Earth's Internal Structure: Thermal Structure of Earth (2.3.3) Flashcards
What is a geothermal gradient?
- Change in temperature with change in depth (^Z) ^ is a triangle today lol.
- Heat floor: Q = K ^T / ^Z (where K is thermal conductivity of rocks.
- Geothermal gradient near surface ~1 degrees C per 30m.
- Earth’s geothermal gradient must decrease with depth.
- We have no direct way of measuring deep internal temperature.
- Must employ indirect evidence for Earth’s internal temperature.
What is the lithosphere?
Crust and uppermost mantle.
- Rigid/brittle (breaks rather than flows).
- Broken into tectonic plates.
What is the asthenosphere?
Upper mantle below lithosphere.
- Plastic/ductile (flows rather than breaks).
- Flows as soft solid.
Determining Earth’s deep geotherm: asthenosphere (1)
- Lithosphere-asthenosphere boundary at ~100km depth.
- Mostly solid.
- Incipient melting (basalt magma) starts at ~1200 degrees c.
- Plastic behaviour results in S-wave velocity drop.
- More pronounced than drop in P-wave velocity.
Determining Earth’s deep geotherm: discontinuity (2)
- P and S wave velocities increase in a jump at 410km.
- Result of stiffer and denser structure.
- Experimentally arrangements of structure changes from olivine to ‘spinel-like’ at ~1400 degrees C.
Determining Earth’s deep geotherm: 660km discontinuity (3)
- P and S wave velocities jump increase again, stiffer and denser structure.
- Coincident with changes from ‘spinel’ to ‘perovskite’ structure.
- From experiments occurs at ~1800 degrees C.
Determining Earth’s deep geotherm: mantle boundary (4)
- S wave velocity falls to 0 at 2900km depth.
- Liquid outer core.
- Experimentally, at this pressure iron melts at ~3600-3700 degrees C.
Determining Earth’s deep geotherm: inner core boundary (5)
- S wave velocity increases again at 5140km depth.
- Solid inner core.
- Experimentally, iron becomes solid at this pressure at ~4300 degrees C.
What is terrestrial heat flow?
- Heat flow outward from Earth interior around 1.5 x 10^21 Joules per year.
- Equivalent to 750 magnitude 9 earthquakes per year.
Heat sources: solar radiation
- Around 5 x 10^24 J per year.
- 3000x as much comes from inside Earth.
- 30% immediately radiated back to space.
- 70% powers global hydrological system.
Heat sources: residual heat
- From Earth’s formation.
- Still contributes 30-60% of Earth heat flow.
- Big uncertainty on actual contribution.
Heat sources: radioactivity
- Heat from breakdown of radioactive elements.
- Elements today are uranium U, thorium Th, and potassium K.
- Radioactive heat-producing elements concentrated in continental crust.
- Makes up 33% to 90% of measured heat flow from Earth. Big uncertainty.
What are the three mechanisms of terrestrial heat transfer?
- Radiation: movement by light.
- Convection: heat moved out core and mantle.
- Conduction: near surface in lithosphere, large part of heat lost via this. Some lost by volcanism, some used in deformation e.g. earthquakes.
Shallow heat transfer: hydrothermal circulation
- Main source of heat transferred out solid Earth occurs on ocean floor (>70%).
- Occurs by conduction and convection (seawater circulating through cracks in ocean crust).
- ‘Black Smokers’ - vents of hot water at mid-ocean ridges emit intense heat.
- Also diffuse flow of warm water emitted from basement across entire ocean basin.
- Hydrothermal fluids react with rock to strip out and concentrate metals; form seafloor mineral deposits.
What are continental geothermal systems?
- Meteoric waters derived from Earth’s atmosphere work in same forced recharge system removing heat from Earth’s interior.
- Geothermal energy important energy source for man, very minor component of global heat budget.
