Fig. 1.21. Metamorphic facies of Winkler (Szakmány 2008b)

1.5. 1.5. Movement processes in the lithosphere

Movements of lithosphere can be originate in two main processes. First one is isostasy, while the other is plate tectonics.

Isostasy is a term used in geology to refer to the state of gravitational equilibrium between the earth's lithosphere and asthenosphere such that the tectonic plates "float" at an elevation which depends on their thickness and density. This concept is invoked to explain how different topographic heights can exist at the Earth's surface. When a certain area of lithosphere reaches the state of isostasy, it is said to be in isostatic equilibrium. Isostasy is not a process that upsets equilibrium, but rather one which restores it (a negative feedback). It is generally accepted that the earth is a dynamic system that responds to loads in many different ways. However, isostasy provides an important 'view' of the processes that are happening in areas that are experiencing vertical movement (Fig. 1.22).

Plate tectonics is a scientific theory that describes the large-scale motions of Earth's lithosphere. The lithosphere is broken up into tectonic plates. On Earth, there are seven major plates (depending on how they are defined) and many minor plates. Mechanically, the lithosphere is cooler and more rigid, while the asthenosphere is hotter and flows more easily. In terms of heat transfer, the lithosphere loses heat by conduction, whereas the asthenosphere also transfers heat by convection and has a nearly adiabatic temperature gradient. This division should not be confused with the chemical subdivision of these same layers into the mantle and the crust. Three types of plate boundaries exist. They are associated with different types of surface phenomena. The different types of plate boundaries are:

1.   Divergent boundariesoccur where two plates slide apart from each other. Mid-ocean ridges (e.g., Mid-Atlantic Ridge) and active zones of rifting (such as Africa's East African Rift) are both examples of divergent boundaries.

2.   Convergent boundaries(or active margins) occur where two plates slide towards each other commonly forming either a subduction zone (if one plate moves underneath the other) or a continental collision (if the two plates contain continental crust). Deep marine trenches are typically associated with subduction zones, and the basins that develop along the active boundary are often called "foreland basins". The subducting slab contains many hydrous minerals, which release their water on heating; this water then causes the mantle to melt, producing volcanism. Examples of this are the Andes mountain range in South America and the Japanese island arc.

Rocks are under pressure in geological units because of the moving of lithosphere plates. These pressure calles stress. In geology, stress is a physical quantity that expresses the internal forces that neighboring particles of rocks exert on each other. For example, when a solid vertical bar is supporting a weight, each particle in the bar pulls on the particles immediately above and below it. These forces are actually the average of a very large number of intermolecular forces and collisions between the molecules in those particles. The answere of rock blocks for these stress call straine. According to the type of strain, two main structural form can be divide.