A short history of nearly everything - Bill Bryson [107]
The crust and part of the outer mantle together are called the lithosphere (from the Greek lithos, meaning “stone”), which in turn floats on top of a layer of softer rock called the asthenosphere (from Greek words meaning “without strength”), but such terms are never entirely satisfactory. To say that the lithosphere floats on top of the asthenosphere suggests a degree of easy buoyancy that isn't quite right. Similarly it is misleading to think of the rocks as flowing in anything like the way we think of materials flowing on the surface. The rocks are viscous, but only in the same way that glass is. It may not look it, but all the glass on Earth is flowing downward under the relentless drag of gravity. Remove a pane of really old glass from the window of a European cathedral and it will be noticeably thicker at the bottom than at the top. That is the sort of “flow” we are talking about. The hour hand on a clock moves about ten thousand times faster than the “flowing” rocks of the mantle.
The movements occur not just laterally as the Earth's plates move across the surface, but up and down as well, as rocks rise and fall under the churning process known as convection. Convection as a process was first deduced by the eccentric Count von Rumford at the end of the eighteenth century. Sixty years later an English vicar named Osmond Fisher presciently suggested that the Earth's interior might well be fluid enough for the contents to move about, but that idea took a very long time to gain support.
In about 1970, when geophysicists realized just how much turmoil was going on down there, it came as a considerable shock. As Shawna Vogel put it in the book Naked Earth: The New Geophysics: “It was as if scientists had spent decades figuring out the layers of the Earth's atmosphere—troposphere, stratosphere, and so forth—and then had suddenly found out about wind.”
How deep the convection process goes has been a matter of controversy ever since. Some say it begins four hundred miles down, others two thousand miles below us. The problem, as Donald Trefil has observed, is that “there are two sets of data, from two different disciplines, that cannot be reconciled.” Geochemists say that certain elements on Earth's surface cannot have come from the upper mantle, but must have come from deeper within the Earth. Therefore the materials in the upper and lower mantle must at least occasionally mix. Seismologists insist that there is no evidence to support such a thesis.
So all that can be said is that at some slightly indeterminate point as we head toward the center of Earth we leave the asthenosphere and plunge into pure mantle. Considering that it accounts for 82 percent of the Earth's volume and 65 percent of its mass, the mantle doesn't attract a great deal of attention, largely because the things that interest Earth scientists and general readers alike happen either deeper down (as with magnetism) or nearer the surface (as with earthquakes). We know that to a depth of about a hundred miles the mantle consists predominantly of a type of rock known as peridotite, but what fills the space beyond is uncertain. According to a Nature report, it seems not to be peridotite. More than this we do not know.
Beneath the mantle are the two cores—a solid inner core and a liquid outer one. Needless to say, our understanding of the nature of these cores is indirect, but scientists can make some reasonable assumptions. They know that the pressures at the center of the Earth are sufficiently high—something over three million times those found at the surface—to turn any rock there solid. They also know from Earth's history (among other clues) that the inner core is very