Volcanoes are openings or cracks in the earth's surface that allow molten magma (or other material) to escape from the mantle beneath. Volcanoes are most well-known for releasing lava, but they may also release volcanic ash, rocks and gases.
Distribution of Volcanoes
As can be seen on the map below, the earth's active volcanoes are located in specific areas, including:
Around the edge of the Pacific Ocean
Down the centre of the Atlantic Ocean
In Southern Europe
Down the east coast of Africa
The reason for the distribution of volcanoes, is that they are located on or near tectonic plate boundaries, specifically destructive and constructive boundaries.
There are a few exceptions in the middle of the Pacific Ocean, noticeably on the islands of Hawaii. These volcanoes are caused by hotspots. The east coast of Africa is also not a plate boundary, but rather a plate (African Plate) ripping itself in half creating a rift valley lined by volcanoes.
Because volcanoes are normally found on plate boundaries their spatial concentration is limited. Their areal extent is also normally limited to areas immediately around the volcanoes, although volcanic ash clouds can potentially have global impacts by disrupting air travel and causing climate change.
Ring of fire: This is the name commonly given to the area around the Pacific Ocean. It gets its name because it has the biggest concentration of volcanoes.
Hot spots: These are volcanoes that are not found on plate boundaries. The most common explanation in mantle plume theory. This is when hot magma melts the crust above and escapes. Because tectonic plate are constantly moving, but mantle plumes stay stationary they normally create a chain of volcanoes e.g. the islands of Hawaii.
Key Volcanic Terminology
Lava: Molten rock above the surface of the earth.
Magma: Molten rock below the surface of the earth.
Magma Chamber: A store of magma found below the surface of the earth. When the pressure becomes to great in the magma chamber, volcanoes occur.
Vent: The main passage by which magma travels from the magma chamber to the crater. You can also get smaller secondary vents that often split off from the main vent.
Crater: A large hole or depression that has been created by a volcano. Lakes will often form in the bottom of lakes, they are known as crater lakes.
Eruption: A release of volcanic lava, ash or gas.
Active volcano: A volcano that regularly erupts and has erupted in recent history.
Dormant volcano: A volcano that has not erupted recently, but may erupt again in the future. It is unclear how long a volcano has to be dormant, before it is classified as dormant.
Extinct volcano: A volcano that will not erupt again in the future. It is unclear when an active or dormant volcano becomes extinct. Some people argue its is extinct if there is no reordered eruptions, others if it has not erupted for 10,000 years.
Volcanic Hazards
Volcanoes can cause multiple hazards (both primary and secondary hazards). Each hazard can have varying impacts. Below is a summary of volcanoes major hazards and their likely impact:
Primary Hazards: Hazards that are a direct result of the eruption and are caused by the released of substances during the eruption.
Lava Flow: The most commonly associated hazard with volcanoes. Lava flows are simply rivers of molten rock. Viscous (thick) lava flows are very slow, which means most lava flows can be avoided by humans. However, they can cause massive damage to land and property and trigger fires.
Tephra (Lava Bombs): Any material that is ejected from a volcano during an eruption. As long as you are standing a safe distance, humans should not be effected by tephra although they can damage buildings and start secondary fires.
Pyroclastic Flow: Probably the most dangerous of all volcanic hazards are pyroclastic flows (sometimes called nuee ardentes) which are superheated clouds of ash, gas and small tephra. They can travel at speeds up to 500km/hr and incinerate anything in their path.
Ash Cloud: Ash clouds are normally released into the atmosphere. Although they don't pose much immediate danger they can disrupt air travel and when the ash falls to ground it can crush buildings and bury farmland and also cause the secondary hazard of acid rain.
Poisonous Gases: Often released before a major eruption these gases can be deadly to animals and humans if inhaled in sufficient quantities.
Secondary Hazards: Hazards that happen as a result of primary hazards.
Lahar (mudslide): Volcanoes ash and/or lava can cause snow to melt or they can mix with river/rain water and create mudslides, commonly known as lahars.
Acid Rain: Gases released during an eruption e.g. sulphur dioxide can mix with water vapour in the atmosphere and create acid rain which can damage buildings and change the pH of soils and lakes killing plant and animal life.
Climate Change: Gases released into the atmosphere e.g. sulphur dioxide can enhance the greenhouse effect causing global warming. However, ash released into the atmosphere can also absorb or reflect incoming solar radiation and reduce global temperatures.
