Volcanoes, earthquakes & tsunami pose the most frightening hazards, which is able to eradicate the lives of thousand within seconds. In this regard let me put across a few words from Antigone, by the Attic tragedian Sophocles (4967-406 B.C.), in the translation of Sir Richard Jebb, C.U.P., 1900 (Jakobsen, p. 57); Wonders are many, and none is more wonderful than man…. only against Death shall he call for aid in vain; but from baffling maladies he hath devised escape.
This year (2010) there were so many earthquakes that it is hardly a day, when we do not hear about it. The occurrence as narrated and visualized in television tells a tale of destruction which still remains visible in the eyes of the beholder. The word ‘tsunami’ may be a much more recent acquisition to our vocabulary, attained as a result of 26 December 2004, when a submarine earthquake near Sumatra displaced the sea water into devastating series of waves – a tsunami – that claimed nearly 300,000 lives around the shores of the Indian Ocean.
These Volcanoes undeniably produce impressive landscapes and those of us who are fortunate enough to have witnessed such an erupting volcano will carry to our graves indelible memories of an erupting volcano, the spectacle, the noise, the smell and the drama. But beauty has its worst side too. To save one from these types of disasters purely lies on ones position. If you are in a wrong place at the wrong time, you may not be able to save yourself. For other people, whose lives, health, homes and livelihoods are being destroyed or put at risk by an eruption – any sense of scientific curiosity is understandably displaced by more pressing personal concerns.
Generally it is often seen that eruptions are always associated with small earthquakes and that in some circumstances the eruption of a volcano is the cause of a small scale tsunami. Everything in this world is related. One cannot mitigate one parameter, without understanding the other’s ecological links. Mitigation can only be done, when we understand the ecology and its biodiversity outlooks and hence require specialists from various fields to come together and act. Natural Catastrophe Management may be the domain of trained civil defense people, but if one needs to mitigate the disasters and save billions of dollars of development from annihilation, one need to have effective understanding of Environment and our immediate surroundings – the things we do and things we should not do. It’s just not deforestation, it’s not about the construction of high rises buildings, it’s not about the exploitation of Mother Nature in the name of development but it’s about understanding the role of all these factors in context to Environmental Impact Assessment (EIA, 5-10 years of human existence). That’s the importance of these EIA.
It was the eruption of Krakatau (popularly referred to as Krakatao) which caused the tsunami in 1883. In addition to generating an ocean-crossing tsunami, a volcanic eruption can teach up and pluck aircraft from the sky. Let’s glance into the reasons as why does a volcano happen.
Molten rock at a depth is known to geologists as magma. Depending on its composition, magma solidifies when its temperature drops below about 1200 – 800 degree centigrade. This type of solidified rocks formed by solidified magma is described as an igneous rock. The term ‘igneous’ is derived from the Latin word ‘ignis’meaning fire. Thus an igneous rock made by solidification inside the earth’s surface is described as intrusive and is said to form an ‘igneous intrusion’. If the magma reaches the surface the resulting rock is called as volcanic.
When the molten rock reaches the surface it is generally called lava rather than the magma and if it flows in a stream across the surface, then this is described as a lava flow. Just to mention here that rocks of all types contain various minerals. When these rocks are in a molten state it is called Magma. These magmas may also have small crystals and bubbles of gas inside them. Magma will tend to rise upwards only if it is less dense than the solid rock that surrounds it. A close study reveals that the movement of the magma is restricted by its viscosity, which is a measure of how freely it is able to flow. One can compare and understand the amount of viscosity by taking the example of basalt (Common variety of magma, in fluid state) is about 100,000 times more viscous than water! This gives it the consistency of very thick porridge, so that it would not be able to escape up a narrow borehole.
The earth is composed of Core (Inner and Outer), Mantle (Lower and Upper) and the crust. The Inner core of the Earth is between 6370 Km to 5155 Km. The Outer Core is between 5155 Km to 2900 Km, the Lower Mantle is between 2900 Km to 670 Km, the Upper Mantle is between 670 Km to 90 Km/25 Km. The Crust is between 6-11 Km. Although the outer core’s chemical composition is uncertain, we can be sure that it is a liquid rather than a solid because of its effect on seismic waves. These are vibrations of various sorts emanating from earthquakes or underground explosions, which travel through the rock at speeds of several kilometers per second. The biggest earthquakes and the underground nuclear detonations generate seismic waves strong enough to pass right through the globe. When seismic waves encounter the outer core, those waves consisting of shearing vibrations (as inside a wobbling jelly), which is called the S waves, cannot travel through it and are either reflected or absorbed. This demonstrates that the outer core offers no resistance to shearing motions, and so must be liquid. Conversely, seismic waves that consist of alternating pulses of compression and dilation (like sound waves in air or water) called P Waves, can travel through it. There are other sorts of seismic waves that can travel only near the Earth’s surface.
