Balanced boulders and seismic hazard

The seismometer invented by early Chinese engineer Zhang Heng

China has been plagued by natural disasters since the earliest historical writings. Devastating earthquakes have been a particular menace, the first recorded having occurred in 780 BC . During the Han dynasty in 132 CE, polymath Zhang Heng invented an ‘instrument for measuring the seasonal winds and the movements of the Earth’ (Houfeng Didong Yi, for short): the first seismometer. A pendulum mechanism in a large bronze jar activated one of eight dragons corresponding to the eight cardinal and intermediate compass directions (N, NE, E etc.) so that a bronze ball dropped from its mouth to be caught by a corresponding bronze toad. The device took advantage of unstable equilibrium in which a small disturbance will produce a large change: akin to a pencil balanced on its unsharpened end. Modern seismometers exploit the same basic principle of amplification of small motions. The natural world is also full of examples of unstable equilibrium, often the outcome of chemical and physical weathering. Examples are slope instability, materials that are on the brink of changing properties from those of a solid to a liquid state (thixotropic materials – see: Mud, mud, glorious mud August 2020) and rocks in which stress has built almost to the point of brittle failure: earthquakes themselves. But there are natural curiosities that not only express unstable equilibrium but have maintained it long enough to become … curious! Perched boulders, such as glacial erratics and the relics of slow erosion and weathering, are good examples. Seismicity could easily topple them, so that their continued presence signifies that large enough tremors haven’t yet happened.

A precarious boulder in coastal central California (credit: Anna Rood & Dylan Rood, Imperial College London)

Now it has become possible to judge how long their delicate existence has persisted, giving a clue to the long-term seismicity and thus the likely hazard in their vicinity (Rood, A.H. and 10 others 2020. Earthquake Hazard Uncertainties Improved Using Precariously Balanced Rocks. American Geological Union Advances, v. 1, ePDF e2020AV000182; DOI: 10.1029/2020AV000182). Anna Rood and her partner Dylan of Imperial College London, with colleagues from New Zealand, the US and Australia, found seven delicately balanced large boulders of silica-rich sedimentary rock in seismically active, coastal California. They had clearly withstood earthquake ground motions for some time. Using multiple photographs to produce accurate digital 3D renditions and modelling of resistance to shaking and rocking motions, the authors determined each precarious rock’s probable susceptibility to toppling as a result of earthquakes. How long each had withstood tectonic activity shows up from the mass-spectrometric determination of beryllium-10 isotopes produced by cosmic-ray bombardment of the outer layer. Comparing its surface abundance relative to that in the rock’s interior indicates the time since the boulders’ first exposure to cosmic rays. With allowance for former support from surrounding blocks, this gives a useful measure of the survival time of each boulder – its ‘fragility age’.

The boulder data provide a useful means of reducing the uncertainties inherent in conventional seismic hazard assessment, which are based on estimates of the frequency of seismic activity, the magnitude of historic ‘quakes, in most cases over the last few hundred years, and the underlying geology and tectonics. In the study area (near a coastal nuclear power station) the data have narrowed uncertainty down to almost a half that in existing risk models. Moreover, they establish that the highest-magnitude earthquakes to be expected every 10 thousand years (the ‘worst case scenario’) were 27% less than otherwise estimated. This is especially useful for coastal California, where the most threatening faults lie off shore and are less amenable to geological investigation.

See also:  Strange precariously balanced rocks provide earthquake forecasting clues. (SciTech Daily; 1 October 2020) 

Monitoring ground motions with satellite radar

By using artificially generated microwaves to illuminate the Earth’s surface it is possible to create images. The technology and the theory behind this radar imaging are formidable. After about 30 years of development using aircraft-mounted transmission and reception antennas, the first high resolution images from space were produced in the late 1970s. Successive experiments improved and expanded the techniques, and for the last decade radar surveillance has been routine from a number of orbiting platforms. Radar has two advantages over optical remote sensing: being an active system it can be done equally effectively day or night; it also penetrates cloud cover, which is almost completely transparent to microwaves with wavelengths between a centimetre and a metre. The images are very different from those produced by visible or infrared radiation, the energy returns from the surface being controlled by topography and the roughness of the surface. One of many complicating factors is that images can only be produced by oblique illumination.  That, together with deployment of widely separated transmission and reception antennas, opens up the possibility of extracting very-high precision (millimetre) measurements of topographic elevation.

In 1992 radar data from two overpasses of the European ERS-1 satellite over California were processed to capture interference due to changes in the ground elevation during the time between the two orbits: the first interferometric radar or InSAR. It revealed the regional ground motions that resulted from the magnitude 7.3 Landers earthquake at 4:57 am local time on June 28, 1992. For the last decade InSAR has become a routine tool to monitor globally both lateral and vertical ground movements, whether rapid, as in earthquakes, or slow in the case of continental plate motions, subsidence or the inflation of volcanoes prior to eruptions. Juliet Biggs and Tim Wright, respectively of the Universities of Bristol and Leeds, UK, have summarised InSAR’s potential (Biggs, J. & Wright, T.J. 2020. How satellite InSAR has grown from opportunistic science to routine monitoring over the last decade. Nature Communications, v. 11, p. 1-4; DOI: 10.1038/s41467-020-17587-6).

Ground motions associated with the 2016 Kaiköuea earthquake on the South Island of New Zealand. Each colour fringe represents 11.4 cm of displacement in the radar line-of-sight (LOS) direction. Known faults are shown as thick black lines (Credit: Hamling et al. 2017. Complex multifault rupture during the 2016 Mw 7.8 Kaikōura earthquake, New Zealand. Science, v. 356, article eaam7194; DOI: 10.1126/science.aam7194)

Since the ERS-1 satellite discovered the ground motions associated with the Landers earthquake, InSAR has covered more than 130 large seismic events. Although the data post-dated the damage, they have demonstrated the particular mechanics of each earthquake, allowing theoretical models to be tested and refined. In the image above it is clear that the motions were not associated with a single fault in New Zealand: the Kaikoura earthquake involved a whole network of them, at least at the surface. Probably, displacement jumped from one to another; a complexity that must be taken into account for future events on such notorious fault systems as those in densely populated parts of California and Turkey.

