The crust beneath the British Isles is made up of several once widely separated terranes, parts of Laurentia, an arc segment called Avalonia that split from Gondwana around 500 Ma ago, and a similar terrane (Armorica) that followed Avalonia across the Iapetus Ocean to accrete to Laurentia at the end of the Palaeozoic Era. Because of its maritime position, modern Britain is cloaked in vegetation so that rock occurrences are few and far between by comparison with less humid areas. Conditions for geological investigations are made yet worse by a mantle of glacial sediments plastered on top of bedrock. So, although having been studied for longer than almost every other piece of continental crust, the evolution of that beneath the British Isles is a subject of continual controversy and surprises. Sitting at the interface between the Laurentian and Avalonian terranes, roughly where the Iapetus suture is thought to have consumed at least half of the eponymous ocean, sit the Lower Palaeozoic rocks of the Southern Uplands of Scotland. They are widely thought to have formed as an accretionary prism on the edge of the plate underidden by subducted Iapetus oceanic lithosphere until Avalonia collided with the north-British terranes at the close of the Silurian. Some of the Ordovician sediments in the pile contain clasts of volcanic rocks, which were long thought to be contemporary and giving evidence of the expected arc volcanism behind the prism. However, they turn out to be much older, now that zircons from the sediments have been dated using high-preciiision methods (Phiilips, E.R. and 7 others 2003. Detrital Avalonian zircons in the Laurentian Southern Uplands terrane, Scotland. Geology, v. 31, p. 625-628). The zircons yielded Neoproterozoic ages (557 to 613 Ma), with evidence that some had been assimilated from older crust (1043 Ma) during volcanism. Taken at face value, the Neoproterozoic ages are similar to those of volcanic rocks in England and Wales, which formed off Gondwana in an arc setting, when the terranes were widely separated. The problem is one of getting the material across the subduction zone that separates the accreted terranes, but that is the issue proposed by the authors (all from the Natural Environment Research Council. However, such a conclusion might stem from the authors’ narrow context; that of British geology. Immediately to the north of the Southern Uplands terrane is another, poorly exposed crustal block that underlies the Scottish Midland Valley. It was directly involved in the Ordovician Grampian orogeny that formed the highly deformed Precambrian rocks of the Scottish Highlands. With a narrow view, that terrane is also a mystery, yet it has a counterpart in the Taconia terrane that is familiar to North American geologists, which was involved in orogenic events contemporary with the Grampian orogeny in Scotland. Taconia has late Neoproterozoic to Ordovician arc volcanics.
Titanic solution to unpalatable water
Currently around a billion people are at severe risk from drinking contaminated water, and whenever there is a major human crisis refugees are placed in the same plight. The main solution would seem to be drilling wells that tap groundwater that aerobic bacterial action cleanses of most pathogens. That is essentially true, but some groundwater is rejected even by people suffering the most extreme privations. It has the appearance of water from the radiator of an aged lorry, because it contains abundant dissolved iron that immediately precipitates as red-orange slime when exposed to the air, tainting food and staining clothes. A solution may arise from studies as far from drought-stricken areas as one could possibly get; concerning the way in which deep-sea wrecks decay away. The discovery of the wreck of the Titanic in 1985 and recovery of parts of it later by marine historian Robert Ballard, revealed that its ironworks were being consumed by bacteria that created stalactite-like masses of iron oxides, known as “rusticles”. Detailed microbiological studies found a highly complex harmony of different bacteria that created and inhabited the rusticles. Effectively, they were eating the mighty ship at a rate of about a tonne every ten days by exploiting the energy released by oxidation of iron. It may prove possible to harness the habits of these iron-loving bacteria to remove iron from groundwater and make it palatable
Source: Fry, C. 2003. Iron rations. New Scientist, 26 July 2003, p. 36-37.
