Radioactive Decay and Thermal Convection

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

processes

What evidence do they have for Earth’s internal temperature?

Starting with student experiences

Starting with their experiences

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)

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

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)

Average 25oC/km

Thermal structure

Tufts.edu

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)

How is Earth’s heat released?

• Conduction• Convection• Sketch expected convection in pan

How to best model mantle material

• Obleck?– Cornstarch and water

• Silly putty?

• What are important criteria for choosing?

Convection and Earth’s interior

• 3D Mantle flow models

• 3D Mantle flow models

• 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

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?