Earthquakes and Volcanoes

The structure of the Earth and the moving plates

The Earth is built in three layers. The crust is the thin, cold, rocky outer shell: only about 5 to 10 km thick beneath the oceans, where it is made of dense basalt, and roughly 30 to 70 km thick beneath the continents, where it is made of lighter granite. Beneath the crust lies the mantle, a layer of hot rock about 2 900 km thick. The mantle is solid under ordinary conditions, but over millions of years it flows extremely slowly, like a very stiff tar, and its upper part is partly molten. At the centre is the core, made mainly of iron and nickel, with a liquid outer part and a solid inner part held solid by enormous pressure. The core is the reason the Earth has a magnetic field, and it is the source of much of the heat that drives the layers above it.

The crust is not one continuous shell. It is broken into about a dozen large tectonic plates, and many smaller ones, which fit together like the pieces of a badly made jigsaw. The plates are slabs of crust and the uppermost mantle, and the continents travel on them: Africa, Antarctica, Asia, Europe, North America, Oceania and South America are not fixed in place but drift with the plates they sit on. A plate moves only a few centimetres a year, about the speed at which a fingernail grows, which sounds harmless until you remember how much rock is being dragged and how much strain builds up when it is held back.

What moves the plates is convection currents in the mantle. Heat from the core, and heat released by the radioactive decay of rocks in the mantle, warms the rock at the bottom of the layer, so it expands, becomes less dense and rises. As it rises it cools, becomes denser and sinks back down, and this slow circulation drags the base of the plates with it. Where hot mantle rock rises towards the surface it pushes plates apart, and where old, cool and dense plate material sinks back into the mantle it pulls the rest of the plate along behind it.

Almost every earthquake and volcano on the planet sits on, or close to, one of these plate margins, because that is where the crust is broken and weak and where the strain and the heat are concentrated. One idea - the plates move, and their edges are the dangerous places - explains most of the map of natural hazards.

• Crust: about 5 to 10 km thick beneath the oceans (basalt) and 30 to 70 km thick beneath the continents (granite).
• Mantle: about 2 900 km of hot rock that flows very slowly; the partly molten upper part is the asthenosphere.
• Core: mainly iron and nickel, with a liquid outer part and a solid inner part, and the source of most of the Earth's internal heat.
• Continents: Africa, Antarctica, Asia, Europe, North America, Oceania and South America.
• Plates move a few centimetres a year, dragged along by convection currents in the mantle.

Plate boundaries and the distribution of the hazards

Constructive (divergent) plate boundaries. Two plates move apart. The gap they leave is filled by hot mantle rock rising towards the surface, which partly melts as the pressure on it falls and erupts as runny basalt lava; the lava solidifies to make new ocean floor, which is why this process is called sea-floor spreading. The classic example is the Mid-Atlantic Ridge, where the North American plate and the Eurasian plate are separating and the Atlantic is widening by a few centimetres a year. Eruptions here are usually quiet and non-explosive, they build broad shield volcanoes and long cracks called fissures, and on land the same movement can stretch the crust until it drops into a rift valley, as in the East African Rift. Iceland sits astride the Mid-Atlantic Ridge, which is one reason it erupts so often.

Destructive (convergent) plate boundaries. Here the plates move together, and one of them is forced down beneath the other. Because oceanic crust is denser than continental crust, the oceanic plate is the one that goes down, in a process called subduction. As it bends downwards it forms a deep ocean trench, and it carries water and sediment down with it. That water lowers the melting point of the mantle rock above, so the mantle partially melts and magma rises through the plate above, producing a line of volcanoes set back from the trench. This is why the edge of the Pacific is ringed with volcanoes: the Pacific Ring of Fire is a horse-shoe of trenches, island arcs and explosive volcanoes running from New Zealand through Japan, the Aleutians and the Americas. Earthquakes here can be very deep, because the cold, brittle sinking slab can still snap hundreds of kilometres down, and where the magma is trapped and sticky the eruption is violent.

Conservative plate boundaries. Two plates slide past each other, either in opposite directions or in the same direction at different speeds. No crust is created and none is destroyed, and no magma is produced, so there are no volcanoes at all. Instead the rough edges of the plates catch on one another; the rock bends elastically for years or centuries, and then suddenly breaks or jerks forward, releasing the stored energy as an earthquake. The San Andreas Fault in California marks the boundary between the Pacific and North American plates, the North Anatolian Fault runs across northern Turkey, and the Enriquillo-Plantain Garden Fault in Haiti lies on the boundary between the Caribbean and North American plates.

