Our direct knowledge is limited only to the upper several km of the Earth (Earth has radius of 6370 km). Therefore we must rely on indirect evidence to learn about Earth's interior.
Average density of whole Earth ~5.5 gm/cm3. However, most rocks at surface have density of ~2.5-3.0 gm/cm3. This indicates that Earth's interior must be denser than 5.5 gm/cm3 . Therefore, the Earth is not homogeneous.
Internal structure, of the Earth (fig. 10-2, table 10-1). The Earth’s major layers are inner core, outer core, mantle, and crust (oceanic and continental). The curst and upper mantle can be divided into asthenosphere and lithosphere.
Seismic Waves- Clues (Indirect Evidence)
Seismic waves- velocity generally increases with increasing density and elasticity of rocks; density increases with increasing depth (therefore seismic velocity generally increases with depth). Seismic waves travel in straight paths if the material they travel in is homogeneous, however, they bent when the material becomes inhomogeneous (fig. 10.3).
Refraction- curving of seismic wave as a result of velocity increase at depth in Earth (fig. 10-4).
Reflection- seismic wave bounces off of a discontinuity (rock boundary). This phenomenon can be used to calculate depth to a discontinuity by measuring velocity and the time it takes for the wave to travel from source to boundary and back to surface.
-some energy of a seismic wave reflected, some refracted (fig. 10-5).
-seismic wave velocity does not increase uniformly with depth; abrupt changes in velocity correspond to discontinuities (fig. 10-6).
-major seismic velocity discontinuities include crust/mantle boundary (Moho), low velocity zone (asthenosphere), mantle/outer core boundary, outer core/inner core boundary.
Seismic waves decrease in velocity at core/mantle discontinuity at 2900 km depth (fig. 10.6); decrease in velocity causes waves to be refracted inward at this boundary; creates a P-wave shadow zone between 103˚ and 143˚ from earthquake focus (figs. 10-7).
S-wave shadow zone- no S-waves can be recorded from focus at distances greater than 103˚ (fig. 10-9). Therefore, S-waves cannot be transmitted through core; S-waves (shear waves) cannot travel through liquids. This is an indication that the Core must be liquid.
Weak (low energy) P-waves observed in the P-wave shadow zone is an indication of solid inner Core (fig. 10.8b).
Density & composition of the Core- 6960 km in diameter; very dense; believed to be made mostly of iron (inner + nickel, outer + sulfur); many meteorites have this composition (Table 10.2).
Seismic waves increase in velocity at average depth of 35 km (crust/mantle boundary.) indicating a sharp change in rock properties. This sharp change is called the Mohorovicic discontinuity (Moho) (fig. 10-10).
Structure, density, and composition of mantle- seismic velocity decreases at 100-250 km depth due to partial melting of rocks. This corresponds to the asthenosphere layer (fig. 10-11). Velocity of seismic waves indicates that density is 3.3 to 5.7 gm/cm3; peridotite has this density; lower parts of crust now on land (ophiolites) have this composition; kimberlites, explosive diamond bearing igneous bodies believed to come directly from mantle, have rocks of this composition; many meteorites also have this composition.
Continental crust- 20-90 km (average 35 km) thick; density of ~2.7 gm/cm3; P-wave velocity of ~6.7 km/sec.; granitic rocks have this P-wave velocity and are known to make up most of the continental crust.
Oceanic crust- 5-10 km thick; density of ~3.0 gm/cm3; p-wave velocity of ~7 km/sec.; Basalt has this P-wave velocity and is known to make up most of the oceanic crust. Immediately below crust (Moho), seismic velocity increases to ~8 km/sec.)