The End of D-Double-Prime?

A new name for the core-mantle boundary

There must be no branch of science with more names for invisible things than geology. The whole long history of Earth, for instance, is a gigantic hierarchy of eons, eras, periods, epochs, ages, and stages, each one known by a specific name. There are the P waves and S waves of seismology, and their various reflections within the Earth. There are the thousands of formally named rock units of stratigraphy—the Smith Formations and Jones Groups of the world, very few of them obvious to the eye.

And there are the Richter magnitude and the Mercalli local intensity of earthquakes.

The layer of the Earth's deep interior known as "D-double-prime" may have the oddest name of them all.

How the D Layer Got Its Double-Prime

I first learned about the name 20 years ago, when I started following studies of the mantle. Where's D-single-prime, I wondered. Where's C or B, for that matter? No one ever said—it was ancient history. Every other part of the Earth has a word for its name, from the crust through the mantle to the inner core. But thanks to NASA's Benjamin Chao and the article he wrote in the 1 February 2000 Eos, I know that Keith Bullen gave it that name and I learned why.

Bullen was a New Zealander who lived from 1906 to 1976, publishing nearly 300 papers during his career. One of his great contributions was to build mathematical models of the Earth's interior, beginning in the 1930s. At that time we knew that Earth has a dense iron core surrounded by a thick rocky mantle, but he set out to add detail to the picture.

Using the mechanical calculators of the time, Bullen laboriously worked out the seismic properties needed to account for the way the Earth bends earthquake waves.

To do his math, Bullen divided the planet into a series of shells or layers, labeled A through G. The crust was layer A and the inner core was G.

In the first model he published, in 1942, the whole lower mantle, 2000 kilometers in thickness, was the D layer. By 1950 he had figured that the lower mantle was actually two very different layers, the lower one being a zone just a couple hundred kilometers thick. So in good mathematical fashion he divided D into D' and D"—D-prime and D-double-prime.

The Troubles of D"

Since that time the community of scientists has used the names we already had instead of Bullen's zones. The crust, upper mantle, transition zone, lower mantle, and inner and outer core fell neatly into his scheme. But that thin D" layer never got its own name, and in recent years it has gained quite a lot of attention. Chao's article recounts all this, and also mentions the trouble the name caused typesetters and indexers.

Not to mention the readers, unless you were already a seismologist. Seismologists had no problem in 1936 when Nature magazine published the famous paper by Inge Lehmann with the shortest title of all: "P'." They knew just from the author and title that Lehmann was reporting on an unusual P-wave she had detected. (It was the first evidence of the Earth's inner core.) But relying on the impenetrable name D" was practically a guarantee that no one outside the field would ever take an interest.

The thing really needs an actual name.

Hell or Bullen?

One workaround long used by the deep-Earth community is to call this zone "the core-mantle boundary" or the CMB. But a boundary is a surface with no thickness, so this otherwise useful name has its basis in a fatal flaw. D" is a transitional zone between the mantle and core. But we already have a "transition zone" between the upper and lower mantle, so that possibility is out too.

A few years ago I suggested in a whimsical way that we should call this layer Hell instead of D". But Chao has a better idea. Call it the Bullen Layer, he says. "In so doing, rather than taking anything away from the originator, we pay tribute to the great seismologist, along with other well-known namesakes in seismology: Rayleigh and Love waves, Slichter and Stoneley modes, Gutenberg-Richter scale, Jeffreys-Bullen table, Benioff-Wadati zone, Mohorovicic discontinuity, and Lehmann discontinuity." So go read Bullen's biography and see if you agree that he should join the pantheon.

PS: Bullen also put a layer inside the core between F, the liquid outer core, and H, the solid inner core. His G layer is known today as the Lehmann discontinuity after another great seismologist, Inge Lehmann. She was lucky enough to enjoy the honor, around the time she turned 100.