Author: zooks777

Former Senior Lecturer at British Open University, research in remote sensing of arid lands, groundwater exploration, Precambrian tectonics and geochemistry

Land vertebrates snuffed at the end of the Permian

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Without doubt, the mass extinction at the Permian-Triassic (P-T) boundary was the most important biological event in the history of Phanerozoic evolution.  Around 80-90% of families disappeared, and perhaps more than 50% of species diversity.  But, the evidence stems largely from marine records.  Marine organisms went down at a hasty rate, as evidenced by the superb boundary sequence in China.  However, such is the inconsistency of preservation on land that matching evidence is sparse from the terrestrial realm.  The best chance of examining the response of land animals to whatever wrought such havoc at sea lies in the Karoo sediments of southern Africa.  Roger Smith of the South African Museum and Peter Ward of the University of Washington have combed the mainly fluvial sediments for evidence (Smith, R.M.H. and Ward, P.D.  2001.  Pattern of vertebrate extinctions across an event bed at the Permian-Triassic boundary in the Karoo Basin of South Africa.  Geology, v. 29, p. 1147-1150).  Rather than supporting the general view that terrestrial P-T extinctions took a few million years, they have been able to show that Permian vertebrates disappeared abruptly, to be replaced by a very different fauna equally suddenly in the lowermost Triassic.  Only one genus (Lystrosaurus) spans the boundary, and the boundary itself contains no evidence of life.  Calculations based on estimates of the rate of sedimentation point to around 50 thousand years for the extinction event, about the same as that affecting marine organisms.  Interestingly, the event sharply separates very different sediments, that Smith and Ward interpret as products of perennially wet Permian flood plains and those experiencing ephemeral flow in the Triassic (see End-Permian devastation of land plants in Earth Pages October 2000).  Whatever its cause, the stresses placed on land vertebrates seem to have included the sudden onset of aridity.

Mesozoic fossil hunting in Madagascar

Most papers on palaeontology report the details of years of research on what the fossil hunters have found, with mentioning the months of patient searching.  John Flynn and André Wyss have provided an insight into the tribulations of palaeontological field work in difficult terrains, as well as a broad account of the context of their finds (Flynn, J.S. and Wyss, A.R. 2002.  Madagascar’s Mesozoic secrets.  Scientific American, v. 286, February 2002, p. 42-51).  Madagascar lingered at the heart of the Gondwana supercontinent until it finally began to split into drifting segments during the early-Triassic.  It lay on the eastern flank of an evolving rift basin that filled with mainly terrestrial sediments until the late-Jurassic.  This particular basin remained uninterrupted by volcanism or erosion, and so is a repository for organic remains trapped in a continuous sedimentary sequence.  This period in geological history, particularly the Triassic, spans the emergence and development of both the dinosaurs and primitive mammals.  The wealth of vertebrate fossils that geologists are beginning to unearth suggests that Madagascar may well become the site where the mysterious origins of both are resolved.

The simplest living ecosystem

Hugely complex as life is, at the cell level it has a profound simplicity, at least as regards its fundamental chemistry.  Cell metabolism receives its power from the transfer of electrons from a high to a low energy level.  High-energy electrons stem from chemically active molecules, atoms or ions able to release them; electron donors or reducing agents.  The metabolic path ends in oxidizing agents accepting these electrons.  This process of donating and accepting electrons takes the form, in most cell types, of “pumping” hydrogen ions, or protons back and forth across the cell wall to create an electrochemical gradient that is continually charged and discharged.  Biochemistry reflects this by the ADP-ATP cycle at life’s core, in many different versions.

