Category: Environmental geology and geohazards

Highly fractionated, silica-rich magma poses the greatest danger of explosive volcanic eruption, characterised by glowing pyroclastic flows that produce the strange rock ignimbrite. For example, in the Andes, ignimbrites extend for large distances from the calderas that emitted them. Fortunately rhyolite eruptions are rare, but that poses a scientific problem – they have not been as well studied as more common magmatic phenomena. Until May 2008 the latest rhyolite eruption had been in Alaska during 1912. In 2008 the Chilean volcano Chaitén erupted for the first time in 9 thousand years. There was no warning. Andesitic and dacitic volcanoes are restless for months before an eruption, though that is not much comfort as exactly when they ‘go off’ is still unpredictable. But any warning helps prepare local populations for the worst. A volcanoes precursory rumblings and shakings reflect the slow upward movement of magma. In the case of Chaitén, magma rose at about 1 m s^-1 that flabbergasted the volcanologists who rushed to study such a rare event (Castro, J.M. & Dingwell, D.B. 2009. Rapid ascent of rhyolitic magma at Chaitén volcano, Chile. Nature, v. 461, p. 780-783). The magma rose 5 km from its source in less than 4 hours. It is generally thought that the more silicic magma is, the more viscous and sluggish, which is certainly the case for rhyolite when it emerges: the melting of impurities in a coal fire produces a very silica-rich melt but such slag certainly does not dribble out of the fire box to pool on the hearth. High viscosity allows an erupting magma to retain gas escaping from solution as pressure drops, which is the source of the catastrophic blasts of massive ignimbrite events. Below the surface the Chaitén magma behaved in an extremely fluid manner, perhaps because it contained so much dissolved gas that it became a fluid froth at quite shallow depth. This unique observation is deeply disturbing for populations living in areas blanketed by ancient ignimbrites, as in the Andes. The very worst terrestrial events imaginable are ignimbrite eruptions that can blast out at such high velocities as to groove the ground and carry over thousands of km² in matter of minutes. Without warning, there is no escape.

Wenchuan earthquake (May 2008) analysed

On 12 May 2008 a magnitude 7.90 earthquake killed more than 80 thousand people and left many more injured and homeless in the Wenchuan area of Sichuan province China. In the worst affected areas up to 60% of the population were killed. The catastrophe occurred at the densely populated western boundary of the Sichuan basin with the Tibetan Plateau, and involved surface displacement that propagated rapidly north-eastwards along a 235 km long zone. There was virtually no warning sign and although crossed by major faults, high-magnitude seismicity was a rarity in the area. Several satellites now repeatedly deploy synthetic aperture radar sensing along their ground swath, so that interferometric methods (InSAR) are able to assess ground motions between separate times of overpass, with sub-centimetre precision. Together with direct measurement of motions at GPS ground stations, InSAR allows an unprecedented ‘post-mortem’ of this dreadful event (Shen, Z-K et al. 2009. Slip maxima at fault junctions and rupturing of barriers during the 2008 Wenchauan earthquake. Nature Geoscience, v. 2, p. 718-724). The structural architecture of the surrounding area is of five fault-bounded blocks that jostled during the event, resulting in profound shifts in the geometry of motion along two parallel faults that ruptured. The event was so sudden and large because what would otherwise have been barriers to propagation of strain failed at the same time. All the strain cascaded through several fault segments. This is not a scenario that could have been easily predicted, the authors judging it to have been a once-in-4000 years concatenation of crustal failure.

Seismic unpredictability is something that seismologists now recognise (Chui, G. 2009. Shaking up earthquake theory. Nature, v. 461, p. 870-872). Active faults turn out not to be ‘creatures of habit’, and nor can we assume that long-quiet segments are the most likely to fail in future. Ominously, there is a growing body of evidence that great earthquakes are able somehow to trigger others, often far distant. An example is the giant Sumatra-Andaman event of 26 December 2004, tsunamis from which caused a toll of hundreds of thousand lives around the Indian Ocean. It was followed quickly by swarms of small tremors on the San Andreas Fault 8000 km away. Rapid successions of great earthquakes around the world, such as the October 2005 Pakistan earthquake 9 months after that in the Indonesian area, can no longer be regarded as ‘bad luck’. Seismic waves are able to weaken far-off segments of active faults.

All forms of asbestos (various serpentines and some amphiboles), but especially the blue variety, are carcinogenic because their dusts consist of minute fibres. Most publicity about the hazard that this mineral presents is from cases that stem from its use as an insulator in housing, shipbuilding and other constructions in developed countries. Areas where it has been mined or outcrops naturally are equally risky if wind can pick up asbestos dust under dry conditions. A large proportion of this now banned industrial mineral was mined in South Africa and many cases of asbestosis and mesothelioma in former mining areas have come to light there since the fall of apartheid. The locations of former asbestos mines are well known, and some attempts are being made to bury the waste. The most tragic cases are where the mining companies have either folded or been engulfed by larger transnational corporations; several legal actions for compensation have been dragging through the courts for a decade or more. However, asbestos minerals are common at what were non-commercial levels in many ultramafic rocks. Such rocks occur in ophiolite complexes and Archaean greenstone belts on every continent, and although ultramafics are in a minority as regards rock outcroppings, they are far from rare. In its natural state such land can shed asbestos-rich dust when dry, and urban and communications developments expose the material to wind action.

