12.2 Mantle convection and plate tectonics

Mantle convection drives plate tectonics, shaping Earth's surface. Hot material rises, cool material sinks, creating convection cells that move lithospheric plates. This process forms divergent, convergent, and transform boundaries, resulting in mid-ocean ridges, subduction zones, and strike-slip faults.

Evidence for mantle convection comes from seismic tomography and geoid anomalies. These reveal temperature and density variations in the mantle, showing large-scale structures like upwelling plumes and descending slabs. Mantle plumes also create hot spot volcanism, forming iconic features like the Hawaiian Islands.

Mantle Convection and Driving Forces

Thermal Convection and Gravitational Instability

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• Mantle convection is the slow, continuous circulation of the Earth's mantle driven by heat transfer from the core to the surface
• The main driving forces of mantle convection are thermal convection and gravitational instability
  • Thermal convection occurs due to temperature differences between the hot lower mantle and the cooler upper mantle, causing the material to rise and sink
  • Gravitational instability arises from density differences, with denser material sinking and less dense material rising
• The convective flow in the mantle is estimated to have velocities ranging from a few centimeters to several tens of centimeters per year

Heat Generation and Convective Patterns

• Radioactive decay of elements within the mantle (uranium, thorium, potassium) generates additional heat, contributing to the convective process
• The convective patterns in the mantle can be described as large-scale convection cells or plumes
  • Convection cells involve the rising of hot, less dense material and the sinking of cooler, denser material
  • Mantle plumes are localized, upwelling columns of hot mantle material that originate from the core-mantle boundary and rise through the mantle
• Numerical models of mantle convection, based on physical principles and observed parameters, produce patterns that closely resemble the observed seismic and geoid anomalies

Mantle Convection and Plate Tectonics

Driving Mechanism and Plate Boundaries

• Mantle convection is the driving mechanism behind plate tectonics, the theory that explains the movement and deformation of the Earth's lithosphere
• The rising and sinking of mantle material creates convection cells, which exert forces on the overlying lithospheric plates
• Divergent boundaries occur where convection cells rise and diverge, pulling plates apart and creating new oceanic crust at mid-ocean ridges (East Pacific Rise)
• Convergent boundaries form where convection cells descend, causing plates to collide, resulting in subduction zones, oceanic trenches, and volcanic arcs (Andes Mountains)
• Transform boundaries develop where convection cells move horizontally past each other, creating strike-slip faults (San Andreas Fault)

Plate Boundary Patterns and Mantle Convection Cells

• The pattern of plate boundaries and their associated geological features closely match the predicted pattern of mantle convection cells
• The geometry and orientation of plate boundaries are influenced by the underlying mantle flow patterns
• Subduction zones and oceanic trenches are located above descending limbs of convection cells, while mid-ocean ridges are situated above ascending limbs
• The interaction between mantle convection and plate tectonics creates a dynamic, self-sustaining system that shapes the Earth's surface features

Evidence for Mantle Convection

Seismic Tomography

• Seismic tomography provides images of the Earth's interior by analyzing the velocity of seismic waves, revealing variations in temperature and density
  • High-velocity anomalies indicate colder, denser regions, while low-velocity anomalies suggest hotter, less dense areas
  • Seismic tomography reveals large-scale structures in the mantle, such as upwelling plumes and descending slabs, consistent with convection patterns
• Seismic imaging of mantle plumes reveals low-velocity anomalies extending from the core-mantle boundary to the surface, supporting the concept of deep mantle upwelling

Geoid Anomalies

• Geoid anomalies, or variations in the Earth's gravitational field, provide insights into the density distribution within the mantle
  • Positive geoid anomalies indicate areas of higher density, while negative anomalies suggest lower density regions
  • The pattern of geoid anomalies correlates with the expected density variations associated with mantle convection, with positive anomalies over subduction zones and negative anomalies over upwelling regions
• The long-wavelength geoid anomalies, with a dominant degree-2 pattern, are consistent with the presence of large-scale mantle convection cells

Mantle Plumes and Hot Spot Volcanism

Characteristics and Examples

• Mantle plumes are localized, upwelling columns of hot mantle material that originate from the core-mantle boundary and rise through the mantle
• Hot spot volcanism occurs when mantle plumes reach the base of the lithosphere, causing melting and the formation of volcanic islands or seamounts
  • Examples of hot spot volcanism include the Hawaiian Islands, Iceland, and the Yellowstone Caldera
  • Hot spot volcanoes are characterized by their distinct geochemical signatures (enriched in incompatible elements), indicating a deep mantle source

Implications for Mantle Dynamics and Plate Motion

• The fixed nature of hot spots relative to the moving plates allows for the reconstruction of plate motion and the determination of absolute plate velocities
• The presence of mantle plumes suggests that convection in the mantle is not limited to the upper mantle but extends to the deep mantle, possibly in a whole-mantle convection regime
• The interaction between mantle plumes and the overlying plates can influence plate motion, as the buoyancy of the plumes may exert additional forces on the plates
• The age progression of volcanic islands and seamounts along hot spot tracks (Hawaiian-Emperor Seamount Chain) provides evidence for the relative motion between plates and the underlying mantle plumes