Revising the Plate Tectonics Unit
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 - 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 - San Francsico Earthquake - 1989 - 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 1989 San Francisco earthquake)
13. How can we monitor and prepare for earthquake activity?
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 (to access previous posts - use blog archive list on the left hand side of the blog - Dec/Jan posts) - 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 some interactive revision quizzes etc. here for you to test yourself.
Plate Tectonics Glossary - Key word flash cards (definition then word)
Plate Tectonics Glossary Key word flash cards (word then definition)
Plate Tectonics Glossary - Key Word Test
Plate Tectonics Crossword Quiz
Tectonic Activity (Penalty Shoot out) - Hull Trinity House school
Yr 10 Plate Tectonics Revision (Penalty Shoot out)
Plate Tectonics Quick Quiz (Multiple Choice)
Sunday, January 28, 2007
Case Study of an Earthquake in an LEDC - Bam 2003
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. Read more about this in this excellent article from the BBC. One of the big criticisms by the domestic press and independent organisations at the time was over the sub-standard housing (with many people dying in collapsed buildings) calling into question the role of the Minister for Housing.
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. So why was the death toll of the Bam earthquake so high? See this excellent articles from the Guardian "Dangerous buildings, lax rules: why Bam death toll was so high" and "Why did so many have to die in Bam?"
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:
- Twin Earthquakes - comparing the California and Bam earthquakes of 2003
- Why did the Bam Earthquake Happen (some excellent links thanks to Noel Jenkins of Court Fields School)
- Preliminary report on the Bam Earthquake - Iran
- Bam: Iran's Ancient City (includes photographs and further information)
- Lessons learnt on the national and international response to the Bam earthquake
- Iran Earthquake - Preliminary Reconnaissance (Using Remotely Sensed Data the Views (Visualising the impacts of earthquakes with satellite images) system) - some amazing before and after images here). and also a very detailed report here ICG Report - Bam Earthquake - December 2003
News Stories from the time (BBC News)
The Earthquake: Causes / Effects / Responses:
Bam: Jewel of Iranian Heritage
Tending to Iran's shattered City
Iran battles to cope with disaster
Concealed fault caused Bam quake
Starting from scratch in Bam
Iran lowers Bam earthquake toll
Rebuilding Bam 'could cost $1bn'
Iran considers moving capital
Bam: a year after the earthquake (includes video clips)
Iran quake survivors battle cold
Rescue Missions / Helping the people of Bam:
Helping the people of Bam (work of aid / relief workers to help survivors)
Devon rescue team on Quake Mission
Helping the people of Bam
Woman helps reunite quake survivors (story of a red cross worker)
Iran Earthquake - How to help (what was needed in the immediate aftermath - who helped?)
Earthquake Rescue (audio tale of Phil Haigh who has been working in Disaster Relief since 1968)
Response to the Earthquake / Controversy over lack of Preparedness:
The politics of earthquakes
Iran press lambasts quake efforts
Tough questions over Iran quake
Press tackles Iran over quake
Earthquake angers Iran media
Survivors recall Iran quake loss
Pictures:
In Pictures: Bam before and after
In Pictures: Symbols of Hope
Your Pictures: Bam after the earthquake
Original source of photo: http://commons.wikimedia.org/wiki/Image:BAM_IR2726.JPG
Saturday, January 20, 2007
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 1989 San Francisco earthquake was around 80km from San Francisco whereas in 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 San Francisco 1989 quake and Bam 2003 quake - both of which were around 6.6-6.9 on the richter scale yet with vast differences in the number of fatalities - 67 in the San Francisco 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
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 1989 San Francisco earthquake was around 80km from San Francisco whereas in 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 San Francisco 1989 quake and Bam 2003 quake - both of which were around 6.6-6.9 on the richter scale yet with vast differences in the number of fatalities - 67 in the San Francisco 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 Marine District in San Francisco during the 1989 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! Also see this article Factors that Affect Damage in an Earthquake for information on effects and factors that can affect them.
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..
Follow Up Links:
Can we predict earthquakes?
Wikipedia - Earthquake Prediction
When will the next earthquake occur?