Asbestos minerals fortunately have distinctive infrared spectra in the short-wave infrared (SWIR), preferentially absorbing photons at around 2.3 micrometres because of their abundance of magnesium-oxygen bonds that such wavelengths cause to vibrate. Remote sensing is therefore a potentially useful means of screening areas of human habitation for asbestos risks (Swayze, G.A. et al. 2009. Mapping potentially asbestos-bearing rocks using imaging spectroscopy. Geology, v. 37, p. 763-766). The authors, from the US geological Survey and the California Department of Conservation, used a sophisticated and costly form of aerial remote sensing that covers the visible and infrared part of the EM spectrum with hundreds of narrow-wavelength bands: so-called hyperspectral imaging. It is possible to highlight areas containing asbestos minerals by matching the measured and mapped surface spectra with laboratory standard spectra of the pure minerals. In the case of the test area in northern California, where suburban expansion is likely to occur or has done already, the geology is known in some detail and the expensive airborne hyperspectral surveys could be focused. The approach gave results sufficiently accurate for preventive measure to be taken; not only for asbestos-rich bare soils, but also the specific kind of vegetation that ultramafic soils encourage.

There is another, far cheaper means of assessing asbestos risks that is not so accurate, but capable of covering very large areas of poorly known geology, especially in less well-off parts of the world. This uses the satellite remote sensing conducted by the US-Japanese ASTER instrument carried on NASA’s Terra satellite. ASTER data include 5 narrow wavebands that bracket the 2.3-micrometre part of SWIR, so that it is capable of assessing the distribution of ultramafic rock outcrops using software similar to that for hyperspectral data. The USGS/California DoC survey could have tested ASTER data to see how effective it would be if more costly airborne data was unaffordable. Sadly the team didn’t foresee how a local test of concept might benefit a great many areas elsewhere by using an ASTER scene that would cover their entire study area, be free to USGS scientists and cost only US$85 for anyone working in the Third World.

Nuclear waste: planning blight writ large

The artificial radioactive isotopes generated in nuclear fission reactors have half lives that range from days (¹³¹I) to a few million years (¹³⁵Cs). They pose a thorny problem for disposal since the radiation that they emit collectively is likely to reach ‘safe’ levels only after tens to hundreds of thousand years, even if they were diluted by leakage into air or water or onto the land surface. They have to be contained, and that demands storage in rock. More over, underground disposal sites must ensure no leakage for geologically significant periods – a great many rare events, such as magnitude 9 earthquakes, large volcanic upheavals and rapid climate changes all become increasing likely the longer the delay time. Apart from Sweden and Finland, no country that uses nuclear energy has a deep disposal site. The focus has been on the temporary measure of reprocessing, and one major facility, that at Sellafield in the UK, is to close down.

In 1987 the US Congress designated only one potential site for investigation as a place for long term water storage in their vast, geologically diverse country: Yucca Mountain in Nevada. The reasoning was that the area is remote and arid, and not so far away from highly secure military sites, so it could be guarded unobtrusively. After 30 years of investigation, Yucca Mountain has been abandoned, with no equally-well researched fallback site (Ewing, R.C. & von Hippel, F.N. 2009. Nuclear waste management in the United States – starting over. Science, v. 325, 151-152). From a geological standpoint, that is not so surprising as Nevada is seismically active; there has been volcanism in the not-so-distant past, it does have groundwater, and that is present in the volcanic ash proposed for storage. Moreover, the water is oxidising and uranium in spent nuclear fuel easily dissolves under those conditions – storage was to be in titanium casks. Clay saturated in anoxic water is a better bet, while the Scandinavian approach seems safer still: galleries and boreholes in dry crystalline basement rock with canisters packed in clay.

Yucca Mountain has been wrangled over for 3 decades, and one component in its abandonment was a change in the proposed ‘regulatory period’ from 10 thousand to a million years. How compliance might be demonstrated for a period five time longer than our species has existed, and 500 time longer than the length of the Industrial Revolution is something of a problem for bureaucrats, as of course is judging the cost and time for decommissioning obsolescent nuclear plant. If nuclear energy is to play any role in cutting carbon emissions, the volume of nuclear waste is set to rise enormously, but this does not seem to concentrate the regulatory group mind wonderfully.

See also: Wald, M.L. 2009. What now for nuclear waste? Scientific American, v. 301 (August 2009), p. 40-47.