Not every volcano sits on a plate margin. A hotspot is a fixed plume of unusually hot rock rising from deep in the mantle, which burns through the plate above it and builds a volcano. Because the plate keeps moving while the plume stays still, the volcano is carried away from the hotspot and a new one forms over it, producing a chain of volcanoes that are older, lower and more eroded the further they are from the plume. The Hawaiian islands are the classic example, and the Yellowstone caldera in the United States sits over a hotspot in the middle of the North American plate, far from any boundary. Hotspots also explain why the map of earthquakes and volcanoes is not a perfect tracing of plate margins: most of the activity follows the margins, but there are isolated islands and swarms of tremors in the middle of plates.

Earthquakes: focus, epicentre, waves and magnitude

An earthquake is the shaking of the ground caused by the sudden release of energy stored in rocks. As plates move, the rocks along the boundary do not slide smoothly; they grip one another, bend and store energy like a stretched spring. When the rock finally breaks, or the two sides jerk past each other, the stored energy is released all at once as seismic waves. The point inside the Earth where the break happens is the focus, and the point on the Earth's surface directly above it is the epicentre. Everything else being equal, the damage is worst at and near the epicentre and gets less with distance from it.

The depth of the focus matters as much as the size of the earthquake. A shallow-focus earthquake, with its focus less than about 70 km down, releases its energy close to the surface, so the waves arrive with most of their energy still in them and the shaking is violent. A deep-focus earthquake of the same magnitude shakes the ground far less, because its energy has to travel through hundreds of kilometres of rock. This is also why destructive boundaries produce the deepest earthquakes: only the cold, brittle top of a sinking slab can snap, and the slab can still be brittle more than 600 km down.

There are three kinds of seismic wave, and they arrive in a fixed order because they travel at different speeds. P waves (primary waves) are the fastest; they push and pull the rock in the direction the wave is travelling, and they pass through solids, liquids and gases, which is how we know the outer core is liquid. S waves (secondary waves) are slower, they shake the rock from side to side, and they cannot travel through liquid at all, so they stop at the outer core. Surface waves travel along the surface, are the slowest of the three, and cause the greatest shaking and the greatest damage to buildings. A seismograph records the arrival of the waves, and because P waves arrive before S waves, the gap in time between them is used to work out how far away the earthquake was; three recording stations are needed to pin down the epicentre.

Magnitude is a measure of the size of an earthquake - the energy it released - and it is not the same thing as intensity, which describes how severe the shaking and the damage were in a particular place. The modern measure is the Moment Magnitude Scale (Mw), which has replaced the older Richter scale. Both are logarithmic, so each whole number represents about 32 times more energy released than the number below it; a magnitude 7 earthquake is therefore enormously more powerful than a magnitude 6, not just a little stronger. Magnitude is the same wherever it is measured, because it is calculated from the seismograph record. Intensity is not: it is high near the epicentre, low far away, and it also depends on how deep the focus was, what the ground is made of and how well the buildings were built. That is why magnitude alone never tells you how many people will die.

Volcanoes: magma, lava and what an eruption produces

Magma is molten rock below the surface. It forms where the mantle partially melts, usually because pressure has fallen where plates are moving apart or because water carried down by a sinking plate has lowered the melting point above it. Being less dense than the rock around it, magma rises, and it collects in a magma chamber beneath the volcano. Pressure from the dissolved gas it contains, and from the weight of the rock above, pushes it up a vent to the surface, where it erupts from a crater or, in a large fissure eruption, from a long crack in the ground. Once magma reaches the surface it is called lava.

What the eruption is like depends mostly on the silica content of the magma, because silica makes magma sticky. Basaltic magma is low in silica and runny, so the gas dissolved in it escapes easily, the lava flows away quietly and the volcano it builds is a broad, gently sloping shield volcano, such as those of Hawaii or Iceland. Andesitic and rhyolitic magma is high in silica and far more viscous, so gas cannot escape and pressure builds up inside it until the magma is shattered and blasted out. The result is an explosive eruption and a steep-sided composite (strato) volcano, such as Mount Fuji in Japan or Mount Merapi in Indonesia. This is the link between the boundaries and the eruption style: destructive margins produce explosive volcanoes because subduction adds water to the mantle and makes sticky, gas-rich magma, while constructive margins produce quiet ones because the basalt is runny and the gas escapes.

