Plate Tectonics and Associated Hazards

Plate movement, vulcanicity and seismicity

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  • Created by: Rachael
  • Created on: 08-06-11 11:03

Earth Structure

Core- densest part of the planet, made of rocks rich in iron and nickle. A semi-molten outer core contains a solid inner core with a temperature cover of over 6,000 deg C

Mantle- surrounds the core. Composed of silicate rocks rich in iron + magnesium. Temp reach 5,000 deg C generate convection currents.

Crust- thinnest layer + coolest, least dense rocks- rich in silicon, oxygen, aluminium, potassium abd sodium. Two tyes of crust : oceanic crust and continental crust. Continental crust can reach 70km thick beneath worlds major mountain ranges.

Lithosphere- Crust + rigid upper mantle

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Plate Techtonics Theory

Wegner- published a theory in 1912 suggesting all continents one fit together to form a supercontinent he called Pangaea. He proposed that at some time land masses had drifted apart until they occupied their current positions

Evidence includes:

Continental fit- continents seemed to fit together ( western seaboard of Afrika and eastern seaboard of South America) if placed by each othe.

Geological evidence- rocks of same age and type+ displaying same formations found in s.e Brazil + S. Africa. Similar glacial ndeposits found in Antarctica, S.America and India =, now many km apart.

Climatological evidence- Antactica, N. America, Avalbard and UK all contain coal deposits of similar age that were formed in tropical climate zones and must of drifted apart since the carboniferous period.

Biological evidence- SImilar fossil formations found on either sides of the Atlantic =. For example, same reptile called Mesosaurus is found only in S.America and southern African sediments of Permian age. Plant remains from the humid swamps that later formed coal deposits have been found in India and Antarctica. Marsupials are found only in Australia because it drifted away from main supercontinent before preditors that wiped themselves out elsewhere had migrated there.

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Evidence from palaeomagnetism

1948- survey of floor of Atlantic Ocean = continuous ridge running largely north to south . It was around 1,000km wide, 2.5km high and composed of volcanic rocks. Similar submarine mountain ranges found in Pacific Ocean extending over 5,000km.

1950s- magnetic surveys of ocean floors showed  regular patterns of palaeomagnetic striping about the ridges.When lava erupts on ocean floor,magnetic domains within iron-rich minerals in the lava are aligned with the magnetic field of the earth. this is fixed as lava cools. However, earths polarity reverses on average every 400,000 years bands or stripes of normal and polarity rocks are mirrored on either side of mid-ocean ridges= new rocks are being added equally on either side.

Evidence of sea floor spreading supported by establishing the age of ocean floor. Surveys recorded very young ages for places on or near the ridges e.g Iceland, less than 1 million years ago.Older crust is being pushed away by new crust. However there is no evidence of the earth growing in size as ocean crust accumulates therefore ocean crust must be comsumed elsewhere.Large ocean floors where found to be subducted. Denser oceanic crust is created in some areas and destroyed in others, the less dense is not consumed.

Higher temp of earths core + heat released by radioactive decay of elements = convection current ( hotter, more liquid magma+ exhibit continuous circulatory motion in atmosphere = plates to move. Pull crust apart at spreading ridges + rift zones + pull slabs of oceanic crust back down into mantle at subduction zones.

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Plate margins

landforms associated with constructive margins-Where convection currents rise and then diverge , create high temp = updoming of crust and tensional forces that pull apart .

Oceanic ridges- Longest continuous uplift feature on surface of earth, combined length of 60,000km. Where two plates pull apart there is a weaker zone in the crust and an increase in heat near surface. Hotter expanded crust = ridge. Central part of ridge may feature a central valley where a section of crust had subsided into magma below.The spilt in the crust provides a low pressure zone where the more liwuid lavas can eruptnto form submarine volcanoes.If persist may eventually reach surface , islands can be formed (Iceland on mid-atlantic ridgeAs crust is pushed away from heat source at mid-ocean ridge, it cools, contracts and sinks towards deeper regions where it becomes covered in fine sediments. Mid- ocean ridges are irregular, curving around the planet. If new ocean crust was created equally on both sides, it would appear to create possibility of overlapping new crust on concave sections and divergance on complex sections. There is no sudden gaps in the sea floor is explained by the fact that the seemingly continuos spreading ridges are frequently bisected by transform faults which allow the crust created at ridges to move outwards at different rates.


