Plate Tectonics: An Introduction

Continental Drift and the Structure of the Earth

Image courtesy of USGS

Our next unit is the study of plate tectonics. In this unit we will be studying the forces of nature which have shaped our planet including the processes behind natural hazards such as earthquakes and volcanoes. We will also be considering the impact that such hazards have on people across the world. It is believed that out continents have not always been in their present configuration and that over millions of years our continents have changed their position (see animation). This theory is known as continental drift.

Millions of years ago there was one supercontinent called Pangea. Over time this has split into smaller continents which have gradually moved into the positions in which they exist today. There are various pieces of evidence for this including the apparent jigsaw fit between the east coast of South America and the west coast of Africa.

In order to understand how this is possible we need to consider the structure of the earth.

The earth is up to 6,000km in radius from the inner core to the surface. It is made up of four main layers. The surface layer is known as the crust. This is the relatively thin layer on which we live and it consists of solid rock. The crust 'floats' on top of the mantle. The mantle has very high temperatures resulting in rock being in a 'molten' state. This 'molten' rock is known as magma and is able to move. At the centre of the earth is the core. This is divided into the outer and the inner core. The outer core is partly molten whilst the inner core is solid, this is due to the extreme temperature and pressures which exist here, with temperatures reaching up to 5,000 oC.

Follow up Links:
Some useful and clear animations showing the movement of the continents over millions of years in the process of continental drift can be found here:

- Continental Drift

- The 'fit of the continents' (evidence for continents having moved)

- Sea floor spreading showing the break up of Pangea

Check out this excellent site for more information about the structure of the earth and its layers. Further detail on the structure can be found here.

Key Terms Check:

Continental Drift - the theory that our continents have changed their postion over time
Crust - the outer layer of the earth (up to 75km thick)
Mantle - the middle and thickest layer of the earth part of which is semi-molten in nature
Outer Core - the outer layer of the core is semi-molten
Inner Core - central part of the earth which is solid due to extreme temperature and pressure
Magma - molten rock in the mantle

Rivers Revision

Revising the Rivers Unit
We have now come to the end of the Rivers Unit and its time to revise! Here are some resources to help you..

Check list of key concepts to revise:

1. Hydrological Cycle - key terms (and understanding of inputs, stores & processes)
2. Drainage Basin - remember this is the land based part of the Hydrological Cycle - learn the key terms (both of drainage basin features e.g. source, watershed etc. and processes e.g. throughflow) and their meanings and be able to distinguish between inputs, stores, processes & outputs
3. River Processes - erosion, transport and deposition (Learn them!)
   
4. The course of a river - you should know the main changes in both river channel and valley as it passes from source to mouth
5. River Features:- (remember you need to be able to describe and explain the characteristics / formation of each - remember to talk about processes involved) - these include...
   
6. Upper Course of the River - v-shaped valleys and waterfalls
7. Middle Course of the River - meanders and ox-bow lakes
8. Lower Course of the River - floodplains and levees
9. Hydrographs - hydrograph features and terms (lag time, discharge, peak rainfall, peak discharge, rising limb, falling limb) and factors affecting hydrographs (e.g. land-use, basin shape etc.)
10. Case study of Flooding in an MEDC - Lynmouth 1952
11. Case study of Flooding in an LEDC - Bangladesh 1998 floods
12. Contrasting flooding between MEDCs and LEDCs (reasons for differences)

Revision Resources:

• - make good use of your class notes
  
• - make use of the blog posts to consolidate your understanding / go over anything you are not sure on (to access previous posts - use blog archive list on the left hand side of the blog) - remember there are various links to animations etc. to help you!

Interactive Revision Quizzes:
You must learn your notes (particularly case study detail) but once you have revised from your notes there are lots of interactive revision quizzes etc. here for you to test yourself! (if you spot any mistake - e-mail me!)

