Editing Gravity anomaly (section)

==Causes==
[[File:Gravity, geoid anomaly synthetic cases with local isostasy 2.gif|thumb|480px|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 with an elevation of either +1000 or −1000 m above the reference level.]]
[[File:Nj cboug.jpg|thumb|right|237px|(Bouguer) gravity anomaly map of the state of [[New Jersey]] (USGS)]]
Lateral variations in gravity anomalies are related to anomalous density distributions within the Earth. Local measurements of the [[gravity of Earth]] help us to understand the planet's internal structure.

===Regional causes===

The Bouguer anomaly over continents is generally negative, especially over mountain ranges.{{sfn|Lowrie|2007|p=95}} For example, typical Bouguer anomalies in the [[Central Alps]] are −150 milligals.<ref>{{cite journal |last1=Werner |first1=Dietrich |last2=Kissling |first2=Eduard |title=Gravity anomalies and dynamics of the Swiss Alps |journal=Tectonophysics |date=August 1985 |volume=117 |issue=1–2 |pages=97–108 |doi=10.1016/0040-1951(85)90239-2 |bibcode=1985Tectp.117...97W }}</ref> By contrast, the Bouguer anomaly is positive over oceans. These anomalies reflect the varying thickness of the Earth's crust. The higher continental terrain is supported by thick, low-density crust that "floats" on the denser mantle, while the ocean basins are floored by much thinner oceanic crust. The free-air and isostatic anomalies are small near the centers of ocean basins or continental plateaus, showing that these are approximately in isostatic equilibrium. The gravitational attraction of the high terrain is balanced by the reduced gravitational attraction of its underlying low-density roots. This brings the free-air anomaly, which omits the correction terms for either, close to zero. The isostatic anomaly includes correction terms for both effects, which reduces it nearly to zero as well. The Bouguer anomaly includes only the negative correction for the high terrain and so is strongly negative.{{sfn|Lowrie|2007|p=95}}

More generally, the Airy isostatic anomaly is zero over regions where there is complete isostatic compensation. The free-air anomaly is also close to zero except near boundaries of crustal blocks. The Bouger anomaly is very negative over elevated terrain. The opposite is true for the theoretical case of terrain that is completely uncompensated: The Bouger anomaly is zero while the free-air and Airy isostatic anomalies are very positive.{{sfn|Kearey|Klepeis|Vine|2009|pp=45–48}}

The Bouger anomaly map of the Alps shows additional features besides the expected deep mountain roots. A positive anomaly is associated with the [[Ivrea]] body, a wedge of dense mantle rock caught up by an ancient continental collision. The low-density sediments of the [[Molasse basin]] produce a negative anomaly. Larger surveys across the region provide evidence of a relict subduction zone.{{sfn|Lowrie|2007|p=97}} Negative isostatic anomalies in Switzerland correlate with areas of active uplift, while positive anomalies are associated with subsidence.{{sfn|Lowrie|2007|p=103–105}}

Over [[mid-ocean ridge]]s, the free-air anomalies are small and correlate with the ocean bottom topography. The ridge and its flanks appear to be fully isostatically compensated. There is a large Bouger positive, of over 350 mgal, beyond {{convert|1000|km||sp=us}} from the ridge axis, which drops to 200 over the axis. This is consistent with seismic data and suggests the presence of a low-density magma chamber under the ridge axis.{{sfn|Lowrie|2007|pp=97–99}}

There are intense isostatic and free-air anomalies along [[island arc]]s. These are indications of strong dynamic effects in subduction zones. The free-air anomaly is around +70 mgal along the Andes coast, and this is attributed to the subducting dense slab. The trench itself is very negative,<ref>{{cite book |last1=Monroe |first1=James S. |last2=Wicander |first2=Reed |title=Physical geology : exploring the Earth |date=1992 |publisher=West Pub. Co |location=St. Paul |isbn=0314921958 |page=326}}</ref> with values more negative than −250 mgal. This arises from the low-density ocean water and sediments filling the trench.{{sfn|Lowrie|2007|p=99}}

Gravity anomalies provide clues on other processes taking place deep in the [[lithosphere]]. For example, the formation and sinking of a lithospheric root may explain negative isostatic anomalies in eastern [[Tien Shan]].<ref>{{cite journal |last1=Burov |first1=E. V. |last2=Kogan |first2=M. G. |last3=Lyon-Caen |first3=Hélène |last4=Molnar |first4=Peter |title=Gravity anomalies, the deep structure, and dynamic processes beneath the Tien Shan |journal=Earth and Planetary Science Letters |date=1 January 1990 |volume=96 |issue=3 |pages=367–383 |doi=10.1016/0012-821X(90)90013-N|bibcode=1990E&PSL..96..367B }}</ref> The Hawaiian gravity anomaly appears to be fully compensated within the lithosphere, not within the underlying aesthenosphere, contradicting the explanation of the Hawaiian rise as a product of aesthenosphere flow associated with the underlying mantle plume. The rise may instead be a result of lithosphere thinning: The underlying aesthenosphere is less dense than the lithosphere and it rises to produce the swell. Subsequent cooling thickens the lithosphere again and subsidence takes place.<ref>{{cite journal |last1=Detrick |first1=Robert S. |last2=Crough |first2=S. Thomas |title=Island subsidence, hot spots, and lithospheric thinning |journal=Journal of Geophysical Research |date=1978 |volume=83 |issue=B3 |pages=1236 |doi=10.1029/JB083iB03p01236|bibcode=1978JGR....83.1236D }}</ref>

===Local anomalies===

Local anomalies are used in [[applied geophysics]]. For example, a local positive anomaly may indicate a body of [[metal]]lic [[ore]]s. [[Salt dome]]s are typically expressed in gravity maps as lows, because [[salt]] has a low density compared to the rocks the dome intrudes.{{sfn|Monroe|Wicander|1992|pp=302–303}}

At scales between entire mountain ranges and ore bodies, Bouguer anomalies may indicate rock types. For example, the northeast-southwest trending high across central New Jersey represents a [[graben]] of [[Triassic]] age largely filled with dense [[basalt]]s.<ref>{{cite book |last1=Herman |first1=G.C. |last2=Dooley |first2=J.H. |last3=Monteverde |first3=D.H. |year=2013 |chapter=Structure of the CAMP bodies and positive Bouger gravity anomalies of the New York Recess |title=Igneous processes during the assembly and breakup of Pangaea: Northern New Jersey and New York City: 30th Annual Meeting of the Geological Association of New Jersey |publisher=College of Staten Island |location=New York |pages=103–142 |url=https://www.researchgate.net/publication/270216459 |access-date=29 January 2022}}</ref>

The largest continental gravity gradient in the world is found across the Woodroffe Thrust-Mann Fault Zone in central Australia, and is attributed to an upthrust of dense [[Earth's mantle|mantle]] material 30 km closer to the present land surface, which occurred during the 630–520 Ma [[Petermann Orogeny]].<ref>Raimondo, Tom. [https://www.abc.net.au/news/science/2017-07-22/five-places-that-mark-australias-extreme-geological-past/8728928 Five places that mark Australia's extreme geological past], ''[[ABC News (Australia)|ABC News - Science]]'', 22 July 2017. Retrieved 1 February 2025.</ref><ref>Aitken, A. R. A., Betts, P. G., Weinberg, R. F., Gray, D. (23 December 2009).  [https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2008JB006194 Constrained potential field modeling of the crustal architecture of the Musgrave Province in central Australia: Evidence for lithospheric strengthening due to crust-mantle boundary uplift] ''[[Journal of Geophysical Research]]''. {{doi|10.1029/2008JB006194}}</ref>