Hazardous Earth While Earthquakes And Volcanic Eruptions example essay topic

A Guide To The End Of TheA Guide To The End Of The World: Everything You Never Wanted To Know By Bill McGuire A Guide to the End of the World: Everything You Never Wanted to Know by Bill McGuire Danger: Nature at work We are so used to seeing on our television screens the battered remains of cities pounded by earthquakes or the thousands of terrified refugees escaping from yet another volcanic blast that they no longer hold any surprise or fear for us, insulated as we are by distance and a lack of true empathy. Although not entirely immune to disaster themselves, the great majority of citizens fortunate enough to live in prosperous Europe, North America, or Oceania view great natural catastrophes as ephemeral events that occur in strange lands far, far away. Mildly interesting but only rarely impinging upon a daily existence within which a murder in a popular soap opera or a win by the local football team holds far more interest than 50,000 dead in a Venezuelan mudslide. Remarkably, such an attitude even prevails in regions of developed countries that are also susceptible to volcanic eruptions and earthquakes. Talk to the citizens of Mammoth in California about the threat of their local volcano exploding into life, or to the inhabitants of Memphis, Tennessee, about prospects for their city being levelled by a major quake, and they are likely to shrug and point out that they have far more immediate things to worry about. The only explanation is that these people are in denial.

They are quite aware that terrible disaster will strike at some point in the future – they just can't accept that it might happen to them or their descendants. When it comes to natural catastrophes on a global scale such an attitude is virtually omnipresent, pervading national governments, international agencies, multinational trading blocks, and much of the scientific community. There is some cause for optimism, however, and in one area, at least, this has begun to change. The threat to the Earth from asteroid and comet impacts is now common knowledge and the race is on to identify all those Earth-approaching asteroids that have the potential to stop the development of our race in its tracks.

Thanks to recent widely publicised television documentaries shown in the UK and United States, the added threats of volcanic super-eruptions and giant tsunamis have now also begun to reach an audience wider than the tight groups of scientists that work on these rather esoteric phenomena. In fact, the Earth is an extraordinarily fragile place that is fraught with danger: a tiny rock hurtling through space, wracked by violent movements of its crust and subject to dramatic climatic changes as its geophysical and orbital circumstances vary. Barely 10,000 years after the end of the Ice Age, the planet is sweltering in some of the highest temperatures in recent Earth history. At the same time, over-population and exploitation are dramatically increasing the vulnerability of modern society to natural catastrophes such as earthquakes, floods, and volcanic eruptions. In this introductory chapter, current threats to the planet and its people are examined as a prelude to consideration of the bigger threats to come.

The Earth is the most dynamic planet in our solar system, and it is this dynamism that has given us our protective magnetic field, our atmosphere, our oceans, and ultimately our lives. The very same geophysical features that make the Earth so life-giving and preserving also, however, make it dangerous. For example, the spectacular volcanoes that in the early history of our planet helped to generate the atmosphere and the oceans have in the last three centuries wiped out a quarter of a million people and injured countless others. At the same time, the rains that feed our rivers and provide us with the potable water that we need to survive have devastated huge tracts of the planet with floods that in recent years have been truly biblical in scale. In any single year since 1990 perhaps 20,000 were killed and tens of millions affected by raging floodwaters, and in 1998 major river floods in China and Bangladesh led to misery for literally hundreds of millions of their inhabitants.

I could go on in the same vein, describing how lives made enjoyable by a fresh fall of snow are swiftly ended when it avalanches, or how a fresh breeze that sets sailing dinghies skimming across the wave tops can soon transform itself into a wailing banshee of terrible destruction – but I think you get the picture. Nature provides us with all our needs but we must be very wary of its rapidly changing moods. The Earth: a potted biography The major global geophysical catastrophes that await us down the line are in fact just run-of-the-mill natural phenomena writ large. In order to understand them, therefore, it is essential to know a little about the Earth and how it functions. Here, I will sashay through the 4.6 billion years of Earth history, elucidating along the way those features that make our world so hazardous and our future upon it so precarious.

To begin, it is sometimes worth pondering upon just how incredibly old the Earth is, if only to appreciate the notion that just because we have not experienced a particular natural catastrophe before does not mean it has never happened, nor that it will not happen again. The Earth has been around just about long enough to ensure that anything nature can conjure up it already has. To give a true impression of the great age of our planet compared to that of our race, perhaps I can fall back on an analogy I have used before. Imagine the entirety of Earth's history represented by a team of runners tackling the three and a half laps of the 1,500 metres.