East to west speed of the Anatolian micro-plate south of the North Anatolian Fault derived from the first five years of the EU’s Sentinel-1 InSAR constellation. Major known faults shown by black lines (Credit: Emre, O. et al. 2018. Active fault database of Turkey. Bulletin of Earthquake Engineering, v. 16, p. 3229-3275; DOI: 10.1007/s10518-016-0041-2)

Since its inception, GPS has proved capable of monitoring tectonic motions over a number of years, but only for widely spaced, individual ground instruments. Using InSAR alongside years’ worth of GPS measurements helps to extend detected motions to much finer resolution, as the image above shows for Asiatic Turkey. An important parameter needed for prediction of earthquakes is the way in which crustal strain builds up in regions with dangerously active fault systems.

InSAR image of the Sierra Negra volcano on Isabela Island in the Galapagos Archipelago, at the time of a magma body intruding its flanks. Each colour fringe represents 2.8 cm of subsidence in the LOS direction (Credit: Anantrasirichai, N. et al. 2019. A deep learning approach to detecting volcano deformation from satellite imagery using synthetic datasets. Remote Sensing of Environment, v. 230, article 111179; DOI: 10.1016/j.rse.2019.04.032)

Volcanism obviously involves the movement of large masses of magma beneath the surface before eruptions. GPS and micro-gravity measurements show that charging of a magma chamber causes volcanoes to inflate so InSAR provides a welcome means of detecting the associated uplift, even if it only a few centimetres, as show by the example above from the Galapagos Islands. A volcano’s flanks may bulge, which could presage a lateral eruption or a pyroclastic flow such as that at Mount St Helens in 1980. Truly vast eruptions are associated with calderas whose ring faults may cause collapse in advance.

The presence of cavities beneath the surface, formed by natural solution of limestones, deliberately as in extraction of brines from salt deposits or after subsurface mining, present subsidence hazards. There have been several series of alarming TV programmes about sinkhole formation that demonstrate sudden collapse. Yet every case will have been preceded by years of gradual sagging. InSAR allows risky areas to be identified well in advance of major problems. Indeed estate agents (realtors) as well as planners, civil engineers and insurers form a ready market for such survey.

Natural sparkling water and seismicity

For all manner of reasons, natural springs have fascinated people since at least as long ago as the Neolithic. Just the fact that clear water emerges from the ground to source streams and great rivers seems miraculous. There are many occurrences of offerings having been made to supernatural spirits thought to guard springs. Even today many cannot resist tossing in a coin, hanging up a ring, necklace or strip of cloth beside a spring, for luck if nothing else. Hot springs obviously attract attention and bathers. Water from cool ones has been supposed to have health-giving properties for at least a couple of centuries, even if they stink of rotten eggs or precipitate yellow-brown iron hydroxide slime in the bottom of your cup. Spas now attribute their efficacy to their waters’ chemistry, and that depends on the rocks through which they have passed. Those in areas of volcanic rock are generally the most geochemically diverse: remember the cringe-making adverts for Volvic from the volcanic Chain des Puys in the French Auvergne. Far more ‘posh’ are naturally carbonated waters that well-out full of fizz from pressurised, dissolved CO2. Internationally the best known of these is Perrier from the limestone-dominated Gard region of southern France. Sales of bottled spring waters are booming and the obligatory water-chemistry data printed on their labels form  a do-it-yourself means of regional geochemical mapping (Dinelli, E. et al. 2010. Hydrogeochemical analysis on Italian bottled mineral waters: Effects of geology. Journal of Geochemical Exploration, v. 107, p. 317–335; DOI: 10.1016/j.gexplo.2010.06.004) But it appears from a study of variations in CO2 output from commercial springs in Italy that they may also help in earthquake prediction (Chiodini, G. et al. 2020. Correlation between tectonic CO2 Earth degassing and seismicity is revealed by a 10-year record in the Apennines, Italy. Science Advances, v. 6, article eabc2938; DOI: 10.1126/sciadv.abc2938).

Italy produces over 12 billion litres of spring water and the average Italian drinks 200 litres of it every year. There are more than 600 separate brands of acqua minerale produced in Italy, including acqua gassata (sparkling water). Even non-carbonated springs emit CO2, so it is possible to monitor its emission from the deep Earth across wide tracts of the country. High CO2 emissions are correlated worldwide with areas of seismicity, either associated with shallow magma chambers or to degassing from subduction zones. There are two possibilities: that earthquakes help release built-up fluid pressure or because fluids, such as CO2 somehow affect rock strength. Giovanni Chiodini and colleagues have been monitoring variations in CO2 release from carbonated spring water in the Italian Apennines since 2009. Over a ten-year period there have been repeated earthquakes in the area, including three of magnitude 6.0 or greater. The worst was that affecting L’Aquila in April 2009, the aftermath of which saw six geoscientists charged with – and eventually acquitted of – multiple manslaughter (see: Una parodia della giustizia?, October 2012). It was this tragedy that prompted Chiodini et al.’s unique programme of 21 repeated sampling of gas discharge rates at 36 springs, matched to continuous seismograph records. The year after the L’Aquila earthquake coincided with high emissions, which then fell to about half the maximum level by 2013. In 2015 emissions began to rise to reach a peak before earthquakes with almost the same magnitude, but less devastation, on 24 August and 30 October 2016. Thereafter emissions fell once again. This suggests a linked cycle, which the authors suggest is modulated by ascent of CO2 that originates from the melting of carbonates along the subduction zone that dips beneath central Italy. They suggest that gas accumulates in the lower crust and builds up pressure that is able to trigger earthquakes in the crust.

The variation in average emissions across central Italy (see figure above) suggests that there are two major routes for degassing from the subduction zone, perhaps focussed by fractures generated by previous crustal tectonic movements. In my opinion, this study does not prove a causal link, although that is a distinct possibility, which may be verified by extending this survey of degassing and starting similar programmes in other seismically active areas. Whether or not it might become a predictive tool depends on further work. However, other studies, particularly in China, show that other phenomena associated with groundwater in earthquake-prone areas, such as rise in well-water levels and an increase in their emissions of radon and methane, correlate in a similar manner.

