Editing Isostasy (section)

==History of the concept==
In the 17th and 18th centuries, French [[geodesist]]s (for example, [[Jean Picard]]) attempted to determine the shape of the Earth (the [[geoid]]) by measuring the [[length of a degree of latitude]] at different latitudes ([[arc measurement]]). A party working in [[Ecuador]] was aware that its [[plumb line]]s, used to determine the vertical direction, would [[Plumb line deflection|be deflected]] by the gravitational attraction of the nearby [[Andes Mountains]]. However, the deflection was less than expected, which was attributed to the mountains having low-density roots that compensated for the mass of the mountains. In other words, the low-density mountain roots provided the buoyancy to support the weight of the mountains above the surrounding terrain. Similar observations in the 19th century by British surveyors in [[India]] showed that this was a widespread phenomenon in mountainous areas. It was later found that the difference between the measured local gravitational field and what was expected for the altitude and local terrain (the [[Bouguer anomaly]]) is positive over ocean basins and negative over high continental areas. This shows that the low elevation of ocean basins and high elevation of continents is also compensated at depth.<ref>{{cite book |last1=Kearey |first1=P. |last2=Klepeis |first2=K.A. |last3=Vine |first3=F.J. |title=Global tectonics. |date=2009 |publisher=Wiley-Blackwell |location=Oxford |isbn=9781405107778 |edition=3rd |page=42}}</ref>

The American geologist [[Clarence Dutton]] use the word 'isostasy' in 1889 to describe this general phenomenon.<ref name=Dutton1882/><ref name=Orme2007/><ref name=cdutton1958/> However, two hypotheses to explain the phenomenon had by then already been proposed, in 1855, one by [[George Airy]] and the other by [[John Henry Pratt]].{{sfn|Kearey|Klepeis|Vine|2009|p=43}} The Airy hypothesis was later refined by the Finnish geodesist [[Veikko Aleksanteri Heiskanen]] and the Pratt hypothesis by the American geodesist [[John Fillmore Hayford]].<ref name=Watts2001/>

Both the Airy-Heiskanen and Pratt-Hayford hypotheses assume that isostacy reflects a local hydrostatic balance. A third hypothesis, [[lithospheric flexure]], takes into account the rigidity of the Earth's outer shell, the [[lithosphere]].{{sfn|Kearey|Klepeis|Vine|2009|pp=44-45}} Lithospheric flexure was first invoked in the late 19th century to explain the shorelines uplifted in Scandinavia following the melting of [[continental glacier]]s at the end of the [[Last Glacial Period|last glaciation]]. It was likewise used by American geologist [[G. K. Gilbert]] to explain the uplifted shorelines of [[Lake Bonneville]].<ref>{{cite journal |last1=Gilber |first1=G.K. |title=Lake Bonneville |journal=U.S. Geological Survey Monograph |date=1890 |volume=1 |doi=10.3133/m1}}</ref> The concept was further developed in the 1950s by the Dutch geodesist [[Vening Meinesz]].<ref name=Watts2001/>