Long-term prediction of volcanic activity

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 20th 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 22nd 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.


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