Abstract. During the first half of 20th century, the dominant global tectonics model based on Earth contraction had increasing problems accommodating new geological evidence, with the result that alternative geodynamic theories were investigated. Due to the level of scientific knowledge and the limited amount of data available in many scientific disciplines at the time, not only was contractionism considered a valid scientific theory but the debate also included expansionism, mobilism on a fixed-dimension planet, or various combinations of these geodynamic hypotheses. Geologists and physicists generally accepted that planets could change their dimensions, although the change of volume was generally believed to happen because of a contraction, not an expansion. Constant generation of new matter in the universe was a possibility accepted by science, as it was the variation in the cosmological constants. Continental drift, instead, was a more heterodox theory, requiring a larger effort from the geoscientists to be accepted.

The new geological data collected in the following decades, an improved knowledge of the physical processes, the increased resolution and penetration of geophysical tools, and the sensitivity of measurements in physics decreased the uncertainty level in many fields of science. Theorists now had less freedom for speculation because their theories had to accommodate more data, and more limiting conditions to respect. This explains the rapid replacement of contracting Earth, expanding Earth, and continental drift theories by plate tectonics once the symmetrical oceanic magnetic striping was discovered, because none of the previous models could explain and incorporate the new oceanographic and geophysical data.

Expansionism could survive after the introduction of plate tectonics because its proponents have increasingly detached their theory from reality by systematically rejecting or overlooking any contrary evidence, and selectively picking only the data that support expansion. Moreover, the proponents continue to suggest imaginative physical mechanisms to explain expansion, claiming that scientific knowledge is partial, and the many inconsistencies of their theory are just minor problems in the face of the plain evidence of expansion. According to the expansionists, scientists should just wait for some revolutionary discovery in fundamental physics that will explain all the unsolved mysteries of Earth expansion.

The history of the expanding-Earth theory is an example of how falsified scientific hypotheses can survive their own failure, gradually shifting towards and beyond the limits of scientific investigation until they become merely pseudoscientific beliefs.