‘Mud, mud, glorious mud’

Earth is a water world, which is one reason why we are here. But when it comes to sedimentary rocks, mud is Number 1. Earth’s oceans and seas hide vast amounts of mud that have accumulated on their floors since Pangaea began to split apart about 200 Ma ago during the Early Jurassic. Half the sedimentary record on the continents since 4 billion years ago is made of mudstones. They are the ultimate products of the weathering of crystalline igneous rocks, whose main minerals – feldspars, pyroxenes, amphiboles, olivines and micas, with the exception of quartz – are all prone to breakdown by the action of the weakly acidic properties of rainwater and the CO2 dissolved in it. Aside from more resistant quartz grains, the main solid products of weathering are clay minerals (hydrated aluminosilicates) and iron oxides and hydroxides. Except for silicon, aluminium and ferric iron, most metals end up in solution and ultimately the oceans.  As well as being a natural product of weathering, mud is today generated by several large industries, and humans have been dabbling in natural muds since the invention of pottery some 25 thousand years ago.  On 21 August 2020 the journal Science devoted 18 pages to a Special Issue on mud, with seven reviews (Malakoff, D. 2020. Mud. Science, v. 369, p. 894-895; DOI: 10.1126/science.369.6506.894).

Mud carnival in Brazil (Credit: africanews.com)

The rate at which mud accumulates as sediment depends on the rate at which erosion takes place, as well as on weathering. Once arable farming had spread widely, deforestation and tilling the soil sparked an increase in soil erosion and therefore in the transportation and deposition of muddy sediment. The spurt becomes noticeable in the sedimentary record of river deltas, such as that of the Nile, about 5000 years ago. But human influences have also had negative effects, particularly through dams. Harnessing stream flow to power mills and forges generally required dams and leats. During medieval times water power exploded in Europe and has since spread exponentially through every continent except Antarctica, with a similar growth in the capacity of reservoirs. As well as damming drainage these efforts also capture mud and other sediments. A study of drainage basins in north-east USA, along which mill dams quickly spread following European colonisation in the 17th century, revealed their major effects on valley geomorphology and hydrology (see: Watermills and meanders; March 2008). Up to 5 metres of sediment build-up changed stream flow to an extent that this now almost vanished industry has stoked-up the chances of major flooding downstream and a host of other environmental changes. The authors of the study are acknowledged in one Mud article (Voosen, P. 2020. A muddy legacy. Science, v. 369, p. 898-901; DOI: 10.1126/science.369.6506.898) because they have since demonstrated that the effects in Pennsylania are reversible if the ‘legacy’ sediment is removed. The same cannot be expected for truly vast reservoirs once they eventually fill with muds to become useless. While big dams continue to function, alluvium downstream is being starved of fresh mud that over millennia made it highly and continuously productive for arable farming, as in the case of Egypt, the lower Colorado River delta and the lower Yangtze flood plain below China’s Three Gorges Dam.

Mud poses extreme risk when set in motion. Unlike sand, clay deposits saturated with water are thixotropic – when static they appear solid and stable but as soon as they begin to move en masse they behave as a viscous fluid. Once mudflows slow they solidify again, burying and trapping whatever and whomever they have carried off. This is a major threat from the storage of industrially created muds in tailings ponds, exemplified by a disaster at a Brazilian mine in 2019, first at the site itself and then as the mud entered a river system and eventually reached the sea. Warren Cornwall explains how these failures happen and may be prevented (Cornwall, W. 2020. A dam big problem.  Science, v. 369, p. 906-909; DOI: 10.1126/science.369.6506.906). Another article in the Mud special issue considers waste from aluminium plants (Service, R.F. 2020. Red alert. Science, v. 369, p. 910-911; DOI: 10.1126/science.369.6506.910). The main ore for aluminium is bauxite, which is the product of extreme chemical weathering in the tropics. The metal is smelted from aluminium hydroxides formed when silica is leached out of clay minerals, but this has to be separated from clay minerals and iron oxides that form a high proportion of commercial bauxites, and which are disposed of in tailings dams. The retaining dam of such a waste pond in Hungary gave way in 2010, the thixotropic red clay burying a town downstream to kill 10 people. This mud was highly alkaline and inflicted severe burns on 150 survivors. Service also points out a more positive aspect of clay-rich mud: it can absorb CO2 bubbled through it to form various, non-toxic carbonates and help draw down the greenhouse gas.

Muddy sediments are chemically complex, partly because their very low permeability hinders oxygenated water from entering them: they maintain highly reducing conditions. Because of this, oxidising bacteria are excluded, so that much of the organic matter deposited in the muds remains as carbonaceous particles. They store carbon extracted from the atmosphere by surface plankton whose remains sink to the ocean floor. Consequently, many mudrocks are potential source rocks for petroleum. Although they do not support oxygen-demanding animals, they are colonised by bacteria of many different kinds. Some – methanogens – break down organic molecules to produce methane. The metabolism of others depends on sulfate ions in the trapped water, which they reduce to sulfide ions and thus hydrogen sulfide gas: most muds stink. Some of the H2S reacts with metal ions, to precipitate sulfide minerals, the most common being pyrite (FeS2). In fact a significant proportion of the world’s copper, zinc and lead resources reside in sulfide-rich mudstones: essential to the economies of Zambia and the Democratic Republic of Congo. But there are some strange features of mud-loving bacteria that are only just emerging. The latest is the discovery of bacteria that build chains up to 5 cm long that conduct electricity (Pennisi, E. 2020. The mud is electric. Science, v. 369, p. 902-905; DOI: 10.1126/science.369.6506.902). The bacterial ‘nanowires’ sprout from minute pyrite grains, and transfer electrons released by oxidation of organic compounds, effectively to catalyse sulfide-producing reduction reactions. NB Oxygen is not necessary for oxidation as its chemistry involves the loss of electrons, while reduction involves a gain of electrons, expressed by the acronym OILRIG (oxidation is loss, reduction is gain). It seems such electrical bacteria are part of a hitherto unsuspected chemical ecosystem that helps hold the mud together as well as participating in a host of geochemical cycles. They may spur an entirely new field of nano-technology, extending, bizarrely, to an ability to generate electricity from moisture in the air.

If you wish to read these reviews in full, you might try using their DOIs at Sci Hub.

