F4 Earth Structure and Global Tectonics

• Wilson Cycle

  • Opening and closing of oceans
  • Shows formation, growth and destruction
  • Based on palaeomagnetic reconstructions
• • Phases of the Wilson Cycle
  • Opening phase
  • A stable craton. Hotspot rises up underneath causing crust to swell and split.
  • Formation of rift valleys involves uplift and crustal extension of continental areas
  • Continent is separated and a new ocean basin is generated. It widens and the stretched and thinned edges where the two continents used to be joined cool, become denser and sink.
  • Narrow, parallel sided sea (e.g. the red sea)
  • Ocean basin widens - mature stage caused by widening of the growing basin and continual production of new, hot oceanic crust along the ridge system
• • Closing phase (wilson cycle)
  • When a subduction zone forms
  • Most of the ocean is subducted and two continents are about to collide.
  • Once subduction outpaces new crust formation the ocean begins to contract creating new mountain ranges.
  • Collision occurs, ocean basin completely subducted, closing phase is over.
  • Mountain is eroded and plate boundary becomes inactive.
• Inner core - solid

  Outer core - liquid

  Mantle - solid (ductile)

  Crust - solid
• • Lithosphere - cold, brittle rock representing crust and the top of the mantle
  • Asthenosphere - warm and ductile/plastic part of the mantle sitting below the lithosphere - plates float on this
• • Rheology - branch of physics in which we study the way in which materials deform or flow in response to applied forces or stresses.
  • Rheological properties - material properties that govern the specific way in which these deformation or flow behaviours occur are called rheological properties.
• Evidence for the structure of the Earth

  • Direct evidence
  • Rocks at the earth’s surface exposed by uplift, erosion and weathering
  • Mines and boreholes provide direct access to the higher levels of the crust. Provide evidence of the geothermal gradient
  • Volcanic vents show where magma generates from in the lower crust or upper mantle. Mantle xenoliths are sometimes ejected
  • Kimberlite pipes are volcanic vents tapping into the mantle.
• Evidence for the structure of the Earth

  • Indirect evidence
  • Meteorites - thought to originate from the same source as material from earth. Differentiated shows the core of a planet and the layers
  • Earth density data - shows the density of continental and ocean crust.
  • Earth’s magnetic field - needs metallic elements in high enough concentration somewhere in Earth’s interior to generate its magnetic field - e.g. iron core
  • Seismological evidence - P and S waves
• • Body waves
  • P-waves
  • Move by compression and expansion - longitudinal
  • Pass through solids, liquids and gases
  • Velocity related to rock density and incompressibility
  • S-waves
  • Slower velocity than P waves
  • Pass through solids only - transverse
  • Velocity dependent on density and rigidity
• RIGIDITY AND INCOMPRESSIBILITY OUTWEIGH THE EFFECT OF DENSITY
• • Surface waves
  • Rayleigh waves
  • cause surface of the ground to move up and down - 90% of the speed of the S waves
  • Love waves
  • side to side motion of the ground - perpendicular to propagation velocity
• • Recording seismic waves
  • Earthquakes are recorded by an instrument called a seismometer - seismogram records the trace and duration of all waves during an earthquake
• • Refraction - means seismic wave changes speed
  • Reflection means the seismic wave stops and bounces back
• • Discontinuities
  • Using earthquakes, boundaries in the Earth can be recognised
  • These boundaries are called discontinuities
  • Resulting in changes in seismic wave velocities as a result of changing rock properties
• • Mohorovicic Discontinuity
  • Mark the position of the crust-mantle boundary
  • Sudden increase in seismic wave velocities as a result of increasing incompressibility rigidity
  • Under oceans
  • Under continents
• • Gutenberg Discontinuity
  • 2900km
  • Boundary between the lower mantle and outer core
  • P Wave velocities slow and S wave velocities stop
  • Change from solid to liquid
• • Lehmann Discontinuity
  • 5100km
  • P wave velocities increase
  • Increase in incompressibility
  • Indicates a solid core
• Seismic shadow zone - the area of the Earth’s surface where seismographs cannot detect an earthquake/seismic wave after the waves have passed through the Earth
• • P waves - refracted by liquid outer core and are not detected between 103 and 143 degrees
  • S waves - cannot pass through the liquid outer core (reflected) and are not detected beyond 103 degrees
• Gravity is the attraction between two objects; this is caused by the objects masses and the square distance between them
• Latitude - due to its spin the Earth is slightly ‘flattened’ at the poles and it bulges slightly at the equator - value for gravity there is slightly higher than the equator
• • Free Air Anomaly - measured gravity anomaly after a free air correction is applied to correct for the elevation at which a measurement is made.
  • Bouguer anomaly - gravity anomaly corrected for the height at which it is measured and the attraction of terrain
• • Positive gravity anomalies result from excess of mass
  • Negative gravity anomalies result from deficit of mass
• • Isostacy - a term used to describe the state of balance of mass
  • Isostacy - the state of equilibrium of Earth’s crust pushing down while the mantle is pushing up
  • Isostatic equilibrium - a condition is reached so that the lithosphere does not want to move up or down
  • Idea that the lighter crust floats on the underlying denser mantle
• • Craton
  • Stable portion of the continental lithosphere
  • Situated far from today’s plate boundaries
  • Shield - area with low relief (little change in elevation). Precambrian crystalline rock
  • Stable platform - area characterised by a covering of sedimentary rocks. Not subject to uplift or tectonic forces.
• • Orogenic belts
  • Mountains that normally occur along the margins of continents
  • Mountains form belts due to collision of two tectonic plates
• • Orogenic belts
  • Mountains that normally occur along the margins of continents
  • Mountains form belts due to collision of two tectonic plates
• Palaeomagnetism

