Transcript Radioactive Decay & Convection Presentation

Radioactive Decay and
Thermal Convection
Thermal structure of the Earth:
Heat is a driver for seismological
processes
Starting with student experiences
What evidence do they have for Earth’s internal temperature?
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
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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?