Category: Environmental geology and geohazards

In the highlands of central Sri Lanka the sediment suspended in rivers suggest rates of soil loss from agricultural land of the order of up to 7000 tonnes per km² each year.  However, it is difficult to judge how much would be eroded under natural conditions, compared with the probable loss as a result of deforestation and human activities, particularly from very rugged landscapes where seasonal rainfall is high..  Radionuclides produced by cosmic-ray bombardment of minerals exposed to them, such as ¹⁰Be and ²⁶Al, accumulate in soil that is being eroded at a rate that is inversely proportional to the rate of erosion.  The nuclides form in the top 0.6 m of soil, which is the depth within which cosmic rays are normally absorbed.  So erosion rates that can be calculated from the cosmogenic nuclides in minerals, such as quartz, in river sediments apply to the times taken to remove that depth of soil.  Essentially, the rates that are measured represent the long-term erosion within a catchment basin.  Swiss and Sri Lankan geoscientists have applied the technique to rivers in central Sri Lanka, whose catchments have different vegetation cover and land usage (Hewawasam, T. et al. 2003.  Increase of human over natural erosion rates in tropical highlands constrained by cosmogenic nuclides.  Geology, v. 31, p. 597-600), such as forest reserves, rice terraces, tea plantations, areas of slash and burn agriculture, and various levels of degraded land.  The unmodified forest catchments give the lowest long-term erosion rates of 5-11 mm per 100 ka (13-30 tonnes per km² per year) as expected, but this is about a quarter of the rate of erosion measured by the same method throughout the highland region.  That probably reflects the antiquity of erosion induced by agriculture, yet current rates measured from sediments being carried by rivers suggests that soil erosion is now between 10 and 100 times faster than would occur under natural conditions.

Remote signs of earthquakes

All manner of ground-based observations have been tried as means of timely predictors of pending earthquakes, ranging from strange behaviour of wildlife to emissions of radon from wells (see Radon emissions and earthquakes, July 2003 issue of EPN).  So far, none of them have been universally useful, although there have been successful evacuations of threatened populations, principally in China, whose seismologists have focused on a wide range of signals.  Ideally, what is needed is some kind of global monitoring, and as with attempts to predict volcanic eruptions the only realistic means is from satellite surveillance.  Long ago, Doug Shearman of the Royal School of Mines at Imperial College, London introduced me to the peculiar properties of the mineral dolomite, as discovered by the man whose name it takes, Count Deodar de Dolomieu.  If you rub two lumps of dolomite together in a darkened room, they emit a sinister glow, and so do other minerals, such as quartz and even sugar.  Excellent for amusing the kids.  But then I learnt of “earth lights”, which had been photographed by Japanese observers just before earthquakes, in the vicinity of active faults – previously they were supposed to be as mythological as the fire balls during thunder storms (also a proven fact now).  At the time, the Landsat remote sensing satellite captured images during its night-time overpasses, on request.  A nice, if a little “blue skies” research project.  I submitted a brief proposal to my department’s research committee for ranking along with other studentship projects.  Perhaps my wry attitude to what had become somewhat dominated by other disciplines than remote sensing coloured my efforts; it was rejected.  So it was with some glee, a decade later, to find that NASA and the US Federal Emergency Management Agency had been testing the idea using weather satellites and the MODIS instrument carried by the Terra platform since 2000 (Enriquez, A. 2003.  The shining.  New Scientist, 5 July 2003, p. 26-29).  Encouragingly, though not for their victims, the devastating 1999 Izmit and 2001 Gujarat earthquakes were preceded by increased infrared emissions, detected from space, 5 days before the event.  Experiments show that when rock is stressed, emissions build up, and then vanish once the rocks fails, as in an earthquake, so the method looks very promising.

Another seismic phenomenon is changing magnetic fields around the site of failure.  This was first noticed from magnetometer records on the ground before the 1989 Loma Prieta earthquake that damaged large tracts of northern California.  Magnetic field variations too can be monitored from orbit.  The privately funded QuakeSat, launched on 30 June 2003 aims to test this possibility, as will a more ambitious French satellite, due to reach orbit in April next year (Reichhardt, T. 2003.  Satellites aim to shake up quake prediction.  Nature, v. 424, p. 478).

