GeoClassroom Physical Geology Historical Geology Structure Lab

Review Questions and Answers; Earth's Interior

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1. List the major differences between P and S waves.

P waves (primary or pressure waves) travel though matter as particle vibrations along lines parallel to the ray path or to the path traced by a point traveling with the wave front. P waves move through solids, liquids, and gases, the "vibrations" being in the form of pressure variations. S waves (secondary or shear waves) move by particle vibrations at right angles to the ray path; they are transmitted only by solids, not by liquids or gases. P waves always travel faster than S waves in a given material.

2. How does the boundary between the crust and mantle (Moho) differ from the boundaries that occur at depths of about 400 and 700 kilometers?

A discontinuity represents an abrupt change in seismic velocities over a short depth interval. The Mohorovicic discontinuity (the Moho) refers to the crust-mantle boundary; it generally lies between 25 and 65 km beneath the continents and at much shallower depths in the ocean basins. The Moho probably represents a fairly sharp boundary between higher velocity, mantle peridotites below and lower velocity, less mafic, lower crustal rocks above. The discontinuity is too shallow to be caused by inversion of common mantle minerals to higher pressure, more dense forms. Thus the Moho is probably largely a compositional rather than a phase-change discontinuity.

Although seismic velocities gradually increase downward from the Moho, significant jumps occur at depths of 400 and 700 km (Fig. 17.12; note the S- and P-wave velocity curves in the region marked "Upper mantle"). These discontinuities are thought to be caused by phase changes. Subjected to increased pressures, common, upper mantle Mg-silicate minerals, such as olivine and pyroxenes, invert to denser, more closely packed, crystalline forms with higher body-wave velocities than the lower pressure forms. High pressure, experimental studies have shown that such inversions would be expected to occur at pressures and temperatures corresponding to depths of about 400 km (the olivine to the spinel structure) and 700 km (the spinel to the perovskite structure).

3. Describe the lithosphere. In what important way is it different from the asthenosphere?

The lithosphere is the cool, rigid, rocky, outer shell of Earth. This shell breaks into lithospheric fragments, plates of plate tectonics, that move in response to convective motions in the deep mantle and asthenosphere. Failure in response to stress is by brittle rupture and fracturing.

The asthenosphere is much warmer, and temperatures are near the lower end of the melting range for rock-forming silicates. Thus asthenospheric rocks are mechanically softer than lithospheric rocks and deform by ductile flowage rather than brittle fracture.

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4. Describe the chemical (mineral) makeup of the four principal layers of Earth.

Crust - The continental crust is much like average andesite or shale in chemical composition. The upper crust is enriched in granitic rocks rich in silicon and potassium and low in magnesium and iron. Quartz, orthoclase, and sodium-rich plagioclase are common minerals. The lower crust is more mafic; silicate minerals like olivine, hornblende, and pyroxene are abundant. The oceanic crust is composed largely of basaltic lavas and intrusive gabbro; iron-magnesium silicates such as olivine and pyroxene and calcium-rich plagioclase are the dominant minerals.

Mantle - The mantle is believed to be mainly ultramafic, silicate rock with magnesium silicates (olivine and pyroxene) being dominant. At great depth, denser minerals with similar compositions exist under higher temperature and pressure conditions.

Outer Core - The outer core is a liquid, mainly iron alloyed with nickel and lower atomic number elements such as sulfur, silicon, and perhaps potassium. Recent theoretical studies have suggested that oxygen might be one of the "needed" lighter elements. Iron generally fits the melting point and density requirements for the core material, and it also conducts electricity, a requirement for generation of Earth's magnetic field.

Inner Core - The inner core is thought to be a solidified, crystalline, iron-rich alloy with a composition similar to that of the liquid, outer core.

5. Why was it difficult for seismologists to obtain precise travel-time data before the turn of the century?

Numerous, evenly distributed, seismic-recording stations and exact locations and times of seismic events are required to generate precise, travel-time data. In the decades before and after the turn of the century, seismic stations were sparse; most were concentrated in North America, Europe, and Japan. Epicentral locations and event times determined for distant earthquakes often lacked precision. Even so, Gutenberg (1914) and Mohorovicic (1909) were able to make important discoveries concerning the fundamental, internal structure of Earth. By the 1930's, additional recording stations and improved communications and seismographs had led to greatly improved precision in travel-time curves at most recording stations.

During the decades of the 1960s and 70s, underground, nuclear weapons testing greatly stimulated interest in seismological research and instrumentation. Many new recording stations were built, including some highly sophisticated, seismic arrays. Detonation times and exact locations were known in advance of, or soon after, many test explosions, making possible significant improvements in travel-time data at recording stations all around the world.

6. Describe the method first used to accurately measure the size of the inner core.

Early in the 1960s following a powerful, underground explosion at the Nevada test site, P waves reflected directly from the inner core-outer core boundary were recorded by the Montana seismic array. Their travel times could be compared to those of waves reflected directly from the core-mantle boundary (Fig. 17.11). The difference represented the "extra" distance that the inner-outer core, reflected phase traveled inside the outer core with a minor correction for the slightly different paths the two phases followed through the mantle. By using outer core, P-wave velocities determined previously, the distance from the core-mantle boundary to the outer core-inner core boundary was determined more precisely than had previously been possible. Even so, the new value was not found to be greatly different from earlier ones estimated by comparing P waves that had traveled directly through the outer and inner core with those that had passed through the outer core only (Figs. 17.10 & 17.12).

