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D Seeing Inside the Earth ESSENTIALS OF GEOLOGY 3rd Edition Stephen Marshak © 2009 W.W. Norton INTERLUDE The Earth’s surface hides a hot interior. © 2009 W.W. Norton Fig. D.CO Fig. D.1 Oceanic crust Continental crust Crust Mantle Mantle Continental and oceanic crust are not the same. A simple 3-layer Earth model. The 19th century image of the Earth’s interior. © 2009 W.W. Norton Core Fig. D.2a,b Seismic ray Wave front An earthquake sends out waves in all directions. Peridotite Seismic waves travel at different velocities in different rock types. The propagation of earthquake waves — 1. © 2009 W.W. Norton Sandstone Fig. D.2c,d S-wave Solid Liquid P-wave P-wave Solid iron alloy P-wave Molten iron alloy P-waves travel faster in solid iron allow than in liquid, such as molten iron alloy. The propagation of earthquake waves — 2. © 2009 W.W. Norton Both P-waves and S-waves can travel through a solid, but only P-waves can travel through a liquid. Fig. D.3a,b Incoming light Air Water Reflected Refracted Faster Slower Slower Faster A ray entering slower material bends away from the boundary, but a ray entering faster material bends toward the boundary. Refraction and reflection of waves. © 2009 W.W. Norton A ray of light reflects and refracts when it crosses the boundary between different materials. Fig. D.4a,b Crust Moho Mantle A nearby seismograph detects crustal seismic waves first. Crust Moho A distant seismograph detects mantle seismic waves first, because waves travel faster in mantle rock. Discovery of the Moho. © 2009 W.W. Norton Mantle Fig. D.5 P-wave velocity (km/s) 7 9 5 11 0 Lithosphere LVZ 200 Upper Mantle 400 Depth (km) Transition zone 600 Lower mantle 1000 The velocity of P-waves in the mantle changes with depth. © 2009 W.W. Norton 800 Fig. D.6a,b,c Seismic ray Focus In a stack of discrete layers, rays bend at each boundary. Mantle If velocity increases gradually with depth, rays curve smoothly. The velocity of seismic waves changes with depth in the mantle . © 2009 W.W. Norton The velocity of seismic waves increases with depth in the mantle, so rays curve. Fig. D.7a Focus 103 ° P-wave shadow zone P-wave shadow zone 143 ° P-waves do not arrive in the P-wave shadow zone. Shadow zones and the discovery of the Earth's core — 1. © 2009 W.W. Norton P-wave shadow zone Fig. D.7b Focus 103 ° S-wave shadow zone S-wave shadow zone S-waves do not arrive in the S-wave shadow zone. Shadow zones and the discovery of the Earth's core — 2. © 2009 W.W. Norton 103 ° Fig. D.7c Source Seismograph Incident ray Mantle Reflected ray Outer core Seismic waves reflect off the inner core-outer core boundary. Shadow zones and the discovery of the Earth's core — 3. © 2009 W.W. Norton Inner core Note the error! The outer core is liquid (the liquidus lies to the left of the geotherm, so it melts). Fig. D.7d 0 Melting curve 1000 Mantle Depth (km) Geotherm 2000 3000 Solid outer core 4000 A graph of the geotherm and melting curve for the Earth. 5000 Liquid inner core 6000 Liquid and solid inside the Earth. 0 2000 4000 Temperature (°C) © 2009 W.W. Norton The inner core is solid (the geotherm is not enough to melt it because of high pressure). Fig. D.8 0 0 4 Velocity (km/s) 8 12 400 670 S-wave 2900 Core-mantle boundary 4155 Inner coreouter core boundary Depth (km) 6371 The velocity-versus-depth profile of the Earth. © 2009 W.W. Norton P-wave Fig. D.9a Mantle Outer core Inner core Faster Tomographic image of the whole Earth. Tomographic images of the Earth's interior — 1. © 2009 W.W. Norton Slower Fig. D.9b,c Pacific Ocean Plate graveyard North America 660 km Convecting cell Mantle plume Lower Mantle Tomographic images of the Earth's interior — 2. Modern view of the Earth's complex and dynamic interior. © 2009 W.W. Norton Close-up tomographic image of the mantle. Fig. D.10a,b Ground surface Trucks thump the ground to generate a seismic signal. 100 ft Seismic-reflection profiling. © 2009 W.W. Norton 1000 ft A seismic-reflection profile. Color stripes are subsurface layers. Fig. BXD.1.1 Geoid high The Earth's geoid is distorted by highs and lows in the gravity field. An exaggerated representation of the geoid. © 2009 W.W. Norton Geoid low Interlude D: Figure Credits © 2009 W.W. Norton BXD.1: ESA; D.10a: John Q. Thompson, courtesy of Dawson Geophysical Company; D.10b: John Q. Thompson, courtesy of Dawson Geophysical Company.
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