The Jebel Hadid Structure is located in southeast Libya, roughly 370 km SSE of Al Kufra. It is made up of five concentric rings, the outermost of which is approximately 4.7 km across. Jebel Hadid means 'Iron Mountain' in Arabic. The area was heavily land-mined during the Chadian-Libyan War of 1978-1987, making it a difficult if not impossible place in which to carry out geological fieldwork. However the structure is clearly visible in satellite images, and a number of studies of the area were made before 1978, the data from which is still available.
A satellite image of the Jebel Hadid Structure.
In 2009 a paper appeared in the journal Marine and Petroleum Geology in which Martim Schmieder and Elmar Buchner of the Institut für Planetologie at Universität Stuttgart and Daniel Paul Le Heron of Royal Holloway, University of London, propose that the Jebel Hadid Structure is an impact structure, caused by an extra-terrestrial object striking the Earth.
There are a number of terrestrial occurrences that can cause circular marks on the surface of the Earth. The most obvious if these is volcanism, but the Jebel Hadid Structure is situated entirely in sedimentary rocks, so this can be ruled out.
Salt or gypsum diapirism can both form circular structures; these are caused by mineral rich waters drying up, causing the minerals to precipitate out of solution. Since these minerals occupy slightly more space as solids than as solutions they can form 'diapirs' circular areas of uplift (bulges). However there are no indications of evaporite deposits at Jebel Hadid.
Sand volcanism, caused by gas bubbles erupting through wet sands, can be ruled out by the large size of the structure; 4.7 km gas bubbles are to improbable for consideration.
Karst structures are formed by the dissolution (chemical weathering) of limestones and can be circular in shape, but there do not appear to be any significant limestone deposits around Jebel Hadid.
Finally circular structures can be formed by the thawing and decompression of deposits which have been covered by glaciers, as sediments expand and are forced upwards in circular bulges called pingos. However the deposits of Jebel Hadid are located in the Libyan Dessert, where glaciation is unlikely, and Africa has been drifting northwards since the breakup of the Gondwana Supercontinent, so that it has had to cross the equator since its last encounter with glacial conditions, which would have been at the very latest in the Devonian.
The structure is partially overlain by Miocene (23-5.3 million year old) fluvial sandstones (sandstones formed in a river basin), and is found in Early Cretaceous sandstones, the older rocks beneath, though it is not completely clear how deep the structure extends. There were apparently no rocks laid down in the area between the Early Cretaceous and the Miocene (not unusual) so if the structure is an impact crater then it would have been caused by an object falling to Earth during this period. There would not necessarily be any remains of the object at the impact site, as an object burning up in the low atmosphere would still cause a considerable pressure wave to strike the surface. Concentric rings within impact structures are caused by uplift of the central area after the initial strike as the rocks decompress, and by collapse of the outer rim.
The structure of Jebel Hadid, as envisioned by Schnieder et al.
Jebel Hadid resembles a number of impact structures that scientists have been able to access, such as Tin Bidar in Algeria and Aorounga in Chad (impact structures are more common in desert areas because they last longer; in other areas they tend to be buried or eroded quickly), supporting the hypothesis that Jebel Hadid is an impact structure.
The Tin Bider Impact Structure in Algeria, which Jebel Hadid resembles.
See also A close encounter with Asteroid 2005 YU₅₅, When Moons Collide and Visiting Vesta.