EARTH HISTORY FROM THE PERSPECTIVE OF A GEODYNAMICIST

Earth scientists attempt to understand present day geological features or the state of the earth's interior in terms of physics or chemistry. But the large-scale rhythms seen in the rock record are not something we usually attempt to rigorously explain in terms of physics. By linking historical geology and deep earth geophysics, I show that we can start to understand the large scale tempo of solid-earth activity, a fundamental part of the earth system.

First, consider large-scale geological events, such as the massive outpouring of magma or the development of sedimentary basins, each occurring at a particular place and at a particular time. We often attempt to explain such processes generically, but never discretely. The magma may have come from a mantle plume, but why did that plume impinge where and when it did ? Why didn't it happen on a different continent or at a different time ? Such questions seem beyond our predictive abilities.

Although apparently unrelated, recall the debate concerning the depth of mantle convection: Does flow associated with plate tectonics extend down to the base of the upper mantle or down to the core mantle boundary? Although seismic images of the whole mantle are now sufficiently well resolved through the whole mantle, the problem has not been obviously solved, because such images reveal enormous, unexpected complexity.

We can now start to understand many events seen in the geological record, as well as mantle complexity, if we think of the largest solid part of the earth as a rock formation with a geological history. The exquisite images of the mantle can now be interpreted with Dynamic Earth Models (DEMs). Such DEMs, spherical models of thermal convection through the solid earth with plate tectonics as the surface manifestation, are akin to GCMs used in studies of the oceans and atmosphere. We will see how such methods and ideas shed new light on the geological record.