Submarine landslides and formation of the East African Rift System

The East African Rift System (Credit: P.C. Neupane, M.Sc thesis 2011; Fig. 1)

East Africa is traversed from the Afar Depression in the north to Malawi in southern Africa by several great depressions bounded by active normal fault systems: grabens in the old terminology. They are regions of active crustal extension and thinning decorated by chains of active volcanoes. The last 50 years has witnessed more than 3400 major earthquakes (magnitude 4 to 7); unsurprising for the Earth’s largest active continental rift system. In Afar, the East African Rift system links to two others that have extended sufficiently to create oceanic crust: the Red Sea and the Gulf of Aden rifts. Afar is the site of the best documented tectonic triple junction. In Ethiopia, the rifting began after the whole of the Horn of Africa and Yemen had been smothered by continental flood basalts 30 Ma ago, during the Oligocene Epoch. The East African rifts are repositories for younger sediments that contain a continuous record of hominid evolution from about 5 Ma ago. This is no coincidence, for adjacent bulging of the continental crust resulted both from its unloading by thinning along the rifts and the buoyancy conferred by high heat flow in the mantle beneath. The uplifted areas have risen as high as 4 kilometres elevation (in Ethiopia), and present some of the world’s most spectacular land forms. This N-S barrier disrupted earlier climatic patterns that had much of tropical Africa blanketed by dense woodland and resulted in a strongly seasonal climate during the last few million years and the development of open savannah land. Put simply, open grassland with widely spaced trees was no place for diminutive forest apes to scamper on all-fours. Being able to leg-it nimbly on two gave the apes that developed such a gait a decisive evolutionary advantage: the rest, as they say, is human evolutionary history.

The extension and rapid uplift along the rift flanks to this day pose severe risk of landslides. Indeed, some are so large as to resemble fault blocks in their own right. Vast amounts of the upper crust have been stripped off by rapid erosion driven by the uplift. The debris has not only ended-up on the rift floors as sedimentary fill but far more has made its way eastward to be deposited on the Indian Ocean continental shelf. Until recently, piecing together the history of rifting and uplift has been restricted to the rifts themselves and their adjacent flanks. Such terrains have extremely complex and usually discontinuous geological sequences, so signs of the onset of extensional tectonics and uplift may differ from region to region. Agreement is limited to some time between 25 and 17 Ma. The whole tectonic process may, in fact, have begun at different times along the length of the rift. A clearer picture should emerge from studies of the post-30 Ma sedimentary pile along the Indian Ocean continent shelf. A sure-fire way of getting the needed data is from offshore areas that are prospective for oil and natural gas. Such is the case off the Tanzanian coastline at the southern limit of the rift system.

Seismic reflection profile parallel to the Tanzanian coastline with the Mafia mega-slide highlighted in green (Credit: Maselli et al. 2020; Fig. 5) Click to view full resolution

The Tanzania Petroleum Development Corporation and Shell have conducted seismic reflection surveys and drilled some test wells to the SE of Zanzibar Island, an area of major deposition from the eastward flowing Ruaha–Rufiji and Rovuma Rivers. Vittorio Maselli of Dalhousie University in Halifax Nova Scotia and colleagues from the UK, Italy and the Netherlands analysed a wealth of data from these surveys, to discover one of the biggest landslides on Earth (Maselli, V. and 10 others 2020. Large-scale mass wasting in the western Indian Ocean constrains onset of East African rifting. Nature Communications, v. 11, article 3456; DOI: 10.1038/s41467-020-17267-5). The Mafia mega-slide is represented in seismic profiles by a sedimentary unit, up to 300 m thick. It has a highly irregular base that cuts across strata in late-Oligocene to early-Miocene (25-23 Ma) sediments. It covers an area of more than 11,600 km2 and has a volume of at least 2500 km3. The unit’s upper surface is also irregular, suggesting that the unit’s thickness varies considerably. Younger sediments are draped across the irregular top of the slide body. In other, parallel sections the deposit is absent. Unlike the clearly bedded nature of sediments above and below it, the seismic response of the slide deposit is featureless, except for zones of chaotic stratification that reveal slump-folds. Nor is this the only sign of major submarine slides: there are others of lesser extent that predate the base of the Pliocene (5.3 Ma).

A mass movement of this magnitude would have generated a tsunami larger than that which possibly wiped out Mesolithic habitation on the east coast of Britain 8200 years ago due to the even larger Storegga Slide at the edge of the Norwegian continental shelf. The Mafia slide event would have flooded wide tracts of the East African coast. Its estimated age, between 22.9 to 19.8 Ma, is roughly coeval with the initiation of volcanism in the Tanzanian segment of the East African Rift and the onset of rifting and uplift of its flanks. It was probably launched by a major earthquake (>7 on the Richter scale). Such is the pace of current deposition and the thickness of sedimentary build-up since the Pliocene, there is a danger of future slides, albeit of lesser magnitude: the system continues to be seismically active, with recently recorded quakes offshore of Tanzania.

Turmoil in Roman Republic followed Alaskan volcanic eruption

That activities in the global political-economic system are now dramatically forcing change in natural systems is clear to all but the most obdurate. In turn, those changes increase the likelihood of a negative rebound on humanity from the natural world. In the first case, data from ice cores suggests that an anthropogenic influence on climate may have started with the spread of farming in Neolithic times. Metal pollution of soils had an even earlier start, first locally in Neanderthal hearths whose remains meet the present-day standards for contaminated soil, and more extensively once Bronze Age smelting of copper began. Global spread of anomalously high metal concentrations in atmospheric dusts shows up as ‘spikes’ in lead within Greenland ice cores during the period from 1100 BCE to 800 CE. This would have resulted mainly from ‘booms and busts’ in silver extraction from lead ores and the smelting of lead itself. In turn, that may reflect vagaries in the world economy of those times

Precise dating by counting annual ice layers reveals connections of Pb peaks and troughs with major historic events, beginning with the spread of Phoenician mining and then by Carthaginians and Romans, especially in the Iberian Peninsula. Lead reaches a sustained peak during the acme of the Roman Republic from 400 to 125 BC to collapse during widespread internal conflict during the Crisis of the Republic. That was resolved by the accession of Octavian/Augustus as Emperor in 31 BCE and his establishment of Pax Romana across an expanded empire. Lead levels rose to the highest of Classical Antiquity during the 1st and early 2nd centuries CE. Collapse following the devastating Antonine smallpox pandemic (165 to 193 CE) saw the ice-core records’ reflecting stagnation of coinage activity at low levels for some 400 years, during which the Empire contracted and changed focus from Rome to Constantinople. Only during the Early Medieval period did levels rise slowly to the previous peak.

