Click on the links below to revise that particular topic:
How to..... use this blog
This blog has been designed to work in connection with your book and class work, the extended answers booklets (paper 1 and 2), G3 and your revision guides (still availabke from the geography team, £3).
Choose a topic to study, click on the link and use the information to make detailed revision cards. There is plenty of case study specific information as well as all the ideas and knowledge you need. Revise in short bursts of 30 minutes. Break your revision into chunks and find ways of learning that suits you, e.g. listen to the podcasts, summarise your notes, highlight key points, draw diagrams, make revision cards, ask someone to test you BUT make it ACTIVE.
Revising the Coasts Unit: We have now come to the end of the Coasts Unit and its time to revise! Here are some resources to help you. Year 11 mind map: COASTS - GCSE GEOGRAPHY
1. Energy at the coast - types of waves and basic principles behind wave formation / factors affecting the strength of waves
2. Coastal Processes - erosion processes (you need to be able to describe the processes and how they work); transport processes (you must be able to talk through the process of longshore drift and it helps if you are able to draw an annotated diagram to show the process) and deposition
3. Coastal Erosion Landforms - you must be able to talk through both the features and formation of the following landforms: cliffs; wave-cut platforms; headlands and bays; caves, arches, stacks and stumps (in your description and explanation of formation always include some examples of named erosion processes that may be at work). You should also be able to draw annotated diagrams of the features to show how they form as well as knowing named examples of each.
4. Coastal Deposition Landforms - you must be able to talk through both the features and formation of the following landforms: beaches; spits; bars and tombolos. You should be able to draw annotated diagrams of the features to show how they form as well as knowing named examples of each.
5. Case Study of Coastal Erosion - learn (including detail - i.e. location, facts and figures) a case study of coastal erosion - Lulworth and Durdle Door.
6. Coastal Defence - you need to be aware of the options for coastal defence - hard engineering, soft engineering or managed retreat. You should be able to describe coastal management techniques and be able to discuss their advantages and disadvantages. You should learn a named case study to back this up - for example the Holderness coast / Holland.
Revision Resources: - make good use of your class notes - make use of your extended answers booklet and mind maps from G3 - make use of blog posts to consolidate your understanding / recap concepts you are less sure of - remember there are various links to animations etc. to help you. -use the film from the Coast trip to revise erosional features
When managing the coastline there are two main options:
1. HARD ENGINEERING- this is where man made coastal defence structures are used to reflect large amounts of wave energy and hence protect the coastline.
2. SOFT ENGINEERING- this is where beaches or naturally formed materials are used to control / re-direct erosion processes.
You need to know examples of coastal management techniques and their advantages and disadvantages:
Hard Engineering Techniques:
1. Re-Curved Sea Wall- concrete wall which is curved on the underside to deflect the power of the waves- these can be very expensive (up to £1-2 million per km) and the deflected waves can scour material at the base of the wall causing them to become undermined- these are however a very effective means of preventing erosion and they reflect rather than absorb wave energy.
2. Rip Rap- large boulders on the beach absorb wave energy and break the power of the waves- although movement of the boulders is expensive this can be a much cheaper method than some other solutions- the boulders can however be undermined easily by waves washing away sand and shingle beneath them. They also can be quite ugly, changing the appearance of a coastline.
3. Groynes- these structures (usually either wooden or steel) are designed to stop longshore drift and therefore act to build up and anchor beach material, protecting the base of cliffs.- they are effective at reducing erosion in the area they are constructed in by causing significant build up of beach material- groynes may however starve areas further down the coast of material by stopping longshore drift, resulting in an increase in erosion in these areas
4. Gabions- these cages of boulders are built into cliff faces to protect the cliff from the force of the waves;- they are cheaper than sea walls and can be very effective where severe erosion is a problem- they are however visually intrusive
5. Revetments - these wooden structures break the force of waves and beach material builds up behind them- they are cheap and effective at breaking waves- as well as being visually intrusive however they do need replacing more frequently than most other defence methods.
Soft Engineering Techniques
Soft engineering includes beach replenishment in which beach material is added to provide a "natural solution".
Environmentally this is a preferred option as it maintains the beauty of the landscape and avoids visual intrusion, however it can be expensive to maintain as longshore drift continues to move beach material down the coast and therefore regular replenishment is required.Sand Dunes and salt marshes can also be encouraged to act as natural barriers to the waves.
Revision Time: Coastal Management at Sidmouth - What forms of coastal management can you see?
Sidmouth seafront west in England Case Study of Coastal Erosion and Coastal Defence: Where is the Holderness Coast? What is the problem?
The Holderness Coast is on the NE coast of the UK, facing the North Sea. To the north one finds Flamborough Head (Headland) and to the south Spurn Head ( a Spit). This is a distance of roughly 60km. Hull is the closest city to Mappleton and is 17km east of it.
The coastline is mainly made up of cliffs (20-30m high), consisting of soft, easily eroded boulder clay. Where the cliff line meets the Humber Estuary, a spit has formed due to the change in the direction of the coastline - Spurn Head.
The cliff line is retreating at an alarming rate
- greater than 1m / yr (fastest rate in Europe)
- 4km of land have been lost since Roman Times, including many villages and farm buildings.
Easington Gas Station (a North Sea Gas terminal) is situated on the cliffs top and its position is under threat.
Click here for an excellent Google Tour of the Holderness coastline (thanks to Simon Renshaw)
Why is Cliff Erosion such a problem here?
1. The cliffs are made up of soft glacial material (Boulder Clay - made up of sands and gravels). This is easily eroded by the waves and the cliffs are easily undermined. 2. The Holderness Coast is very exposed, approaching waves have a long fetch over the North Sea. 3. The waves are mainly destructive - eroding the base of the cliffs (hydraulic action etc.) 4. Most of the Material eroded from the cliffs is washed out to sea, the rest is moved by longshore drift - the beaches are therefore narrow and do little to protect the coastline. (If the beaches were wider, the waves would break on the beaches reducing their erosive power). 5. The coastline is threatened further by sea-level rise.
Attempts at Coastal Management along the Holderness Coast include:
-use of groynes to trap moving beach material and provide a protective beach in front of the cliff
-the construction of sea walls and revetments as wave-resistant structures at the base of the cliffs -artificial off-shore breakwaters like tyres and concrete blocks, forcing waves to break off-shore. -sea wall used to protect Easington Gas Station (cost £4.5 million) -Due to extensive costs - only the most valuable areas of land are protected. Much of the area is farmland which is not protected.
Example of the impacts of Coastal Management: Mappleton
The village of Mappleton is greatly under threat by coastal erosion along the coastline and by 1998, the main road running through the village was only 500m from the cliff top and in places it is now only 50m. Mappleton is 3km south of Cowden. There are roughly 30 properties. The village is under threat due to the easily eroded boulder clay (glacial till) which makes up the cliff line. The area suffers from erosion rates of up to 1m per year.
