Tag: Africa

The risk of landslides in Africa

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The most widespread risk from natural hazards is, with little doubt, that posed by ground instability; landslides and landslips; mudflows; rock avalanches and a range of other categories in which large volumes of surface material are set in motion. They can be triggered by earthquakes, volcanism or heavy rainfall that changes the physical properties of rock and soil. Not only steep slopes pose a risk, for some affect ground with quite gentle topography, as witness the terrible scenes from Sulawesi triggered by the 28 September 2018 magnitude 7.5 earthquake beneath the Minhasa Peninsula. This set in motion mudflows on gently sloping ground when the seismic waves caused liquefaction of unconsolidated sediments that not only shattered dwellings by the lateral motion, but whole communities sank into the slurry with little trace. The rapid events left a death toll confirmed at 2010 people with about 5000 missing, feared to have been swallowed by the earth. In the last 9 months mass movement has resulted in fatalities in many places, the most publicised being in Uganda, Japan, Philippines, Sulawesi, Ethiopia, Sumatra, South India, Bangladesh, California, Nepal, and the list grows as it does every year.

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Types of mass movement (Credit: US Geological Survey

As well as purely natural causes, human activities, such as deforestation, excavations and dumping of materials, greatly exacerbate risks. The South Wales former coal-mining communities commemorate every year the collapse of a mine spoil heap on a steep hillside on 21 October 1966 that engulfed a primary school at Aberfan, killing 116 small children and 28 adults. Wherever they occur, there seems to be little chance of escape for those in their path. Slowly it has become possible for geoscientists to outline areas that are potentially at risk from catastrophic mass wastage, sometimes from the distribution of scars of previous events on remotely sensed images, but increasingly by multivariate analysis of landscapes in terms of the factors that may contribute to future ground failures. The principal ones are: topographic slope and relief; annual rainfall, especially the likely precipitation in a single day; vegetation cover, particularly by trees; strength of surface rock and soils, including degrees of consolidation, interbedding and water content; geological structure, such as the trajectory of faults, degree of  jointing and the dip of strata. Modelling risk has to grapple with the global scale of the problem, which cannot be addressed in the least developed regions by piecemeal local studies, although those are urgent, of course, in areas with recorded instances of catastrophic ground failure. Regional studies can screen vast areas of probably low risk so that meagre resources can focus on those that appear to be most dangerous to populated places.

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Degree of risk from landslides of all types in the northern part of the East African Rift System (Credit: Broeckx et al. 2018; Fig. 6)

Belgian engineering geologists and GIS specialists have assembled a monumental risk assessment of Africa, together with a bibliography of all published work on mass movement across the continent (Broeckx, J. et al. 2018. A data-based landslide susceptibility map of Africa. Earth-Science Reviews, v. 185, p. 102-121; DOI: 10.1016/j.earscirev.2018.05.002). They point out that Google Earth’s 3-D viewing potential at fine spatial resolution provides a free and rapid means of mapping scars of previous earth movements in considerable detail over areas that data analysis suggests to be susceptible. Their paper provides continent-scale maps of the parameters that they used as well as maps showing several versions of their risk analysis. The supplementary data to the paper include downloadable, full-resolution maps of landslide susceptibility.

Multiregional human evolution in Africa

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Africa is not only a large continent, but is subdivided into many different climatic zones and ecosystems and these have changed drastically over the last 2 Ma. It is further subdivided by terrain features, such as the courses of major rivers, large plateaus, tectonic rift systems and the mountains that frequently define their flanks. Getting around Africa is not easy today, was more difficult before modern transport, and many geomorphic provinces may have been mutually inaccessible in the distant past. For instance, the Sahara Desert forms a major barrier to travellers on foot because access to surface water is non-existent except at widely spaced oases. Without boats or rafts the Nile and Congo cannot be crossed for a thousand miles or more. Migration was perhaps a very rare event outside of periods of widespread humid climates or when great environmental stress forced people either to move or perish. Despite these physical and ecological divisions and barriers palaeoanthropologists have, until recently, tended to regard the evolution of Homo sapiens and earlier human and hominin species as having occurred within single populations: a linear view forced on them by scanty fossil remains and limited methodologies. Logically, when human numbers were small Africa probably had several isolated population Physical isolation would have engendered genetic isolation in which our ancestors evolved for tens of thousand years.

