Plate tectonic theory
- Revolutionised the study of Earth science.
- People noticed that continents seemed to fit together remarkably well.
- Francis Bacon noticed this fit as early as the 17th century but it did not att5ract serious attention as no one thought the continents could move.
Theory of plate tectonics
- 1912, Alfred Wegener published his theory that a single continent existed about 300 million years ago (Pangaea).
- Later split into two continents, Laurasia in the North and Gondwanaland in the South.
- Former splitting of these 2 masses created todays continents.
- Wegener published this theory of continental drift.
- Supported by several peices of evidence that these areas were once joined.
- The fit of South America and Africa
- Evidence of glaciation of the late carboniferous period, deposits of which are found in South America, Antarctica and India. Must have been formed together and then moved. There are also striations pn rocks in Brazil and West Africa which point to a similar situation.
- Rock sequences in Northern Scotland closely agree with those found in Eastern Canada, indicating that they were laid down under the same conditions in one location.
- Fossil brachiopods found in Indian limestones are comparable with similar fossils in Australia.
- Fossil remains of the reptile Mesosaurus are found in both South America and Southern Africa. It is unlikely that the same reptile could have developed in both areas or that it could have migrated.
- The fossilised remains of a plant which existed when coal was being formed have been located only in India and Antarctica.
Deelopment of the thoery
Wegeners theories were unable to explain how continental movement could have taken place however from the 1940's onwards, evidence began to accumulate to show that he may have been correct.
The mid-Atlantic ridge was discovered. A similar feature was then discovered in the Pacific Ocean. Examination of the ocean crust either side of the mid-Atlantic ridge suggested that sea floor spreading was occuring. The evidence for this is the alternating polarity of the rocks that form the ocean crust. Iron particles in lava erupted on the oceans floor are aligned with the Earths magnetic field. As the lavas solidify, these particles provide a permanent record of the Earths polarity at the time of the eruption (palaeomagnetism). However the Earths polarity reverses at regular intervals. The result is a series of magnetic stripes with rocks aligned alternatively towards the North and South poles. The striped pattern which is mirrored exactly on either side of the mid-Atlantic ridge, suggests that the ocean crust is slowly spreading away from this boundary. Moreover, the ocean crust gets older with distance from the mid-oceanic ridge. Plates must be being destroyed somewhere to accomodate the increase in size at mid-oceanic ridges. Evidence of this was found with the discovery of huge oceanic trenches , with large areas of ocean floor being pulled downwards.
The Earths layers
Before the development of plate tectonic theory, earth scientists divided the interior of the Earth into three layers, the crust, the mantle and the core. The core is made up of dense rocks containing iron and nickel alloys and is divided into a solid inner core and a molten outer one, with a temperature of over 5000c. The mantle is made up of molten and semi-molten rocks containing lighter elements, such as silicon and oxygen. The crust is even lighter because of the elements that are present, the most abundant being silicon, oxygen, aluminium, potassium and sodium. The crust vaires in thickness - beneath the oceans it is only 6-10 km thick but below the continents this increases to 30-40 km. Under the highest mountain ranges the crust can be up to 70 km thick.
The theory of plate tectonics has retained this simple threefold division, but new research has suggested that the crust and upper mantle should be divided into the lithosphere and the asthenosphere. The lithosphere consists of the crust and the rigid upper section of the mantle and is approximately 80-90 km thick. It is divided into seven very large plates and a number of smaller ones. Plates are divided into 2 categories, oceanic and continental, depending on the type of material from which they are made. Below the lithosphere is the semi-molten asthenosphere, on which the plates float and move.
Hot spots around the core of the Earth generate thermal convection currents within the asthenosphere, which cause magma to rise towards the crust and then spread before cooling and sinking. This circulation of magma is the vehicle upon which the crustal plates move. The crust can be thought of as 'floating' on the more dense material of the asthenosphere. This is a continuous process, with new crust being formed along the line of constructive boundaries between plates (where plates move away from each other) and older crust being destroyed at destructive boundaries (where plates are moving towards each other).