Protecting Mappleton To reduce the amount of erosion threatening Mappleton, 2 rock groynes were constructed in 1991 to encourage the build up of beach in front of Mappleton by trapping longshore drift. Rock armour was also placed at the foot of the cliffs to create a sea wall.
The following images show a rock groyne and rock armour (rip rap) which creates a sea wall and protects the cliffs.
This meant that that waves would break on the beach rather than attacking the cliffs.
Problems for further down coast - Cowden
Those living south of Mappleton village have experienced the 'knock-on' effects of the coastal management. The groynes at Mappleton have disturbed the natural longshore drift movement, trapping the coastal material. Therefore whilst material is still being moved south of Mappleton, there is no fresh sediment to replace it. Beaches have become even narrower and the cliffs are unprotected. Estimates suggest that it has accelerated cliff erosion south of Mappleton to 10m / yr.
Click here for excellent images from the Holderness coastline
Here is a video showing a farm south of the Mappleton rock groyne being lost to the sea
Follow this link to gather excellent information on Mappleton and the Holderness coastline in general Mappleton Case Study Sheet
There are 4 main depositional features that you need to learn. These are: 1. Beaches
2. Spits
3. Bars
4. Tombolos
Beaches
Beaches are the main feature of deposition found at the coast, these consist of all the material (sand, shingle etc.) that has built up between the high and low tide mark. There are number of different sources of beach material - the main source being rivers, where fine muds and gravels are deposited at the river mouth. Other sources of beach material include longshore drift (bringing material from elsewhere along the coast); constructive waves (bringing material up the beach from the sea) and from cliff erosion.As constructive waves build up beaches, they often form ridges in the beach known as berms. The berm highest up the beach represents the extent to which the water has reached during high tide.
Click on the diagram below to see the main sources of beach material
SPITS
Spits are long narrow ridges of sand and shingle which project from the coastline into the sea.The formation of a spit begins due to a change in the direction of a coastline - the main source of material building up a spit is from longshore drift which brings material from further down the coast.Where there is a break in the coastline and a slight drop in energy, longshore drift will deposit material at a faster rate than it can be removed and gradually a ridge is built up, projecting outwards into the sea - this continues to grow by the process of longshore drift and the deposition of material.A change in prevailing wind direction often causes the end of spits to become hooked (also known as a recurved lateral).On the spit itself, sand dunes often form and vegetation colonises (for example Blakeney Point - North Norfolk)Water is trapped behind the spit, creating a low energy zone, as the water begins to stagnate, mud and marshland begins to develop behind the spit;Spits may continue to grow until deposition can no longer occur, for example due to increased depth, or the spit begins to cross the mouth of a river and the water removes the material faster than it can deposited - preventing further build up.
Examples of Spits-
Spurn Head - Holderness Coast
Hurst Castle Spit Model answer on how spits are formed - shown as a "word cloud"
Click below for an annotated diagram of spit formation:
BARS
These form in the same way as a spit initially but bars are created where a spit grows across a bay, joining two headlands. Behind the bar, a lagoon is created, where water has been trapped and the lagoon may gradually be infilled as a salt marsh develops due to it being a low energy zone, which encourages deposition.
Example of a Bar: Slapton Sands - Devon.
TOMBOLOS
Tombolos are formed where a spit continues to grow outwards joining land to an offshore island.
Example of a Tombolo: - Chesil Beach - which joins the South Dorset coast to the Isle of Portland.
Remember - as well as being able to describe the formation of each feature of coastal deposition, you should be able to give a named and located example e.g. a spit - Spurn Head (Holderness Coast). You should also try and learn a labelled diagram to show the formation of each feature.
- Beach - Dawlish Warren (Devon)
- Spit - Spurn Head (Holderness Coast)
- Bar - Slapton Sands (Devon)
- Tombolo - Chesil Beach (joining S Dorset Coast to Isle of Portland)
Having now learnt both erosion and deposition features you need to make sure that you can distinguish between them.
Coastal Erosion Features
There are 3 main groups of coastal features which result from coastal erosion:
1. Headlands and Bays
2. Caves, Arches, Stacks and Sumps
3. Cliffs and Wave-cut platforms
Before you revise the formation of these landforms, have a look at this video and make sure you are able to identify the landforms from their distinctive features.
1. HEADLANDS AND BAYS
Headlands are resistant outcrops of rock sticking out into the sea, whilst bays are indents in the coastline between two headlands.
So how do headlands form?
- Headlands form along discordant coastlines in which bands of soft and hard rock outcrop at right angles to the coastline.- Due to the presence of soft and hard rock, differential erosion occurs, with the soft, less resistant rock (e.g. shale), eroding quicker than the hard, resistant rock (e.g. chalk)- Where the erosion of the soft rock is rapid, bays are formed- Where there is more resistant rock, erosion is slower and the hard rock is left sticking out into the sea as a headland.- The exposed headland now becomes vulnerable to the force of destructive waves but shelters the adjacent bays from further erosion.Named Examples of Headlands and Bays: (LEARN!)The Dorset coast has excellent examples of Headlands and Bayse.g. Swanage Bay and the Foreland (a headland)
2. CAVES, ARCHES, STACKS and STUMPS
Once a headland has formed it is then exposed to the full force of destructive waves and it gradually begins to erode. you need to be able to describe the erosion of a headland and the features that form.For the sequence of formation see the animation below:
So how does a headland erode and caves, arches, stacks and stumps form?
- Firstly, the sea attacks the foot of the cliff and begins to erode areas of weakness such as joints and cracks, through processes of erosion such as hydraulic action, wave pounding, abrasion and solution;
- Gradually these cracks get larger, developing into small caves;
- Further erosion widens the cave and where the fault lines runs through the headland, two caves will eventually erode into the back of each other forming an arch, passing right through the headland.
- A combination of wave attack at the base of the arch, and weathering of the roof of the arch (by frost, wind and rain), weakens the structure until eventually the roof of the arch collapses inwards leaving a stack, a stack is a column of rock which stands separate from the rest of the headland.
- The stack will continue to erode, eventually collapsing to form a stump which will be covered by water at high tide.
Named Examples:The Foreland (Dorset Coastline) is a great example of a headland which shows these features - there is a distinctive stack called Old Harry and a stump known as Old Harry's Wife.
A good example of a distinctive arch, also found on the Dorset Coast is Durdle Door.
3. CLIFFS AND WAVE-CUT PLATFORMS
Cliffs are steep rock faces along the coastline, they tend form along concordant coastlines with resistant rocks parallel to the coast.
So how do cliffs and wave-cut platforms form?
- The erosion of a cliff is greatest at its base where large waves break - here hydraulic action, scouring and wave pounding actively undercut the foot of the cliff forming an indent called a wave-cut notch whilst the cliff face is also affected by abrasion as rock fragments are hurled against the cliff by the breaking waves.
- This undercutting continues and eventually the overhanging cliff collapses downwards - this process continues and the cliff gradually retreats and becomes steeper.