Methane: the dilemma of Lake Kivu

A massive discharge of carbon dioxide from the small but deep Lake Nyos in Cameroon in 1986 killed 1700 local people after a small earthquake and landslide disturbed the bottom water.  The lake is stagnant, and carbon dioxide released by exhalation from deep magma chambers beneath it had dissolved under pressure in in deepest levels. Once disturbed, the gas came out of solution to reduce bottom water density so a large volume rose to blurt out gas and deal silent death in the lake’s immediate surroundings.

Lake Kivu in the western branch of the East African Rift system borders the Democratic Republic of Congo (DRC) and Rwanda. With an area of 2700 km² and a depth of over 400 m it is far larger than Lake Nyos, but similar in having stagnant water below a depth of about 75 m, in which gases are dissolved under pressure. Lake Kivu contains an estimated 256 km³ of carbon dioxide derived from magmas beneath the Rift and 65 km³ of methane that probably arises by anoxic bacterial reduction of the CO₂. Cores into Lake Kivu’s sedimentary floor indicate massive biological die-offs at roughly millennial intervals, which probably result from magmatic destabilisation of the gas-rich lower waters. Experimental vent pipes have been installed in Lake Nyos and nearby Lake Monoun to remove gas from the deep water (see Taming Lake Nyos, Cameroon and Letting Cameroon’s soda-pop lakes go flat in EPN issues for April 2001 and March 2003, respectively), but such a solution for the much larger Lake Kivu would be far less predictable and extremely expensive (Nayar, A. 2009. A lakeful of trouble. Nature, v. 460, p. 321-323). Energy companies based in DRC and Rwanda are now starting to use the ‘soda siphon’ approach that relieved Cameroon’s deadly lakes to capture the methane potential in Lake Kivu. Perhaps that will dampen down the lake’s potential for explosive gas surges, but no one knows if it could instead destabilise its uneasy equilibrium. Furthermore, the deep cool water is nutrient rich and may set off planktonic blooms in Lake Kivu’s surface waters. DRC is notorious for bandit mining and politics and security even more unstable than the lake that it shares with its tiny neighbour Rwanda. Population density on the lake’s shore, always high because of the fisheries and agricultural potential, rose explosively in the aftermath of the Rwandan genocide of 1994.

An option much touted as a means of having our cake (power stations fired by fossil fuels, especially coal) and eating it (escaping runaway global warming while enjoying a high-energy lifestyle) is extracting carbon dioxide from flue gases, or even the atmosphere itself, and safely disposing of it in long-term storage. Carbon capture and storage (CCS) is not a well-tried technology. Yet some authorities claim it is at the least a means of ‘tiding-over’ an economy that depends to such a degree on fossil carbon burning as an energy source that it seems unlikely that alternative, carbon-neutral sources can be deployed in time to stave off increasingly awful and plausible climate and thereby social scenarios. There are others who are convinced that CCS is merely an excuse to continue with ‘business as usual’, and therefore fraught with dangers. Whichever, there are elements of CCS that do concern geoscientists, such as where should it be stored and in what form. Leaving aside some of the geological issues of storage, such as depleted natural petroleum fields or deep aquifers, what happens to CO₂ at depth? There are five possibilities: it remains as a gas; under high pressure it may take on liquid form (CO₂ can exist only as gas or ‘dry ice’ at atmospheric pressure); it reacts with the rock itself to form some kind of carbonate; under moderate pressure and low temperature it may combine with water to form a gas-hydrate ‘ice’, as does methane; or it may dissolve in water under high pressure.

The ideal form for long-term storage would be in the form of solid carbonate, but that demands bicarbonate ions combining with calcium, magnesium or perhaps sodium ions. One possibility is through dissolution in highly saline groundwater. The chemical reactions are not complex, but depend on the solubility of carbonates being exceeded because of massive increases in bicarbonate concentrations. However, experiments have had little success. Another means of solid storage is by the combination of atmospheric CO₂ with calcium hydroxide to form calcium carbonate, which is what happens when lime plaster slowly ‘cures’. The downside is that the only means of making Ca(OH)₂ is by kilning limestone: no free lunch there. To cut a long story short, a view is emerging that CO₂ pumped, in whatever form, into wet rock will end up dissolving in groundwater, to form vast quantities of ‘sparkling’ water, or ‘soda pop’ (Gilfillan, S.M.V. and 10 others 2009. Solubility trapping in formation water as dominant CO₂ sink in natural gas fields. Nature, v. 458, p. 614-618). The British, Canadian, US and Chinese team investigated nine natural gas fields in which CO₂ is present as well as petroleum gas, using noble gases and carbon isotopes as tracers of the chemical fate of the natural CO₂ as the reservoir rocks filled with oil and natural gas during maturation. They discovered that the bulk of CO₂ ended up dissolving to form a weakly acidic water under pressure. This is a recipe for filling huge analogies of soda siphons. They did discover that some CO₂ ended up as solid carbonate, but no more than 15%. As those who add Perrier or Volvic to their Scotch should know, carbonated springs are not unknown. Consequently, CCS that uses confined aquifers poses the danger of eventual leakage, whether CO₂ is stored as gas, liquid or in solution. Petroleum geologists often claim that no trap is leak proof, and extensive areas of gas leakage are known over most oil fields; they are an important sign for explorationists, if they can be detected. The other issue is that fans of CCS set much store in re-use of depleted commercial oil and gas fields for sequestration. Such fields have already been depressurised, and nobody knows whether or not they were leaky to gas and water.

