North Atlantic physiographic diagram

About this item

Marie Tharp joined the Lamont Geological Observatory at Columbia University in 1948. Barred as a woman from the research vessels that gathered the data, she worked ashore converting echo-sounding records into drawn profiles of the seafloor, from soundings collected at sea by Bruce Heezen. She did not sail on a research cruise until the mid-1960s.

Around 1952, plotting several North Atlantic crossings side by side, she identified a narrow V-shaped valley running continuously along the crest of the mid-ocean ridge, in the same relative position on every profile. Heezen is reported to have initially dismissed the finding as “girl talk.” The dismissal reflected the stakes: a continuous axial rift is very hard to explain unless the seafloor is spreading, and continental drift was at that time a discredited position in American geology.

Independent confirmation came from within the same group, when Howard Foster’s plots of oceanic earthquake epicentres were overlaid on Tharp’s rift and the two coincided. Two unrelated instruments were indicating the same line. The first physiographic diagram of the North Atlantic was published under both names in 1957; Harry Hess proposed seafloor spreading a few years later, and the rift became its most legible evidence.

Tharp and Heezen published oblique, shaded “physiographic diagrams” rather than conventional bathymetric contour charts. Part of the reason was communicative. Part was that detailed ocean-floor contours had military value and were restricted; a perspective drawing conveyed the geology without publishing a navigable depth chart.

Why it is in the collection

Because the rift was not lying on the seabed waiting to be transcribed. It existed as a perceptible feature only once several hundred unremarkable depth readings were drawn in a form where a pattern could emerge. The map is doing the reasoning, which places it alongside Hooke’s plates and Cajal’s drawings in this collection.

It also documents a specific mechanism of misattribution. We read a paper as an argument with authors, and a map as a description of something already there. That habit made it easy to describe Tharp’s observation as draftsmanship in service of someone else’s insight, a framing only substantially corrected near the end of her life and after her death in 2006.