Chemical changes are all based on the increased kinetic energies of the ions.
Heat as an agent of metamorphism Temperatures increase with depth at a rate known as the geothermal gradient. Hence, degree of metamorphism increases as depth increases.
The deepest mine in the world The Western Deep Levels Mine in South Africa at 4 km is the deepest in the world. At these depths, the rock is actually hot enough to burn human skin. Miners work in pairs- one digging, the other operating a large fan to keep cool.
Primordial heat – trapped when Earth’s crust first cooled
Radioactivity – energy released when subatomic particles are spontaneously emitted by radioactive elements
Solar radiation – absorbed and converted to thermal energy by the Earth’s surface
Primordial heat Earth initially formed as a molten ball of rock. When the surface cooled to become the crust, the heat became trapped. This trapped heat now drives the convection cells in the mantle and the movement of the molten outer core round the inner core.
Solar radiation – the Earth’s crust absorbs about 50% of the Sun’s energy
The absorbed solar radiation is distributed unevenly. The greatest concentration occurs in the tropical belt (red zone).
Radioactivity Radioactive elements contribute to Earth’s internal heat. Background radiation (not enough to harm) exists all around us. Shown at right is a geologist measuring background radiation with a portable Geiger counter.
Pressure as an agent of metamorphism Two types of pressure:
Confining pressure – experienced by buried rocks; forces are applied equally in all directions
Differential stress – forces are unequal in different directions
Confining pressure produces rock layers that are undeformed.
Directonial stress deforms rock layers as shown in C. (the series shows the deformation of sediments deposited in a river flood plain)
The layers of rock shown below have been deformed by directional stress. Such pressure is most active in convergent plate boundaries.
Rocks in shallow depths are pulverized when subjected to differential stress. At greater depths, on the other hand, rocks are more ductile and are compressed and elongated rather than crushed. A sample of conglomerate becomes metaconglomerate when subjected to differential stress at depth (note the elongated rock fragments).
Heat and pressure Heat increases with increasing pressure. This means that areas of mountain building (continental vs continental convergent plates) are hotter due to greater pressure.