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.