Consider the example of global atmospheric circulation. Certainly, the existence and orientation of trade winds, a fundamental observation, was known from long ago. Yet the northward counterflow at high altitude cannot have been documented observationally by those pioneers; they must have inferred it and included it in their mental picture because the picture did not make physical sense without it. Some of the great leaps of visualization of three-dimensional Earth processes have progressed by a hybrid process, in which the visualization is partly shaped by data and observations and partly by intuitive understanding of driving forces. Modern models of oceanic and atmospheric circulation try to formalize this hybrid cognitive process. The models are built around equations that purport to represent physical forces, and then data are used to “train” the models through a process of “data assimilation.”

Envisioning the Processes by Which Objects Change Shape

Whereas the fluid parts of the Earth system generally respond to imposed forces by moving through space, the solid parts may respond by changing their shape, by deforming, by folding, and by faulting. After struggling to visualize the internal three-dimensional structure of a rock body, the geologist’s next step is often to try to figure out the sequence of folding and faulting events that has created the observed structures. This task may be attacked either forwards or backwards: backwards, by “unfolding” the folds and “unfaulting” the faults; or forwards, by applying various combinations of folds and faults to an initially undeformed sequence of rock layers until you find a combination that resembles the observed structure (Figure 3.27). The forward approach resembles the paper-folding task used by psychologists who study spatial thinking (see Chapter 2).

Elements of solid Earth also change their shape through erosion, through the uneven removal of parts of a whole. Thinking about eroded terrains requires the ability to envision negative spaces, the shape and internal structure of the material that is no longer present.

Using Spatial Thinking to Think About Time

It is common in thinking about Earth to find that variation or progression through space is closely connected with variation or progression through time. For example, within a basin of undeformed sedimentary rocks, the downward direction corresponds to increasing time since deposition. On the seafloor, distance away from the mid-ocean ridge spreading center corresponds to increasing time since the formation of that sliver of seafloor.

As a consequence, geologists often think about distance in space when they really want to think

FIGURE 3.27 Mentally manipulating a volume by folding, faulting, and eroding: geometry of the lower duplex at Hénaux. SOURCE: Ramsay and Huber, 1987. Copyright 1987, reprinted with permission from Elsevier.

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