Magnetotelluric Investigation of Northern Harrat Rahat, Kingdom of Saudi Arabia

Professional Paper 1862-L
Jointly published with the Saudi Geological Survey [as Saudi Geological Survey Special Report SGS–SP–2021–1]
By: , and 



Volcanism within the harrats (Arabic for “volcanic field”) of the Kingdom of Saudi Arabia includes at least one historical eruption occurring close to the holy city of Al Madīnah in 1256 C.E. As part of a volcanic- and seismic-hazard assessment of northern Harrat Rahat, magnetotelluric (MT) data were collected to investigate the structural setting of the area, the presence or absence of melt within the crust, and the mantle-derived magmatic source. Collected MT data were modeled in both two dimensions, where anisotropy can be estimated, and three dimensions. Interpretation of the preferred resistivity model includes a shallow sediment-filled graben beneath northern Harrat Rahat lavas, a melt-free upper crust, and a region of decompression melting in the asthenosphere below 60–70 kilometers depth. Models in two dimensions image the lower crust as anisotropic, demonstrating that a series of elongate conductivity anomalies with a strike of N. 10° E. within the lower crust of the three-dimensional model are artifacts of inverting anisotropic data with an isotropic modeling algorithm. Careful examination of the resistivity models, in combination with regional geological and geophysical data, suggests an anisotropic lower crust that is free of large zones of melt. Azimuthal anisotropy in the lower crust extends well beyond the limits of Harrat Rahat volcanic rocks, with a conductive direction oriented N. 10° E. and an anisotropy factor of 2–5 between the most and least conductive directions. Enhanced conductivity is likely caused by interconnected grain-boundary graphite, where the direction of anisotropy reflects either frozen-in fabric from the Neoproterozoic stabilization of the Arabian Shield or ductile deformation driven by channelized asthenospheric flow coupled with a thin rigid mantle lid. Asthenospheric melt is interpreted to transect the crust largely through diking, with limited melt storage and short residence times within the crustal column.

Suggested Citation

Peacock, J.R., Bedrosian, P.A., Al-Dhahry, M.K., Shareef, A., Feucht, D.W., Taylor, C.D., Bloss, B., and Zahran, H.M., 2023, Magnetotelluric investigation of northern Harrat Rahat, Kingdom of Saudi Arabia, chap. L of Sisson, T.W., Calvert, A.T., and Mooney, W.D., eds., Active volcanism on the Arabian Shield—Geology, volcanology, and geophysics of northern Harrat Rahat and vicinity, Kingdom of Saudi Arabia: U.S. Geological Survey Professional Paper 1862 [also released as Saudi Geological Survey Special Report SGS–SP–2021–1], 111 p.,

ISSN: 2330-7102 (online)

ISSN: 1044-9612 (print)

Study Area

Table of Contents

  • Abstract
  • Introduction
  • Method
  • Data
  • Modeling
  • Results and Discussion
  • Summary
  • Acknowledgments
  • References Cited
  • Appendix 1. Anisotropic Modeling
  • Appendix 3. Data and Model Responses
Publication type Report
Publication Subtype USGS Numbered Series
Title Magnetotelluric investigation of northern Harrat Rahat, Kingdom of Saudi Arabia
Series title Professional Paper
Series number 1862
Chapter L
DOI 10.3133/pp1862L
Year Published 2023
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Central Mineral and Environmental Resources Science Center, Crustal Geophysics and Geochemistry Science Center, Geology, Minerals, Energy, and Geophysics Science Center, Volcano Hazards Program
Description Report: vi, 111 p.; Data Release
Country Kingdom of Saudi Arabia
Other Geospatial northern Harrat Rahat
Online Only (Y/N) N
Additional Online Files (Y/N) N
Google Analytic Metrics Metrics page
Additional publication details