Fires: Tephra and lava flows can start fires which can cause widespread damage to buildings and land.
Predicting and Measuring Volcanoes
Volcanoes are easier to predict than earthquakes, volcanologits can try and predict volcanoes by looking for:
Changes in the shape of a volcano
Changes in the amount of gas being released
Changes in the temperature
Tectonic activity (earthquakes)
Animal behaviour
Changes in local hydrology
Mass movements
Because there are many types of volcanoes and types of volcanic hazard, it is hard to compare one volcano with another. However, one method that has been used is the volcano explosivity index (VEI) shown to the right. The VEI was developed by Stephen Self and Chris Newhall in 1982. The scale is open-ended and measured by looking at:
Volume of material released in eruption
cloud height
and qualitative observations
All material released (tephra, ash, etc) is all treated the same when calculating volume. The largest volcano over recorded in history was an 8.0 (Yellowstone and Toba). Both of these eruptions were classified as supervolcanoes with a frequency of 10,000 years.
Mount St. Helens Volcano
Mount St. Helens is located in the Cascades mountain ranges which is part of the North American Rockies. It sits on a destructive plate boundary (convergent plate boundary) where the Pacific plate and Juan de Fuca plate subduct under the North American plate. Mt St. Helens had been dormant for nearly 120 years when on the morning of 18th May 1980 a 5.0 earthquake triggered a huge landslide and pyroclastic flow. The pyroclastic flow travelled for 25km and flattened everything in its path. Ash and gas continued to be released from the volcano over the course of the day and reached the east coast of the US three days later.
Sixty one people lost their lives in the tragedy, mainly residents refusing to leave and scientists monitoring the volcano. Spirit lake was destroyed along with 250km of fishing rivers. 250km2 of forest was destroyed and 10 million trees had to be replanted. No animals survived in the blast zone and many crops were destroyed by falling dust.
Earthquakes are caused by sudden releases in energy from the earth's crust, resulting in seismic waves.
Distribution of Earthquakes
Earthquakes tend to follow a fairly distinct pattern. The map to the right shows that there have been a lot of earthquakes:
Around the edge of the Pacific Ocean
Down the centre of the Atlantic Ocean
Along the Caribbean archipelago
Southern Europe
South Asia
East Africa
The Philippine and Indonesian archipelagos
The reason for their fairly organised distribution is that they tend to be focused along plate boundaries, where there are build ups and releases of pressure as plates collide or diverge. Unlike volcanoes, earthquakes can be found at any plate boundary. There are a few exceptions to the rule like the earthquakes found in Central Australia or the east coast of the US. These exceptions are called intraplate earthquakes.
Intraplate earthquake: Earthquakes found in the centre of plates. The quakes might be on old fault lines or weaknesses that are often unknown. These earthquakes can cause large damage because people aren't expecting them.
Humans can also cause earthquakes through explosions, the building of dams and mining.
Epicentre: The location on the earth's surface directly above the hypocentre.
Hypocentre (focus): The actual site/location that an earthquake takes place.
Aftershock: A smaller earthquake that takes place in the coming hours, days and weeks after the main earthquake.
Seismic Waves: These are waves of energy that travel through the earth as a result of an earthquake. There are two types of waves; body waves that travel through the earth and can be divided into p-waves (more longitudinal) and s-waves (more transverse) and surface waves that travel across the surface.
Tremor: A tremor is another name for an earthquake, but is all sometimes the name given to a lesser earthquake or the felt effects of a big earthquake by people living further from the epicentre.
USGS: The United States Geological Survey is an organisation that records many of the natural disasters that take place around the world. Go to the link below to look at the world's most recent earthquakes: USGS - Recent World Earthquakes
Earthquake Hazards
Primary Hazards
Ground Shaking: The movements of the ground caused by the seismic waves can fell buildings, bridges, trees, etc. killing and trapping people.
Secondary Hazards
Tsunamis: The sudden shifting of tectonic plates under the sea can displace large amounts of water which can trigger massive tsunamis. The 2011 Japanese tsunami and the 2004 Indian Ocean tsunami both caused more deaths than the earthquakes that triggered them.
Fire: Earthquakes can break gas cables and knock over ovens and open fires which trigger secondary uncontrolled fires.
Liquefaction: This is where a saturated soil loses strength and rigidity because of applied stress, normally an earthquake. The changes in its state causes the ground to behave like water allowing things to sink into it. The recent earthquake in Christchurch New Zealand saw large scale liquefaction
Mass movements (landslides): The sudden movement of the earth and subsequent seismic waves can trigger landslides and avalanches which can bury and kill many people. The El Salvador earthquake of 2001 triggered a landslide in Santa Tecla which killed hundreds of people.
