AQA A2 Geography


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Notes on Plate Tectonics and Associated Hazards

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AQA A2 Geography

  1. 1. Plate MovementThe theory of plate tectonics 1912, Alfred Wegener published theory of single continent (Pangaea) that existed 300 million years ago o Later split into 2 continents  Laurasia in north  Gondwanaland in south Different evidences to prove single continent o Geological evidence  Bulge of south America fitting into indent below west Africa  Evidence of glaciation of late Carboniferous period – deposits found in south America, Antarctica and India  Rock sequences in northern Scotland and Eastern Canada o Biological evidence  Fossil brachiopods found in Indian limestones – comparable with fossils in Australia  Fossil remains in South America and southern Africa  Fossil remains in coal in India and AntarcticaEarth’s layers o Core – made up of dense rocks containing iron and nickel alloys  Divided into solid inner core / molten outer core o Mantle – made up of molten / semi-molten rocks containing lighter elements such as silicon and oxygen o Crust – even lighter because of elements  Most abundant = silicon, oxygen, aluminium, potassium, sodium  Varies in thickness Oceanic crust – 6-10 km thick Continental crust – 30-40km thick Under highest mountain ranges – 70km thick o Lithosphere – consists of crust and the rigid upper section of the mantle  Approximately 80-90 km thick o Asthenosphere – below lithosphere  Semi-molten Continental Crust Oceanic CrustThickness 30-70 km 6-10 kmAge Over 1,500 million years Less than 200 million yearsDensity 2.6 (lighter) 3.0 (heavier)Composition Mainly granite; silicon, Mainly basalt; silicon, aluminium, oxygen (SIAL) magnesium, oxygen (SIMA) o Hot spots – generate thermal convection currents within asthenosphere Page 1
  2. 2.  Seen in Hawaii (see picture below)Features of plate marginsConstructive (divergent) margins o Plates move apart in oceanic areas o RIDGE VALLEYS  Longest continuous uplifted features  Precise form influenced by rate at which plates move apart Slow rate – 10-15mm/year, produces wide ridge axis (30-50km) and deep central rift valley (3,000m) Intermediate – 50-90mm/year, produces well-marked rifts (50-200m deep) with smoother outline Rapid rate – >90mm/year, produces smooth crest and no riftDestructive (convergent) margins OCEANIC / CONTINENTAL CONVERGENCE  Where oceanic and continental plates meet, denser oceanic plates is forced under continental crust – known as subduction  Downwarping of oceanic crust creates a trench (deep part of sea)  Sediments accumulate and continental crust is uplifted to from fold mountains  Further the rock descends the hotter the surrounding becomes. Andesitic lava then creates complex, composite, explosive volcanoes. If eruptions take place offshore a line of volcanic islands form called island arc Page 2
  3. 3. OCEANIC/OCEANIC CONVERGENCE  Ocean trenches and island arcs are the features associated – takes place offshore CONTINENTAL/CONTINENTAL CONVERGENCE  Plates forming continental crust have much lower density than underlying layers, not much subduction where they meet  Due to there being no subduction – no volcanic activity. Can create shallow- focus earthquakesConservative margins o Where 2 crustal plates slide past each other / movement of plates is parallel – no destruction/creation of crust o At these margins – no volcanic activity o However creates stresses as they rub past each other – causes shall-focus earthquakes such as San Andreas Fault, San FranciscoHot Spots In centre of pacific ocean, find Hawaiian islands Hot spot – concentration of radioactive elements inside the mantle; plume of magma rises to eat into plate above. When lava breaks through to the surface, active volcanoes occur above the hot spot Hot spot is stationary – only plates move above. Hence the row of Hawaiian islands Page 3
  4. 4. VulcanicityDistribution Most volcanic activity associated with plate tectonic processes, mainly located along plate margins Such activity therefore found: o Along ocean ridges o Associated with rift valleys o On or near a subduction zones o Over hot spotsVolcanic Eruptions Vary in form, frequency and type of volcanic eruptionIntrusive volcanic landforms o When magma forced the surface, only small amount of lava reaches that level o Most magma is intruded into crust where it solidifies o Often exposed after erosion o Batholith – formed deep below surface where large amounts of magma cools and solidifies. Large crystals then formed in rock (e.g. granite). Often dome shaped and exposed by erosion o Metamorphic Aureole – transformed from Batholiths o Dykes - o Dykes – vertical intrusions with horizontal cooling racks. Cluster of dykes called ‘dyke swarm’ o Sills – horizontal intrusions with vertical cooling racks A diagram to show intrusive volcanic landformsExtrusive volcanic landforms Involves two forms of lava o Basaltic lava – formed when magma is low in silica. effusive o Andesitic/rhyoltic lava – silica rick magma. More explosive Page 4