See also: Aeschbach-Hertig, W. 2009. Clean coal and sparkling water. Nature, v. 458, p. 583-4.

Shortly before the start of the Younger Dryas cold period, around 12.9 ka, the Palaeoindian Clovis culture of North America seems to have come to an abrupt halt. The North American mammoths on which the Clovis people preyed also disappear from the fossil record. Some folk reckon that early immigrants from NE Asia devoured the last of the mammoths, as they ate their way through two continents en route to Tierra del Fuego. Equally imaginative scientists have been suggesting since 2007 that an extraterrestrial cataclysm was responsible for climate change and the demise of both mammoths and the Clovis people (see Whizz-bang view of Younger Dryas and Impact cause for Younger Dryas draws flak in EPN July 2007 and May 2008). Evidence found just beneath a sediment layer that marks the outset of the Younger Dryas included: excess iridium; tiny spherules; fullerenes containing extraterrestrial helium; nanodiamonds and evidence for huge wildfires. Neither crater nor shocked mineral grains have been found, and the proponents of this controversial idea have opted for a cometary airburst as culprit – an impact would have produced shocked debris. The authors have had a ‘bad press’, but remain undeterred and have published photomicrographs of diamonds in minute spherules made of amorphous carbon (Kennett, D.J. and 8 others 2009. Nanodiamonds in the Younger Dryas boundary sediment layer. Science, v. 323, p. 94). There is a problem or two with the hypothesis: mammoths, albeit little ones, lived on Wrangel Island in the Arctic Ocean until 1650 BC; had some kind of cosmic encounter in North America set global cooling in motion at 12.9 ka, then the best place to look for evidence would be in the Greenland ice cores, in which diamonds have yet to be found. No-one doubts that diamonds do occur in the sediments formed just before the Younger Dryas, but experts don’t accept them as irrefutable evidence for impacts (Kerr, R.A. 2009. Did the mammoth slayer leave a diamond calling card? Science, v. 323, p. 26). But the plot thickens. A Belgian and German team has discovered that forest topsoils, grasslands and swamps, no more than a few thousand years old, from 70 sites across Europe also contain nanodiamonds. Although one member of that team reportedly has no idea where they came from, a website (http://www.chiemgau-impact.com/) hints that a very young (2500 years) impact site in Bavaria may be the source. While the end-Clovis diamonds may not have triggered global cooling and killed off mammoths, they could well set off a research line aimed at documenting hazardous extraterrestrial events of the recent past and puzzling occurrences in the archaeological record.

See also: Herd, C.D.K et al. 2009. Anatomy of a young impact event in central Alberta, Canada: Prospects for the missing Holocene impact record. Geology, v. 36, p. 955-958.

Chinese dam implicated in the 2008 Sichuan great earthquake

Four years after the completion of the Koyna Dam in India’s Maharashtra State in 1963, the surrounding area experienced a magnitude 6.5 earthquake. Because the region is free of active tectonics, the earthquake was a surprise. The possibility that it could be linked to filling of the reservoir behind the Koyna Dam became a proven fact when the region subsequently became plagued by minor seismicity. In the immediate aftermath of the magnitude 7.9 Wenchuan earthquake in Sichuan, China on 12 May 2008, which killed 80 thousand people, there were alarms about the possible failure of weakened dams and lakes blocked by landslides in the Longmen Shan mountains. But now suspicion has fallen on the earthquake having been caused by the load that filling a new reservoir created only 5 km from the epicentre and 500 m from the fault that failed during the disaster (Kerr, R.A. & Stone, R. 2009. A human trigger for the great quake of Sichuan? Science, v. 323, p. 322). Calculations of the stress from this loading suggest that it was 25 times that of the tectonic stresses in the region.

