Plate Tectonics

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  • Created by: gemma-t96
  • Created on: 01-11-14 15:33

Geological Time & Earth Structure

Time: Eons, Eras, Periods & Epochs. Divisions of time mark major changes in life form

Age of earth: 3.8b earliest human remains: 1m Oldest rocks: 2m

Structure

Core: Inner = Solid, D= 13.6gkm. Iron and Nickel 5500 Outer: Liquid, 5-12g Iron

Mantle: D=3.4g, Silicates up to 5000 Asthernosphere, inner solid, outer: semi liquid

Lithosphere: Oceanic 5-10km thick, mostly basalt SIMA
Continental: 35-70km less dense, mostly granite SIAL
  Granite - solid, D= 3gkm 5-70km thick

Convection currents
Sit on top of the mantle = tectonic plates, liquid rock is heated, particles rise but as they get nearer the crust they cool and sink = more dense. Up and down circular motion are convection currrents and this is what drives tectonic movement

 

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Plate tectonic theory

Alfred Wegener
 Continental drift published in 1912, continents fit together PANGAEA 225m yrs ago.
Start of Dinosaurs caused the split into LAURASIA & GONDWANALAND by the end continents almost seperate and continue to split to the present day

Plate tectonics: 1945, M,A ridge mapped. 1960's new seal floor created = earth's expanding
 Crust is being destroyed and earth seperated into no, of plates & move due to C,C

Geological evidence: rocks of same age/ type fround in Brazil & S, Africa. Glacial deposits found in Antartica, S, America and India

Climatological Evidence: Antartica, N America & UK have coal deposists of similar age that were once formed in tropical areas, must have drifed apart during Carboniferous period

Biological Evidence: Reptiles - Nesasarous found in S Am & S Africa sediments of Permian age
 Marsupials only found in Australia, as it drifted apart from main super continent before preditors that would have otherwise wipe them out before they've migrated 

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Plate tectonic theory

Alfred Wegener
 Continental drift published in 1912, continents fit together PANGAEA 225m yrs ago.
Start of Dinosaurs caused the split into LAURASIA & GONDWANALAND by the end continents almost seperate and continue to split to the present day

Plate tectonics: 1945, M,A ridge mapped. 1960's new seal floor created = earth's expanding
 Crust is being destroyed and earth seperated into no, of plates & move due to C,C

Geological evidence: rocks of same age/ type fround in Brazil & S, Africa. Glacial deposits found in Antartica, S, America and India

Climatological Evidence: Antartica, N America & UK have coal deposists of similar age that were once formed in tropical areas, must have drifed apart during Carboniferous period

Biological Evidence: Reptiles - Nesasarous found in S Am & S Africa sediments of Permian age
 Marsupials only found in Australia, as it drifted apart from main super continent before preditors that would have otherwise wipe them out before they've migrated 

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Palaeomagnetism & Hotspots

1948 - continous ridge running N-S was revealed. It was composed of volcanic rocks & magnetic surveys of the ocean floors causing striping. 
 When lava erupts on the sea floor the minerals align to earth's magnetic field.
 Strips of normal & reverse polarity are added equally to both sides
 Basalt also strips and runs parallel to the ridge
Rocks closest to the ridge are younger, older ones are pushed out but no evidence suggest that the earth is expanding
 Ocean Trenches = large areas of the ocean floor are subducted

Hotspots
 Area of crust with high volcanic activity caused by magma plumes. Not usually located on boundaries
 Responsilbe for : Hawaii, Iceland and relic features e.g. Deccan Traps, IndiaTheories: Radioactive elements in the mantle burn through the crust and releases lava creating islands of geothermal activity, hotspots stationary, crust moves over them
Island Chains- New volcano created and the plate moves left and plume rises = Volcano 2 etc
Modern theories suggest hotspots move/ stem from weaknesses in the crust caused by tectonics
Coral Atoll= corals settle and grow around volcanic islands, fringing in reefs can subsides island beneath leaving a lagoon  

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Plate Margins

Constructive: ocean ridges 2 plates pull apart. Crust expands = ridge. Central part may have a valley crust subsidised into margin. Crust pushed away = cools and sinks
 Transform fault: crust created at ridge moves out at different rates.
Rift Valleys: Continued rifting = new oceaic floorm sudes rife apart

Destuctive: O-O, C-O, C-C = subduction zone.
C-O dense O subducts, friction =EQ, rocks scraped off and C folds to create fold mountains. Ocean trenches marks the point where O descends. Basaltic margins less dense than mantle rise through weaknesses and forms composite volcanoes & assists fold mountain formation
 O-O volcanoes erupt to form islands, formed in curved lines = island arcs alongside deep sea ocean trenches
 C-C collision zone. volcanics from subduction events = fold mountain with chains & deep roots in the lithosphere
Mt Everest grows 2.5cm a year due to isostatic uplift

Conservative
S.A fault, 10,000 EQ yr. 800km long 16km wide. LA on W getting close to San Fran on east
Transform faults connect sea floor spreading, ridge of Gulf of California with spreading of Washington

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Intrusive & Extrusive Features