7. How were the first samples (in place) of the deep-ocean floor obtained?

During the historic oceanographic expeditions of the nineteenth century and into the early decades of the twentieth century, samples of ocean floor rocks and sediments were obtained by crude dredging and grab-sampling techniques; coring devices to extract an undisturbed, vertical section of unconsolidated sediment were developed before the Second World War. In situ samples of lithified sedimentary rocks and igneous rocks from beneath the ocean floor were first obtained in 1968 using new technology and newly developed, deep sea, core-drilling equipment carried aboard the Glomar Challenger, an oceanographic research ship especially designed for deep-water drilling.

8. What evidence did Beno Gutenberg use for the existence of Earth's central core?

The S-wave shadow zone (Fig. 17.10) indicated that the core or the outer portion of the core was a liquid. Utilizing geometrical constraints imposed by the P-wave shadow zone, expected patterns of wave refraction, and travel times of P waves that had passed through the core region (Fig. 17.9), Gutenberg (1914) estimated the core-mantle boundary to be at a depth of 2900 km. This remarkable study gave a core radius of 3470 km, not greatly different from the currently accepted value determined from waves reflected directly back from the core-mantle boundary (Fig. 17.11).

9. Suppose the shadow zone for P waves was located between 120 and 160 degrees, rather than between 105 and 140 degrees. What would this indicate about the size of the core?

The explanation for the P-wave shadow zone is shown in Figure 17.9. P-wave rays that just graze the mantle-outer core boundary can be visualized as splitting into two components; one remains entirely in the mantle and emerges at the 105° location. The other component is refracted into the outer core and emerges at the 140° location. The diameter of the core must lie between the diameters of two spheres centered at Earth's center and tangent to right-circular cones with apices at the earthquake epicenter (the north pole in Fig. 17.9) and basal diameters equal to the 105 and 140 degree angular distances as represented by the boundaries of the P-wave shadow zone (Figs. 17.9 & 17.10).

If a fictitious planet with the same diameter as Earth showed P-wave shadow-zone boundaries at angular distances of 120 and 160 degrees, its core would be smaller than Earth's. As noted, the core diameter would have to lie between limiting values imposed by the smaller diameters of the 120 to 160 degree, shadow-zone boundary circles.

10. Explain why the asthenosphere is able to flow like a fluid yet has the ability to transmit S waves which cannot travel through fluids.

S waves are elastic waves in solids. Passage of the wave energy induces small deformations, but the material quickly recovers its original configuration once the vibrations have stopped. For any material, the speed of S waves is determined by the ratio of the mechanical rigidity to density. Since liquids have zero rigidity, they do not transmit S waves.

Asthenospheric flow is a very slow form of plastic deformation in solids driven by long lasting stresses and made possible by elevated temperatures. Strains are cumulative, finite, and not recoverable; much of the strain is accommodated by recrystallization. Liquids and liquid flow are not involved. Warm, silicate rocks are solids. They respond elastically to small, highly transient stresses such as S-wave vibrations and flow plastically to stresses applied continuously for long periods of time.

11. Why are meteorites considered important clues to the composition of Earth's interior?

The Earth is thought to have accumulated by impacts of meteorites and other matter orbiting nearby in the early solar system. Thus the bulk composition (average composition of the whole earth) is probably close to the average composition of meteoritic material. Differentiation into the core and mantle occurred soon after the planet had grown to include most of its present-day mass.

12. What evidence is provided by seismology to indicate that the outer core is liquid? What other evidence exists for a molten outer core?

The outer core does not transmit S waves, as demonstrated by the S-wave shadow zone. However S waves, initiated by P waves striking the inner core-outer core boundary, do propagate in the inner core. Thus the outer core is a liquid and the inner core is a solid. Fluid motions necessary to sustain Earth's magnetic field also require that the outer core be liquid.

13. Why is it possible for the outer core to be molten when the inner core (which has a higher temperature) is in the solid state?

In general for any given temperatures, crystalline solids are more dense than liquids of equivalent composition. Thus melting temperatures increase with increasing pressures. Ice at atmospheric pressure, however, is a glaring exception because at 0° C, ice is less dense than pure water. In addition at constant pressure, freezing point temperatures of multi-component liquids are usually lower than the melting temperatures of the individual solidified components.

Pressures and temperatures increase gradually with depth; and obviously, the highest pressures and temperatures are found at the center of Earth. The core is thought to be dominantly iron alloyed with smaller amounts of lighter elements such as silicon, carbon, and sulfur. These dissolved components lower the temperature at which pure iron will crystallize from the outer core liquid. At greater depths in the core, increased melting temperatures of iron induced by the higher pressures overcome the freezing point depression caused by the dissolved components, allowing metallic iron to crystallize from the core liquid at the inner core-outer core boundary. The heat of crystallization released during this process provides a convenient heat source to drive the vigorous convection envisioned to occur in the outer core.

 

 

 


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