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Rift Valleys

Where speading occurs beneath a major land mass, the heating and subsequent updoming of the crust leads to fracturing and rifting. As the sides of the rift move apart, cenral sections drop down to form rift valleys.The Great East African Rift Valley indicates where the crust has begun to pull apart. Active volanoes such as Mount Kilimanjaro and Mount Kenya are surface evidence of the igneous activity beneath. At 4,000km long up to 50km wide and 600m in depth , this feature might widen still, allowing the sea to inundate it.

 To the north, two rifts have widened into the Red Sea and the Gulf of Arabia. Here the rifting has continued and new ocean floor is forming between Africa on the s.w side and Arabia on the n.e side.

Other examples of past rift zones include the steep-seided valley of the River Rhine between Germany and France where the river runs in a down faulted rift valley, it is no longer subject to rifting.

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Destructive plate margins

There are three types of convergent margin    1) Oceanic/ continental plate convergence- oceanic crust is denser then continental crust therefore when they collide the oceanic crust is subducted into the upper mantle. This creates several features. As oceanic crust descends, friction with the overlying continental crust builds ip and causes major earthquakes. Destructive margins are some of the most seismically active zones in the world, with shallow to deep-focus earthquakes charting the descent of subducted crust into the mantle. Rocks scraped of the descending plate and folding of the continental crust help to create young fold mountains chains on the leading edge of continental masses such as the Andes along the west coast of S.America. Deep ocean trenches are found along the seaward edge of destructive margins, they mark where the plate begins to decend beneath another and can reach great depths :Peru-Chile trench is more than 8km deep.The friction created by the decending slab of ocean floor also generates enormous heat, leading to partial melting of the rust. Magmas derived from the melting of the old ocean floor basalts are less dense than the mantle; they try to rise up through fissures and by burning their way through overlying rock until they reach the surface. Where volcanoes erupt on land they help to create young fold mountains such as the Andes.Because the magmas from which the volcanic lavas originate have incorporated elements of older crust and continental rocks as they rose, they are more silica rich, more intermediate or acidic in type. These magmas are more viscous and flow easily leaving intrusice features such as batholiths within the mountain masses and generating extrusives such as andesitic lavas to erupt through volcanoes.Stickier lavas frequently block of their own vents until erupting violently to form conical shaped volcanoes of alternating layers of ash and lava i.e composite corner .

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Oceanic plate/oceanic plate convergence

Two pieces of oceanic crust on different plates collide, one is subducted beneath the other. The crust that is subducted may be marginally denser or is moving more quickly. The process that accompany subduction are much the same as in the case of the ocean/continental plate collision, but where the volcanoes usually erupt on crust covered by oceans they form islands. These form characteristically curving lines of new volcanic land known as island arcs with deep ocean trenches such island chains may develop over millions of years to become major landmasses. Subduction produces frequent shallow- to deep - fucus earthquakes , some of which are immensly powerful: Indonesia, where the Australian Plate is being subducted beneath the Eurasian Plate. 

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Continental plate/continental plate convergence

Where subduction of oceanic crust draws two continental masses together, a collision margin may develop. Continents have similar density and thus buoyancy, they will not be subducted.Instead they collide with each other. Volcanics associated with earlier subduction and sediments scraped off the varnishing ocean floor are mixed up and compressed to form young fold mountain chains with deep routesin the lithospere.Example, subcontinent of India,proppelled by sea floor spreading of the Indo-Australian Plate in a north-easterly direction until it collided with the eurasion plate  some 40 million years ago, this formed the Himalayan mountain chain home of the highest mountain on earth, Everest.

 Himalayas are constantly changing because they are highly folded and faulted regions do not become seismically quiet after first impact . Weathering and erosion reduce height, but isostatic lift ( uplift of land mass resulting from techtonic processes) in some area, produced by continuing plate motion, means that some scientists think everest is increaing by 2.5cm per year. whole area experiences high levels of seismicity causing devestating earthquakes e.g China 2008.

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Conservative margins

Referred to as passive or slip margins and occur where two plates meet and the direction of plate motion is either parallel. E.g San Andreas fault in Calafrnia and the Alpine fault in New Zealand. No crust is destroyed of created, although these are areas of frequent seismic activity as the build-up of friction as plates pass each other is released by earthquakes. They are not associated with active vulcanism.

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Hot spots

A volcanic area on the surface of the Earth created by a rising plume of magma. Others suggest they are created from far less depth and in fact, are moving slowly.

For example, the Hawaiian Islands, which are entirly of volcanic origin, were formed in the middle of the Pacific Ocean more than 3,200km from the nearest plate boundary.

