Submarine landslides and formation of the East African Rift System

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Turmoil in Roman Republic followed Alaskan volcanic eruption

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Arsenic hazard on a global scale

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The dilemma of Rwanda’s Lake Kivu

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Ecological hazards of ocean-floor mining

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The distribution of potential ocean-floor metal-rich resources (Credit: Hefferman 2019)

 

Anthropocene edging closer to being ‘official’

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The issue of erecting a new stratigraphic Epoch encompassing the time since humans had a global effect on the Earth System has irked me ever since the term emerged for discussion and resolution by the scientific community in 2000. An Epoch in a chronostratigraphic sense is one of several arbitrary units that encompass all the rocks formed during a defined interval of time. The last 541 million years (Ma) of geological time is defined as an Eon – the Phanerozoic. In turn that comprises three Eras – Palaeozoic, Mesozoic and Cenozoic. The third level of division is that of Periods, of which there are 11 that make up the Phanerozoic. In turn the Periods comprise a total of 38 fourth-level Epochs and 85 at the fifth tier of Ages. All of these are of global significance, and there are even finer local divisions that do not appear on the International Chronostratigraphic Chart . If you examine the Chart you will find that no currently agreed Epoch lasted less than 11.7 thousand years (the Holocene) and all the others spanned 1 Ma to tens of Ma (averaged at 14.2 Ma). Indeed, even Ages span a range from hundreds of thousands to millions of years (averaged at 6 Ma).

The Vattenfall lignite mine in Germany; the Anthropocene personified

In the 3^rd week of May 2019 the 34-member Anthropocene Working Group (AWG) of the International Commission on Stratigraphy (ICS) sat down to decide on when the Anthropocene actually started. That date would be passed on up the hierarchy of the geoscientific community  eventually to meet the scrutiny of its highest body, the executive committee of the International Union of Geological Sciences, and either be ratified or not. In the meantime the AWG is seeking a site at which the lower boundary of the Anthropocene would be defined by the science’s equivalent of a ‘golden spike’; theGlobal boundary Stratotype Section and Point (GSSP).

Several options were tabled for discussion and decision, summarised by a 2015 paper in Nature. A case against the erection of an official Anthropocene Epoch on stratigraphic grounds appeared in a GSA Todaypaper in 2016. Despite the fact that there is evidence for the start of human geological, geochemical and biological influences as far back as 8 000 years ago (in effect the Holocene is the Epoch of rapid human growth and transformations), the 2015 paper concludes that there are two candidates for the base of the Anthropocene. The earliest is the decline in atmospheric CO₂ that began around 1570 CE and its recovery around 1620 CE recorded in Greenland ice cores. This is suggested to mark a fall in the indigenous population of the Americas from ~60 to ~6 million that followed the completion of European conquest, as a result of genocide, disease and famine. Regeneration of the American forest lands (~5 x 10⁷ hectare) that the dead had once occupied drew down CO₂.  However this overlaps with the coolest part of the Little Ice Age which may also have resulted in absorption of the greenhouse gas by cooled ocean water. The beginning of the industrial revolution was discounted on the grounds that it was diachronous as well as being difficult to define, having arisen first in Europe at some time in the 18^th century. The second candidate was the period when ~500 nuclear weapons were tested above-ground, beginning in 1945 and ending by treaty between the then nuclear powers in 1963. These distributed long-lived plutonium globally, which resides in sediments as a ‘spike’. Around 1963 there are also clear signs that plastics, aluminium, artificial fertilisers, concrete and lead from petrol began to increase in sediments. It is this last option upon which the AWG settled, with 29 members for and 5 against, and is to forward up the ‘chain of command’ in the geoscientific bureaucracy. A detailed and sometimes amusing account of the AWG’s deliberations appeared in the online Guardian newspaper on 30 May 2019.

The decision, in my opinion, signifies that the Anthropocene is an Epoch that includes the future, which is somewhat pessimistic as well as being scientific nonsense. Yet, coinciding as it does with rapidly escalating efforts, mainly by young people, to end massive threats to the Earth System, that can only be welcomed. It is an essentially political statement, albeit with a learned cloak thrown over it.  The only way to erase the exponentially growing human buttock print on our home world is for growth-dependent economics to be removed too. That is the only logical basis for the ‘green’ revolt that is unfolding. If that social revolution doesn’t happen, there will be a mass extinction to join the ‘Big Five’, and society in all its personifications will collapse. That is known as barbarism…

 

Frack me nicely?

