Geochemists have worked closely with cosmologists to obtain a better understanding of the chemical composition of the universe and Earth-Moon system in the field of cosmochemistry. These cosmochem-ists have produced models for how the chemical composition of the universe began, initially consisting almost exclusively of hydrogen and helium, and evolved through stellar processes and supernovas to include heavier elements. studies of meteorites have yielded data on the average composition of the rocky planets in the inner solar system, and these data have been used to derive a model for the average composition of the Earth. The Earth is thought to have an overall composition close to that of a carbonaceous chondrite meteorite. The planet experienced early heating due to the decay of short-lived radioactive isotopes and heat from gravitational compaction and the collection in the core of metallic phases that sink and release heat. This early heating formed a melt phase within the Earth, and the outer core is still molten, as shown by seismic waves. This causes a chemical and density zonation in the Earth.
The chemical and density zonation has broken the Earth into several different shells with different properties. These layers include the crust, upper mantle, transition zone, lower mantle, outer core, and inner core. The Earth's magnetic field is formed by motions in the liquid outer core, where a dynamo effect generates a magnetic field by the motion of electrically charged fluid. The lower mantle has a composition that includes magnesium and iron silicate concentrations, similar to chondritic meteorites.
There are a couple of major polymorphic transitions within the Earth that are of great importance for determining overall Earth structure and behavior. At 250 miles (400 km) depth pyroxene minerals attain a garnet structure by the increase in pressure from higher in the Earth, but they maintain a pyroxene composition. Also the crystal lattice structure of olivine—one of the most abundant elements in the mantle—changes from an olivine to a denser crystal lattice structure known as a spinel structure at the same depth. Deeper, at 435 miles (700 km), the garnet and spinel structures react together and change into progressively denser crystal lattices including ilmenite, then to a perovskite structure, still retaining the olivine and pyroxene composition, although possibly with more iron present.
Geochemists have determined that the Earth is not yet completely chemically fractionated. Volcanoes continue to release gases such as sulfur and carbon dioxide, and deep eruptions such as those
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