- As the cliff retreats, a gently-sloping rocky platform is left at the base, this is known as a wave-cut platform which is exposed at low tide.
Named Examples:Good examples of cliffs and wave-cut platforms can be found at Hunstanton (North Norfolk) and Flamborough Head (Yorkshire) -------------------------------------------------------------------------------
REVISING COASTAL EROSION FEATURES
Remember - for each erosion feature try and learn a labelled diagram to show its formation, make sure that you also mention examples of erosion processes when describing how the features are actually formed. Finally to access the highest marks remember to name and locate examples of each feature.
- Swanage Bay (Dorset Coast)
- The Foreland (Headland) (Dorset Coast)
- Old Harry (Stack) (Dorset Coast - off of the Foreland)
- Old Harry's Wife (Stump) (Dorset Coast - off of the Foreland)
- Durdle Door (South Dorset Coast)
- Cliffs and Wave-cut platforms - Flamborough Head (Yorkshire)
Remember, there are 3 main processes that cause a coastline to change:
1. Erosion
2. Transport
3. Deposition.
There are number of factors which affect each of these processes - we are going to start by exploring erosion processes and the factors that can affect the amount of erosion that may take place along a coastline.
COASTAL EROSION
Erosion Processes:
Erosion is the wearing away of rocks, at the coast there are 6 main types of erosion processes in action :
1. ABRASION (this is also known as corrasion) - this is where rock fragments are hurled at cliffs by breaking waves, gradually scraping away at the cliff face;
2. HYDRAULIC ACTION - as waves break against the cliff face, the pressure of the breaking wave can compress air in cracks. This compressed air gradually forces open the crack in the rock - as this process continues, the rock becomes increasingly weakened.
3. SOLUTION (this is also known as corrosion) - this occurs where the salt water is able to dissolve some of the chemicals in rocks - for example, limestone cliffs are gradually weakened as the salt water dissolves the calcium carbonate in the limestone.
4. SCOURING - this occurs at the base of the cliff as the waves break and swirl around, gradually removing loose rock.
5. ATTRITION - this is where rock fragments carried by the waves hit against each other and gradually wear down to form sand and silt
6. WAVE POUNDING - the sheer force of waves hitting against the cliff face
These processes of erosion form a series of distinctive landforms at the coast.
Rates of Coastal Erosion
So what are the factors that determine how much erosion can take place at the coast?
1. The Resistance of the Rocks - e.g. limestone, chalk and granite are resistant rocks (often forming cliffs and headlands) and erode relatively slowly, whilst less resistant rocks such as clay are easily eroded.
2. The Strength of the waves - affected by the wind strength and duration and its fetch
3. The shape of the coastline (which is dependent on its geology) - on concordant coastlines, rocks are parallel to the wave front and therefore rates of erosion are similar along the coastline. On discordant coastlines, differential erosion may occur, where bands of hard and soft rock outcrop at right angles to the sea. Consequently headlands and bays form along discordant coastlines and whilst headlands remain exposed to the force of the waves, bays are sheltered.
Look at the diagram below for a summary of factors affecting coastal erosion.
COASTAL TRANSPORT
The second process operating at the coast is transport. Material eroded by the sea is carried within the water in a number of ways, minerals dissolved from rocks are carried in solution, whilst small rock fragments, light enough to be held within the water, float in suspension. The largest rock fragments which are too heavy to be picked up by the waves, are transported by the process of traction, this is where they roll along the bed when the waves pick up enough energy. Finally, medium sized rock particles, which cannot be carried by the waves all the time, are moved by saltation. This is where during times of higher wave energy the particles are picked up and then dropped again as the wave looses its energy.
The main form of transport operating at the coast is that of LONGSHORE DRIFT.
Longshore drift is the process by which sand and pebbles are moved along a beach by the movement of the waves.
Watch the following film to revise:
COASTAL DEPOSITION
Material is moved up the beach by the swash at an angle which is controlled by the prevailing wind. The backwash then carries material back down the beach at right angles to the coastline under the influence of gravity. Gradually the material is moved along the coastline, its direction being controlled by the prevailing wind direction.The final process operating at the coast is that of deposition - this is where material that is too heavy to be transported any more is left behind, building up the beach. Due to the importance of energy in transporting sand and shingle, it is the largest material that is deposited first. A number of distinctive features may form due to coastal deposition.
Erosion - the wearing away and removal of material
Deposition - the dropping of material
Abrasion - the wearing of rock due to rock fragments being hurled against cliffs
Attrition - the breakdown of rocks as they hit against each other
Hydraulic Action - the force of waves causing rocks to split apart as waves compress air in cracks in the rocks
Wave Pounding - sheer force of water hitting rocks
Solution - where minerals in rocks are dissolved by the action of sea water
Scouring - occurs where water and broken rock fragments swirl around at the base of cliffs gradually wearing rock away.
Longshore Drift - the movement of material along a coastline
Podcast: Coastal ProcessesYou can listen to a podcast of this post below - to download a copy to listen to on your .mp3 player click here.
The coast represents the meeting point between the land and sea. Coasts are very dynamic areas and they are constantly change. This change is due to 3 main processes which operate at the coast, 1. Erosion; 2. Transport and 3. Deposition. These 3 processes are all driven by the amount of energy that is available at the coast. The main agents of change at the coasts are waves. Waves are movements of energy throughout the water, but where do waves get their energy from? The answer to this is wind.
As wind blows over the surface of the sea, it creates friction. This frictional drag causes water particles to begin to rotate and energy is transferred forward in the form of a wave. Whilst the water moves forward, the water particles return to their original position. As a wave reaches shallow water, friction between the sea bed and the base of the wave causes the wave to begin to slow down and its shape becomes more eliptical. The top of the wave however, unaffected by the friction, becomes steeper until it eventually breaks. When the wave breaks, water washes up the beach, this is called the swash. The movement of water backdown the beach is called the backwash.It is the rate at which waves reach the coast which determine whether the main process acting on the coastline is erosion or deposition.
There are two main types of waves:(i) CONSTRUCTIVE WAVES - tend to arrive at the coast at a rate of less than 8 waves per minute, they are low energy waves and are small in height. They have a strong swash and a weak backwash. This means that constructive waves tend to deposit material and build up a beach.
(ii) DESTRUCTIVE WAVES , have much higher energy and tend to arrive at the coast at a rate of more than 8 per minute. They are much larger in height often having been caused by strong winds and a large fetch. These high energy waves have a weak swash but a strong backwash, which erode the beach but pulling sand and shingle down the beach as water returns to the sea.
There are 3 main factors which will affect the strength of a wave and therefore whether it is more likely to erode or build up the coastline:
(i) the strength and speed of the wind - the faster the wind, the more energy is transferred and therefore the bigger the wave that is produced.
