Earth, Interior Structure (World of Earth Science)
It is 3,950 miles (6,370 km) from the earth's surface to its center. The rock units and layers near the surface are understood from direct observation, core samples, and drilling projects. However, the depth of drill holes, and therefore, the direct observation of Earth materials at depth, is severely limited. Even the deepest drill holes (7.5 mi, 12 km) penetrate less than 0.2% of the distance to the earth's center. Thus, far more is known about the layers near the earth's surface, and scientists can only investigate the conditions within the earth's interior (density, temperature, composition, solid versus liquid phase, etc.) through more indirect means.
Geologists collect information about Earth's remote interior from several different sources. Some rocks found at the earth's surface, known as kimberlite and ophiolite, originate deep in the crust and mantle. Some meteorites are also believed to be representative of the rocks of the earth's mantle and core. These rocks provide geologists with some idea of the composition of the interior.
Another source of information, while more indirect, is perhaps more important. That source is earthquake, or seismic waves. When an earthquake occurs anywhere on Earth, seismic waves travel outward from the earthquake's center. The speed, motion, and direction of seismic waves changes dramatically at different levels within Earth, known as seismic transition zones. Therefore, scientists can make various assumptions about the earth's character above and below these transition zones through careful analysis of seismic data. This information reveals that Earth is composed of three basic sections, the crust (the thin outer layer), the mantle, and the core.
The outermost layer of Earth is the crust, or the thin "shell" of rock that covers the globe. There are two types of crust: the continental crust, which consists mostly of light-colored rock of granitic composition that underlies the earth's continents; and the oceanic crust, which is a dark-colored rock of basaltic composition that underlies the oceans. One of the most important differences between continental and oceanic crust is their difference in density. The lighter-colored continental crust is also lighter in weight, with an average density of 2.6 g/cm3 (grams per cubic centimeter), compared to the darker and heavier basaltic oceanic crust, which has an average density of 3.0 g/cm3. It is this difference in density that causes the continents to have an average elevation of about 2,000 ft (600 m) above sea level, while the average elevation (depth) of the ocean bottom is 10,000 ft (3,000 m) below sea level. The heavier oceanic crust sits lower on the earth's surface, creating the topographic depressions for the ocean basins, while the lighter continental crust rests higher on the earth's surface, causing the elevated and exposed continental land masses.
Another difference between the oceanic crust and continental crust is the difference in thickness. The heavier oceanic crust forms a relatively thin layer of 3 mi (50 km), while the continental crust is lighter, and the underlying material can support a thicker layer. The continental crust averages about 20 mi (35 km) thick, but can reach up to 40 mi (70 km) in certain sections, particularly those found under newly elevated and exposed mountain ranges such as the Himalayas.
The base of the crust (both the oceanic and continental varieties) is determined by a distinct seismic transition zone called the Mohorovic discontinuity. The Mohorovic discontinuity, commonly referred to as "the Moho" or the "M-discontinuity," is the transition or boundary zone between the bottom of the earth's crust and the underlying unit, which is the uppermost section of the mantle called the lithospheric mantle. Like the crust, the lithospheric mantle is solid, but it is considerably more dense. Because the thickness of the earth's crust varies, the depth to the Moho also varies from 3 mi (50 km) under the oceans to 200 mi (35-70 km) under the continents.
This transition between the crust and the mantle was first discovered by the Croatian seismologist Andrija Mohorovicic in 1908. On October 8, 1908, Andrija Mohorovicic observed seismic waves that emitted from an earthquake in Croatia. He noticed that both the compressional, or primary (P), waves and the shear, or secondary (S), waves, at one point in their journey, picked up speed as they traveled
farther from the earthquake. This suggested that the waves had been deflected, as if they had encountered something that had affected their energy. He noted that this increase in speed seemed to occur at a depth of about 30 mi (50 km). Since seismic waves travel faster through denser material, he reasoned that there was an abrupt transition from the rocky material in the earth's crust, to denser rocks below. In honor of Andrija Mohorovicic's discovery, this transition zone marking the base of the earth's crust was named after him.
The Moho is a relatively narrow transition zone estimated to be somewhere between 0.1.9 mi (0.2 km) thick. Currently, the Moho is defined by the level within Earth where P wave velocity increases abruptly from an average speed of about 4.3 mi/second (6.9 km/second) to about 5.0 mi/second (8.1 km/second).
