🪨Intro to Geophysics Unit 4 – Earth's Heat and Thermal Structure

Key Concepts and Terminology

• Geothermal gradient measures the rate of temperature increase with depth in Earth's interior
• Heat flow quantifies the amount of heat energy transferred from Earth's interior to the surface per unit area and time
• Thermal conductivity describes a material's ability to conduct heat (measured in W/m·K)
• Radioactive decay of unstable isotopes (uranium, thorium, potassium) generates heat within Earth
  • Accounts for ~80% of Earth's internal heat production
• Mantle convection involves the transfer of heat through the motion of hot, buoyant material rising and cooler, denser material sinking
• Adiabatic compression heats material as it moves deeper into Earth due to increasing pressure
• Lithosphere refers to Earth's rigid outer layer, including the crust and uppermost mantle
• Asthenosphere is the weak, ductile layer of the upper mantle beneath the lithosphere

Earth's Internal Heat Sources

• Primordial heat remaining from Earth's formation and early differentiation (~20% of total heat)
• Radioactive decay of long-lived isotopes (uranium-235, uranium-238, thorium-232, potassium-40) in the mantle and crust (~80% of total heat)
  • Half-lives range from 0.7 to 14 billion years, providing a steady heat source over geologic time
• Gravitational energy released during Earth's accretion and differentiation
• Latent heat from crystallization of the inner core at the expense of the outer core
• Tidal heating caused by the gravitational pull of the Moon and Sun on Earth
• Frictional heating along fault planes during earthquakes and plate boundary interactions
• Chemical reactions and phase changes within Earth's interior can absorb or release heat

Heat Transfer Mechanisms in the Earth

• Conduction transfers heat through direct contact between particles, without bulk motion of material
  • Dominant mechanism in the lithosphere and uppermost mantle
  • Heat flows from high-temperature regions to low-temperature regions
• Convection is the transfer of heat through the bulk motion of fluids or ductile solids
  • Primary heat transfer mechanism in the mantle and outer core
  • Driven by buoyancy differences due to temperature and density variations
• Radiation involves the emission and absorption of electromagnetic waves
  • Significant at very high temperatures (e.g., Earth's core) or in transparent materials
• Advection transfers heat through the bulk motion of material, such as magma or hydrothermal fluids
• Adiabatic compression heats material as it moves to greater depths and experiences higher pressure
• Decompression melting occurs when hot mantle material rises and partially melts due to reduced pressure

Temperature Distribution in Earth's Interior

• Temperature increases with depth due to the geothermal gradient
  • Average geothermal gradient in the crust is ~25-30°C/km
• Lithosphere exhibits a steep, conductive geothermal gradient
  • Temperature increases rapidly with depth
• Asthenosphere has a more gradual temperature increase due to the influence of convection
• Mantle temperature increases with depth, reaching ~1300-1400°C at the core-mantle boundary
• Outer core is extremely hot, with temperatures estimated at 4000-5000°C
• Inner core is slightly cooler than the outer core due to its solid state and higher pressure
• Lateral temperature variations exist due to differences in tectonic settings, heat production, and mantle dynamics
  • Higher temperatures beneath mid-ocean ridges and mantle plumes
  • Lower temperatures beneath subduction zones and cratonic lithosphere

Geothermal Gradients and Heat Flow

• Geothermal gradient varies with location and depth
  • Influenced by factors such as heat production, thermal conductivity, and tectonic setting
• Continental crust has an average geothermal gradient of ~25-30°C/km
  • Higher gradients in tectonically active regions (volcanic areas, rifts)
  • Lower gradients in stable cratonic regions
• Oceanic crust exhibits higher geothermal gradients (~50-60°C/km) due to thinner lithosphere and proximity to hot mantle
• Heat flow is highest at mid-ocean ridges and lowest in ancient continental shields
  • Global average heat flow is ~87 mW/m²
• Geothermal gradients and heat flow provide insights into Earth's thermal structure and dynamics
  • Used to constrain models of mantle convection and plate tectonics
• Borehole temperature measurements and thermal conductivity data are used to calculate geothermal gradients and heat flow

Thermal Structure of Lithosphere and Mantle

• Lithosphere acts as a thermal boundary layer, conducting heat from the hot mantle to the surface
  • Thickness varies from ~50-250 km, depending on age and tectonic setting
• Oceanic lithosphere cools and thickens as it moves away from mid-ocean ridges
  • Thermal structure controlled by conductive cooling and plate age
• Continental lithosphere has a more complex thermal structure
  • Influenced by heat production, composition, and tectonic history
• Asthenosphere is characterized by lower viscosity and higher temperatures compared to the lithosphere
  • Allows for ductile deformation and mantle convection
• Mantle temperature increases with depth due to adiabatic compression and heat from the core
  • Thermal boundary layers at the lithosphere-asthenosphere boundary and core-mantle boundary
• Mantle plumes are localized upwellings of hot material that originate from the lower mantle
  • Associated with intraplate volcanism (Hawaii) and large igneous provinces

Implications for Plate Tectonics and Convection

• Earth's internal heat drives mantle convection, which is the primary driver of plate tectonics
• Convection currents in the mantle cause the lithospheric plates to move and interact
  • Divergent boundaries form where upwelling mantle reaches the surface (mid-ocean ridges)
  • Convergent boundaries occur where cooler lithosphere sinks back into the mantle (subduction zones)
• Subducting slabs transfer cold material into the mantle, influencing convection patterns and thermal structure
• Mantle plumes transport hot material from the lower mantle to the surface
  • Can cause rifting, volcanism, and the formation of hotspots (Yellowstone)
• Plate tectonics and mantle convection play a crucial role in Earth's heat loss and thermal evolution
  • Efficient heat transfer through the creation and destruction of lithosphere
• Variations in Earth's thermal structure can influence plate velocities, deformation styles, and magmatism

Methods for Studying Earth's Thermal Structure

• Borehole temperature measurements provide direct observations of geothermal gradients
  • Temperature logs from deep boreholes (several km) in various tectonic settings
• Heat flow measurements use a combination of borehole temperatures and thermal conductivity data
  • Fourier's law: $q = -k \frac{dT}{dz}$, where $q$ is heat flow, $k$ is thermal conductivity, and $\frac{dT}{dz}$ is the geothermal gradient
• Seismic wave velocities are sensitive to temperature variations in the Earth
  • Higher temperatures generally correspond to lower seismic velocities
• Geophysical modeling incorporates various datasets (seismic, gravity, magnetic) to constrain thermal structure
• Geochemical and petrological studies of mantle-derived rocks (xenoliths, volcanics) provide insights into mantle temperatures and composition
• Numerical simulations of mantle convection and lithospheric dynamics help understand the thermal evolution of the Earth
• Satellite-based thermal infrared imaging can map surface heat flow and detect thermal anomalies
• Geothermal exploration and drilling projects provide valuable data on subsurface temperatures and heat flow