Although the molten iron stew of the outer core has a surprisingly low viscosity (little more than that of water), it is much too dense to find its way up to the surface at volcanoes. However, it does make its presence felt at the surface through Earth’s magnetic field. This is a product of electrical currents in the outer core, which are generated because the molten material is in rapid circulation and is a good conductor of electricity. The core is surrounded by the mantle and overlying this mantle is the crust, which is relatively thin skin at the Earth’s Surface, accounting for less than 0.5 per cent of the Earth’s mass. The crust is richer in silicon and certain other elements than the mantle, so the varieties of silicate materials that are most common in the crust differ from those that characterize the mantle. However, the compositional difference between mantle and crust is trivial compared to the difference between mantle and core. There are two types of crust: one is the Oceanic Crust which is about 6-11 Km thick and mostly composed of basalt and constitutes the floor of deep oceans. Continental crust makes up the continents and floors of the shallow seas that are adjacent to most major land masses. It can be as thin as 25 Km where it has been thinned and stretched and as much as 90 Km thick below the highest mountain ranges where it has been buckled and compressed.
The elements which are mostly found in the earth’s crust are Silicon, Titanium, Aluminum, Iron, Magnesium, Calcium, Sodium, Potassium etc. Volcanoes generally occur where magma that has been generated at isolated patches in the mantle collects into sufficient volumes to be able to rise into the crust and make its way to the surface. The theory of plate tectonics describes the way in which the plates slide around and explains why most volcanoes occur where they do and the nature of the ground displacement during earthquakes. The Earth crust is firmly joined to the part of the mantle immediately beneath it. In most places, the top 100 Km or so of the mantle is just as strong and rigid as the crust, so that the crust and thus this uppermost mantle constitute a single mechanical layer. This layer is known as the lithosphere, a term chosen because it includes ‘lithos’, the Greek word for rock.
The lithosphere is rocky (in the familiar sense) in terms of both its composition and is strong and rigid nature. It ranges between 20 and 50 Km thick in the oceans and is typically about 150 Km thick under the continents. Each tectonic plate is a slab of lithosphere that can move around because the part of the mantle immediately beneath it is much weaker. This layer of the mantle is called the Asthenosphere(constructed from the Greek word for weak). The part, which is weak of the mantle, lies in few tens of Kilometers immediately below the base of the lithosphere, where there is evidence that a few percent of molten material may permeate along the interfaces between crystals. However, the proportion of this melt is so small that it is no more valid to think of this zone as molten or rather it is better to describe it as water-sodden brick as a liquid. However below the lithosphere there is an important change in the properties of the Earth’s rock that persists all the way to the core – although deep mantle is solid but it is not at rest. It is circulating at a speed of a few centimeters a year. However, that does not mean it is a liquid, certainly not so far as the transmission of seismic waves is concerned. The deep mantle’s slow flow is usually described as ‘solid-state convention’.
It’s this convention of current, what makes warm air to rise and cold air sink or water circulate in a saucepan (even before it boils). It is a way of transporting heat outwards. In the Earth’s solid mantle, convective forces cause it to circulate and thereby transfer the Earth’s internal heat outwards much more effectively than could be achieved simply by conduction through a motionless mantle. In fact, it is the efficiency of solid-state convention in the mantle that actually prevents the temperature getting quite hot enough to cause widespread melting. Put simply, hot mantle rises upwards & transfers its heat to the base of the lithosphere. Mantle that has lost heat in this way becomes slightly denser and sinks downwards again. Most of the heat deposited at the base of the lithosphere trickles through to the surface by conduction, but some is carried higher by pods of magma that can intrude high into the crust or even reach the surface at volcanoes. Often it is seen that most volcanoes occur independently of convection in the mantle and are a result of movements of the tectonic plates and these movements are possible only because only because the top of the Aesthenosphere is weak enough to allow them to happen. Volcanoes tend to be concentrated in well defined belts. These volcanoes during eruption also disturb the plate boundaries and are the cause of earthquakes and tsunami. A sudden change can be drastic and can eliminate thousands of human life.
According to computer models, somewhere near Toba, along the fault line there may be another super volcano getting ready for eruption. 3.1 mile sinking of Indo-Australian plate under the Euresian Plate in the last 74,000 years has created enough magma for a super volcano.
In the words of poet Stefanie Zammit,
‘Where distant screams haunt the nights,
And streets are filled with empty homes.
Where starving dogs are left to fight
Over lost men’s meat and children’s bones…
…When the smoke of burning men fills the air:
A smoke that no wind can fend.
When you take a breath and you declare:
This is when it really ends.’
Though these is just an assumption till now, but who knows when these volcanoes in well defined belts starts erupting and cause huge earthquakes all around the world to tell the final tale of human beings last annihilation story 2012.
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(Please Note: Incase, there is any mistake in the above data, kindly feel free to mail me at the e-mail address given below)
Thanks and Regards,
MAINAK MAJUMDAR
Disaster Management Specialist and Writer
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