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Tectonic Hazards - Part 1

  • Continental Drift
  • Structure of the Earth
  • Plate Tectonics and Boundaries
  • Causes of Plate Movement
  • Effects of Earthquakes in a LIC (Haiti, 2010) and HIC (L’Aquila, 2009)
  • Responses to Earthquakes in a LIC (Haiti, 2010) and HIC (L’Aquila, 2009)

What is a natural hazards?

Natural events such as earthquakes, volcanic eruptions and tsunamis occur everyday. However, when they pose risk to people or property they become a natural hazard. 

 

Natural Hazards occur due to processes inside Earth.

What are the layers of earth

Earth is made up of layers. There are four layers.

  • At the centre of the earth is a solid ball of iron and nickel; this is the earth's inner core. It is around 6000°C and 1260km thick
  • This is surrounded by an outer core which is a liquid mixture of iron and nickel. It is around 5000-6000°C and 2220km thick.
  • The largest layer of earth is the mantle. This is 4000°C and 2900km thick. It is made up of molten (melted) rock and has a viscous texture (a bit like the texture of jam). 
  • The outer layer of the earth is the crust. There are two types of crust; continental and oceanic. The temperature can range between 35 and 70°C

What is the plate tectonic theory?

 

Around 225 million years ago, all seven of our continents made up one big supercontinent known as the Pangaea. Around 150 million years ago, Pangaea started to break apart into the seven continents we have now. This is known as continental drift and in 1912, German meteorologist; Alfred Wegener put forward four pieces of evidence to support it. He argued:

  • some of the continents look as though they could fit together like a jigsaw. For example, the east coast of South America and the west coast of Africa.
  • similar fossils of animals that cannot swim in deep waters have been found on each continent, suggesting that at one time, animals must have been able to walk across all continents. 
  • there are mountain ranges and coal beds of similar ages in the west of the UK and the east of North America. 

This happened due to the natural processes in the earth's mantle and the continents still move today.

What causes tectonic plates to move?

 

At the time, Alfred Wegener could not explain why the continents moved and so his theory of continental drift was not accepted by scientists. However, some years later during World War Two, the United States Navy mapped the earth's ocean floor to help them locate enemy submarines. They noticed large mountain ranges and deep ocean trenches down the centre of ocean floors. The mountain ranges (known as ocean ridges) and ocean trenches mark the coming together of what we now know to be tectonic plates. 

 

Earth's crust is split into 9 major tectonic plates and several smaller plates. These plates are named after the land masses found on them. 

Scientists believed this was due to a new ocean floor being formed at the ridges and pushing the tectonic plates away from each other (this is known as ridge push). 

 

There are three major processes that cause tectonic plates to move. These are:

  • Convection Currents - this is circular movement of rocks in the mantle dragging the plates apart. Molten (melted) rock in the lower mantle is heated by the outer core, this makes the rock become less dense and therefore, it rises. As it moves further away from the outer core, it looses its heat source and becomes cooler and denser (heavier). As at result it sinks back down to the lower mantle, then it heats back up again and rises. The cycle is continuous. This circular motion pulls tectonic plates apart.
  • Ridge Push - at constructive plate margins, plates move away from each other creating a gap. This allows magma and molten rock to erupt from the earth's mantle. As it erupts it cools to forms new land. This is how Iceland was formed. This new land pushes the older land either side of it away from each other. 
  • Slab Pull - this is when the denser ocean plate is pulled by the force of gravity beneath a lighter continental plate. This is known as subduction and happens at destructive plate margins.

What are the tectonic plate margins and what happens at them?

 

There are four types of plate tectonic margins, this is where two tectonic plates meet. Different processes and landforms happen at each type of tectonic plate.

 

Constructive Margins

  • Two plates move apart.
  • Old crust is pushed away by new crust forming in the gap (hence the term ridge push).

