, varying between ±3 and 8 per cent. However, low velocities are not restricted to the Harrat. We observed a difference between Rayleigh- and Love-wave velocities that extends north from the site of the 1256 CE eruption and coincides with a low gravity anomaly. We obtain a shear-wave velocity increase of 10–15 per cent between 15 and 25 km depth consistent with the Conrad discontinuity, the interface between andesitic upper crust and the mafic lower crust of the Arabian Shield. The average velocities of the upper and lower crust are estimated to be 3.64 and 3.95 km s–1 using Rayleigh waves and 3.53 and 4.16 km s–1 using Love waves, which are in good agreement with the results of other geophysical studies of this area. The magnitude of the low-velocity anomalies, their location away from the Harrat, and the lack of reversals in the shear-velocity inversions suggest that the presence of a crustal magma chamber is not likely. If a magma chamber exists, it is smaller than can be imaged with a secondary microseism source (approximately 15 km wavelength), deeper than 30 km, or shallower than 5 km with a small velocity contrast.