The Okmok caldera on the Aleutian island of Umnak (Credit: Desert Research Institute, Reno, Nevada USA)

Earth-logs has previously summarised how natural events, mainly volcanic eruptions, had a profound influence in prehistory. The gigantic eruption of Toba in Sumatra (~73 ka ago) may have had a major influence on modern-humans migrating from Africa to Eurasia. The beginning of the end for Roman hegemony in the Eastern Mediterranean was the Plague of Justinian (541–549 CE), during which between 25 to 50 million people died of bubonic plague across the Eastern Empire. This dreadful event followed the onset of famine from Ireland to China, which was preceded by signs of climatic cooling from tree-ring records, and also with a peak of volcanogenic sulfate ions in the Greenland and Antarctic ice caps around 534 CE. Regional weakening of the populace by cold winters and food shortages, also preceded the Black Death of the mid-14th century. In the case of the Plague of Justinian, it seems massive volcanism resulted in global cooling over a protracted period, although the actual volcanoes have yet to be tracked down. Cooling marked the start of a century of further economic turmoil reflected by lead levels in ice cores (see above). Its historical context is the Early Medieval equivalent of world war between the Eastern Roman Empire, the Sassanid Empire of Persia and, eventually, the dramatic appearance on the scene of Islam and the Arabian, Syrian and Iraqi forces that it inspired (see: Holland, T. 2013. In the Shadow of the Sword: The battle for Global Empire and the End of the Ancient World. Abacus, London)

An equally instructive case of massive volcanism underlying social, political and economic turmoil has emerged from the geochemical records in five Greenlandic ice cores and one from the Siberian island of Severnaya Zemlya (McConnell, J.R. and 19 others 2020. Extreme climate after massive eruption of Alaska’s Okmok volcano in 43 BCE and effects on the late Roman Republic and Ptolemaic Kingdom. Proceedings of the National Academy of Sciences, recent article (22 June 2020); DOI: 10.1073/pnas.2002722117). In this case the focus was on ice layers in all six cores that contain sulfate spikes and, more importantly, abundant volcanic dust, specifically shards of igneous glass. Using layer counting, all six show major volcanism in the years 45 to 43 BCE. The Ides (15th) of March 44 BCE famously marked the assassination of Julius Caesar, two years after the Roman Republic’s Senate appointed him Dictator, following four years of civil war. This was in the later stages of the period of economic decline signified by the fall in ice-core levels of Pb (see above). The Roman commentator Servius reported “…after Caesar had been killed in the Senate on the day before, the sun’s light failed from the sixth hour until nightfall.” Other sources report similar daytime dimming, and unusually cold weather and famine in 43 and 42 BCE.

As well as pinning down the date and duration of the volcanic dust layers precisely (to the nearest month using laser scanning of the ice cores’ opacity), Joseph McConnell and the team members from the US, UK, Switzerland, Germany and Denmark also chemically analysed the minute glass shards from one of the Greenlandic ice cores. This has enabled them to identify a single volcano from 6 possible candidates for the eruption responsible for the cold snap: Okmok, an active, 8 km wide caldera in the Aleutian Islands of Alaska. Previous data suggest that its last major eruption was 2050 years ago and blasted out between 10 to 100 km3 of debris, including ash. Okmok is an appropriate candidate for a natural contributor to profound historic change in the Roman hegemony. The authors also use their ice-core data to model Okmok’s potential for climate change: it had a global reach in terms of temperature and precipitation anomalies. Historians may yet find further correlations of Okmok with events in other polities that kept annual records, such as China.

See also: Eruption of Alaska’s Okmok volcano linked to period of extreme cold in ancient Rome (Science Daily, 22 June 2020); Kornei, K. 2020. Ancient Rome was teetering. Then a volcano erupted 6,000 miles away. (New York Times, 22 June 2020)

Arsenic hazard on a global scale

I have been following the harrowing story of how arsenic gets into domestic water supplies for 20 years (see: Earth-logs Geohazards for 2002; 2003; 2004; 2005; 2006; 2008; 2009; 2011; 2013; 2017). In my opinion, it is the greatest natural hazard in terms of the numbers at risk of poisoning. In 2006 I wrote about the emergence in Bangladesh of arsenic poisoning on a huge scale during the mid 1990s for a now defunct Open University course. If people depend for drinking water on groundwater from tube wells driven into alluvium they would not know of the risk, unless the water is rigorously analysed for levels of As greater than 10 micrograms per litre (μg l-1), the WHO recommended maximum. The sad fact is that the affected population were advised to switch from surface water supplies, which carry a high risk of biological infection, to well water. That is because during downward percolation from the surface oxidation destroys bacteria and viruses as well as parasites. Opportunities provided by a massive UN-funded drilling programme and local well digging made the choice seemingly obvious. Most people came to prefer well water as gastro-intestinal infections and child mortality fell rapidly.

Arsenic adds no taste, which is why it was once the ‘poison of choice’. How it gets into groundwater is difficult to judge, unless wells are downflow of areas riddled with metal mines. Years of research uncovered an unsuspected mechanism. The most common colorant of mineral grains, and thus sedimentary rocks, is brownish iron hydroxide (goethite), and that is able to adsorb a range of dissolved elements, including arsenic. One would think, therefore, that groundwater should be made safe by such a natural ‘filtering’ process: indeed goethite can be used in decontamination. The problem is that iron hydroxide, which contains Fe-3, is only stable in water with a high capacity for oxidation. Under reducing conditions it breaks down to soluble Fe-2 and water, thereby releasing to solution any other element that it has adsorbed. In alluvium, beds containing organic matter are prone to this ‘reductive dissolution’ of goethite. If weathering upstream has released even seemingly insignificant amounts of arsenic during the build up of alluvium, there is a potential life-threatening problem as arsenic builds up in the goethite coating of sedimentary grains to become ‘locked in’, with the potential to be released in high concentrations if subsurface chemical conditions change. The colour of the alluvial sediments penetrated by wells is a clue. If they are reddish brown, groundwater is safe, if they are greyish and goethite-free then, ‘beware’. But it is rare to examine ‘cuttings’ from a drill site aimed at groundwater, unlike those aimed at ores or oil

