VOLCANICITY AND EARTHQUAKES

·         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 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, volcanologist can try and predict volcanoes by looking for:

ü  Changes in the shape of a volcano

ü  Changes in the amount of gas being released

ü  Changes in the temperature

ü  Tectonic activity (earthquakes)

ü  Animal behaviour

ü  Changes in local hydrology

ü  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 scale is open-ended and measured by looking at:

ü  Volume of material released in eruption

ü  Cloud height and

ü  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 super volcanoes with a frequency of 10,000 years.

Key Earthquake Terminology

ü  Epicentre: The location on the earth's surface directly above the hypocentre.

ü  Hypocentre (focus): The actual site/location that an earthquake takes place.

ü  Aftershock: A smaller earthquake that takes place in the coming hours, days and weeks after the main earthquake.

ü  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.

ü  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.

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 defenses or even dams can cause widespread flooding.

Factors Affecting the Impact of Earthquakes

Depth: If the hypocenter 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.

Can humans cause an earthquake?

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:

ü  Microearthquakes

ü  Changes in rock stress

ü  Ground subsidence, uplift or tilt

ü  Changes in magnetic field and electrical resistivity of rocks

ü  Animal behaviour

ü  Seismic history

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.