Underlying the crust is the mantle. The uppermost section of the mantle, which is a rigid layer, is called the lithospheric mantle. This section extends to an average depth of about 40 mi (70 km), although it fluctuates between 300 mi (5000 km). The density of this layer is greater than that of the crust, and averages 3.3 g/cm3. But like the crust, this section is solid and brittle, and relatively cool compared to the material below. This rigid uppermost section of the mantle (the lithospheric mantle), combined with the overlying solid crust, is called the lithosphere, which is derived from the Greek word lithos, meaning rock. At the base of the lithosphere, a depth of about 40 mi (70 km), there is another distinct seismic transition called the Gutenberg low velocity zone. At this level, the velocity of S waves decreases dramatically, and all seismic waves appear to be absorbed more strongly than elsewhere within the earth. Scientists interpret this to mean that the layer below the lithosphere is a softer zone of partially melted material (with between 10% molten material). This "soft" zone is called the asthenosphere, from the Greek word asthenes meaning weak. This transition zone between the lithosphere and the asthenosphere is named after Beno Gutenberg, a mid-twentieth century geologist who made several important contributions to the study and understanding of the earth's interior. It is at this level that some important Earth dynamics occur, affecting those at the surface. At the Gutenberg low velocity zone, the lithosphere is carried "piggyback" on top of the weaker, less rigid asthenosphere, which seems to be in continual motion. This motion creates stress in the rigid rock layers above it, and the slabs or plates of the lithosphere are forced to jostle against each other, much like ice cubes floating in a bowl of swirling water. This motion of the lithospheric plates is known as plate tectonics, and it is responsible for many of the earth's activities that we experience at the surface today, including earthquakes, certain types of volcanic activity, and continental drift.
The asthenosphere extends to a depth of about 155 mi (250 km). Below that depth, seismic wave velocity increases, suggesting an underlying denser, but solid phase.
The rest of the mantle, from the base of the asthenosphere at 155 mi (250) km to the core at 1,800 mi (2,900 km), is called the mesosphere (or middle sphere). There are mineralogical and compositional changes suggested by sharp velocity increases within the mesosphere. Notably, there is a thin zone at about the 250 mi (400 km) depth attributed to a possible mineralogical change (presumably from an abundance of the mineral olivine to the mineral spinel), and there is another sharp velocity increase at about the 400 mi (650 km) level, attributed to a possible increase in the ratio of iron to magnesium in mantle rocks. Except for these variations, down to the 560 mi (900 km) level the mesosphere seems to contain predominantly solid material that displays a relatively consistent pattern of gradually increasing density and seismic wave velocity with increasing depth and pressure. Below the 560 mi (900 km) depth, the P and S wave velocities continue to increase, but the rate of increase declines with depth.
At a depth of 1,800 mi (2,900 km) there is another abrupt change in the seismic wave patterns, known as the Gutenberg discontinuity, or more often referred to as the core-mantle boundary (CMB). At this level, P waves decrease while S waves disappear completely. Since S waves cannot be transmitted through liquids, it is believed that the CMB denotes a phase change from the solid mantle above, to a liquid outer core below. This phase change is believed to be accompanied by an abrupt temperature increase of 1,300° F (704° C). This hot, liquid outer core material is denser than the cooler, solid mantle, probably due to a greater percentage of iron. It is believed that the outer core consists of a liquid of 802% iron, alloyed with a lighter element. The composition of the remaining 80% is not well understood, but it must be a compressible element that can mix with liquid iron at these immense pressures. Various candidates proposed for this element include silicon, sulfur, or oxygen.
The actual boundary between the mantle and the outer core is a narrow, uneven zone that contains undulations that may be 3 mi (5 km) high. These undulations are affected by heat-driven convection activity within the overlying mantle, which may be the driving force for plate tectonics. The interaction between the solid mantle and the liquid outer core is very important to Earth dynamics for another reason. It is the eddies and currents in the core's iron-rich fluids that are ultimately responsible for the earth's magnetic field.
Although the core-mantle boundary is currently situated at a depth of about 1,800 mi (2,900 km), this depth has not been constant through geologic time. As the heat of the earth's interior is constantly but slowly dissipated, the molten core within the earth gradually solidifies and shrinks, causing the core-mantle boundary to slowly move deeper and deeper within the earth's core.
There is one final, even deeper transition evident from seismic wave data. Within Earth's core, at the 3,150 mi (5,100 km) level, P waves speed up and are reflected from yet another seismic transition zone. This indicates that the material in the inner core below 3,150 mi (5,100 km) is solid. The phase change from liquid to solid is probably due to the immense pressures present at this depth.
In addition to this phase change in the inner core from liquid to solid, seismic wave velocities, as well as the earth's total weight, suggest that the inner core has a different composition than the outer core. This could be accounted for by a relatively pure iron-nickel composition for the inner core. Although no direct terrestrial evidence for a solid iron-nickel inner core exists, comparative evidence from meteorites supports this theory. Numerous meteorites, fragments presumably from the interior of shattered extraterrestrial bodies within our solar system, often contain relatively pure iron or iron-nickel compositions. It is likely that the composition of the core of our own planet is very similar to the composition of these extraterrestrial travelers.
See also Earth (planet); Earth science; Seismograph; Seismology; Volcanic eruptions; Volcanic vent