The Effects of High Pressure

Pressure increases with depth in the earth, to a maximum at the centre. The rate of increase depends on the material present along the path followed; it is lower, for example, in sea water than in rocks. But a useful generalization, adequate for most petrologic purposes to moderate depth, is that the pressure increases by about 3 kbar for every 10 km. Inverting this, depth increases at a rate of about 3.3 km/kbar. A bar is defined as 105 pascals and is about equal to an atmosphere of pressure. The bar and its multiples are therefore convenient units for Earth science, ranging from millibars (mb) in the atmosphere through bars in the oceans, kilobars in the crust and upper mantle, and megabar (Mbar) in the deep mantle and core. The core–mantle boundary lies at a pressure of about 1.27 Mbar (127 GPa) and a temperature about 3500 K. Despite Earthly convenience, the uniform code for scientific publication is based on multiples of Pascals. Some Earthly journals still accept kbar for upper-crustal pressures. A summary of Earth pressures and mantle minerals versus depth is given in Table 18.1.