Features of plate margins - Constructive (divergen
Constructive - Where plates move apart in oceanic areas they produce mid-oceanic ridges. Where they move apart in continental crust they produce rift valleys. The space between the diverging plates is filled with basaltic lava upwelling from below. Constructive margins are therefore some of the youngests parts of the Earths surface, where new crust is being continuously created.
Oceanic ridges are the longest uplifted features on the surface of the planet, and have a total length of 60,000 km. In some parts they rise to 3,000m above the ocean floor. Their precise form appears to be influenced by the rate at which the plates separate:
- a slow rate (10-15mm per year), as seen in parts of the mid-atlantic ridge, produces a wide ridge axis (30-50km) and a deep (3,000m) central rift valley with inward facing fault scarps.
- an intermediate rate (50-90mm per year), such as that on the Galapagos ridge (Pacific), produces a less well marked rift (50-200m deep) with a smoother outline.
- a rapid rate (>90mm per year), such as on the East Pacific rise, produces a smooth crest and no rift.
Volcanic activity also occurs along the ridge, forming submarine volcanoes, which sometimes rise above sea level e.g. Surtsey to the South of Iceland (Iceland itself was formed in this was and is the largest feature produced above sea level on a divergent margin). These are volcanoes with fairly gentle sides because of the low viscosityt of basaltic lava. Eruptions are frequent but relatively gentle (effusive).
As new crust forms and spreads, transform faults occur at right angles to the plate boundary. The parts of the spreading plates on either side of these faults may move at differing rates, leading to friction and ultimately to earthquakes. These tend to be shallow-focus earthquakes, originating near the surface.
At constructive margins in continental areas, such as East Africa, the brittle crust fractures as sections of it move apart. Areas of the crust drop down between parallel faults to form rift valleys. The largest of these features is in the African rift valley which extends 4,000 km from Mozambique to the Red Sea. From the Red Sea it extends North into Jordan, a total distance of 5,500 km. In some areas, the inward facing scarps are 600m above the valley floor and they are often marked by a series of parallel step faults.
The area is also associated with volcanic activity (for example the highest mountain in Africa, Mt Kilimanjaro). The crust here is much thinner than in neighbouring areas, suggesting that tension in the lithosphere is causing the plate to thin as it starts to split. The line of the African rift is thought to be an emergent plate boundary, the beginning of the formation of a new ocean as Eastern Africa splits away from the rest of the continent.
There are two types of plates, so there are three different convergent situations:
- Oceanic plate moves towards continental plate
- Oceanic plate moves towards oceanic plate
- Continental plate moves towrads continental plate
Where oceanic and continental plates meet, the denser oceanic plate is forced under the lighter continental one. This process is known as subduction. The downwarping of the oceanic plate forms a very deep part of the ocean known as a trench. A good example of an ocean trench is off the Western coast of South America where the Nazca plate is subducting under the South American plate, forming the Peru-Chile trench.
Sediments that have accumulated on the continental shelf on the margin of the land mass are deformed by folding and faulting. Along with the edge of the continental plate, these are uplifted to form fold mountains, such as the Andes alond the Pacific side of South America. As the oceanic plates descends, the increase in pressure can trigger major earthquakes along the line of the subducting plate; these may be shallow, intermediate or deep focus.
The further the rock descends, the hotter the surroundings become. This, together with the heat generated from friction, begins to melt oceanic plate into magma in a part of the subduction zone known as the Benioff zone. As it is less dense than the surrounding asthenosphere, this molten material begins to rise as plutons of magma. Eventually, these reach the surface and form volcanoes. The andesitic lava, which has viscous nature (flows less easily), creates complex, composite, explosive volcanoes (contrast this with the basaltic emissions on constructive margins which tend to be gentle eruptions). If the eruptions take place offshore, a line of volcanic islands known as an island arc can appear, e.g. the West Indies.
Where oceanic plates meet, one is forced under the other and the processes involved with subduction begin. Ocean trenches and island arcs are the features associated with this interaction, as it takes place well offshore. A good example is on the Western side of the Pacific Ocean where the Pacific plate is being subducted beneath the smaller Phillippine plate. Here the ocean floor has been pulled down to form the very deep Marianas trench. A line of volcanic islands, including Guam and the Marianas, has been formed by upwelling magma from the Benioff zone.
