Editing Geoid (section)

==Relationship to mass density==
{{further|Gravity anomaly}}
[[File:Gravity, geoid anomaly synthetic cases with local isostasy 2.gif|thumb|[[Gravity anomaly|Gravity]] and Geoid anomalies caused by various crustal and lithospheric thickness changes relative to a reference configuration. All settings are under local [[isostasy|isostatic]] compensation.]]

Variations in the height of the geoidal surface are related to anomalous density distributions within the Earth. Geoid measures thus help understanding the internal structure of the planet. Synthetic calculations show that the geoidal signature of a thickened crust (for example, in [[orogen|orogenic belts]] produced by [[continental collision]]) is positive, opposite to what should be expected if the thickening affects the entire [[lithosphere]]. Mantle convection also changes the shape of the geoid over time.<ref>{{cite journal |last1=Richards |first1=M. A. |first2=B. H. |last2=Hager |year=1984 |title=Geoid anomalies in a dynamic Earth |journal=Journal of Geophysical Research |volume=89 |issue=B7 |pages=5987–6002 |doi=10.1029/JB089iB07p05987|bibcode=1984JGR....89.5987R }}</ref>

[[File:Gravity anomalies on Earth.jpg|thumb|Three-dimensional visualization of [[Gravity anomaly|gravity anomalies]] in [[Gal (unit)|units of Gal.]], using [[pseudo color]] and [[shaded relief]].]]

The surface of the geoid is higher than the [[reference ellipsoid]] wherever there is a positive [[gravity anomaly]] or negative [[disturbing potential]] (mass excess) and lower than the reference ellipsoid wherever there is a negative gravity anomaly or positive disturbing potential (mass deficit).<ref>{{cite book|last=Fowler|first=C.M.R.|author-link=Mary Fowler (geologist)|title=The Solid Earth; An Introduction to Global Geophysics|date=2005|publisher=[[Cambridge University Press]]|location=United Kingdom|isbn=9780521584098|page=214}}</ref>

This relationship can be understood by recalling that gravity potential is defined so that it has negative values and is inversely proportional to distance from the body.
So, while a mass excess will strengthen the gravity acceleration, it will decrease the gravity potential. As a consequence, the geoid's defining equipotential surface will be found displaced away from the mass excess.
Analogously, a mass deficit will weaken the gravity pull but will increase the geopotential at a given distance, causing the geoid to move towards the mass deficit.

The presence of a localized inclusion in the background medium will rotate the gravity acceleration vectors slightly towards or away from a denser or lighter body, respectively, causing a bump or dimple in the equipotential surface.<ref name="Lowrie 1997 p. 50">{{cite book | last=Lowrie | first=W. | title=Fundamentals of Geophysics | publisher=Cambridge University Press | year=1997 | isbn=978-0-521-46728-5 | url=https://books.google.com/books?id=7vR2RJSIGVoC&pg=PA50 | access-date=2022-05-02 | page=50}}</ref>

The largest absolute deviation can be found in the [[Indian Ocean Geoid Low]], 106 meters below the average sea level.<ref name="Raman 2017">{{cite web | last=Raman | first=Spoorthy | title=The missing mass -- what is causing a geoid low in the Indian Ocean? | website=GeoSpace | date=2017-10-16 | url=https://blogs.agu.org/geospace/2017/10/16/missing-mass-causing-geoid-low-indian-ocean/ | access-date=2022-05-02}}</ref>
Another large feature is the North Atlantic Geoid High (or North Atlantic Geoid Swell), caused in part by the weight of ice cover over North America and northern Europe in the [[Late Cenozoic Ice Age]].<ref name="x153">{{cite journal | last1=Carminati | first1=Eugenio | last2=Doglioni | first2=Carlo | title=North Atlantic geoid high, volcanism and glaciations | journal=Geophysical Research Letters | volume=37 | issue=3 | date=2010 | issn=0094-8276 | doi=10.1029/2009GL041663 | page=| bibcode=2010GeoRL..37.3302C }}</ref>