Caption: Abyssal hills, which run parallel to the mid-ocean rift
(yellow, center) in this seafloor map, formed when lower sea levels led to large magma outpourings at mid-ocean ridges.
When she inked in and cross hatched the spaces, the topographical profile of the North Atlantic was evident, with mountains and plains and a very obvious mid-ocean rift
valley indicating continental drift (a horrifying concept for geologists to grasp in 1952).
Only the largest of these quakes are detectable by seismic stations on land, but they are part of a continuous mountain building process along the 50,000-kilometer-long mid-ocean rift
that circles the globe.