An eruption produces more than lava, and a good description names the materials. Lava flows out of the vent and can travel several kilometres, burying farmland, roads and houses and setting fire to whatever it touches. Pyroclastic flows are the most lethal hazard of all: mixtures of hot gas, ash and rock fragments at several hundred degrees Celsius that sweep down the slopes faster than a car can drive, destroying everything in their path. Volcanic bombs are blobs of molten lava thrown out of the vent which spin and cool as they fly and land as rounded or twisted lumps of rock. Ash is pulverised rock and magma blasted high into the atmosphere, where it may drift thousands of kilometres; it collapses roofs under its weight, chokes engines, kills crops and stops aircraft flying.

The gases matter too. Most of the gas released is water vapour, which is harmless by itself, but eruptions also release carbon dioxide and sulfur dioxide. Carbon dioxide is denser than air, so it flows downhill and collects in valleys, hollows and cellars, where it can suffocate people and animals; at Lake Nyos in Cameroon in 1986, a cloud of carbon dioxide released from a volcanic crater lake flowed into the surrounding valleys and killed around 1 700 people in a single night. Sulfur dioxide reacts with water vapour and oxygen in the atmosphere to form droplets of sulfuric acid, so a large eruption causes acid rain over a wide area downwind, burning leaves and lowering crop yields, and in a very large eruption the same droplets reflect sunlight and cool the climate slightly for a year or two.

• Magma below the surface becomes lava when it reaches the vent; the lava may flow, or be blasted out as ash, bombs and pyroclastic flows.
• Basaltic magma (low silica): runny, gas escapes easily, quiet eruptions, broad shield volcanoes.
• Andesitic and rhyolitic magma (high silica): viscous, gas trapped, explosive eruptions, steep composite volcanoes.
• Ash: pulverised rock and magma, carried high into the atmosphere, collapsing roofs and stopping aircraft.
• Volcanic bombs: lumps of molten lava that spin and cool in flight and land as rounded or twisted rocks.
• Pyroclastic flows: fast, searing clouds of gas, ash and rock that destroy everything on the slopes they cross.
• Gases: water vapour, carbon dioxide (denser than air, collects in valleys) and sulfur dioxide (forms sulfuric acid and acid rain).

Hazards: what tectonic events do to people and the environment

The hazards of a tectonic event are sorted into primary hazards, which are the direct result of the event itself, and secondary hazards, which follow from it and often kill more people than the event did. The primary hazards of an earthquake are ground shaking, the tearing of the surface along the fault, and the collapse of buildings, bridges and pipelines that the shaking causes; the primary hazards of an eruption are lava flows, ash fall, volcanic bombs, pyroclastic flows and the gases released from the vent.

The secondary hazards are where most of the deaths are. Fires break out when gas mains and power cables are ruptured and the water mains that would fight them are broken at the same time, as in San Francisco in 1906 and in Kobe, Japan, in 1995. Tsunamis are generated where an earthquake lifts or drops the sea floor, sending a wave across an ocean at the speed of a jet aircraft; the 2004 Indian Ocean tsunami reached the coast of Sumatra within minutes and killed more than 200 000 people in 14 countries. Landslides and mudflows follow when shaking or heavy rain on an ash-covered slope makes the ground slide; on volcanoes, rain mixing with ash and loose debris makes lahars, mudflows that move down valleys like wet concrete and bury everything in them, as happened at Armero in Colombia in 1985 when a lahar from the Nevado del Ruiz volcano destroyed the town and killed more than 20 000 people. Contamination of drinking-water supplies is the quiet killer: broken pipes, sewers and treatment works mix sewage into the water that people drink, which is how a tectonic event leads directly to water-related disease such as cholera and typhoid.

The impacts on the environment and on livelihoods take years to repair. Crops are flattened, buried in ash or killed when ash coats the leaves and stops them photosynthesising; livestock are killed and injured; habitats such as forests, coral reefs, mangroves and river banks are burnt, buried or smothered, and the loss of vegetation on slopes leaves them exposed to erosion and further landslides for years afterwards. Rivers can be dammed by landslides and altered in course, and farmland downstream is then flooded or starved of water and sediment. Evacuation of people and livestock places a further burden on the region: families are moved into temporary camps or to relatives in other districts, animals that cannot be moved are lost, and the area loses the labour it needs for the harvest.

