Category Archives: Science in Context

Volcanism does not cause glaciations; let’s turn this statement on its head

It is almost a truism that volcanic eruptions influence climate. Cold winters and even failed crops, particularly in the northern hemisphere, followed on the heels the Tambora, Krakatoa, and Pinatubo eruptions.  But these climate aberrations were relatively short-lived, counted in years; the stratospheric aerosols and fine volcanic ash that reflect solar radiation back into space, eventually succumb to gravity and fall to earth.  Eruptions of this kind do not result in long-lived, or permanent changes; they are temporary blips on an evolving earth and an evolving climate.

There have been stupendous volcanic outbursts in the more distant geological past, that have wreaked havoc on the global climate and threatened life itself. One such event was the eruption of the Siberian Traps, 250 million years ago, an outpouring of lava, volcanic ash, and gas that lasted almost one million years.  Individual lava flows, in a single outpouring, produced upwards of 1500-2000 cubic kilometres of basalt.  The cumulative volume of lava and ash would have been sufficient to cover all western Europe or USA in a layer more than a kilometre thick. The Siberian event is strongly implicated as the cause of mass extinctions at the end of the Permian Period, commonly referred to as The Great Dying; it was an event of truly global proportions. Events like this are orders of magnitude greater than the kinds of volcanic activity recorded over the last few thousand years.

So, my cliched introduction is true – to an extent. But how about turning this statement on its head; the excesses of a changing climate can influence volcanism. The possible links between global climate change, volcanism, and the carbon cycle have been argued for decades. First, we need to establish some time scales. Events like a Krakatoa-induced cooling persist for only a few years – usually less than 10. Contrast this with the time frame of your average glaciation, and we are now dealing with (roughly) 100,000 years. How do signals of volcanism compare with these longer-term cooling and warming episodes (glaciations and interglacials)? It turns out that there is a relationship between volcanic activity and glaciations. But it is also apparent that volcanism lags the onset of climate change that herald glaciations or interglacials, in some cases by several thousand years. Therefore, volcanism cannot be a primary cause of such climate changes, at least on a time scale of 1000s to 10,000s of years (a suggestion made almost 4 decades ago by Rampino, Self, and Fairbridge in their paper Can Rapid Climate Change Cause Volcanic Eruptions? published in Science, volume 206, 1979).  These authors hypothesis that the temporal association between volcanism and major climate change is caused not by the eruptions themselves, but changes in water-ice budgets and stress associated with the subsequent loading and unloading of the earth’s crust. The hypothesis is appealing, but until recently it has lacked any reasonable kind of testing and verification.

As happens so often in science, research in one direction can lead to unexpected results that point investigations to new and exciting directions. One such project along the subducting coast of South America, undertaken by S. Kutterolf and colleagues at GEOMAR (Helmholtz Centre for Ocean Research Kiel) provides good evidence for links between glacial cycles and volcanism.  The Initial data base consists of volcanic ash layers retrieved from cores into the sea floor off South and Central America.  The more than 80 ash layers identified provide a history of major, landward eruptions up to one million years ago.  This data set was augmented by volcanic ash – tephra records from various ocean drilling projects at several sites around the Pacific Ring of Fire (more than 400 ash layers were eventually identified). Once the age of all the ash layers had been established, the frequency of eruptions was analysed and plotted. What surprised the researchers, was the prominence of volcanic activity close to, but consistently lagging by about 1-5 thousand years, an important astronomical cycle – the Milankovitch Obliquity Cycle.  Obliquity records the gradual shift in the earth’s axis from a tilt of 21.5o to 24.5o, a change that takes 41,000 years.  Shifts in the tilt axis ‘force’ an increase or decrease in solar radiation, particularly at the poles.

The Kutterolf et al. analysis makes an explicit connection between peak volcanism and the Milankovitch obliquity cycle.  Note however, that it is not changing obliquity itself that causes the increase in volcanism, but the changing climate. For example, during deglaciation ice-sheet mass decreases as melting progresses; this means that the load on the earth’s crust will also decrease.  The opposite affect takes place in the oceans where the water load increases.  Load distributions are reversed during the subsequent glaciation. The crust needs to maintain equilibrium by balancing the changing masses, such that the land surface beneath the ice-sheet rises, and the ocean basins deepen.  This balancing act is called Isostasy.

However, the balancing of loads on the crust also results in changing patterns of stress. If stress levels decrease in regions of active volcanism, like the Ring of Fire, it may be easier for magmas to ascend through the crust, promoting increased volcanic activity.  As a final piece to their argument, the authors create a model of changing crustal stress at a site along the west coast of central America; the model calculates stress for the last 120,000 years, a period that includes the last glaciation and the interglacial that we now find ourselves in.  They also plot actual eruptions along the same time-line. There is a striking increase in actual volcanic activity and modelled stress beginning about 20,000 to 22,000 years ago – a time that corresponds to the maximum extent of glaciation. Melting began soon after (significantly in the Laurentide Ice-sheet that covered a large swath of North America), with subsequent redistribution of ice-water loads and patterns of stress.

