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EARTH'S INTERIOR Because most of the Earth is not directly accessible, Earth scientists must use a combination of remote sensing techniques, inverse theory, theoretical models, and laboratory experiments to infer the properties of the Earth's interior.
The crust ranges from 6 to 70 kilometers thick; it is the most accessible, and thus, best studied part of the Earth. The crust is primarily composed of rocks and minerals similar to those exposed at the surface. The deepest mine is only about 12 kilometers below the surface of the Earth, so our knowledge of the composition of even the lower part of the crust is from indirect observation. The spherical shell from about 50-2,900 kilometers depth, comprised of magnesium-iron silicates, is called the mantle, and it accounts for about 80% of the volume of the Earth. As one descends into the mantle, pressure and temperature increase rapidly. Pressure increases primarily due to the overlying mass of rock (hydrostatic pressure) and temperature increases both from heat generated by the decay of radioactive elements and heat generated during its formation and trapped deep in the Earth.At 2,900 km depth, there is an abrupt change in both density and seismic velocity. This marks a chemical boundary between silicate minerals and the liquid iron outer core.  The most important constraint on the composition of the interior of the Earth comes from measuring the velocity of seismic waves generated in large earthquakes. In actuality, seismologists measure arrival times of seismic wave energy and invert of the velocity structure of the Earth through which the wave traveled. Seismic velocities generally increase as a function of depth, and the lateral variations in velocity decrease with depth. This is generally taken to mean that the composition of the Earth becomes more uniform with depth. Geochemical models based on cosmochemical abundances of elements provide an important constraint on the composition of the deep interior of the Earth. The Earth is believed to have a bulk composition similar to that of primitive chondritic meteorites. These meteorites have elemental abundance ratios similar to the ratios of the rock-forming elements (Mg, Fe, Si, Al, Ni, O) in the Sun's outer corona and show no indication of chemical alteration since their formation nearly 4.5 billion years ago. Models of solar system formation suggest that small planetesimal bodies, ranging in size from from meters to kilometers formed from the condensing solar nebula. The primitive chondritic meteorites are believed to represent samples of the primitive solar nebula, thus providing a sample of the bulk composition of the starting material for the terrestrial planets. It is generally accepted that the Earth formed by the collision of many smaller planetesmal bodies. These collisions produce an enormous amount of heat, perhaps enough to melt the entire planet early in the history of the solar system. In the mantle at present, pressures range from 10-1,000 Mbars and temperatures range from 1,000-5,000 degrees Celsius. Based on the moment of inertia, bulk density, and seismic properties of the Earth, we know that the core must be composed of a dense element, (primarily iron), plus a small fraction of some lighter element (oxygen, sulfur, and silicon are all candidates). The fact that seismic shear-waves do not travel through the core tells us that at least the outer part of the core is a liquid. The gradual freezing of the inner core releases gravitational and thermal energy which drives flow in the outer part of the core. Because liquid iron is an electrically conducting fluid, fluid flow in the outer core generates the Earth's magnetic field by a dynamo process. An excellent overview of the interior of the Earth has been put together by Prof. J. Louie. Another excellent resource is in the Views of the Solar System site. The page describing the Earth's interior from that site is here. |