Since the awful discovery in the 1980s that millions of people in the delta plains of the northern Indian subcontinent were at risk of chronic arsenic poisoning if they drank water drawn from wells in alluvium, that hazard has been found to exist in other alluvial areas close to sea level. The arsenic is of natural origin and is released when iron hydroxide, the most common sediment colorant and powerful medium for adsorption of many elements including arsenic, breaks down. Iron hydroxide is destabilised in strongly reducing environments, when its component Fe³⁺ gains an electron to become soluble Fe²⁺. The most common source of reducing conditions is vegetation buried in alluvial sediments. In Bangladesh and West Bengal, India, the problem is peat layers buried by rapid sedimentation since about 7 thousand years ago that filled channels cut by rivers when sea level was much lower during the ast glacial maximum. The risky areas in the Mekong Delta are more complex (Papacostas, N.C. et al. 2008. Geomorphic controls on groundwater arsenic distribution in the Mekong River Delta, Cambodia. Geology, v. 36, p. 891-894). Areas at risk are strongly focused by recent landforms associated with channel migration, rather than extending across entire flood plains as in Bangladesh. Features such as meander scrolls, point bars and islands that have grown to be incorporated in older floodplains show the highest arsenic concentration in groundwater. These accumulate organic debris in large amounts, whose decay releases arsenic from iron hydroxide veneers on sand grains. Older features of the same kinds show less arsenic contamination in their groundwater, suggesting that eventually either the reductants become exhausted or available arsenic is flushed out. So, careful mapping and dating of fluviatile geomorphology may be a means of screening for arsenic risk in the Mekong and other low-lying delta plains.

Since the Indian Ocean disaster of 26 December 2004, coastal areas world wide are increasingly examined for signs of past tsunamis. Much the most common focus is on large boulders on low-relief shorelines never subject to glaciation. On the Bahamas large blocks of coral scattered above sea level suggest past tsunamis perhaps caused by collapse of volcanoes on Atlantic islands such as the Canaries or Azores. Yet, ordinary storm waves, if focused by coastal inlets can literally blast large boulders from well-jointed outcrops and carry them hundreds of metres inland. So peculiar boulders on a coast do not necessarily show that a tsunami once struck, although many around the shores of eastern Britain may well have been dislodged by tsunami triggered by a submarine landslide off western Norway about 7 thousand years ago. In an attempt to get more reliable signs of past tsunamis, the devastated coasts of northern Sumatra and western Thailand have been searched for tangible signs of the 2004 event (Monecke, K. et al. 2008. A 1,000-year sediment record of tsunami recurrence in northern Sumatra. Nature, v. 455, p. 1232-1234. Jankaew, K. et al. 2008. Medieval forewarning of the 2004 Indian Ocean tsunami in Thailand. Nature, v. 455, p. 1228-1231).

Both teams homed in on boggy depressions or swales between fossil beach ridges on broad low-lying shores. There, debris carried by the huge 2004 waves could be trapped and then preserved by regrowth of vegetation. The generally low energy in the swales is also likely to prevent erosion, so that deep superficial sediment can build up that may preserve signs of past tsunamis. This focus paid dividends, in the form of coarse sand just beneath a regrown vegetation mat, with distinctive signs that the sand had been deposited by transport from the seaward side of swales. Coring and trenching then unearthed deeper, older sands with exactly the same structure. The surprise was the antiquity of the tsunami sands: layers carbon-dated around 1300-1400, 780-990 AD and 250 BC. Clearly, more extensive surveys of this kind are necessary wherever coastal conditions permit good preservation. That would give an idea of the periodicity of earthquakes and landslips energetic enough to produce coastal catastrophes around major ocean basins. Yet there is a danger: if, as suggested by the Thai and Indonesia data, several centuries have lapsed between such dreadful events, it presents an excuse not to install costly monitoring devices or permanently shift coastal townships to foretell or prevent future disasters.

See also: Bondevik, S. 2008. The sands of tsunami time. Nature, v. 455, p. 1183-1184.

Chinese PM is a geo

Like me, many EPN readers may have admired the swift, effective and open response of the government of the People’s Republic of China to the Szechuan earthquake disaster in May 2008. They may also be surprised to learn that Wen Jiabao, the prime minister of the PRC, is geologist who worked for 14 years with a provincial geological survey. To read an abbreviated transcript of a dialogue between the editor of Science and Wen Jiabao was refreshing, and quite probably unique in a world where most senior politicians are, to say the least, not science-savvy (Alberts, B. & Jiabao, W. 2008. China’s scientist premier Q&A. Science, v. 322, p. 362-364; full transcript at www.sciencemag.org/cgi/content/full/322/5900/362/DC1).

Millions of people in Bangladesh and West Bengal have unwittingly drunk groundwater that is contaminated with arsenic as a result of natural processes for up to 20 years. They are potential victims of the greatest mass poisoning in human history. Dreadful as the possible fate awaiting them might be – they may develop various cancers – discovery and ten years of research into their problems has alerted geoscientists to the hazard of environments like those in which they live. That arsenic poses great dangers is common knowledge, but until unmistakable signs of arsenic poisoning appeared there (black wart- and mole-like skin lesions), the hazard was thought to be restricted to former mining areas where oxidation of iron sulfides released the traces of arsenic locked within those minerals. From studies in West Bengal and Bangladesh has emerged a cause that was completely unexpected: it involves one of the commonest minerals at the Earth’s surface, goethite or FeOOH. This yellow-brown colorant of many sediments has the remarkable property of being able to adsorb or ‘mop-up’ a large range of elements dissolved in water with which it comes into contact. Among these is arsenic. In the oxidising conditions that sponsor the formation of goethite as a coating on sedimentary grains the mineral actually prevents a great deal of natural, geochemical pollution. Yet, exposed to reducing conditions, commonly developed when buried organic material begins to rot, goethite may dissolve and release its potentially toxic load into groundwater. This is precisely the source of arsenic at levels more than 100 times the safe level in some wells on the Ganges-Brahmaputra plains. The story does not stop there, however.

