Pure Geophysics

• • Earth emerged from a swirling nebula of interstellar gas and dust that also gave rise to the Sun and other bodies in our solar system.
  • Countless tiny mineral grains in the solar nebula grew to form rocks of increasing size and then merged into planetesimals 10 km or so across.
  • Gravity pulled the planetesimals together causing them to collide and coalesce into planetary embryos.
• Differentiation is the process, driven by gravity, by which random chunks of primordial matter were transformed into a body whose interior is divided into concentric layers that differ from one another both physically and chemically.
• In the final stages the Earth grew by the collisional accretion of tens of Moon- to Mars sized planetary embryos. The interiors of these planetary embryos were sufficiently hot to have substantially melted, allowing the segregation of dense, immiscible molten iron to form a core and a residual overlying silicate mantle. As the Earth grew, metal from the cores of accreted embryos likewise sank to its centre, after temporarily accumulating in the so-called “Metallic pond” at the base of a silicate magma ocean

  
• • The earliest formed fragments of the solid outer layer are known as cratons. (between 4.4-2.5 Ga).
  • Linked to diamonds
• It is believed that water is found on Earth from late icy meteorite collisions
• The continents grew by the later addition of younger continental material by “plate tectonic-like processes” around the >2.5 Ga Cratons. This younger continental crust is known as “Juvenile crust” and is about half the thickness of cratons. From Diamond inclusions – all those older than ~3 Ga indicate the rocks around them at the time of formation was peridotite (olivine, pyroxene), whereas some younger than ~3 Ga were surrounded by ecolgite (garnet-rich rocks) which could only have come from subducted oceanic crust
• What was the two-layer mantle convection system in early Earth characterized by?


  No transfer of material across the phase boundary between the upper and lower mantle
• What happened at discrete periods in the early Earth regarding subducted slabs?


  Large volumes of subducted slabs accumulated at the boundary, causing massive mantle overturns
• What was the result of the massive overturns of the mantle in early Earth?


  Very high-temperature lower mantle material was brought to shallow depths, producing voluminous melting
• What was the state of the mantle convection system around 1.2 Ga?


  The mantle convection transformed from a layered to a modern whole-mantle convection regime
• What are the key stages in the evolution of the mantle convection system from early Earth to 1.2 Ga?


  • Early Earth had a two-layer mantle convection system
  • No material transfer across the 670 km discontinuity
  • Accumulation of subducted slabs at the boundary
  • Massive overturns brought lower mantle material to shallow depths
  • Resulted in voluminous melting at upper mantle depths
  • Transformation to whole-mantle convection regime around 1.2 Ga
• • Subduction Zones are the primary construction of continental crust, with processes including: trench accretion, arc-magmatism, arc-continent collision
  • Hot spots can cause oceanic lithosphere to become thick and buoyant. When this material eventually reaches a subduction zone it may collide and become part of continental crust
• > 4.5 Ga Collisions

  4.5-4.0 Ga Magma ocean and differentiation. First crust formed

  4.0-2.5 Ga Formation of cratons

  2.5-1.0 Ga Start of mantle convection and rapid growth of the continents by mantle overturn. Primitive plate tectonics.

  < 1.0 Ga Start of modern plate tectonics
• Generalised seismic velocity profile of continental crust:

  Velocities generally < 6 km/s

  Very variable

  Upper, mid and lower crust (sometimes mid layer not distinguished)

  Average thickness 40 km, but ranges 30-80 km thick worldwide
• • Due to the inaccessibility of oceanic crust we rely on ophiolites – sections of oceanic crust now on land to study it
  • They are typically linked to subduction
• Isostasy:

  • There is a level within the asthenosphere where no horizontal pressure gradient can be sustained over geological time; such a level is called the compensation level
  • Gravity data tells us whether an area is in isostatic compensation
• What is transported by seismic waves during an earthquake?


  Energy produced from an earthquake
• What causes elastic deformation in rocks during an earthquake?


  Seismic waves passing through the rocks
• Why is it important to understand the properties of rocks in relation to elastic deformation?


  Because they control the elastic deformation
• What does Hooke's law describe?


  The nature of elastic deformation
• According to Hooke's law, how is strain related to applied stress?


