|Today's Mantle, a Guided Tour|
All of the evidence that I've been talking about in this series, all of the experiments and theoretical work, have put a lot of detail into that big blank space between the crust and the core. Let's see what we've got.
The clearest picture we have of the mantle is the one outlined by a century of seismic data. It shows that between the crust and the top of the core 2,888 kilometers down (give or take a few kilometers), there are several distinct boundariesscientists use the more neutral name "seismic discontinuities," and so will I. You can follow along on this diagram as we move downward. The mantle has at least four layers, and all of them are subjects of vigorous scientific discussion.
The Upper Mantle
The crust, of course, is a scrambled mess of rock types, and so is the uppermost mantle beneath it. The strong discontinuity at the bottom of the crust is named to honor the seismologist Andrija Mohorovičić, the man who found it early in this century. I've never once heard a geologist call it the Mohorovičić (mo-ho-ROV-i-chich) discontinuity, but that's its proper name. Everyone calls it the Moho. The rock above and below the Moho is solid, but as we go deeper the upper mantle gradually turns softthat soft zone is what allows the tectonic plates of the crust to move about. (In plate tectonics, the soft zone is the asthenosphere and the hard rocks above it make up the lithosphere.)
The soft zone bottoms out around 220 km depth, but as with the rocks above, there's a lot of variation in the material there. That comes from the processes of plate tectonicsdownward subduction of plates at the deep ocean trenches and formation of plates at the mid-ocean ridges. Slabs of subducted plates going downward are cooler than the rock around them, and in other places hotspots are rising. The upper mantle is as busy in its own way as the surface, constantly mixing and being stirred like stew simmering.
The Transition Zone
There is a worldwide seismic discontinuity at about 410 km depth, which has no name yet, just "the 410-km discontinuity." And there's another at 660 km (or 670 km, depending on who you askI prefer to round it off at 666 km). These mark the transition zone. Above is the upper mantle, beneath is the lower mantle. Earthquakes occur all the way down to 660 km, but not below.
These two discontinuities appear at those depths because they mark pressure thresholds, where the abundant mineral olivine suddenly changes to denser crystal forms. Mantle rock gains density by several percent at these discontinuities, and that density change affects the mixing action of the upper mantle. Descending crustal slabs tend to stall in the transition zone, and after a while, a few dozen million years, they mix with surrounding rocks and return to circulation. Many argue that some slabs do seem to push down past the 660-km level, and some deeper rocks may cross it upward, so it's not a complete barrier. Opponents say that the barrier prevents any exchange of material, although exchange of heat takes place. On the whole it appears that most of the mantle's physical and chemical activity is confined to the upper part.
The Lower Mantle and D''
Beneath 660 km, there's nothing much in the seismic picture. The lower mantle appears to have little structure. Many researchers claim to see old crustal slabs there, falling toward the base of the mantle, but the evidence is subtle and disputed. Others argue that the high pressure prevents the rock of the lower mantle from convecting at all. Basic ideas about the Earth's heat budget depend strongly on the lower mantle. Some argue that the minerals of the lower mantle are large enough and clear enough that heat moves through there not by conduction or convection, but mainly by radiation.
Near the bottom of the lower mantle, at 2,700 km or so, we find a strange pair of discontinuities. The top one has no name at all, and the lower one is just called "the core-mantle boundary" or CMB. The rock in between seems to be quite different from the mantle above it. Part of that may, again, reflect a change in mineral structure from the so-called perovskite structure to a recently discovered form, called post-perovskite for the time being. Beneath this layer lies the liquid outer core, an iron-nickel alloy. This lowermost part of the mantle is called D'', "D-double-prime," for lack of a better name (here's the whole story).
The scientific picture of D'' is fuzzy, so I'll be a bit loose describing it. The most popular idea is that it's the dregs of the Earth, a place where slabs from the crust come to die and where iron-silicate slag builds up along the edge of the core. Slow stirrings in the deepest mantle, and chaotic iron swirlings in the core's magnetic dynamo, both seem to push the stuff of D'' into heaps here and thin spots there. Sometimes, it is conjectured, this allows a huge pulse of heat energy to rise from the core, like the activity of a Lava Lamp. The whole mess is squeezed at 135 gigapascals (20 million pounds per square inch) and is white-hot. If there is a Hell on Earth, D'' is where it is, and maybe "hell" is the name we should give it.
The names of all these features are an unsatisfying mishmosh. But there's a deeper discontinuity left, one with a proper name that adds pleasing symmetry to the picture. The discontinuity between the liquid outer core and the solid inner core, 5,150 km down, is named the Lehmann discontinuity for Inge Lehmann, the seismologist who found it early in the 1900s. The name was given recently, but Lehmann was still around, in her nineties, to enjoy the occasion.