  • Palaeomagnetism - study of past variations in the Earth’s magnetic field form rock and archaeological records
  • Some rocks contain a record of the direction of Earth’s magnetic field at the time of their formation
  • Iron minerals - becomes magnetised when groups of molecules align in particular arrangement. Can take on Earth’s magnetism
  • Curie temperature - temperature at which minerals lose their permanent magnetic properties
• Evidence to support Continental Drift

  • Continental geographic fit - continents fit together
  • Rock types - similarities in rock types across continents show they were once together
  • Mountain chains - Similarities in orientation of mountain chains show they were once part of one chain
  • Fossil records - fossils of the same type now found at separated boundaries e.g. mesosaurus
  • Glacial striations - scratches that form by bits of rock on bottom of glacier leaving groves in direction of travel. They match up across continents
• Alfred Wegener’s continental drift hypothesis was rejected initially by scientific community because he could not explain mechanism for the movement of continents
• The Process of Seafloor Spreading

  • New ocean floor/ basalt formed by injection of magma into the ridge from below
  • Splits the older material in half, moves it sideways
  • One side may move faster than the other
• Evidence for seafloor spreading

  • Mid ocean ridges
  • Warmer rocks at Mid Ocean Ridges where magma reaches the surface. Crust gets cooler away from the Mid Ocean Ridge
  • Increase in thickness and density of rocks away from MOR - cooling ocean lithosphere becomes denser and ocean sediment is thicker on older crust
  • Increase in age of rocks from the MOR
  • Magnetic stripes - symmetrical suggesting spreading from centre. Age of stripes helps to confirm that symmetry formed at the same time
• Crust is divided into a set of large moving plates called plate boundaries/plate margins
• Divergent plate boundaries

  • Shallow focus earthquakes
  • Basaltic magma, shield volcanoes
  • Pillow basalt
  • Symmetrical magnetic stripes
  • Thickens, increases in age away from MOR
• Destructive boundaries

  • Series of triangles on plate boundary maps
  • Benioff zone - area of seismicity corresponding to descending slab
  • Trench
  • Accretionary wedge
  • Fold mountains formed during continental collision
  • Explosive volcanism in ocean-continent and ocean-ocean
• Conservative boundaries

  • Shallow focus earthquakes
  • No creation or destruction of crust
  • No magma generation
  • Plates slide past each other, different directions or same direction at different speeds
• Geothermal gradient = change in temperature/ change in depth
• • Magma - molten rock below the surface
  • Lava - molten rock on the surface
• • Melting by adding volatiles
  • Dehydration melting - water transferred from ocean crust to mantle. Lowers melting point of mantle peridotite and produces wet solidus
  • Mantle plume - hotspot
  • Upwelling of abnormally hot rock within Earth’s mantle. Heads of mantle plumes partially melt when they reach shallower depths and result in hotspots
  • Decompression melting - decrease in pressure lowers melting point
• Hotspot - area at core/mantle boundary where heat is generated. Believed to be a concentration of radioactive elements. Heating of rocks causes a rising thermal current called a mantle plume