  Strain is directly proportional to the applied stress
• What is the definition of stress in the context of Hooke's law?


  Force per unit area
• What happens to a deformed body when the force is removed according to Hooke's law?


  It returns to its original length or shape
• What is the term used for the relationship between stress and strain in rocks?

  Rheology
• What are the two parameters that describe the properties of rocks in relation to elastic deformation?

  Bulk modulus and shear modulus
• What does the bulk modulus measure in rocks?


  Incompressibility
• What does the shear modulus measure in rocks?


  Rigidity
• There are 2 main types of body waves, P (primary) and S (secondary), and these are directly related to the fact that there are 2 ways to strain material by volume change or shape change (distortion).
• • Compressional (P) Wave: Particle motion consists of alternating compression and dilation. Particle motion is parallel to the direction of propagation (longitudinal). Material returns to its original shape after wave passes.
  • Shear (S) Wave: Particle motion consists of alternating transverse motion, perpendicular to the direction of propagation (transverse). Transverse particle motion shown here is vertical but can be in any direction; however Earth’s layers tend to cause mostly vertical (SV) or horizontal (SH) shear motions. Material returns to its original shape after wave passes.
• On Earth there are two modes of propagation; body waves and surface waves.
• There are two types of surface waves:

  • Rayleigh waves: Particle motion consists of elliptical motions in the vertical plane and parallel to the direction of propagation. Amplitude decreases with depth. Material returns to its original shape after wave passes.
  • Love Waves: Particle motion consists of alternating transverse motions. Particle motion is horizontal and perpendicular to the direction of propagation (transverse). Amplitude decreases with depth. Material returns to its original shape after wave passes.
• Surface waves (Rayleigh and Love) are dispersive, which means that their velocities depend on frequency. In the Earth the elastic constants generally increase with depth, so surface waves with longer wavelengths sample deeper and so are faster. This gives the appearance of ‘stretching’ out the wave train. Because of their particle motions dispersion is usually most noticeable in the Rayleigh waves compared to the Love waves.

  
• • The 410km seismic discontinuity is due to mineral phase changes: dominantly olivine ($\alpha$ phase) -> spinel ($\beta$ phase)
  • Origin well known as temperature and pressure conditions can be recreated in the laboratory. The phase change results in closer packing of the atoms, and results in a 14% density increase.

  
• • At about 550km $\beta$-phase spinel changes to y-phase spinel, but doesn't result in major changes of physical properties.
  • At about 660km depth the y-phase spinel changes to pervoskite - the most abundant mineral in the lower mantle.

  
• • Exact depth at which the phase change happens also depends on temperature
  • 410km phase change is exothermic, whilst 660km is endothermic

  
• At 410km - Slab is colder than surrounding mantle and undergoes the phase change and so becomes denser before 410 km depth (so drops through easily)

  At 660km - Slab is colder than surrounding mantle and so wont undergo the phase change until deeper than 660 km (so it is lighter than surrounding mantle and gets stuck)

  abs and plumes “stir the mantle” and so this phenomena may separate the upper and lower mantle convection systems at 660 km depth.

  
• • The lower mantle (sometimes known as the Mesosphere) is characterised by a steady increase in seismic velocity but without any mineral phase changes
• The core mantle boundary (CMB) - A highly anomalous zone at the base of the mantle (100-200km thick) known as D"

  • Largest contrast in physical properties in the planet (density, elasticity, conductivity, viscosity)
  • May be the source of mantle plumes and a 'slab graveyard'
• Inner core has 2 seismological qualities:

  • Separated from the rest of Earth by the fluid outer core, recent investigations detected the differential rotation of the inner core relative to the mantle.…the inner core rotates faster than the rest of the planet by about 0.3° yr−1
  • Seismic waves that propagate through the inner core (PKIKP) travel up to 5 seconds faster when going from south to north than from east to west.
• Flexural load examples:

  • Quaternary Ice Sheets - Pattern of glacio-isostatic rebound following the retreat of the Laurentide icesheet (15-8ka). This shows compensation was regional rather than local
  • Foreland basins (sediments that fill depression)
  • Ocean Island - The bending of the plate is indicated by the moat of low gravity (-50 mGal ) adjacent to the seamounts and an outer gravity high (+30 mGal ) created by the outer bulge