The plates forming continental crust have a much lower density than the underlying layers, so there is not much subduction where they meet. Instead, as the plates move towards each other, their edges and the sediments between them are forced up into fold mountains. As there is little subduction, there is no volcanic activity, but the movement of the plates can trigger shallow focus earthquakes. Material is also forced downwards to form deep mountain roots.
The best example of such a margin is where the Indo-Australiam plate is being forced northwards into the Eurasian plate. The previous intervening ocean, known as the Sea of Tethys, has had its sediments forced upwards in large overfolds to form the Himalayas, an uplift that is continuing today. The Himalayan range of fold mountains, containing the highest mountain on the planet (Mt Everest 8,848m), is up to 350km wide and extends for 3,000km.
Where two crustal plates slide past each other and the movement of the plates is parallel to the plate margin, there is no creation or destruction of crust. At these conservative margins (sometimes called passive) there is no subduction and therefore no volcanic activity.
The movement of the plates, however, creates stresses between the plates edges and, as sections of the plates rub past each other, the release of friction triggers shallow-focus earthquakes, for example San Francisco 1906 and 1989, Los Angeles 1994. These earthquakes occured at the best-known example of a conservative margin - the San Andreas fault in California, where the Pacific and North American plates move parallel to each other. Both plates are moving in the same direction but not at the same speed. Stresses set up by this movement cause transform faults to develop, running at right angles to the main San Andres fault.
Vulcanicity is normally associated with plate margins but, in the centre of the Pacific Ocean, we find the volcanic Hawaiian islands that are not connected with any plate boundary. It is believed that this volcanic area is caused by a localised hot spot within the Pacific plate. A concentration of radioactive elements inside the mantle may cause such a hot spot to develop. From this a plume of magma rises to eat into the plate above. Where lava breaks through to the surface, active volcanoes occur above the hot spot.
The hot spot is stationary, so as the Pacific plate moves over it, a line of volcanoes is created. The one above the hot spot is active and the rest form a chain of islands of extinct volcanoes. The oldest volcanoes have put so much pressure on the crust that subsidence has occured.
This, together with marine erosion, has reduced these old volcanoes to seamounts below the level of the ocean. Evidence shows clealry that the Pacific plate is moving Northwest. This is further proof that the Earths crust is moving, as originally suggested by Alfred Wegener.
Vulcanicity - Distribution
Most volcanic activity is associated with plate tectonic processes and is mainly loctaed along plate margins. Such activity is therefore found:
- Alongs are moving apart. The best example is the mid-Atlantic ridge - Iceland represents a large area formed by volcanic activity.
- Associated with rift valleys. The African rift valley has a number of volcanoes along it including Mt Kenya and Mt Kilimanjaro
- On or near subduction zones. The line of volcanoes or 'ring of fire', that surrounds the Pacific ocean is associated with plate subduction. This tends to be the most violent of all activity.
- Over hot spots such as the one in the middle of the Pacific Ocean which has given rise to the Hawaiian islands.
There are variations in the form, frequency and type of volcanic eruption. These are related to the different kinds of plate margin, emissions and lava, and this leads to different types and frequencies of eruption. How these different plate margins affect volcanic eruptions and landforms:
Constructive plate margins - Plates move away, magma is basaltic, lava is runny, little violence as gases easily escape, mainly lava flow, regular frequency and can be continuous, lava plateau/shield volcano.
Destructive plate margins - Plates move towards each other involving subduction zone, magma is acidic e.g. rhyolite, lava is slow flowing and very viscous, potentially explosive as lava shatters into pieces, can include lava bombs, ash and dust, erupt from time to time, can have long dormant periods, acid lava dome/composite cone.
Intrusive volcanic landforms
When magma is forced to the surface, only a small amount of the mass actually reaches that level. Most of the magma is intruded into the crust where it solidifies into a range of features. These are often exposed at the surface by later erosion.