• - Hydrological Cycle - flash cards
• - Hydrological Cycle - key word test
• - Hydrological Cycle Countdown Conundrums
• - Rivers Glossary - key word flash cards
• - Rivers Glossary - key word test
• - River Erosion - simple drag and drop
• - River Transport - simple drag and drop
• - River Features - simple drag and drop
• - Drainage Basin Features - Drag and Drop
• - Drainage Basin Key Terms Countdown Conundrum
• - Rivers Revision Quiz
• - Rivers Crossword Quiz
• - Penalty Shootout - Rapid Rivers Revision (Quiz 1)
• - Multiple Choice - Rapid Rivers Revision (Quiz 2)
• - Walk the Plank - Hydrological Cycle/ Drainage Basins

Comparing flooding in MEDCs and LEDCs

Comparing the effects of flooding in MEDCs and LEDCs

Having studied two flood case studies in countries with contrasting levels of economic development you will have seen that MEDCs usually have much better flood protection than LEDCs and subsequently the effects of flooding are often not as severe in terms of loss of life. However flooding still occurs in MEDCs and as well as some loss of life, the level of economic disruption can still be significant. You need to be aware of the main reasons for the differences in the level of disruption caused by flooding in LEDCs and MEDCs:

Problems faced in LEDCs which make the effects of flooding worse and flood management difficult:

• - poor quality housing can't withstand flood waters
• - poor infrastructure is easily damaged with roads, bridges and communications destroyed by flooding
• - lack of sanitation and clean water supplies resulting in further loss of life during floods through the spread of diseases such as cholera, dysentery etc.
• - difficult to mobilise rescue teams - lack of funding for training but made more difficult by many areas being isolated during flooding due to damage to infrastucture and inundated by flood waters;
• - little political co-operation between Bangladesh and its neighbouring countries - makes it difficult to reduce flood risks by tackling issues in the headwaters of the major rivers which are located in India and Nepal;
• - the country relies on government aid and aid from other countries - with a lack of money many necessary flood defences can not be constructed
• - in order to tackle poverty the government have focused much of their funds on improving exports - again reducing the money available for flood protection.

Reasons why the effects of flooding usually less severe in MEDCs and flood protection is better:

• - homes and possessions are able to be insured against flood damage
  
• - good water and sewage systems are in place providing back up supplies of clean water when local supplies become contaminated - means that disease is not the problem it is during flooding in LEDCs;
• - good infrastructure and communication networks means it is easier to get aid and helpworkers to affected areas increasing survival through rescues and evacuation;
• - planning restrictions are usually in place to discourage new building of houses on floodplain areas or areas prone to flooding;
• - governments in MEDCs are able to invest more heavily in flood defence systems - including channelisation projects; the construction of artifical levees and the development of prediction and warning systems.

You should be prepared in an exam to be able to contrast the effects of flooding in MEDCs and LEDCs and to be able to suggest reasons for the differences.

Flooding in an LEDC - The 1998 Floods in Bangladesh

BANGLADESH FLOODS

Between July-September 1998, Bangladesh suffered one of its worse ever floods. Despite being flooding being common in this country, the floods of 1998 were particularly severe resulting in over 1000 deaths and 30 million people being made homeless and newspapers / media sources were full of headlines like the following; South Asia - Bangladesh Floods Rise again (BBC Article) and Floods threaten 20 million lives in Bangladesh.

So why is Bangladesh so prone to flooding? Well the answer to this requires consideration of both the physical landscape and conditions of the country and the impact of its population.

CAUSES OF FLOODING IN BANGLADESH

Physical (Natural) causes of flooding in Bangladesh

1. Bangladesh is a very low lying country, with 70% of its land area being less than 1m above sea level and 80% of it being floodplain.
2. Bangladesh receives large amounts of water passing through it with two major rivers (the Ganges and Brahmaputra) converging and forming a huge delta (see picture) formed from silt deposited by the river as it enters the sea. Both rivers have large volumes of water flowing through them to the sea as they have large drainage basins which increasing the flood risk;
3. Bangladesh has a monsoon climate and the annual torrential rains which result often result in the rivers exceeding their capacity and flooding;
4. In the spring, melting snow from the Himalayas further increases the flood risks as torrents of melt water enter the rivers at their source.
   

