A channelled plume under Africa

Large areas of east Africa are and have been highly active geologically, but the underlying processes are debated. New modelling work may shed light on those processes and also illustrates the growing maturity of such studies.

For Earth scientists, the African continent and its surroundings are a distinctive and rich laboratory. Among many noteworthy features, Africa has the highest mean elevation of all of the continents; it hosts the great east African rift system; there is young and active seafloor spreading in the Red Sea and Gulf of Aden; great volumes of volcanic deposits are found in the northeast; and minor volcanism and associated rifting of the crust are scattered across a huge area of the eastern, central and northern parts of the continent. Because all of these features are suggestive of warm or upwelling mantle under Africa, they have been seen for some time as potentially involving mantle plumes — narrow, hot upwellings inside the Earth that were first proposed by W. Jason Morgan1 in 1971.

Opinion has been divided as to how many plumes there might be under Africa. At one extreme, in 1976 Burke and Wilson2 proposed up to about 40 volcanic ‘hotspots’ in Africa, but dynamicists debate whether so many plumes could rise beneath one continent. Most other counts were lower, with Duncan and Richards3 in 1991 including only one clearly identified hotspot within the African continent. However, if that were the case, it is not clear how all of the scattered volcanism could be accounted for by only one or a few plumes. On page 788 of this issue4, Ebinger and Sleep propose a way in which much of the volcanism might be explained by a single, large plume. The novel feature of their story is the way topography on the base of the lithosphere, some 100-150 km below the surface, might channel the plume material into streams and pools.

Mantle plumes are now widely thought to rise from a hot thermal boundary layer at the base of the mantle (Fig. 1). When a new plume starts, it is led by a large spherical ‘head’ as much as 1,000 km in diameter. As the head nears the top of the mantle, it pancakes under the lithosphere, which is the cool, strong outer part of the Earth comprising the crust and the upper part of the mantle down to 100 or 200 km depth. The pancaking plume head spreads to a diameter of about 2,000 km and thins to about 200 km vertically. For most of its ascent, the plume material is in the solid state, but its high temperature allows it to deform slowly and so to behave like a fluid on geological timescales. However, as this hotter material reaches shallow depths, it may begin to melt because of the reduction in pressure, producing magma that rises and erupts on the Earth's surface. It is widely believed that the arrival of a new plume head is the cause of ‘flood basalt’ eruptions that spew millions of cubic kilometres of magma onto the Earth's surface within the relatively short period of a million years. Worldwide, about a dozen of these giant eruptions are known to have occurred within the past 250 million years.

Figure 1: Ebinger and Sleep's4 proposal for how a mantle plume interacting with topography on the base of the lithosphere might explain scattered volcanism across a very broad region of east Africa.
figure1

A large plume head followed by a thin plume tail rises from the base of the mantle. The head flattens and spreads as it approaches the base of the lithosphere, and it tends to flow up and under the thinnest parts of the lithosphere. Melting of the hot plume material only begins above a critical depth, and it only occurs as the plume material is rising and decompressing. Thus, melting tends to be concentrated where the plume material flows up a slope towards thinner lithosphere, as well as over the plume tail.

One of the younger flood basalts, about 30 million years old, is on the Ethiopian plateau, near the junction of the Red Sea, the Gulf of Aden and the east African rift system. The Afar hotspot (see map on page 789), inferred to be the trace of the thin plume ‘tail’ that followed the plume head (Fig. 1), is inferred to be still active in this region. The arrival of the Afar plume may not only have produced the Ethiopian flood basalts but also possibly triggered or promoted the rifting that radiates from the region.

Ebinger and Sleep's proposal is a bold extension of this picture. They suggest that a much broader region has been affected by the Afar plume, a region extending down the east African rift, offshore and south to the Comoros Islands near Madagascar, west to the Darfur uplift, and possibly much further to the Adamawa plateau and the Atlantic coast (see Figs 1 and 2 on pages 789 and 790). They explain the scattered nature of the associated volcanism as being due to variations in the thickness of the lithosphere through this region.

There are two aspects to their mechanism. First, the plume material will tend to rise into regions of thinner lithosphere because of its buoyancy (Fig. 1). Second, melting occurs once the plume material reaches a critical depth and where it is still rising, so that the pressure it is under is decreasing. The latter point is crucial in determining where the melting will actually occur. It will not necessarily be focused where the lithosphere is thinnest, but rather where the base of the lithosphere slopes upwards. If the plume initially rises under thick lithosphere, it may melt very little or not at all (Fig. 1). If the material then flows horizontally, it will not melt any more. If later it reaches thinner lithosphere and can flow upwards again, it can melt further (Fig. 1). Subsequent rifting may trigger more melting, and may also promote the flow of plume material over greater distances.

Ebinger and Sleep's proposal is a stimulating one that is well worth exploring, but many uncertainties remain. Their modelling of plume flow and melting is plausible, but (appropriately at this stage) quite simplified. Geophysical information on lithosphere thickness is rather sparse. Subsurface magma, intruded into thick sediments, may be more extensive than is known at present, but more seismic data are required to find out. The details of melting relationships are not well constrained. Finally, the ages and geochemistry of the erupted rocks have some broad consistency with the idea, but much more detail needs to be filled in.

A potential complication is that George et al.5, in a paper published earlier this month, argue on the basis of new dating results that there are actually two plumes in this region. There were eruptions on the southern Ethiopian plateau as early as 45 million years ago which George et al. attribute to an older plume now located under Lake Victoria, which lies to the south. They ascribe later eruptions in southern Ethiopia, 19-12 million years ago, to the spreading influence of the more northerly Afar plume.

If George and colleagues' interpretation is confirmed, then Ebinger and Sleep's one-plume model would be a bit too simple, but their channelling mechanism still shows us a way in which scattered and apparently unrelated activity may have a common cause. In the meantime, Ebinger and Sleep's paper is a good illustration of the growing maturity of mantle dynamics modelling. The proposal is quantified, and testable in many respects. Its full evaluation will require a conversation between dynamicists, geologists, geochemists and geophysicists. In other words, mantle dynamics is becoming a useful part of geology, in the broad sense.

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Davies, G. A channelled plume under Africa. Nature 395, 743–744 (1998). https://doi.org/10.1038/27320

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