 As basaltic shield volcanoes erupt through the drifting oceanic crust, they may build up from the ocean floor to form an island over time. However they become part of the plate and are gradually moved away from the heat source. Some islands will become eroded away by waves and form flat toped sea mounts called guyots.Newer volcanoes erupt over the hot spot and a new island is formed. This sequence can form a chain of mountains if the plate is moving quickly in one direction over the hot spot (Hawaiian chain) or a cluster if the plate is rotating more slowly around a hot spot.

In the case of Hawaii, the main island is currently closest to the hot spot and is most volcanically active. Older, less active or volcanically extinct island extend to the north-west of the main island, having been moved away from the hot spot as the Pacific Plate moves in that direction at aprox. 10cm/year.

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Vulcanicity

Extrusive landforms= above the ground.

Basaltic (basic) lavas-upward movement of mantle material.Most common along spreading ridges but also found at hot spots (hawaiian, 2 types of lava pahoehoe and aa), not that dangerous, runny presure, realeased quickly) and within more developed rift systems. Fissure or Icelantic volcano- very fluid basaltic magma almost horizontal flows

Andesitic (intermetiate) lavas- typical of destructive plate margins where crust is being destroyed.

Rhyolitic (acid) lavas- destructive and collision margins. Explode less frequently usually have viscous lava- sticky, more dangerous, lava cone.  E.g Stombolian, Vulcanian, Vesuvian and Plinean and Pelean ( most dangerous)

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Volcanoe classification

Shape

Fissure eruptions- elongated crack in the crust allows lava to spill out over a large area.Typically these are found around spreading ridges where tension pulls the crust apart e.g Heimaey, Iceland 1973- Eurasian and N. American plates pulled apart, existing topography was drowned in a vest lake of basaltic lava. Eruptions are gentle but persistant.

Shield volcanoes-basaltic rock and form gently slooping cones from layers of less viscous lava, largest volcanoe in world, Mauna Loa in Hawaii atands 4,170m above sea level with a volume of 40,000km3. Eruptions gentle and predictable. Location- hot spots and where oceanic crust meets oceanic crust.

Composite volcanoes- most common type on land. Created by layers of ash from initial explosive phases of eruptions and subsequent layers of lava from the main eruption phases. E.g mount Etna and Vesuvius in Italy. Location- destructive margins, eruptions explosive and unpredictable.

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Dome volcanoes- steep sided volcanoes formed from very viscous lava. As the lava cannot travel far, it builds up convex cone shaped volcanoes. Lava may solidify in the vent and be revealed later by erosion. E.g Mount Pelee. continental crust, eruptions explosive and unpredictable.

Calderas- gases that have built up beneath a blocked volcanic vent result in a catastrophic eruption that destroys the volcanoe summit, leaving an enormous crater where later eruptions may form smaller cones e.g Krakatoa in Indonesia. Destructive ,margins , eruptions very explosive and unpredictable.

Minor extrusive features

Geysors and hot springs- water heated at depth in the crust by magma chambers can periodically escape as steam and hot water. A geyser is an intermittent, turbulant discharge of superheated water ejected and accompanied by a vapour phase. Where hot water on its way upwards mixes with muds near the surface, a bubbling, boiling volcano may form. In some places hot springs have become tourist attractions e.g Turkey and Japan.

Boiling mud- hot water mixes with mud and surface deposits.

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Volcanic Frequency

Active- have erupted in living memory Dormant- have erupted within historical record Extinct- will not erupt again

Basalt volcanoes

Fissure- very fluid basaltic magma  gives almost horizontal flows along a crack.Erupt regular in parts of iceland are typical of those which occur along spreading oceanic ridges and continental rift valleys during the splitting of the continental crust.There is no single cone with a central vent, instead lava issues from a number of different points along a fissure in the earths surface. Hawaiian- molten lava erupts from a 'lake; in the central crater. Occasional bursts of incandescent spray; little tephra. Produces shield volcanoes. They are dangerous, runny, pressure released quickly.

Rhyolite volcanoes

Strombolian- sporadic eruptions of gas; lava forms bumbs; tephra (ash) mainly bombs and lava. Vulcanian- viscous lava which rapidly solidifies; leads to violent eruption of gases; abundent dust and bombs- lots of dust. Vesuvian and Plinean- Very violent expulsion of magma charged with gas, following prolonged quiessence. Abundant dust and bombs. May reach great height in Plinean eruptions. Pelean- very highly viscous lava results in the blockage of main vent. Lava charged with gas escapes from weak points as nuees ardentes.

