Chapter 12 Earth s Interior

1. Chapter 12Earth’s Interior

2. How do we gain knowledge about the interior of the Earth? Drill a well – obtain actual samples Distance from the surface to the center of the Earth is about 6456 km (4035 mi) Deepest well drilled so far reached the astonishing depth of 12.3 km (7.7 mi) Thus, not a viable method Volcanic activity brings samples from the depths; however, only about 200 km (125 mi) deep Study of meteorites

3. Most information obtained by analysis of seismic (earthquake) waves Remember them? Measuring the travel times of P and S waves from an earthquake, nuclear explosion, or other large source, to various seismographic stations Nuclear tests and mining blasts are best, as the exact time and location of the source energy is known

4. Nature of Seismic Waves Basics of wave (energy) propagation Builds on info presented in Chapter 10 Seismic energy travels away from the source in all directions as waves (toss a pebble into a body of water and observe the waves) Danged physics Wave fronts are hard to visualize Instead, we use raypaths – a line drawn perpendicular to the wavefront (a diagram is in order here)

6. Characteristics of seismic waves Velocity of seismic waves depends on the density and elasticity of the material Within a layer of the same composition, the velocity GENERALLY increases with depth (pressure increases, making the material more dense)

7. P waves (compressional waves) (particle vibration back & forth in the direction of travel) travel through both solids and liquids Material behaves elastically, resists change in volume S waves (shear waves) (particles vibrate at right angles to direction of travel) pass through solids only Liquids have no shear strength When subjected to forces that can change material’s shape, the material flows (apply pressure to ice, and do the same to water)

8. Always, P-wave velocity is faster than S-wave velocity When a seismic wave passes from one material to another, the wave path is refracted (bent); also, some of the energy is reflected

9. If the Earth was homogeneous (all the same throughout) (don’t write this stuff down….) Seismic waves would spread in all directions Raypaths would be straight lines, with the waves travelling at a constant speed Therefore, the farther away a seismograph is from a source, the longer the travel time would be (both P- and S-waves)

11. This is not what is seen on seismograms Some seismographs farther from the source record the wave arrival before seismographs closer to the source Due to increase in seismic velocity with depth (pressure) Assumed gradual change in seismic velocity Refraction of the wave energy Some waves not recorded at certain places (we’ll ‘cus this in a bit)

13. Development of more sensitive seismographs (as well as more of them) Data indicate abrupt velocity changes at particular depths (physics of wave propagation) These changes detected world-wide, suggesting the Earth is composed of layers having varying compositions and/or mechanical properties

14. Layers of the Earth Based on chemical composition From early melting; denser elements sank, while less dense elements rose Three major regions: Crust – 3 to 70 km (2-40 mi) thick Mantle – solid, rocky, magnesium-iron silicate shell to a depth of about 2900 km (1800 mi) Core – iron-rich sphere

16. Based on physical properties (derived from physics & seismic data) Temperature, pressure, and density increase with depth These increases affect the physical properties & mechanical properties of materials Increase in temperature weakens chemical bonds, reducing mechanical strength Increase in pressure increases rock strength; also increases melting temperature because of confining pressure – melting requires increase in volume

17. Five main layers – lithosphere, asthenosphere (upper mantle), mesosphere (lower mantle), outer core, inner core Lithosphere – crust & uppermost mantle; rigid Up to 250 km (156 mi) thick Asthenosphere – base of lithosphere to about 660 km (410 mi) Comparatively weak Temperatures/pressures in uppermost part results in small amount of melting Mechanically detached from lithosphere

18. Mesosphere (lower mantle) – from 660 to 2900 km (410 to 1810 mi) Increased pressure counteracts higher temperatures Rocks become stronger with depth Still very hot, able to flow gradually Outer core About 2270 km (1420 mi) thick Mainly iron-nickel alloy Liquid Magnetic field generated here

19. Inner core Radius of about 1216 km (760 mi) Also iron-nickel alloy Higher pressures than outer core solid

20. Boundaries in the Earth By analysis of seismic data from seismograph stations worldwide Laboratory studies determine properties of Earth materials under extreme temperature/pressure conditions Data continues to be collected and analyzed

21. The Moho The long form – Mohorovicic discontinuity A layer at about 50 km (31 mi) where seismic velocities abruptly increase Explanation not simple, will attempt in next slide Analogy of a driver taking the bypass route around a large city during rush hour (the author obviously doesn’t live in Atlanta)