  5. 5. Main types of extrusive volcanic landforms o Basic/shield volcanoes – formed from free flowing lava. Have gentle sides and cover a large area o Fissure/lava plateaux – extensive lava flows are basaltic in nature, flow great distances o Acid/dome volcanoes – steep sided convex cones, viscous lava that is rhyoltic o Ash and cinder cones – formed from ash, cinders and volcanic bombs ejected from crater o Composite cones – classic pyramid shaped volcanoes. Consisting of layers of ash and lava o Calderas – occur when build-up of gases becomes extreme- huge explosions removes summit of volcano, leaving a ‘crater’ at the top Nature of volcanic eruptions o Vulcanologists classify volcanoes according to nature of eruption o Classification based on degree of violence of explosion Minor volcanic forms o Solfatara – small volcanic areas without cones, produced by gases (sulphurous) escaping to surface o Geysers – occur when water heated by volcanic activity, explodes onto surface o Hot springs/boiling mud – sometimes water heated below does not explode on surface. If water mixes with surface deposits, boiling mud is formedIntrusive and extrusive volcanic activity in the UK UK has no current volcanic activity Granites / other examples of intruded rocks occur across Grampians in Scotland, in Ireland and southwest of England o Exposed batholith in Dartmoor – known as a tor Dykes and sills also common o Dykes generally occur as small ridges in landscape as more resistant than surrounding rocks Basaltic flows – when lava cools, vertical cracks in flow result in hexagonal columns Volcanic plug – build-up of magma that has solidified and blocked the top of the volcanoImpact of volcanic activity Primary effects: o Tephra – solid material of varying grain size – volcanic bombs to ash ejected into the air o Pyroclastic flows – very hot (800C), gas-charged, high-velocity flows made up of a mixture of gases and tephra. Usually have a rock avalanche too o Lava flows Page 5
  6. 6. o Volcanic gases – include carbon dioxide, carbon monoxide, hydrogen sulphide, sulphur dioxide and chlorineSecondary effects : o Lahars – volcanic mud flows o Flooding – melting of glaciers and ice caps o Tsunamis – giant sea waves generated after violent caldera-forming events o Volcanic landslides o Climatic change – ejection of vast amounts of volcanic debris into atmosphere can reduce global temperatures and is believed to have been an agent in past climatic changeVolcanic effects become a hazard when they have an impact on the human and builtenvironments, killing and injuring people, burying and collapsing buildings, destroyinginfrastructure and bringing agricultural activities to a halt Page 6
  7. 7. SeismicityCauses of earthquakes As crust of Earth is mobile, tends to be a build up of stress within the rocks When pressure is suddenly released part of the surface experience an intense shaking motion Point of pressure release is known as focus – point immediately above on earth’s surface is called epicentre Depth of focus is significant and 3 broad categories of earthquake are recognised: o Shallow focus (0-70km deep) – tend to cause the greatest damage / account for 75% of all earthquake energy released o Intermediate focus (70-300km deep) o Deep focus (300-700km deep) Seismic waves radiate from focus rather like ripples in water Three main types of seismic wave, each travelling at different speeds: o Primary (P) waves travel fastest and are compressional, vibrating in direction in which they are travelling o Secondary (S) waves travel at half speed of P waves and shear rock by vibrating at right angles to direction of travel o Surface (L) waves travel slowest and near ground surface. Some surface waves shake ground at right angles to direction of wave movement / some have a rolling motion that produces a vertical ground movement P and S waves travel through interior of Earth and recorded on seismographDistribution Vast majority of earthquakes occur along plate boundaries – most powerful at destructive margins Conservative margins, boundary marked by a fault – movement along which produces the earthquake Some earthquakes occur away from plate boundaries and are associated with reactivation of old fault lines Human activity could be cause of some minor earthquakesMagnitude and frequency Magnitude measured on 2 scales o Richter scale – logarithmic scale  Event measured on a 7 point scale  Has an amplitude of seismic waves 10 times greater than once measured at 6 on a scale  Energy release is proportional to magnitude – for each unit increase in scale, energy released increases by approximately 30 times  Mercalli scale - Measures intensity of event and its impact (12 point scale) Page 7
  8. 8. Seismic records enable earthquake frequency to be observed – records only date back to 1848 when instrument to record seismic waves was first developedEffects of earthquakes Initial effect is ground shaking Severity depends on magnitude of earthquake, distance from epicentre, local geological conditions Secondary effects: o Soil liquefaction when violently shaken, soils with high water content lose mechanical strength / start to behave like fluid o Landslides/avalanches slope failure as a result of ground shaking o Effects of people and built environment  Collapsing buildings  Destruction of road systems / other forms of communication  Destruction of service provision (gas, electricity)  Fires from gas pipes and collapsed electricity lines  Flooding  Disease  Food shortages  Disruption to local economy o Tsunamis giant sea waves generated by shallow-focus underwater earthquakes, volcanic eruptions, underwater debris slides and large landslides into the seaTsunamis Have a very long wavelength (sometimes more than 100km) and low wave height (under 1m), travel quickly at speeds greater than 700km/h-1 On reaching shallow water bordering land they increase rapidly in height When a tsunami reaches land, its effect will depend upon: o Height of the waves / distance they have travelled o Length of event that caused tsunami o Extent to which warnings can be given o Coastal physical geography, both offshore and in coastal area o Coastal land use / population density Effects of most tsunamis are felt at least 500-600m inland depending upon coastal geography Buildings, roads, bridges, harbour structures, trees and soil washed away Around 90% if all tsunamis are generated within Pacific basin / are associated with tectonic activity taking place around its edges Page 8