(ii) the duration of the wind - this is the length of time for which the wind has blown - the longer the wind blows, the more energy is transferred to the wave
(iii) the fetch - this is the distance over which the wind has blown and therefore how far the wave has travelled. The longer the fetch, the larger the wave is likely to be.
Swash - the movement of water and material up the beach (in direction of prevailing wind)Backwash - the movement of water and material back down the beach (straight back down due to gravity
Constructive wave - low energy wave with greater swash than backwash - tends to build up the beach
Destructive wave - high energy wave with greater backwash than swash - tends to erode beach
Podcast: Energy at the Coast - Wave FormationYou can listen to a podcast of this post below - to download a copy to listen to on your .mp3 player click here.
We have now come to the end of the Plate Tectonics Unit and its time to revise!Here are some resources to help you.
Check list of key concepts to revise: 1. Structure of the Earth - what are the four main layers of the earth 2. Plates - named examples of plates and reasons for plate movements (convection currents) 3. Plate Boundaries - make sure you learn the four main types of plate boundaries. For each you need to be able to (i) describe movement of the plates (ii) explain what happens - processes and landforms created (iii) named example of each. 4. Distribution of earthquakes and volcanoes - be able to describe the distribution - linked to plate boundaries and examples (remember the anomaly for volcano distribution is the Hawaiian Islands in the middle of the Pacific Plate - due to hotspot activity) 5. Volcanoes - what are volcanoes? What are the key characteristics of a volcanoes structure? 6. Where do volcanoes occur? (be able to talk through what happens at a destructive and constructive plate boundary resulting in volcano formation) 7. Case Study of a Volcanic Eruption - Montserrat (Caribbean) and Mount St Helens (Cascade Range, USA 1980) - learn causes, effects and responses (remember you must learn specific facts and figures and be able to show locational knowledge to get the highest marks) 8. Why do people continue to live near volcanoes? (be able to illustrate with examples) 9. What techniques are used to monitor and predict volcanic eruptions? 10. Earthquakes - theory of formation and key terms 11. Case Study of an Earthquake - Kobe Earthquake, Japan - 1995 - again learn causes effects and responses (and as before you must learn specific facts and figures and be able to show specific locational knowledge to get the highest marks) 12. Why do some earthquakes cause more damage than others? - be able to compare MEDC / LEDC earthquakes (learn facts and figures for Bam 2003 earthquakes - to compare with 1995 Kobe earthquake) 13. How can we monitor and prepare for earthquake activity? 14. Fold Mountains - formation / characteristics / human uses and problems of living in a fold mountain area - The ALPS
Revision Resources: - make good use of your class notes - make use of blog posts to consolidate your understanding / recap concepts you are less sure of - remember there are various links to animations etc. to help you.Interactive
Click here for a link to the KEY WORDS and their meanings.
The cause of Earthquakes
Earthquakes are sudden ground movements which result from the sudden release of built up energy. This energy is released in the form of seismic waves. Earthquakes are caused due to tectonic motions in the earth's crust. Earthquakes are found at all four of the major plate boundaries (constructive, destructive, collision and conservative boundaries), due to the forces of collision between plates as well as the irregular movement and build up of friction as plates move past each other. Earthquakes also occur away from plate boundaries at weaknesses in the earth's crust known as faults.
As plates move past each other, friction between them results in the build up of pressure. As the plates continue to move and the pressure builds up, eventually the pressure is great enough to overcome friction and the plate jolts forward releasing the pent up energy in the form of seismic waves. The point at the rocks break apart and shock waves start is known as the focus of the earthquake. The point on the surface directly above the focus is known as the epicentre of the earthquake. For further explanation of how earthquakes occur, see this excellent animation from the BBC. Measuring Earthquakes
We measure the magnitude (strength) of an earthquake using a seisometer, the results of which are recorded on a seismograph.
The magnitude of the earthquake, reflecting the energy released, is measured on the Richter Scale (from 1-10). This is a logarithmic scale and therefore each point on the scale is 10x greater than the previous one. Therefore an earthquake measuring 8 on the richter scale is 10 times more powerful than an earthquake measuring 7 on the richter scale. The Effects of Earthquakes
The effects of earthquakes are far ranging and often involve death and destruction. In 1995, an earthquake measuring 7.2 on the Richter Scale devastated the port city of Kobe. Kobe lies on Honshu, close to a destructive plate boundary. It is here that the Phillipines Plate (oceanic crust) is forced downwards on contact with the Eurasian Plate (continental crust). Kobe was very close to the focus of the earthquake (about 15km). The earthquake struck at 5.46am on the morning of the 17th January, 1995. It lasted for 20 seconds. During this time the ground moved 18cm horizontally and 12cm vertically. There was so much devastation because the FOCUS was so close to Kobe. Fire caused widespread damage.We can divide the effects of an earthquake into those known as the primary effects and those known as the secondary effects.
Primary effects of an earthquake are those resulting directly from the earthquake itself. These include; buildings collapsing; roads cracking; bridges giving way; shattering of glass and injuries / deaths resulting from these.
SPECIFIC EFFECTS for Kobe Earthquake:
Primary effects: 5 500 people killed, 40 000 injuries, 200 000 houses destroyed. 1km of the Hanshin Expressway collapsed and numerous bridges, 130km of railway broken, 120 out of 150 quays in important port of Kobe were destroyed. This was damaging because the soil liquefied ( turned to liquid), especially along the port where the land had been reclaimed and was not as solid.
Secondary effects are those that result from the primary effects. For example ground shaking may result in the cracking of gas and water pipes (primary effects) this can result in severe fires due to explosion from escaping gas and difficulties in putting out fires due to lack of water from burst mains (secondary effects). Other secondary effects include, homelessness, business going bankrupt and closing etc.
SPECIFIC SECONDARY EFFECTS for Kobe:
Electricity, gas and water supplies were disrupted. Fires raged (500m wall) caused by broken gas pipes - 7500 further houses destroyed. Earthquake proof water pipes failed. Roads were at gridlock, delaying ambulances and fire engines. 230 000 people made homeless - living in temporary shelters even when night time temperatures dropped to -2 degrees. Short - term shortage of blankets, clean water and food. People were afriad to go home as there were an estimated 716 after shocks.High tech buildings like the Kansai International Airport were unaffected- earthquake proof.
RESPONSES TO THE KOBE EARTHQUAKE
One week after the earthquake - fires still burning, bulldozers started to clear the rubble and knock down unsafe buildings. Electricity and telephones restored. Some shops and schools reopened.Two weeks later - highways and railways re-opened. Overcrowding in the shelters became a health hazard and cold weather led to flu epidemic. Trained people worked with survivors on stress and shock of the quake.After 3 years - parts still being rebuilt, government was critized for slow response and lack of emergency plan. Now sophisticated monitoring equipment in place and Kobe criss crossed with this. High rise buildings re-designed to absorb shock waves and not break as before.
