Читать книгу Engineering Physics of High-Temperature Materials - Nirmal K. Sinha - Страница 22

The top section of Earth's crustal layer mainly consists of water, rocks, and ice. Rocks, depending on their types and mineral contents, have components of crystalline and amorphous phases. Plummer and McGeary (1985) provide a simple, but comprehensive, description of geology that will be useful for readers. Silica (SiO₂) is the most abundant component of rocks, and the common extrusive rocks are classified into silica‐rich (felsic) rocks and silica‐poor (mafic) rocks. Ice or the solid state of water (H₂O) is crystalline and belongs to the hexagonal system in Earth's crust; however, individual ice crystals are extremely pure because ice lattice is very intolerant to inclusions. Water, of course, is a liquid with amorphous structure, but oceans in the Arctic and Antarctic regions can be “ice rich” (often described for wrong reasons as “ice‐infested”), depending on the season.

Interestingly, viscous molten rocks or magma at extremely high temperatures at great depths below Earth's surface could be similar in behavior to ice‐rich water (specially with thick layers containing frazil ice crystals) on Earth's crust. The structure and fluidity of magma depends on location, depth, and temperature. As temperature and pressure decrease with decrease in depth, the viscosity of magma and its constitution change. The viscosity of the mass depends on its structure, because different types of crystals form depending on the eutectic points and float, separated or trapped within the melt, and are extruded if possible. Structural features of magma are very complex simply because of pressure, thermal state, and different mineral contents with wide‐ranging concentrations. It is impossible to conduct in situ studies of magma, but laboratory investigations are often performed on quenched melts or glasses of simple compositions as analogs (Henderson et al. 2006; Greaves and Sen 2007).