For the first lap our planet would be a barren wasteland of impacting asteroids and exploding volcanoes. During the next the planet would begin to cool, allowing the oceans to develop and the simplest life forms to appear. The geological period known as the Cambrian, which marked the real explosion of diverse life forms, would not begin until well after the bell has rung and the athletes are hurtling down the final straight of the last lap. As they battle for the tape, dinosaurs appear and then disappear while the leaders are only 25 metres from the finish. Where are we? Well, our most distant ancestors only make an appearance in the last split-second of the race, just as the exhausted winner breasts the tape.

Since the first single-celled organisms made their appearance billions of years ago, within sweltering chemical soups brooded over by a noxious atmosphere, life has struggled precariously to survive and evolve against a background of potentially lethal geophysical phenomena. Little has changed today, except perhaps the frequency of global catastrophes, and many on the planet still face a daily threat to life, limb, and livelihood from volcano, quake, flood, and storm. The natural perils that have battered our race in the past, and which constitute a growing future threat, have roots that extend back over 4 billion years to the creation of the solar system and the formation of the Earth from a disc of debris orbiting a primordial Sun. Like our sister planets, the Earth can be viewed as a lottery jackpot winner; one of only nine chunks of space debris out of original trillions that managed to grow and endure while the rest annihilated one another in spectacular collisions or were swept up by the larger lucky few with their stronger and more influential gravity fields. This sweeping-up process – known as accretion – involved the Earth and other planets adding to their masses through collisions with other smaller chunks of rock, an extremely violent process that was mostly completed – fortunately for us – almost 4 billion years ago.

After this time, the solar system was a much less cluttered place, with considerably less debris hurtling about and impacts on the planets less ubiquitous events. Nevertheless, major collisions between the Earth and asteroids and comets – respectively rocky and icy bodies that survived the enthusiastic spring cleaning during the early history of the solar system – are recognized throughout our planet's geological record. As I will discuss in Chapter 5, such collisions have been held responsible for a number of mass extinctions over the past half a billion years, including that which saw off the dinosaurs. Furthermore, the threat of asteroid and comet impacts is still very much with us, and over 300 Potentially Hazardous Asteroids (or PHAs) have already been identified that may come too close for comfort. The primordial Earth would have borne considerably more resemblance to our worst vision of hell than today's stunning blue planet.

The enormous heat generated by collisions, together with that produced by high concentrations of radioactive elements within the Earth, would have ensured that the entire surface was covered with a churning magma ocean, perhaps 400 kilometres deep. Temperatures at this time would have been comparable with some of the cooler stars, perhaps approaching 5,000 degrees Celsius. Inevitably, where molten rock met the bitter cold of space, heat was lost rapidly, allowing the outermost levels of the magma ocean to solidify to a thin crust. Although the continuously churning currents in the molten region immediately below repeatedly caused this to break into fragments and slide once again into the maelstrom, by about 2.7 billion years ago more stable and long-lived crust managed to develop and to thicken grad- u ally. Convection currents continued to stir in the hot and partially molten rock below, carrying out the essential busi- ness of transferring the heat from radioactive sources in the planet's deep interior into the growing rigid outer shell from where it was radiated into space.

The disruptive action of these currents ensured that the Earth's rigid outer layer was never a single, unbroken carapace, but instead comprised separate rocky plates that moved relative to one another on the backs of the sluggish convection currents. As a crust was forming, major changes were also occurring deep within the Earth's interior. Here, heavier elements – mainly iron and nickel – were slowly sinking under gravity towards the centre to form the planet's metallic core. At its heart, a ball made up largely of solid iron and nickel formed, but pressure and temperature conditions in the outer core were such that this remained molten. Being a liquid, this also rotated in sympathy with the Earth's rotation, in the process generating a magnetic field that protects life on the surface by blocking damaging radiation from space and provides us with a reliable means of navigation without which our pioneering ancestors would have found exploration – and returning home again – a much trickier business. For the last couple of billion years or so, things have quietened down considerably on the planet, and its structure and the geophysical processes that operate both within and at the surface have not changed a great deal.

