1429399
Description
Mind Map by sebastian1862, updated more than 1 year ago
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Created by sebastian1862
about 10 years ago
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Earth Structure, Magnetism, Continental
Drift and Plate Tectonics
- Earth Structure
- Changes with depth
- Pressure-weight of the overlying rock increases with depth
- Temperature-Heat is generated in Earth's interior by radioactivity
- Geothermal gradient: Rate of T change varies with depth, geothermal gradient varies at about High 50 celsius per Km to low 15 celsius determined by tectonic setting
- Geothermal gradient is the rate of increasing temperature with respect to increasing depth in the
Earth's interior.
- Geothermal gradient is the rate of increasing temperature with respect to increasing depth in the
Earth's interior.
- Pressure-weight of the overlying rock increases with depth
- Investigation into the Earth
- Seismology-use of earthquake waves to map interior of Earth
- Seismic wave velocities change with density allowing us to determine depth of seismic velocity changes indicates density changes)
- 2 Types
- P-waves-compressional-travel through sold and liquid
- S-waves-shear-travel through solid NOT liquid
- Liquids not rigid enough to transmit shear
- Liquids not rigid enough to transmit shear
- P-waves-compressional-travel through sold and liquid
- 2 Types
- Seismic wave velocities change with density allowing us to determine depth of seismic velocity changes indicates density changes)
- Seusmological evidence for liquid outer core as no s-waves. Shadow waves caused via refraction to give us a size of the core to outer core.
- Seismic waves measured due to 1950's Global seismographic network to monitor nuclear tests
- Liquid outer core drives Earth's Magnetic field
- Core radius: 3,471Km
- Outer core:
- Liquid iron-nickel-sulphur
- 2,255km thick
- Density 10-12g/cm3
- Liquid iron-nickel-sulphur
- Inner core
- Solid iron-nickel alloy
- Radius of 1,220Km
- Density 13g/cm3 like core of proto-planets (iron meteorites)
- Solid iron-nickel alloy
- Outer core:
- Core radius: 3,471Km
- Seismic waves measured due to 1950's Global seismographic network to monitor nuclear tests
- Seismology-use of earthquake waves to map interior of Earth
- Changes with depth
- Magnetic Field
- Molten iron in core circulates along a spiraling path in columns aligned in the N-S direction generating electrical currents to set up the dipolar magnetic field
- Known as Geodynamo
- Known as Geodynamo
- Like a dipole bar magnet: N pole is near Earth's geographic S pole due to opposites attracting
- Magnetic ffield lines extend into space, weaken with distance.
- Form a shield known as the Earth's magnetosphere
- Solar wind distorts the magnetosphere (shaped like a tear drop)
- Deflects solar wind, protecting Earth
- Magnetic field concentrates charged particles of the solar wind in the Van Allen belts
- Lack of protection would lead to atmosphere being "hosed away"
- Van Allen belts concentrate deadly plasma from sun and expand when increased solar activity
- Inner Van Allen Belt: Protons and electrons. Outer is mostly electrons
- Inner Van Allen Belt: Protons and electrons. Outer is mostly electrons
- Van Allen belts concentrate deadly plasma from sun and expand when increased solar activity
- Lack of protection would lead to atmosphere being "hosed away"
- Magnetic field concentrates charged particles of the solar wind in the Van Allen belts
- Deflects solar wind, protecting Earth
- Solar wind distorts the magnetosphere (shaped like a tear drop)
- Form a shield known as the Earth's magnetosphere
- Revealed by spectacular auroe where charged particles are channeled along magnetic lines where they ionise atmospheric gases in polar regions and glow
- Northern Lights: aurora borealis. Southern Lights: aurora australis
- Northern Lights: aurora borealis. Southern Lights: aurora australis
- Molten iron in core circulates along a spiraling path in columns aligned in the N-S direction generating electrical currents to set up the dipolar magnetic field
- Earth's Crust
- Crust: Continental, Oceanic
- Continental underlies continents:
- Average thickness 35-40km - thickest 70km under mountain belts
- Average density of 2.7g/cm3 (more buoyant)
- Composition is Felsic (granite) to intermediate
- Average thickness 35-40km - thickest 70km under mountain belts
- Oceanic underlies ocean basins
- Average thickness 7-10km - thinnest under ridges
- Average density of 3.0g/cm3 less buoyant
- Average thickness 7-10km - thinnest under ridges
- Composition is Mafic (basaltic and gabbroic) in composition
- 99.5% of crust is comprised of 8elements, Oxygen being the most abundant (reflects
importance of silicate minerals), occupies 93% of crustal volume
- Continental underlies continents:
- Mantle: Upper, Transitional, Lower
- Lithosphere
- Outermost 100-150km
- Behaves rigidly - non-flowing material
- Makes up Tectonic plates
- Outermost 100-150km
- Asthenosphere
- Upper mantel below lithosphere
- Shallow under oceanic lithosphere, deeper under continental
- Flows as a soft solid
- Upper mantel below lithosphere
- Lithosphere
- Core: Outer-Liquid, Inner-solid
- Layers sub-divided on basis of seismic waves
- Alfred Wegener - Continental Drift
- German meteorologist wrote 'The Origins of Oceans and Continents in 1915'
- Radical ideas of Continental drift and formation of supercontinent of Pangea
- Before, oceans and continents thought as FIXED
- Led to Plate tectonics where in 1960's 'paradigm' shift, Harry Hess proposed sea floor spreading, 1968
complete model made linking continental drift to sea floor spreading and subduction. 20 plates!
