Download to read offline
Structure of the Earth.ppt
- 1. Structure of theEarth
- 2. Gravity reshapes the proto-Earthinto a sphere. The interior of the Earth separates into a core and mantle. Forming the planets from planetesimals: Planetessimals grow by continuous collisions. Gradually, an irregularly shaped proto-Earth develops. The interior heats up and becomes soft.
- 3. The NEAR Mission TheNear Earth Asteroid Rendezvous Mission
- 4. Why is theEarth (near) spherical? • Accretion: the gradual addition of new material • When the Earth first accreted, it probably wasn’t spherical • What happened? HEAT was generated and retained
- 5. Sources of InternalHeat • Accretionary Heat Proto-earth 1) Gravity attracts planetesimal to the proto- earth 2) Planetesimals accelerate on their journey, gaining kinetic energy (KE=1/2mv2) 3) They strike the proto-earth at high speed 4) Their kinetic energy is converted to thermal energy (HEAT)
- 6. Sources of InternalHeat • Accretionary Heat
- 7. Sources of InternalHeat • Radioactive Decay – The natural disintegration of certain isotopes to form new nuclei – Time for nuclei to decay given by a “half-life” Radioactive decay is an important source of the Earth’s internal heat
- 8. Sources of InternalHeat • Radioactive decay – Short-lived Isotopes 26Al 26Mg + Energy + … (t1/2 = 0.72 x 106 yrs) 129I 129Xe + Energy + … (t1/2 = 16 x 106 yrs) – Long-lived Isotopes 40K 40Ar + Energy + … (t1/2 = 1270 x 106 yrs) 232Th (t1/2 = 1400 x 106 yrs) 235U (t1/2 = 704 x 106 yrs) 238U (t1/2 = 4470 x 106 yrs)
- 9. The Differentiated Earth Theearth differentiated into layers by density: 1) Crust 2) Upper Mantle 1) Lithospheric 2) Asthenospheric 3) Lower Mantle 4) Outer Core 5) Inner Core Least Dense Most Dense Because different minerals have different composition and densities, physical partitioning of the earth led to: chemical differentiation High Si High Fe Low Si Low Fe
- 10. The Differentiated Earth WholeEarth Density ~5.5 g/cm3 Surface Rocks 2.2 - 2.5 g/cm3 Core: Nearly pure Fe/Ni Mantle: Fe/Mg rich, Si/Al poor Crust: Si/Al rich, Na/K/Ca rich
- 11. Another Source ofInternal Heat • Residual heat from the formation of the core Gravitational Settling E=GMm/r (gravitational potential energy) • Practically speaking: – A 1-kg ball of iron, settling from the surface to the center of the earth produces enough energy to heat a 10-kg piece of rock (granite) to 750°C, where it would begin to melt. • Heat capacity of granite = 840 J/kg K
- 12. The Crust Continental Crust •35 - 40 km • Less Dense Oceanic Crust • 7 - 10 km •More Dense
- 13. The Mantle The asthenospheremay contain a few percent molten rock, but the mantle is by and large solid Despite this, given time, it will flow
- 15. Loss of InternalHeat • All celestial bodies lose heat – Asteroids > Moon > Mars > Earth • There are three main mechanisms – Conduction – Convection – Radiation • Conduction is the transfer of heat without movement of material
- 16. Temperatures in theEarth The geotherm is the description of how the temperature of the earth increases with depth. Near the surface (to 8 km depth): 2-3 °C/100 m depth Heat loss by conduction! Pure conduction geotherm
- 17. Convection Heating at thebottom: • Increases temperature • Decreases density Less dense hot water rises… • Displacing the cooler, denser water at the top Denser, cool water descends… • Where it is heated
- 18. The Core &The Earth’s Magnetic Field The core is almost completely Fe/Ni alloy. The outer core is liquid, while the inner core is solid. Convection of the outer, liquid core gives rise to the Earth’s magnetic field
- 19. The Atmosphere Present Atm. N2(78%) O2 (21%) Ar (1%) CO2 (0.04%) H2O (varies) …others Early Atm. N2 CO2 H2O H2S HCN …others Where’s the H and He?
- 20. The importance oflife to the development of the planet