Detailed Notes on Lithosphere and Plate Tectonics

Lithosphere Definition and Properties

  • Isacks, Oliver, and Sykes (1968) defined the lithosphere as the "near surface layer of strength of earth".
  • The lithosphere is the strong, outermost layer of the Earth that deforms elastically.
  • The upper part of the lithosphere behaves elastically when stress is applied.
  • Below the elastic part is an elasticoviscous portion.
  • The thickness of the elastic part changes with the load and duration of the load.
  • The lower boundary of the lithosphere (LAB – lithosphere-asthenosphere boundary) is a narrow zone.
  • Below the LAB is the asthenosphere, a weak viscous material that flows under small stress.
  • The lithosphere is the rigid outermost layer that transmits stress without significant deformation.
  • It is broken into plates that fit together like a jigsaw puzzle.
  • Plates move on the asthenosphere with different velocities, causing relative motion between neighboring plates.
  • The lithosphere comprises the crust and uppermost mantle.
  • The asthenosphere is weaker and reacts to stress like a fluid.
  • Oceanic and continental lithosphere have different characteristics; this discussion focuses on oceanic lithosphere.
  • Oceanic lithosphere results from partial melting of the mantle.
  • Partial melting produces:
    • Melt that is buoyant and rises.
    • Residual mantle that is less buoyant than the melt but more buoyant than the unmolten mantle.
  • The melt freezes to form the oceanic crust.
  • The residual solid forms a layer below the crust.
  • The crust-mantle boundary is the ‘Moho’.
  • These layers make up the oceanic lithosphere.
  • The asthenosphere is a viscous part of the mantle below the lithosphere.
  • When oceanic lithosphere forms, the partial melt (oceanic crust) has the lowest density, and the unmolten mantle (asthenosphere) has the highest density.

Thermal and Mechanical Properties

  • The lithosphere acts as a conductive cover over the hot, convective asthenosphere.
  • It is considered a thermal boundary layer (TBL) separating the asthenosphere and the hydrosphere/atmosphere.
  • Oceanic lithosphere cools and becomes denser with age.
  • After approximately 20 million years, it becomes denser than the asthenosphere.
  • The LAB is a surface of mechanical disequilibrium, where older lithosphere is denser than the underlying asthenosphere.
  • A large portion (>=70%) of oceanic lithosphere is older than 20 million years and out of mechanical equilibrium.
  • Temperature primarily controls the strength of subsurface materials.
  • Hydrostatic pressure increases linearly with depth, raising the melting point of rocks.
  • The asthenosphere is where the melting point is most closely approached, represented by a homologous temperature T<em>HT<em>H close to 1, where \TH = \frac{\text{Temperature in K}}{\text{Melting point in K}}.
  • The asthenosphere is not fully molten but may contain a small amount of melt.
  • S-wave velocity drops drastically below the LAB, indicating a decrease in rigidity like a liquid.
  • The asthenosphere contains ~0.04% water by weight, contributing to its fluid-like behavior.
  • The depth of the asthenosphere depends on the geothermal gradient and melting temperature of mantle materials (Le Pichon et al., 1973).
  • Beneath ocean ridges, high temperature gradients result in a shallow asthenosphere and thin lithosphere.
  • The lithosphere thickens towards deep ocean basins as it cools and subsides.
  • The mean lithosphere thickness beneath oceans is approximately 60–70 km.
  • Beneath continents, heat flow is produced within the crust, leading to a lower temperature gradient in the sub-crustal lithosphere.
  • The continental lithosphere is thicker (100–250 km), with maximum thickness beneath cratonic areas (>2500 million years old).
  • Solidus: the minimum temperature required for partial melting of a solid under given pressure-temperature conditions.
  • The LAB is a temperature-controlled boundary with a temperature between 1200 and 1300°C.
  • The lithosphere–asthenosphere interface is not sharply defined and occupies a zone several kilometers thick.
  • Oceanic lithosphere comprises oceanic crust and depleted mantle, both less chemically dense than the asthenospheric mantle.

Plate Tectonics and Plate Boundaries

  • Plate tectonics studies the geometry, movement, effects, and causes of lithospheric fragment movement.
  • These fragments are called plates.
  • J. Tuzo Wilson (1965) first described these fragments as ‘PLATES’.
  • Lithosphere can be defined rheologically, thermally, or by earthquake wave velocities.
  • Common perception: lithosphere is an average 100 km thick outermost rigid layer.
  • Each plate moves relative to its neighbors.
  • Relative velocity: the velocity of plate A with respect to plate B.
  • Absolute velocity: the velocity of a plate with respect to the geographic north pole.
  • Major plates (area in million square kilometers):
    • Pacific (106)
    • Africa (79)
    • Eurasian (69)
    • North American (60)
    • Indo-Australian (60, now India and Australia are separate)
    • Antarctic (59)
    • South American (41)
    • Nazca (15)
  • Smaller plates: Arabian, Philippine, Cocos, Scotia, Caribbean, Juan-de-Fuca, etc.
  • Three types of plate boundaries (faults):
    • Divergent (Rift):
      • Plates move away from each other.
      • Example: mid-ocean ridge (MOR) – a 60,000 km long mountain chain, 2 km high from the sea-floor.
      • MORs are zones of extension where new sea-floor (oceanic lithosphere) is created by sea-floor spreading.
      • Age of sea floor increases away from the MOR.
    • Convergent (Subduction Zone):
      • Plates move toward each other.
      • Found around the Pacific Ocean, Indonesia, and the western margin of South America.
      • Marked by thrust faults and compressional earthquakes.
      • The denser plate bends and goes underneath the lighter plate along a thrust fault.
      • Destructive boundaries: one plate disappears from the surface.
      • Subduction: the process of one plate going underneath another.
      • The subducting plate is usually the oceanic part (2900 kg/m3) as it is denser than the continental part (2700 kg/m3).
      • Trench: a deep, long furrow marking the contact between two plates in a subduction zone.
        If there is a convergent boundary between two oceanic plates, the more dense plate (usually the older plate) gets subducted.
    • Transform (Conservative):
      • Plates move past one another.
      • Large strike-slip fault.
      • No new material is added or old material destroyed.
      • Connects two non-transform plate boundaries (MORs, trenches, or one MOR and a trench).
      • Traces of dead transforms are called fracture zones.

Earthquake and Volcanism at Plate Boundaries

  • Plate boundaries are sites of earthquakes due to constant movement.
  • Tension across MORs lowers pressure, causing decompression-melting of mantle material, resulting in volcanism.
  • In subduction zones:
    • The subducting oceanic plate carries water into the asthenosphere.
    • Water is released from the subducting plate due to higher temperatures.
    • Released water reduces the melting temperature of the material in the asthenosphere.
    • Melts rise and erupt as volcanoes on the overriding plate, forming a volcanic arc.
    • Example: the Andes Mountains along the western boundary of South America.
    • The Cocos, Nazca, and Antarctic plates subduct below South America.
    • The Peru-Chile trench (~6000 km long) marks the western plate boundary of South America.
    • Lava from volcanic arcs is charged with steam and gases and is more siliceous (rich in \SiO_2).
    • Siliceous melts (andesitic lava) are more viscous than basaltic melts from MORs, causing explosive volcanism.
    • When a subduction zone develops between two converging oceanic plates, volcanoes rise from the ocean bottom, and if they reach above the sea level, form an island arc.
    • Example : Japan, Philippines, Indonesia,etc.