Models abound for predicting earthquakes from past seismicity and detailed tectonic maps, analogous to those suggested for prediction of volcanic hazards.  The Izmit earthquake of 17 August 1999 in Turkey was among the most savage in recent years and killed thousands.  It was as powerful (magnitude 7.8) as the celebrated 1906 San Francisco earthquake, and like it stemmed from movement on a continental-scale strike-slip fault.  The North Anatolian Fault is almost as well studied as the San Andreas line, and seismicity was known to be heading westwards well before the Izmit catastrophe.  Indeed, the Izmit area was predicted to be next on the list, yet no preparation had been made, even by Turkish tectonicians who had been involved in seismic analysis.  Chinese geoscientists take a different approach to seismic prediction than those in the west – over the last few centuries, hundreds of thousand Chinese people have perished in earthquakes.  They are trying to organise local people to monitor possible precursors to earthquakes, such as rises in water levels in wells and strange behaviour of animals.  They have had some notable successes, including preparation for one earthquake in recent years that saved an estimated 80 thousand people in one particularly hazard prone city.  The Geological Survey of Israel has been testing a well known correlation between the times of anomalous radon emissions from the ground and earthquakes along the Aqaba Fault that controls the Dead Sea.  Over a 7-year period, hourly scintillation-counter readings of radon emissions from springs, wells and especially gravels near known active faults allowed a rigorous test of a possible prediction system, because in that time there were almost 800 minor earthquakes (Steinitz, G. et al. 2003.  Statistically significant relation between radon flux and weak earthquakes in the Dead Sea rift valley.  Geology, v. 31, p, 505-508).  For events beneath the Dead Sea rift, there is a good correlation between the start of radon emission increases and earthquakes, which suggests that about 3-days warning could be given, if the monitoring was widely deployed.  The same cannot be said for small tremors with a source outside of the active fault zones.  The success may possibly be because sufficient radon to be easily detected is generated by radioactive decay of uranium in a phosphorite bed that underlies the study area. Radon escape to the surface is possibly eased when microfractures begin to open as strains build before an earthquake.

Unless it is possible to give people who live near dangerous volcanoes sufficient warning that they can escape disaster, eruption prediction might be looked on as a lugubrious topic.  Up to now, there have been very few predictions that have been better than a few hours or days.  Mexico’s Popocatapetl gave two days warning in late 2001, and that was sufficient for a completely successful evacuation of those threatened.  In the case of the eruption of Nyirangongo in eastern Congo, a few months later, warning signs preceded eruption by 5 days, but the people of Goma were not told and 45 people died trying to rescue possessions from the quiet, but relentless movement of a lava stream (see EPN February 2002, Is volcanic eruption predictable?).  In both cases it was abnormal seismicity that presaged the events.  John Murray, of the British Open University, has analysed the statistics of seismic events and eruptions of possibly the world’s most monitored volcano, Etna on Sicily (Murray, J.B. 2003.  Seismicity and time-lagged lava output at Mount Etna: A new method of long-term forecasting at a destructive volcano.  Geology, v. 31, p. 443-446).  Energy released during 19-year periods by earthquakes beneath the volcano since 1870 shows a inverse relationship with 9-year lava production, which suggests that seismicity and eruption are widely separated in time over long periods.  However, by examining the correlation of seismic energy with eruption volume for time differences between the two from 0 to 50 years, Murray has been able to show that Etna increases its productivity roughly 25 years after major releases of seismic energy.  Using this as an input to a model that might predict eruption intensity, he has been able to mimic the actual volcanism through the 20^th century with fair accuracy.  In his opinion, the very high eruption rate since 1950, which reached a peak in the 1990s, is only likely to decline a quarter of a century after large earthquakes (> magnitude 6) return to Sicily.  So, Sicilians have a difficult choice.  Should they worry about lava flows or earthquake damage?  Sadly, data suitable for broadening Murray’s method are available for very few volcanoes, all in quite prosperous countries.

