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Continental drift theory

 

We know that Earth formed around 4.5 billion years ago when swirling cosmic dust and gases surrounding the newly formed Sun were pulled together by gravity to form a sphere of rock. We know that Earth increased in size due to the gradual accumulation of rocks and that this process is known as accretion. We also know that the impact of these rocks smashing into each other created intense heat that caused the early Earth to melt, becoming a ball of hot soft rock. The rock contained elements, which are substances that cannot be broken down into any other substance. Examples of elements found in rocks include iron, aluminium, magnesium, and silicon.

 

During the meltdown of early Earth, gravity pulled most of Earth’s heaviest, metallic elements down and the lightest elements remained close the Earth’s surface. This separation of the elements created four layers, the inner core, outer core, mantle, and crust, or lithosphere. The heaviest metals sank to form the Earth’s core, which is about 5000-6000°C. At this point there was no land, no air, no water.

 

After the meltdown, Earth started to cool. This cooling caused the upper mantle to solidify and form a brittle, but solid crust. Processes in the mantle created openings in the Earth’s crust where lava erupted and formed the first volcanoes. In time, these volcanoes got bigger until they emerged from the oceans creating volcanic islands, like Iceland and the Azores. These island arcs (chains of islands) accreted to form continents. It is believed by some, that the first continents started forming around the same time as life on Earth, so 3.8 billion years ago.

 

Around 240 million years ago, between the Palaeozoic and Mesozoic Eras, almost all of Earth’s land masses joined to make one giant supercontinent. The supercontinent was named Pangaea and it was home to the dinosaurs. The name Pangaea comes from the Greek words ‘pan’ which means all, and ‘Gaia’ which means Earth. So, Pangaea means all of Earth. Around 150 million years ago towards the end of the Jurassic Period, the Pangaea started to break up into two continents, Laurasia and Gondwana.

 

About 100 million years ago, the two continents continued to break up into the recognisable continents we know today. Laurasia became North America and Eurasia, which consists of Europe and Asia. Gondwana became South America, Africa, Antarctica and Australasia, or Oceania. About 140 million years ago, part of east Africa broke away and was carried north across the Indian Ocean before it collided with Asia some 40 billion years ago and became India.

 

In 1912, Alfred Wegener, a German Meteorologist (someone that studies the atmosphere) noticed that if we closed the Atlantic Ocean, the nose of Brazil in South America would fit into the bight of West Africa, while the east coast of North America would border the Sahara Desert. Wegener proposed the idea that the continents were not fixed in place and did in fact drift across the Earth’s surface.

 

Wegener based his suggestion on much more than the fitting together of the continent’s coastlines. He also used evidence from fossils. For instance, fossils of Mesosaurus, a freshwater crocodile-like reptile that lived during the Permian Period of the Palaeozoic Era, are found in Southern Africa and Eastern South America. With small limbs, it would have been physiologically impossible for Mesosaurus to swim across the Atlantic Ocean. This supports the argument that Africa and South America were once joined.

 

Other evidence included mountains. Wegener argued that mountains with similar geology (rock type and age) had formed side-by-side and that the land had since drifted apart.

 

The proposed theory of continental drift was rejected by many because Wegener could not explain how the continents had moved apart.

Tectonic plate theory

 

We know that around 240 million years ago, between the Palaeozoic and Mesozoic Eras, almost all of Earth’s land masses joined to make one giant supercontinent, the Pangaea. Towards the end of the Jurassic Period, some 150 million years ago, the Pangaea started to break up into the recognisable continents we know today.

 

In 1912, using evidence from the shape of the continents, fossils and mountain ranges, Alfred Wegener proposed the theory of continental drift, but it was rejected because he could not explain how the continents had moved apart.

 

To explain how the continents had moved, we needed to turn to the oceans – until recently, we did not really know what lay below. It was during World War Two that Bruce Heezen, an American geologist, and oceanographer used sonar technology to collect thousands of depth measurements from across the Atlantic Ocean that colleague, Marie Tharp used to create the first map of the Atlantic Ocean floor. The map revealed a chain of underwater mountains that divided the Atlantic seafloor from north of Iceland to the tip of the Antarctic Peninsula. Peninsula comes from the Latin words ‘paene’ which means almost, and ‘insula’ which means island. Similar mountains can be observed in the Pacific, Indian and Southern oceans.