There is an interesting corollary to the Milankovitch-volcanism hypothesis. Milankovitch cyclicity predicts that we are now in a cooling phase, which means that stress levels will increase on land as ice accumulates; there should in this case be a decrease in volcanic activity. But if surface temperatures continue to increase…?  The Milankovitch time-frame is in 1000s of years, which gives us a bit of time to work on it.

 

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A burp and a hiccup; the volcanic contribution of carbon dioxide to the atmosphere

Of the two certainties in life, volcanoes offer the most excitement (death and taxes are basically the same thing).  They are magnificent while asleep; a primeval ruggedness that stirs the imagination. We paint them, we eulogise them. And when they awaken, we run for cover. Whether in a state of dormancy or high agitation, they leave an impression on our inner and outer landscapes.

All active volcanoes emit gas; pre-, during and post-eruption. On average, 96% of volcanic gases are water vapour, the remaining components being CO2, SO2 (most common), plus a little helium, nitrogen, carbon monoxide, hydrogen sulphide, and a few halides. Volcano-derived carbon dioxide is frequently cited as a culprit for increasing atmospheric CO2 concentrations in climate change debates.  However, it is sulphur dioxide, not carbon dioxide that does most of the short-term damage to climate. Continue reading

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Contrails, analogues, and visualizing groundwater flow

Analogues and analogies.  Standard dictionaries define these as a comparison, correspondence, or similarity between one thing and another, that can apply to concepts, ideas or physical entities. They are tools, used to illustrate concepts, particularly abstract ideas, to help explain phenomena or theories. Science makes frequent use of analogies. It does so because many phenomena that it attempts to investigate and explain, extend beyond normal human experience, beyond what is visible to the unaided eye, beyond what we can touch.  Well-chosen analogies can help us understand the universe without, and the universe within. Continue reading

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A misspent youth serves to illustrate groundwater flow

Groundwater is always on the move. Under some conditions, in fractures or other large conduits, it can move quickly; almost at a walking pace. Under other conditions it moves inexorably slowly, like fractions of a millimeter a year. Regardless, it is always compelled to move. Movement requires energy.  Where does this energy come from?  What drives the flow of groundwater?  Answers to these questions provide the foundations to the science of hydrogeology. Continue reading

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Tsunamis behave as shallow-water waves

Tsunami statistics make grim reading, which is why I am not going to quote any.  There are some great documentaries and websites that will regale you with all the stats you need. There’s even a couple of movies, where, if you sift through the hype, you may see a smidgen of science, or hear a bit of terminology added to the dialogue to give the impression of knowledgeable heroes.

The word Tsunami derives from two Japanese words; Tsu meaning harbour, and nami wave; an appropriate etymology given that these forces of nature really come into their own along shallow coasts and harbours. About 80% of tsunamis are generated by powerful earthquakes (particularly those beneath the sea floor); the remaining 20% result from large landslides, volcanic eruptions, and less frequently (fortunately) meteorite impacts. They are sometimes referred to, incorrectly, as tidal waves. Tides result from astronomical forces.  We can think of the succession of high and low tides as the passing of a wave that has a period of about 12 hours (the time from one high tide to the next). Tidal waves move along coasts such that a high tide at one location (i.e. the crest of the wave) will occur at a different time to that at a more distant location.  Tides also move water masses; waves do not.

Sea and lake surface waves are generated by wind. The wind provides the energy which is transferred to surface waters.  As a general rule, the stronger the wind, the greater are wave amplitude, wavelength, and speed. Water particles beneath waves have a circular or elliptical motion (referred to as orbitals); the larger circles occurring immediately below the crest, and decreasing in size to a depth that equates to about half the wavelength.  This means that in deep water, waves do not interact with the sea floor. This kind of surface wave is given the name deep-water wave, the speed of which depends only on the ratio of wavelength to wave period. Deep-water waves occur where water depth is greater than half the wavelength.

As waves approach the coast, the wave orbitals begin to touch the sea floor (also referred to as wave-base) and wave speed decreases.  At these depths (depth is less than half the wavelength), loose sediment can be moved by the wave orbitals. Some energy is transferred to the sea floor, but to conserve energy, the height, or wave amplitude must also increase. As you can see in the diagram, the orbitals also become flattened. At this stage, the waves have become shallow-water waves.