Share this:
Glaciers of Mars
The world has been agog these last few years as evidence has mounted to suggest that Mars still has abundant water buried beneath its dusty surface, in the form of permafrost. Early in its history there are many signs of vast floods that carved huge meandering canyons and may have filled basins with moderately long-lived seas. Yet Mars has probably always been pretty cold, as it is now, and the most likely form that surface water would have taken is in glaciers; that is, if there was ever sufficient atmospheric water to precipitate snow. As on Earth, the likeliest places to look are in mountainous regions, and Mars is not lacking in very high places. By far the largest, and indeed they are the highest mountains in the Solar System, are the shield volcanoes of the Tharsis Rise, topping out around 18 km above the Martian version of the geoid. The volcanoes have gnarled surfaces, which until recently have been regarded by most as the result of volcano-related processes. Imaging of the Martian surface has stepped up several notches in resolution in recent years, and details of the small-scale features of the volcanoes are very clear. Above all else, they resemble aspects of the nearest analogue to Martian conditions on Earth – the Dry Valleys of Antarctica. Although the Dry Valleys are now largely free of ice sheets, they show many features of former glaciation, perhaps extending back 30 Ma to the Oligocene. Their frigidity has ensured that any glaciers there were frozen to the surface, rather than having zones of incipient melting at their bases. Such cold-based glaciers move sluggishly, and produce peculiar features. Among these are moraines produced by sublimation rather than melting of the ice – they evidence no reworking by melt water – and rock glaciers that are also products of sublimation and sometimes rest on relics of former glaciers. Probable examples of both occur on the flanks of the Tharsis volcanoes, together with weird track-like assemblies of concentric ridges, that are likely to have formed on the flanks of ablating glaciers as they reached a standstill and then retreated. (Head, J.W. & Marchant, D.R. 2003. Cold-based mountain glaciers on Mars: Western Arsia Mons. Geology, v. 31, p. 641-644). Interestingly, the relationship of the glacial features to impact craters suggests that glaciation took place during the period since about 1.8 billion years ago (the Amazonian phase of Mars’ history) when bombardment had slackened to almost terrestrial rates and liquid water was unable to form on the red planet. Of course, glaciers do not have to be made of water ice, and there is still a possibility that at such immense altitudes any glaciers might have been made of solid carbon dioxide. Head and Marchant speculate that some of the features might still sit upon relics of the glaciers. It could be a bit of a disappointment if future explorers of Mars landed there expecting a water supply.
Share this:
Smithsonian Dynamic Earth site
The Smithsonian Institute’s National Museum of Natural History has a new and evolving Earth science website at www.mnh.si.edu/earth (Flash 6 and printable versions). Currently only the Rocks and Mining section is up and running, but it is instructive at the introductory level. To come are sections on gemstones, plate tectonics and the Solar System. There are also downloads and a geogallery. It is somewhat slow in Flash using a normal dial-up connection., but the printable version has no images. With a fast connection, this is likely to become a favourite for elementary visualisation of Earth processes.
Share this:
Cosmogenic nuclides and tropical erosion
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).
Share this:
Iron isotopes and ocean evolution
The main driver for biological activity in the oceans far from land is the availability of iron, and this helps control the burial of organic carbon and hence aspects of global climate. At low Fe concentrations, as they have been since the oxygenation of the surface environment from 2 billion years ago, iron is cycled in the marine environment in a matter of a few hundred years. So, ocean water responds very quickly, in geological terms, to changes in the source of any dissolved iron. There are two main sources, discharge of hydrothermal fluids from the oceanic lithosphere and delivery of river water and dust derived from the continents. Of the last, riverine sources probably end up in near-shore sediments and only dust contributes significantly to deep ocean water. The slowly growing nodules and crusts, composed mainly of iron and manganese compounds, on the ocean floor can chart variations in the relative proportions of these sources, because their growth produces zonation. Measurements of d56Fe in various materials show that the two sources are different in isotopic composition (Beard, B.L. et al. 2003. Iron isotope constrains on Fe cycling and mass balance in oxygenated Earth oceans. Geology, v. 31, p. 629-632). While continent derived materials exude iron that is essentially the same as that in terrestrial volcanic rocks (d56Fe ~0.0‰), ocean-floor hydrothermal activity is significantly depleted in 56Fe (‰56Fe ~ -0.38‰). From 6 Ma to 1.7 Ma iron-manganese crusts record iron with a dominant hydrothermal origin, but during the glaciation-dominated period since 1.7 Ma the contribution of continent-derived dusts becomes overwhelming – cooling forces drying on a global scale. Because hydrothermal contributions probably stay much the same over very long periods, because of the sluggishness of plate tectonics, iron isotopes in deep marine sediments, such as Fe-Mn crusts, may be important tracers for glacial events in the distant past, such as the glaciations during the Neoproterozoic and Palaeozoic. Interestingly, the largest iron-rich deposits on the planet, the BIFs that peaked during Archaean and Palaeoproterozoic times, record far larger excursions in iron isotopes than any other. The very low d56Fe values of some BIFs (down to – 2.4‰) probably signify the dominance of sea-floor sources, although a non-oxidising atmosphere would have mobilised dissolved iron from the continents too, which explains the range in BIFs up to +1.0‰.