Modelling the duration and extent of mining contaminants

Release of high concentrations of heavy metals and other pollutants to drainages is a natural consequence of geochemical anomalies associated with mineralization.  However, these have come to balance with the rest of the environment over periods measured in thousands of years or even longer.  The pose perpetual hazards, some of which are known, some not.  Environmental disturbance by mining and associated activities scales up releases of pollutants many times over those of natural origin.  Even with modern means of waste containment, escapes occur, sometimes of very large magnitude, such as the breaching of tailings dams or landslips in spoil heaps.  Of course, these hit the news when they happen, but assessing how long the pollution dwells in downstream areas and how it moves is not easy.  It requires some kind of model of the hydrology, erosion and sediment-transport characteristics of the affected drainage basins, that takes into account catchment topography and the size-distribution and density of escaped wastes.  Such a modelling tool is now available, having been developed at the University of Wales in Aberystwyth (Coulthard, T.J. & Macklin, M.G. 2003.  Modelling long-term contamination in river systems from historical metal mining.  Geology, v. 31, p. 451-454).  It is complex, because it combines the 3-D shape of basins with water discharge and depth, vegetation cover, depth to bedrock and the properties of released materials. In a simulation of hydrological dynamics.  TRACER is able to take account not just of the fate of grains that enter drainages, but how they are deposited in alluvium and then reworked by later changes in hydrology.  Coulthard and Macklin apply the model to the base-metal mining district of Swaledale in North Yorkshire, England, where production began in 1700 and ended 200 years later.  Swaledale was a minor producer of lead and zinc in modern terms, and the miners paid scant attention to environmental protection.  Results suggest that contamination spread downstream to the flat land of the Vale of York in only 10 years after mining started, but the pollution lingers, and seems likely to stay above safe limits until well after the start of the 22^nd century.  When possible increases in rainfall through global warming are factored in, the simulation remains much the same for 10 to 25 % rises, and only moves towards clean-up with 50 to 100 % increases in precipitation, when clean sediments should dilute the pollutants.  As well as predicting the general effects of contaminant releases, TRACER is able to highlight parts of a drainage basin that are particularly at risk due to trapping of sediments.  Mining in Swaledale produced, at most, only about 600 thousand cubic metres of metal-rich waste, fine enough to be transported by water.  Recent escapes from tailings dams and landslipped spoil heaps, as in Spain and OK Tedi in Papua New Guinea, were orders of magnitude larger.

Unlike some natural catastrophes, there is no stopping a volcanic eruption.  The best that can be done is to give people who live in the danger zones sufficient warning that they can escape disaster.  Many volcanic areas are densely populated, largely because soils derived from lavas and ash are extremely fertile, and high volcanoes create decent rainfall because of their orographic effect.  Naturally, nobody likes to up sticks, whatever the dangers, least of all if there are false alarms.  As with seismic prediction, volcanologists do not have a good track record of foretelling big eruptions, even though a great many geologists cluster on and around volcanoes.  Most of them flock to areas with active lavas, pyroclastic flows and other lugubrious after effects of major activity.  However some do the painstaking work of trying to monitor the plumbing of volcanoes, to get a handle on which parameters are most likely to be authentic warnings of impending doom.  It is no longer a matter of experienced volcano watchers and their instinctive feel for when one is about to blow its top, but one of ever more sophisticated instruments and software to analyse data and model volcanoes’ inner workings.  The 28 March 2003 issue of Science (p. 2015-2030) devotes 16 pages to a review of volcano monitoring.  While advances are being made, there is still a long way to go before they can pay dividends by reducing the loss of life.  What is not going to go away, even in the best of all possible scientific worlds, is the economic devastation that follows any geohazard.

The April 2001 issue of Earth Pages News (Taming Lake Nyos, Cameroon) announced attempts to release CO₂-rich water from the bottom of the notorious Lake Nyos, by setting in motion a sort of soda siphon.  A massive discharge of gas from Lake Nyos in 1986 killed 1700 local people, possibly after a small earthquake and landslide disturbed the bottom water.  Nearby Lake Monoun had already asphyxiated 37 people two years previously.  Both lakes are stagnant, and carbon dioxide released by exhalation from deep magma chambers dissolves under pressure in their deepest levels.  If the water rises, then it belches out dissolved gas, with potentially disastrous results.  Taming these killer lakes by bringing gas-rich water up pipes works because as the gas bubbles out of solution it rushes up the pipe dragging water with it, to create a fountain.  This is slowly relieving the danger of Lake Nyos, and there have been no problems caused by disturbing the deep water by the pipe’s presence, so far.  A French team from the University of Savoie is now installing a similar device in Lake Monoun, which poses a greater threat than Nyos, because the gas-rich water is only 60 metres down.  Potentially far more dangerous are the lakes of the East African Rift system, where magma exhalation is far more widespread and seismicity more common.  Lake Kivu, near Goma on the border between Rwanda and the Democratic Republic of Congo, threatens far more people with a massively greater threat, which also includes huge volumes of buried methane.  Luckily, the lava flow there during early 2002 did not reach the gas-rich level.  The experience from Cameroon promises an eventually easing of the dangers elsewhere.