Human causes of flooding in Bangladesh

1. Increasing population pressure in the foothills of the Himalayas where the rain contributes to the source of the River Ganges and Brahmaputra has resulted in intense deforestation. It is believed that this reduction in interception has resulted in more water entering the rivers - indeed with 92% of the area drained by the rivers being in countries other than Bangladesh, Bangladesh's proneness to flooding is exacerbated by population and environmental issues in countries other than its own, making it increasingly difficult to target the problems.
2. Indeed deforestation in the headwaters is also believed to be responsible for the increased soil erosion which has led to large amount of silt being washed into the rivers and subsequently being deposited on the river bed, reducing its channel capacity and increasing the likelihood of flooding.
   
3. Increasing population pressure in Bangladesh itself has resulted in the sinking of many new wells resulting in the lowering of the water table and the subsequent subsidence of land making it even more prone to flooding;
4. Bangladesh is an LEDC and its lack of money and heavy national debt means that little money is available to spend on flood protection methods / defences and many existing defences lack upkeep and are of questionable use.
   

(click on the digram below for a summary of these)

EFFECTS OF FLOODING IN BANGLADESH

Remember - you must learn place specific detail when writing answers to case study questions if you are to be awarded the full marks.

POSITIVE EFFECTS OF FLOODING

It is important to remember that whilst flooding has serious impacts on human life in Bangladesh it is also instrumental in the wellbeing of Bangladesh's economy and the survival of its people. So what are these positive effects of flooding?

1. As well as providing water for crops, when flooding occurs, as there is friction between the water and the surface of the land, the water slows down and loses its energy. This loss of energy results in the deposition of rich fertile soil resulting in the providing important nutrients enabling people to grow crops;
2. This deposition of silt also creates land upon which people can live - for example the Ganges delta has been formed in this way as deposition has occured where the river has entered the Bay of Bengal.

EFFECTS OF THE 1998 FLOODS:

1. Over two thirds of the land area was covered by water and the capital, Dhaka, was 2m underwater.
2. 30 million people were made homeless in the floods with many losing all their belongings.
   
3. 1,070 people died - this death toll resulted from a number of things. As well as people being killed by drowning in the flood waters, health problems increased the number of deaths further. Contamination of water by waste and dead bodies / animals, and the lack of a clean water supply resulted in the spread of disease such as cholera and typhoid. Further deaths from snake bites and other injuries which led to death through the lack of access to medical care.
4. Food supplies were severely affected as flooding destroyed the rice stocks with a total of 668,529ha of crops being destroyed;
5. The impact on the economy was signifcant with Bangadesh's export industries seeing a 20% decrease in production with over 400 clothing factories forced to close.
   
6. Communications became difficult, with shopping impossible in the main port, as well as roads and railways having been swept away making the distribution of aid and the rescue operation very difficult;

The effects can clearly be seen in the following links:
Photographic presentation of the floods of 1998
Flood '98 - Bangladesh Photo Gallery

and all though very detailed this report provides an over view of the Disaster Impacts, Household coping and response. This chapter from the report provides specific detail on the impacts of the flood on agricultural production, employment and wealth.

FLOOD RELIEF / MANAGEMENT IN BANGLADESH

As has already been mentioned Bangladesh's low level of economic development means Bangladesh's flood protection is insufficent and a number of factors as discussed in this post have exacerbated the problems.