Share this:
Imaging radar and WMD
A short article in New Scientist (Morris, H. 2003. Satellites hunt for buried treasure. New Scientist 12 July 2003, p. 12-13) reminded me of the puzzling failure of British and US forces in Iraq to discover any buried caches of weapons of mass destruction, either before the invasion of Iraq or in the aftermath of Saddam Hussein’s disappearance. Researchers at the Ben Gurion University of the Negev in Israel have tested the ground-penetrating capabilities of imaging radar that uses microwave pulses with various wavelengths. One of the principles of radar remote sensing is that microwaves can penetrate beneath the Earth’s surface, provided the materials contain little liquid water. The longer the wavelength the greater the depth from which information can be sensed. Ground-penetrating radar is a common tool in archaeological investigations and in glaciology (ice is “dry”), but is usually deployed along ground traverses. The Israeli experiments, which duplicated work done by remote sensing researchers at NASA’s Jet Propulsion Laboratory, used airborne imaging radar to detect buried metal target, which are highly reflective to microwaves. They used microwaves with moderately long wavelength, and showed that objects half a metre deep were easily detected.
Radar with a wavelength of around 70 cm is called P-band radar, and has the greatest potential for sub-surface mapping, with penetration up to 9 metres. In 1987, NASA’s Jet Propulsion Laboratory first flew an airborne radar imaging system (AIRSAR) that uses P-band, partly to exploit its ability to “see through” dense vegetation but also to produce ground-penetrating images in dry regions. AIRSAR has the potential to produce images with a resolution of 3.3 metres, and data produced by it have been available freely to civilian users. It would be no surprise, therefore, if there were imaging radar systems with P-band radar being used for intelligence gathering. The US National Imagery and Mapping Agency (NIMA), in conjunction with JPL and EarthData International, Inc., developed in 2000 the 2-metre resolution Geographic Synthetic Aperture Radar (GeoSAR) mapping system, that also includes a P-band imager. GeoSAR is funded by the US Defense Advanced Research Projects Agency (DARPA). NIMA, formerly the US Defense Mapping Agency, is a Department of Defense Combat Support and National Intelligence Community agency that provides imagery, image intelligence and geospatial information in support of US national security objectives. The French and Italian space agencies are also discussing the development such systems, perhaps to be deployed from orbit by the European Space Agency.
It was NASA/JPL’s Shuttle Imaging Radar missions in the 1980s and 90s that revealed dramatic evidence for former tributaries of the Nile River System that are buried beneath the sands of the arid eastern Sahara desert in Egypt and Libya. Although not so dry, the Tigris-Euphrates plain is a desert, and it would be very surprising if P-band radar imaging has not been used in the search for buried WMD. Since radar energy is barely affected by the atmosphere, and the microwaves used in radar imaging are effectively highly focussed laser beams, systems carried on satellites have the same spatial resolution as those carried on aircraft. Had a P-wave system been deployed on a military surveillance aircraft or satellite, then sizeable buried caches would have been difficult to miss. Even if the ground was damp, one of radar’s other features is that it responds to variations in the texture of the ground surface. Reworked soil over excavations would be easily spotted by any radar imaging system, either orbiting or on an aircraft. So it was somewhat odd when the US Secretary of State, Colin Powell, did not use any imaging radar evidence in his submission to the UN Security Council on 5 February 2003.
Share this:
Impact database
The University of New Brunswick, Canada, maintains an illustrated archive of information on terrestrial impact craters. It lists 169, with exact co-ordinates for each and much other information besides. Many have satellite, aerial and/or ground images, plus full lists of references for each. The URL is http://www.unb.ca/passc/ImpactDatabase/
Share this:
Rodinia muddles
In the early 1990s, Ian Dalziel, Eldridge Moores and Paul Hoffman speculated on the former existence of a supercontinent comparable with Pangaea, between about 1100 and 750 Ma. The name Rodinia, from the Russian for Motherland, seemed appropriate. They based sketchy reconstructions on the way in which orogens formed almost globally between 1300 and 1000 Ma could be fitted together by shuffling older crustal fragments, along with evidence from sediments in North America, and Antarctica that the supercontinent began to disassemble around 800 Ma. A great conundrum of later Neoproterozoic times seemed to be partly resolved by what might have happened when Rodinia broke apart and its fragments drifted across the globe. This was the event that welded together the southern supercontinent of Gondwana between 800 to 500 Ma ago, forming the web of orogens known colloquially as the Pan African and Brazilide belts of Africa and South America. Palaeomagnetic pole positions for the 1200-750 Ma period, from the supposed components of Rodinia, were an obvious test of Rodinia’s former existence and its gross structure. As they appeared the palaeomagnetic data seemed to confirm the early ideas that were based on Wegener’s method of linking now far-separated orogens to reassemble his Carboniferous Pangaea supercontinent. A reasonable consensus existed by the early years of the 21st century. One of the main contributors of palaeopole data for Rodinia reconstruction has been Trond Torsvik of the Geological Survey of Norway, so it is noteworthy that he has cast the first shadows of doubt on what seemed to be an elegant general solution to more than half a billion years of global tectonics (Torsvic, T.H. 2003. The Rodinia jigsaw puzzle. Science, v. 300, p. 1379-1381).