You need to learn a case study of the causes, effects (short and long term) and responses ( short and long term) of a major earthquake (KOBE)
Bam, the ancient historic city in Iran, was hit by an earthquake measuring 6.6 on the Richter scale on December 26th, 2003 resulting in the deaths of over 43,000 people and leaving over 60,000 people homeless. Many of the mud-brick buildings in Bam collapsed resulting in the high loss of life. The mud-brick disintegrates easily into rubble, making rescue difficult and hopes of survival low. The survivors had not only lost friends and family, but their homes and everything else they had. Many were left destitute on the streets, some forced to spend the cold nights wrapped in blankets; whilst some were given tents, others made use of any shelter they could find. 90% of the buildings in the ancient citadel was completed destroyed (photo shows view over the city prior to the quake. After the quake there was criticism of the coordination of relief efforts by the government and of the lack of preventative measures. The Iranian press spoke out about the controversy surrounding this and the lack of a national plan to make buildings quake proof.
Following the quake in Bam, 'tented cities' were constructed on the outskirts of the city by relief workers to try and house the many homeless. Many survivors were however reluctant initially to leave the sites of their destroyed homes with family members, friends and property still buried. As well as battling to survive in the below freezing temperatures of the Iranian winter, one of the concerns at the time was over the lack of access to decent sanitation, with many sharing outside toilets and a lack of clean water. However, ironically it is believed that the freezing temperatures may have actually helped to reduce the incidence of water bourne disease expected. With large numbers of volunteers coming in from all over Iran after the quake, the main plea for help after the earthquake was for medicine and equipment to help the tens of thousands injured in the quake.
You need to be aware of the reasons for differences in earthquake impact between countries at idfferent levels of development and you need to be prepared in an exam to discuss the differences with examples.
Here is a summary of some of the reasons for the high death toll (see article and links below for further detail)
- poor construction of buildings
- lack of earthquake proof buildings
- buildings made of mud-brick (collapse easily into rubble)
- lack of enforcement of building codes / regulations
- lack of research into techniques to protect the buildings from earthquakes
- population boom and competition for houses (resulted in rapid building of sub-standard housing)
- lack of national plan for the event of a disaster
- extreme cold temperatures made conditions for survivors difficult
To find out more about the quake, including the effects and considerations as to why the effects may have been so severe try exploring some of the following links and news stories:
Earthquake - a sudden ground movement Epicentre - this is the point on the surface directly above the focus of the earthquake Fault - a weakness in the earth's crust where an earthquake may occur Focus - this is the point underground where the earthquake starts Richter Scale - a logarithmic scale used to measure the magnitude of an earthquake Seismic Waves - waves of energy released in the event of an earthquake Seismograph - used to measure seismic waves released during an earthquake
Comparing Earthquakes
Why do more people die in some earthquakes than others?
One of the crucial factors in determing the severity of the effects caused by an earthquake is the magnitude of the quake. The magnitude of an earthquake is measured on the richter scale, which is logarithmic (hence each level of magnitude is 10 times greater than the one before it on the scale). However, magnitude is not the only factor to be taken into consideration; indeed in December 2003 - 2 quakes of a simillar size resulted in very different death tolls - in California 3 people died in a quake measuring 6.5 whilst around 40,000 died in Bam (Iran) in a quake measuring 6.6.
So why do some earthquakes result in more fatalities than others?
What are the factors which contribute to the severity of the effects of a quake?
1. Location of the Epicentre The epicentre is the point on the surface directly above the focus (start of the earthquake). It is at this point where the energy from an earthquake is usually at its greatest. The distance from the epicentre therefore has a big impact. The epicentre of the Kobe earthquake was very close to Kobe as was the 2003 Iranian earthquake, the epicentre was very close to the city of Bam (accounting for the high death toll)
2. Level of development of the Country Earthquakes which occur in the richer countries of the world often have fewer fatalities simply due to the greater state of preparedness which is facilitated by the greater amount of money available to put into earthquake research, monitoring and preparation. You should be able to compare examples of earthquakes in MEDCs and LEDCs (see posts on the Kobe 1995 quake and Bam 2003 quake - both of which were around 6.6-7.2 on the richter scale yet with vast differences in the number of fatalities - 5500 in the Kobe quake compared to around 40,000 in the Bam quake).
Some examples of reasons for an often greater death toll in LEDCs: - buildings are often not earthquake proof and may be built out of flimsy materials not suited to quakes (e.g. mud brick used in Bam); -with pressures on population, buildings are also often built quickly and as a result are often sub-standard and not built to meet building codes; - emergency services in LEDCs are usually not as well funded and therefore not as able to cope, due to fewer training opportunities and less money for essential equipment / supplies; - lack of money for prediction technology and monitoring of sesimic activity - many LEDC cities are very densely populated with houses packed close together, resulting in great danger from collapsing buildings and the rapid spread of fire; - in some LEDCs, difficult political situations can mean response to earthquakes by government officials is not as quick as it should be. Also have a look at this interesting article from the BBC "Can money stop an earthquake?"
3. Time of the Day / Time of Year If an earthquake occurs at night, most people are in bed. In areas where buildings collapse easily this can result in a higher death toll, although in areas where fewer buildings are likely to collapse and where deaths are often higher due to collapsing roadways / falling debris, fewer people may die if the quake occurs at night. The time of year can also be important due to seasonal differences in temperature which can exacerbate the effects of a quake. Following the Bam 2003 quake almost 60,000 were left homeless, forced to take shelter in simply blankets and makeshift tents in freezing evening temperatures. Where conditions are much warmer, this can facilitate more rapid decay of bodies and lead to an increase in the spread of disease following a quake, particularly in areas where access to clean water is poor.
4. Population Density
An area of dense population is likely to experience more deaths than a rural area simply due to a greater liklihood of people being affected by the quake and more buildings, road networks and bridges which may collapse. A major difficulty however in earthquakes which occur in rural areas is getting rescue teams and aid to the affected areas.
5. Land that buildings are constructed on
Where buildings are constructed on soft granular sediments or areas of landfill, the effects of an earthquake maybe more severe due to the process of liquefaction. This process, which results in ground failure, occurs when ground shaking causes water to rise, filling pore spaces between granular sediments, increasing pore water pressure and causing the sediment to act as a fluid rather than a solid. This can result in the collapse of overlying buildings, roads etc., such as occured in the Port District in Kobe during the 1995 quake due to it being built on landfill from the 1906 quake (see this excellent article on liquefaction in earthquakes for examples)Likewise, if earthquakes occur in areas with steep surrounding slopes, ground movements can trigger landslides causing great loss of life.
For further information there is an excellent article on "The Geography Site" called "What determines the impact of an earthquake?" - has a good level of detail and lots of examples - well worth reading!
