A COMPARISON OF SMITH'S GEOLOGICAL MAPPING AND THARP/HEEZEN'S SEAFLOOR MAPPING: WHY DID THESE MAPS ENABLE NEW THINKING ABOUT EARTH PROCESSES?

William Smith's geological map revolutionized humanity's ability to visualize a portion of the Earth's continental surface and thus make evidence-based inferences about the processes that had shaped the mapped terrain. Similarly, the seafloor maps of Marie Tharp and Bruce Heezen revolutionized humanity's ability to visualize the Earth's seafloor and make evidence-based inferences about causal processes. Smith's map is credited as part of the enabling foundation underlying Darwin's theory of natural selection, and the Tharp/Heezen map is credited as one of the enabling factors in the plate tectonic paradigm shift.

Both Smith and Tharp worked by assembling an enormous number of snippets of information into an organized spatial array. In Smith's case, the snippets were observations of the rock strata that were visible in limited exposures in outcrops and canal cuttings. In Tharp's case, the snippets were water depth measurements from sparse soundings made by primitive echo sounders, and roughly-located earthquake epicenters. They made maximum use of the cartographic technologies of their times—notebooks, large sheets of paper, colored pencils, pen and ink, and manual measuring devices—but in the absence of computers they must also have had remarkable capacity to use their human minds to encode, retrieve and synthesize vast quantities of spatial information. In both cases, the spatial coverage of available information was far less than the intervening data gaps, and the cartographers boldly bridged between available observations.

Why did organizing tiny snippets of information about the spatial distribution of attributes of the Earth's surface into a horizontal map array turn out to be so illuminating? In both cases, the potential for insight derived from the intimate relationship between space and time, with horizontal position on the Earth's surface conveying information about temporal sequence in geological time. On Smith's continental map, distance towards the southeast equates to transitioning through geologic time from the distant past towards the less distant past. On Tharp’s seminal Atlantic map, the earthquake strewn bathymetric notch of the mid-Atlantic Ridge corresponds to time zero, the present; distance east and west from that axis correspond to transitioning backwards through geologic time.