Safer Structures, Engineering and Building Codes (USGS)
Earthquake Preparedness (USGS)
Preparedness & Response (USGS - Earthquake Hazards Program)
Earthquake Survival Manual (Tokyo)
What to do in case of an earthquake in Tokyo
Earthquake Preparedness Handbook (LA Fire Department)
Photo Source: Chuetsu Earthquake 2004 (http://commons.wikimedia.org/wiki/Image:Chuetsu_earthquake-Yamabe_Bridge.jpg)
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)
- Earthquake proof modern buildings - this may include reinforcement of foundations; counter-weights; and use of fireproof construction materials. (see here for animation on buildings resisting earthquakes)
- Building Regulations - avoid building on unstable ground (such as landfill) to avoid liquefaction
- Practice Disaster Routines - annual earthquake drills - e.g. San Francisco - April 18th (also see this news article on Japan's quake drill)
- Automatic shut off switches - for gas mains - to try and minimise the likelihood of fire
- Public Education - public information posters / information on emergency procedures / preparing house / survival kits etc. (e.g. Is Your Home Protected from Earthquake Disasters?)
- Strengthen Routeways - strengthen foundations of bridges / suspended roadways etc.
- 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.
See also this excellent factsheet from the USGS looking at efforts to reduce the impact of future large quakes in the San Francisco Bay Region (Progress Toward a Safer Future Since the 1989 Loma Prieta Earthquake)
Follow Up Links:
Can we predict earthquakes?
Wikipedia - Earthquake Prediction
When will the next earthquake occur?
Safer Structures, Engineering and Building Codes (USGS)
Earthquake Preparedness (USGS)
Preparedness & Response (USGS - Earthquake Hazards Program)
Earthquake Survival Manual (Tokyo)
What to do in case of an earthquake in Tokyo
Earthquake Preparedness Handbook (LA Fire Department)
Photo Source: Chuetsu Earthquake 2004 (http://commons.wikimedia.org/wiki/Image:Chuetsu_earthquake-Yamabe_Bridge.jpg)
Earthquake Case Study: 1989 San Francisco Earthquake
On 17th October 1989, at 5.04 pm, an earthquake measuring 6.9 on the Richter scale, and lasting only 15 short, but devastating seconds, hit San Francisco (see shake intensity map) The following YouTube video gives an overview of some of the images of the aftermath of the earthquake. You will need to learn the case study of the San Francisco Earthquake. You need to have an understanding of both the causes of the quake and its effect on people, the environment and the economy.Earthquakes
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. To see how a seismograph works, watch this excellent animation. 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 1906, a particularly large earthquake, measuring 8.2 on the Richter scale hit San Francisco
(California, USA). Whilst at the time around 500 deaths were actually reported, the actual figure is believed to be around 3,000, the largest death toll from a natural disaster to be recorded in California. Although the actual shaking of the ground during the earthquake was incredibly damaging, it is estimated (wikipedia source) that 90% of the destruction was caused by the severe fires (see photograph) which raged following the earthquake, many starting as a consequence of ruptured gas mains.
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. 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.
You need to learn a case study of the causes and consequence of a major earthquake - see post on the 1989 San Francisco earthquake.
Follow up links:
USGS - The Richter Magnitude Scale
National Earthquake Information Centre
Earthquakes - General interest publication from the USGS
Wikipedia - 1906 San Francisco Earthquake
Discover our Earth - Earthquakes
How earthquakes work
Understanding Earthquakes
Earthquakes do occur in Britain - see the British Geological Survey site Earthquakes in the British Isles
Map of recent earthquakes in the UK (BGS)
Earthquake Animation
Key Term Check:
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
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. To see how a seismograph works, watch this excellent animation. 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 1906, a particularly large earthquake, measuring 8.2 on the Richter scale hit San Francisco
(California, USA). Whilst at the time around 500 deaths were actually reported, the actual figure is believed to be around 3,000, the largest death toll from a natural disaster to be recorded in California. Although the actual shaking of the ground during the earthquake was incredibly damaging, it is estimated (wikipedia source) that 90% of the destruction was caused by the severe fires (see photograph) which raged following the earthquake, many starting as a consequence of ruptured gas mains.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. 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.
You need to learn a case study of the causes and consequence of a major earthquake - see post on the 1989 San Francisco earthquake.