The simplest provision of electrons is by hydrogen, and arguably a supply of hydrogen gas is a highly likely precondition for the origin of life.  Surprisingly, hydrogen is generated by many geological reactions, although little survives some form of oxidation for long.  In a few places hydrogen gas escapes abundantly, as in the weathering of ultramafic rocks by groundwater.  The essential process is the breakdown of iron and magnesium silicates to various kinds of clay, by the interaction of hot water with fresh igneous rocks.  Geochemists and microbiologists from the USA  analysed such a hydrothermal system 200 m beneath a volcanic area in Idaho, and found a thriving and diverse ecosystem dominated by simple organisms that do depend on hydrogen (Chapelle, F.H. et al. 2002.  A hydrogen-based subsurface microbial community dominated by methanogens.  Nature, v. 415, p. 312-315).  More than 90% of the organisms are methane-producing Archaea, which reduce carbon dioxide to methane, using hydrogen.  No other hot-spring system comes close to this probably highly primitive community.  It is a handy analogue for the kind of ecology that may have developed if life has arisen deep beneath the icy surface of Europa – a target for future NASA missions.

Incidentally, the exploitation of electron and proton transfer that underpins cell metabolism potentially forms a source of electrical power.  Younger readers may have experimented with using fruit as the basis of a simple galvanic battery, thereby exploiting low pH conditions.  Investigation of the potential of bacteria for direct electricity generation recently made a breakthrough (Bond, D.R. et al. 2002.  Electrode-reducing microorganisms that harvest energy from marine sediments.  Science, v. 295, p. 483-485).  A member of the family Geobacteraceae (Desulfuramonas acetoxidans)  has been found to readily produce excess electrons as it metabolises organic material in oxygen-free muds.   Daniel Bond and co-workers from the University of Massachusetts and the US Naval Research Laboratory introduced graphite electrodes into airless fish-tank muds and the upper oxygenated sediments.  Even with such a crude experiment, sufficient current flowed to power a small calculator.  Moreover, D. acetoxidans colonised the electrodes within a matter of days, showing that they were directly involved in the oxidation-reduction system at the root of such a fuel cell.  As well as raising the possibility of powering submarine monitoring devices using bioelectricity, such geobacteria are able to metabolise a range of common organic pollutants.  Marine organic sediments are virtually limitless, so it is not inconceivable that the process may result in yet another renewable power source, albeit difficult to convert to high-power supplies, with the blessing of pollution control as a sideline.

Meltdown for Snowball Earth?

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Following on from their linking carbon-isotope excursions associated with Neoproterozoic diamictite-cap carbonate sequences to methane release (see Methane and Snowball Earth in Earth Pages, December 2001), Martin Kennedy, Nicholas Christie-Blick and Anthony Prave turned to the d13C values in the diamictites for which a glaciogenic interpretation forms the main plank of the Snowball Earth hypothesis (Kennedy, M.J. et al. 2001.  Carbon isotope composition of Neoproterozoic glacial carbonates as a test of paleoceanographic models for snowball Earth phenomena.  Geology, v. 29, p. 1135-1138).  Complete ice cover of the oceans would chemically isolate ocean water from the atmosphere, and would effectively shut down the organic sinks for atmospheric carbon dioxide.  While they operate, the exclusion of 13C relative to lighter carbon by organisms drives up d13C in sea water, to be preserved in carbonate sediments.  The Snowball Earth model predicts negative d13C, approaching the -5‰ of the mantle, in carbonates deposited during all-enveloping glacial epochs.  However, few researchers have made the measurements needed to test that part of the hypothesis.

Kennedy and co-workers show from three such diamictite sequences that the carbonate precipitated as cement in them has consistently positive d13C.  Although that does not disprove the existence of glaciation at tropical latitudes, it is not consistent with the dreadful scenario of totally ice-bound oceans devoid of life.  Nor, for that matter, is there any evidence from strontium isotope variations in carbonate cap rocks for the massive continental weathering that the Snowball Earth devotees propose as a means of escape from the eventual hot-house that build up of volcanic CO2 emissions to release Earth from the mothers of all cold snaps would create.  Expect interesting news in later Earth Pages of how the greatest Earth science debate of the 21st century develops.

