The Bangui magnetic anomaly is a local variation in the

Earth's magnetic field Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magnetic f ...

centered at

Bangui Bangui () (or Bangî in Sango, formerly written Bangi in English) is the capital and largest city of the Central African Republic. It was established as a French outpost in 1889 and named after its location on the northern bank of the Ubangi ...

, the capital of

Central African Republic The Central African Republic (CAR; ; , RCA; , or , ) is a landlocked country in Central Africa. It is bordered by Chad to the north, Sudan to the northeast, South Sudan to the southeast, the DR Congo to the south, the Republic of th ...

. The

magnetic anomaly In geophysics, a magnetic anomaly is a local variation in the Earth's magnetic field resulting from variations in the chemistry or magnetism of the rocks. Mapping of variation over an area is valuable in detecting structures obscured by overlying ...

is roughly

elliptical Elliptical may mean: * having the shape of an ellipse, or more broadly, any oval shape ** in botany, having an elliptic leaf shape ** of aircraft wings, having an elliptical planform * characterised by ellipsis (the omission of words), or by conc ...

, about , and covers most of the country, making it one of the "largest and most intense crustal magnetic anomalies on the African continent". The anomaly was discovered in the late 1950s, explored in the 1970s, and named in 1982. Its origin remains unclear.

History

In 1962, Raymond Godivier and Lucien Le Donche reported on a magnetic anomaly in Central African Republic, which they identified by analyzing their surface magnetic activity data of 1956. These results were confirmed and built upon by the high-altitude aeromagnetic surveys carried out by the US Naval Oceanographic Office, as well as by the satellite measurements conducted in 1964 with Cosmos 49 and in the 1970s with the

Orbiting Geophysical Observatory Orbiting Geophysical Observatory (OGO) Program of NASA refers to the six satellites launched by the United States that were in use from September 1964 to 1972, designed to study the Earth's magnetosphere. The satellites successfully studied th ...

at altitudes. This data was combined in 1973 and yielded a spatial map of Earth's magnetic field, which was then updated after the launch of the

Magsat Magsat (Magnetic field Satellite, Applications Explorer Mission-C or AEM-C or Explorer 61) was a NASA/USGS (United States Geological Survey) spacecraft, launched on 30 October 1979. The mission was to map the Earth's magnetic field, the satelli ...

satellite with an accuracy of 15 nT at an altitude of . In 1982, Robert D. Regan and Bruce D. Marsh named the anomaly after the city located at its center. The anomaly is sometimes called the Bangui negative anomaly, owing to its negative peak-to-trough difference, and is compared with the positive anomalies observed at the

Benue Trough The Benue Trough is a major geological structure underlying a large part of Nigeria and extending about 1,000 km northeast from the Bight of Benin to Lake Chad. It is part of the broader West and Central African Rift System. Location The ...

and

Congo Basin The Congo Basin (french: Bassin du Congo) is the sedimentary basin of the Congo River. The Congo Basin is located in Central Africa, in a region known as west equatorial Africa. The Congo Basin region is sometimes known simply as the Congo. It con ...

where

Lower Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago (m ...

geological formations are exposed.

Salient features

The Bangui anomaly is bounded to the south by the

Walvis Ridge The Walvis Ridge (''walvis'' means whale in Dutch and Afrikaans) is an aseismic ocean ridge in the southern Atlantic Ocean. More than in length, it extends from the Mid-Atlantic Ridge, near Tristan da Cunha and the Gough Islands, to the African ...

, the north by the Cameroon–St. Helena volcanic line, and to the west by the

Mid-Atlantic Ridge The Mid-Atlantic Ridge is a mid-ocean ridge (a divergent or constructive plate boundary) located along the floor of the Atlantic Ocean, and part of the longest mountain range in the world. In the North Atlantic, the ridge separates the North Ame ...