Follow up links:
USGS - The Richter Magnitude Scale
National Earthquake Information Centre
Earthquakes - General interest publication from the USGS
Wikipedia - 1906 San Francisco Earthquake
Discover our Earth - Earthquakes
How earthquakes work
Understanding Earthquakes
Earthquakes do occur in Britain - see the British Geological Survey site Earthquakes in the British Isles
Map of recent earthquakes in the UK (BGS)
Earthquake Animation
Key Term Check:
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
Sunday, January 14, 2007
Volcanoes - Prediction Technology and Satellite Imagery
Advances in science and monitoring techniques have meant that scientists have made great advances in forecasting eruptions. A number of different techniques are used to monitor changes in physical processes which may indicate increasing volcanic activity. However, due to the availability of expertise, cash and technology, only 20% of volcanoes are currently monitored, most of which are in MEDCs. Prediction / Monitoring Techniques:
Remote Sensing
Landsat images which show the amount of energy from the sun being reflected from the earth's suface when an image is taken by a satellite, can be used to indicate environmental change and can be used to identify areas affected by an eruption. This can then be used for future hazard mapping (see below).
Other satellite imagery can be used to predict volcanic eruptions. Infra-red images of volcanoes can be made every 15 minutes by geo-stationary satellites allowing thermal mapping to be used to detect hot spots where magma is rising to the surface, enabling a warning to be given. In 1998, the eruption of Pacya in Guatemala was detected a week before it happened by a satellite using infra-red detectors. A heat signal was picked up indicating hot magma rising to the surface. Success in predicting eruptions in this way has been experienced in other areas as well. However, in some areas the sheer number of volcanoes combined with the poverty of the area means the technique is less useful. Where it can be used, it can however provide valauble hours or even days in which to evacuate people from areas facing an impending eruption.
See also the USGS overview of Remote Sensing for Monitoring Volcanoes
Seismicity
Earthquake activity can often increase prior to a volcano as magma and gas rises. The rising magma and gas can cause vibrations and trigger earthquakes. Scientists monitoring seismicity (earthquakes activity) in volcanic areas can detect an increase in earthquakes and the type and intensity of the activity can be used to determine when an eruption is occuring. For further information see the USGS overview of Monitoring Volcano Seismicity
Ground Deformation
As magma rises it can cause a change in the shape of a volcano. Changes in the ground may include subsidence, tilting, or even the formation of a bulge when the magma rises. Remember, during the lead up to the eruption of Mt St Helens a cryptodome formed on its side due to the rising magma. Tilt metres, surveying techniuqes and even satellite imagery can all be used to detect and monitor ground deformation. For more information on the techniuqes used, see the USGS overview of Monitoring Volcano Ground Deformation
Volcanic Gases
The build up of gases are one of the driving forces of volcanic eruptions and monitoring gases such as sulphur dioxide can be important in the prediction / monitoring of volcanic activity. An increase in sulphur dioxide emissions can reflect rising magma. Likewise, a sudden decline in sulphur dioxide emissions following a period of rapid increase can suggest some blockage which may result in the build up of pressure prior to eruption. For more information on the measuring of gas emissions, see the USGS overivew of Monitoring Volcanic Gases
Hazard Assessment
Hazard Mapping
Being prepared for volcanic events can ensure the saving of many lives through successful evacuation. As well as careful monitoring of volcanoes and the implementation of carefully thought out evacuation plans, hazard mapping can help to determine the areas most at risk from an eruption by taking into account the behaviour and hazards associated with previous eruptions in order to help determine which areas most at risk and thus to determine evacuation zones.
This is an example of a Hazard Zonation map for Mount St Helens - it comes from the USGS's report on Volcanic-Hazard Zonation for Mount St Helens, Washington, 1995 (USGS) - the report considers areas that are most likely to be at risk (known as hazard zones) during future eruptions. The zonation takes into account experience during previous major eruptions such as the 1980 eruption as well as monitoring of changes in the volcanoes hydrology, topography and geology.
See USGS - Volcano Hazard-Assessment Techniques.
Research into volcanic risk and hazard management can help in a number of ways, but the messages don't always work, for example due to distrust in information sources or public apathy resulting from long periods of inactivity by some volcanoes. This poster explores the dangers of volcanoes, why lives are still lost, how research can help and why messages of warning / prediction don't always work.