Following the 1998 floods a number of short term flood relief measures were put in place to try an minimise loss of life - these included:

• international food aid programmes
• the distribution of free seed to farmers by the Bangladesh govenrment to try and reduce the impact of food shortages - the government also gave 350,000 tonnes of cereal to feed people;
• volunteers / aid workers worked to try and repair flood damage (see OCR A textbook - p.39 for further details)

In the long term a number of flood prevention measure are possible:

• the creation of embankments (artificial levees) along the river to increase channel capacity and restrict flood waters - however since 1957, 7,500km of flood embankments have been constructed and yet many were breached in the 1998 floods;
• constructing flood protection shelters (large buildings raised above the ground) to shelter both people and animals
• emergency flood warning systems and plans made for organising rescue and relief services;
• providing emergency medical stores in villages
• building flood proof storage sheds for grain and other food supplies
• dam construction upstream and major embankments around Dhaka have been suggested however lack of money has meant that these suggestions have not been taken further.
  

Further Links:

Conclusions and lessons from the 1998 floods
Lessons learning from the 1998 Bangladesh Floods

Map source: US CIA World Factbook (Creative Commons)
Photo source: Ganges delta - screenshot from NASA World Wind (Creative Commons)

Flooding in an MEDC - The 2004 Boscastle Flood

The Boscastle Floods of 2004

Flash floods such as those that resulted in the flooding Lynmouth back in 1952 are often caused by a combination of factors which as well as sustained heavy rainfall, includes consideration of the relief and drainage of an area. In 2004, almost exactly 52 years to the day after Lynmouth's disaster, Boscastle, a town in north Cornwall suffered a fate simillar to that of Lynmouth as 6cm of rain fell in two hours resulting in a 3m high wall of water rushing through the village. This BBC newspaper article summaries the causes of the Boscastle flood - look for similarities in the cause between this flood and the one you have studied that affected Lynmouth in 1952.

The village of Boscastle suffered extensive damage after the flood and the Environment Agency have released this excellent (but detailed) document summarising the causes, effects and responses to the Boscastle flood entitled "living with the risk".

6 buildings and many cars were washed into the sea and boats and other debris was washed into the sea. Thankfully unlike Lynmouth, in the Boscastle floods there were no major injuries or loss of life - even more amazing when you see the pictures and video footage of the devastation caused. If you are interested in finding out more the following links provide further information on the causes and effects of the Boscastle flood - there are also some great pictures and short video clips showing the devastation.

Articles
North Cornwall Flooding Updates - provides a useful summary of day by day updates from the time of the flood
Wikipedia Article - Boscastle Flood 2004
Boscastle a 'tourist ghost town' (BBC Article)
Devastation in Boscastle (BBC Cornwall) - great site including audio file interviews with villagers
Villagers describe flood horror (BBC article)
Dozens rescued from flash floods (BBC article)
Village 'unlikely to flood again' (BBC article)

Photo Galleries
In Pictures - Then and Now (Boscastle Flood) (BBC) - excellent site showing very clear photographs of the devasted areas before and after the floods;
The devastation and clean up operation (BBC)
Repair work being undertaken in Boscastle
Photographs of the 2004 Boscastle Flood










Video Reports on the flooding
BBC 2004 video report on the Boscastle Floods (see top side link)
BBC video report on Boscastle as it is now (see bottom video link)

Mr Allway of geographyalltheway.com has added an excellent activity to his website comparing the Boscastle and Lynmouth floods - well worth a look!

Source of Photographs: Wikipedia Creative Commons - Benjamin Evans

Flooding in an MEDC - The 1952 Lynmouth Flood

LYNMOUTH FLOOD 1952 (Devon)

The Lynmouth Flood
It happened in August of 1952,
The size of the river suddenly grew;
It tossed and turned and ruined lives,
It killed husbands, children and wives;
The sun disappeared, they sky turned grey;
This was no longer an ordinary day.
The water rose to 40ft high,
You could here all around the people cry.
In the months before there had been lots of rain,
The ground no longer became a drain.
The ground trembled, water came crashing,
Parents were runnning, children were splashing;
When the water came in there was no place to, hide
The river was loud, fast and wide;
It came thundering in and over took the land,
Destroying houses like a giant's hand. (Chloe - Yr 8)

The flood which hit the Devon village of Lynmouth on August 15th 1952 was one of the worst in living memory in Britain. This BBC article from the time gives an introduction to the disaster. Floods the size of that which struck Lynmouth may occur only every 100 - 200 years. For your exam you will need to know the causes of the floods and the effects on the people, environment and economy of Lynmouth.

