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Exploring Earth's Thermal Dynamics: Radioactive Decay and Mantle Convection in Seismological Understanding

Delve into the intricate thermal structure of the Earth, examining the role of heat in driving seismological processes. Explore historical perspectives, from Darwin to Lord Kelvin, and challenges to Kelvin's models. Uncover the sources of Earth's heat, including radioactive decay and residual heat from accretion. Understand Earth's energy budget and how heat is released through conduction and convection. Discover methods for modeling mantle materials, such as using oobleck or silly putty. Dive into 3D mantle flow models and the imaging of mantle convection through seismic waves. Reflect on Earth's interior and engage with questions about the crust, mantle, lithosphere, and asthenosphere, enhancing your understanding and teaching approach.

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Exploring Earth's Thermal Dynamics: Radioactive Decay and Mantle Convection in Seismological Understanding

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  1. Radioactive Decay and Thermal Convection


  2. Thermal structure of the Earth: Heat is a driver for seismological processes

  3. Starting with student experiences What evidence do they have for Earth’s internal temperature?

  4. Starting with their experiences

  5. Earth’s heat and the age of the Earth • Uniformitarianism (Lyell’s Principles of Geology) • same geological processes occurring today have existed throughout geologic time • Darwin (Origin of Species) estimated that it took 300 million years to erode a chalk deposit in southern England • Lord Kelvin - estimate time from molten state to solidification via cooling • temperature at Earth's core = melting point of rocks • temperature gradient with regard to depth below the surface (1 degree/50’) • thermal decrease through conductivity of rocks* • Estimate of 20 myo to 400 myo)

  6. Challenges to Kelvin’s model • Assumption of a solid Earth • Some argued that the Earth had never been a molten sphere; rather Earth had formed from the slow accumulation of solid material like asteroids. • Some attacked Kelvin's assumption about a closed system of dwindling initial heat • Others offerred the possibility that the then-unknown internal structure of atoms could contain massive amounts of potential energy

  7. Where does the heat come from? • 20% Residual heat from accretion and gravitational collapse • 80% Radioactive decay • Uranium-238 (4.47 × 109) • Uranium-235 (7.04 × 108) • Thorium-232 (1.40 × 1010) • Potassium-40 (1.25 × 109)

  8. Average 25oC/km

  9. Thermal structure Tufts.edu

  10. Earth’s Energy Budget • Solar Radiation - (99.978%, or nearly 174 petawatts; or about 340 W m-2) • Geothermal Energy - (0.013%, or about 23 terawatts; or about 0.045 W m-2) • Tidal Energy – (0.002%, or about 3 terawatts; or about 0.0059 W m-2). • Waste Heat - (about 0.007%, or about 13 terawatts; or about 0.025 W m-2)

  11. How is Earth’s heat released? • Conduction • Convection • Sketch expected convection in pan

  12. How to best model mantle material • Obleck? • Cornstarch and water • Silly putty? • What are important criteria for choosing?

  13. Convection and Earth’s interior

  14. 3D Mantle flow models

  15. 3D Mantle flow models

  16. Mantle convection • Can be imaged using seismic waves • Complex • Sometimes both upper and lower mantle together • Some subduction zones can be imaged to base of mantle

  17. Reflection on Earth’s interior • First by yourself, and then with your table, consider 1, 2 or all 3 of the following questions: • What is the difference between the crust and the mantle? • What is the difference between the lithosphere and the asthenosphere? • Why are both sets of terms used, and which would be simplest for your students to understand?

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