Minor Extrusive Features
Solfatara: emits sulphurous gases, yellow coloured deposits e.g. Solfatara di Pozzuoli, Italy
Geyser: hot water & steam through vent, by geothermal heating e.g. Strokkur Geyser, Iceland
Hot Springs: e.g. Japan
Boiling water/ water pools: heated water under no pressure mixed with mud e.g. New Zealand

Intrusive Features
Batholith: large magma mass cools slowly = coarse rock, exposed through weathering/ erosion e.g. Shap and Skiddaw, Lake District
Laccolith: igneous intrusion goes between layers & bubble up, dome of igneous rock between 2 layers of sedimentary rock
Monadnock: mountain that rises out of a surrounding plain and that develops because it consists of more resistant rock than the bedrock of the surrounding region
Dyke: Vertical sheet of rock created when magmas thrust into body of rock
Sills: long, thin intrusions of rock to form between sedimentary layers
Vein:  magma forces its way through cracks in the rock. When it cools vein like patterns may remain.

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Volcanoes

Fissure Eruptions: elongated crack, lava can flow back into the crack e.g. Iceland. Flood Basalts Form from fissure eruptions. Basaltic lava, Flow over great distances. Flat and featureless landscapes
Shield Volcanoes: Constructive boundary, gentle slow, thin lava. Mauna Loa, Hawaii
Acid: Steep sided, convex cone, sticky lava e.g. Puy France (Extinct)
Ash/Cinder: Volcanic bombs, symmetrical sides, thick lava e.g. Paricutin, Mexico
Composite: Alternate lava & ash. Destructive boundary, explosive pyroclastic flow. Mt Etna, Italy
Caldera: formed by collapse and removal of cone summit, can be flooded e.g. Krakatoa, Indonesia

Monitoring: Remote sensing, seismicity,gas, hydrology & geophysical measurements

Prediction: Hazard mapping, shock waves, gas, 
Preperation: land use planning, evacuation, react before, during and after the event

Primary: immediate damage caused by disaster, death, buildings collapsing, roads destoryed
Secondary: After effect, fire, disease, fammine, lack of food & water 

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Hazards

Geomorphological- floods           Meteorological – tornadoes        Tectonic – tsunami
Event – a natural/geophysical phenomenon.
Hazard – ‘perceived natural/ geophysical event with potential to threaten both life and property.’
Disaster – the realisation of a hazard,  causes ‘a significant impact on the vulnerable population.’
Risk – ‘the probability of a hazard occurring and creating a loss of life and livelihoods.’
Vulnerability – ‘a high risk of exposure to hazards combined with an inability to cope.’
Risk: Determined by: economy, preparedness, population growth/density, lack of training/ skills Perception – weighing up the pros and cons
Vulnerability: Dependent on resilience of people and reliability of management systems. The poor, young, old, women and ill are most likely to be affected
Human activity and physical processes do not interact, so there is no hazard. Human activity and physical processes become closer together so disaster can be the result. The more severe the hazard or more vulnerable to population the more the two overlap

Response varies due to: past exp, wealth, resources, and attitude. Scales vary e.g. local, national
Responses: Modify cause, Modify event - engineering/technology, Modify vulnerability, Modify loss

Prediction: Forecasting the event             Preparation: reducing the risk 

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Earthquakes

Caused when movement of the crust is concentrated into a sudden single shock or series of shocks. Aftershocks redistribute resultant stresses. Mostly from convergent margin activity

Types of Fault: Land plates (strike slip, San Andreas Fault) Move away diagonally
Normal fault: extension of land Reverse: shortening of land

Waves: P: fast, moves through everything, compressional wave.             
S: solids only, distort & move perpendicular to wave material
Surface Waves: travel just below or along the ground’s surface & slower than body waves

Epicenter location: Time-distance graph showing the average travel times for P- and S-waves. The farther away a seismograph is from the focus of an earthquake, the longer the interval between the arrivals of the P- and S- waves

Magnitude & Frequency: Magnitude – the amount of energy released. This is usually measured by the Richter scale. A logarithmic scale, Frequency - number of occurrences of a repeating event per unit time. Mercalli Scale – measures effects

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Tsunami's

Tsunami -a large wave, or series of waves, caused when an earthquake causes massive undersea crust movements and/or collapses which displace the water above. Earthquakes deform the ocean floor, pushing the overlying water up into a tsunami wave.

Liquefaction: Needs three factors. Loose, granular sediment. Saturation by groundwater. Strong shaking. Saturated loose sand and silt can behave like a liquid when shaken.
 Earthquake waves cause water pressures to increase in the sediment and the sand grains to lose contact with each other. Sediment loses strength and behaves like a liquid. It can then flow down even very gentle slopes.

TWS: issues warnings to prevent loss of life/property.
2 compounds: Network of sensors and communication infrastructure to issue warnings and cue evacuations. International WS use data from deep sea bouys and coastal bouys. Regional WS use recent data to determine the possible threat of TS and can issue warnings in 15 minutes  

What detects TS?
 Siesmometers, Tide gauges (measures sea level)
DART Bouy: sea levels to TWS, info's processed = accurate forecast & plans put in place quickly 

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