American Geophysical Union 2001 Fall Meeting

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The regular AGU Meetings are the largest of all Earth science bun fights, some 8500 souls attending the latest In San Francisco from 10-14 December 2001.  Without attending or browsing the abstracts, the rest of us might wait months if not years for ideas presented at such conferences to emerge as papers, and many presentations never reach the press.  Summaries of conference proceedings shortly after the event are a useful, although rare notice of “what’s up” to the rest of us.  Surely it is in the interest of organisers to arrange postscripts, perhaps through a general website – conferences cost an arm and a leg to attend, let alone organise, and paying someone to write them up would have a minuscule cost. But there has always been a cachet to being one of the “in crowd”, and no one satirised that better than David Lodge, in his 1980 novel Small World (Penguin Press).  In the case of the AGU and GSA Meetings the crowd is around that at a Nationwide League Division 1 match at 3 pm on a winter Saturday, but is somewhat less focussed.

So a summary of AGU 2001 Fall was a welcome sight in Science (Kerr, R.  2002.  Of ocean weather and volcanoes.  Science, v. 295, p. 260-261), even though it covered only some of the themes in the poster sessions.  A sizeable number of AGU attendees do sea time, and they must have been delighted to learn that the US Naval Research Laboratory  now provides 30-day “sea forecasts” on-line (www.73320.nrlssc.navy.mil/global_nlom/).  An outcome of work from the Ocean Drilling Program along the Hawaii-Emperor sea-mount chain is clear palaeomagnetic evidence that the Hawaiian hot spot has not always been fixed relative to moving lithospheric plates.  From late-Cretaceous to early-Oligocene times it was shifting southwards relative to the north magnetic pole at a rate comparable with that of sea-floor spreading, thereby helping to explain the 60° bend in the chain, and perhaps that less well seen in other Pacific hot-spot tracks.  As well as demanding an explanation for lateral dynamics in the deep mantle, hot spots that move challenge some basics of plate tectonics.  The current mood among plate tectonicians seems somewhat similar to that of a zoologist colleague in the 1960s.  He had ended up on a psychiatrist’s couch when he stoutly maintained that a red blemish on his torso moved, but only when it was he who was examining it.  That turned out to be a giraffe parasite, picked up during field work in the Serengeti.  More immediately worrying for the US public was news in spring 2001 that a sizeable bulge was growing in Oregon, close to dormant volcanoes.  Pushing 30 mm a year, the carbuncle’s growth rate suggested that 0.02 cubic kilometres of magma were on the move – that is a lot of potential volcano.  USGS volcanologists have been on a bulge watch for 2 decades, and were the first to announce a far larger manifestation beneath a northern Californian caldera.  At the AGU, worries were damped down by reassurances that neither seemed likely to amount to a can of beans (but see Is volcanic eruption predictable? below).

Interstellar carbonates and “fossils” from Mars

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Part of the argument used to support the notion that life may have arisen on Mars early in its history depends on the presence of carbonates in the notorious meteorite ALH84001 found in Antarctica.  Supposedly having been ejected by an impact on the Martian surface (based on its oxygen isotope composition and the blend of noble gases trapped within it), ALH84001 also contains the minute structures that were prematurely announced in a blaze of publicity as fossilized alien life forms by US and British meteorite specialists in 1996.  The discoverers claimed that carbonate minerals within it clearly evidenced the rotting of silicates by liquid water containing dissolved CO2; so they do in terrestrial rocks. However, carbonates also occur in meteorites that by no shred of the imagination can have formed within sizeable planets.  Many probably accreted in a near vacuum from dusts that occur in clouds within our galaxy, while the solar system was forming.

Using infrared spectra to assess the mineral composition of dust clouds surrounding stars, a team of European and American cosmochemists has found that in two cases such dust contains calcite and perhaps dolomite (Kemper, F. et al. 2002.  Detection of carbonates in dust shells around evolved stars.  Nature, v. 415, p. 295-297).  Because liquid water cannot exist in a near vacuum, production of these carbonates cannot have taken place by the familiar silicate-rotting process.  More likely, they formed on the surfaces of silicate dust or ice grains by reactions between calcium and magnesium ions and those in which carbon and oxygen were combined.

