Abstract

The present study reveals two sites of the lava tubes that conducted successfully using geophysical techniques in Umm El-Quttein area within Harrat El-Jabban (total area 11.400 km²), NE Jordan. This Harra is part of the large intra-continental Harrat Ash Shaam (total area 45.000 km²), which is the largest Cenozoic volcanic lava field of the twelve basaltic plateaus mostly separated from each other in the western parts of the Arabian Peninsula. Harrat Ash Shaam is extending from Syria in the north, through Jordan to northwestern part of Saudi Arabia; it consists of six basaltic flows (B1 to B6), basaltic dikes, tephra cones and volcanic ridges of Oligocene to Holocene age.

The Fahda Vesicular Basalt Formation (FA) of Al-Bishriyya Group is characterized by slightly weathered linear pressure ridges, joints up to 2 m, fissures, and is very important lava tubes form readily within the Pahoehoe and "aa" lava flows. Two locations of the lava tubes were chosen in the study area, the first is known as Azzam cave that is formed in a pressure ridge, and the second is known as Al-Howa tunnel which resulted from lava sheets by repeated inflation of initial lava delta, as indicated by previous geological studies. These tubes across the terrain at slopes of less than 2º, and can reach width of 5 to 10 m, which have transported molten lava to several kilometers from the eruptive vents with little cool. Survey grid and three geophysical techniques were performed in the study area. These techniques included the Very Low Frequency-Electromagnetic (VLF), Vertical Electrical Sounding (VES), and Electrical Imaging (EI). Each technique has limitations in depth and resolution accuracy based on geological factors, dimensions of the lava tubes, and directions. In order to define these limitations, the geophysical data were collected and interpreted over two known lava tube locations.

From the qualitative interpretation of VLF-EM mapping, three anomaly zones can be recognized, according to their shapes, amplitudes, frequency, and current density at different depths and orientations; these are: (1) Elongated anomalies that are characterized by very low conductivity zones. These anomalies are related to the lava tubes at different depth levels. (2) Linear and circular anomalies reflect a good correlation between the VLF conductors and the structural features. The two main linear conductive zones are distinguished by inferred fault structures. These anomalies coincide with results of Fraser, Karous and Hjelt filters, tilt angle, and phase angle maps, which may suggest a normal fault with possible minor strike-slip movements in a NE-SW direction as also suggested by previous geological studies. Further, linear conductive zone of pressure ridge suggest a relation with Azzam cave, and trending in the E-W direction. The VLF conductors also confirm the presence of circular anomalies associated with fault structures that may represent minor basaltic dikes. (3) Semi-circular anomalies which are characterized by high conductivity zones.

The results of the VLF-EM mapping indicate that these tubes are closely spaced and are limited in the orientations within Al-Bishriyya Group, where the anomalies related to the extension of Al-Howa tunnel have trending in the NE-SW direction, and a continuation to the tunnel flows in length and thickness, while the anomalies related to the extension of the Azzam cave have trending in the NNW-SSE direction that adjacent to Jabal Quis volcano. The quantitative interpretation of VLF-EM models indicates that the elongated anomalies are different in the high peak frequency responses, where the values of maximum in-phase response for the extension of Al-Howa tunnel is ranging from 16% to 19%, while the extension of the Azzam cave is ranging from 1% to 15%. Results of the VLF-EM modeling suggest that the variation in high peak frequency response reflect varying the extensions of the dimensions of these tubes within the basaltic flows. However, the radii of the lava tubes vary with depth from north to south direction of the study area, that due to the topography, overburden thickness, and that is controlled by elevation of the eruptive vents.

The interpretation of Electrical Imaging (EI) confirms the maximum depth for the lava tubes at 19 m below the surface, and has a high resistivity values ranging from (932 to greater than 3300 Ohm-m). The VLF-EM and EI anomalies also correlated with many fissures which are distinguished by shallow conductive zones, and decreasing in resistivity values that have values of (373 to less than 156 Ohm-m). Moreover, the depths of Al-Bishriyya Group was recognized, where the average depth of basalt (FA) formation ranges from 10 to 40 m below all the transverse profiles, while the scoriaceous (HN) formation of the Rimah Group has a depth of 12 to 24.9 m. The integration of geophysical data interpretation indicates that most of the lava tube structures in Umm El Quttein area are sub-parallel to the last phases of youngest eruptive flows. This study is providing a new insight in the explorer's geophysicists, geologists, and volcanologist.