Source: Krajick, K. 2003.  Efforts to tame second African “killer lake” begin.  Science, v. 299, p. 805.

Millions of Bangladeshi people risk arsenic poisoning if they drink water drawn from tube wells (see British Geological Survey sued over arsenic, October 2002 Earth Pages News).  Since the disaster first came to light, UNICEF have tested 1.3 million of the estimated 10 million tube wells that are potentially hazardous.  Those deemed safe are painted green, while those which are risky are now red.  Unfortunately, doubts are being cast on the reliability of the commercial test kits that UNICEF use to estimate dissolved arsenic concentrations.  It is claimed that the analytical method have never been validated by controlled field experiments, and also that the minimum level of arsenic that they can detect is ten times higher than the safe level set by the WHO.  A positive contribution to solving the problem is to drill deeper, since it seems as if the condition for release of arsenic from bonding in sedimentary iron minerals is related to bacterial action that creates reducing conditions.  Although deep by comparison with traditional hand-dug wells, the tube wells go down only 50 to 80 metres and do not penetrate the zone in which reducing bacteria survive.

Source:  Pearce, F. & Hecht, J. 2002.  Flawed water tests put millions at risk.  New Scientist, 16 November 2002, p, 4-5.

Seismic bathymetry and Mediterranean debris flows

Tsunamis are an ever present threat in coastal areas, and can be set in motion by submarine debris flows as well as by earthquakes.  As more evidence for ancient tsunamis emerges on coastlines, such as characteristic features in Alaska (seismically induced), the Hawaiian islands and Bahamas (induced by landslips on unstable volcanic islands), and even the east coast of Britain (submarine debris flow off western Norway) their perceived threat has grown.  A team of oceanographers from Spain, Canada, Belgium, Britain and France has re-examined seismic reflection data from the western Mediterranean, to extract detailed topography of the sea floor (Lastras, G. and 6 others 2002.  Seafloor imagery from the BIG’95 debris flow, western Mediterranean.  Geology, v. 30, p. 871-874).  Although the western Mediterranean is seismically quiet, compared with around Italy and Greece, it is floored by products of turbidity flows.  A particularly large example (BIG’95) off the Spanish coast has an estimated volume greater than 26 km³.  Lastras et al. provide exceptional detail of the internal structure and surface shape of this debris flow, which enables them to suggest how it formed.  It coincides with an interface between deep volcanic rocks and a thick cover of soft sediments, along which gradual detachment eventually resulted in a normal fault propagating to the sea floor.  The mechanical instability seems to have been due to rapid deposition from the Ebro river system at a time of low sea level in the Mediterranean around the beginning of the Holocene.  The flow is marked by fluid escape structures, which the authors suggest may have been connected with a rise in bottom-water temperatures.  Is this another example of gas hydrate being involved, as seems likely for the Storegger slide that caused tsunamis along Britain’s east coast (see Collapsing islands, March 2002 Earth Pages News) about 7200 years ago?  The authors do not speculate on that.  However, the detail that they provide about the conditions that culminated in BIG’95 should provide a benchmark for seeking areas prone to such massive and potentially catastrophic events.

In an number of well-studied areas of chronic seismicity it appears from historical records that earthquakes recur with regularity.  If that was so, it might be possible at least to prepare to throw many methods of detecting imminent movements at such areas, when they are “due” to go off.  The theory behind time-predictability is that earthquakes relieve tectonic stresses along faults, and that if the forces are maintained, stress builds up again, to be released after a roughly fixed time (the same might apply to volcanism where magma production stays constant).  A corollary is that high-magnitude events have longer periodicities than those lower on the Richter scale.  One of the best cases thought to support this view is a 25-km stretch of the San Andreas Fault near Parkfield in California.  The area has had  5 or 6 earthquakes greater than magnitude 6 since 1857, roughly every 22 years, the last being in 1966.  There ought to have been one in 1988, but the poor statistics give an uncertainty either way of 10 years.  By now there should have been a magnitude-6 event in the area, but it hasn’t happened.  Jessica Murray and Paul Segall of Stanford University have analyzed the physics of the last event, and of the period that followed it.  (Murray, J. & Segall, P 2002.  Testing time-predictable earthquake recurrence by direct measurement of strain accumulation and release.  Nature, v.  419, p. 287-291).

