Wednesday, 1 April 2009

Madrid weight loss brings trouble to the city.

The Retiro Park's light and airy Palacio de Cristal

Now here's some intriguing news: Madrid is lighter and higher than it used to be.

I have spent most of my life in the field of Geophysical exploration; the study of the earth’s sub-surface using techniques involving small movements of the earth’s crust, and I still take a great interest in any new information that comes my way.

So I was intrigued recently when I read in the IGJ, the Iberian Geological Journal, that certain seismic events have brought change to the geology of Madrid.

Any change in the geology of an area is a slow process. That continents move about the globe, floating on “rafts” of rock on a deep sea of magma is now well known. They move very slowly, but as Galileo would have said, they do move. It was the northern drift of the Iberian peninsular slowly pushing into the continent of Europe that folded and buckled the surface rocks to form the Pyrenees chain of mountains in the north and the massive plate that supports Africa pushing against the Iberian plate in turn that created the Sierra Nevada in the south.



The Iberian peninsular has not stopped moving, but its travels are now predicted to take it more to the northwest, squeezed out from between the those two massive plates to eventually break away from mainland Europe altogether and drift isolated into the Atlantic. According to a report I read last year (and it pains me that I can’t remember where) Spain’s global location in thirty million years time is predicted to be somewhere north of where Iceland is now, so it’s too early to be buying the cold weather clothing just yet.

However, according to the report in the IGJ some movement in the Spanish geophysical record is speeding up, and an important result of that is being experienced right here in Madrid.

The author of the report, Doctora Señora Avril Primero, suggests that Madrid is undergoing a severe vertical geological shift. In non-technical language, this means that the height of Madrid above sea level, already the highest capital in Europe, is actually increasing. And increasing fast.

The studies used a relatively new form of geological survey called GSI3D, or Geophysical Survey and Investigation in Three Dimensions, which has pin-pointed with some accuracy exactly what is happening under the city.

However, first, let me explain a basic principle of plate tectonics, the system proposed and eventually accepted in the 1970s (despite much opposition) that describes continental drift. Imagine a toy boat in the bath. Untouched it floats perfectly level in the water, but apply pressure or weight to one end, say you sit the soap on the stern, the other end, or bow, will rise in the water. This is the principle geologists call “Isostasy”.

Derived from the Greek; “isos” meaning equal, and “stásis” meaning standstill, the term refers to the state of gravitational equilibrium between the earth’s lithosphere (the solid bit we live on) and the asthenosphere, (the crumbly part way below us). In this way the earth’s tectonic plates float like icebergs with their median height determined by their thickness and density. And like the toy boat in the bath, if the weight of one area is altered, there will be a corresponding change elsewhere. For instance, if the Himalayas vanished southern India would likely begin a journey to the centre of the earth.

Pulling together a combination of the results obtained by the new GSI3D techniques and borehole tests exploring the possibilities of using geothermal energy close to Madrid, plus Elevation Mapping by Satellite, using high frequency radar to monitor the height of the earth’s topological surface, Señora Primero has shown that Madrid is slowly rising. However, her measurements have also demonstrated that land masses to the north and west of the city, the Guadarrama range near El Escorial and El Monte de el Pardo are, at the same time, descending.
Legend: 1. Plutonic rocks; 2.Slates, marbles, quartzites and gneisses; 3.Slates and metagreywacke; 4. Slates,quartzites and, metavulcanites; 5. Mesozoic;6. Early Tertiary; 7. UndifferentiatedMiocene; 8. Lower Miocene Unit; 9. Intermediate Miocene unit; 10. Upper Miocene unit; 11. Pliocene; 12.Quaternary.
Explanation: “Plutonic Rocks” are Rocks formed deep in the earth’s magma. Granite is such a rock. “Cuenca” is Spanish for a fluvial basin, or an area mostly formed by water erosion and deposit of sedimentary rocks. From this map it is very easy to see the differentiation of the lighter rock of the Madrid plain and the heavier material of the mountains to the northwest.

Map after La Instituto Geológico y Minero de España. Ríos Rosas 23, 28003 Madrid, Spain

Geological studies have shown conclusively that the annual rates of erosion in the mountains has stayed constant, meaning they are getting lighter, not heavier, and so should show an increase in elevation, but this is not the case. The only explanation for this, according to Doctora Avril Primero, is that Madrid is becoming lighter than the mountains and at a faster rate.

This is a problem of balance. The Madrid Basin, an area of sedimentary geological deposits, sand, gypsum and clay, known locally as “Peñuela”, and on which the city rests, is of a relative lower density than the granitic mountains to the northwest. Madrid is the smaller child on the end of the see-saw. (Subibaja in Spanish) However, until recent excavations, this balance has been maintained, but now the sheer volume of rock and soil deposits being removed from under the city is causing severe problems. Put simply, Madrid now weighs less than it used to.

What could be the cause of this? Well Señora Primero thinks she has the answer. As the subsurface of Madrid has been removed in new tunnelling operations, the building of the new cercanías station at Sol, further extensions to the metro system, and excavations for new underground car parks, the density and mass of Madrid has decreased alarmingly. These excavations have led to Manileños describing their city as “Like a Gruyere cheese”. It is a reasonable analogy.

Excavating yet another station for the Madrid Metro.

Finding a spot near El Escorial, the famous town about 60 Km northwest of Madrid, where the elevation has not changed at all, but where the verticality of older buildings are now showing a demonstrable “lean”, Doctora Primero has declared this the site of a “Geological Fulcrum”.

Calculations using elementary physics show that although Madrid has not lost a similar amount of weight as the mountains, the lever principle, taught in schools to young children everywhere, would easily account for the vertical movement.

“Madrid’s elevation is increasing”, Señora Primero declared, “and like the rising end of a child’s see-saw is beginning to tilt alarmingly. Either the comunidad must stop removing soil and rock from beneath Madrid, or we should start urgent quarry work in the Guadarrama Mountains to redress the balance. The rise in the elevation to the south of the city has already led to the waters of the Manzanares River slowing to a trickle. If the plateau of Madrid continues to tilt in this manner, the consequences will be disastrous. How far do the Cuatro Torres have to lean before they fall over?” she asked.

The Cuatro Torres in the north of the city already show a slight lean.

That is a very good question to ask, this first day of April 2009.

11 comments:

  1. How can I keep up with all this information in my brain?.....you're killing me.

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  2. Great post. Fascinating material.

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  3. Jaja Señora Avril Primero, muy bueno!!!

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  4. I wish I knew who the anonymous commenters were. Why are you so shy?

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  5. There is no bovine defeacation quite like well educated bovine defeacation

    Nice one Richard

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  6. Had me intrigued until I twigged I was being duped when i saw the photos of the buildings. Nice one mate. You can buy me a beer now when I am over.

    Ian E. Facebook friend/Pueblo Friend/Nics friend/

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  7. Very cool - from one geophysicist to another, it is good to read about Madrid's geology. More shots of rocks would be cool.

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  8. Ha-Ha! Love it! Will I have to walk on a tilt when I come to Madrid in 3 weeks? Will the cerveza in my glass show a definite decline from the horizontal in my glass?

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  9. Hi
    interesting article, I am interested to know the full reference for the IGJ articel you mention.
    Thanks

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  10. I got there lots of helpful content and great information. Thanks a lot for your help.
    www.just4ubootcamp.com

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  11. Hi
    I have heard about GSI3D and want to find out more about how it was used in this study, can you help?

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