Quantitative plate tectonics. Physics of the Earth - plate kinematics – geodynamics / Количественная тектоника плит. Физика Земли - плитная кинематика - геодинамика

Plate tectonics focuses on the dynamics and kinematics of the most external layers of the solid Earth: the relatively thin oceanic crust (7–10 km thickness), the continental crust (5–70 km thickness), the mantle lithosphere, whose lower boundary can be found at depths ranging between 80 and 250 km, the underlying asthenosphere (up to 410 km depth), the transition zone (410–670 km depth), and the very huge lower mantle, which extends to a depth of 2,900 km (Fig. 1.1). All these rock layers are formed by solid-state mixtures of minerals that are chemically and structurally stable only within determined intervals of pressure and temperature. Rocks can eventually contain liquid phases in the existing pores between grains or in cracks.In general, chemical composition, crystalline structure, and the physical state of Earth materials change from point to point, so that
rocks are strongly heterogeneous systems. However, any rock system can be ideally resolved into a finite number of phases that are physically and chemically homogeneous. For example, seismology studies and research on high-temperature (HT) and high-pressure (HP) minerals indicate that the upper mantle mostly consists of two phases: Mg2SiO4 (the fosterite end-member of olivine) and Mg2Si2O6 (orthopyroxene). The chemical composition of mineral assemblages is usually described by the socalled components,which represent the minimum number of chemical formulae that are needed to describe the set of phases composing the rock. For example, both olivine and orthopyroxene can be represented by a mixture of MgO and SiO2, because Mg2SiO4 D2MgOCSiO2 and Mg2Si2O6 D2MgOC2SiO2. We emphasize that this decomposition is arbitrary and represents only a useful way to describe the chemical composition of a rock through its constitutive elements, independently from the real crystalline structure of the mineral phases included in the solid state mixture. <...>