 

Harry Hess, an American Geologist suggested that the underwater mountain belts, or ocean ridges marked the places where oceanic crust is being created, slowly pushing the continents on either side away from each other. When magma rises to create new ocean crust, it can sometimes form wide and flat shield volcanoes as well as ocean ridges. As a consequence of this seafloor spreading and ridge push, the North American Plate and the Eurasian Plate move apart at 2-3cm per year. This ridge push partially explains how the continents have drifted from one another. Shallow earthquakes can occur because of friction as the ocean crust moves over the upper mantle.

 

Although new ocean crust is being created at ocean ridges, Earth is not getting bigger. This is because old ocean crust is being destroyed elsewhere. The graveyards for older ocean crust are subduction zones, or ocean trenches that mark the point at which oceanic crust sinks beneath continental crust, returning to the mantle. It is the subduction of the older crustal slabs that pulls the ocean crust apart making way for newer crust to form at the ridges. As the subducting crust sinks back into the mantle, it melts and magma erupts through the overriding crust to form tall, cone shaped composite volcanoes. As the crust subducts, friction with the overriding crust can cause the two slabs to become locked and the continuing force of the subducting slab causes pressure to build. When movement continues, the pressure is released, and deep, violent earthquakes occur. Mountain ranges can form where oceanic crust subducts beneath continental crust – one such example is the Andes. Mountains can also develop where continental crust collides – the Himalayas being an example.

 

From mapping ocean ridges, ocean trenches, and earthquakes it became clear that Earth’s surface was made up of rigid tectonic plates that move. The tectonic plates consist of the crust and the uppermost part of the mantle, which together form the lithosphere. Constructive plate margins occur where plates move away from each other while destructive plate margins occur where plates move towards each other. As well as moving apart and together, plates can also slide past each other, this does not result in the creation or destruction of crust, but the plates can become locked and large earthquakes can occur – the San Andreas fault serves as a good example. Where plates slide past each other, conservative plate margins occur.

 

All this knowledge is known as tectonic plate theory, and it explains how the continents have drifted. They are passengers on the moving tectonic plates. Plates move because of slab pull and ridge push.

Tectonic plate margins and earthquakes

 

We know that the Earth’s lithosphere is split into rigid (not much room for movement) tectonic plates that move across the heavier, semi molten rock in the mantle. Tectonic plate theory is a relatively new theory that explains how the continents have drifted. The continents themselves have not moved, but the tectonic plates they are on have moved. The major and bigger tectonic plates are named after the continents or oceans that are on top of them – there are around seven large and eight small tectonic plates. The main drivers of the movement are slab pull and ridge push.

 

Slab pull occurs where oceanic crust moves towards continental crust creating a destructive plate margin. The oceanic crust is denser (heavier) so sinks beneath the continental crust creating deep subduction zones known as ocean trenches. The edge of the continental crust is crumpled and folded upwards creating spectacular mountain belts such as the Andes. As the oceanic plate sinks back into the mantle, the crust is destroyed and it melts causing magma to rise through the overriding continental plate to form tall, cone shaped composite volcanoes. The subduction of the oceanic plate pulls the rest of the slab or tectonic plate along behind it. This forms openings where two plates start moving away from each other creating a constructive plate margin. It is important to note that two plates with continental crust can move towards each other creating collision zones and mountain ranges such as the Himalayas.   

 

Meanwhile, where two plates move apart at constructive plate margins, a rift is created. This allows semi molten rock in the mantle to melt and erupt as lava. Once cooled, the lava creates new oceanic crust lined with underwater mountains and wide, flat shield volcanoes. As new oceanic crust is formed, the stretching and fracturing of earlier formed crust and friction as the plates move over the mantle causes shallow and low-magnitude earthquakes. Over millions of years, the underwater mountains build up in height and emerge from the oceans to create volcanic islands such as Iceland and the Azores.