- EVIDENCE FOR ALFRED WEGENER
- Present shorelines fit: Bullard 1965 continental shelf edges make a better fit
- Glacial evidence of Permain glaciers found on 4 continents. Glacial deposits converge in Pangea model
- Fossil evidence of identical fossils on widely spearted landmasses:
- Mesosaurus: freshwater reptile
- Glossopteris: subpolar fern with heavy seeds
- Cynognathus and Lystrosaurus-mammal like reptiles
- Mesosaurus: freshwater reptile
- Geological units
- Distinctive rock assemblages across Atlantic of structures, rock types and ages: Proterozoic 2,500-542Ma and Archean 4-2.5 Ga
- Distinctive rock assemblages across Atlantic of structures, rock types and ages: Proterozoic 2,500-542Ma and Archean 4-2.5 Ga
- Present shorelines fit: Bullard 1965 continental shelf edges make a better fit
- EVIDENCE FOR ALFRED WEGENER
- Before, oceans and continents thought as FIXED
- Radical ideas of Continental drift and formation of supercontinent of Pangea
- Criticism though due to No Mechanism thus hypothesis died
- Revived in WW2 via Polar wandering, Palaeomagnetism Bathymetry of the oceans via echo
sounding, global seismology, sea-floor spreading detected by magnetometers
- Palaemogagnetism: Geographic and magnetic poles not parallel where the difference (declination) between geographic North and Magnetic North is constantly varying: 11.5 degrees declination atm)
- Curved magnetic fields causes magnetic needle to tilt this is called inclination influenced via latitude, normal or reverse polarity
- Magnetic signals archived by iron minerals: Hot magma no magnetization due to kinectic energy of atoms very high, magnetic dipoles randomly oriented
- Cools through 'curie temperature) 540 celsius for basalts, dipoles align with mag field, magnetic dipoles 'frozen' into rock
- Cools through 'curie temperature) 540 celsius for basalts, dipoles align with mag field, magnetic dipoles 'frozen' into rock
- Magnetic signals archived by iron minerals: Hot magma no magnetization due to kinectic energy of atoms very high, magnetic dipoles randomly oriented
- Curved magnetic fields causes magnetic needle to tilt this is called inclination influenced via latitude, normal or reverse polarity
- Polar Wandering
- Layered basalts record magnetic changes overtime
- Inclination and declination indicate change in position
- Known to be an artifact as it is not the signature of a wandering pole on a fixed continent,
- Signature of a fixed pole on a wandering continent
- Apparent polar wandering is strong evidence for drift!
- Each contient has a separate polar wandering path, paths align when continents joined as Pangea
- Each contient has a separate polar wandering path, paths align when continents joined as Pangea
- Apparent polar wandering is strong evidence for drift!
- Signature of a fixed pole on a wandering continent
- Known to be an artifact as it is not the signature of a wandering pole on a fixed continent,
- Inclination and declination indicate change in position
- The slow erratic movement of the earth's poles relative to the continents throughout geological
time, due largely to continental drift.