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‘There’s a seaside place they call Blackpool that’s famous for fresh air and fun’. Well, maybe, not any more. If you, dear weekender couples, lie still after the ‘fun’ the Earth may yet move for you. Not much, I’ll admit, for British fracking regulations permit Cuadrilla, who have a drill rig at nearby Preston New Road on the Fylde coastal plain of NW England, only to trigger earthquakes with a magnitude less than 0.5 on the Richter scale. This condition was applied after early drilling by Cuadrilla had stimulated earthquakes up to magnitude 3. To the glee of anti-fracking groups the magnitude 0.5 limit has been regularly exceeded, thereby thwarting Cuadrilla’s ambitions from time to time. Leaving aside the view of professional geologists that the pickings for fracked shale gas in Britain [June 2014] are meagre, the methods deployed in hydraulic fracturing of gas-prone shales do pose seismic risks. Geology, beneath the Fylde is about as simple as it gets in tectonically tortured Britain. There are no active faults, and no significant dormant ones near the surface that have moved since about 250 Ma ago; most of Britain is riven by major fault lines, some of which are occasionally active, especially in prospective shale-gas basins near the Pennines. When petroleum companies are bent on fracking they use a drilling technology that allows one site to sink several wells that bend with depth to travel almost horizontally through the target shale rock. A water-based fluid containing a mix of polymers and surfactants to make it slick, plus fine sand or ceramic particles, are pumped at very high pressures into the rock. Joints and bedding in the shale are thus forced open and maintained in that condition by the sandy material, so that gas and even light oil can accumulate and flow up the drill stems to the surface.

Shale, being dominated by ultra-fine clay minerals, is slippery when wet. Consequently, any elastic strain built-up in the rock, either by active tectonics or from long in the past, is likely to be released by fracking. The fractures that release the gas also facilitate the escape of formation water locked in the shale from when it was originally deposited. Being rich in organic matter, target shales maintain highly reducing chemical conditions. So as well as being salty, such formation water may contain high abundances of heavy metals and arsenic, unlike the groundwater in naturally permeable and oxygenated rocks, such as sandstones and limestones. Fracking carries a pollution risk too. Toxic waste fluid is generally disposed of by pumping into permeable strata beneath the well site. There is no knowing where such noxious water might go, other than to follow lines of least resistance, such as large joints and dormant faults that may well be unsuspected at the depths to which drilling might penetrate. That too poses seismic rick by lubrication of the pathways taken by the fluids.

The sheer scale of shale-gas fracking in the US is indicated by the light emitted at night by well-lit installations and gas flares in a mature shale-gas basin in Texas targeting the mature, gas-rich Eagle Ford shale. (see: https://geology.com/articles/eagle-ford/)

Britain has barely been touched by fracking or conventional petroleum drilling, unlike large swathes of North America. Fracking began in Kansas, USA in 1947 but got underway in earnest in the 1970s to dominate US natural gas production since the 1990s. The effects of fracking in the long term [July 2013] show up in the active shale-gas basins there. Even in geological settings as quiescent as the Fylde seems to be, the picture is one of repeated earthquakes induced by fracking, which often exceed magnitude 3.0, including one of magnitude 5.6 in Oklahoma that destroyed 14 homes in 2016. A recent paper in Science examines how fluid migration induces dormant structures to move again (Bhattacharya, P. & Viesca, R.C. 2019. Fluid-induced aseismic fault slip outpaces pore-fluid migration. Science, v. 364, p. 464-468; DOI: 10.1126/science.aaw7354). The authors, from Tufts University in the US, used experimental fluid injection in France to indicate that aseismic slip resulting from fluid injection transmits stress far and wide, and more quickly than expected from the outward movement of the injected fluids. This explains why earthquakes produced by deliberate fluid injection into the crust often occur more frequently in active shale-gas basins than they do in areas of naturally high seismic activity

Related article: Fracking: Earthquakes are triggered well beyond fluid injection zones (Science News)

Gravity signals of earthquakes

A sign that an earthquake is taking place is pretty obvious: the ground moves. Seismometers are now so sensitive that they record significant seismic events at the far side of the world. The Richter magnitude scale commonly used to assign the power of an event is logarithmic, and the difference between each unit represents an approximately 32-fold change in the energy released at the source, so that a magnitude 6.0 earthquake is 32 times more powerful than one rated as magnitude 5.0. Because seismic motion affects a mass of rock it also perturbs the gravitational field. So, theoretically, gravimeters should also be able to detect an earthquake. Seismic waves travel at a maximum speed of about 6 to 8 km s^-1 about 20 times the speed of sound, yet changes in the gravitational field propagate at the speed of light, i.e. almost instantaneously by comparison. The first ground disturbances of the magnitude 9.0 Tohoku-Oki earthquake of NE Japan on 11 March 2011 hit Tokyo about 2 minutes after the event began offshore. Although that is a quite short time it would be sufficient for people to react and significantly reduce the earthquake’s direct impact on many of them. A seismic gravity signal would give that warning. The full horror of Tohoku-Oki was unleashed by the resulting tsunami waves, whose speed in the deep ocean water off Japan was about 800 km hr-1 (0.22 km s^-1). An almost real-time warning would have allowed 40 times more time for evasion.

