Wednesday, October 03, 2012

Of oceans and crystals

kw: geology, earth, crystal chemistry

In a number of recent discussions some basic questions of Earth science have arisen. Putting together a number of ideas, I realized that on the grossest scale, the Earth consists of three oceans and five crystals.

From outermost inward:
  • The first ocean is the atmosphere. This is of geophysical and biological interest, but air is not a "rock". However, atmospheric processes have a lot to do with the movement of mineral materials on and in the upper parts of the crust.
  • The second ocean is "the Ocean", primarily the saltwater that covers 71% of the surface to an average depth of 4 km. Low-salinity and fresh waters make up a fraction of a percent, and ice constitutes more than half of that. When I lived in South Dakota, we thought of snow as a special kind of sand, and just drove on it. The interaction of the water ocean with oceanic crust produces continental crust.
  • The first crystal is the continental crust. To a first approximation, it is composed of feldspar with impurities. It averages about 20 km thick, ranging up to 40 km or so beneath large mountain ranges. Why do I call it a crystal? Most of the "rock forming" minerals are actually a framework of oxygen ions held together with metal ions, chiefly silicon, aluminum, magnesium and iron. Due to the activity of water and living things on shallow minerals, there is also a considerable component of calcium oxide, AKA limestone. But this is also a framework of oxygen held together by calcium ions. This immense oxygen has a comparatively low density, around 2.6. It was distilled from the minerals in the next crystal by tectonic activity coupled with water's dissolving power.
  • The second crystal is the oceanic crust. It is primarily a mixture of olivine and pyroxene, which are also oxygen frameworks held together by the same metallic ions, but in a denser configuration, with a density of around 3.2 to 3.4. The thickness of oceanic crust is about 8 km, but quite variable. It is formed by expulsion of mantle materials into the depths of the water ocean by tectonic activity.
  • The next two crystals constitute the mantle. The upper mantle grades from primarily pyroxene to enstatite, as the pressure and temperature increase downward. The density of enstatite approaches 4. 
  • The lower mantle is primarily magnesium silicate with the structure of Perovskite, and its density reaches 6 in the very deep mantle. This mineral is a close approximation to a close-packed structure. Think of cannon balls stacked in a 3-sided pyramid. This is "cubic closest packing" (CCP); the pyramid is a corner cut off a cube. Each ball is touching 12 others. In high-pressure Perovskite, each oxygen is touching eight other oxygens and four magnesium (or iron) ions, which at this pressure are similar in size to oxygen. Together they form a CCP network with the tiny silicon ions fitting into the spaces between.
  • Next we have the third ocean, the outer core. This is generally considered a liquid, particularly because it will not transmit S-type seismic waves, only P-waves. However, it is a stiff liquid, and a chunk of it suddenly brought to the lab would sit there (burning a hole in even an asbestos bench) and flow even more slowly than cold molasses – although that might change very rapidly once the huge pressure is released. Nonetheless, its fluid motions are apparently what create the Earth's magnetic field. It is composed of iron and nickel, with up to 10% sulfur. Although it is less than a tenth of Earth's volume, with a density more than 10, it is more than a quarter of the mass of the planet.
  • Finally, the fifth crystal is the inner core. Also composed of iron and nickel, it probably has much less of a sulfur constituent. It is crystalline, and measurements of its seismic anisotropy indicate it may indeed be a single huge (2,400 km diameter) metallic crystal. It is not known if its rotation rate exactly matches that of the rest of the Earth. Its density is about 15, and its temperature may exceed 6,000K, or 11,000°F. Subtracting the amount of heat we think is generated within the earth by radioactive elements (U, Th and K, mainly), it is thought that this central crystal is growing at a rate of a few cm per century.
To sum up simply: an ocean of air, an ocean of salty water, four crystals consisting of ever-denser packings of oxygen held by metal ions, an ocean of nickel-iron, and a metallic crystal.

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