 

Plates also move past each other, side-by-side. This does not destroy or create crust, but it does result in very strong earthquakes. So, earthquakes happen in linear belts at all three tectonic plate margins.

 

An earthquake is a sudden or violent movement in the Earth’s crust that causes the ground to shake. Earthquakes happen when plates on either side of a tectonic plate margin interact and become stuck due to friction. Although stuck, the plates continue to move so pressure builds up. When the pressure becomes too much, one tectonic plate gives way and slips or jerks upwards. This causes all the stored energy to travel to the Earth’s surface as seismic waves. The point at which the plate slips, and the seismic waves start is called the focus. The point at the Earth’s surface directly above the focus is called the epicentre.

 

Earthquakes are measured using seismometers. They record the ground shaking. The amount of energy an earthquake gives out is called its magnitude. Earthquakes can have a magnitude between 0 and 10, although there have been no magnitude 10 earthquakes recorded. The stronger the seismic waves, the higher the magnitude. The strength of the earthquake reduces the further away it is from the focus.

Earthquake in L'Aquia, Italy, 2009

 

Earthquakes can be deadly and dangerous – they are natural hazards. A natural hazard is a natural event such as an earthquake that poses a risk to people and property. When we study the effects of an earthquake, we categories them into primary and secondary effects. Primary effects happen as a direct impact of the event, for instance, buildings collapsing as the ground shakes. Secondary effects happen as an indirect impact of the event, for instance, landslides blocking rivers.

 

Most earthquakes need some type of response. We categorise responses into immediate and long-term. Immediate responses involve the reactions of people, organisations, and governments as the disaster happens or in the immediate aftermath. These responses tend to focus on preserving life. Long-term responses occur in the weeks, months, and years after the event. These responses tend to focus on building back better.

 

L’Aquila is a 13th century city located in central Italy, some 60 miles north east of Rome, Italy’s capital. L’Aquila is located between the African and Eurasian tectonic plates. These tectonic plates are colliding at a destructive plate margin. This coming together of continental crust has created the Apennine Mountains that run throughout L’Aquila and Italy. Geologically, the Apennine Mountains are related to the Atlas Mountains in North Africa Africa.

 

On the 6th April 2009, a magnitude 6.3 earthquake struck central Italy at 03.32 AM with its epicentre near L’Aquila. The shaking of the ground caused damage to over 10,000 buildings with several collapsing. A total of 308 people lost their lives, and 1500 people were injured. Around 67,500 people were made homeless.

 

Powerful aftershocks were felt for at least two days after the main earthquake, some measuring 4.8 in magnitude. Aftershocks are earthquakes that occur as the displaced plate adjusts to its new position and are generally lower in magnitude. The aftershocks triggered landslides that caused damage to towns, individual houses, and the transport network. Unsafe buildings in the central business district (CBD) meant that it was temporarily cordoned off and closed to the public. Later, as a consequence of residential buildings being damaged or collapsing, the demand for housing increased. This led to higher rents and house prices.

 

Within an hour after the earthquake, the Italian Red Cross established search and rescue. The Italian Post Office offered free phone calls, raised funds, and provided free postage of small items to support businesses. Hotels provided accommodation for 10,000 people and 40,000 tents were given out. The EU gave $552.9 million in aid.

 

Some time after the earthquake, students were given free public transport and were exempt from university fees for three years. People were moved from tent camps and into houses within six months of the earthquake. The government announced a $42 million ‘Home Italy’ project focussed on building back better and restoring L’Aquila’s community.

Earthquake in Haiti, 2010

 

Not all countries can respond to earthquakes in the same way Italy did. For some, the effects are much worse, and the recovery takes much longer. In 2010, disaster struck Haiti, one of the poorest countries in the western hemisphere. Haiti occupies the western third of the Hispaniola Island in the West Indies/Caribbean. On the 12th January, a magnitude 7.0 earthquake occurred on a destructive plate margin between the Caribbean and North American tectonic plates. The focus was just under 13km deep, that is about 8 miles. The epicentre was 25.75km, that is about 16 miles south west of the capital, Port-au-Prince.