- Layered basalts record magnetic changes overtime
- Sea-Floor Bathymetry
- Until 1940-lead weight used to measure depth
- Echo-sounding (sonar) created Bathymetric maps revealing:
- Mid-ocean mountain range (every ocean), Deep Ocean trenches occur near volcanic island chains,
submarine volcanoes ocean floor, huge fracture zones segment the mid ocean ridge, oceanic transform faults.
- Belts of subsea earthquakes found limited to mid ocean ridge axes, parts of oceanic fracture zones, deep ocean trenches
- 1920's seismology identified zones of earthquakes parallel to oceanic trenches inclined
40-60 horizontal and extending several 100km into Earth - subduction zones
- 1920's seismology identified zones of earthquakes parallel to oceanic trenches inclined
40-60 horizontal and extending several 100km into Earth - subduction zones
- Belts of subsea earthquakes found limited to mid ocean ridge axes, parts of oceanic fracture zones, deep ocean trenches
- Mid-ocean mountain range (every ocean), Deep Ocean trenches occur near volcanic island chains,
submarine volcanoes ocean floor, huge fracture zones segment the mid ocean ridge, oceanic transform faults.
- Echo-sounding (sonar) created Bathymetric maps revealing:
- Until 1940-lead weight used to measure depth
- Sea-Floor Spreading
- 1960, Harry Hess published 'Essay in Geopoetry', theory 'Sea-floor spreading'
- Upwelling mantel erupts at mid ocean ridges
- New crust moves away from ridges gathering sediment
- At trenches, sea-floor subducts back into mantle
- Provides mechanism for continental drift
- Contient move apart as sea-floor spreading occurs, continents move together as sea-floor is subducted
- Contient move apart as sea-floor spreading occurs, continents move together as sea-floor is subducted
- Provides mechanism for continental drift
- At trenches, sea-floor subducts back into mantle
- New crust moves away from ridges gathering sediment
- Upwelling mantel erupts at mid ocean ridges
- Layered lava flows reveal reversals in magnetic polarity which are geologically rapid - time markers
- Magnetism in sea floor varies across MORs, stripes of positive (stronger) and negative (weaker) magnetic intensity (symmetrical about MOR)
- Sea floor spreading explains stripes as magnetic polarity are imprinted in sea-floor as sea floor continues to spread.
- Determine rate of sea floor spreading: Mid Atlantic Ridge 2cm/yr, East Pacific Rise 10cm/yr
- Age-Dating rock gives timing of polarity reversals which occur in uneven intervals
- Longer intervals (500 to 700+ Ka) called chrons
- Shorter intervals (200Ka) are subchrons
- Magnetic reversals time scale extended to 170 Ma
- Longer intervals (500 to 700+ Ka) called chrons
- Age-Dating rock gives timing of polarity reversals which occur in uneven intervals
- Determine rate of sea floor spreading: Mid Atlantic Ridge 2cm/yr, East Pacific Rise 10cm/yr
- Sea floor spreading explains stripes as magnetic polarity are imprinted in sea-floor as sea floor continues to spread.
- Magnetism in sea floor varies across MORs, stripes of positive (stronger) and negative (weaker) magnetic intensity (symmetrical about MOR)
- 1960, Harry Hess published 'Essay in Geopoetry', theory 'Sea-floor spreading'
- Palaemogagnetism: Geographic and magnetic poles not parallel where the difference (declination) between geographic North and Magnetic North is constantly varying: 11.5 degrees declination atm)
- Revived in WW2 via Polar wandering, Palaeomagnetism Bathymetry of the oceans via echo
sounding, global seismology, sea-floor spreading detected by magnetometers
- German meteorologist wrote 'The Origins of Oceans and Continents in 1915'
- Crust: Continental, Oceanic
- Plate Tectonics
- 1968, evidence overwhelming
- Mechanism explains: Origin of continents and ocean basins, distribution of earthquakes and volcanoes, distribution of igneous, sedimentary and
metamorphic rocks, distribution of fossil platts and animals and the genesis and destruction of mountain chains "orogeny"
- Mechanism explains: Origin of continents and ocean basins, distribution of earthquakes and volcanoes, distribution of igneous, sedimentary and
metamorphic rocks, distribution of fossil platts and animals and the genesis and destruction of mountain chains "orogeny"
- Tectonics in Lithosphere
- Principles of Buoyancy and "isostacy"
- Archimedes 2000 years ago of floating solids displace water equal to their mass.
