Seeing Inside the Earth INTERLUDE D ESSENTIALS OF GEOLOGY 3rd Edition Stephen Marshak Seeing Inside the Earth

The Earth’s surface hides a hot interior. Fig. D.CO The Earth’s surface hides a hot interior.

The 19th century image of the Earth’s interior. Fig. D.1 Oceanic crust Continental crust Mantle Core Crust A simple 3-layer Earth model. Continental and oceanic crust are not the same. The 19th century image of the Earth’s interior.

The propagation of earthquake waves — 1. Fig. D.2a,b An earthquake sends out waves in all directions. Seismic waves travel at different velocities in different rock types. Wave front Seismic ray Sandstone Peridotite The propagation of earthquake waves — 1.

The propagation of earthquake waves — 2. Fig. D.2c,d Both P-waves and S-waves can travel through a solid, but only P-waves can travel through a liquid. P-waves travel faster in solid iron allow than in liquid, such as molten iron alloy. S-wave P-wave Solid Liquid Solid iron alloy Molten iron alloy The propagation of earthquake waves — 2.

Refraction and reflection of waves. Fig. D.3a,b Air Water Incoming light Faster Slower Reflected Refracted A ray of light reflects and refracts when it crosses the boundary between different materials. 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.

Fig. D.4a,b A nearby seismograph detects crustal seismic waves first. A distant seismograph detects mantle seismic waves first, because waves travel faster in mantle rock. Crust Mantle Moho Discovery of the Moho.

The velocity of P-waves in the mantle changes with depth. Fig. D.5 P-wave velocity (km/s) Depth (km) 1000 200 400 600 800 5 7 9 11 LVZ Upper Mantle Transition zone Lower mantle Lithosphere The velocity of P-waves in the mantle changes with depth.

The velocity of seismic waves changes with depth in the mantle . Fig. D.6a,b,c In a stack of discrete layers, rays bend at each boundary. If velocity increases gradually with depth, rays curve smoothly. The velocity of seismic waves increases with depth in the mantle, so rays curve. Seismic ray Mantle Focus The velocity of seismic waves changes with depth in the mantle .

P-waves do not arrive in the P-wave shadow zone. Fig. D.7a P-waves do not arrive in the P-wave shadow zone. P-wave shadow zone 103° 143° Focus Shadow zones and the discovery of the Earth's core — 1.

S-waves do not arrive in the S-wave shadow zone. Fig. D.7b S-waves do not arrive in the S-wave shadow zone. S-wave shadow zone 103° Focus Shadow zones and the discovery of the Earth's core — 2.

Seismic waves reflect off the inner core-outer core boundary. Fig. D.7c Source Seismic waves reflect off the inner core-outer core boundary. Seismograph Mantle Outer core Inner Incident ray Reflected ray Shadow zones and the discovery of the Earth's core — 3.

Note the error! The outer core is liquid (the liquidus lies to the left of the geotherm, so it melts). The inner core is solid (the geotherm is not enough to melt it because of high pressure). Fig. D.7d A graph of the geotherm and melting curve for the Earth. Depth (km) 6000 Temperature (°C) 1000 2000 3000 4000 5000 Geotherm Mantle Melting curve Solid outer core Liquid inner core Liquid and solid inside the Earth.

Inner core-outer core boundary Fig. D.8 Depth (km) Velocity (km/s) 400 2900 4155 6371 670 P-wave S-wave Core-mantle boundary Inner core-outer core boundary 4 8 12 The velocity-versus-depth profile of the Earth.

Tomographic image of the whole Earth. Fig. D.9a Tomographic image of the whole Earth. Inner core Outer core Faster Slower Mantle Tomographic images of the Earth's interior — 1.

Tomographic images of the Earth's interior — 2. Fig. D.9b,c Lower Mantle North America Convecting cell Mantle plume Plate graveyard Pacific Ocean 660 km Close-up tomographic image of the mantle. Modern view of the Earth's complex and dynamic interior. Tomographic images of the Earth's interior — 2.

Seismic-reflection profiling. Fig. D.10a,b Trucks thump the ground to generate a seismic signal. A seismic-reflection profile. Color stripes are subsurface layers. Ground surface 1000 ft 100 ft Seismic-reflection profiling.

The Earth's geoid is distorted by highs and lows in the gravity field. Fig. BXD.1.1 The Earth's geoid is distorted by highs and lows in the gravity field. Geoid high Geoid low An exaggerated representation of the geoid.

Interlude D: Figure Credits BXD.1: ESA; D.10a: John Q. Thompson, courtesy of Dawson Geophysical Company; D.10b: John Q. Thompson, courtesy of Dawson Geophysical Company.