Landforms occurring here include:

  • Shield Volcanoes
  • Mountains
  • Ocean Ridges

Example:

  • North American Plate and Eurasian Plate

Destructive Margins

  • An oceanic plate and a continental plate move towards each other.
  • The oceanic plate is denser (heavier) and is pulled beneath the continental plate by the force of gravity. This is called subduction.

Landforms occurring here include:

  • Composite Volcanoes
  • Mountains
  • Ocean Trenches

Example:

  • Nazca Plate (oceanic) and South American Plate (continental)

Conservative Margins

  • Two plates move past each other, this could be in opposite directions or the same direction.

Example:

  • Pacific Plate and North American Plate (San Andreas Fault)

Collision Margins

  • Two continental plates move towards each other.
  • This creates uplift of the land.

Landforms

  • Fold Mountains

 

Example:

  • Indian Plate and Eurasian Plate

What is an earthquake?

 

An earthquake is a sudden and violent shaking of the ground that lasts for seconds. Earthquakes happen when tectonic plates move. Sometimes, two plates move past each other and get stuck because of friction. This builds up pressure, when the pressure gets too much, one plate slips. This causes energy to be released in the form of seismic waves. 

 

The point where the earthquake starts is called the focus, this is within the earth's crust. The point on the earth's surface directly above the focus is called the epicentre. 

What are the effects and responses to earthquakes?

 

Earthquakes can effects humans in many ways. A primary effect is an effect that happens during or immediately after the event such as collapsed buildings, homelessness, burst pipes and death. Secondary effects happen after the event such as spread of diseases, tsunamis, landslides or fires.

 

Immediate responses happen straight away such as search and rescue, setting up evacuation centres and shelter or brining in medical supplies and field hospitals. Long-term responses happen later, weeks or months after the event. Examples of long-term responses include using international aid to help survivors, rebuilding the economy and building back better.

 

Effects of and responses to earthquakes differ between countries due to differences in GDP, strength of buildings, population density and disaster planning. Some countries are better equipped to cope with them than others.

L’Aquila, Italy 2009. 

Italy is a high income country.

Magnitude 6.3

 

Primary Effects

  • 308 dead and 1500 injured.
  • 67,500 made homeless.
  • 10,000 – 15,000 buildings collapsed.

Secondary Effects

  • Aftershocks triggered landslides.
  • House prices and rents increased.
  • CBD was cordoned off due to unsafe buildings.
  • Total damage cost of $16billion.

Immediate Responses

  • Italian Red Cross established search and rescue within an hour.
  • Italian Post Office offered free mobile calls.
  • EU gave $552.9million in aid.

Long-term Responses

  • Students were given free public transport, and were exempt from university fees for three years.
  • People were moved from camps to houses in 6 months.
  • $42million initiative ‘Home Italy’ to rebuild communities.

Haiti, 2010.  

Haiti is a low income country.

Magnitude 7.0

 

Primary Effects

  • Over 316,000 dead or missing and 300,000 injured.

  • 1.5 million made homeless.

  • Around 300,000 buildings collapsed.

Secondary Effects

  • Cholera broke out and claimed the lives of hundreds of children.

  • Total damage cost of $11.5billion.

Immediate Responses

  • Search and rescue.

  • UK sent fire and rescue teams with sniffer dogs.

  • UK raised over £100million in aid to send.

Long-term Responses

  • 3/4 of damaged buildings were inspected and repaired.

  • 200,000 people received cash or food and water for clearing rubble.

Questions

1) What is the difference between a natural event and a natural hazard?

2) Why didn't scientists believe Alfred Wegener's continental drift theory?

3) How did World War Two help us to support Alfred Wegener's continental drift theory?

4) Why were the effects of the earthquakes in Italy, 2009 and Haiti, 2010 different?

5) Are primary effects more significant than secondary effects?

6) Why do tectonic plates move?

7) How can responses to earthquakes reduce the effects?

8) How can countries reduce the effects of earthquakes?