Hydrocarbon source rocks and ocean anoxia events

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Much of the world’s oil resources formed by maturation and migration of hydrocarbons from organic-rich, marine mudrocks, which seem to have formed episodically during Earth history.  A widely accepted view is that such source rocks’ content of organic matter fell to the ocean floor as the remains of tiny organism.  That they were not oxidized by bacterial action seems to suggest that the periods when source rocks accumulated were characterized by low oxygen levels in bottom waters.  Each major source rock has been linked to such ocean-anoxia events, and to periods when deep-ocean circulation effectively stopped, so cutting off oxygen supplies to deep levels.  However, studies of modern deposition of organic matter in marine sediments at continental margins reveals that discrete particles of organic matter are far outweighed by biological molecules that coat the surfaces of minerals, particularly those of clay minerals.  The amount of organic carbon in a modern sediment depends largely on its content of clay minerals derived from intense chemical weathering of continental rocks.  Such coatings are protected from normal processes of decay, so that the adsorbed organic carbon compounds can be buried, more or less intact

It should be possible to check whether ancient source rocks are similar to modern carbon-rich sediments by seeking a strong correlation between clay content and organic content – mudrocks also contain fine silt particles made of non absorbent quartz. It seems that in at least one Cretaceous source rock in the US Mid-West such a correlation is clear (Kennedy, M.J., Pevear, D.R. and Hill, R.J. 2002.  Mineral surface control of organic carbon in black shale.  Science, v. 295, p. 657-660).  This suggests that oil-shale deposition is as much related to the intensity of continental weathering of silicates as it is to ocean-water chemistry.  Since clays, especially the sponge-like smectites, adsorb organic molecules from solution in seawater, they draw on a vast pool of material, so that enhanced biological productivity need not be linked to oil-shale formation either.  The fact that most organic material in such rocks is structureless kerogen, rather than identifiable particles, also supports this alternative hypothesis.

Both petroleum geologists and palaeoclimatologists have assumed a source rock – ocean anoxia connection in both exploration strategies and assessment of past climate shifts.  So Martin Kennedy et al.’s painstaking findings are sure to cause a major stir.  However, what cannot be avoided is that increased chemical weathering of the continents is likely to accompany globally warm conditions, and they in turn sponsor growth in planktonic productivity.  Likewise, global warmth does not favour the formation of dense, cold and therefore oxygenated sea-surface water, which sinks to aerate deep oceans when the planet is cool.

Measuring erosion rates.

So many landscapes show evidence of changes in the rate of erosion, such as terraces, waterfalls and signs of changing rates of sediment deposition, that a means of accurately measuring rates opens up an important new phase in geomorphological research.  Precise dating of modern surfaces is not possible using stratigraphic or radio-carbon methods, and this has hidden much of landform history.  Once a surface is exhumed, it becomes exposed to cosmic ray bombardment.  These particles travel at near-relativistic speeds, and so have sufficient energy to transmute common element nuclei to unstable isotopes.  The longer the exposure of a surface material, the more radioactive it becomes, albeit very weakly so  Since erosion and sedimentary processes move and quickly bury particles dislodged from a surface, material has a finite time during which it can be irradiated.  The particles themselves carry the isotopic signature of their surface residence time, the slower erosion is the more radioactive are particles derived from the surface.

Cosmogenic dating uses sedimentary grains from sands deposited in a drainage basin, particularly those of quartz that are common and stable.  Oxygen and silicon in silica can become 10Be and 26Al when struck by cosmic rays.  Although sampling is fraught with pitfalls, essentially it amounts to scooping up a handful of sand that represents the past erosion of the entire catchment above a sample point.  Measuring the minute concentrations of new isotopes  costs of the order of $1000 per sample, using a high-energy accelerator mass spectrometer.  Since dozens of samples provide sufficient data for meaningful interpretation, this is not a method that will spread widely to places that come anywhere near fully reflecting the intricacies of erosional shifts over the large age range that cosmogenic dating can address.  Nonetheless, its early results are astonishing.  Work in Idaho suggests that through the period of the last glacial maximum into the early Holocene the average rate of erosion was 17 times faster than it is at present.  That possibly signifies either continual high erosion, that has petered out, or, more likely, that erosion has had episodic, catastrophic pulses.  As might be expected, anthropogenic disturbance of the surface enhances erosion rates, but a cosmogenic study of river sediments in Sri Lanka indicates that 200 years of intensive farming in rugged highland areas have resulted in a 20- to 100-fold acceleration.  Most awkward of all, another study of long-term erosion in California’s Sierra Nevada showed no relation between weathering and erosion rates and climate change.  Geochemists contributing to the debate over climate controls by weathering take note.  It seems that the primary control of erosion rates in western California was purely tectonic, which could tally with the notion that newly rising mountains have a major influence over sequestering of CO2 by silicate breakdown. 

