Tectonic and Hydrological Change

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2.1 Processes associated with plate tectonics

Plates and Plate boundries

  • 1912 Wegener- 'Continental Drift'- Pangaea- Laurasia & Gondwanaland.  Plate Tectonics
  • 7 Major tectonic plates;
    • African                                                                                        - Antarctic
    • Eurasia                                                                                       - Indo-Australian
    • North America                                                                            - South American
    • Pacific
  • Plate Properties;
    • Oceanic Plates- Thin & New
    • Continental Plates- Thick & Old
  • Lithosphere= Earth's Crust & Outer mantle (rigid material)
  • Aesthenosphere= Lower mantle (molten/semi-molten
  • The movement of tectonic plates is driven by thermal convection currents in the upper mantle
    • using heat derived from radioactive decay of minerals deep within the earth and residual heat from the Earth's formation
    • heat causes plumes of hot magma to rise.  If the crust is thinner on mid-ocean floor, the hotter and less dense magma breaks through to form new crust as it is cooled by the water
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2.1 Processes associated with plate tectonics

Constructive, Destructive and Conservative Plate Margins

Constructive- moves away

  • Plates move away from one another
  • Magma rises to the surface, forming oceanic crust (oceanic ridges) or continental crust (rift valleys)
  • Characterised by shallow-focus earthquakes and volcanic activity (sheild)
  • Iceland lies across mid-Atlantic ridge and new islands e.g Surtsey and Heimey have been formed due to volcanicism.
  • 2010 Eyjafjallajokull erupted along this plate boundary

Destructive and Collision margins- moves towards eachother

  • 3 Types: oceanic-continental; oceanic-oceanic; continental-continental
  • first two are destructive plate boundaries and last is a collision margin
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Destructive & Collision Margins

Destructive Plate Boundries and Collision Margins

  • Oceanic-Continental
    • denser oceanic crust subducts beneath lighter continental crust, creating deep ocean trenches and volcanic mountain chains and batholiths beneath the surface
    • ocean plate is partially melted, producing andesitic or granitic magma
    • shallow-to-intermediate-focus earthquakes commonly occur
    • E.g. Chile, February 2010- Nazca plate was subducted beneath South American plate
  • Oceanic-Oceanic
    • one plate subducts and a volcanic island arc with an adjacent deep-sea trench may form and shallow-to-deep-focus earthquakes occur in the Benioff Zone (trench towards island arc)
    • Aleutian Islands form an arc from Alaska, USA to Kamchatka Peninsula, Russia along the northern edge of the Pacific Ring
  • Continental-Continental
    • crust cannot subduct so is forced upwards to form fold mountains e.g. Himalayas.
    • shallow-focus earthquakes occur but with little volcanic activity.
    • Kashmir, Pakistan 2005 earthquake occured in the region of the colliding Eurasian and Indo-Australian Plates
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2.1 Processes associated with plate tectonics

Conservative margins

  • plates slide laterally past one another
  • no volcanic activity but shallow-focus earthquakes are common along faults running parallel to the plate margin
  • San Andreas Fault along the west coast of North America where the Pacific and North American plates meet, puts a string of major cities e.g. San Francisco and Los Angeles at risk
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2.2 Hazards associated with tectonic events

Earthquakes & Volcanoes

Volcanoes & their impact;

  • occur at constructive and destructive plate margins, or over hotspots in the Earth's crust (hot magma, ash and gases escae from below Earth's surface)
  • Type of eruption and the nature of its product depend on location
  • Can be classified by their size, shape or nature of their eruptions- features connected as type of eruption affects consequent landform
  • Eruptions can either be classified as;
    • Effusive: outpouring of magma, relatively low in viscosity & gas content
    • Explosive: magma is more viscous and acidic
  • 2 basic types of valcano: Shield & Composite
  • Vary according to how explosive they are and the volume of their eruptions
  • More explosive volcanoes often create pyroclastic flows (dangerous, fast-moving flows of hot gas and rock) recent examples include Mount St Helens 1980
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Shield Volcanoes