Batholiths are formed deep below the surface when large masses of magma cool and solidify. As the magma cools slowly, large crystals are formed in the rock e.g. Granite. Batholiths are often dome-shaped and exposed by later erosion. This is the case on Dartmoor and the Isle of Arran. Batholiths can be several hundreds of kilometres in diameter. The area surrounding the batholith is altered by the heat and pressure of the intrusion to form a metamorphic aureole. Batholiths are unaffected by the characteristics and structure of existing rock. Sometimes smaller injections of magma form a lens shape that is intruded between layers of rock. This then forces the overlying strata to arch upwards, forming a dome. This feature is known as a laccolith, and it may be expoosed by later weathering and erosion to form a small range of hills, for example the Eildon Hills on the Scottish Borders.
Intrusive volcanic landforms
Dykes are vertical intrusions with horizontal cooling cracks. They cut accross the bedding planes of the rocks into which they have been intruded. Dykes often occur in groups where they are known as dyke swarms. Many Scottish islands, such as Mull and Skye, have clusters of dykes, all associated with one intrusive event.
Sills are horizontal intrusions along the lines of bedding planes. Sills have vertical cooling cracks. Examples include the Great Whin Sill (which carries part of Hadrians Wall) and Drumadoon on the Isle of Arran. Both sills and dykes are commonly made up of dolerite.
Extrusive volcanic landforms
Extrusive vulcanicity involves two forms of lava:
- Basaltic lava is formed from magma that is low in silica. This makes for a more fluid magma that allows gas bubbles to expand on the way up to the surface, so preventing sudden explosive activity.
- Andesitic and rhyolitic lavas are formed from silica-rich (acid) magma that is very viscous. This often solidifies before reaching the surface, leading to a build-up of pressure and, ultimately, to a violent explosion.
The main types of extrusive volcanic landforms are as follows:
- Lava plateaux are formed from fissure eruptions. The extensive lava flows are basaltic in nature, so they flow greawt distances. A good example is the Antrim lava plateau in Northern Ireland, the edge of which can be seen at Giants Causeway. Lava plateaux are generally flat and featureless.
- Basic/shield volcanoes are also formed from free-flowing lava. The resulting volcanoes have gentle sides and cover a large area, for example Mauna Loa, Hawaii.
Extrusive volcanic landforms
- Acid/dome volcanoes are steep-sided convex cones, consisting of viscous lava, which is probabaly rhyolite. The best examples can be seen in the Puy region of central France.
- Ash and cinder cones are formed from ash, cinders and volcanic bombs ejected from the crater. The sides are steep and symmetrical, for example Paricutin, Mexico.
- Composite cones are the classic pyramid-shaped volcanoes, consisting of layers of ash and lava that is usually andesitic. Examples include Mt Etna on Sicily and Mt Fuji in Japan.
- Calderas occur when the build-up of gases becomes extreme and a huge explosion removes the summit of the cone, leaving an opening several kilometres in diameter. The caldera may become flooded by the sea, or a lake may form within it. Examples include Krakatoa in Indonesia and Santorini in Greece.
The nature of volcanic eruptions
Vulcanologists have traditionally classified volcanoes according to the nature of the eruption. This classification is based on the degree of violence of the explosion, which is a consequence of the pressure and amount of gas in the magma.
Minor volcanic landforms
- Solfatara - small volcanic areas without cones, produced by gases (mainly sulphurous) escaping to the surface, for example around the Bay of Naples in Italy.
- Geysers - these occur when water, heated by volcanic activity, explodes onto the surface, for example Old Faithful, Yellowstone National Park, USA.
- Hot springs/boiling mud - sometimes the water, heated below, does not explode onto the surface. If this water mixes with surface deposits, boiling mud is formed. Such features are very common in Iceland. There are hot springs in Bath in the West of England.
Volcanic activity in the UK
Apart from hot springs, the UK has no current volcanic activity. However, there is much geological evidence of such activity, which occured during the mountain-building periods of the Caledonian, Hercynian and Alpine orogenies.
- Granites and other examples of intruded rocks occur across the Grampians in Scotland, in Ireland and particularly in the southwest of England where the top of an exposed batholith is seen in areas such as Dartmoor and Bodmin Moor. Here, weathering and erosion have combined to give a distinctive landscape of upland plateaux capped by rock outcrops, which are known as tors.