e.g. iceberg sinks until mass of water it displaces is equal to the total mass of
iceberg, applies to lithosphere plates
- Maintaining "Isostatic equilibrium"
- Glaciation:
- 1) Isostatic equilibrium destroyed by loading of ice sheet, mantle moves to restore
- 2) Ice sheet melts destroys equilibrium, mantle moves to restore equilibrium e.g. Post-glacial rebound (glacial-isostatic adjetment)
- 2) Ice sheet melts destroys equilibrium, mantle moves to restore equilibrium e.g. Post-glacial rebound (glacial-isostatic adjetment)
- 1) Isostatic equilibrium destroyed by loading of ice sheet, mantle moves to restore
- via erosion and redeposition of sediment:
- Flow in asthenosphere acts to form isostatic equilibrium
- Erosion of mountains, transport and redeposition destroys equilibrium, mantle flows to compensate
- Movement of asthenosphere mantle restores isostatic equilibrium
- Movement of asthenosphere mantle restores isostatic equilibrium
- Erosion of mountains, transport and redeposition destroys equilibrium, mantle flows to compensate
- Flow in asthenosphere acts to form isostatic equilibrium
- Glaciation:
- Archimedes 2000 years ago of floating solids displace water equal to their mass.
e.g. iceberg sinks until mass of water it displaces is equal to the total mass of
iceberg, applies to lithosphere plates
- Principles of Buoyancy and "isostacy"
- 20 Tectonic plates, average 15cm/yr, identified via active margins, subduction zones, volcanoes, plate interiors almost equarthquake free.
- Volcanoes mark subduction zones and some spreading ridges and rifts
- Continental margin is where land meets ocean
- Margins near plate boundaries are 'active', far are 'passive'
- Passive-margin continental crust thins seaward: transition into oceanic crust, traps eroded
sediment, develops continental shelf (the area of seabed around a large land mass where the sea is
relatively shallow compared with the open ocean. ) (mainly Atlantic)
- Margins near plate boundaries are 'active', far are 'passive'
- Continental margin is where land meets ocean
- Volcanoes mark subduction zones and some spreading ridges and rifts
- Plate Boundaries:
- Divergent Boundary-Plate moves apart, Lithosphere thickens away from ridge axis, spreading boundary, mid ocean ridge
- How plates break up
- Phase 1: Rifting, Lithosphere stretched, doming and faulting, crust thins, mantle upwells, pressure causes melting thus volcanism develops e.g. East African Rift Valley
- Phase 2: Drifting - Ocean crust formation, develops MOR, initiation of sea-floor spreading, development of new continental margins
- Fast spreading=smoother topography (EPR) 55mm/yr, slow=faulted central rift valley and more wide spread volcanism 12mm yr (MAR), Ultraslow spreading 7mm/yr (Southwest Indian Ridge)
- Fast spreading=smoother topography (EPR) 55mm/yr, slow=faulted central rift valley and more wide spread volcanism 12mm yr (MAR), Ultraslow spreading 7mm/yr (Southwest Indian Ridge)
- Phase 2: Drifting - Ocean crust formation, develops MOR, initiation of sea-floor spreading, development of new continental margins
- Phase 1: Rifting, Lithosphere stretched, doming and faulting, crust thins, mantle upwells, pressure causes melting thus volcanism develops e.g. East African Rift Valley
- How plates break up
- Convergent Boundary - Plates move together, subduction zone, trench, etc
- Subducting plate descends at an 45 degree angel revealed by Wadati-Benioff Earthquakes (lanar zone of seismicity
corresponding with the down-going slab in a subduction zone.) marks frictional contact and mineral
transformations, earthquakes deepen away from trench
- Quakes cease below 660km, plate descent may continue past earthquake limit, lower mantle may be plate graveyard
- Subduction features - Accretionary prisms (sediment scraped off), Back-arc basins, Volcanic arcs (Andes) (RESEARCH)
- Plate collision: Subduction, ocean closure, continental collision, continental crust too buoyant to subduct
- If continental and continental, mountains uplift e.g. Himmalayas
- If continental and continental, mountains uplift e.g. Himmalayas
- Plate collision: Subduction, ocean closure, continental collision, continental crust too buoyant to subduct
- Subduction features - Accretionary prisms (sediment scraped off), Back-arc basins, Volcanic arcs (Andes) (RESEARCH)
- Quakes cease below 660km, plate descent may continue past earthquake limit, lower mantle may be plate graveyard
- Subducting plate descends at an 45 degree angel revealed by Wadati-Benioff Earthquakes (lanar zone of seismicity
corresponding with the down-going slab in a subduction zone.) marks frictional contact and mineral
transformations, earthquakes deepen away from trench
- Transform Boundary - Plates move sideways, Plate material neither created or destroyed, transform fault.