Shield Volcanoes- e.g. Mauna Loa, Hawaii

  • flat shape & wider diameter as layers of liquid lava spread out over a wide area
  • produce lavas that are hotter an dless viscous
  • Gas release is rare, so gentle slopes are built up with layers of basaltic lava
  • Often the crater reamins open & filled with bubbling lava & typically there is just one central vent
  • Formed over a hotspot & also found at constructive plate margins, forming rift volcanoes with open fissures, e.g. Iceland
  • Speed of lava flow and (in Iceland) potential flooding in colder climates as ice ias melted, are the main hazards
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Composite Volcanoes

Composite Volcanoes- e.g. Mount Fuji, Japan

  • more pyramidal in cross-section and built up with layers of lava, rock fragments and ash
  • produce lavas that are cooler and more visous, allowing gas pressure to build up between infrequent eruptions, causing explosive ones
  •  contribute to the formation of volcanic arcs ar destrictive plate margins
  • Examples are all located around the Pacific Ring of Fire
  • sequnce of slow-flowing lava and pyroclastic material erupted produces steep layers of ash and lava domes- increasing chance of landslides and avalanches
  • common hazard: lahars or mudflows- move quickly and usually hot
  • Caldera may be formed in the greatest eruptions, when the top of the symmetrical cone is blown off leaving a collapsed central crater.
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2.2 Hazards associated with tectonic events

Volcanic Event- Mount Pelee, Martinique (1902)

  • 29,000 dead
  • Eruption generated massive pyroclastic flows
  • destroyed the coastal town of Saint Pierre
    • Everyone either burned alive, suffocated or burried by fast-moving ash flow

Earthquakes and their Impact

  • Result of sudden release of pressure as tectonic plates move against each other creating seismic waves
  • initial point of rupture= focus/hypocentre within the crust, described as deep or shallow focus
  • Epicentre= ground surface directly above the focus

Moment magnitude scale:

  • measured energy released- each step x10 greater than the previous on a lagarithmic scale
  • Richter Scale: amplitude of shaking on a seismograh to estimate earthquake size
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2.2 Hazards associated with tectonic events

Mercalli Scale

  • measures the intensity of skaking
    • descriptive judging (extent of damage) relying on people's interpretation of events- if no one is there, nothing will be measured
  • Different undderlying rock and sediments give varying results for the same magnitude
    • difficult to compare earthquakes using this scale (I-XII)

Indonesia tsunami, 26 December 2004

  • large earthquake's focus beneath seabed and causes displacement
  • undersea earthquake off west coast of Indonesia with 9.3 magnitude (2nd largest recorded)
    • triggered a series of tsunamis affect Indian Ocean coast lines
  • 230,000 people dead
  • Large earthquakes can trigger volcanic eruptions up to 500km away as well as many months later
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2.3 tectonic hazards percieved and managed

why live in potentiall hazardous places?

  • risk has increased since they moved
  • benefits outweigh the hazardous cost
  • unable to move for social, economic, cultural or political reasons
  • Engineers and scientists calculate risk: degree of hazard x cost
  • risk equation: Risk (R) =

frequency/magnitude of Hazard (H) x level of vulnerability (v)

capacity of population to cope and adapt (C)

  • Those who are fatalistic (accepts hazards as a natural part of life) will stay and accept whatever happens
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2.3 tectonic hazards percieved and managed

Strategies to manage tectonic hazards and their effectiveness

  • Management is partly influenced by the perception of risk
  • No warning system for countries around Indian Ocean in 2004 tsunami as such events were percieved as less common in that region
  • Levels of preparedness and response are closely related to the affluance of a country and to its past experience
  • 3 stages in management: preparation, immediate response & long-term response


  • emergency equipment (tents, rations, first aid) in public buildings and in homes
  • specialist workers
  • raise public awareness
  • warning systems (evacuation plans)
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2.3 tectonic hazards percieved and managed

immediate response

  • coordinate search-and-rescue
  • establish medical reception centres
  • alternative accommodataion & emergency shelters
  • householders and businesses turn off gas supplies to prevent fire

Longer-term response

  • safe buildings e.g. earthquake-resistant structures (flexible steel frames, shock absorbers, cross-bracings & counterweights
  • improve prediction and monitoring systems

Responding to a tectonic event

  • rich countries can usually cope with the consequences of disasters using own resources
    • poorer nations are reliant on support of international organisations, charities and other countries to help in the immediate aftermath of a hazard and the long-term reconstruction
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2.3 tectonic hazards percieved and managed

