About 2,900 kilometers away--less than three days drive, if that were possible--lies Earths most dramatic structure. Largely ignored in past research, the remote region between the lowermost mantle and the upper core is proving to be crucial in understanding the chemical and thermal evolution of the planet. No longer regarded as simply a contact delineating the liquid-iron outer core from the rocky mantle, the core-mantle region may actually be the most geologically active zone of Earth. Its features seem to have changed immensely during Earth's history, and its physical properties vary from place to place near the bottom surface of the mantle. In fact, the physical changes across the interface between the core and mantle are more pronounced than those across the planetary surface separating air and rock.
The strong heterogeneity of the core-mantle boundary region is thought to influence many global-scale geologic processes [see "The Earth's Mantle," by D. P. McKenzie; Scientific American, September 1983]. The dynamics of the zone affect the slight wobbling of Earths axis of rotation and characteristics of the geomagnetic field. Variations in the core-mantle region also modulate the convection in Earth's mantle, which is responsible for the movement of continents and tectonic plates.
The first hint that something unusual was going on at the depth where the core and mantle meet came in the mid-1930s. Vibrations generated by earthquakes provided the clue. Throughout most of the mantle, the speed of seismic waves increases as a function of depth. Furthermore, lateral variations in seismic-wave velocity are only minor. One can interpret these characteristics as meaning that Earth gets "simpler" with respect to depth--that is, the composition and structure of the planet become more uniform. In contrast, the great diversity of geologic structures and rocks observed underfoot reveal the surface to be the most complicated region.
Yet the velocity behavior of seismic waves holds only to a certain point. At the lowermost few hundred kilometers of the mantle, just before the core begins, the average speed of seismic waves does not increase appreciably, and more meaningful changes in velocity appear from region to region [see box on page 38]. The effect is subtle, amounting to only a few percent difference. Yet by geologic standards, these few percent represent enormous variations in structure, temperature or both. Early workers recognized the significance of the changes from the simple behavior in the overlying lower mantle and consequently named this region, which was deduced to be about 200 to 400 kilometers thick, the D" layer.
The origin of the layers name (pronounced "dee double prime") is more historic than poetic. Early geologists had labeled the parts of the deep Earth with letters of the alphabet, rather than as crust, mantle and core. This form of identification, however, meant that any intervening layer subsequently discovered had to incorporate a "prime" symbol to distinguish it. Although other layers were eventually renamed, the D" nomenclature has endured.
Investigators proposed numerous interpretations to account for the seismic properties of the D" layer. Unfortunately, there were far too many possible explanations and far too little information to permit a definitive characterization of the layer. Better descriptions of the D" layer had to wait until the technological breakthroughs of the 1980s. Then, using arrays of recording instruments deployed around the world, seismologists could for the first time collect and process enough data to derive three-dimensional images of Earth's interior. They used seismometers that primarily operate in the range between about one and 0.0003 hertz, or cycles per second. (These acoustic frequencies are far below the range of human hearing, which extends from about 20 to 20,000 hertz.) Seismic tomography is often compared to computed tomographic scans used in medicine. But because it relies on sound waves, seismic tomography is more akin to the ultrasonic imaging done during pregnancy. The main drawback is its resolution: images of features smaller than 2,000 kilometers tend to be smeared out.