Earth's Core
If it were possible to drill a hole to the center of the Earth, about 1,800 mi (2,900 km) below the surface the drill bit would reach the Earth's core. First the drill would bore through the solid, relatively low density rock of the crust. Then anywhere from 4.5 to 30 mi (7 to 50 km) below the surface the bit would encounter the much denser rock of Earth's mantle. Finally it would reach the core, which consists of an outer molten layer beginning at about 1,800 mi (2,900 km) and then an inner solid crystalline mass starting at about 3,150 mi (5,100 km).
Sometimes called the centrosphere, or the Earth's innermost layer, the core was almost a complete mystery until the development of the science of seismology and seismic instruments. Seismographs reveal the nature of the vibrations, or seismic waves, produced during an earthquake. As seismologists learned more about seismic waves, they realized they could use them to interpret the density and structure of the Earth's interior.
In 1897, a discovery by the Irish scientist Richard Dixon Oldham provided early clues about the nature of Earth's interior. He found that (seismic) waves generated by explosions or earthquakes travel through the interior of the Earth in different directions and at different speeds.
The two basic types of seismic waves discovered by Oldham are known as body waves--those that move through the Earth's interior, and surface waves--those that travel only along the surface. There are two kinds of body waves: primary and secondary waves. Primary or P waves, cause compressional movement emanating from the source of the disturbance. Secondary or S waves, produce shear motion in a direction perpendicular to the P wave. While P waves can pass through gases, liquids, and solids, S waves can only penetrate through solid matter.
Seismic waves have also helped scientists learn the various densities of the Earth's many layers because the speed of primary shock waves moving through the Earth generally increases with depth. This is because as density increases, seismic wave velocity increases. While P-waves travel through the Earth's crust at an average of about four miles (6.4 km) per second, they reach an average of seven miles (11.3 km) per second at the center of the Earth. When the shock waves suddenly shift in direction and speed, scientists are able to determine the depths at which Earth's various layers are located.
In 1906, Oldham recognized the existence of the Earth's core and made a preliminary, but incorrect, estimate of its size. He also noted a seismic "shadow zone," on the side of the Earth opposite the earthquake, where no P waves were recorded. Although uncertain, he presumed this resulted from refraction, or bending of waves, similar to light passing through a glass lens. In 1926, British seismologist Harold Jeffreys recognized an S wave shadow zone that begins 103 degrees from an earthquake and forms a "bullseye" shaped shadow on the backside of the Earth where no S waves are recorded. This indicated that the core was molten, since shear waves cannot pass through liquids.
Oldham's work, which included partially successful attempts to determine the existence of a thin outer crust and an inner core, helped other scientists accurately map the Earth's different layers. In 1909, Andrija Mohorovicic (1857-1936) published important findings from his study of an earthquake that hit Croatia. Based on his analysis of P and S wave speeds and arrival times, he was able to calculate the depth of the boundary where material changes from the Earth's crust to its mantle. This important discovery resulted in his name being applied to that boundary, called the Mohorovicic discontinuity, or the Moho for short. In 1914, based on seismic wave studies, Beno Gutenberg estimated the diameter of the core to be about 4,375 mi (7,000 km), a figure that still stands today. In tribute to that discovery, the boundary where the mantle and the core meet is referred to as the Gutenberg Discontinuity.
In 1936, Danish seismologist Inge Lehmann hypothesized the existence of an inner core within the Earth's deepest interior, based on receipt of weak P waves in the P shadow zone and presented her arguments using elementary trigonometry. She believed that by passing through the boundary from an outer core to an inner core, P waves could be refracted and received within the P wave shadow zone. For the next two years, Gutenberg and Charles F. Richter (1900-1985) worked on the problem and helped confirm Lehmann's hypothesis that the core had both an inner and an outer shell. The size of the inner core, however, and whether it was solid, liquid, or a mixture of both could not be established at that time. Further studies during the 1940s by K.E. Bullen showed that the inner core was solid, while the outer core was liquid. This change in phase probably results from the immense pressures at this depth.
Unlike the rock of Earth's mantle and crust, the core is thought to be composed almost entirely of metal, largely iron in the outer core, with an alloy of iron and nickel in the inner core. It is, of course, impossible to sample the core directly. What are believed to be minute inclusions of mantle material are sometimes found in diamonds, as well as from other sources; however, no such fragments of the Earth's core have ever been found, nor are they ever likely to be. Instead, geologists have had to find another source for study. Planetary geologists believe that Earth formed from material very similar to that of meteorites. Stony meteorites are considered to be representative of the mantle, while iron meteorites are representative of the core. Therefore, geologists base their estimates of the composition of the Earth's core on studies of meteorites.
In 1996 researchers Xiaodong Song and Paul Richards of the Lamont-Dougherty Earth Observatory discovered that the inner core rotates freely within the low viscosity fluid of the outer core. The finding confirmed what seismologists had suspected for years: the highly fluid iron of the outer core allowed the inner core to move independently of the rest of the Earth. They discovered this by noting that on seismic records for earthquakes from different years, P waves following the same path through the inner core had different travel times. These waves must be traveling along different paths through the inner core, a result of its rotation. In addition, they determined that the inner core rotates about one-quarter turn faster than the crust and mantle each century, or about one degree faster per year. Computer models suggest this is due to two jet stream-like currents flowing through the outer core. These carry a magnetic field that "tugs" on the inner core and gives its rate of rotation an extra "boost." Later that same year other researchers confirmed their findings.
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