Their work involved using precise geodetic measurements obtained over the last four decades to assess the 1966 Parkfield earthquake’s size, which combines the movement then along the San Andreas Fault, the area involved in the slip and how “stiff” the crust is locally.  Comparing this with geodetic data since then suggests strongly that the strain released in 1966 must have recovered between 1973 and 1987.  They have shown that another Parkfield earthquake is long overdue.  Their method rigorously allows for the effects of movements along other nearby fault, and inherent unpredictability seems inescapable.  While other tests of the time-predictability principle, theoretically the most plausible approach, will continue, most devastating earthquakes continue to occur without forewarning.  That reflects the fact that there are only enough seismologists with fancy equipment to cover threatened areas in a few extremely rich countries.  Most people who live along active fault zones know whether or not high-magnitude earthquakes occur in their vicinity, yet will not have the privilege of scientists and equipment to provide warnings of this kind for a very long time, for simple economic reasons.  Perhaps some effort and funds should be diverted to providing warnings within days of a serious event, using less “robust” methods.

See also:  Stein, R.S. 2002.  Parkfield’s unfulfilled promise. Nature, v.  419, p. 257-258.

British Geological Survey sued over arsenic

The world’s largest ever class action has been launched in London against the British Geological Survey, over claims that it failed to spot arsenic contamination during a 1992water survey in Bangladesh. As many as 40 million Bengalis risk arsenic poisoning, following a major groundwater development programme in the 1970s and 80s.  Arsenic poisoning at non-fatal doses often shows first as water blisters on hands and shins. Long-term exposure via drinking-water causes cancer of the skin, lungs, urinary bladder, and kidney.

Aid agencies, led by UNICEF sank four million wells deep into alluvium, in the hope that groundwater use would alleviate the chronic problem of heavily polluted surface water in Bangladesh.  The arsenic is of natural origin, and stems from leaching of the toxic element from sulphide minerals by deep, reducing waters.  The case hinges on BGS’ failing to test for arsenic, which is easily detected using low-cost semi-quantitative methods, only 3 years after they had completed a comprehensive evaluation of groundwater quality in Britain that did include arsenic measurements.  Accusations of double standards have been flying.  However, UNICEF also failed to test for arsenic during the original drilling, because they did not expect to find it in the water.  World Health Organization guidelines are very clear that arsenic does pose a threat in groundwater, but most cases in the past have been associated with former mining areas.

Considerable work on measures to clean up well water has been conducted since the Bengal arsenic crisis surfaced.  Under oxidizing conditions, arsenic is adsorbed by ferric hydroxide, and a simple remedy is passing the water through iron wool or over ground-up rust or natural ochres.

Lots of attention has focused on impacts by Earth-crossing asteroids and comets as potential causes of economic and biological catastrophe, as too on hazards from climate change induced by major volcanic activity.  To these fears can be added the effects of tsunamis, but not those caused by even the largest conceivable eathquake.  Oceanic islands can fall apart by a process that is identical to, though vastly bigger than a landslip, thereby displacing their equivalent volume of seawater.

Britain has experienced the effects of tsunamis driven by collapse of part of the Norwegian continental slope, triggered by massive methane release from gas hydrates in sea-floor sediments.  The last of these was when its shores were colonised by Bronze Age people, and left its mark in the form of high-level sand beds on the flanks of eastern Scotland’s firths.  The north-east part of the Isle of Skye preserves spectacular results of landslips of volcanic rocks, that represent the largest mass movement known in Europe.  However both examples are dwarfed by features off the Hawaiian islands, that sonar has revealed.  There are some 70 debris fields that date back to 20 Ma, some of which contain up to 5 000 cubic kilometres of rock from collapses of the flanks of the growing volcanic islands.  Surveys around other large oceanic islands of volcanic origin suggest that such flank collapses occur around every 10 000 years.  Movement of masses so large involves energy equivalent to the world’s arsenal of nuclear weapons, so flank collapses are comparable in magnitude with moderately sized impacts.  They would generate tsunamis waves as high as 30 metres, that would devastate coastal areas around large ocean basins.