When sea level stood about 130 m lower than now, at the last glacial maximum, rivers rising in the Himalaya cut deep valleys in the coastal areas. As sea-levels rose these rapidly filled with new sediments, most of which were stained with goethite. But they were interbedded with thick organic-rich peats that formed during periods of slow sea-level rise. It is the peats and more finely dispersed vegetable matter that caused the reduction and solution of goethite, and thus the arsenic that it carried. Especially high arsenic levels develop in sediments derived from specific areas in the Himalaya. So a suite of conditions conducive to arsenic hazard have emerged from unravelling the tragedy of the northern plains of the Indian subcontinent. It is possible to use that suite as a means of predicting other risky areas, one of the first to be revealed being in the Red River delta of northern Vietnam: the population of Hanoi is at risk from well water drawn from the Red River sands and gravels. Systematic computer screening of known geology, topography and soil conditions in Southeast Asia is beginning to throw up other problematic areas (Winkel, L. et al. 2008. Predicting groundwater arsenic contamination in Southeast Asia from surface parameters. Nature Geoscience, v. 1, p. 536-542) where concentrations of arsenic in drinking water are highly likely to exceed the maximum recommended level of 10 μg l^-1 (parts per billion). The pilot study highlights the known areas, but also the deltas of Mekong River in Cambodia and southern Vietnam, the Irrawaddy in Burma (Myanmar) and the Chao Phraya basin of Thailand. Hopefully, geochemical testing will reveal in details which wells are at risk and which are not, in these three regions: it would be easy to reject perfectly safe groundwater that often occurs close to contaminated areas, as found in Bangladesh, without careful testing. The implicated mineral, goethite, is itself a cheap and abundant means of remediation if contaminated water is passed through goethite-rich filters. But the large areas at risk in SE Asia, together with others discovered by epidemiologists in northwestern India, the Indus plains of Pakistan and in Mongolia, create a chilling scenario for many other populous, sediment-rich areas elsewhere. Winkel et al’s approach surely needs to be refined and applied globally.

See also: Polizzotto, M.L. et al. 2008. Near-surface wetland sediments as a source of arsenic release to ground water in Asia. Nature, v. 454, p. 505-508. Harvey, C.F 2008. Poisoned waters traced to source. Nature, v. 454, p. 415-416.

Cause of Javan mud volcano

Since May 2006 the largely urban Sidoarjo area of eastern Java has been plagued by continuous eruption of hot mud and steam from a vent that suddenly appeared. Around 7 km² have been buried by up to 20 m of noxious mud, giving a total emission of about 0.05 km³ at a rate of 100 thousand m³ per day. Although nobody has been killed, the mud volcano is an economic and social disaster, 30 thousand people having been displaced. The area is one of active petroleum exploration, and locals blame a blow out from a nearby gas exploration well, though scientists and the exploration company point to the eruption having begun a couple of days after a magnitude 6.3 earthquake in the area around the capital Yogyakarta, 250 km away. If the latter, economic losses may be difficult to recover from insurers; if the former, there will be a rare old furore. So, a thorough evaluation of what the cause may have been is welcome (Tingay, M. et al. 2008. Triggering of the Lusi mud volcano: Earthquake versus drilling initiation. Geology, v. 36, p. 639-642). Being a mix of Australian, German and British geologists, the authors have no axe to grind. They consider that seismic influence was highly unlikely, in this case, although many mud volcanoes have formed close to earthquake epicentres in other areas. On the other hand, the well that was being drilled at the time suffered a loss of drilling mud shortly before the volcano began to erupt, suggesting escape to fractures at depth around the well. Moreover, the hole was not cased at depth. The most likely trigger was creating a passageway up the well for high-pressure fluids to escape from the 3 km deep target limestone sequence into shallower unconsolidated clays. They were liquefied and escaped as a lateral blow out

Beneath the Dragon’s Gate (Longmenshan) Mountains of Sichuan Province, China an apparently ‘stuck’ segment of a major fault complex failed on 12 May 2008 (Stone, R. 2008. An unpredictably violent fault. Science, v. 320, p. 1578-1580). Unprecedented access to the world’s media resulted in our exposure to the full horror of the results of major seismic events in mountainous terrain and on habitations, especially schools, whose building standards were unable to withstand ground shaking. &0 thousand souls died, thousands more are still unaccounted for and more than 1.5 million people have become refugees in a country that is rapidly emerging from Third World status. Now that aftershocks have subsided massive threats remain from the many landslide-blocked rivers and fractured dams. Yet we also witnessed enormous mobilisation of the People’s Army within hours of the earthquake and truly heroic attempts to rescue as many trapped people as possible. Without that swift response the casualties would undoubtedly have been worse.