 

According to the Haitian government, the death toll was 316,000 but others put it at 220,000. Around 300,000 people were injured and 1.5 million were made homeless after some 300,000 buildings collapsed, including 8 hospitals.

 

Prior to the earthquake, Haiti had been free of cholera for 100 years. Cholera is a water-borne disease that causes severe diarrhoea and dehydration. Left untreated, it can kill. In October 2010, Haiti confirmed an outbreak of cholera. In 2016, the United Nations accepted that the outbreak was caused because raw sewage from peacekeeper’s camps was dumped directly into rivers from which thousands of Haitians collected water from for cooking and drinking. The outbreak has resulted in some 10,000 deaths.

 

With over a million people living in tent camps, there was an increase in crime, especially sexual violence against women and looting.

 

Immediately after the earthquake, the neighbouring Dominican Republic provided medical assistance and humanitarian aid (food, water, and shelter). The USA sent 10,000 troops to support with search and rescue and the distribution of aid. The UK sent fire and rescue teams with sniffer dogs and donated over £20 million. Later, over 200,000 Haitians were paid in wood and water to clear the rubble that lay on the streets. The EU donated $330 million, and the World Bank waived the country’s debt repayments for 5 years. Six months after the earthquake rubble continued to block vital access roads and no housing had been built for those living in tent camps.

Reducing the risks of earthquakes

 

Now that we have looked at the effects and responses to two earthquakes, one in a high-income country and the other in a low-income country, we are going to explore how the effects can be reduced. Earthquakes do not kill people. Buildings do. High-income countries have more money to spend on better infrastructure and emergency response efforts that reduce the effects after an earthquake. HICs can also build back quicker and are less reliant on aid and donations.

 

It is important to remember that earthquakes are natural events. Earthquakes only become a hazard when they pose a risk to people and property. We can reduce the effects of earthquakes, and other natural hazards through monitoring, prediction, protection, and planning.

 

Earthquakes are monitored using seismometers that record the ground shaking and this is how we determine their magnitude. Most earthquakes occur along tectonic plate margins so we know where they are likely to happen, but tectonic plates can stay locked for a long time. This makes it hard to predict exactly when and where an earthquake will happen. It is not possible to accurately predict earthquakes. This is one of the reasons why they are so deadly.

 

Perhaps a more effective strategy to reduce the effects from earthquakes is protection. Protection involves constructing buildings so that they are safe to live in and will not collapse. Some examples of building improvements are:

  • rubber shock absorbers in the foundations to absorb the earth tremors,
  • steel frames that can sway during earth movements,
  • open areas outside of the buildings where people can assemble during an evacuation,
  • buildings and bridges are built to resist the ground shaking because of an earthquake. 

The purpose of earthquake resistant buildings is to prevent death and injury. The buildings can still incur structural damage and need repairing. However, these buildings are expensive and LICs cannot afford it. Instead, LICs can use bamboo, a more affordable resource to construct bridges, or LICs can train masons to construct buildings with the largest bricks towards the bottom.

 

Planning and preparing what to do during and after an earthquake helps the authorities, emergency services and individuals to act quickly and calmly. Hazard maps can be created to show where earthquakes are likely, this will prevent buildings being built in earthquake prone areas. Regular earthquake drills help people keep alert and be prepared – the 1st September in Japan is Disaster Preparation Day. Individuals can also prepare by using a checklist provided by the Red Cross detailing what they should have in an emergency survival kit. This could help a family in the days after the event while waiting for aid to arrive.

Tsunamis

 

Tsunami comes from the Japanese words ‘tsu’ which means harbour, and ‘nami’ which means wave. The main cause of a tsunami is an underwater earthquake. Many earthquakes happen on the ocean floor near deep ocean trenches, or subduction zones where an oceanic plate sinks beneath another and back into the mantle due to slab pull. In some places, the oceanic plate may sink harmlessly beneath the overriding plate, but in others, the friction can be enormous, and two plates can become stuck, or locked together.

