A scar in the Earth's crust
Beneath the rainforests of the Central African Republic lies a geophysical feature of continental scale: the Bangui Magnetic Anomaly. It is completely invisible from the surface, buried under thick layers of younger rock, but its presence is unmistakable to sensitive instruments. The anomaly is a vast, roughly elliptical zone, measuring approximately 700 by 1,000 kilometers, where the planet's magnetic field is intensely distorted. First noted in ground surveys in 1956 and confirmed by the Soviet Cosmos 49 satellite in 1964, it is one of the largest and most powerful crustal magnetic anomalies on the planet.
At ground level, the magnetic field intensity drops by more than 1000 nanoteslas (nT). Even at an altitude of 400 kilometers, NASA's Magsat satellite recorded a significant negative anomaly of -20 nT. The structure of the anomaly is complex, with a large central negative field flanked by two smaller positive lobes, a classic pattern for a magnetic body located near the magnetic equator. This immense disturbance originates deep within the Earth's crust, with its sources estimated to be between 15 and 35 kilometers down. The rocks responsible are believed to be part of the Precambrian basement, ancient foundations of the African continent formed billions of years ago.
A tale of two theories
The origin of the Bangui Anomaly is a subject of intense scientific debate, with two primary competing hypotheses. One theory proposes that the anomaly is the remnant of a colossal meteorite impact that occurred in the Precambrian era, before 540 million years ago. If correct, the impact structure would be around 800 kilometers in diameter, far larger than any other known impact crater on Earth. Proponents of this idea point to a subtle, ring-like topographic feature with a diameter of 810 km that coincides with the anomaly. They also note a corresponding negative gravity anomaly of -120 milligals, which suggests lower-density material beneath the surface, consistent with a massive, fractured impact zone.
The alternative hypothesis attributes the anomaly to purely terrestrial processes. It suggests the feature is a massive igneous intrusion or a complex of highly metamorphosed rocks emplaced during the Pan-African orogeny—a period of mountain-building and continental collision. This event, which occurred between 650 and 550 million years ago, involved the collision of ancient cratons, creating long, deformed zones called mobile belts. Such a collision could have injected vast quantities of magnetic mafic or ultramafic rocks, like basalt and gabbro, into the crust. These iron-rich rocks, cooling in the presence of Earth's magnetic field, would acquire a strong permanent magnetization, creating the large-scale disturbance seen today. The lack of undisputed shock-metamorphosed rocks, the typical "smoking gun" for an impact, keeps the debate active.