Key Term Check: Liquefaction - where sediments act like as a liquid rather than a solid due to ground shaking causing water to rise and increase pore water pressure. Epicentre - the point on the surface directly above the focus Focus - the point at which an earthquake starts Magnitude - this refers to the strength of an earthquake and is measured on the Richter Scale
Preparing for Earthquakes Earthquake Preparedness
Can we predict earthquakes? Although we cannot predict the exact timing of an earthquake and they can occur unexpectedly, we can undertake monitoring of seismic activity to try and determine where earthquakes are most likely to occur and to try and provide some sort of forecast to ensure preparations can be made where possible to try and minimise death and damage caused. This includes:
- using special instruments to measure / monitor possible earth movements (use of seismometers / seismographs)
- plotting the regularity of earthquakes to look for possible patterns
- mapping centers of earthquakes to indicate where earthquakes may be overdue and where pressure may be building up
- observing natural phenomena which can sometimes indicate the likelihood of earthquake activity (odd animal behaviour has been acknowledged - for example snakes in China as indicated in this BBC article also see this National Geographic article Can animals sense earthquakes? ) There is an excellent article on Earthquake Prediction on the Geography Site here.
Preparing for Earthquakes
There are many things that can be done to take precautions and prepare for the event of an earthquake in order to try and minimise any potential damage.
These include.
- Monitoring ground movements - to identify possible seismic activity (as discussed above)
.- Emergency Service Planning - ensure emergency services (medical and rescue) are fully trained to cope with such a disaster and that specialist emergency equipment is available.
Complete the following paragraphs in your revision book.
Earthquakes happen when the plates that make up the earth suddenly move against each other. On the 26th of December the biggest earthquake in forty years occurred between the Australian and the plates. The quake triggered a that caused large waves to spread across the ocean in a matter of hours.
The Tsunami was formed when the vertically jolted the seabed by several metres, forcing up the ocean. Large waves started to move away from the earthquake’s . The Tsunami’s journey had begun.
In deep water the Tsunami moved at 500mph. When it reached shallow water near the coast it slowed but increased in . The only sign that a Tsunami is coming is when the waterline suddenly , exposing hundreds of metres of beach and seabed.
Several waves of the Tsunami came at intervals of between five and forty minutes. Some of the waves reached one kilometre inland, causing widespread .
Eurasian Indian Retreats.
Tsunami Epicentre Destruction
Earthquake Height
Activity 2:
Read through these 16 statements carefully.
Organise the statements into their THREE groups:
1. Causes - what made the tsunami happen?
2. Effects - what problems did the tsunami bring?
3. Responses and Solutions - what did people do afterwards?
1) An earthquake measuring 9.0 on the Richter Scale occurs in the Indian Ocean 2) Many bridges and railways were swept away 3) There was no warning system to inform people a tsunami was going to strike 4) Construction begins on 5000 emergency shelters in Thailand 5) The UK population raise over £100 million in the 2 weeks following the disaster. 6) Over 150,000 people have died including over 400 Britons 7) Most people were drowned 8) Drinking dirty water has brought about many diseases 9) The earth’s crust is formed from floating pieces of rock that rub against each other 10) People made homeless 11) Part of the Earth’s crust was jolted upwards causing a swell that turned into a Tsunami 12) A tsunami warning system will be implemented that covers the Indian Ocean 13) Britain sent boats and helicopters 14) Water purification tablets were sent to the area 15) The UN call an emergency conference in Jakarta, Indonesia to decide what to do about the disaster 16) The force of the tsunami picked up cars and boats and dumped them up to 1km inland
Incredible footage of the tsunami. Watch and look for effects:
Volcanoes Volcanoes are simply vents at the earth's surface through which lava and other volcanic products are erupted. Although many volcanoes are cone-shaped, different types of volcano exist according to their location and the products they are made up of.
The distribution of volcanoes Volcanoes occur in narrow, linear belts and are mostly found along destructive boundaries with large numbers found around the Pacific Ring of Fire (the area marking the boundary of the Pacific plate). They are also found at constructive plate margins such as the Mid-Atlantic Ridge. An exception to the general distribution of volcanoes is those found in the middle of the Pacific Plate (the Hawiian islands) which are formed due to hotspot activity.
As well as describing the distribution of volcanoes you will need to be able to describe and explain their occurence at plate boundaries:
Volcanoes at Destructive Boundaries:
1. Plates move together due to convection currents 2. Heavier oceanic plate is subduced 3. Friction between the plates and heat from the interior causes the subducting plate to met 4. Melting of the plate creates molten magma 5. This magma is less dense than its surrounding and therefore rises and is erupted at the surface through a weakness in the crust creating a volcano. 6. Volcanoes at Destructive boundaries tend to be quite explosive due to the build up of pressure and gases.
Volcanoes at Constructive Boundaries:
1. Plates move away from each other due to convection currents 2. This creates a weakness / 'gap' in the crust 3. Magma is a able to rise to plug the gap forming lava flows and submarine volcanoes 4. As magma continues to build up above the surface of the ocean, volcanic islands (such as Surtsey) may form. 5. Eruptions at constructive boundaries tend to be gentle with little pressure build up
Volcanoes at Hotspots:
As mentioned in the last post, volcanoes may also be created at Hotspots - for example the Hawaiian Islands (with volcanoes such as Kilauea).
See this animation as a reminder of how hotspot activity can result in the formation of volcanoes. Structure of a Volcano:
Some Volcanic eruptions are more explosive than others due to the type of magma. At destructive margins andesitic magma gives rise to acid lava which is thick and sticky. As gases can't escape easily pressure builds up resulting in violent explosions. In contrast at constructive margins, basaltic lava gives rise to basic lava which is thin and runny and from which gases escape easily. These eruptions are therefore more gentle in nature and less explosive.
Key Term Check: Volcano - a vent on the earth's surface through which lava erupts Hotspot - a thermal plume of magma which rises underneath a tectonic plate
Magma Chamber - an underground chamber storing molten rock Secondary Crater - a small crater often cut into the side of a large cone (may form due to a blockage in the main cone) Crater - a large opening at the top of a volcano from which gases, lava and ash etc. escape Vent - pipe taking magma towards the main crater Pyroclastic Flow - a cloud of burning ash, gases and other volcanic material which can travel downslope at great speeds. Volcanic Bombs - large, hot boulders ejected during an eruption Magma - molten rock under the ground Lava - molten rock which reaches the ground surfaceThe photographs in this post are courtesy of the USGS
Case Study 1 – Volcanic eruption in an LEDC – Soufriere Hills, Montserrat
Causes:
Case study: Chances Peak, Montserrat, 1995-97
Volcanic ash covers the town of Plymouth, Montserrat Montserrat is a small island in the Caribbean. There is a volcanic area located in the south of the island, called Soufriere Hills. Population of 11 000 in 1995. It is an island arc on a destructive plate margin between the North American and Caribbean plates.