Floods: damage to flood defences or even dams can cause widespread flooding.
Factors Affecting the Impact of Earthquakes
Depth: If the hypocentre of an earthquake is close to the surface then it is more likely to cause greater damage than a deep earthquake.
Duration: A longer earthquake is likely to cause greater damage than an earthquake that lasts only a few seconds.
Magnitude: Obviously a stronger earthquake is going to have a greater impact than a weaker one.
Time of Day: Time of day can be important. If people are sleeping and get trapped in their beds more people can be killed. In Japan an earthquake that struck while people were cooking their evening dinner caused widespread secondary hazards (fire) that caused more deaths.
Epicentre Location: If the epicentre of an earthquake is an uninhabited region it is going to have a lesser effect than one under a densely populated city.
Geology: If an earthquake occurs in solid bedrock it is likely to cause less damage than one centred below an alluvial floodplain which may lead to liquefaction.
Economic Development (buildings, planning, preparedness): Generally speak more developed countries have better zonal planning, building codes and preparedness mean the effects of the earthquake are less.
Earthquakes are extremely hard to predict. Scientists can normally predict where earthquakes are likely to happen, but they can not predict when they will happen and how strong they will be. Scientists can attempt to predict by looking at:
Microearthquakes
Changes in rock stress
Ground subsidence, uplift or tilt
Changes in magnetic field and electrical resistivity of rocks
Richter Scale: The Richter Scale was developed by Charles Richter in 1935. It uses a base 10 logarithmic scale. The scale is normally seen from 0-10, but in theory could go above this. The scale measures the amplitude of waves on a seismograph. An earthquake of 5.0 is ten times stronger than one of 4.0. The largest earthquake ever recorded was a 9.5 of the coast of Chile in 1960.
Seismograph: The name commonly given to seismometers. It records movements in the earth caused by seismic waves. A picture of a seismograph is shown to the right.
Mercalli Scale: Instead of measuring an earthquakes energy like the Richter scale the Mercalli scale looks at the effects of an earthquake. The scale goes from 1 (hardly felt) up to 12 (total destruction). The scale is obtained by looking at the effects on humans, nature and structures.
Earthquakes and Volcanoes
Volcanoes
Volcanoes are openings or cracks in the earth's surface that allow molten magma (or other material) to escape from the mantle beneath. Volcanoes are most well-known for releasing lava, but they may also release volcanic ash, rocks and gases.
Distribution of Volcanoes
As can be seen on the map below, the earth's active volcanoes are located in specific areas, including:
The reason for the distribution of volcanoes, is that they are located on or near tectonic plate boundaries, specifically destructive and constructive boundaries.
There are a few exceptions in the middle of the Pacific Ocean, noticeably on the islands of Hawaii. These volcanoes are caused by hotspots. The east coast of Africa is also not a plate boundary, but rather a plate (African Plate) ripping itself in half creating a rift valley lined by volcanoes.
Because volcanoes are normally found on plate boundaries their spatial concentration is limited. Their areal extent is also normally limited to areas immediately around the volcanoes, although volcanic ash clouds can potentially have global impacts by disrupting air travel and causing climate change.
Ring of fire: This is the name commonly given to the area around the Pacific Ocean. It gets its name because it has the biggest concentration of volcanoes.
Hot spots: These are volcanoes that are not found on plate boundaries. The most common explanation in mantle plume theory. This is when hot magma melts the crust above and escapes. Because tectonic plate are constantly moving, but mantle plumes stay stationary they normally create a chain of volcanoes e.g. the islands of Hawaii.
Key Volcanic Terminology
Lava: Molten rock above the surface of the earth.
Magma: Molten rock below the surface of the earth.
Magma Chamber: A store of magma found below the surface of the earth. When the pressure becomes to great in the magma chamber, volcanoes occur.
Vent: The main passage by which magma travels from the magma chamber to the crater. You can also get smaller secondary vents that often split off from the main vent.
Crater: A large hole or depression that has been created by a volcano. Lakes will often form in the bottom of lakes, they are known as crater lakes.
Eruption: A release of volcanic lava, ash or gas.
Active volcano: A volcano that regularly erupts and has erupted in recent history.