Since the tragedy of Bangladesh, which resulted after 5 years or so in obvious signs of arsenicosis – dark wart-line keratoses on hands and feet or black blotches on facial and torso skin – several alluvial basins in large river systems have had their well water tested. But by no means all such basins have been screened in this way, and there are many less-obvious signs of arsenic poisoning. After long exposure to the lower range of dangerous arsenic levels a variety of cancers develop in known areas of arsenic risk. There are also high levels of endemic respiratory problems, cardiovascular disease, reduced intellectual development in children and even diabetes. Geochemical monitoring of all populated and farmed river systems is a huge task that is far beyond the resources of many countries through which they run. One approach to ‘screening’ for hazard or safety is to use geological, hydrological, soil, climate and topographic data. Those from known arsenic-prone basins and those where its levels are shown to be consistently below the 10 μg l-1 danger threshold help to develop a predictive model (Podgorski, J. & Berg, M. 2020. Global threat of arsenic in groundwater. Science, v. 368, p. 845-850; DOI: 10.1126/science.aba1510).

Modelled global probability of arsenic concentration in groundwater exceeding 10 μg l-1. Click to display a larger map in a separate browser tab. (credit: Podgorski & Berg; Fig 2A, with enhanced colour)

Rather than trying to model the full range of arsenic concentrations, Joel Podgorski and Michael Berg of the Swiss Federal Institute of Aquatic Science and Technology focussed on assessing probabilities that arsenic in well water exceeds the WHO recommended maximum safe level of 10 μg l-1. Their global map highlights areas of concern for environmental health. Thankfully, huge (blue) areas are suggested to present low risk, the pale, yellow, orange and red patches signifying areas of increasing concern. No populated continent is hazard-free. What is very clear is that Asia presents the greatest worries. Most of the Asian ‘hot zones’ are spatially close to large mountain ranges and plateaus. In the case of the Indus and Ganges-Brahmaputra plains the sources for excessive arsenic in groundwater implicated by previous geochemical investigations lie in the Himalaya. The factor common to all major hot spots seems to be rapid transport of huge amounts of sediment released by weathering from areas of high topographic relief, rather than local large-scale mining operations. There are hazardous areas related to historic and active mining, such as the Andes of Bolivia, Peru and Chile and the western USA, but they are tiny by comparison with the dominance of natural arsenic mobilisation.

Despite the WHO recommended maximum of 10 μg l-1 of arsenic, many countries base their policy on levels that are five times higher, largely because of the difficulty of analysing for the lower concentration without expensive analytical facilities. Field analyses are often done using simple semi-quantitative tests based on paper impregnated with reagents that show a colour range for different concentrations, which are unreliable for those lower than 100 μg l-1. Thankfully, despite the many risky areas, most of them have population densities less than 1 per km2.

If you are interested in the geological details of the arsenic problems of Bangladesh, the course text that I produced for the Open University (Drury, S. 2006. Water and well-being: arsenic in Bangladesh. The Open University: Milton Keynes, UK. ISBN 0-7492-1435-X), the course itself (S250 Science in Context) was withdrawn some years ago.  It may be possible to arrange a PDF for private use.

See also: Zheng, Y. 2020. Global solutions to a silent poison. Science, v. 368, p. 818-819; DOI: 10.1126/science.abb9746

The dilemma of Rwanda’s Lake Kivu

In 1986 the small, roughly circular Lake Nyos in the Cameroon highlands silently released a massive cloud of carbon dioxide. Being a dense gas it hugged the ground and flowed down valleys for up to 25 km. 1700 local people perished by suffocation, together with their livestock (See Geohazards 2000). Having a recent volcanic origin, the lake is fed by springs in its bed that contain dissolved CO2 emitted from the residual magma chamber below. At 200 m deep the bottom water is sufficiently pressurised to retain the dissolved gas so that signs of the potential hazard remain hidden until such a limnic eruption occurs. Far larger, with a surface area of 2700 km2, Lake Kivu bordered by Rwanda and The Democratic Republic of Congo, is even deeper (up to 470 m). It too lies within a volcanically active zone, in this case the western arm of the East African Rift System. Being one of the most nutrient-rich bodies of fresh water on Earth, its biological productivity is extremely high, so as well as bottom water enriched in dissolved CO2 – a staggering 256 km3 – methane (CH4) is also present in very large amounts (~65 km3). This comes partly from anaerobic decay of dead organisms and from microbial reduction of the magmatic CO2 passing through its bottom sediments. Sulfate-reducing bacteria also generate toxic hydrogen sulfide (H2S) in the anoxic bottom waters – Lake Nyos contains less dissolved salts and did not emit H2S.

So Kivu presents a far greater hazard than the volcanic lakes of Cameroon and an emission of a dense gas mixture might fill the rift valley in the area to a depth of about a hundred metres. Being highly fertile the valley around the lake has a high population (2 to 3 million), so the death toll from a limnic eruption could be huge. A further hazard stems from tsunamis generated by such gas bursts. Once bubbles form at depth the bulk density of water drops, so large masses of water surge to the surface rather than the gas itself; a phenomenon known to happen in the periodic eruptions of Lake Nyos. What might trigger such an event in Lake Kivu? The East African Rift System is seismically active, but recent earthquakes did not result in limnic eruptions. Subaqueous volcanic eruption is the most likely to set one off. A surface lava flow from the nearby Mount Nyiragongo entered the lake at the town of Goma in 2002 but, fortunately, did not reach the threatening deeper part of Kivu. Sediment samples from the lake reveal periodic transport of land vegetation to its deeper parts, roughly every thousand years. The sediments with plant fossils also contain abundant remains of aquatic animals, suggesting both tsunamis accompanied by toxic emissions.