Although it may seem counterintuitive, tsunamis behave as shallow-water waves. They have long wavelengths, commonly measured in 10s to 100s of kilometres. The speed of shallow-water waves, including tsunamis, is independent of their wavelength, but is dependent on water depth in the following way:

Speed = (g . depth) (g = gravitational constant, 9.8m/s2; depth in metres)

In the case of tsunamis, the wavelength is many times greater than water depth, even in oceans more than 4000m deep. For example, a tsunami traveling across ocean that is 4000m deep will have a speed of 198m/second, or 713 km/hour. This animation of the 2010, M8.8 Chile earthquake and tsunami gives an impression of the speed of wave propagation across oceans, and the shape of the wave fronts. Tsunami waves commonly pass unnoticed beneath ships at sea or offshore rigs.  As they approach shallower water, their speed decreases to between 40-80km/hour (because speed is dependent on water depth), but the amount of energy in the wave changes very little; to compensate, the wave amplitude must increase. Earthquakes that generate tsunamis create several waves that spread out from the epicentre. All these waves can be destructive, and in some cases the first wave is the least harmful. It is also possible for a wave trough to reach the coast before the first wave crest; this results in a rapid drawdown of the water-level, exposing parts of the foreshore that would not normally be seen at even the lowest tides. Unfortunately, in all too short a time, the absence of water is replaced by a more menacing prospect.

Landslides can also produce monster waves; Lituya Bay in Alaska, 1958 is a good example with first-hand witnesses to the 15-22m wave. A prime example of volcanic eruption-derived waves is the cataclysmic 1883 Krakatoa eruption; a 30m tsunami wreaked havoc in Indonesia and across Sunda Strait.

Tsunami warning systems generally involve an international effort to, in the first instance, detect and pinpoint the epicentre of large earthquakes, and secondly, to detect tsunamis and predict their arrival times at different locations. There is a particular focus on submarine and near-coast, shallow crust seismic events of magnitude 7 and greater; high magnitude earthquakes deeper than about 100km generally do not produce destructive tsunamis.  Tsunami detection buoys have installed in 59 deep ocean locations, most around the Pacific rim.  The map shows the buoys to be located along tectonically active plate margins, such as the west coasts of North and South America, the Aleutian Arc, and other volcanic arcs – subduction zones from Japan through to New Zealand.

The deep-water buoys are anchored to the sea floor; for each sea-bottom buoy there is a linked surface buoy that relays data via satellite.  The deep buoys measure subtle changes in water pressure that can be used to calculate changes in sea-surface height.  The latest models have two-way communications so that a particular buoy can be programmed to search for pressure changes if an earthquake is known to have occurred.  Of course, all this is fine if a region has several hours to prepare for possible inundation.  Those close to epicentres may only have a few minutes to react.

The technology for tsunami prediction and warning is always improving. This is particularly the case for new generations of satellite that are tasked with collecting all manner of climate-related data, data relating to short- and long-term sea-level changes, and subtle changes in gravity and magnetic fields associated with earth’s ever-changing profile.

National Tsunami Warning Centre

Some Tsunami video clips

Boxing Day tsunami 2004 (Cornell Univ. animation)

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Stabilisation of an architectural icon; the Leaning Tower of Pisa

Sunday in Pisa proved to be a welcome change from the usual tourist-cramped, shoulder-barging throngs of popular attractions in Tuscany.  No problem finding a seat in a decent café, en route to the Piazza del Miricoli.  Cross the street, turn a corner and there – the massive, white-marbled Pisa Duomo, Romanesque grandeur with a veneer of 21st Century scaffolding.  But the sense of balance normally attributed to cathedrals, is disrupted by the stand-alone bell tower that leans precariously, like a drunk looking for a lamppost.  The Leaning Tower of Pisa has been looking for a lamppost for almost one thousand years.  And for a thousand years, people have been drawn to the tower not because it is particularly beautiful, but because it looks like it is about to fall over. Continue reading

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A measure of the universe; Renaissance slide-rules and Heavenly spheres

Measurement is a cornerstone of science, in fact of pretty well everything we do: How far? How fast? How long?  We take most measurement for granted, with little thought to how the process originated.  We demand accuracy and precision, forgetting that these are relatively modern luxuries.  Before the universal clock chimed GMT in 1884, there were more than 200 time zones in the US.  A league in France was shorter than a league in Spain, a discrepancy for which the 16th C French scribe François Rabelais had an imaginative, if rollicking explanation.  In his tale, The Life of Gargantua and Pantegruel (1532-1564), a king required a standard distance to be determined (after all, if he was going to send his armies to battle it would be best if his advisors new how far they had to go).  He sent a trusted Knight, instructing him to ride to Spain, stopping every league to “roger and swive”; hence the discrepancy.  The leagues gradually became longer. The amusing satire of this explanation had its roots in real Medieval measures; the width of a hand, the distance one could walk in an hour. Continue reading

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