Devastation in NE Japan caused by the Tohoku-Oki tsunami in March 2011

Japan is particularly well endowed with advanced geophysical equipment because of its notorious seismic and volcanic hazards. The first data to be analysed after Tohoku-Oki were understandably those from Japan’s large array of seismometers. The records from two super-sensitive gravimeters, between 436 and 515 km from the epicentre, were examined only recently. These instruments measure variations in gravity as small as a trillionth of the average gravitational acceleration of the Earth using a superconducting sphere suspended in a magnetic field, capable of detecting snow being cleared from a roof. Masaya Kimura and colleagues from Tokyo University and other geoscientific institutes in Japan undertook the analyses of both seismic and gravity data (Kimura, M. et al. 2019. Earthquake‑induced prompt gravity signals identified in dense array data in Japan. Earth Planets and Space, v. 71, online publication. DOI: 10.1186/s40623-019-1006-x). The gravimeter record did show a statistically significant perturbation at the actual time of the earthquake, albeit after complex processing of both gravity and seismographic data.

That only 2 superconducting gravimeters detected the event in real-time is quite remarkable, despite the need for a great deal of processing. It amounts to a test of the concept that such instruments or others based on different designs and deployed more widely may eventually be deployed to give prompt warnings of seismic events that could save thousands.

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Volcanism and the Justinian Plague

Between 541 and 543 CE, during the reign of the Roman Emperor Justinian, bubonic plague spread through countries bordering the Mediterranean Sea. This was a decade after Justinian’s forces had had begun to restore the Roman Empire’s lost territory in North Africa, Spain, Italy and the present-day Balkans by expeditions out of Byzantium (the Eastern Empire). At its height, the Plague of Justinian, was killing 5000 people each day in Constantinople, eventually to consume 20 to 40% of its population and between 25 to 50 million people across the empire. Like the European Black Death of the middle 14^th century. The bacterium Yersinia pestis originated in Central Asia and is carried in the gut of fleas that live on rats. The ‘traditional’ explanation of both plagues was that plague spread westwards along the Silk Road and then with black rats that infested ship-borne grain cargoes. Plausible as that might seem, Yersinia pestis, fleas and rats have always existed and remain present to this day. Trade along the same routes continued unbroken for more than two millennia. Although plagues with the same agents recurred regularly, only the Plague of Justinian and the Black Death resulted in tens of million deaths over short periods. Some other factor seems likely to have boosted fatalities to such levels.

Monk administering the last rites to victims of the Plague of Justinian

Five years before plague struck the Byzantine historian Procopius recorded a long period of fog and haze that continually reduced sunlight; typical features of volcanic aerosol veils. Following this was the coldest decade in the past 2300 years, as recorded by tree-ring studies. It coincides with documentary evidence of famine in China, Ireland, the Middle East and Scandinavia.. A 72 m long ice core extracted from the Colle Gnifetti glacier in the Swiss Alps in 2013 records the last two millennia of local climatic change and global atmospheric dust levels. Sampled by laser slicing, the core has yielded a time series of data at a resolution of months or better. In 536 an Icelandic volcano emitted ash and probably sulfur dioxide over 18 months during which summer temperature fell by about 2°C. A second eruption followed in 540 to 541. ‘Volcanic winter’ conditions lasted from 536 to 545, amplifying the evidence from tree-ring data from the 1990’s.

The Plague of Justinian coincided with the second ‘volcanic winter’ after several years of regional famine. This scenario is paralleled by the better documented Great Famine of 1315-17 that ended the two centuries of economic prosperity during the 11^th to 13^th centuries. The period was marked by extreme levels of crime, disease, mass death, and even cannibalism and infanticide. In a population weakened through malnutrition to an extent that we can barely imagine in modern Europe, any pandemic disease would have resulted in the most affected dying in millions. Another parallel with the Plague of Justinian is that it followed the ending of four centuries of the Medieval Warm Period, during which vast quantities of land were successfully brought under the plough and the European population had tripled. That ended with a succession of major, sulfur-rich volcanic eruption in Indonesia at the end of the 13^th century that heralded the Little Ice Age. Although geologists generally concern themselves with the social and economic consequences of a volcano’s lava and ash in its immediate vicinity– the ‘Pompeii view’ – its potential for global catastrophe is far greater in the case of really large (and often remote) events.

Chemical data from the same ice core reveals the broad economic consequences of the mid-sixth century plague. Lead concentrations in the ice, deposited as airborne pollution from smelting of lead sulfide ore to obtain silver bullion, fell and remained at low levels for a century. The recovery of silver production for coinage is marked by a spike in glacial lead concentration in 640; another parallel with the Black Death, which was followed by a collapse in silver production, albeit only for 4 to 5 years.

Related article: Gibbons, A. 2018. Why 536 was ‘the worst year to be alive’. Science, v. 362,p. 733-734; DOI:10.1126/science.aaw0632

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The risk of landslides in Africa

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.

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.

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.