Financial losses and the effects on human health run through everything. Houses, schools, hospitals, roads, bridges, power stations, ports and factories are damaged or destroyed; businesses close and jobs disappear; a government loses tax revenue at exactly the moment it must spend the most; and the cost of rebuilding can be greater than the country's entire annual income. Health suffers in three waves: deaths and injuries from crush injuries and burns at the time of the event; disease, dehydration and hypothermia in the days afterwards, especially in crowded shelters with no clean water or sanitation; and, long after the rubble has been cleared, depression, anxiety and post-traumatic stress among survivors who have lost their homes, their families and their livelihoods. Money is what separates these outcomes. A wealthy country can afford buildings that stay standing, warning systems, drills and search and rescue teams, while a poor country hit by an earthquake of the same magnitude loses many more people, and the financial loss as a share of national income is far greater.

Managing the impacts before, during and after the event

Monitoring and warning. Volcanoes can be watched, because magma on the move gives warning signs: swarms of small earthquakes as the rock cracks, the ground swelling and tilting as the magma chamber fills, changes in the temperature and chemical composition of the gases and springs, and swelling of the whole mountain that satellites can measure from space. A network of seismometers, tiltmeters, GPS stations and gas sensors allows an eruption to be forecast - not to the exact hour, but well enough to evacuate the area in the days before it happens, which is what happened at Grindavik in Iceland, where the town was evacuated ahead of the Reykjanes fissure eruptions; most of it was saved, although lava destroyed some homes on its edge. Earthquakes are the harder problem: after decades of research there is still no reliable precursor, so they cannot be predicted, and effort goes instead into detecting the shaking within seconds and sending an automatic warning that reaches people before the slower, more damaging waves arrive, as in Japan's earthquake early warning system. The limitations are equally clear: monitoring hardware and trained staff are expensive, instruments have to be maintained and powered, an eruption may begin with no useful warning at all, and false alarms are both costly and destructive of public trust.

Hazard mapping and land-use zoning. A hazard map divides the land around a fault or a volcano into zones according to the risk, from the valleys that a pyroclastic flow or lahar would follow and the ground immediately beside a fault, to land that is unlikely to be affected. Planning rules then use the map to keep new housing, schools and hospitals out of the most dangerous zones, to reserve land for evacuation routes, and to mark the safe ground where shelters and medical posts will be set up. The limitation is enforcement: the highest-risk land is often the cheapest and the most densely settled, poor families cannot afford to move, and a government with limited resources struggles to stop informal building. A hazard map also protects nobody who is already living inside the red zone, and maps are only as reliable as the eruption history and the seismic data behind them.

The structure of buildings, education and preparation. Building codes make the difference between an earthquake that kills thousands and one that damages property. Reinforced concrete frames, steel cross-bracing, shear walls and base isolation allow a building to sway without collapsing, and light roofs, strong joints and flexible gas connections remove the weight and the fire risk. Older buildings can be retrofitted by tying brickwork together and wrapping columns in steel or carbon fibre. Alongside the buildings, people must be prepared: schools teach the warning signs, run drills so that people know to drop, cover and hold on or to move to high ground, families keep emergency supplies of water, food, a torch and a radio, and emergency rescue teams train with hydraulic cutting gear, listening devices and thermal cameras so that they can pull people out of collapsed buildings in the first hours, when most survivors are still alive. The limitation of all of this is cost and upkeep: codes add to the price of every building and are only as good as the inspectors who enforce them, retrofitting is expensive and disruptive, and drills and supplies are soon forgotten in a country that has not been shaken for a generation.

Evacuation, shelters and the response during and after the event. When a warning is issued, evacuation is the most effective single measure available: getting people and, where possible, livestock out of the danger zone before the event costs almost nothing and saves the most lives, provided there is a warning, a road, transport, somewhere safe to go and people willing to leave their homes, animals and belongings. After the event, shelters on safe ground with clean water, latrines, food and medical care stop the death toll rising through disease, and rebuilding programmes are the chance to build back better, by rehousing people on safer land, enforcing the building codes properly this time, restoring the water and sewerage systems first and strengthening schools and hospitals. International aid arrives as search and rescue and medical teams in the first days, as food, water, tents and medicine in the following weeks, and as money and technical help for reconstruction later. The limitations are real: aid is slow to arrive and is unloaded at a port or airport, not on the street where it is needed, it is often badly coordinated between many agencies, it can be diverted by corruption or by politics, and in the long run it can create dependency and leave the government less prepared for the next event.