The obvious next step is blending cosmogenic sediment dating with that of crustal exhumation from Ar-Ar and U-Th/He dating of cooling due to uplift and erosion.

Source:  Greensfelder, L. 2002.  Subtleties of sand reveal how mountains crumble.  Science, v. 295, p. 256-258.

Vertical tectonics and formation of Archaean crust

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Since Pentti Eskola’s recognition in 1949 that many Precambrian granitic rocks form domes surrounded by cusp-like synclines of supracrustal rocks, such mantled gneiss domes have been found in most cratons.  Probably the best example characterizes the 3.5 Ga Pilbara province of the West Australian Shield.  How they formed has long been a vexed topic, the most popular views being as a result of low-density basement rising through denser cover that contains abundant volcanic rocks, or as a result of regional-scale fold interference.  Precise dating of the Pilbara granitic rocks and greenstones shows a common age range, with some older greenstones,  The age data suggest that the dome and cusp structure is a product of the co-evolution of both, probably from a primary oceanic-like crust of mafic composition (Zegers, T.E. and van Keken, P.E. 2001.  Middle Archean continent formation by crustal delamination.  Geology, v. 29, p. 1083-1086).

Archaean rocks of broadly granitic composition (dominantly tonalites, trondhjemites and granodiorites, or TTG) have geochemical features setting them apart from post-Archaean varieties.  Rather than signifying their origin by supra-subduction melting of the mantle wedge with fractional crystallization and crustal assimilation in the lower crust (the dominant crust-forming process in post-Archaean times), all Archaean TTG seem to have formed by partial melting of a garnet-rich mafic source.  One of several possibilities is that their source was eclogite.  Based on the peculiar regional structure of the Pilbara and its dominance of the whole crust, as shown by maps of gravitational potential and magnetic field strength, Zegers and van Keken revisit earlier ideas of dominantly vertical tectonics that underlay early crust formation.  They suggest that efficient cooling by hydrothermal circulation allowed thick mafic crust (similar in some respects to that formed in the Mesozoic beneath ocean plateaux) to enter the field of eclogite stability at its base to form a layer denser than ultramafic mantle.  Once sufficiently thick, this layer would begin to founder, or delaminate, to be replaced by hot mantle.  Rebound of the remaining crust would set in motion rapid crustal uplift and extension, together with decompression melting of rising mantle (to form high-magnesium basalts high in the crustal sequence)and melting induced in the remaining mafic crust (to generate TTG magmas).  Indeed, the kimberlites that puncture other Archaean cratons carry abundant eclogite xenoliths from mantle depths.  Seemingly well-documented, this tectonic model does not explain all Archaean crust formation, for other cratons, such as that of west Greenland, are more readily accounted for by seemingly familiar subduction-zone processes.

Is volcanic eruption predictable?

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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 12th January 2002, p. 29-31; Horizon, BBC2 17th 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 18th 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 3rd 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.

