How does a scientist know how to draw isochrons to connect basalt flows on land with the ocean floor

 Sea-floor spreading — In the early 1960s, Princeton geologist Harry Hess proposed the hypothesis of sea-floor spreading, in which basaltic magma from the mantle rises to create new ocean floor at mid-ocean ridges. ... A test of the hypothesis of sea-floor spreading was provided by studies of the Earth's magnetism.

The Beginning of 1960s, Princeton geologist Harry Hess suggested the sea-floor spreading theory, in which basaltic magma rises from the mantle to create new ocean floor on mid-ocean ridge. Studies of the Earth's magnetism have provided a test of the sea-floor spreading hypothesis.

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81 is a composite number.