23. Core-mantle boundary Also called the Gutenberg discontinuity, after its discoverer P-waves diminish & disappear about 105º away from an earthquake focus About 140º away, they reappear but about 2 minutes later than expected based on distance traveled The P-wave shadow zone

25. P-wave velocity decreases by about 40% on entering the core Also, S-waves are not observed over 105º away from the focus These data combined suggest change from a solid to a liquid

27. Inner core boundary Also called the Lehmann discontinuity after its discoverer Discovery based on both the reflection and refraction of seismic waves Reflection of seismic waves is much akin to the formation of echos

28. Original estimates of depth weren’t accurate Underground nuclear tests in the early 1960s allowed refinement of estimates Data also indicated that P waves passing through this inner core traveled much faster than those passing through the outer core

30. Some details on the layers of Earth Crust Averages less than 20 km (12 mi) thick; ranges from 3 km (1.9 mi) to over 70 km (40 mi) thick Continental rocks average 2.7 g/cm3, and are up to 4 b.y. old Average composition of upper continental crust is that of granodiorite (a mix of granite and diorite) Lower continental crust is likely closer to basalt

31. Oceanic rocks are more dense, 3.0 g/cm3, rather young (less than 200 m.y.) Composition predominantly basalt

33. Mantle Both layers of the mantle comprise over 82% of Earth’s volume What is “known” is based on experimental data and examining material brought to the surface by volcanic activity i.e., kimberlite pipes – likely from about 200 km depth, roughly peridotite in composition Overall composition thought to be that of the mineral olivine

34. Break between the upper & lower mantle defined by an abrupt increase in seismic velocity About 410 km (256 mi) depth Result of a phase change Crystal structure of a mineral changes as a result of changes in temperature and/or pressure The mineral olivine’s structure is changed to one that resembles that of the mineral spinel A denser packing results in an increase in seismic velocity Another velocity change at about 660 km (412 mi) Another phase change olivine converts to perovskite structure

36. Lowermost 200 km (125 mi) of mantle Called the D” layer P-wave velocities show a sharp decrease (and, this is looking at the data REALLY closely) Explanation – this zone is partially molten, at least in places

37. The Core Overall, larger than the planet Mars 1/6 of Earth’s mass, but 1/3 of its density Average density is about 11 g/cm3 Meteorites helped decipher this Assumed to represent samples of the material from which Earth formed Composition: mostly iron-nickel, to stony ones resembling peridotite

38. How did the core form? Don’t know that for sure Best idea: During accretion, heat released as material collided with proto-Earth At some point, internal temp high enough to melt materials Metal-rich material, being denser, collected and sank toward the center, while less dense material floated upward

39. Entire core may have been molten initially As a material crystallizes, heat is released This is part of the source of heat within the Earth

40. The Magnetic Field Earth acts as if there is a large, permanent magnet at the center Some stuff about magnets Need a material that can be magnetized Something very hot will not retain magnetism Ideas for Earth Core material needs to be able to conduct electricity The material needs to be mobile And, this gets deeply into physics

41. Dynamo theory This takes place in the OUTER core Moving, molten metal generates electric fields When you have an electric field, you also have an associated magnetic field The magnetic field generates an electric field, which then generates a magnetic field And so on And that is the simple explanation

43. Heat in the Earth Geothermal gradient – the gradual increase in temperature as you go deeper in the Earth Varies considerably place to place, especially in the crust; increase thought to be less in the mantle and core (we can’t measure it directly) 3 main sources: Decay of radioactive isotopes Heat of crystallization of iron in the core Kinetic energy due to collisions during formation

44. Heat flow – in the crust Heat is transferred through matter by molecular activity Put a metal spoon in a hot pan, come back a few minutes later, then pick it up Rock materials are poor conductors of heat Crust acts as an insulator (cool on top, hot on bottom)

45. Heat flow – in the mantle Models of the mantle need to explain temperatures calculated for the specific layers (note the underlines) Temperatures increase with depth in the mantle, but more gradually than in the crust

47. Rocks are poor conductors of heat – how is the heat distributed? Convection is the best explanation (transfer of heat by circulation) Hot rock rises, pushing cooler rock downward Suggests the rocks are not solid, yet they transmit S-waves Indicates the rocks are plastic – behave as both a solid and a liquid Ex.: taffy – hit a piece with a hammer, it shatters; slowly pull it apart, it flows

49. End of chapter