One area on Hawaii is indeed liable to collapse, and in November 2000 it moved, only to stop short of a full collapse.  Geoscientists from the US Geological Survey and Stanford University used GPS receivers to monitor movement on the southern flank of Kilauea, and after a series of barely detectable earthquakes they recorded slips of up to 6 centimetres per day (Cervelli, P. et al. 2002.  Sudden aseismic slip on the south flank of Kilauea volcano.  Nature, v. 415, p. 1014-1018).  Careful analysis of many kinds of motion sensors suggests that the moving block sits on top of a low-angle fault or detachment, that may eventually carry the block seawards to unleash tsunamis.  It is uncertain how much warning there would be of a fully fledged collapse, but is does seem sensible to establish such monitoring on active volcanic islands in the world’s oceans.  Since expansion of humanity following the retreat of the last continental ice sheets would have largely been along coasts, with their easy terrain and abundant food supplies, tsunamis would have been an ever present, but never suspected risk.  Britain’s example is minor in comparison to those that would stem from flank collapses, and perhaps the near-miss of November 2000 may encourage searches for the scars that huge tsunamis generate in relation to maritime archaeological records.

Inhabitants of the eastern Congolese town of Goma have suffered three disasters in 8 years – the aftermath of the Rwanda massacres of 1994, the episodic war centred on control of Congo’s immense physical resources since 1995, and now the devastating eruption of the Nyiragongo volcano that threatens half a million people.  The last is a grim reminder of the difficulty in predicting geological disasters, and follows closely on claims that spotting impending volcanic eruptions is now “sorted” (Marshall, T. 2002.  There she blows.  New Scientist 12^th January 2002, p. 29-31; Horizon, BBC2 17^th January 2002, Volcano Hell).  There are four phenomena that have been investigated as signifying threats of  eruption.  Most obvious are increases in temperature at existing vents that can easily be measured using infrared images from daily orbits of meteorological and environmental satellites.  A remote sensing approach is so cheap that it ought to be applicable world-wide, yet most devastating eruptions emerge with insufficient  time following thermal signs for emergency evacuations to begin.  Fundamentally the clearest evidence that magma beneath a volcano is rising is that the edifice swells.  Interferometric radar can detect millimetre-scale changes in surface topography, and such pre-eruption inflation is detectable (see Interferometric radar and faults of the Mojave Desert in Earth Pages, December 2001).  However, the lengthy periods between overpasses by radar imaging satellites (two images are a minimum for radar interferometry), and the need for immensely powerful computer processing has rendered this approach one of retrospection rather than early warning.  Individual volcanoes’ ground motions, and the minute changes in their gravitational potential that also relate to magma movements can be monitored at permanent ground stations, but apart from a select few on which volcanologists conduct long-term research, some thousands of dangerous volcanoes go unwatched.

The central theme of both the Horizon programme and the New Scientist article was a method based on monitoring low-energy seismicity emanating from magmatic movements.  The observation of low-frequency, long-period seismicity  by US Geological Survey volcanologists while Mount St Helen’s was active in 1980 is probably connected to a natural resonance of each volcano as magma begins to move.  Follow-up work at a small number of volcanoes has fine tuned such signals to the timing of eruptions, with sufficient confidence levels that believable warnings are possible.  Believability is essential, for a mass evacuation followed by no threat to life could deter future responses by endangered people, on the “crying Wolf” principle.  Mexican volcanologists were able to give two day’s warning of the immense eruption of Popocatapetl on 18^th December 2001, and evacuation prevented any loss of life.  However, none would have been threatened, as it happened, for the eruption on the vast massif was far from habitations.  Yet so spectacular were the fire fountains, that the exercise served to habituate locals to take such warnings very seriously indeed.

Nyiragongo volcano and its companions in the western African Rift regularly erupt low-viscosity lavas that flow quietly over long distances.  They pose less violent threat to life than explosive volcanoes, such as those around the Pacific rim, but chance may channel such flows through inhabited areas disrupting communications and destroying buildings.  Many of the 45 confirmed deaths in Goma arose when people tried to rescue belongings from their engulfed homes.  The current Goma disaster is not one primarily of volcanic origin, but of poverty, poor communications and fragile provision of basic necessities, such as unpolluted water and emergency food supplies.  After the 1994 humanitarian tragedy, and threats from Nyirangongo to the 800 thousand Rwandan refugees camped around Goma, the US Geological Survey and Japanese volcanologists set up seismometers to monitor the volcano’s internal activity.  Five days before the eruption, only two remained functional, yet transmitted signs of abnormal seismic activity (Clarke, T. 2002.  Seismic rumbling foretold Congo eruption.  Nature. v. 415, p, 353).  Despite that, warning did not get through to Goma in time for local people to flee, or any assistance to arrive. There was nowhere for the victims to go and relief followed only days and weeks after the event, when the damage was done.  The same fate hangs over millions of people living in volcanic areas in poor countries – they favour such risky areas to live because of the richness of soils and the encouragement of rainfall by high mountains..  As things stand, communities in volcanic areas of  North America, New Zealand, Japan and a few of the richer 3^rd World countries stand a good chance of escaping magmatic events because of believable warnings and efficient communication.  For the majority, survival is a matter of luck alone.