Popper refuted

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In mid-Victorian times, Lord Kelvin peered down his nose at Charles Lyell’s estimation of sedimentation rate from the historic silting of the port of King’s Lynn, as a means to judge the vast time span represented by the stratigraphic column.  His words were not kind; “…when you cannot measure [what you are speaking about], when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind”.  Geologists cringed, particularly when Kelvin went on to reckon an age of 20 to 40 Ma for the Earth based on its cooling from a molten mass, using the physical laws of conduction and radiation.  He was fundamentally wrong on most counts, partly because he knew nothing of radioactive heat generation nor convective heat transfer.  Sadly his corpse could not be revived to eat his mean-spirited words.  Nonetheless, the gibe of Earth scientists’ being “unscientific” has stuck.  We rarely stick to the “scientific method”, reputedly stemming from the Elizabethan philosopher, Francis Bacon and his rationalization of the inductive method of reductionist experimentation.  There are few universal “truths” in Earth history, and the interweaving of limitless processes with a vast spectrum of rates, scales and magnitudes renders reductionism absurd.  Even more prone to reductio ad absurdem is the chemist Karl Popper’s supposedly logical insight that “proper” science rigorously subjects hypotheses to a “risky test”; an experiment that should yield evidence of refutation if the notion is unsound.  Popper’s method of falsification consigns to the dustbin of research any hypothesis which fails the test, with the corollary that in is not “best practice” to seek confirmation for a hypothesis.

Carol Cleland of the University of Colorado (Cleland, c.e. 2001.  Historical science, experimental science, and the scientific method.  Geology, v. 29, p. 987-990) demolishes the “recipe-book” approach to science, which has laid a dead hand on not only the Earth sciences, from the standpoint of philosophy and reality.  She starts from the position of Thomas Kuhn, by pointing out that, for Popper, the whole of Newtonian celestial mechanics should have bitten the dust when 19th century astronomers discovered that the orbit of Uranus deviated from Newtonian prediction.  A sustained search for reasons why concluded that there must be gravitational forces from planets beyond Uranus, and sure enough astronomers discovered Neptune.

There is an air of bullying about the “scientific method”, which has warped investigations and dulled imagination and curiosity for centuries.  It provides ammunition for those who carp and pontificate from the sidelines, and in many cases from positions of considerable power.  Cleland does us all a service by discussing philosophical matters of science in the context of the realities that confront us all, in an accessible way.  Her analogy is Holmesian detection (Sherlock was a deductionist, by the way, proceeding from the general to the particular), which discovers events and proceeds to trace their circumstances – the search, to my mind, for the artillery rather than a single “smoking gun” is far richer than the events themselves, because that deepens our sense of context for particular events, however dramatic they might seem to be.

 

US security clamp on vital data

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The geopolitical realities of remotely sensed data became plain in the aftermath of the 11th September attack by terrorists on the United States.  The US National Imagery and Mapping Agency (formerly the Defense Mapping Agency of the Department of Defense) placed a moratorium on release of digital elevation data derived from NASA’s February 2000 Shuttle Radar Topography Mission, “in the interests of national security”.  The SRTM, which used radar interferometry from dual antennae on a 60 metre long boom, was intended to satisfy the huge demand from Earth scientists for digital elevation models of the continents for a large range of applications, ranging from accurate hydrological mapping to sophisticated mathematical analyses of landforms.  An accurate, high-resolution DEM is central to rapid topographic mapping of those many parts of the world where published scales do not exceed 1:250 000.  NIMA also maintains the classified DTED Level-1C global elevation data set, derived from a variety of sources, including clandestine aerial and satellite photography, and which has a resolution as precise as 30 metres.  SRTM data are reported to be more revealing.  At the heart of cruise-missile guidance and the real-time imaging radar used for navigation in low-flying, all-weather military aircraft lies DTED Level-1C data.  Such facilities are not known to be in the possession of, or under development by any agencies other than the military of a small number of developed countries, for obvious economic reasons.  Oddly, elevation data as revealing as DTED Level-1C for the whole of the USA and its territories are still available freely from the US Geological Survey.  Anyone “targeting” installations, either for military or more innocent purposes, need look no further than the growing number of commercial image providers who sell satellite images with spatial resolutions as good as 1 metre.  Indeed, some such companies currently promote their wares through images of Manhattan Island in the aftermath of 9th September, and there is a thriving business in selling aerial photographs of real estate with resolutions up to the 10 centimetre level.