KIVUWATT’s methane extraction rig on Lake Kivu. (Credit: Contour Global)

Mitigating the hazard of limnic eruptions at Lake Nyos was made possible in 2002 by linking its bottom waters to the surface by plastic piping. After initial pumping, the release of bubbles at shallower depths and the resulting fall in bulk water density set off something akin to a large soda siphon, slowly relieving the deeper layers of their load of dissolved CO2. This resulted in 50 m high fountains of what was effectively soda ‘pop’. In 2009 this was repeated on a far larger scale on Lake Kivu, the operation being paid for by separation and sale of methane. Yet even this attempt at mitigation has its risks: first of destabilising what may be a fragile equilibrium to trigger a limnic eruption; second by lifting nutrient-rich bottom water that would encourage algal blooms at the lake surface and potential deoxygenation. The current issue of the Journal of African Earth Sciences includes a detailed review of the issues surrounding such dual-purpose hazard mitigation (Hirslund, F. & Morkel, P. 2020. Managing the dangers in Lake Kivu – How and why. Journal of African Earth Sciences, v. 161, Article 103672; DOI: 10.1016/j.jafrearsci.2019.103672). By 2015 the Rwandan KivuWatt Methane Project had a capacity for 25 MW of electrical power generation.

Running at full capacity, degassing the depths of Lake Kivu would provide the economic benefit of low-cost electricity for Rwanda and the DRC, at a maximum generating capacity of 300 mW using the most efficient power plant, as well as removing the risk of a catastrophic gas release. Yet the release of CO2 from the lake and from methane burning would increase atmospheric greenhouse warming significantly, albeit less than if the methane was simply released, for CH4 has 25 times the potential for trapping outgoing heat. Hence the dilemma. Either way, there remains the risk of turning Kivu’s surface water into an anoxic algal ‘broth’ with devastating effects on its fishery potential. Burial of the dead phytoplankton, however, might generate more methane by bacterial decay; a possible source of renewable biofuel that ‘recycles’ the atmospheric CO2 consumed by algal photosynthesis. The geohazards, according to Hirslund and Morkel, are really the ultimate driver for development of Lake Kivu’s fossil fuel potential, now that they are better understood as a real and present danger to millions of people. The authors calculate that a catastrophic gas release may be on the cards in the late 21st century. Yet there are other resource issues bound up with the health of the lake’s surface waters. Preserving the layered structure of the lake water to some extent is also important. Until the rates of natural infiltration of volcanic CO2 and biogenic production of methane are known, a minimum rate of gas extraction to make the lake safe is impossible to calculate. Perhaps matching those rates with gas removal should govern future operation. The total methane content of Lake Kivu is just 1.5 times the annual production from the UK sector of the North Sea. It is sufficient for power generation at 300 MW, at most, for 50 years, which would roughly double Rwanda’s current installed generation capacity – mainly from hydropower. Although Kivu is shared equally between Rwanda and the DRC even half of the short term power potential would be a significant benefit to Rwanda’s ~11 million people, though considerably less to the ~81 million living in the DRC; if access was equitable.

Should you worry about being killed by a meteorite?

In 1994 Clark Chapman of the Planetary Science Institute in Arizona and David Morrison of NASA’s Ames Research Center in California published a paper that examined the statistical hazard of death by unnatural causes in the United States (Chapman, C. & Morrison, D. 1994. Impacts on the Earth by asteroids and comets: assessing the hazard. Nature, v. 367, p. 33–40; DOI:10.1038/367033a0). Specifically, they tried to place the risk of an individual being killed by a large asteroid (~2 km across) hitting the Earth in the context of more familiar unwelcome causes. Based on the then available data about near-Earth objects – those whose orbits around the Sun cross that of the Earth – they assessed the chances as ranging between 1 in 3,000 and 1 in 250,000; a chance of 1 in 20,000 being the most likely. The results from their complex calculations turned out to be pretty scary, though not as bad as dying in a car wreck, being murdered, burnt to death or accidentally shot. Asteroid-risk is about the same as electrocution, at the higher-risk end, but significantly higher than many other causes with which the American public are, unfortunately, familiar: air crash; flood; tornado and snake bite. The lowest asteroid-risk (1 in 250 thousand) is greater than death from fireworks, botulism or trichloroethylene in drinking water; the last being 1 in 10 million.

Chapman and Morrison cautioned against mass panic on a greater scale than Orson Welles’s 1938 CBS radio production of H.G. Wells’s War of the Worlds allegedly resulted in. Asteroid and comet impacts are events likely to kill between 5,000 and several hundred million people each time they happen but they occur infrequently. Catastrophes at the low end, such as the 1908 Tunguska air burst over an uninhabited area in Siberia, are likely to happen once in a thousand years. At the high end, mass extinction impacts may occur once every hundred million years. As might be said by an Australian, ‘No worries, mate’! But you never know…

Michelle Knapp’s Chevrolet Malibu the morning after a stony-iron mmeteorite struck it. Bought for US$ 300, Michelle sold the car for US$ 25,000 and the meteorite fetched US$ 50,000 (credit: John Bortle)

How about ordinary meteorites that come in their thousands, especially when the Earth’s orbit takes it through the former paths taken by disintegrating comets? When I was a kid rumours spread that a motor cyclist had a narrow escape on the flatlands around Kingston-upon-Hull in East Yorkshire, when a meteorite landed in his sidecar: probably apocryphal. But Michelle Knapp of Peeskill, New York, USA had a job for the body shop when a 12 kg extraterrestrial object hit her Chevrolet Malibu, while it was parked in the driveway. In 1954, Ann Hodges of Sylacauga, Alabama was less fortunate during an afternoon nap on her sofa, when a 4 kg chondritic meteorite crashed through her house roof, hit a radiogram and bounced to smash into her upper thigh, badly bruising her. For an object that probably entered the atmosphere at about 15 km s-1, that was indeed a piece of good luck resulting from air’s viscous drag, the roof impact and energy lost to her radiogram. The offending projectile became a doorstop in the Hodge residence, before the family kindly donated it to the Alabama Museum of Natural History. Another fragment of the same meteorite, found in a field a few kilometres away, fetched US$ 728 per gram at Christie’s auction house in 2017. Perhaps the most unlucky man of the 21st century was an Indian bus driver who was killed by debris ejected when a meteorite struck the dirt track on which he was driving in Tamil Nadu in 2016 – three passengers were also injured. Even that is disputed, some claiming that the cause was an explosive device.