The volcanic peak in this area is called Chances Peak, which had been dormant for over 300 years. Then in 1995, the volcano began to give off warning signs of an eruption (small earthquakes and eruptions of dust and ash). Once Chances Peak had woken up it then remained active for a period of 5 years. The most intense eruptions occurred in 1997. Watch this film which shows the effects, responses and monitoring of the Montserrat eruption:
Effects:
During this time, Montserrat was devastated by pyroclastic flows. The small population of the island (11,000 people) was evacuated in 1995 to the north of the island, neighbouring islands and the UK. The evacuees became refugees.Despite the evacuations, 23 people were killed by the eruptions. This is because a small group of people chose to stay behind on the island and watch over their crops.Volcanic eruptions and lahars have destroyed large areas of Montserrat. The capital, Plymouth, has been covered in layers of ash and mud. Homes and buildings have been destroyed. The only hospital and airport was destroyed. Many roads were destroyed under ash and dust covered the whole island, making it difficult to breathe. The graphic shows the progress of the eruption and its impact on the island.
Map of Montserrat
Volcanic activity has calmed down in recent years and people have begun to return to the island.
Short term responses:
The authorities and people were totally unprepared since the volcano had been dormant for 400 years
People evacuated to the north of the island, were housed in tents and makeshift homes with little food, poor sanitation and no power
The UK government gave £55 million in compensation and redevelopment and 250 prefabricated houses
The hospital reopened in a former school
Communications were difficult and expensive to repair
Many people are unemployed since the tourist industry collapsed
Little farmland to reclaim in the south
Plymouth – capital city abandoned
Long term responses:
The volcano is still active with a new lava dome growing in 2006. A volcanic observatory has been established to monitor activity and advise residents and tourists of the dangers
v An exclusion zone exists over much of the southern part of the island and for 2km off shore
v The population is about 5000, under half of that in 1995.
v There is no capital city. Plymouth remains in the exclusion zone
v Services have been expanded in the north – there are reliable electricity and water services. Education exists for all 5 years.
v New roads have been built in the south and are being extended
v Agriculture and fishing once employed nearly 500 people, now it is only about 200. The government is concentrating on poultry production (chickens) to reduce the island’s dependency on imports
v Construction of an industrial park is in progress
v Tourism is being encouraged with “volcano experiences”, “diving and snorkelling” and “weddings and honeymoons” as attractions – international access is via Antigua and then a 1hour ferry trip.
Case Study 2-Mt. St. Helens 1980, volcanic eruption in an MEDC (USA)
Mount St Helens, Washington State, NW USA is located in the Cascade mountain range and prior to its eruption in 1980 it had been active for over 100 years. The volcano sits on a destructive boundary where the Juan de Fuca plate meets the North American plate.
Mount St Helens erupted on May 18th 1980 following a period of activity which began in March 1980 with an earthquake measuring 4.0 on the richter scale. What followed was 3 months of seismic activity as magma rose within the mountain. As the magma rose, a large bulge grew on the north flank of the volcano, this was due to a blockage in the main vent resulting in the growth of a cryptodome (mound of viscous lava) in the side of the volcano. On May 18th, an earthquake measuring 5.1 on the richter scale caused a landslide on the northern flank of the volcano, which in turn exposed the cryptodome below, resulting in a sudden release of pressure and a cataclysmic eruption in the form of a lateral (sideways) blast. The blast zone consisted of 230 square miles with the eruption leaving a 'lunor' landscape in its wake.
Watch the short video clip below to remind yourself of the nature of the lateral blast:
The effects of the eruption included: * laval flows (lahars) and ash filling in Spirit Lake and log jams and ash blocking the channel of the Toutle River. Pyroclastic flows covered 6 square miles. * 57 people died in the eruption - most from poisonous gases; * large number of wildlife were killed by the blast and the volcanic ash with nothing surviving in the blast zone. 2 million birds, animals and fish are killed. * flooding resulting from blocked rivers washed away road and rail bridges, 200 homes, 27 bridges, 15miles of railways and 185miles of road destroyed. * crops were ruined and livelihoods of loggers were devastated with large areas of trees being flattened like matchsticks. Damage to crops estimated at $175m
The responses to the eruption are:
Immediate responses:
molilising helicopters for search and rescue
rescuing survivors and giving emergency treatment
stranded tourists had to be given shelter
Roads were blocked - drifts of ash were up to a metre had to be cleared.
2 million masks were sent
Long term responses:
buildings and bridges were re-built
The forest was replanted - 10 000 trees.
Roads were re-built and tourists were encouraged to return. The government spent $1.4m to transform the area.
The soils fertility was improved by all the ash and so people moved back and wanted to live next to this volcano again.
Despite their danger many people choose to live close to volcanoes. This is because soils are very fertile. The tourist industry thrives in volcanic regions as people like to see volcanic springs, geysers and boiling mud. Geothermal energy provides electricity.
Monitoring and predicting volcanoes.
You can never predict the exact time of a volcanic eruption, as was shown with the Mt St Helens eruption. Earthquakes are a frequesnt sign of an impending eruption and their frequency and strength can be recorded.
Mt St Helens had a bulge on its side which was an indication that magma was moving below ground. Tiltmeters were used to measure this bulge. Global positioning systems (GIS) can detect movements as little as 1mm. Satellite images can show changes in temperature on the volcano. Digital cameras placed on the rim can take photos and monitor changes. Gases that are relased (especially sulphur dioxide) from the vent change before an eruption and these can be monitored as well. Robots called "spiders" enter the crater and collect samples.
We can tell how the volcano is likely to behave by past eruptions. All of this technology can help to evacuate people prior to an eruption.
Watch this film, listen to the lyrics and use this as a revision tool:
For your exam you will need to learn a detailed case study of a volcanic eruption, using Mount St Helens as your eruption. You will need to be able to discuss causes and effects of the eruption and the responses of people to the event. It is important that you learn some place specific detail / facts and figures to put into your exam answer in order to reach the highest marks.
CREATING YOUR CASE STUDY Through the use of class notes and independent research you now need to create your case study. You task is set out below and there are a number of links for you to follow up for further information.
TASK: Your task is to write an article for a magazine. You should give your work the title "Volcanic Fury - the 1980 eruption of Mount St Helens" and you need to ensure that you include labelled diagrams / pictures in your work. You need to ensure that you structure your work using the sub-heading given on the task sheet (which can be downloaded here).
Wikipedia - 1980 eruption of Mount St Helens - includes some very useful information on aftermath, including impacts such as cost etc. and a good overview of the build up to disaster - worth exploring!
To view Mount St Helens in Google Earth download this .kmz file (you will need Google Earth on your computer to be able to view this). See this fantastic panorama from the top of Mount St Helens after the eruption.
The point at which two tectonic plates meet is called a plate boundary. It is at these locations where tectonic activity results in earthquakes, volcanoes and the formation of mountain ranges due to the movement of the plates.The diagram below shows the major plates and their boundaries. The arrows indicate the direction of movement at each plate. It is the direction of movement as well as the difference in crust which determine the variations in processes and landforms at the different plate boundaries.(animation from USGS)There are a number of different types of plate boundaries.