Dormant volcano: A volcano that has not erupted recently, but may erupt again in the future. It is unclear how long a volcano has to be dormant, before it is classified as dormant.
Extinct volcano: A volcano that will not erupt again in the future. It is unclear when an active or dormant volcano becomes extinct. Some people argue its is extinct if there is no reordered eruptions, others if it has not erupted for 10,000 years.
Volcanic Hazards
Volcanoes can cause multiple hazards (both primary and secondary hazards). Each hazard can have varying impacts. Below is a summary of volcanoes major hazards and their likely impact:
Primary Hazards: Hazards that are a direct result of the eruption and are caused by the released of substances during the eruption.
Lava Flow: The most commonly associated hazard with volcanoes. Lava flows are simply rivers of molten rock. Viscous (thick) lava flows are very slow, which means most lava flows can be avoided by humans. However, they can cause massive damage to land and property and trigger fires.
Tephra (Lava Bombs): Any material that is ejected from a volcano during an eruption. As long as you are standing a safe distance, humans should not be effected by tephra although they can damage buildings and start secondary fires.
Pyroclastic Flow: Probably the most dangerous of all volcanic hazards are pyroclastic flows (sometimes called nuee ardentes) which are superheated clouds of ash, gas and small tephra. They can travel at speeds up to 500km/hr and incinerate anything in their path.
Ash Cloud: Ash clouds are normally released into the atmosphere. Although they don't pose much immediate danger they can disrupt air travel and when the ash falls to ground it can crush buildings and bury farmland and also cause the secondary hazard of acid rain.
Poisonous Gases: Often released before a major eruption these gases can be deadly to animals and humans if inhaled in sufficient quantities.
Secondary Hazards: Hazards that happen as a result of primary hazards.
Lahar (mudslide): Volcanoes ash and/or lava can cause snow to melt or they can mix with river/rain water and create mudslides, commonly known as lahars.
Acid Rain: Gases released during an eruption e.g. sulphur dioxide can mix with water vapour in the atmosphere and create acid rain which can damage buildings and change the pH of soils and lakes killing plant and animal life.
Climate Change: Gases released into the atmosphere e.g. sulphur dioxide can enhance the greenhouse effect causing global warming. However, ash released into the atmosphere can also absorb or reflect incoming solar radiation and reduce global temperatures.
Fires: Tephra and lava flows can start fires which can cause widespread damage to buildings and land.
Predicting and Measuring Volcanoes
Volcanoes are easier to predict than earthquakes, volcanologits can try and predict volcanoes by looking for:
Because there are many types of volcanoes and types of volcanic hazard, it is hard to compare one volcano with another. However, one method that has been used is the volcano explosivity index (VEI) shown to the right. The VEI was developed by Stephen Self and Chris Newhall in 1982. The scale is open-ended and measured by looking at:
All material released (tephra, ash, etc) is all treated the same when calculating volume. The largest volcano over recorded in history was an 8.0 (Yellowstone and Toba). Both of these eruptions were classified as supervolcanoes with a frequency of 10,000 years.
Mount St. Helens Volcano
Mount St. Helens is located in the Cascades mountain ranges which is part of the North American Rockies. It sits on a destructive plate boundary (convergent plate boundary) where the Pacific plate and Juan de Fuca plate subduct under the North American plate. Mt St. Helens had been dormant for nearly 120 years when on the morning of 18th May 1980 a 5.0 earthquake triggered a huge landslide and pyroclastic flow. The pyroclastic flow travelled for 25km and flattened everything in its path. Ash and gas continued to be released from the volcano over the course of the day and reached the east coast of the US three days later.
Sixty one people lost their lives in the tragedy, mainly residents refusing to leave and scientists monitoring the volcano. Spirit lake was destroyed along with 250km of fishing rivers. 250km2 of forest was destroyed and 10 million trees had to be replanted. No animals survived in the blast zone and many crops were destroyed by falling dust.
1980: Nine dead after Mount St Helens eruption - BBC article
Mount St Helens - BBC
Earthquakes
Earthquakes are caused by sudden releases in energy from the earth's crust, resulting in seismic waves.
Distribution of Earthquakes
Earthquakes tend to follow a fairly distinct pattern. The map to the right shows that there have been a lot of earthquakes:
The reason for their fairly organised distribution is that they tend to be focused along plate boundaries, where there are build ups and releases of pressure as plates collide or diverge. Unlike volcanoes, earthquakes can be found at any plate boundary. There are a few exceptions to the rule like the earthquakes found in Central Australia or the east coast of the US. These exceptions are called intraplate earthquakes.