Anatomically modern human (AMH) remains found at Jebel Irhoud in Morocco turned out to be 315 ka old, displacing those from Ethiopia (190 ka) as the earliest known examples of AMH. Several more archaic H. sapiens fossils have turned up in southern Africa and as far afield as the Middle East, suggesting that the early evolution of AMH was in an Africa-wide context rather than in one area – the rift system of Ethiopia and Kenya – from which a new species radiated outwards. This breadth of finds has encouraged Eleanor Scerri of Oxford University and her many international colleagues to resurrect what was once a widely discarded hypothesis; a multiregional model of modern human origins, originally proposed to have arisen from pre-sapiens groups in Eurasia by Milford Wolpoff but which was sunk once genetic connections among living humans turned out to be rooted in Africa. (Scerri, E.M.L. and 22 others 2018. Did our species evolve in subdivided populations across Africa, and why does it matter? Trends in Ecology & Evolution, v. 33, p. 582-594; (PDF) doi: 10.1016/j.tree.2018.05.005). Scerri et al’s model is sited in Africa and the paper’s authors include several leading palaeoanthropologists who once opposed multiregionalism and established the Recent African Origin hypothesis on the back of the early genetic data.

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Different early AMH cranium shapes: left Jebel Irhoud, Morocco (315 ka), right Qafzeh, the Levant (85 ka) (credit: Scerri et al, 2018; Figure 1)

From region to region in Africa, the oldest AMH crania show significant differences from each other, but within a distinct combination of features that clearly distinguish us from our fossil relatives and ancestors, such as Homo heidelbergensis from Zimbabwe and the primitive-looking H. naledi found in a South African cave in 2015. Improved dating now shows that the Zimbabwean H. heidelbergensis and H.naledi remains are roughly the same age as the Jebel Irhoud AMH specimens. The first has long been held as the progenitor of AMH and descended from H. antecessor, perhaps the common ancestor for AMH, Neanderthals and Denisovans about 700 ka ago. The three human species cohabited Africa early in the evolutionary history of AMH. It is now abundantly clear from ancient and modern genomes that AMH, Neanderthals and Denisovans interbred in Eurasia. The proximity in time and space of earlier African AMH to two more ancient human species opens up a similar possibility earlier in the emergence of all living humans. There is evidence for that too: Yoruba people living in West Africa, whose genomes have been analysed, carry up to 8% of genetic ancestry that originated in an unidentified ancient population that was non-sapiens. At present, DNA analysis with the same high precision and information content from other living Africans has not been performed, and deterioration of ancient DNA in African climates has so far thwarted genomic studies of ancient African fossils.

The new view of our origins points to repeated hybridisation involving other coexisting human species, as well as evolution in isolation, from the outset. It continued through later times while Neanderthals and Denisovans survived. Even recent human genetic history is peppered with intermingling of a great variety of migrants passing through all the habitable continents. Another issue: In the earliest times, were cultures exchanged as well as genes? The first appearance of AMH coincides with that of a new stone technology (Levallois technique), moving away from the earlier dominance by handaxes towards more delicate, leaf-shaped points, that characterise the African Middle Stone Age. Similar techniques reached Europe with the Neanderthals. Was this an invention of the earliest AMH or a joint venture?

You can find an excellent review of these issues in the September 2018 issue of Scientific American (Wong, K. 2018. Last hominin standing.  Scientific American, v. 319(3), p. 56-61) along with several other articles on human evolution.

A revised and updated edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

Last common paternal and maternal ancestors closer in time

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One of the oddities of using human genetic material passed down the male (from Y chromosomes) and female lines (from mitochondria) to assess when fully modern humans originated is that they have hitherto given widely different dates: 50 to 115 ka and 150 to 240 ka respectively. Twice to three-times the age for a putative ancestral ‘mother’ compared with such a ‘father’ for humanity raised all kinds of problematic issues for palaeoanthropology, such as a possibly greater ‘turnover’ of lines of descent among males perhaps due to riskier lifestyles. Y-chromosome data  limited speculation on the timing of human colonisation outside of Africa to a maximum of 60 ka, even though there is fossil and archaeological evidence for a much earlier presence in the Levant and India.  The difference also questions the validity of molecular-clock approaches to evolutionary matters. Two new studies have lessened the phylogenetic  strains.