China boasts one of the most sophisticated seismic warning systems outside of California and Japan, deploying robotic seismometers and GPS recorders in the most risky regions, and with a 10-thousand strong Earthquake Administration. Sadly, Chinese seismologists regarded the faults shown to be accumulating displacement most quickly as those most likely to fail. It is generally ‘stuck’ segments that fail catastrophically. China has a long-respected reputation for gathering data generally regarded as ‘non-scientific’, such as well water levels, and animal behaviour, that might give empirical clues to impending earthquakes. The Tangshan earthquake of 28 July 1976, which killed a quarter of a million people 160 km from Beijing, was preceded by reports of shifts in the water table, odd ‘earthlights’ and unusual animal behaviour. Paying serious attention to reports by ordinary people of such oddities is reported to have avoided untold numbers of deaths in the period since Tangshan, but not in the case of Sichuan. Strangely, a Taiwanese weather satellite detected decreased electrical activity in the ionosphere above Sichuan hours before the recent earthquake (see Clouds and large earthquakes in May 2008 issue of EPN). Geophysicists have noted increased emissions of radon in the period immediately preceding some major earthquakes which might conceivably have an effect on the ionosphere. Whatever, prediction of catastrophic earthquakes has had very few successes in terms of lives saved, and the signal lesson from Sichuan, as from that which destroyed the Japanese city of Kobe in 1995, is that building standards in zones of active faulting must take account of the risk of ground movement.

See also: Stone, R. 2008. Landslide, flooding pose threats as experts survey quake’s impact. Science, v. 320, p. 996-997.

Extraterrestrial impactors

July 2008

June 30, 2008 was the centenary of the mysterious Tunguska event that devastated more than 2000 km2 of forest 1000 km north of Lake Baikal in Siberia at 7 am a hundred years before. Much of the mystery stems from there being no sign of a crater and therefore of the process involved. Speculation about the cause of a massive explosion between 5-10 km above the surface still goes on (Steel, D, 2008. Tunguska at 100. Nature, v. 453, p. 1157-1159). Ideas have ranged over a gamut of high-energy physical processes involved in the explosion: a deuterium-rich, fluffy comet that was ignited as a thermonuclear explosion by hypersonic atmospheric entry; a lump of antimatter; a miniature black hole; explosive release and ignition of natural gas; a ‘Verneshot’, and even an alien space craft involved in an accident. The chances are that the explosion was more mundane, and akin to what occurs inside a diesel engine. Compressive heating of the air in front of a small asteroid or comet travelling at more than 15 km s-1 would generate temperatures around 50 thousand degrees. Flash vaporisation of a small comet or asteroid would add to a massive shock wave at the epicentre, rather than by an intact projectile. It is thought that many small craters, such as Meteor Crater in Arizona, result from impacts by strong metallic asteroids, whereas stony ones or comets easily disintegrate. Whatever, research still goes on at the site, now completely reforested.

The centenary spurred Nature to devote pages 1157-1175 in its 26 June 2008 issue to impact-induced features from Earth and other planets, together with three Letters and two reviews. Topics covered include the search for near-Earth objects and the Spaceguard survey, which is beginning to suggest that humanity can concentrate on global warming for the next century or so, and truly monster impact structures from the Moon and Mars, including evidence for one that may have ‘scalped’ northern Mars. In one of the reviews it is said that a sci-fi novel (Niven, R. & Pournelle, J. 1977. Lucifer’s Hammer. Harper Collins) inspired the Alvarez father-and-son team that first postulated an impact origin for the K-T mass extinction event. The second review is of a highly realistic sculptural depiction of a pope (John Paul II) knocked over by a meteorite: perhaps planetary science’s first involvement, literally, in what some might consider lèse majesté. So, in many ways, quite an event…

See also: Cohen, D. 2008. The day the sky exploded. New Scientist, v. 198, 28 June 2008 issue, p. 38-41.

The press announced in April that the USGS and other western US geoscience institutes had issues the first ever comprehensive earthquake forecast for California (see http://www.scec.org/ucerf/) , but it was cautiously phrased in terms of probabilities of destructive magnitudes (>6.7) over the next 30 years. That might be fine and dandy for administrators and civil engineers, but not so good for anyone who becomes a victim at the precise time this or that Californian fault ‘goes off’. People world-wide have rarely chosen where to live based on knowledge of geological risks; indeed most threatened communities have little choice, for many reasons. What would be useful is being warned that a devastating earthquake is definitely due where one lives, and it will happen sometime in the next few days or weeks. Even an hour’s warning will save many lives. But no geological survey will commit itself to that kind of pronouncement, except perhaps some of the many surveys in China. The fact that all kinds of phenomena, such as nervousness among animals, rising water levels in wells and so-on have been shown to occur shortly before many big earthquakes has prompted a kind of ‘barefoot’ monitoring that is officially co-ordinated in some parts of China. It is said that lives have been saved on a number or recent occasions.

