CPT Q. 023: Why is the mantle not stably stratified and hence resistant to any vertical flow of rock?

Q. 23. Why should density differences due to temperature differences even matter in the mantle when the density differences among the different minerals are so large by comparison? Would not the minerals rise and sink in accordance with their specific gravities during the 1650 pre-Flood years to establish a stable vertical density stratification which lithospheric slabs afterward would be incapable of penetrating?

Response: The answer actually is quite simple. The rock material of the mantle is in the solid, not liquid, state, except for some extremely tiny pockets near the surface where some partial melting is taking place. Where the rock is solid, no density stratification can occur! Almost everywhere in the mantle, the ultramafic rock consists of an interlocking matrix of solid mineral grains. Moreover, the rock consists of a mixture of different mineral types. In the upper mantle above the transition zone, for example, the dominant mineral types are olivine and pyroxene, but there are commonly smaller amounts of several other minerals such as garnet. In the lower mantle, the dominant minerals are magnesiowuestite and silicate perovskite. As one goes deeper into the mantle, the mineral assemblages change because of the increasing pressure, but available observational evidence indicates that the bulk chemical composition remains very close to uniform.

Even in the small volumes where partial melting is taking place, for example, just below spreading ridges and about 100 km deep just above the top side of a subducting plate, the rock is still, for the most part, a coherent solid of interlocking mineral grains. In these volumes, it is only the minerals with the very lowest melting points that are melting. The remainder of the crystals in the solid matrix are unmelted and solid. Typically, the degree of melting in these special places in the uppermost mantle is only on the order of a few percent.

How do we know that the mantle is solid and that the rock is strong? The primary evidence comes from seismology. Waves of the type known as shear waves propagate throughout the entire mantle. From the shear wave speeds we observe, it is possible to determine the shear modulus of the rock material and to conclude that, on the time scale of seismic waves, the rock strength is comparable to that of high grade steel. By contrast, shear waves fail to propagate at all in the outer core, that is, in the region just below the mantle. From this we infer with a high level of certainty that the outer core is liquid. From its density and other characteristics, in view of high pressure/temperature measurements we can make in the laboratory, we can further infer that its composition is largely molten iron.

Significant chemical contrast does exist, however, between the mantle and the crust. While the oceanic crust is basaltic in composition and derived from the partial melting of mantle rock, continental crust is radically different from mantle rock. Its density (about 2700-2800 kg/m3) is about 15-18% less than that of mantle rock (about 3300 kg/m3), and it tends to have dramatically higher concentrations of what are called incompatible elements. Such elements are referred to as incompatible because they do not conveniently fit into the lattices of most common rock-forming mantle minerals. These incompatible elements includes those with a large ionic radius, such as potassium, rubidium, cesium, strontium, and barium, as well as those with large ionic valences such as zirconium, niobium, hafnium, the rare earth elements, thorium, uranium and tantalum. Because of its high concentration of incompatible elements, continental crust typically displays at least 100 times the level of the main radioactive elements relative to rock from the mantle.

In summary, why is the mantle not stably stratified according to mineral density? The basic answer seems to be that God created it nearly chemically homogeneous and solid. Being solid and nearly homogeneous, there is no significant tendency for denser minerals within a small local parcel of rock to sink relative to the others.