One examines Y chromosomes in 69 males from nine diverse populations from Africa, Eurasia and Central America (Poznik, G.D.  and 10 others 2013. Sequencing Y chromosomes resolves discrepancy in time to common ancestors of males versus females. Science, v. 341, p. 562-565). The US-French team applied sophisticated statistics as well as the elements of a molecular clock approach to both Y-chromosome and mitochondrial DNA, discovering in the process a hitherto unresolved feature in the African part of the male ‘tree’. The outcome is a significant revision of both male and female paths of descent: 120 to 156 ka and 99 to 148 ka to the last common ancestor in both lines. The upper limit is somewhat lower than the age of fossil evidence for the earliest anatomically modern humans.

The second study zeros-in on the European story, by examining the Y-chromosome data of 1200 men from Sardinia (Francalacci, P. and 38 others. Low-pass DNA sequencing of 1200 Sardinians reconstructs European Y-chromosome phylogeny. Science, v. 341, p. 565-569) calibrated to some extent by the date when Sardinia was first colonised (7.7 ka). It too revealed new detail that enabled the Italian-US-Spanish team to refine the time when features of Sardinian Y-chromosome DNA would coalesce with those from the rest of the world. In this case the date for a last common paternal ancestor goes back to between 180 to 200 ka, more similar to the old dates for ‘African Eve’ and the earliest modern human fossils than to either that for male or female lines arrived at by Posnik et al. (2013), which are significantly younger.

Map of early migrations of modern humans
Map of early migrations of modern humans based on Y chromsome data (credit: Wikipedia)

Equally interesting are the comments on both papers in the Perspectives section of the issue of science in which they appear (Cann, R.L. 2013. Y weigh in again on modern humans. Science, v. 341, p. 465-7).Rebecca Cann of the University of Hawaii Manoa considers the two sets of results from Y-chromosomes potentially capable of refining models for the migration times of modern humans out of Africa and their interactions with the archaic populations that they eventually displaced from Europe and central and southern Asia (Neanderthals, Denisovans and Homo erectus respectively). She believes that will include signs of earlier excursions that the generally accepted diaspora between roughly 60 and 50 ka seemingly constrained by the previous 50 to 115 ka estimate for the last common paternal ancestor. That would help explain the presence of modern humans in India at the time of the Toba eruption (71 ka).

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Groundwater in Africa

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English: Mwamanongu Village water source, Tanz...
Drinking water for many rural Africans often comes from open holes dug in the sand of dry riverbeds, and it is invariably contaminated. (Bob Metcalf on Wikipedia)

Sub-surface water supplies have rarely, if ever, figured in Earth Pages except in passing or in relation to the on-going crisis of arsenic pollution in drinking-water supplies. That is largely because of the paucity of groundwater publications that have a general interest. So it was welcome news to learn that hydrogeologists of the British Geological Survey and University College London have produced a continent-wide review of groundwater prospects for Africa, probably in most need of good news about water supplies (MacDonald, A.M. et al. 2012. Quantitative maps of groundwater in Africa. Environmental Research Letters, v. 7 doi:10.1088/1748-9326/7/2/024009. They used existing hydrogeological maps, publications and other publically available data to estimate total groundwater storage in a variety of aquifer types and the yield potentials of boreholes. Details can be seen at http://www.bgs.ac.uk/research/groundwater/international/africanGroundwater/maps.html