Follow Up Links:
Wikipedia - Volcano Prediction - a comprehensive account of general principles and methods of volcano prediction / monitoring
Discovery Channel - Monitoring and Predicting Volcanoes
USGS Cascades Volcano Observatory - Volcano Monitoring Overview
Deadly Shadow of Vesuvius - Can we predit Eruptions?
Prediction of Danger - methods and instructments of monitoring
Volcanoes - Can we predict Volcanic eruptions?
Volcano Monitoring Techniques
USGS Overview - Volcano-Monitoring Techniques
Key Term Check:
Remote Sensing - the use of satellite imagery to detect clues of volcanic activity which are beyond the normal range of human revision, using ultra-violet, infra-red and microwave sensors. As well as tracking eruption clouds, thermal-sensing can be used to detect hot features on volcanoes which can indicate areas where magma is rising closer to the surface.
Ground Deformation - this refers to changes in the slope / shape of a volcano which can indicate the rising / build of magma. (think about the cryptodome which grew on the North flank of Mount St Helens prior to its eruption in 1980)
Hazard Mapping - a technique used to map out areas most at risk during future eruptions and the likely hazards (this mapping is based on experiences during past eruptions)
Seismicity - this simply refers to earthquake activity (which frequently preceeds / accompanies volcanic eruptions).
Photo courtesy of USGS
Living with Volcanoes
Despite the well known hazards associated with volcanic eruptions, it is estimated that 360 million people still live in volcanically active areas. So why do people still live near volcanoes?
Reason: Fertile Soils
Explanation: Volcanic soils are some of the most fertile in the world due to the weathering of volcanic products such as ash lava and rock, which release valuable nutrients and minerals which enrich the soil as well as improving soil characteristics such as moisture retention. In tropical areas in particular, for example Hawaii, climate conditions mean that the weathering of lava etc. is fairly quick resulting in the growth of lush vegetation due to the rapid soil formation. As volcanic areas are therefore ideal for growing crops, they attract large populations.
Example: In Java (Indonesia) some of the best rice growing areas are in the shadow of volcanoes such as Mt Merapi which attract large numbers due to the rich farming opportunities (1 million live within 20 miles of Merapi). Likewise, in Italy large numbers live on the slopes of Vesuvius and Etna (one in five Sicilians are belived to live on the slopes of Etna) due to the fertile soils which provide rich opportunities for growing products such as Olives and fruit.
Reason: Geothermal Energy
Explanation: Heat from magma sources close to the surface in volcanic areas can be used as a source of Geothermal energy which can be harnessed to produce electricity. In these instances, superheated steam, created by the heating of water in permeable rocks in magma can be used to drive turbines. This use of energy is renewable and sustainable, it has the added advantage of being pollution free.
Example: Over 20 countries around the world generate geothermal power, including the US, Italy, New Zealand and Iceland. In fact 17% of Iceland's electricity is created in this way.
In farming areas around Reykjavik (Iceland), geothermal energy is also used to heat greenhouses enabling the growing of fruit, vegetables and flowers.
Reason: Tourism
Explanation: Due to the spectacular scenery associated with volcanic landscapes and unique features such as lava flows and geyers, volcanoes, particularly those having experienced recent eruptive activity are particularly popular with tourists. This is a huge economic benefit due to the resulting multiplier effect. Tourism attracts curstom for businesses such as hotel, cafes etc. creating jobs and improving the local economy.
Example: Yellowstone National Park in the USA with the famous Old Faithful geyser receives around 3 million visitors a year. Iceland is famous for its volcanic landscpae and its hot springs and geysers have attracted many tourists. The Blue Lagoon, near Reykjavik is a spa popular with tourists for its known positive effectives on the skin.
Reason: Minerals
Explanation: Valuable minerals such as copper, gold, silver, lead, zinc and even diamonds are all associated with volanic regions as they are associated with the rising magma which may cool and harden beneath the volcano. As hot water circulate within the cooled magma, the metals are taken by the water and re-deposited in greater concentrations. Thus volcanic areas are excellent areas for mining creating economic activites through job opportunities and the value of the mined minerals.
Example: Copper, Gold and Silver mining began around Mount St Helens as early as 1892.
Reason: Apathy / Unwillingness to Leave Home
Explanation: Due to the infrequency of some volcanic eruptions, some people, particularly those who have not experienced a volcanic eruption in their lifetime are reluctant to leave their homes in order to move to safety and ignore warning, preferring to live with the threat of a volcanic eruption. Some believe that there will be time to move / be resuced should an eruption begin.