- Dykes and sills are also common. The dyke 'swarms' that radiate across the Isle of Arran in Scotoland contain around 500 such features in a 20km stretch of coastline. Dykes generally occur as small ridges in the landscape because they are more resistant than the surrounding rocks. The Great Whin Sill runs across large distances in the north of England, forming an upstanding cliff-like feature. Many rivers produce high waterfalls as they plunge over it, for example High Force and Cauldron Snout in the Tees valley in the Pennines. It is also the defensive base for man-made features such as Hadrians Wall and Bamburgh Castle.
Volcanic activity in the UK
- Basaltic flows can be seen where the Antrim lava plateau formed in Northern Ireland. When the lava cooled, vertical cracks in the flow resulted in hexagonal columns. These are exposed at the coast - The Giants Causeway. The same volcanic feature can be seen in Fingals Cave on the Isle of Staffa in Scotland.
- A volcanic plug from a long-extinct volcano (active over 300 million years ago) forms the site of Endinburgh Castle. Stirling Castle is also built on a volcanic plug.
The impact of volcanic activity
A volcanic event can have a range of impacts, affecting the area immediately around the volcano or the entire planet. Effects can be categorised into primary and secondary. Primary effects consist of:
- tephra - solid material of varying grain size, from volcanic bombs to ash particles, ejected into the atmosphere.
- pyroclastic flows - very hot (800c), gas-charged, high velocity flows made up of a mixture of gases and tephra.
- lava flows
- volcanic gases - including carbon dioxide, carbon monoxide, hydrogen sulphide, sulphur dioxide and chlorine. Emissions of carbon dioxide from Lake Nyos in Cameroon in 1986 suffocated 1,700.
The impact of volcanic activity
Secondary effects include:
- lahars - volcanic mud flows such as those that devastated the Columbian town of Armero after the eruption of Nevado del Ruiz in Noovember 1985.
- Flooding - melting of glaciers and ice caps such as the Grimsvotn glacial burst on Iceland in Novemeber 1996
- Tsunamis - giant sea waves generated after violent caldera-forming events such as that which occured on Krakatoa in 1883 - the tsunamis from the eruption are believed to have drowned 36,000 people.
- volcanic landslides
- climate change - the ejection of vast amounts of volcanic debris into the atmosphere can reduce global temperatures and is believed to have been an agent in past climatic change.
Volcanic effects become a hazard when they impact upon the human and built environments, killing and injuring people, burying and collapsing buildings, destroying the infrastructure and bringing agricultural activities to a halt.
Case Study - Mt Nyiragongo, Congo, Jan 2002
- lies in Virunga mounrtains in DRC, associated with African rift valley
- main crater is 250m deep, 2km wide and often contains a lava lake
- since records began in 19th century, the volcano has erupted more than 30 times.
- Lava erupted by Mt Nyiragongo is very fluid and has been known to flow downhill at speeds greater then 90km/h.
- Eruption in Jan 2002 was unexpected.
- Large eruption opening a fissure 13km long, spewing lava up to 2m deep which flowed in the direction of Goma and Lake Kivu.
- Lava flows destroyed at least 1/3 of Goma, a town with over 200,000 inhabitants.
- commercial centre of the town was destroyed, along with water and power supplies and medical facilities.
- lava covered the Northern third of the runway at Goma airport.
- Death toll reached 147.
- estimated that more than 350,000 people fled the area.
- Sulphurous lava entered Lake Kivu, polluting the lake which is a major source of drinking water
- Several earthqaukes accompanied the eruption, one measuring over 5 on the richter scale.
- Thousands of people required medical attention.
Case Study - Mt Nyiragongo, Congo, Jan 2002
- there was much looting from abandoned homes and commercial properties.
The authorities initial response to the eruption was to issue a 'Red Alert' for Goma and the surrounding area. This enabled a full evacuation to take place. This prompt response was one factor in keeping the death rate relatively low.
Two days after the eruption, the UN began to bring in humanitarian aid. Emergency rations were initially of high-energy foods such as biscuits, which were followed by more substantial food aid (maize, beans and cooking oil) as communications began to improve. The UN also set up camps to house displaced people. The organisation has estimated the cost of providing food, blankets, household utensils, temporary shelter, clean water, sanitation and healthcare to the refugees at $15 million. However, a much higher cost will be incurred in rebuilding Goma's infrastructure, homes and livelihoods. The lava flow destroyed many businesses, resulting in a massive increase in unemployment in the area.