Internally, the Earth has a threefold structure. A crust made up of low-density, mainly silicate, minerals incorporated into rocks formed by volcanic action, sedimentation, and burial; a partly molten mantle consisting of higher-density minerals, also silicates, and a composite core of iron and nickel with some impurities. Ultimately, the hazards that constantly impinge upon our society result from our planet's need to rid itself of the heat that is constantly generated in the interior by the decay of radioactive elements. As in the Earth's early history, this is carried towards the surface by convection currents within the mantle. These currents in turn constitute the engines that drive the great, rocky plates across the surface of the planet, and underpin the concept of plate tectonics, which geophysicists use to provide a framework for how the Earth operates geologically. The relative movements of the plates themselves, which comprise the crust and the uppermost rigid part of the mantle (together known as the lithosphere), are in turn directly related to the principal geological hazards – earthquakes and volcanoes, which are concentrated primarily along plate margins.

Here a number of interactions are possible. Two plates may scrape jerkily past one another, accumulating strain and releasing it periodically through destructive earthquakes. Examples of such conservative plate margins include the quake-prone San Andreas Fault that separates western California from the rest of the United States and Turkey's North Anatolian Fault, whose latest movement triggered a major earthquake in 1999. Alternatively, two plates may collide head on. If they both carry continents built from low-density granite rock, as with the Indian Ocean and Eurasian plates, then the result of collision is the growth of a high mountain range – in this case the Himalayas – and at the same time the generation of major quakes such as that which obliterated the Indian city of Bhuj in January 2001.

On the other hand, if an oceanic plate made of dense basalt hits a low-density continental plate then the former will plunge underneath, pushing back into the hot, convecting mantle. As one plate thrusts itself beneath the other (a process known as subduction) so large earthquakes are generated. Subduction is going on all around the Pacific Rim, ensuring high levels of seismic activity in Alaska, Japan, Taiwan, the Philippines, Chile, and elsewhere in the circum-Pacific region. This type of destructive plate margin – so called because one of the two colliding plates is destroyed – also hosts large numbers of active volcanoes. Although the mechanics of magma formation in such regions is sometimes complex, it is ultimately a result of the subduction process and owes much to the partial melting of the sub ducting plate as it is pushed down into ever hotter levels in the mantle. Fresh magma formed in this way rises as a result of its low density relative to the surrounding rocks, and blasts its way through the surface at volcanoes that are typically explosive and particularly hazardous.

Strings of literally hundreds of active and dormant volcanoes circle the Pacific, making up the legendary Ring of Fire, while others sit above subduction zones in the Caribbean and Indonesia. Virtually all large, lethal eruptions occur in these areas, and recent volcanic disasters have occurred at Pinatubo (Philippines) in 1991, Rabaul (Papua New Guinea) in 1994, and Montserrat (Lesser Antilles, Caribbean) from 1995 until the time of writing. To compensate for the consumption of some plate material, new rock must be created to take its place. This happens at so-called constructive plate margins, along which fresh magma rises from the mantle, solidifies, and pushes the plates on either side apart. This occurs beneath the oceans along a 40,000-kilometre long network of linear topographic highs known as the Mid-Ocean Ridge system, where newly created lithosphere exactly balances that which is lost back into the mantle at destructive margins. A major part of the Mid- Ocean Ridge system runs down the middle of the Atlantic Ocean, bisecting Iceland, and separating the Eurasian and African plates in the east from the North and South American plates in the west.

Here too there are both volcanoes and earthquakes, but the former tend to involve relatively mild eruptions and the latter are small. Driven by the mantle convection currents beneath, the plates waltz endlessly across the surface of the Earth, at about the same rate as fingernails grow, constantly modifying the appearance of our planet and ensuring that, given time, everywhere gets its fair share of earthquakes and volcanic eruptions. Hazardous Earth While earthquakes and volcanic eruptions are linked to how our planet functions geologically, other geophysical hazards are more dependent upon processes that operate in the Earth's atmosphere. Rather than the heat from the interior, our planet's weather machine is driven by energy from the Sun.

Our nearest star is the ultimate instigator – aided by the Earth's rotation and the constant exchange of energy and water with the oceans – of the tropical cyclones and floods that exact an enormous toll on life and property, particularly in developing countries. Still other lethal natural phenomena have a composite origin and are less easy to pigeonhole. The giant sea waves known as tsunamis (or sometimes incorrectly as ' tidal waves'), for example, can be formed in a number of different ways; most commonly by submarine earthquakes, but also by landslides into the ocean and by eruptions of coastal and island volcanoes. Similarly, many landslides result from a collusion between geology and meteorology, with torrential rainfall destabilizing already weak slopes.

