The expanding Earth or growing Earth hypothesis asserts that the position and relative movement of continents is at least partially due to the volume of the Earth increasing. Conversely, geophysical global cooling was the hypothesis that various features could be explained by the earth contracting.
Different forms of the hypothesis
There are 3 forms of the expanding earth hypothesis.
- Earth's mass has remained constant, and thus the gravitational pull at the surface has decreased over time;
- Earth's mass has grown with the volume in such a way that the surface gravity has remained constant;
- Earth's gravity at its surface has increased over time, in line with its hypothesized growing mass and volume;
Expansion with constant mass
During the second voyage of HMS Beagle, in 1834–1835 Charles Darwin hypothesized that an expanding earth could explain the elevation of the landmass of South America as shown by mountain building in the Andes and stepped plains featuring raised beaches in Patagonia. Later in 1835 he abandoned this idea, and proposed that as mountains rose, the ocean floor subsided.
In 1889 and 1909 Roberto Mantovani published a hypothesis of earth expansion and continental drift. He assumed that a closed continent covered the entire surface of a smaller earth. Thermal expansion led to volcanic activity, which broke the land mass into smaller continents. These continents drifted away from each other because of further expansion at the rip-zones, where oceans currently lie. Although Alfred Wegener noticed some similarities to his own hypothesis of continental drift, he did not mention earth expansion as the cause of drift in Mantovani's hypothesis.
A compromise between earth-expansion and earth-contraction is the "theory of thermal cycles" by Irish physicist John Joly. He assumed that heat flow from radioactive decay inside the Earth surpasses the cooling of the Earth's exterior. Together with British geologist Arthur Holmes, Joly proposed a hypothesis in which the Earth loses its heat by cyclic periods of expansion. In their hypothesis, expansion led to cracks and joints in the Earth's interior, that could fill with magma. This was followed by a cooling phase, where the magma would freeze and become solid rock again, causing the Earth to shrink.
The late Australian geologist S. Warren Carey advocated expansion from the 1950s (before the development of plate tectonics provided the generally accepted explanation of the movement of continents) to his death, denying subduction and other events that could balance the sea-floor spreading at oceanic ridges. Bruce Heezen initially interpreted his work on the mid-Atlantic ridge as supporting S. Warren Carey's Expanding Earth Theory, but later withdrew his support. The remaining proponents after the 1970s, like the Australian geologist James Maxlow, are mainly inspired by Carey's ideas.
In 1888 Ivan Osipovich Yarkovsky suggested that some sort of aether is absorbed within the earth and transformed into new chemical elements, forcing the celestial bodies to expand. This was connected with his mechanical explanation of gravitation. Also the thesis of Ott Christoph Hilgenberg (1933, 1974) and Nikola Tesla were based on absorption and transformation of aether-energy into normal matter.
S. Warren Carey, starting in 1956, proposed some sort of mass increase in the planets and said that a final solution to the problem is only possible in a cosmological perspective in connection with the expansion of the universe.
Decrease of the gravitational constant
Paul Dirac suggested in 1938 that the universal gravitational constant had decreased in the billions of years of its existence. This led German physicist Pascual Jordan to a modification of general relativity and to propose in 1964 that all planets slowly expand. Contrary to most of the other explanations this one was at least within the framework of physics considered as a viable hypothesis. 
Measurements of a possible variation of the gravitational constant showed an upper limit for a relative change of 5•10−12 per year, excluding Jordan's idea.
The theory had never developed a plausible and verifiable mechanism of action, but neither had any of its competing theories. During the 1960s, the theory of plate tectonics made all other theories obsolete following the discovery of subduction, which was found to be an important part of a mechanism of action.
Generally, the scientific community finds that there is no evidence in support of the Expanding Earth theory, and there is evidence against it:
- Measurements with modern high-precision geodetic techniques show that the Earth is not currently increasing in size to within a measurement accuracy of 0.2 mm per year. The lead author of the study stated "Our study provides an independent confirmation that the solid Earth is not getting larger at present, within current measurement uncertainties". The motions of tectonic plates and subduction zones measured by a large range of geological, geodetic and geophysical techniques supports plate tectonics.
- Mass accretion on a scale required to change the Earth's radius is contradicted by the current accretion rate of the Earth, and by the Earth's average internal temperature: any accretion releases a lot of energy, which would warm the planet's interior.
- Expanding Earth models based on thermal expansion contradict most modern principles from rheology, and fail to provide an acceptable explanation for the proposed melting and phase transitions.
- Paleomagnetic data has been used to calculate that the radius of the Earth 400 million years ago was 102 ± 2.8 percent of today's radius.
- Examinations of data from the Paleozoic and Earth's moment of inertia suggest that there has been no significant change of earth's radius in the last 620 million years.
Present day advocates
In 2005 J. Marvin Herndon postulated what he calls whole-earth decompression dynamics, which he describes as a unified theory combining elements of plate tectonics and earth expansion. He suggests that Earth formed from a Jupiter-sized gas giant by catastrophic loss of its gaseous atmosphere with subsequent decompression and expansion of the rocky remnant planet resulting in decompression cracks at continental margins which are filled in by basalts from mid-ocean ridges.
Another present day advocate of an expanding Earth is comics artist Neal Adams, who suggests the Earth is growing and not merely expanding, and proposes his ideas within a "Growing Earth-Growing Universe" Theory. Adams has made video animations that graphically illustrate his hypothesis, in which new mass is manufactured by a hypothesized electron/positron pair production process within the core of the Earth and all celestial bodies.
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- Media related to Expanding Earth at Wikimedia Commons
- G. Scalera: Roberto Mantovani an Italian defender of the continental drift and planetary expansion
- G. Scalera, Braun: Ott Christoph Hilgenberg in twentieth-century geophysics
- G. Scalera: Samuel Warren Carey – Commemorative memoir
- Andrew Alden: Warren Carey, Last of the Giants
- G. Scalera: The expanding Earth: a sound idea for the new millennium
- New Concepts in Global Tectonics
- Database of Expansion Tectonic Scientists, living and deceased
- Chris Rowan, Supercontinent cycles 3, Expanding Earth 0
- a Neal Adams's video on YouTube, Neal Adams's channel on YouTube