solid inner core must be at the melting temperature of the core (Question 4), although the temperature is not known precisely due to uncertainty in the composition of the core and the difficulties of exploring these high temperatures in the laboratory. The temperature of the most important changes of seismic wave velocity in the mantle, which happen at depths of about 400 and 660 km, is well constrained by laboratory studies of the conversion of olivine and pyroxene to higher density minerals. These phase transformations are so drastic that they can influence mantle convection; a phase transformation that causes a large change in density can work either for or against the thermal buoyancy that drives convection.
Although the effects of phase transitions on mantle convection are generally appreciated, we still do not know how the natural system actually works—for example, the extent to which the phase transitions impede or enhance the sinking of subducted slabs or change the size and shape of mantle plumes as they rise. A previously unknown phase transformation was recently discovered at pressures well beyond those previously probed (Murakami et al., 2004). The new transformation, from perovskite, the main mineral structure of the deep mantle, to a higher pressure postperovskite form, occurs at the top of the D″ region, an anomalous zone above the core-mantle boundary (corresponding to some 100-GPa pressure) that exhibits intriguing and highly variable seismological features (see Question 4), some of which may be caused by the transformations.
What is the melting temperature of rocks under pressure? Much of what we know about how Earth’s interior works is based on knowledge of the melting temperature of rock and metal, and how this temperature changes with pressure (Question 4). To expand this knowledge,