Although there remains an enormous amount to learn about natural hazards, their causes and characteristics, our current level of knowledge is truly encyclopedic – and if so desired you can indeed consult weighty and authoritative tomes focused entirely on specific hazards. Here, as a taster, my intention is to gallop you through the principal features of the major natural hazards at a pace which I hope is not too great, before placing their current and future impact on our society in some perspective. At any single point and at any one time the Earth and its enclosing atmospheric envelope give the impression of being mundanely stable and benign. This is, however, an entirely misleading notion, with something like 1,400 earthquakes rocking the planet every day and a volcano erupting every week. Each year, the tropics are battered by up to 40 hurricanes, typhoons, and cyclones, while floods and landslides occur everywhere in numbers too great to keep track of.

In terms of the number of people affected – at least 100 million people a year – floods undoubtedly constitute the greatest of all natural hazards, a situation that is likely to continue given a future of rising sea levels and more extreme precipitation. River floods are respecters of neither wealth nor status, and both developed and developing countries have been severely afflicted in recent years, across every continent. Wherever rain is unusually torrential or persistent, it will not be long before river catchments fail to contain surface run-off and start to expand across their flood plains and beyond. In fact, the intensity of rainfall can be quite astonishing, with, in 1970, nearly 4 centimetres of rain falling in just 60 seconds on the French Caribbean island of Guadeloupe – a world record. On another French island, Reunion, in the Indian Ocean, a passing cyclone dropped close to 4 metres of rain during a single 24-hour period in March 1952. As flood plains all over the world become more crowded, the loss of life and damage to property caused by swollen rivers has increased dramatically.

In the spring of 1993, the Mississippi and Missouri rivers burst their banks, inundating nine Midwest states, destroying 50,000 homes and leaving damage totalling 20 billion US$. Massive floods occurred in many parts of the UK in autumn 2000 as rain fell with a ferocity not seen for over 300 years. River flooding continues to pose a major threat in China, and has been responsible for over 5 million deaths over the last 150 years. Bangladesh has it even worse, with the country often finding two-thirds of its land area under water as a result either of floodwaters pouring down the great Ganges river system or of cyclone-related storm surges pouring inland from the Bay of Bengal. Coastal flooding due to storms probably takes more lives than any other natural hazard, with an estimated 300,000 losing their lives in Bangladesh in 1970 and 15,000 at Orissa, northeast India, in 1999.

Partly through their effectiveness at spawning floods, but also through the enormous wind speeds achieved, storms constitute one of the most destructive of all natural hazards. Furthermore, because they are particularly common in some of the world's most affluent regions, they are responsible for some of the most costly natural disasters of all time. Every year, the Caribbean, the Gulf and southern states of the USA, and Japan are struck by tropical storms, while the UK and continental Europe suffer increasingly from severe and damaging winter storms. In 1992, Hurricane Andrew virtually obliterated southern Miami in one of the costliest natural disasters in US history, resulting in losses of 32 billion US$. This epic storm brought to bear on the city wind speeds of up to 300 kilometres per second, leaving 300,000 buildings damaged or destroyed and 150,000 homeless.

Destructive windstorms are not only confined to the tropics, and hurricane-force winds also accompany low-pressure weather systems at mid-latitudes. Many residents of southern England will remember the Great Storm of October 1987 that felled millions of trees with winds whose average speeds were clocked at just below hurricane force. More recently, in 1999, France suffered a similar ordeal as winter storm Lothar blasted its way across the north of the country. Across the ocean, the US Midwest braces itself every year for a savage onslaught from tornadoes: rotating maelstroms of solid wind that form during thunderstorms in the contact zone between cold, dry air from the north and warm, moist air from the tropics. No man-made structures that suffer a direct hit can withstand the average wind speeds of up to 500 kilometres an hour, and damage along a tornado track is usually total. Although rarely as lethal as hurricanes, in just a few days in April 1974 almost 150 tornadoes claimed over 300 lives in Kentucky, Tennessee, Alabama, and adjacent states.

