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[edit] Introductory Note
The Earth, the Sun, and the rest of the solar system, was formed 4.54 billion years ago by accretion from a rotating disk of dust and gas. The immense amount of heat energy released from gravitational energy and from the decay of radioactive elements melted the entire planet, and it is still cooling off today. Denser materials like iron (Fe) sank into the core of the Earth, while lighter silicates (Si), other oxygen (O) compounds, and water rose near the surface.The earth is divided into four main layers: the inner core, outer core, mantle, and crust.
The core is composed mostly of iron (Fe) and is so hot that the outer core is molten, with about 10% sulphur (S). The inner core is under such extreme pressure that it remains solid. Most of the Earth's mass is in the mantle, which is composed of iron (Fe), magnesium (Mg), aluminum (Al), silicon (Si), and oxygen (O) silicate compounds. At over 1000 degrees C, the mantle is solid but can deform slowly in a plastic manner. The crust is much thinner than any of the other layers, and is composed of the least dense calcium (Ca) and sodium (Na) aluminum-silicate minerals. Being relatively cold, the crust is rocky and brittle, so it can fracture in earthquakes. How was the Earth's core discovered? Recordings of seismic waves from earthquakes gave the first clue. Seismic waves will bend and reflect at the interfaces between different materials, just like the prism below refracts and scatters light waves at its faces.In addition, the two types of seismic wave behave differently, depending on the material. Compressional P waves will travel and refract through both fluid and solid materials. Shear S waves, however, cannot travel through fluids like air or water. Fluids cannot support the side-to-side particle motion that makes S waves. The fact that the Earth has a magnetic field is an independent piece of evidence for a molten, liquid core. A compass magnet aligns with the magnetic field anywhere on the Earth. The earth cannot be a large permanent magnet, since magnetic minerals lose their magnetism when they are hotter than about 500 degrees C. Almost all of the earth is hotter, and the only other way to make a magnetic field is with a circulating electric current. Circulation and convection of electrically conductive molten iron in the Earth's outer core produces the magnetic field. To make the magnetic field, the convection must be relatively rapid (much faster than it is in the plastic mantle), so the core must be fluid. Much of the energy to drive this convection comes from growth of the solid inner core, with the release of energy as the iron changes from solid to liquid.
[edit] Reason
The evidence for the composition of the core is all indirect because no means have yet been devised for directly sampling the deep interior of the Earth. The moment of inertia of the Earth indicates that there is a concentration of mass around the centre, and seismic data have shown that below the Wiechert–Gutenberg Discontinuity the density of the material is high, ranging upwards from 9.7. The only heavy element with high
cosmic abundance is iron, and because an iron–nickel alloy is an important meteorite component, it is reasonable to conclude that the Earth’s core consists largely of metallic iron with a minor admixture of other elements. This conclusion is supported by geophysical evidence that indicates that the mean atomic number of the material of the core is about 22. The atomic number of iron is 26, so this implies that the core also contains elements of lower atomic number. Sulfur, with atomic number 16, and carbon, 6, are relatively abundant in meteoritic matter, and the presence of minor amounts of these elements in the core would effectively
reduce the mean atomic number. Some authorities have advocated silicon (atomic number 14) as the major alloying component in the core, but this seems less likely; if silicon were the sole alloying element, then the core would have to contain more than 30 percent silicon in order to reduce its mean atomic number to 22. In addition, free silicon requires extremely reducing conditions (lack of oxygen), and the presence of ferrous iron in the mantle is inconsistent with this requirement.
It is not possible to give definite figures for the abundances of the elements in the Earth’s core. It is certainly made up largely of metallic iron, however, probably with some nickel, a little cobalt, and appreciable ammount of such lighter element as carbon and sulphur.
[edit] Related Articles
- "We know more about the surface of the sun than the deep earth," says Rich Muller of the Lab's Physics Division, a professor of physics at UC Berkeley. "We can probe it by seismography and by looking at heat signatures, and we have the evidence from changes in the magnetic field. But mostly it's a mystery."
- Most scientists agree that Earth's magnetic field arises from convection currents in the liquid outer core, a good conductor of electricity. These currents constitute an amplifying, self-sustaining "geodynamo."
- Convection probably starts as iron crystallizes on the surface of the inner core, about 5,000 kilometers beneath Earth's surface; lighter components like oxygen, sulfur, and silicon are excluded and rise toward the core-mantle boundary (CMB) 2,000 kilometers higher, where temperatures are a thousand degrees cooler.
- Here the lighter components cool and condense as slushy sediments. Muller theorizes that tens of meters of these buoyant sediments accumulate each million years, "falling" upward onto the uneven topography at the base of the mantle. Even if the slopes of the CMB hills are shallow, like sand dunes, eventually the sediments will slip and slide.
- "It's a little like an avalanche on the sea floor, where mud mixes with water, causing turbidity flow," Muller says. The turbid mixture of cool sediments and hot liquid iron causes cooled-off, denser iron to sink back toward the inner core.
- The sinking iron would perturb the geodynamo's convection cells, causing frequent "excursions" of the dipolar magnetic field as measured at the surface. This normal process of sediment accumulation and slippage probably goes on all the time.
- Rare events could trigger really big avalanches at the CMB, however. When a massive asteroid or comet slammed into Earth's surface at an oblique angle, the lower mantle would jerk sideways, shearing off whole mountains of sediment. As the sediments slide up, a downward-sinking mass of cool iron could completely disrupt large convection cells. Although variously oriented local fields within the core would remain strong, at the surface Earth's dipole magnetic field would collapse.
[edit] References
1. Earth's Interior
2. The Earth's core
3. Fact about earth's core