Browsing through the archive of data from the Terra satellite, particularly those from the ASTER instrument (visit http://edcimswww.cr.usgs.gov/pub/imswelcome/ ), reveals a disproportionate focus on Afghanistan compared with much of the rest of the world.  The majority of Afghan images were captured before 11th September, and the area is hardly a priority for scientific research.  ASTER produces stereographic images with a 15 metre resolution, suitable for producing high-quality digital elevation models that rival those of SRTM.  It would not be surprising to discover that US and British Special Forces engaged in Afghanistan not only carried large-scale topographic maps derived from ASTER images, but also commercial Ikonos 1-metre images, that are capable of pinpointing vehicles and concentrations of people.  Nor is it surprising that relief agencies, intent on delivering humanitarian supplies to emergencies of many different kinds in nearly unknown terrain, rarely if ever have such sophisticated navigational aids.

Interferometric radar and faults of the Mojave Desert

Though it requires considerable computing power and specialized software, the use of “before” and “after” radar data to detect small-scale subsidence or shifts in the horizontal plane, is a potentially powerful tool in neotectonics (see Radar analysis of Turkish earthquake, August 2001 Earth Pages).  Motion detection by such radar interferometry becomes even more useful as historic radar images accumulate.  The workhorse for radar interferometry is the European Space Agency ERS series of satellites, which produce synthetic aperture radar images about 150 km wide along the same track, orbit after orbit.  The system has operated since 1992, so there are rich possibilities for multitemporal use of the distance-measuring capacity inherent in radar imagery.  Means of assessing the regional build-up of strain in seismically active areas are important in earthquake prediction, and such synopses help understand the tectonics at work there.

In terms of seismicity and tectonics there is no better studied area than that extending from the Pacific coast of southern California across the San Andreas Fault and the Mojave Desert.  Radar interferometry provided by 25 pairs of ERS images from 1992 to 2000 produces a spectacular picture of the gradual development of ductile strain underlying this risky area (Peltzer, G. et al. 2001.  Transient strain accumulation and fault interaction in the Eastern California shear zone.  Geology, v. 29, p. 975-978).  Unsurprisingly, shear strain along the San Andreas fault system shows up well.  The Garlock Fault that marks the NW flank of the Mojave is apparently resting after 10 thousand years of motion that averaged 7 mm per year.  The authors focus on displacements associated with the diminutive, by Californian standards, Blackwater and Little Lake fault systems, which trend SE-NW to link the epicentres of the 1872 Owens Valley earthquake and that at Landers in 1992.  Within 10 km of these aligned faults are clear signs of a step in strain rate, that suggests that the lineament lies above a major, active ductile shear zone; perhaps the birth of a new fault system.  Should this system fail in a brittle fashion it is likely to result in an event with a magnitude greater than 7 on the Richter scale, and a surface break more than 100 km long.  Peltzer et al. have achieved a test of concept for interferometric radar’s use in seismic risk assessment, that can be deployed anywhere, given the computing resources.  Their work transcends after-the-event studies that do little to assist the victims of earthquakes.

New map resource for Earth scientists

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Researchers at Cornell University have been compiling digital maps of a wide range of data for the last 8 years.  The Digital Earth project’s web site is http://atlas.geo.ecornell.edu .  There, it is possible not only to download various data sets for use in a GIS, and to track down primary sources for data, but also to build your own maps.  Digital Earth comprises over 100 data sets, on global, regional and North American scales, that include geographical, geological and geophysical themes.  The mapping tool takes a while to get used to, and runs slowly with a 56k connection, but should behave well with broadband access.  I tested the tools by creating a geological map of NE Africa.  This was pleasingly up to date and showed moderate detail, but the lack of a legend is something of a drawback.  Understandably, the level of stratigraphic division is limited, so that all Precambrian areas appear in the same colour.  Similar detail is not yet available for Europe, only a coarser resolution world geology data set covering it.  Downloads are in either Postscript or jpeg form, the latter suffering from artefacts generated by compression.   This is a site well worth a visit.