Insurance and insurance-like schemes spread the cost of the damage over many years and many places, and they influence what is built: an insurer will charge less for a building that meets a code and may refuse to insure one that does not. They are, however, unavailable or unaffordable to the poorest households, and after a very large disaster insurers may simply withdraw cover from the affected region, which makes rebuilding without state help impossible. Weighing the strategies. Split them into two groups and the judgement writes itself. The strategies that work before an event - monitoring, warning, hazard maps, land-use zoning, building codes, education, drills, supplies and evacuation - prevent the disaster, are permanent, are far cheaper than rebuilding and protect everyone in the region whatever their income, but they must be funded every year whether or not anything happens, which is hard to justify to a government, and they are weakened by corrupt or unenforced planning rules. The strategies that work during and after an event - rescue, shelters, aid, insurance and rebuilding - are visible, politically popular and unavoidable once people have been buried, but they arrive after the deaths, they cost far more, and they leave the same risk in place for the next earthquake. The defensible judgement is therefore that money spent before the event matters most, and that it matters almost entirely in poor countries with fast-growing cities, which are the places where safe buildings, enforced planning and working evacuation plans are least affordable and where the next earthquake will kill the most people.

Living with volcanoes: the opportunities they provide

Fertile soils. Volcanic ash and lava weather into some of the most fertile soil on Earth. They release potassium and phosphate as they break down, and the resulting andosols are deep, well drained and easy to work, so the land around active volcanoes is often densely farmed even though it is dangerous. Java in Indonesia, one of the most densely populated islands in the world, farms the slopes of its volcanoes for rice, vegetables and coffee on soils renewed by each eruption, and the same is true around Mount Etna in Sicily and in the volcanic highlands of East Africa. This is the central trade-off of the topic: the fertility that attracts farmers is produced by the same processes that kill them.

Extraction of minerals. Hot water and gases circulating through volcanic rock dissolve metals and deposit them in veins, which is why many of the world's copper, gold, silver, lead and zinc deposits sit in old volcanic and subduction belts; the copper mines of the Andes in Chile and Peru are the clearest example. Sulfur is quarried from the craters and fumaroles of active volcanoes for use in fertilisers, sulfuric acid and rubber, and the same sulfur is a hazard as well as a resource. Where whole towns depend on the mine, the geological wealth becomes an economic reason to stay on ground that has been destroyed before.

Building materials. The rock an eruption leaves behind is useful as well as fertile. Basalt and pumice are quarried for road stone, aggregate and building blocks, volcanic tuff is soft enough to cut into blocks that harden when exposed to air, and ash is blended into cement. Pozzolana, a volcanic ash from the Bay of Naples, was mixed with lime by the Romans to make concrete that sets underwater, and volcanic ash is still used in some countries as a cheap substitute for part of the cement in concrete.

Geothermal power. Where magma lies close to the surface, it heats the rock and the groundwater around it. Wells are drilled down into the hot rock and the steam and hot water that come up drive turbines to make electricity or are piped through the streets to heat buildings directly. Iceland heats the great majority of its homes with geothermal water, and the Wairakei field in the Taupo Volcanic Zone of New Zealand, opened in 1958, was one of the first geothermal power stations in the world. The fuel is free and available day and night, unlike wind or solar power, and it produces no sulfur dioxide or carbon dioxide, but the resource exists only where the geology provides it, the drilling and plant are expensive, the hot water can carry dissolved sulfur and other chemicals that corrode equipment and must be handled, and over-use of a field can cool it down.

Tourism. Volcanoes attract visitors: Yellowstone National Park in the United States draws millions of people to its geysers and hot springs, the geothermal pools of Iceland and the hot springs of Japan are major attractions, and people climb or trek to the craters of Mount Fuji, Mount Rinjani and Santorini. Tourism brings hotels, restaurants, guiding, transport and park fees into regions with few other sources of income, and it makes the volcano visible to politicians, which helps to keep the monitoring budget funded. It also brings risks of its own: tourists who do not know the local hazard arrangements place themselves in danger and can delay the evacuation of residents. The sensible conclusion for a country is neither to abandon the volcano nor to ignore it, but to farm, drill, quarry and visit it with monitoring, hazard maps, building codes and evacuation plans in place.