Case Study - Mt Etna, 1991-93
- Mt Etna (3,323m), towering above Catania, the second largest city in Sicily.
- has one of the worlds largest documented records of eruptions.
- The stratavolcano, truncated by several smaller calderas, was constructed over an older shield volcano.
- The most prominent feature is the Valle del Bove, a 5-10km horseshoe-shaped caldera open to the East.
- Created when the volcano experienced a catastrophic collapse during an eruption, generating an enormous landslide.
- The volcano can be destructive but is not regarded as dangerous.
- Thousands of people live on its slopes working the fertile volcanic soils.
- Towards the end of 1991, lava began to pour from vents high on the eastern flank in the Valle Del Bove, and to advance on the settlement of Zafferana.
- A large earth barrier was constructed accross the end of the Val Calanna which held back lava for several months.
- During the spring of 1992, the accumulated lava began to spill over the barrier. Smaller barriers erected were rapidly overwhelemed and a few small buildings and orchards were destroyed.
Case Study - Mt Etna. 1991-93
- it was decided to cut off the flow by blocking the primary feeder channel. This was first attempted by dropping concrete blocks from helicopters through the roof of the upper lava tube.
- Finally, in May 1992, engineers blasted openings in the lava tube to encourage a new direction of flow. The lava front stopped advancing on Zafferana and the eruption ended 10 months later in early 1993.
- It is not always possible to stop lava flows on Mt Etna.
- In 2002, there was a more serious eruption and lava flows completely destroyed a ski station including two hotels, two restaurants, several ski hire firms, a dozen souvenir shops and a ski school.
- Clouds of ash rained down on the area, effecting the city of Catania in particular.
Seismicity - Causes of earthquakes
A the crust of the Earth is mobile, there tends to be a slow build up of stress within the rocks. When this pressure is suddenly released, parts of the surface experience an intense shaking motion that lasts for just a few seconds. This is an earthquake. The point at which this pressure release occurs is knwon as the focus, and the point immediately above that on the Earths surface is called the epicentre. The depth of the focus is significant and three broad categories are recognised:
- shallow-focus (0-70km deep) these tend to cause the greatest damage and account for 75% of all the earthquake energy released.
- imtermediate-focus (70-300km deep)
- deep-focus (300-700km deep)
Seismic waves radiate from the focus rather like the ripples in water when a rock is thrown into a pond. There are three main types of seismic wave, each travelling at different speeds.
- Primary (P) waves travel fastest and are compressional, vibrating in the direction in which they are travelling.
- Secondary (S) waves travel at half the speed of P waves and shear rock by vibrating at right angles to the direction of travel.
- Surface (L) waves travel slowest and near to the ground surface. Some surface waves shake the ground at right angles to the direction of wave movement and some have a rolling motion that produces vertical ground movement.
P and S waves travel through the interior of the Earth and are recorded on a seismograph. Studying earthquakes and the seismic waves they generate has made it possible to build up a picture of the interior of the Earth.
The vast majority of earthquakes occur along plate boundaries, the most powerful being associated with destructive margins. At conservative margins, the boundary is marked by a fault, movement along which produces the earthquake. Perhaps the most famous of these is the San Andreas fault in California which represents the boundary between North American and Pacific plates. In reality, the San Andres system consists of a broad complex zone in whcih there are a number of fractures of the crust.
Some earthquakes occur away from plate boundaries and are associated with the reactivation of old fault lines. An example is the event that occured on 23 September 2002 in the UK midlands. This earthquake measured 4.8 on the Richter Scale, and the epicentre was located in Dudley, west of Birmingham. It is believed that the cause was movement along an old fault line known as the Malvern lineament.
It has been suggested that human activity could be the cause of some minor earthquakes. Examples are the building of large resevoirs in whcih the water puts pressure on the surface rocks, or subsidence of deep mine workings.