Intraplate earthquake: Earthquakes found in the centre of plates. The quakes might be on old fault lines or weaknesses that are often unknown. These earthquakes can cause large damage because people aren't expecting them.
Humans can also cause earthquakes through explosions, the building of dams and mining.
Virginia earthquake felt in Washington and New York - BBC article
Key Earthquake Terminology
Epicentre: The location on the earth's surface directly above the hypocentre.
Hypocentre (focus): The actual site/location that an earthquake takes place.
Aftershock: A smaller earthquake that takes place in the coming hours, days and weeks after the main earthquake.
Seismic Waves: These are waves of energy that travel through the earth as a result of an earthquake. There are two types of waves; body waves that travel through the earth and can be divided into p-waves (more longitudinal) and s-waves (more transverse) and surface waves that travel across the surface.
Tremor: A tremor is another name for an earthquake, but is all sometimes the name given to a lesser earthquake or the felt effects of a big earthquake by people living further from the epicentre.
USGS: The United States Geological Survey is an organisation that records many of the natural disasters that take place around the world. Go to the link below to look at the world's most recent earthquakes: USGS - Recent World Earthquakes
Earthquake Hazards
Primary Hazards
Ground Shaking: The movements of the ground caused by the seismic waves can fell buildings, bridges, trees, etc. killing and trapping people.
Secondary Hazards
Tsunamis: The sudden shifting of tectonic plates under the sea can displace large amounts of water which can trigger massive tsunamis. The 2011 Japanese tsunami and the 2004 Indian Ocean tsunami both caused more deaths than the earthquakes that triggered them.
Fire: Earthquakes can break gas cables and knock over ovens and open fires which trigger secondary uncontrolled fires.
Liquefaction: This is where a saturated soil loses strength and rigidity because of applied stress, normally an earthquake. The changes in its state causes the ground to behave like water allowing things to sink into it. The recent earthquake in Christchurch New Zealand saw large scale liquefaction
Mass movements (landslides): The sudden movement of the earth and subsequent seismic waves can trigger landslides and avalanches which can bury and kill many people. The El Salvador earthquake of 2001 triggered a landslide in Santa Tecla which killed hundreds of people.
Floods: damage to flood defences or even dams can cause widespread flooding.
Factors Affecting the Impact of Earthquakes
Depth: If the hypocentre of an earthquake is close to the surface then it is more likely to cause greater damage than a deep earthquake.
Duration: A longer earthquake is likely to cause greater damage than an earthquake that lasts only a few seconds.
Magnitude: Obviously a stronger earthquake is going to have a greater impact than a weaker one.
Time of Day: Time of day can be important. If people are sleeping and get trapped in their beds more people can be killed. In Japan an earthquake that struck while people were cooking their evening dinner caused widespread secondary hazards (fire) that caused more deaths.
Epicentre Location: If the epicentre of an earthquake is an uninhabited region it is going to have a lesser effect than one under a densely populated city.
Geology: If an earthquake occurs in solid bedrock it is likely to cause less damage than one centred below an alluvial floodplain which may lead to liquefaction.
Economic Development (buildings, planning, preparedness): Generally speak more developed countries have better zonal planning, building codes and preparedness mean the effects of the earthquake are less.
Predicting and Measuring Earthquakes
Earthquakes are extremely hard to predict. Scientists can normally predict where earthquakes are likely to happen, but they can not predict when they will happen and how strong they will be. Scientists can attempt to predict by looking at:
Italy Scientists on trial over LĂquila earthquake - BBC article
Did toads predict the earthquake? - France 24 article
Richter Scale: The Richter Scale was developed by Charles Richter in 1935. It uses a base 10 logarithmic scale. The scale is normally seen from 0-10, but in theory could go above this. The scale measures the amplitude of waves on a seismograph. An earthquake of 5.0 is ten times stronger than one of 4.0. The largest earthquake ever recorded was a 9.5 of the coast of Chile in 1960.
Seismograph: The name commonly given to seismometers. It records movements in the earth caused by seismic waves. A picture of a seismograph is shown to the right.
Mercalli Scale: Instead of measuring an earthquakes energy like the Richter scale the Mercalli scale looks at the effects of an earthquake. The scale goes from 1 (hardly felt) up to 12 (total destruction). The scale is obtained by looking at the effects on humans, nature and structures.
For a case study about the Haiti earthquake go to: Measuring Disasters.