Dominated by the vast sedimentary aquifers of Libya, Algeria, Egypt and Sudan, such as the Nubian Sandstone, around 0.66 million km3 may lie below the continental surface: more than 100 times the annually renewable freshwater resources, including the flows in  three of the world’s largest rivers, the Nile, Congo and Niger. Though only a fraction of this subsurface potential may be available for extraction through wells, the arithmetic, or rather the statistics, suggest that small diameter boreholes and simple handpumps, as well as traditional wells, can sustainably satisfy the drinking water needs of the bulk of Africa’s rural populations with yields of individual wells between 0.1 to 1 l s-1. However, groundwater use in irrigation and for large urban supplies demands well productivities an order of magnitude higher from thick sedimentary sequences, which rarely coincide in Africa with areas suitable for large-scale agriculture or existing cities and large towns. Both the humid tropical lowlands with thick unconsolidated sediments and the deep sedimentary rock aquifers beneath the Sahara and other arid areas match great groundwater potential with either little need for groundwater or virtually no potential for agricultural development and very few people. Moreover, the truly vast reserves of North Africa that are an order of magnitude or more greater than in any other countries are at such depths and so remote that development needs commensurately huge investment, in the manner of oil-rich Libya’s Great Man Made River Project projected at more than US$25 billion investment. To say that reserves, convenience and yields are inequitably distributed in Africa would understate the hydrogeological difficulties of the continent.

Average well productivity predicted by MacDonald et al from Africa’s regional geology

Much of Africa has crystalline basement at the surface that has useful yields (>0.1  l s-1) only when deeply weathered, and even then rarely yields better than 1 l s–1. An exception to this general rule is where basement has been shattered by large faults and fractures. Sedimentary cover is generally thin across the continent and with highly variable yield potential. The other issue is that of sustainability, for if extraction rates exceed those of recharge then groundwater effectively becomes a non-renewable resource. About half of the African surface, mainly in its western equatorial region, has sufficient rainfall and infiltration potential to outpace universally high evapotranspiration to give recharge rates of more than 2.5 cm of annual rainfall. For all the areas repeatedly hit by drought and famine, average recharge through the surface that escapes being literally blown away on the wind is less than half a centimetre.

To have synopses of all the important issues surrounding African groundwater – the best choice for safe domestic supplies in hot, poor areas – would seem to be very useful to those engaged in development and relief strategies; i.e. to governments, the UN ‘family’ and World Bank. But there are important caveats. An obvious one is the antiquity of many of the surveys drawn on by MacDonald et al. Some 23 out of 33 were published more than 20 years ago using data that may be a great deal older: such has been the snail-like pace of publication by all geological surveys, including BGS. That is compounded by the small scale of the maps (mainly smaller than 1:1 million) and the extremely sparse geophysical data concerning subsurface geology across most of Africa. ‘Quantitative’ is not the adjective to use here, for unlike in most of the developed world, groundwater reserves and locations in Africa have not been measured, but estimated from pretty meagre data. In fact to be brutally realistic, most of the source maps are based on educated guesswork by a few hard-pressed geoscientists once personally responsible for areas that would cripple most of their colleagues working in say Europe or North America.

If there is a truism about water exploration in Africa, outside the well-watered parts, it is this: sink a well at random, and it will probably be dry. The stats may well be encouraging, as MacDonald et al. clearly believe, but finding useful groundwater supplies relies on a great deal more. Outside cities, people survive as regards groundwater often as a result of traditional means of water exploration and well digging: they or at least some locals are experts at locating shallow sources. Yet to improve their access to decent water in the face of both rising populations and climate change demands sophisticated exploration techniques based on geological knowledge. Most important is to ensure supplies to existing communities, whose locations do not necessarily match deeper groundwater availability, bearing in mind that a universal problem for most African villagers is the sheer distance to wells with safe water. Rigs used to drill tube wells are expensive to hire, so the likelihood of success needs to be maximised. In the absence of large-scale (1:50 000) geological maps – rarities throughout Africa – only skilled hydrogeological interpretation of aerial or satellite images followed-up by geophysical ground traverses offer that vital confidence.

Geologically useful ASTER image of the Danakil Block in Eritrea/Ethiopia, showing Mesozoic and Recent sedimentary aquifers and crystalline basement (Steve Drury)

In fact, thanks to the joint US-Japan ASTER system carried in sun-synchronous orbit, geologically-oriented image data are available for the whole continent. Interpretation requires some skills but few if any beyond learning in a practical, field setting. Indeed, the African surface in its arid to semi-arid parts, most at risk of drought and famine, lends itself to rapid hydrogeological reconnaissance mapping using ASTER data. Given on-line training in image interpretation, a ‘crowd-source’ approach coordinating many interpreters could complete a truly life-giving and easily available map base for local people to focus their own well-construction programmes.

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