Magnitude and frequency
The magintude of earthquakes is measured on two scales. The Richter Scale is a logarithmic scale - an event measured at 7 on the scale has an amplitude of seismic waves 10 times greater than one measure at 6 on the scale. The energy release is proportional to the magnitude, so that for each unit increase in the scale, the energy released increases by approximately 30 times.
The largest event ever recorded was measured at 8.9 on the scale. The earthquake in Dudley, at 4.8 on the scale, was large for the UK but small compared with major earthquakes such as the 1999 Turkish earthquake that measured 7.4 on the Richter scale. This earthquake killed more than 14,000, injured 25,000 and completely destroyed more than 20,000 buildings.
The Mercalli scale measures the intensity of the event and its impact. It is a 12-point scale that runs from Level 1 (felt by very few people - approx 2 on the Richter scale) to level XII (total destruction - approx 8.5 on the Richter scale)
Seismic recors enable earthqiake frequency to be observed, but these records only date back to 1848 when an instrument capable of recording seismic waves was first developed.
The effects of earthquakes
The initial effect of an earthquake is ground shaking. The severity of this will depend upon magnitude of the earthquake, the distance from the epicentre and the local geological conditions. In the Mexico City earthqauke of 1985, for example, the seismic waves that devastated the city we amplified several times by the ancient lake sediments upon which the city is built.
Secondary effects are as follows:
- soil liquefaction - when violently shaken, soils with a high water content lose their mechanical strength and start to behave like a fluid.
- landslides/avalanches - slope failure as a result of ground shaking
- effects on people and the built environment - collapsing buildings, destruction of road systems and other forms of communications, destruction of service provision such as gas, water and electricity, fires resulting from ruptured has pipes and collapsed electricity transmission systems, flooding, disease, food shortages, disruption to the local economy. Some of the effects on the human environment are short term; others occur over a longer period.
- tsunamis - giant sea waves generated by shallow-focus underwater earthquakes, volcanic eruptions, underwater debris slides and landslides into the sea.
In the open ocean tsunames have a very long wavelength and a low wave height, and they travel quickly at speeds greater than 700km. On reaching shallow water bordering land they increase rapidly in height.
Often, the first warning is the wave trough in front of the tsunami, which causes a reduction in sea level, known as a drawdown, followed by the tsunami itself which can reach heights in excess of 25m. The event usually consists of a number of waves, the largest not necessarily being the first. When a tsunami reaches land its effects will depend upon:
- the height of the waves and the distance they have travelled.
- the length of the event that caused the tsunami.
- the extent to which warnings can be given.
- coastal physical geography, both offshore and in the coastal area.
- coastal land use and population density.
- wave will wash boats and coastal structures inland, and the backwash may carry them out to sea.
- the water itself and debris can cause drowning and injuries.
- effects of most tsunamis are felt 500-600m inland.
- buildings, roads, bridges, harbour structures, trees and even soil are washed away.
Tsunamis generated by the volcano Krakatoa in 1883 are estimated to have drowned more than 35,000 people and produced waves that travelled around the world, the highest being over 40m.
Around 90% of all tsunamis are generated within the Pacific basin and are associated with the tectonic activity taking place around its edges. Most are generated at convergent plate boundaries where subduction is taking place, particularly off the Japan-Tawain island arc. Since the devestating tsunami of December 2004, the area has been effected by at least 2 major tsunamis.
- July 2006, south Java coast - generated by an earthquake of magnitude 7.7 on the Richter scale 180km offshore, a tsunami devestated the area around Pangandaram, resulting in more than 600 deaths.
- April 2007, Solomon islands - the tsunami swept across the islands, killing at least 15 people.
The geological record indictaes that huge tsunamis have affected areas such as the Meditteranean basin and the North Sea area. Around 7250 BP the Storegga slide, caused by huge submarine debris slides off Norway, produces tsunamis more than 6m high in Scotland and other areas bordering the North Sea. It is believed that these tsunamis continued across the Atlantic to affect the coastlines of Spitsbergen, Iceland and Greenland.