. It is shaped approximately as an

ellipse In mathematics, an ellipse is a plane curve surrounding two focus (geometry), focal points, such that for all points on the curve, the sum of the two distances to the focal points is a constant. It generalizes a circle, which is the special ty ...

in size. It has three sections, and the magnetic equator runs through its center. It has a short axis diameter of about , and its amplitude varies between –1000 nT at ground level and –20 nT at satellite altitude, about . Its features include a Bouguer

gravity anomaly The gravity anomaly at a location on the Earth's surface is the difference between the observed value of gravity and the value predicted by a theoretical model. If the Earth were an ideal oblate spheroid of uniform density, then the gravity meas ...

of −120 mGal, a topographical surface feature shaped as a ring of diameter, rock features of Late

Archean The Archean Eon ( , also spelled Archaean or Archæan) is the second of four geologic eons of Earth's history, representing the time from . The Archean was preceded by the Hadean Eon and followed by the Proterozoic. The Earth Earth ...

and

Proterozoic The Proterozoic () is a geological eon spanning the time interval from 2500 to 538.8million years ago. It is the most recent part of the Precambrian "supereon". It is also the longest eon of the Earth's geologic time scale, and it is subdivided ...

periods in the central part of the anomaly,

granulite Granulites are a class of high-grade metamorphic rocks of the granulite facies that have experienced high-temperature and moderate-pressure metamorphism. They are medium to coarse–grained and mainly composed of feldspars sometimes associated w ...

s, and

charnockite Charnockite () is any orthopyroxene-bearing quartz-feldspar rock formed at high temperature and pressure, commonly found in granulite facies metamorphic regions, ''sensu stricto'' as an endmember of the charnockite series. Charnockite series Th ...

s rock formations supplemented by

granite Granite () is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies undergro ...

s at the lower crust level, and greenstone belts, and metamorphosed basalts seen as rock exposures. A zone of thinner crust bounds the anomaly to the north and a zone of relatively thicker crust is on the southern edge. Pannotia

Origin

Two theories have been suggested for the origin of the Bangui anomaly, neither being conclusive. One theory points to a large

igneous intrusion In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...

and the other to a

meteorite A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or Natural satellite, moon. When the ...

impact in the

Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the ...

(before 540 Ma).Girdler, R.; Taylor, P.; Frawley, J. (1992
A possible impact origin for the Bangui magnetic anomaly (Central Africa)
''

Tectonophysics Tectonophysics, a branch of geophysics, is the study of the physical processes that underlie tectonic deformation. The field encompasses the spatial patterns of stress, strain, and differing rheologies in the lithosphere and asthenosphere of th ...

'', Volume 212, Issue 1, p. 45-58 To support the latter theory, a connection was drawn with a meteorite impact that may have occurred in

Brazil Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...

Bahia state Bahia ( , , ; meaning "bay") is one of the 26 states of Brazil, located in the Northeast Region of the country. It is the fourth-largest Brazilian state by population (after São Paulo, Minas Gerais, and Rio de Janeiro) and the 5th-largest b ...

causing formation of

carbonado Carbonado, commonly known as black diamond, is one of the toughest forms of natural diamond. It is an impure, high-density, micro-porous form of polycrystalline diamond consisting of diamond, graphite, and amorphous carbon, with minor crysta ...

s (black diamond aggregates) which are only found in the Central African Republic and Brazil.

References

{{Reflist, refs= {{cite book, title=Geological Investigations in West Africa , series=Geological Survey Professional Paper, volume=1275, url=https://books.google.com/books?id=UfRUAAAAYAAJ&pg=PA255, year=1982, publisher=U.S. Government Printing Office, page=255 {{cite book, first1=David , last1=Gubbins, first2=Emilio , last2=Herrero-Bervera, title=Encyclopedia of Geomagnetism and Paleomagnetism, url=https://books.google.com/books?id=O-wA0ocxAiIC&pg=PA39, year=2007, publisher=Springer, isbn=978-1-4020-4423-6, pages=39–40 {{cite book, last=Langel, first= R. A. , last2=Hinze, first2= W. J. , title=The Magnetic Field of the Earth's Lithosphere: The Satellite Perspective, url=https://books.google.com/books?id=R0Fa_YcIPt8C&pg=PA11, year= 1998, publisher=Cambridge University Press, isbn=978-0-521-47333-0, pages=11–12 {{cite book, last=Lowman, first=Paul D., title=Exploring Space, Exploring Earth: New Understanding of the Earth from Space Research, url=https://books.google.com/books?id=BAcG2Z4F41AC&pg=PA95, accessdate=8 April 2013, date=15 August 2002, publisher=Cambridge University Press, isbn=978-0-521-89062-5, pages=95, 97 Bangui Geology of the Central African Republic Magnetic anomalies Geology of Africa

History

Salient features

Origin

See also

References