CHAPTER 19: The Earth’s Interior


Paths of Seismic Waves in the Earth


Waves Traveling Through the Earth

1. If the entire earth was of uniform composition, then P and S waves would travel through the earth along essentially straight lines.

2. Figure 19.2: The earth, however, is compositionally layered and the density of rocks, particularly in the mantle, generally increases with depth. As a result, seismic waves bend and reflect as they travel through the earth.

3. Seismic waves from an earthquake's focus travel through the earth along bent paths and are eventually recorded by distant seismograph stations. The character of the waves and the time it takes for them to reach a particular location reveals important clues as to the nature of the earth’s interior.

4. Figure 19.2a: P-waves generally bend outward as they travel through the mantle due to the increased density of mantle rocks with depth. When P-waves strike the outer core, however, they bend downward when traveling through the outer core and bend again when they leave. This indicates that P-waves slow down in the outer core, suggesting that this layer has a significantly different composition from the mantle and may actually be liquid. This bending in the outer core creates a P-wave shadow zone where no P-waves are detected.

5. The bending of seismic waves is called refraction.

6. Figure 19.2b: S-waves do not travel through the outer core, creating an even bigger shadow zone for S-waves. The fact that S-waves do not travel through the outer core suggests that the latter is liquid.


Waves Reflected in the Earth

1. Figure 19.3: Some seismic waves also reflect when reaching the boundary between two different materials.

2. A PcP wave is a P-wave that had bounced off the mantle-core boundary and returned to the surface as a P-wave.

3. PP and SS waves are reflected at the surface without reaching the core and are returned to the mantle.

4. A PKP wave is transmitted through the liquid outer core whereas a PKIKP wave traverses the solid inner core.


Composition and Structure of the Earth's Interior


The Crust

1. Figure 19.6: Seismic studies of the outermost layer of the earth indicate that the crust varies extensively in thickness.

(a) The crust is thin (~5 km average) under oceans and composed primarily of basalt.

(b) The crust is much thicker (~40-65 km) under continents and has an average composition of granite. Continental crust is therefore lighter (more buoyant) than oceanic crust.

2. Figure 19.7: Beneath the crust, seismic waves increase abruptly indicating a sharp boundary between the crust and upper mantle. This is due to the compositional change from granite, or basalt, to peridotite that comprises the upper mantle. The boundary between the crust and upper mantle is called the Moho.

3. Figure 19.7: The crust and very top portion of the upper mantle, which also includes the Moho, comprise the lithosphere. The lithosphere is the rigid outer layer of the earth and constitutes the lithospheric plates.


The Mantle

1. Figure 19.7: The velocity of S-waves decreases within a zone just below the lithosphere. This suggests that the peridotite within this zone contain a few percent partial melt, but not enough to completely stop the S-waves. This region is therefore referred to as the low velocity zone or asthenosphere.

2. From 200-400 km depth, the velocity of S-waves gradually increases again until reaching the 400 km transition zone where the S-wave velocity increases rapidly. This increase may the associated with a change in the crystal structure of olivine to a closer atomic packing referred to as the spinel structure.

3. Another abrupt increase in S-wave velocity occurs at the ~670 km transition zone, indicating another change to even closer atomic packing where the spinel structure changes to that of perovskite.

4. Below the 670 km transition zone, S-wave and P-wave velocity increase in a less dramatic manner until reaching the mantle-core boundary at ~2900 km depth.


The Core

1. Figure 19.5: The slowing down of P-waves in the outer core, coupled with the failure of S-waves to pass through it, tells us that the outer core is liquid. Experimental measurements of seismic waves through various materials, coupled with the fact that the core contains one third of the Earth’s mass, suggests that the outer core is composed of molten iron.

2. P-waves speed up again through the inner core and S-waves also travel through it, suggesting that the inner core is composed of solid iron and nickel.

3. Figure 19.10: The increase of temperature with depth in the earth is indicated by a curve called the geotherm.

4. The geotherm is generally below the melting curve of mantle until ~2900 km depth where the two curves cross at the mantle-core boundary. Within the outer core, the geotherm is above the melting curve of iron.

5. At the boundary between the outer and inner core, the two curves cross again and the geotherm is again below the melting curve of iron so that the inner core is composed of solid Fe.

6. Figure 17.C: The liquid iron in the outer core is stirred into convective motion by heat generated from radioactivity in the core. Circulation of the liquid iron in the outer core produces electric currents that, in turn, generate the earth’s magnetic field.

7. Figure 19.11: The earth can therefore be envisioned as containing a bar magnet tilted at a slight angle to the rotational axis. The magnetic lines of force travel from the magnetic south to the magnetic north pole.