Case Study - Indian ocean tsunami, 26th Dec 2004
Pressure has been building up for some time where the Indo-Australian plate subducts beneath the Eurasian plate south of Myanmar, and on Boxing Day 2004 there was a slippage along the plate edge some 25km beneath the Indian Ocean. A section of sea bed on the Eurasian side of the fault rose several metres, generating a powerful earthquake which measured about 9.0 on the modified Richter scale. This makes it one of the biggest earthquakes ever recorded.
The epicentre of this earthquake was just off the northwestern tip of the island of Sumatra. The earthquake triggered a tsunami that raced across the Indian Ocean, devastating islands and the coastlines of the countires bordering the ocean, particularly Indonesia, Malaysia, Thailand, Myanmkar, India and Sri Lanka. In some places the wall of water that came ashore was more than 25m in height. Tsunami warning systems are in place in the Pacific basin but no such system had been set up in the Indian Ocean. The populations of these countires had no idea what was about to arrive.
Case Study - Indian ocean tsunami, 26th Dec 2004
- The main effects of the tsunami were:
- estimated 300,000 people killed by the waves
- tens of thousands of people were injured by the waves and debris
- many of these coastal areas, especially Thailand, Sri Lanka and the Maldives are popular tourist destinations especially over Christmas
- whole towns and villages were swept away - more than 1,500 villages destroyed
- destruction of property resulting in millions of people being made homeless
- massive damage to the tourist infrastructure
- widespread damage to coastal communications, particularly bridges and railway lines
- damage to the economies of these coastal areas, particularly agriculture and fishing
- many hospitals and clinics were washed away or damaged
on the Western side of the Indian Ocean, countries did recieve a warning of what was to come and were able to take action. One positive result of the tsunami is a warning system has now been set up among the countries that border the Indian Ocean.
Case Study - Gujarat earthquake, 26 Jan 2001
One of the most powerful earthquakes to strike the Indian subcontinent in the last 100 years was centered near the small town of Bhuj in Gujarat. This earthquake, with a focus of 17km below the Earths surface, was measured at 7.9 on the Richter scale. The shock waves from the event were felt over the border in Pakistan, where several people were killed. They were also felt on the other side of the subcontinent in Bangladesh and Nepal.
The death toll was high as many buildings were not able to withstand the tremors, even in an area that was known to be seismically active. One resident complained that 'ours was not a well-designed building, it was built 12 years ago but it just fell to pieces as all the beams and pillars buckled instantly'. A month after the event, the Indian government issued the following information about the effects of the earthquake:
- death toll just iner 20,000 although some say at least 30,000 died.
- more than 160,000 were seriously injured
- over 1 million made homeless
- 345,000 dwellings were destroyed
- 800,000 buildings suffered some form of damage
Case Study - Gujarat earthquake, 26 Jan 2001
- small towns such as Bhuj, Bhachau and Anjar had at least 90% of their dwellings destroyed and many smaller villages totally devestated
- all four hospitals in Bhuj were destroyed
- cultural heritage of the area was destroyed
- communications were severly disrupted
In the aftermath, one of the most disturbing aspects of the event was widespread looting of damaged property, which the authorities struggled to bring under control. In the days following the earthquake there were several hundred aftershocks, most of them small but one over 5 on the Richter scale. These aftershocks caused sconsiderable damage to buildings weakened by the main earthquake.
Rescue teams were sent from many parts of the world to save people trapped beneath fallen buildings. Britain sent a 69-strong team sponsored by the Department for International Development. In addition to rescue operations, disinfectant was sprayed on the collapsed buildings to prevent the spread of disease from so many rotting bodies. Water purification tablets were also issued.
Case Study - Gujarat earthquake, 26 Jan 2001
The Indian government sent 5,000 troops into the area along with 40 military aircraft and three naval vessels, two of which were to act as floating hospitals, each capable of treating more than 200 patients. Military personnel transported medical aid, food, tents and communication equipment by air to the worst effected areas.
The authorities feared widespread epidemics of typhoid and cholera following the event. However, prompt action meant that these did not occur, although there was evidence of widespread diarrhoea and gastroenteritis. In an area still heavily dependent on agriculture, the loss of 20,000 cattle had an enormous impact. The overall cost of the earthquake has been estimated at $4-5 billion and it has been suggested that more than 1 million people have been in reciept of some form of aid as a result of the event.