CAUSES OF THE LYNMOUTH FLOOD


So what did cause the River Lyn following through Lynmouth to burst its banks in such a devasting way in August 1952? The answer to this question involves consideration of several contributing factors which had increased the likelihood of such a flood taking place (click on diagram for a summary of the factors).

• The small but steep sided drainage basin in which Lynmouth was situated increased the risk of flooding in the area. The steep sides encouraged greater surface runoff and combined with the small drainage basin size meant any water could reach the river fairly quickly;
• This was made worse by the high drainage density of the area due to the impermeable rocks of the area around Exmoor which formed the source of the river; again increasing the amount of surface runoff following rainfall;
• Prior to August 15th 1952 Lynmouth had received above average rainfall for 12 out of the first 14 days of the month meaning the soils were already saturated and the river levels high.
• On August 15th a heavy thunderstorm resulted in 200mm falling in 14 hours, one of the three heaviest rainfalls recorded in the UK. This heavy rain combined with the saturated ground and rapid surface runoff resulted in a huge volume of water flowing down the river. As Lynmouth is situated at the confluence of the East and West Lyn rivers the volume of water was increased further at this point and the was far beyond the capacity of the river channel causing the river to burst its banks. This resulted in devastating floods as the West Lyn which had been diverted during the construction of parts of Lynmouth retook its natural course, flowing straight through the village.

EFFECTS OF THE LYNMOUTH FLOOD

You will need to make sure you learn place specific facts about both causes and effects in order to achieve the full marks in case study questions - here are a list of some of the effects of the Lynmouth Flood:

• Debris built up behind bridges resulting in the build up of a flow of water which eventually burst resulting in torrents of water flowing through Lynmouth with 34 people being killed in the disaster
  
• The West Lyn river took its original course flowing straight through Lynmouth destroying 90 houses and hotels;
• 130 cars and 19 boats were also lost, swept into the river or out to sea as the force of water was able to transport large amounts of debris (including gigantic boulders)
• Debris transported by the River Lyn resulted in the enlargement of the River Delta

This link to audio clips features interviews with some survivors of the Lynmouth Flood

Flood Management

Following the Lynmouth flood disaster, flood management plans were put in place to try and ensure such a disaster could not happen again by managing any excess rain water so that the River could cope in the future.


A number of flood management strategies were put in place:

1. The mouth of the East Lyn was widened to increase capacity and allow water to quickly pass into the Bristol Channel
2. The West Lyn was straightened to increase channel efficiency - straightening the channel reduces friction and increases velocity, enabling water to travel through the channel as quickly as possible making it more efficient in coping with flood waters;
3. The West Lyn was not rediverted, instead being allowed to follow its natural course
4. Floodplain zoning was used to identify areas around the river most at risk from flooding. Building restrictions were then put in place with areas close to the river which are most prone to flooding being left as open spaces such as car parks.
5. Bridges were made wider and taller to allow flood water to tr avel quickly beneath them and to reduce the likelihood of debris becoming trapped and acting like a dam as had happened in 1952;
6. Embankments were built by the river to increase channel capacity and reduce the likelihood of flooding;
7. More trees were planted upstream in the source area to try and reduce initial surface runoff through interception and the soaking up of water. Tree roots also help to improve infiltration by opening up the soil and slowing down the rate at which water reaches the ground;
   

Follow up links:
The following links provides further useful background on the Lynmouth Flood
Lynmouth Flood of 1952
Exmoor National Park - the Lynmouth Flood
2002 Memorial for the Lynmouth Disaster
Rain Making link to killer floods - an interesting twist in the tale which some people have said could explain the higher than usual levels of rainfall!

Source of Photographs www.geographyphotos.com / A Lawson