Editing Geophysics (section)

=== Gravity ===
[[Image:Earth gravity.png|thumb|A map of deviations in gravity from a perfectly smooth, idealized Earth|alt=Image of globe combining color with topography.]]
{{Main|Gravity of Earth}}
{{further|Physical geodesy|Gravimetry}}
The gravitational pull of the Moon and Sun gives rise to two high tides and two low tides every lunar day, or every 24 hours and 50 minutes. Therefore, there is a gap of 12 hours and 25 minutes between every high tide and between every low tide.<ref>{{harvnb|Ross|1995|pp=236–242}}</ref>

Gravitational forces make rocks press down on deeper rocks, increasing their density as the depth increases.<ref name=Poirier>{{harvnb|Poirier|2000}}</ref> Measurements of [[gravitational acceleration]] and [[gravitational potential]] at the Earth's surface and above it can be used to look for mineral deposits (see [[gravity anomaly]] and [[gravimetry]]).<ref name=Telford/> The surface gravitational field provides information on the dynamics of [[tectonic plates]]. The [[geopotential]] surface called the [[geoid]] is one definition of the shape of the Earth. The geoid would be the global mean sea level if the oceans were in equilibrium and could be extended through the continents (such as with very narrow canals).<ref name=Lowrie/>