Geology Reference
In-Depth Information
pressuresinthelowermantlearesohigh—hundredsofthousandsoftimesthesurfacepres-
sure—thatsilicon-oxygenbondsadoptanevendenser,moreefficientpackingarrangement
of atoms called perovskite.
Seismic studies document the nature and extent of each of these mineralogically distinct
mantle layers, and by and large the transitions from one to the next are neat and tidy.
The exact depths of the transitions vary slightly by ten or twenty miles from place to
place—beneath the continents versus the oceans, for example—but each boundary ap-
pears to be relatively smooth and well behaved. By contrast, seismology provides tantaliz-
ing evidence to suggest that the core-mantle boundary is an especially complicated zone,
ratherdifferentfromthecleanmantle-mantletransitions.Toafirstapproximation,thecore-
mantle boundary produces the expected strong echo. Indeed, the density contrast between
silicate mantle and metal core is so extreme as to create a physical boundary as sharp as
that between air and water—producing the strongest reflected seismic signal from Earth's
deep interior. More than a century ago, that divide was one of the first hidden features of
Earth's deep interior that seismologists discovered.
Aperfectlysmoothandregularboundarywouldproduceasharp,focusedseismicreflec-
tion—an echoing response that could be recorded as a distinctive spike on a seismomet-
er. But seismic signals reflecting off the core-mantle boundary are often messy, smeared
out, broken up. There's extra structure down there, like irregular lumps or piles of debris.
Geophysicists, who are not always known for employing the catchiest terminology, call
this lumpy chaotic zone the D´´ (D double prime) layer. (Astrophysicists, who coined such
imaginative terms as
brown dwarf, red giant, dark energy,
and
black hole,
are rather more
successful at the scientific name game.)
The complexity of this deep D´´ feature is in part the result of the sharp density contrast
between the core's homogeneous iron metal and the mantle's varied oxygen-rich minerals.
All mantle minerals float on the dense core like corks on water, but these diverse minerals
can differ widely in their densities. In the primordial magma ocean, some silicates sank,
andsomefloated.Asaresult,bigchunksoftheearliestcrystallizedsolidssankthroughthe
mantle all the way down, to float like rafts on the metal core. Some seismologists envision
three-hundred-mile-high “mountains” with irregular piles of dense minerals resting on the
core-mantle boundary, where they chaotically deflect seismic signals.
Surprisingly, there may also be big core-mantle boundary puddles and ponds of unusu-
ally dense silicate liquid, perhaps rich in the elements aluminum and calcium as well as a
slew of “incompatible elements” that seem to be missing from inventories of Earth's outer
layers. We have no easy way to be sure, but seismologists point to deep, localized “ultra-
low velocity zones,” in the D´´ layer just above the core-mantle boundary, where seismic
waves travel about 10 percent slower than they do in adjacent rocks. Slow seismic waves
are often a telltale sign of liquid. Such deep liquid lakes and ponds could also provide a
