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 procescknown as accretion-involved the Earth and other planets adding to their masses through collisionswith other smaller chunks of rock, an extremely violent process that was mostly completed-fortunately for ucalmost 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 dino— saurs. 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 colli— sions, 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— ually. Convection currents continued to stir in the hot and partially molten rock below, carrying out the essential business 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.

Asa 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 thatwhich 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 (aprocess 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 subducting 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

A Volcanic eruptions ' Landslides i Earthquakes T Tornadoes

Q. Tropical cyclones ujj Floods

^ Tsunami jit Impacts ---Plate margins

Tunguska 1908 ^

Bangladesh -f

1998 X it -Tokyo 1923


^ Tsunami jit Impacts ---Plate margins

y f^Heimaey v Mt St Helens

y f^Heimaey v Mt St Helens

/ Vshensif J>t

Guiar at ¡^aW""" 1"1

' Tambora *"■«. katoa 1815 \

Kobe 1995 Northridge 1994'

Toba 73,000 BP



Kobe 1995 Northridge 1994'

/Huascarán 197oJ I

Mevado Del Ruiz-1985

/Lisbon 175S .

Oklahoma 1999

/ New Madrid f7- 1911

G Miami Soufrière Hills, 1992 { Montserrat 1995


/AD79 Armenia 1988

V Gizmit

Mevado Del Ruiz-1985

* Chile 19601

/Huascarán 197oJ I

8razil 1931

i Map of the Earth's plates with locations of recent natural disasters: the locations of many natural disasters coincide with the plate margins

Constructive plate margin (e.g. Mid-Atlantic ridge)

Mantle plume Destructive plate margin ot 'hot-spot' (e.g. Japan, western coast of (e.g. Hawaii) South America)

Constructive plate margin (e.g. Mid-Atlantic ridge)

Mantle plume Destructive plate margin ot 'hot-spot' (e.g. Japan, western coast of (e.g. Hawaii) South America)

2 The lithosphère, the Earth's outer rigid shell, is created at mid-ocean ridges and destroyed in subduction zones

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 plateswaltz 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.

Continue reading here: Hazardous Earth

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