 

Although the two plates are locked, movement continues. As the lower plate subducts, it drags the locked section with it. This pulls the edge of the overriding plate downwards, distorting it so that another part of the same plate pushes upwards. This distortion creates a build-up of tension and energy.

 

Eventually, the locked section gives way. The edge of the overriding plate thrusts, or springs upwards and back to its original position. The creates an earthquake and shock waves caused from this movement pushes up a large column of water. This water comes back down and travels across the ocean towards the shore at a speed of up to 500 miles per hour – just as fast as a jet plane. These waves will become a tsunami. At 500 mph, tsunami waves can travel the entire Pacific Ocean in less than a day.

 

In the deep ocean, tsunami waves may be unnoticeable to those at sea because they start off small. As the waves approach the shore, friction with the rising seabed causes them to slow down but build up in height. Most tsunami waves are no bigger than 3 metres, but they can be as high as 10 metres. The tsunami’s trough, the low point beneath the crest, often reaches the shore first. This produces a vacuum effect that sucks coastal water towards the sea and exposes the harbour and sea floors – this retreating water is an important warning sign. When the wave arrives, it usually looks like a surging flood.

 

Tsunamis are not single waves. Tsunamis consist of multiple waves called a wave train. The first wave may not be the biggest.

 

The vast majority of tsunamis occur around the Pacific Ocean. However, they can and do occur elsewhere. On the 26th December 2004, a magnitude 9.2 earthquake triggered a 500mph tsunami that affected 14 countries around the Indian Ocean – it killed over 230,000 people. This is the deadliest tsunami on record and is sometimes known as the Boxing Day or Christmas Day Tsunami.

 

While the majority of tsunamis are caused by earthquakes in deep ocean trenches, tsunamis can also be caused by underwater volcanic eruptions, underwater landslides and even asteroids. Around 3.5 billion years ago, during the Archean Eon (one third of the Precambrian Eon), an asteroid hit our young Earth and triggered massive tsunamis. The asteroid was twice the size of the one that caused the extinction of the dinosaurs 66 million years ago at the end of the Mesozoic Era.

Tsunami in Japan, 2011

 

We know what causes a tsunami. On the ocean floor near deep ocean trenches, or subduction zones, an oceanic plate sinks beneath the overriding plate and back into the mantle due to slab pull. In some places, the oceanic plate may sink harmoniously beneath the overriding plate, but in others, the friction can be huge, and two plates can become stuck, or locked together. Although the two plates are locked, the movement continues. As the oceanic plate continues to subduct, it drags the locked section with it. This pulls the edge of the overriding plate downwards, distorting it so that another part of the same plate pushes upwards. This distortion creates a build-up of tension and energy.

 

Eventually, the locked section gives way. The edge of the overriding plate thrusts, or springs upwards and back to its original position. The creates an earthquake and shock waves caused from this movement pushes up a large column of water. This water comes back down and travels across the ocean towards the shore at a speed of up to 500 miles per hour – just as fast as a jet plane. These waves will become a tsunami.

 

In the Japan ocean trench, the Pacific plate is subducting beneath the overriding Eurasian plate because of slab pull – this creates a destructive plate margin. The Pacific plates moves fast, at a rate of 9cm a year. As the Pacific plate moves back into the mantle, it pulls the end of the Eurasian plate down with it. However, the plates become locked, and tension and energy build up. On the 11th March 2011, this energy was released, and the seafloor sprung upwards by 10 metres. This was the cause of a deadly tsunami. The sudden up thrust of the seafloor displaced the water above which spawned a series of waves in all directions. Estimates suggest that the largest waves were up to 39 metres high.

