The deep interior of the Earth remains somewhat of a mystery as we have only penetrated the very most outer portion with our deep drilling exploration. What knowledge we do have comes from seismic wave data or lava that has extruded onto the surface. What we do know is that the Earth's interior is somewhat like a concentric series of rings, progressing from the dense and intensely hot inner core toward the brittle outer shell of the crust.
EM.1 Seismograph recording seismic activity. (Courtesy
USGS Hawaii Volcano Observatory)
When seismic waves pass through rock, their amplitude and direction changes. For instance, wave velocity generally increases as rock density increases. Shear waves do not penetrate molten masses and when they encounter a boundary between two rock types of differing densities, a portion of the wave travels along the boundary while another part returns to the surface. Such changes in seismic wave velocities led Yugoslavian geophysicist Andrija Mohorovicic (1857-1936) to discover the boundary between the crust and underlying mantle. Wave velocity increases through the "Moho" discontinuity. It is believed that the discontinuity represents a zone where sima-type minerals undergo a phase change that produces a new and denser combination of minerals. "Examine P and S waves moving through Earth's interior." (Courtesy NSF/TERC/McDougall Littell)
Figure EM.2 Interior Structure of the Earth (Click image to enlarge)
The outer brittle shell of the Earth is the crust that forms the "skin" of the lithosphere. The crust is broken into several continental and oceanic tectonic (lithospheric) plates. These plates ride atop the more pliable mantle beneath, colliding to create great mountain systems and spreading apart to form rift valleys.
The crust is divided into a basal zone called the sima layer, and a less dense sial layer. The sima is primarily composed of a heavy, dark group of basaltic rocks. Primarily composed of silica and magnesium, their high density (2800 to 3300 kg/m3) is due to the large amounts of iron and magnesium. The sial, named for the two predominate elements silicon and aluminum, is lighter in weight with densities around 2700 - 2800 kg/m3. Often geoscientists refer to rocks of the sial as "granitic rock" as granite is a predominant rock type. The lower boundary of the sial grades into the upper portion of sima. The sial actually has quite a diversity of rock types, including large amounts of basaltic rocks. The sima however is almost exclusively basaltic in composition.
The mantle comprises 80% of the Earth's total volume. It is mainly composed of a dark, dense ultramafic rock called peridotite that is rich in iron and magnesium. Seismic wave velocity increases steadily through this zone. The upper mantle is divided into three fairly distinct layers. The lithosphere is a rigid cool layer composed of the outer crust and the uppermost mantle. The asthenosphere is the least rigid portion of the mantle. It is a soft, easily deformed layer that is susceptible to slow convection caused by pockets of increased heat from the decay of radioactive elements. Separating the upper mantle from the oceanic crust is the Moho Discontinuity. Seismic waves passing though this boundary increase their wave velocity from 4 mi (7 km) per second to 5 mi (8 km) per second. The shift of wave velocity is due to the change in rock composition and density. The rock of the oceanic crust is somewhat less dense than the mantle and referred to as mafic rock due to the smaller proportion of iron and magnesium. Below the asthenosphere is the rest of the upper mantle composed of rigid, solid rock called the mesosphere (not to be confused with the atmospheric layer of the same name).
The core is divided into the inner and outer cores. Though intense heat is generated at such great depths, geoscientists believe that under the enormous overlying pressure the inner core is made of solid iron and nickel. The outer core is thought to be molten iron because shear-wave velocities drop to zero which occurs when they encounter a liquid. The interaction between the inner and outer core is though to produce Earth's magnetic field.