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Seismicity

Earthquakes- occur when a build up of pressure within the earths crust is suddenly released and the ground shakes violently. The point within the crust where the pressure release occurs is known as the focus. Shallow focus: 0-70km deep Intermediate focus: 70-300km deep and deep focus: 300-700km deep

 The seismic shock waves have their highest level of nergy at the focus; energy decreases as they waves spread outwards. The place on the earths surface immediatly above the focus is called the epicentre, it receives the highest amount of energy and so its the most potential hazardous location.

Seismic waves which travel out from focus- P-waves- (primary) fastest and shake the earth backwards and forwards, they move through solids and liquids.

S-waves-(secondary) slower and move sideways motion, shaking the earth at right angles to the direction of travel. Cannot move through liquids but do much more damage than P-waves.

Surface waves- travel much nearer to the surface and more slowly but are more destructive. They include L-waves (long waves) which cause the ground to move sideways and Ralaigh waves which make it move up and down.

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Magnitude and frequency

Magnitude- amount of energy released by the event and is usually measured on the richter scale which is a logarithmic scale with each unit representing a 10 fold increase in strength and a 30 fold increase in energy is released.

Frequency- varies between seismically active regions and seismic zones within the shield of ancient crust. Seismometers are instruments that measure the record the shock waves created by earthquakes. They locate and measure the size of the shock waves and are used in establishing patterens of seismic activity that may help to predict future earthquakes.

Aftershocks- earthquakes that follow on from the main event and may last for months afterwards. They are generated by the earth settling back after the disruption and the first displacement.Some aftershocks are significant evernts e.g Indonesia 2004 9.3 magnitude  was followed by a number of after shocks one of which was 6.1 magnitude.

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Effects of earthquakes

Tsunamis- enormous sea waves generated by disturbances on the sea floor. They are most often triggered by earthquakes and submarine landslides. 2011 Japan

Liquefaction- Violent disruption of the ground causes it to become liquid-like when strongly shaken. Such extreme shaking causes increased pore water pressure which reduces the effective stres, and reduces the shear strengh of the soil so it fails more easily . Can sometimes cause movement of groundwater. Evan though surface may appear dry , excess water will sometimes come to the surface through cracks, bringing liquefied soil with it, creating 'soil volcanoes'. The liquefied soil may flow and the ground may crack and move, causing damage to surface structures and underground utilities. Buildings can 'sink'. San Francisco where development has occured on reclaimed land in bay area the land is more likely to fail. estimated US$100,000 million damage was caused by secondary effect of the 1989 earthquake.

Landslides and avalanches- slope failure as result of ground shaking.

Human Impact- depends upon pop density and distance from epicentre. Strong shaking of ground can cause buildings, roads and buildings to collapse and disruption to gas, electricity and water supplies. Some occur imediatly (primary) others develop after often as a consequence of primary events, e.g fires from uprouted gas mains, scarce or contaminated water suplies and loss of trade from cessation of industry.

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Distribution

Not evenly distributed over the planet but tend to occur in broad, uneven belts, this is because vast majority of earthquakes are related to plate motion and are, therefore, found around plate boundaries. Most powerful are related to destructive plate motion where the descent of oceanic crust creates frequent shallow- to deep focus earthquakes.

Earthquakes at constructive margins are often submarine and usually distant from human habitation, presenting a relitively minor hazard.

Conservative margins a series of fault lines marks where the crust has failed catastophically. San Andreas fault is not a single feature but a broad shatter zone of interrelated faults . Earthquakes occur in regions what do not appear to be near active plate margins . Earthquakes in China and central Asia occur along extensive lones of weakness related to the collision on India with the Eurasion Plate over 50 million years ago.

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Earthquake management

Prediction

No reliable way to predict earthquakes, however scientists are able to indicate where they are likely to be found.

Seismic records- studying patterns of earthquakes and using these to predict the next event. Seismic shock waves are recorded on a seismometer or seismograph. When the earth shakes the entire unit shakes with it , except for the mass on the string which has inertia, and remains in the same place. As the seismograph shakes under the mass, the device records the relative motion between itself  and the rest of the instrument, thus recording the ground motion.

Radon gas emissions- radon is an inert gas that is released rocks such as granite at a faster rate when they are fractured by deformation.

Ground water- deformation of ground can cause water levels to rise (compression) or fall (tension) independently of atmospheric conditions.

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remote sensing- some evidence that electromagnetic disturbances in atmosphere directly above areas about to have an earthquake can be detected.

low frequency electromagnetic activity- detection of electromagnetic emissions transmitted from earthquake regions satellite has made observations that show strong correlations between certain types of low frequency electromagnetic activity and the seismically most active zones on the earth.

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Comments

lovebiology101

this is great thanks

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