  • Disasters Emergency Committee (DEC): 13 humanitarian aid agencies,bringing aid, corporate, public and broadcasting sectors to raise funds for relief
    • Red Cross
  • United Nations International Strategy for Disaster Reduction (UNISDR): build disaster-resilient communities- promoting awareness of the importance of disaster reduction to limit loses due to natural hazards.  Improve preparedness and resistance to the consequences.
  • Protection: digging trenches and building artificial barriers to limit the impact of lava flows and lahars
  • Pouring water on lava to solidify it in Iceland (small-scale reasonably successful actions
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2.4 Hydrological processes and drainage basin

Drainage Basin: area of land drained by a river & its tributaries.  Has measurable inputs, flows, stores and outputs and is part of the hydrological cycle that involvese the land.


Precipitation: water & ice that fall from the clouds in the form of rain, snow or hail


  • Throughfall, stemflow and drip occur through the vegetation
  • Infiltration: movement of water from above to below the surface of the ground.  Rate of infiltration depends on saturation, porosity and structure of the soil, and the type and extent of vegetation.  After a long, dry period the ground surface may be 'baked' and relatively impermeable
  • Percolation: movement of water down through the soil and permeable rock
  • Surface runoff/overland flow: water travels across saturated/impermeable land surfaces.  Depends on the ability of ground surface to absorb water.
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2.4 Hydrological processes and drainage basin

  • Throughflow: movement of water through soil towards the surface, often emerging on valley sides to form springs
  • Groundwater flow/base flow: movement of water through rocks towards the surface


  • Interception: vegetation and other surfaces above ground catch falling precipitation. Water may evaporate or continue falling to the ground
  • Surface storage: any body of water (puddle-lake)
  • Soil water storage: essential for plant growth
  • Groundwater storage: (permeable rocks) aquifers- large groundwater stores.                         Water Table- surface of underground water
  • River channel: combing water that flows into it from the surface, soil and ground- water flows in the channel to become river runoff or discharge
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2.4 Hydrological processes and drainage basin


  • river runoff: water carried overland to the sea
  • evaporation: water changes into water vapour
  • transpiration: evaporation of moisture through the stomata on leaf surfaces
  • evaporation and transpiration are difficult to measure individually, so are often combined as evapotranspiration

Water Budget

  • balance between water inputs and outputs
  • changes throughout the year

river regimes & physical and human factors influencing them

  • variations in river regimes or discharge over the course of the year
  • several factors influence river regimes; climate, geology, land use & river management
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2.4 Hydrological processes and drainage basin


  • humid environments: constant flow of groundwater (perennial flow)
  • drier environments: insufficient water to maintain water flow (intermittent flow)
  • driest environments: occasionally fed by flash flooding (ephemeral flow)


  • rocks underlying a drainage basin. Permeable rocks: porous & pervious (joints & cracks)
  • Impermeable: only pass through fractures in such rocks

Land use

  • presence/absence of forestry- level of interception

River Management

  • Dam-building: downstream flow controlled, dredging; channel straightening
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2.4 Hydrological processes and drainage basin

Flood hydrographs

  • river discharge in several parts of the drainage basin
  • show rainfall as a histogram and discharge as a line graph
  • distance between peak rainfall and the peak discharge is called the lag time
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2.5 Causes and consequences of flooding

  • Floods occur when volume of rainfall or snow melt exceeds the capacity of the drainage basin
  • Climate change increasing risk of flooding with rising sea levels and more intense storms

Tewksbury 20 July 2007                                                           Boscastle, august 2004

  • uvial flooding                                                                 - Flash flooding (banks overflowed)
  • equivalent 2 months rain in 1 day.                                 - intense storm (2m in 1 hour) 3m wave  
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2.6 Management

          Hard engineering                                                        Soft Engineering

  • flood walls & embankments                                             - Afforestation
  • River channelisation                                                        - Landuse management
  • Diversion channels                                                          - Provision of wet lands
  • Dams and reservoirs

Reduce vulnerability: flood warning systems; preparedness & emergency response; insurance

three levels of local flood warnings of the Environmental Agency:

  • flood watch- flooding of low-lying land and roads expected
  • flood warnings- flooding of homes and businesses expected
  • severe flood warnings- severe flooding expected (extreme danger to life and property)
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