- Lithosphere slides past, not created or destroyed: many transforms offset spreading ridge segments and can cut through continental crust (Continental tranforms e.g. San Andreas Fault)
- Characterised by: Earthquakes, absence of volcanism
- Oceanic Transforms: MOR axis is offset by transform faults, a geometric necessity for a line spreading on a sphere
- Abundant earthquakes common when offsets are opposed, earthquakes vanish when either side of fault moves in same direction: Strong evidence of sea floor spreading
- Abundant earthquakes common when offsets are opposed, earthquakes vanish when either side of fault moves in same direction: Strong evidence of sea floor spreading
- Oceanic Transforms: MOR axis is offset by transform faults, a geometric necessity for a line spreading on a sphere
- Characterised by: Earthquakes, absence of volcanism
- Lithosphere slides past, not created or destroyed: many transforms offset spreading ridge segments and can cut through continental crust (Continental tranforms e.g. San Andreas Fault)
- Spreading ridges: Black smokers which are metal and sulphide rich plumes, unstable in seawater where precipitation of metal ores occur, energy can support life without light - chemosynthesis
- Oceanic lithosphere, hot asthenosphere at base of MOR, aging ocean crust moves away from heat, moves cools causing increased density and sinking, accumlating increasing thickness of sediment
- Oceanic lithosphere, hot asthenosphere at base of MOR, aging ocean crust moves away from heat, moves cools causing increased density and sinking, accumlating increasing thickness of sediment
- Subduction: Old oceanic lithosphere more dense than mantel, flat lying oceanic crust doesn't subduct easily, once bent downward leading edge sinks
- Driving mechanism: Old idea of convecting mantle but plate motions too complex for this, not prime mechanism. Model of a custard shape (see google images)
- Plate tectonics started as rigid crust - continuous 'stagnant lid regime' (like Mars). crust cracks and becomes a mobile regime, Venus is episodic regime (crust cracks and entirely subducts to reform
- Modern Thinking: Two froces
- Ridge Push-elevated MOR pushes lithosphere away
- Slab-pull-gravity pulls a subducting plate downward
- Convection in the asthenosphere adds or subtracts
- Ridge Push-elevated MOR pushes lithosphere away
- Modern Thinking: Two froces
- Plate tectonics started as rigid crust - continuous 'stagnant lid regime' (like Mars). crust cracks and becomes a mobile regime, Venus is episodic regime (crust cracks and entirely subducts to reform
- Triple Junctions: Where 3 plate boundaries intersect, multiple boundary combinations, these migrate and change across time
- Hot Spots: Plumes of deep mantle material independent of plates burn through plates and add
lines of volcanoes to them. Hot spot seamounts age away from originating hot spot, line of
seamount indicates direction of plate motion and age change marks rate of plate motion
- Plate Velocities mapped by Plotting plate motion relative to a fixed spot in the mantle or measuring ages/distance along a hot spot track
- Plate Velocities mapped by Plotting plate motion relative to a fixed spot in the mantle or measuring ages/distance along a hot spot track
- Tectonics essential for life: Drives rock cycle by providing a global thermostat recycling chemicals crucial to
keeping CO2 content relatively uniform. Enabled liquid Water, avoided runaway greenhouse gases (Venus),
controls long term sea level chage, also important for CO2
- Fostered the development and maintenanceof complex life by creating and maintaining a changing
variety of different environments ranging for mountain chains to deep ocean to allow and maintain
global biodiversity, best defense against mass extinction. Overtime time, continents provide key to
terrestrial environments for animal life to develop.
- Fostered the development and maintenanceof complex life by creating and maintaining a changing
variety of different environments ranging for mountain chains to deep ocean to allow and maintain
global biodiversity, best defense against mass extinction. Overtime time, continents provide key to
terrestrial environments for animal life to develop.
- Divergent Boundary-Plate moves apart, Lithosphere thickens away from ridge axis, spreading boundary, mid ocean ridge
- 1968, evidence overwhelming
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