The problem that Torsvik recognises is that superficially convincing geological jigsaw fits are coming into increasing conflict with better evidence for the palaeolatitudes of different segments. This is compounded by a lack of palaeomagnetic data for some of the 13 major continental segments that had formed earlier in Precambrian times. The central element in the original Rodinia model was the way that India, Antarctica and Australia’s 1300-1000 Ma orogens fitted in what appeared to be a rational reconstruction of East Gondwana. The first fly in the ointment is that revision of Australia’s palaeolatitude seems to make its fit with India impossible. Likewise the position of the geologically fitted Congo and Kalahari cratons, that now make up West Africa, is less certain. Amazonia is also not “behaving” as expected, and Baltica may have been rotated by 180 degrees relative to its former orientation in the old Rodinia model. As well as varying quality of palaeomagnetic data, and its lack from crucial components such as Siberia and North China, their dates vary so much that it is impossible to allow for large-scale readjustments through the lifetime of the putative supercontinent. Torsvik figures a “worst case” scenario, in which the whole Rodinia concept becomes merely continents that were near one another and separated by a variety of active rifts; something of a dog’s breakfast that should spur more dating, palaeomagnetism and tectonic research on the orogens that first suggested a grand unification. That is, if the main proponents do not become so profoundly depressed that they simply give up!
Share this:
Hydrological madness
Regular readers of New Scientist know that Fred Pearce is the scourge of dam builders, especially those with near-megalomania about vast barriers and reservoirs. Back in the late 1960s Canadian environmentalists were horrified to learn of plans being developed to divert southwards water that naturally flows along the great rivers of the Canadian Shield to the Arctic Ocean and Hudson’s Bay. This was NAWAPA, the North American Water and Power Alliance. NAWAPA is still a live ambition for supplying the water-hungry west and mid-west states of the USA. The former Soviet Union put such grandiose plans into effect, one outcome being the dramatic shrinkage of the inland Aral Sea. Pearce returns to continental water transfer in an important review in the weekly for whom he has worked for many years (Pearce, F. 2003. Replumbing the planet. New Scientist, 7 June 2003, p. 30-34). His trigger is the filling of the giant Three Gorges reservoir on the Yangste, one of whose aims is to channel water northwards to augment supplies to the increasing parched plains of central eastern China. But this is only the start of an awesome venture, that will also shift the equivalent of 25% of the Nile’s flow from Tibet’s glacial meltwater that feeds the Yangste into the Yellow River, which now barely trickles into the Yellow Sea. India seems bent on snaffling much of the flow from the Ganges and Brahmaputra catchments into the drought-prone south of the subcontinent. As well as the huge disruption of people and environment that schemes such as these must entail, Pearce highlights the vast economic costs. India’s continental engineering will eat up the equivalent of 40% of its GNP.
Obviously, such huge ventures throw up equally large political and ethical questions, which are not easy to resolve. In many cases the perceived needs for regional water transfers stem from very wasteful water use, particularly in agriculture. Using drip or trickle irrigation, which needs large-scale application but relatively low-cost and simple technology can reduce water requirements dramatically, simply by reducing losses by evaporation from canals. In semi-arid areas as much as 70 % of channelled water never reaches the crops for which it is intended. Governments such as those of India and China depend so much on rural support that they might commit political suicide by pressing for changes to practices that date back millennia, so they opt for the spectacular, quick fixes. Yet there are other such schemes that might transform the livelihoods of some of the worlds most destitute people in the Sahel and Horn of Africa. One suggestion is to divert part of the largely unused river flow through humid tropical Central Africa across the Sahel to reach Lake Chad. Another, not mentioned by Pearce, is to dig a channel that will flood the Danakil Depression of Ethiopia and Eritrea, which lies about 100 m below sea level. Topographically, this would be relatively easy, because only about 30 km of low-lying coastal plain separates the Red Sea from the Depression. The flow could generate hydropower in a power-starved region, and evaporation from the resulting saline lake would boost rainfall in the world’s hottest place, and perhaps allow harvesting of the many salts that would be precipitated, including potash fertilisers. Solar energy could also be used for low-cost desalination. However, no-one can guess at the climatic and ecological consequences of changing humidity in both the Chad and Danakil basins. Yet, water is becoming the most strategically important physical resource so rapidly that the enormous economic implications for transnational contractors, and political prestige associated with regional transfer schemes will drive them ever onwards. There is one glimmer of hope, which Pearce mentions; ordinary people in Rajasthan, India’s driest state, have resurrected old practices of water harvesting, and find that they are more secure than those who rely on state-sponsored canal supplies. The root issue is that rainfall disappears either by run-off or evaporation in a matter of days, unless it is stored somehow. Any habitable place has rainfall, albeit irregular in drought-prone areas, and quite low-cost ingenuity can “bank” the transient spates where the water is needed.
Earth-logs 