 

Japan’s investment in earthquake resistant buildings and design meant that only 100 people died as a direct result of the earthquake – significantly low for a magnitude 9.0 earthquake. However, the tsunami that surged over sea walls caused over 15,000 deaths and over 6,000 injuries, many of which were life changing. Over 2,500 people are still missing. More than 330,000 buildings, 2,100 roads, 56 bridged and 26 railways were damaged or destroyed. 300 hospitals were damaged and 11 were destroyed. Hundreds of thousands were made homeless or displaced. Over 23,000 hectares of farmland, mainly rice paddies were destroyed and salt left in the soil from sea water could affect crop growth for years. About 70,000 trees were lost to the sea. The waves flooded a nuclear power plant causing a nuclear disaster and leaving many without power. Radiation leaked from the plants forcing thousands to evacuate and an exclusion zone was put in place. Contaminated water is being stored in tanks with a vision to release it in the Pacific. The total damage to the Japan economy was over $200 billion.

 

Rescue teams and the army cleared roads and creates access paths to the worse affected areas. However, the amount of silt (fine sand) caused by the tsunami made efforts difficult. Field hospitals were set up and medics were flown in from other parts of Japan and many patients were flown out of the affected areas to receive treatment. The army helped to build temporary shelters. The Japanese Red Cross gave out 30,000 emergency relief kits. Later, the original 12m high tsunami barriers were replaced with 18m ones.

Earthquakes and economic development

 

We have looked at earthquakes in Italy and Haiti and tsunamis in Japan as examples of tectonic hazards. Tectonic hazards are a type of natural hazard, a natural event that poses a risk to people and property. A natural hazard becomes a natural disaster once it affects people and property.

 

Countries with high levels of development (HICs) can typically reduce the impact natural disasters have with regards to amount of people killed and affected, and damage to property and roads. This was the case in Japan after a magnitude 9.0 earthquake killed 100 people. Although it triggered one of the deadliest tsunamis on record, the actual impact of the initial earthquake was low for magnitude 9.0. This is because of Japan’s sector leading earthquake-proof design. Furthermore, Japan was able to respond to the devastating tsunami immediately and without relying on other countries. For LICs, this is not the case, and many are reliant on international support and humanitarian aid.

 

Natural disasters can have a significant impact on a country’s development, especially in low income countries. In Geography, development means improving the standard of living and quality of life in a place through economic growth, the use of technology and human welfare. For countries to sustain long-term economic growth, the standard of living must be maintained even after a natural disaster. This is difficult because the factors that contribute to economic growth such as people, farmland, transport networks, hospitals, and schools may have undergone substantial damage.

 

For example, destruction of hospitals and the deaths and casualties of doctors and nurses affects a country’s ability to treat patients after the event. Similarly, damage to schools and teacher deaths and casualties affects a country’s ability to supply education. For businesses, natural hazards destroy buildings and facilities, and the loss of people means a loss of workers. Furthermore, in the immediate aftermath of a natural disaster, many people are without food and clean water. Children are at risk of becoming malnourished which would affect their biological development and could result in illnesses and higher infant mortality rates. In addition, many children lose parents and therefore, their source of income.

 

Immediately then, a natural disaster would interrupt a country’s development and economic growth. This does not mean that GDP per capita will suddenly fall. GDP is the total sum of money made from the production of goods and services over a specified time, usually a year. Per capita means per person, so the GDP is divided by the total population. Sometimes, GDP per capita increases because there is a sudden surge of jobs involving the clearing of rubble and the rebuilding of the affected areas – many people migrate to the affected areas to fill those jobs. However, the speed at which the GPD per capita increases year-to-year, known as the growth, might fall. This means that the country’s development has been negatively affected by the natural disaster. For example, the GDP per capita in Haiti increased by 4.6% from 2008 to 2009 but decreased by 5.2% from 2009 to 2010. So, Haiti’s economic growth was impacted during 2010. GDP per capita did then increase again by 4.5% from 2010 to 2011 bringing economic growth back to levels before the earthquake. However, the rate of growth after 2010 has not exceeded the rate of growth before 2010, and Haiti has been negatively affected since.

 

However, there is plenty of evidence to suggest that these effects do not last and that actually natural hazards can boost economic growth. Disasters open up opportunities to rebuild and improve outcomes including reducing the risks from future natural hazards.