Risks of sudden changes linked to climate

The Earth system comprises a host of dynamic, interwoven components or subsystems. They involve processes deep within Earth’s interior, at its surface and in the atmosphere. Such processes combine inorganic chemistry, biology and physics. To describe them properly would require a multi-volume book; indeed an entire library, but even that would be even more incomplete than our understanding of human history and all the other social sciences. Cut to its fundamentals, Earth system science deals with – or tries to – a planetary engine. In it, the available energy from inside and from the Sun is continually shifted around to drive the bewildering variety, multiplicity of scales and variable paces of every process that makes our planet the most interesting thing in the entire universe. It has done so, with a variety of hiccups and monumental transformations, for some four and half billion years and looks likely to continue on its roiling way for about five billion more – with or without humanity. Though we occupy a tiny fraction of its history we have introduced a totally new subsystem that in several ways outpaces the speed and the magnitude of some chemical, physical and organic processes. For example: shifting mass (see the previous item, Sedimentary deposits of the ‘Anthropocene’); removing and modifying vegetation cover; emitting vast amounts of various compounds as a result of economic activity – the full list is huge. In such a complex natural system it is hardly surprising that rapidly increasing human activities in the last few centuries of our history have hitherto unforeseen effects on all the other components. The most rapidly fluctuating of the natural subsystems is that of climate, and it has been extraordinarily sensitive for the whole of Earth history.

Cartoon metaphor for a ‘tipping point’ as water is added to a bucket pivoted on a horizontal axis. As water level rises to below the axis the bucket becomes increasingly stable. Once the level rises above this pivot instability sets in until the syetem suddenly collapses

Within any dynamic, multifaceted system-component each contributing process may change, and in doing so throw the others out of kilter: there are ‘tipping points’. Such phenomena can be crudely visualised as a pivoted bucket into which water drips and escapes. While the water level remains below the pivot, the system is stable. Once it rises above that axis instability sets in; an external push can, if strong enough, tip the bucket and drain it rapidly. The higher the level rises the less of a push is needed. If no powerful push upsets the system the bucket continues filling. Eventually a state is reached when even a tiny force is able to result in catastrophe. One much cited hypothesis invokes a tipping point in the global climate system that began to allow the minuscule effect on insolation from changes in the eccentricity of Earth’s orbit to impose its roughly 100 ka frequency on the ups and downs of continental ice volume during the last 800 ka. In a recent issue of Nature a group of climate scientists based in the UK, Sweden, Germany, Denmark, Australia and China published a Comment on several potential tipping points in the climate system (Lenton, T.M. et al. 2019. Climate tipping points — too risky to bet against. Nature, v. 575, p. 592-595; DO!: 10.1038/d41586-019-03595-0). They list what they consider to be the most vulnerable to catastrophic change: loss of ice from the Greenland and Antarctic ice sheets; melting of sea ice in the Arctic Ocean; loss of tropical and boreal forest; melting of permanently frozen ground at high northern latitudes; collapse of tropical coral reefs; ocean circulation in the North and South Atlantic.

The situation they describe makes dismal reading. The only certain aspect is the steadily mounting level of carbon dioxide in the atmosphere, which boosts the retention of solar heat by delaying the escape of long-wave, thermal radiation from the Earth’s surface to outer space through the greenhouse effect. An ‘emergency’ – and there can be little doubt that one of more are just around the corner – is the product of ‘risk’ and ‘urgency’. Risk is the probability of an event times the damage it may cause. Urgency is the product of reaction time following an alert divided by the time left to intervene before catastrophe strikes. Not a formula designed to make us confident of the ‘powers’ of science! As the commentary points out, whereas scientists are aware of and have some data on a whole series of tipping points, their understanding is insufficient to ‘put numbers on’ These vital parameters. And there may be other tipping points that they are yet to recognise.  Another complicating factor is that in a complex system catastrophe in one component can cascade through all the others: a tipping may set off a ‘domino effect’ on all the others. An example is the steady and rapid melting of boreal permafrost. Frozen ground contains methane in the solid form of gas hydrate, which will release this ‘super-greenhouse’ gas as melting progresses.   Science ‘knows of’ such potential feedback loops in a largely untried, theoretical sense, which is simply not enough.

A tipping point that has a direct bearing on those of us who live around the North Atlantic resides in the way that water circulates in that vast basin. ‘Everyone knows about’ the Gulf Stream that ships warm surface water from equatorial latitudes to beyond the North Cape of Norway. It keeps NW Europe, otherwise subject to extremely cold winter temperatures, in a more equable state. In fact this northward flow of surface water and heat exerts controls on aspects of climate of the whole basin, such as the tracking of tropical storms and hurricanes, and the distribution of available moisture and thus rain- and snowfall. But the Gulf Steam also transports extra salt into the Arctic Ocean in the form of warm, more briny surface water. Its relatively high temperature prevents it from sinking, by reducing its density. Once at high latitudes, cooling allows Gulf-Steam water to sink to the bottom of the ocean, there to flow slowly southwards. This thermohaline circulation effectively ‘drags’ the Gulf Stream into its well-known course. Should it stop then so would the warming influence and the control it exerts on storm tracks. It has stopped in the past; many times. The general global cooling during the 100 ka that preceded the last ice age witnessed a series of lesser climate events. Each began with a sudden global warming followed by slow but intense cooling, then another warming to terminate these stadials or Dansgaard-Oeschger cycles (see: Review of thermohaline circulation, Earth-logs February 2002). The warming into the Holocene interglacial since about 20 ka was interrupted by a millennium of glacial cold between 12.9 and 11.7 ka, known as the Younger Dryas (see: On the edge of chaos in the Younger Dryas, Earth-logs May 2009). A widely supported hypothesis is that both kinds of major hiccup reflected shuts-down of the Gulf Stream due to sudden influxes of fresh water into North Atlantic surface water that reduced its density and ability to sink. Masses of fresh water are now flowing into the Arctic Ocean from melting of the Greenland ice sheet and thinning of Arctic sea ice (also a source of fresh water). Should the Greenland ice sheet collapse then similar conditions for shut-down may arise – rapid regional cooling amidst global warming – and similar consequences in the Southern Hemisphere from the collapse of parts of the Antarctic ice sheets and ice shelves.  Lenton et al. note that North Atlantic thermohaline circulation has undergone a 15% slowdown since the mid-twentieth century…

See also: Carrington, D. 2019. Climate emergency: world ‘may have crossed tipping points’ (Guardian, 27 November 2019)