Example: Harry Truman was an 83 years old man who lived by Spirit Lake in the Shadow of Mount St Helens. He died in the 1980 eruption due to his failure to heed the warnings of the government and evacuate the area.
Reason: Lack of Choice
It should also be recognised that some people have no choice but to live in these areas. In areas of poverty, people do not have the resources available to move and for many farming on the fertile soils in the shadow of a volcano may be the only livelihood they know.
Follow up Links:
USGS - The "Plus Side" of Volcanoes
Geography Pages - Benefits of Living in Volcanic Areas (thanks to Alan Parkinson)
Photo Source: USGS
Sunday, January 07, 2007
Volcano Case Study: Mount St Helens 1980
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.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 and ash filling in Spirit Lake and log jams and ash blocking the channel of the Toutle River;
* 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
* flooding resulting from blocked rivers washed away road and rail bridges
* crops were ruined and livelihoods of loggers were devastated with large areas of trees being flattened like matchsticks.
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).
The following websites should provide useful information and photographs to help you, but you should also make good use of your video notes and information from classwork.
USGS Background Information on Mt St Helens
Mount St Helens National Volcanic Monument - includes tourist information related to Mount St Helens and a useful digital library with pre and post eruption images (useful for comparions / exploring effects).
Global Volcanism Programme - St Helens (basic facts)
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!
Mount St Helens - from the 1980 eruption to 2000 (USGS)
Vegetation around the volcano - before and after (comparative photographs)
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.
Photograph courtesy of the USGS
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.
Follow up Links:
Some great images and information on individual volcanoes at Volcano World
Animation - eruption of a stratovolcano (Savage Earth)
Volcanoes Online - a detailed source of information on types of volcanoes
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 surface
The photographs in this post are courtesy of the USGS
Plate Boundaries
Plate Boundaries
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
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 arc will 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))
Animation: click here to see an animation of the processes at a destructive plate boundary (with thanks to Wycombe High School)
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)
Animation: click here to see an animation of the processes at a constructive plate boundary
3. COLLISION BOUNDARY
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)
Animation: Click here to see an animation of the processes at a collision boundary
4. CONSERVATIVE BOUNDARY
Movement: two plates moving alongside each other
Processes: crust is neither created or destroyed here but as both pressure and friction results 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
Animation: See this BBC visualisation of the San Andreas Fault conservative boundary
HOTSPOTS
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.
Follow up Links
An excellent site from the USGS called "Understanding Plate Motions" provides further information
A Flash animation showing the major plate boundaries.
Plate Tectonics on Wikipedia also provides a good overview of the processes at plate boundaries
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
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
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 arc will 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))
Animation: click here to see an animation of the processes at a destructive plate boundary (with thanks to Wycombe High School)
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)
Animation: click here to see an animation of the processes at a constructive plate boundary
3. COLLISION BOUNDARY
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)
Animation: Click here to see an animation of the processes at a collision boundary
4. CONSERVATIVE BOUNDARY
Movement: two plates moving alongside each other
Processes: crust is neither created or destroyed here but as both pressure and friction results 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
Animation: See this BBC visualisation of the San Andreas Fault conservative boundary
HOTSPOTS
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.
Follow up Links
An excellent site from the USGS called "Understanding Plate Motions" provides further information
A Flash animation showing the major plate boundaries.
Plate Tectonics on Wikipedia also provides a good overview of the processes at plate boundaries
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 Convection Currents
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)
Follow up links:
The Earth's Crust in Motion - an excellent page from St Vincent's College - exploring the nature of the crust and its movement due to convection currents - well worth a read!
A simple animation of convection and the resulting movement of plates
A clear and detailed animation of convection currents in the mantle
An animation of convection in the mantle (Exploring Earth)
The Mantle A good overview of the characteristics and influence of the mantle in tectonics
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 state
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)
Follow up links:
The Earth's Crust in Motion - an excellent page from St Vincent's College - exploring the nature of the crust and its movement due to convection currents - well worth a read!
A simple animation of convection and the resulting movement of plates
A clear and detailed animation of convection currents in the mantle
An animation of convection in the mantle (Exploring Earth)
The Mantle A good overview of the characteristics and influence of the mantle in tectonics
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 state