It is easy for western scientists to make the analogy with homeopathy, and pooh-pooh such methodology. Also, there has been a succession of observations from space that could prove useful, such as ‘earth lights’ and magnetic-field fluctuations that accompany some seismic events (see Remote signs of earthquakes in EPN August 2003, Early warning of earthquakes in EPN December 2005). The latest odd, but conceivably useful connection is an association of unusual cloud formations with earthquakes in Iran (Guo, G. & Wang, B. 2008. Cloud anomaly before Iran earthquake. International Journal of Remote Sensing, v. 29, p. 1921-1928). The authors, from Nanyang Normal University in China, scrutinised free, hourly images from the geostationary Meteosat-5 satellite covering the whole of Iran, where seismicity is concentrated on a single large zone of deformation that trends NW-SE through the Zagros mountains. On several dates they found cloud formations parallel to the fault zone. Between 60 to 70 days later large eathquakes took place along the fault, including the highly destructive Bam earthquake of 26 December 2003. Indeed, a noticeable thermal anomaly in clouds directly above Bam occurred 5 days before the disaster.

How often do tsunamis occur?

Fortunately, truly destructive tsunamis on the scale of that of 26 December 2004 are rare events. So much so that nobody has a clear idea of their average frequency at different exposed shorelines; a vital statistic for risk analysis. Tsunamis produce high energy marine deposits, but unless they are preserved in accessible locations their incidence would be difficult to estimate, and they may be confused with tempestites generated by hurricanes. One characteristic of tsunamis is that they are waves that affect the entire ocean volume, unlike wind waves whose effects are restricted to a few tens to hundred of metres, which can create unique features. Canadian, US and Omani sedimentologists have examined a sediment deposited in Oman by a recorded tsunami generated by a large earthquake off Pakistan in 1945 and have discovered one such signature (Donato, S.V et al 2008. Identifying tsunami .deposits using bivalve shell taphonomy.  Geology, v. 36, p. 199-202). The deposit, a coquina rich in bivalve shells, contains an unusually high proportion of still-articulated shells, suggesting that living animals were ripped from the seabed and then flung into a lagoon. Along with oddities in fragmentation of other shells and the sheer size and extent of the coquina, this feature seems to be characteristic of tsunamites. Features in the Oman example closely match those in another on the eastern shore of the Mediterranean Sea in Israel.

In the centenary year of the 1906 San Francisco earthquake a lot of attention has been paid to the northern part of the infamous San Andreas Fault. That avoids the fact the its southerly extension to the south-east of Los Angeles has not ruptured in a devastating way for at least 250 years. Faults break after protracted build-up of elastic strain. Such strains are detectable using data from spaceborne radar systems. These have been available since 1992 from the European Space Agency ERS-1 and ERS-2 satellites. A sequence of data sets provides information about the annual rate of deformation (Fialko Y. 2006. Interseismic strain accumulation and the earthquake potential on the southern San Andreas fault system. Nature, v. 441, p. 968-971). Fialko shows that the parallel San Andreas and San Jacinto faults near the Salton Sea are building up strain at about 3 cm per year, so that about 7 to 10 metres will have accumulated since the last major earthquake in that part of the system. This exceeds the largest known seismic movement on the system, thereby suggesting that Los Angeles is likely to experience a ‘big one’ shortly.

Supervolcanoes

Outside of a major meteorite impact, the greatest danger posed by geological processes is a monster volcanic eruption. As well as the close-by effects of massive debris avalanches and ash falls, explosive eruptions blast sulphur gases into the stratosphere where they reside for a long time as sulphuric acid aerosols. Clouds of these tiny particles reflect a proportion of solar radiation back into space and so cause global cooling. The eruptions of Pinatubo and Krakatau in recent historic times did just that, as have several others with more devastating global effects such as famine. Yet these are tiny compared with eruptions known from the recent geological past that are marked by ash deposits over vast areas. About 71 ka ago, Toba in Indonesia  blasted out a 30 by 100 km caldera and its ash extends across much of south Asia and surrounding ocean floors. Genetic evidence from human Y-chromosomes suggests a massive decline in human numbers at the time, to create an evolutionary bottleneck. This near-extinction may have been connected in some way to eruption of the Toba supervolcano. Such events are a more likely risk than impacts, and a recent review of research into them highlights those that are well-known (Bindeman, I.N. 2006. The secrets of supervolcanoes. Scientific American, v. 294 (June 2006 issue), p. 26-33). The western USA has two potential threats: calderas in Yellowstone National Park and Long Valley. Between 760 and 640 ka both exploded to blanket the whole southern USA and northern Mexico with around 1000 cubic kilometres of ash. Bindeman’s own research sheds light on the details of magma evolution during such eruptions using isotopic signals in zircons contained within ash deposits..