For each plate boundary you will need to be able to describe (i) the movement (ii) processes which occur and (iii) an example
Watch this video to revise each one:
1. DESTRUCTIVE BOUNDARY (also known as a convergent boundary)
Movement: Two plates moving towards each other (continental and oceanic crust)(note where two oceanic plates meet one will be subducted and an island arcwill form)
Processes: The denser oceanic crust is subducted underneath the continental crust forming a subduction zone and oceanic trench. As it is subducted it melts due to heat and pressure. The heat sources are friction between the two plates and from the earth's interior. Melting of the subducting plates creates magma which is lighter than the mantle and therefore rises resulting in the formation of volcanoes. Earthquakes also occur at this type of boundary due to the friction and pressure during subduction.
Landforms Created: Fold Mountains and Ocean Trench
Example: South American and Nazca Plates (forming the Andes and a deep sea trench (Peru-Chile trench))
2. CONSTRUCTIVE BOUNDARY (also known as a divergent boundary
Movement: two plates moving away from each other (see animation opposite - courtesy of USGS) Processes: As the two plates separate, hot magma is able to rise to fill the 'gap' creating new crust. As magma continues to build up, new mountain ranges form under the sea creating a mid-oceanic ridge. Where rising magma continues to build up above the ocean surface, a volcanic island is formed (for example Surtsey, Iceland). Both earthquakes and volcanoes occur at this type of boundary.
Landforms Created: Ocean Ridge; Volcanic Islands
Example: North American and Eurasian Plate - (forming the Mid-Atlantic Ridge)
Movement: two plates moving towards each other (both continental crust)
Processes: As both plates consist of continental crust they both resist subduction and buckle and fold, being forced upwards to create fold mountains, such as the Himalayas. Although there is no volcanic activity at these locations, due to the forces of collision major earthquakes often occur here. Example: Indo-Australian and Eurasian Plate (forming the Himalayas)
Movement: two plates moving alongside each other Processes: crust is neither created or destroyed here but as both pressure and friction resulting during the movement of the plates side by side, a 'stick-slip' motion results in the creation of significant earthquakes. Pressures builds up due to friction between the plates and when the plates break apart the energy is sent through the earth as seismic waves in the form of an earthquake. Example: San Andreas Fault - North American and Pacific Plates
You should be aware that whilst most volcanoes / earthquakes occur along plate boundaries, there are exceptions. For example the volcanic Hawaiian islands which can be found in the middle of the Pacific Plate are formed due to a Hotspot. Hotspots are plumes of molten rock which rise underneath a plate causing localised melting and the creation of magma resulting in volcanic activity.
See this animation for further explanation of hotspot activity.
Key Term Check Constructive Boundary (Divergent) - where two plates move away from each other resulting in new crust being formed. Destructive Boundary (Convergent) - where two plates move towards each other - in the case of a plate consisting of continental crust meeting a plate consisting of oceanic crust, the oceanic crust will be subducted and destroyed as it is less dense. Conservative Boundary - where two plates move alongside each other - although crust is neither created or destroyed here, earthquakes usually occur here. Collision Boundary - where two plates of continental crust move towards each other creating fold mountains. Volcano - a vent through which lava, ash etc. is erupted (often, but not always cone-shaped) Earthquake - a sudden ground movement
Plates and Types of Crust The earth's crust is divided up into a series of slabs of crusts known as plates. These plates consist of two different types of crust: continental crust and oceanic crust.
There are important differences between the two types.
Oceanic crust is constantly being created and destroyed, it is therefore younger than continental crust and its higher density means that it can be subducted and destroyed. As continental crust is ligher with a lower average density, it is permanent and cannot sink. It is also much older and thicker, reaching up to 70km under mountains. In terms of rock type, continental crust is mainly granite, whereas oceanic crust is mainly made up of basalt.
Plate Movements and Convection Currents
The earth's tectonic plates are in motion, moving like giant 'rafts' on top of the semi-molten mantle below. However this movement is slow and rates vary from less than 2.5cm /yr to over 15cm/yr.The movement of the earth's crustal plates is believed to be due to convection currents which occur in the semi-molten mantle. These convection currents are created by heat from within the earth - much of which is generated by radioactive decay in the core.
So how do convection currents cause plate movements?
As semi-molten rock in the mantle is heated it becomes less dense than its surroundings and rises. As it reaches the crust above, it spreads out carrying the plates above with it. As the semi-molten rock then cools, it gradually sinks back down to be re-heated. (see diagram above)
Key terms check: Tectonic plate - individual slab of the earth's crust
Convection Current - transfer of heat throughout the mantle resulting in the rising and falling motion of semi-molten rock
Mantle - the area between the crust and the earth's core the majority of which is in a semi-molten statePlate Tectonics: An IntroductionContinental 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.
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
Click here for an excellent 
-the construction of sea walls and revetments as wave-resistant structures at the base of the cliffs
SPITS
Named Examples:Good examples of cliffs and wave-cut platforms can be found at Hunstanton (North Norfolk) and Flamborough Head (Yorkshire)
As wind blows over the surface of the sea, it creates friction. This frictional drag causes water particles to begin to rotate and energy is transferred forward in the form of a wave. Whilst the water moves forward, the water particles return to their original position. As a wave reaches shallow water, friction between the sea bed and the base of the wave causes the wave to begin to slow down and its shape becomes more eliptical. The top of the wave however, unaffected by the friction, becomes steeper until it eventually breaks. When the wave breaks, water washes up the beach, this is called the swash. The movement of water backdown the beach is called the backwash.It is the rate at which waves reach the coast which determine whether the main process acting on the coastline is erosion or deposition.
Bam, the ancient historic city in Iran, was hit by an earthquake measuring 6.6 on the Richter scale on December 26th, 2003 resulting in the deaths of over 43,000 people and leaving over 60,000 people homeless. Many of the mud-brick buildings in Bam collapsed resulting in the high loss of life. The mud-brick disintegrates easily into rubble, making rescue difficult and hopes of survival low. The survivors had not only lost friends and family, but their homes and everything else they had. Many were left destitute on the streets, some forced to spend the cold nights wrapped in blankets; whilst some were given tents, others made use of any shelter they could find. 90% of the buildings in the ancient citadel was completed destroyed (photo shows view over the city prior to the quake.
Some Volcanic eruptions are more explosive than others due to the type of magma. At destructive margins andesitic magma gives rise to acid lava which is thick and sticky. As gases can't escape easily pressure builds up resulting in violent explosions.
Mount St Helens, Washington State, NW USA is located in the Cascade mountain range and prior to its eruption in 1980 it had been active for over 100 years. The volcano sits on a destructive boundary where the Juan de Fuca plate meets the North American plate.
As semi-molten rock in the mantle is heated it becomes less dense than its surroundings and rises. As it reaches the crust above, it spreads out carrying the plates above with it. As the semi-molten rock then cools, it gradually sinks back down to be re-heated. (see diagram above)
