Monthly Archives: May 2017

A misspent youth serves to illustrate groundwater flow

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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

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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 been 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

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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 lamp-post 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

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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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Galileo’s finger

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“Their final resting place…” a sepulchral phrase, redolent of a fate that awaits us all.  There is no doubt as to its finality, but resting…?  A nice metaphor that may convey a sense of comfort to the living, rather than the deceased.  Wander through any church or cathedral in Europe and Britain, and you will inevitably walk over cold marble slabs, engraved with the details of those who lie beneath, polished by the feet of a myriad worshipers and tourists.  The Basilica di Santa Croce in Florence is, in many respects, like any other magnificent church; it is old, construction beginning in 1295, with alterations and additions during the 14th -15th century overlapping the earlier Gothic forms.  The Basilica is stunning, but differs from many of its contemporaries in that it became THE place in Italy to be buried. Continue reading

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The life of a Tuscan wall

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Montefioralle, Chianti country, Tuscany, Italy, and from where I’m sitting (happily sampling a Chianti Classico) I see rolling, wooded hills, next season’s vintage, olive groves a scattering of farm dwellings, and rock walls. Quintessential Tuscany. Except for a few ratty road cuts, there is little native rock exposed in this part of Tuscany from which a keen geologist might ascertain something of ancient pre-Tuscan history (farther south this changes).  But in fact, there are rocks aplenty. Most walls (houses, defensive, retaining, decorative) are made of limestone and sandstone, some quarried and deliberately shaped as in churches and castello (that date back to the 10th -11th century), and others that made use of whatever was handy at the time. Most of these materials were collected locally; stones that littered the hillsides, and stones brought to the surface during ploughing.  Even today, ploughs bring stones to the surface; the local clay-loam soils are incredibly stony (an important part of Chianti terroir).

So, despite the paucity of hard-rock exposure, one can make a reasonable guess at the geology beneath the hills and vineyards, based on the stone composition in local buildings. About 50-60% of the stones are cream-coloured marls; marl is an old name (medieval Latin) given to very fine grained, usually muddy limestone that breaks along curved, sharp-edged surfaces (referred to as conchoidal).  The Tuscan marls are very hard – ideal for building stone.  Variations on this theme include sandy limestones, some of which contain intricate contorted layering, and small crossbeds that indicate flowing water many millions of years ago.

Grey sandstone is also common; in fact it is found as paving stone throughout most of Tuscany.  All kinds of structures are visible in these stones, especially cut stones in larger buildings and roads; fossil ripples that indicate flowing water, trails and burrows of critters that moved across or below the ancient seafloor in search of food or finding a place to live. Some of the sandstones are not as hard as the marls, and in places show quite advanced damage where bits of rock fritter away with the vagaries of weather.

An assortment of red bricks, some rumoured to be of Roman or Etruscan derivation, has been used in most walls. It looks like odd-shaped bricks are filling equally odd-shaped gaps, but they have also been used to replace stone arches over doors, or fill holes in walls left by marauding armies (of which there were many) or neglect.

The rocks were originally deposited as sediment in an ancient and vast ocean called Tethys, that separated two supercontinents – Gondwana, and Laurasia (most of Europe and Asia).  The Tethys was closed when the Africa plate (part of Gondwana) drifted north and crunched into Laurasia, beginning about 65 million years ago. The resulting uplift produced the Apennine Ranges that now course the length of Italy.

Montefioralle is a picturesque hill-top village, typical of many in Tuscany.  Its medieval origins are still visible, but frequent battles between neighbouring villages, as well as larger fracas between Florence and Siena, put a few dents in the outer wall and houses. The hill top is crowned by a small church and tower; the last refuge in the event of siege.  There is clear evidence of repairs made over the last 800 plus years, including, I suspect, some from a more recent European conflict.

Stones in these Tuscan walls weave their tales in different threads. The limestones and sandstones have a geological story that spans 10s of millions of years, the disappearance of an ocean and the collision of continents.  Each stone and brick can also relate centuries of local history; each was carefully placed by someone, a stone-mason or perhaps a Renaissance DIY. Nameless, we can admire their handy-work, wonder what they talked about with their fellow workers, what they ate, who they loved.  There are centuries of these former lives everywhere in Tuscany. Chianti Classico loosens all their tongues.

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