On 21 August 1986 a huge cloud of carbon dioxide gas released from Lake Nyos in the Highlands of Yaounde District of Cameroon, killed 1,700 local people by suffocation

Lake Nyos is one of several maars produced by one-off explosive events in the recent past.  Isotopic analyses of gas remaining dissolved in the lake show that the CO₂ is of volcanic origin.  The lakes are fed by springs on their beds, which is where the CO₂ enters, so that CO₂-rich water builds up at the bottom.  A thermal overturn of Lake Nyos may have caused dissolved gas to come out of solution as pressure decreased. 

Since 1986, gas levels have built up, so Lake Nyos once again threatens the local people and their livestock.  An international team, headed by George Kling a geologist at Michigan University, USA, has devised a means of venting the gas harmlessly.  This involves polyethylene pipes that descend to the lake bed.  Once primed by pumping, gas bubbles form as pressure drops.  Their rise up the pipe drags more water upwards, as in a soda siphon.  Fifteen years after the disaster, the first such siphon began operating with spectacular effects (Jones, N.  2001.  The monster in the lake.  New Scientist, 24 March 2001, p 36-40).  This only keeps pace with addition of CO₂ and a full solution requires several siphons.

Some scientists worry that siphoning itself may disturb a precarious balance in the lake, so the French engineers who built it have included sensors and shut-off valves.  Not everyone agrees that the 1986 disaster resulted from processes within the deep lake itself.  That should have led to a regular succession of gas releases, for which there is little evidence.  Landslips or a gaseous eruption might have been the trigger.  Reducing dissolved CO₂ levels in Lake Nyos and nearby Lake Monoun would seem to lessen risks of a future disaster, but could also lull locals into a false sense of security.

On 21 August 1986 a huge cloud of carbon dioxide gas was released from Lake Nyos located at 300 metres in the Highlands of Yaounde District of Cameroon. Because carbon dioxide is more dense than air it hugged the ground and flowed down valleys. The cloud travelled as far as 15 miles (25 km) from the lake. It was moving fast enough (as much as 80 kph) to flatten vegetation. 1,700 local people died by suffocation, probably unaware of their plight.   Two years earlier 37 people died similarly in a gas release from nearby Lake Monoun

Lake Nyos is in the Oku volcanic field, and is one of several maars produced by one-off explosive events in the recent past.  Isotopic analyses of gas remaining dissolved in the lake show that the CO₂ is of volcanic origin.  The lakes are fed by springs on their beds, which is where the CO₂ enters.  Being extremely deep (about 200 metres) and with no surface inlet the lake water is strongly stratified, so that CO₂-rich water builds up at the bottom.  The gas release must have involved an overturn of the stratification, so that dissolved gas came out of solution as pressure decreased.  What triggered the overturn is hard to establish, but one possibility is that during August (both catastrophes occurred in that month) cold weather cools surface waters so that they sink.  Other possibilities are storms, landslides or earthquakes, but there are no records of any of these preceding either event; they came completely unannounced.

Since 1986, gas levels have built up, and now stand at twice their concentration following the disaster, so danger threatens the local people and their livestock once again.  An international team, headed by George Kling a geologist at Michigan University, USA, has devised a means of venting the gas harmlessly.  This involves sinking 15 centimetre diameter polyethylene pipes to the lake bed.  Once pumping starts, gas bubbles forming as pressure releases will drag the water upwards, as a self-sustaining siphon, similar to the air-lift dredges used in marine archaeology.  Four such pipes would rid the lake of its lethal gas content in two years, and even one would reduce the hazard considerably.

Sources:  Observer, 20 August 2000, University of Michigan (http://www,biology.lsa.umich.edu/~gwk/research/nyos.html)