Of the so-called geological hazards – earthquakes, volcanic eruptions, and landslides – there is no question that earthquakes are by far the most devastating. Every year about 3,000 quakes reach magnitude 6 on the well-known Richter Scale, which is large enough to cause significant damage and loss of life, particularly when they strike poorly constructed and ill-prepared population centres in developing countries. As previously mentioned, most large earthquakes are confined to distinct zones that coincide with the margins of plates. In recent years, sudden movements of California's San Andreas Fault have generated large earthquakes in San Francisco (1989) and southern California (1994), the latter causing damage totalling 35 billion US$ – the costliest natural disaster in US history. Just a year later, a magnitude 7.2 quake at the western margin of the Pacific plate devastated the Japanese city of Kobe, killing 6,000 and engendering economic losses totalling a staggering 200 billion US$ – the most expensive natural disaster of all time. Four years after Kobe, the North Anatolian Fault slipped just to the east of Istanbul, generating a severe quake that flattened the town of Izmit and neighbouring settlements and took over 17,000 lives.

Large earthquakes can also occur, however, at locations remote from plate margins, and have been known in northern Europe and the eastern USA, which are not regions of high seismic risk. The last such intra plate quake devastated the Bhuj region of India's Gujarat state in January 2001, completely destroying 400,000 buildings and killing perhaps as many as 100,000 people. There is a truism uttered by earthquake engineers: it is buildings not earthquakes that kill people. Without question this is the case, and both damage to property and loss of life could be drastically reduced if appropriate building codes were both applied and enforced.

Earthquakes, however, also prove lethal through the triggering of landslides as a result of ground shaking, and by the formation of tsunamis. The latter are generated when a quake instantaneously jerks upwards – perhaps by just a metre or so – a large area of the seabed, causing the displaced water above to hurtle outwards as a series of waves. When these enter shallow water they build in height – sometimes to 20 metres or more – and crash into coastal zones with extreme force. In 1998, Sis sano and neighbouring villages on the north coast of Papua New Guinea were wiped out and 3,000 of their inhabitants drowned or battered to death by a 17-metre-high tsunamis that struck within minutes of an offshore earthquake.

Estimates of the number of active volcanoes vary, but there are at least 1,500 and possibly over 3,000. Every year around 50 volcanoes erupt, some of which – like Kilauea on Hawaii or Stromboli in Italy – are almost constantly active. Others, however, may have been quiet for centuries or in some cases millennia and these tend to be the most destructive. The most violent volcanoes occur at destructive plate margins, where one plate is consuming another. Their outbursts rarely produce quiet flows of red lava and are more likely to blast enormous columns of ash and debris 20 kilometres or more into the atmosphere. Carried by the wind over huge areas, volcanic ash can be extremely disruptive, making travel difficult, damaging crops, poisoning livestock, and contaminating water supplies.

Just 30 centimetres or so of wet ash is sufficient to cause roofs to collapse while the fine component of dry ash can cause respiratory problems and illnesses such as silicosis. Close to an erupting volcano the depth of accumulated ash can total several metres, sufficient to bury single-storey structures. This was the fate of much of the town of Rabaul on the island of New Britain (Papua New Guinea), during the 1994 eruptions of its twin volcanoes Vulcan and Tavurvur. For years following the 1991 eruption of Pinatubo in the Philippines, thick deposits of volcanic debris provided a source for mudflows whenever a tropical cyclone passed overhead and dumped its load of rain.

Almost a decade later, mud pouring off the volcano was still clogging rivers, inundating towns and agricultural land, and damaging fisheries and coral reefs. Somewhat surprisingly, mudflows also constitute one of the biggest killers at active volcanoes. In 1985 a small eruption through the ice and snow fields of Columbia's Nevado del Ruiz volcano unleashed a torrent of mud out of all proportion to the size of the eruption, which poured down the valleys draining the volcano and buried the town of Ar mero and 23,000 of its inhabitants. Even scarier and more destructive than volcanic mudflows are pyroclastic flows or glowing avalanches. These hurricane-force blasts of incandescent gas, molten lava fragments, and blocks and boulders sometimes as large as houses have the power to obliterate everything in their paths.

In 1902, in the worst volcanic disaster of the twentieth century, pyroclastic flows from the Mont Pele volcano on the Caribbean island of Martinique annihilated the town of St Pierre as effectively as a nuclear bomb, within a few minutes leaving only two survivors out of a population of 29,000. The threat from volcanoes does not end there: chunks of rock collapsing from their flanks can trigger huge tsunamis, while noxious fumes can and have locally killed thousands and their livestock. Volcanic gases carried into the stratosphere, and from there around the planet, have modified the climate and led to miserable weather, crop failures, and health problems half a world away. On the grandest scale, volcanic super-eruptions have the potential to affect us all, through plunging the planet into a frigid volcanic winter and devastating harvests worldwide.