Editing Isostasy (section)

==Implications==

===Deposition and erosion===
When large amounts of sediment are deposited on a particular region, the immense weight of the new sediment may cause the crust below to sink. Similarly, when large amounts of material are eroded away from a region, the land may rise to compensate. Therefore, as a mountain range is eroded, the (reduced) range rebounds upwards (to a certain extent) to be eroded further. Some of the rock strata now visible at the ground surface may have spent much of their history at great depths below the surface buried under other strata, to be eventually exposed as those other strata eroded away and the lower layers rebounded upwards.{{sfn|Kearey|Klepeis|Vine|2009|pp=45-46}}

An analogy may be made with an [[iceberg]], which always floats with a certain proportion of its mass below the surface of the water. If snow falls to the top of the iceberg, the iceberg will sink lower in the water. If a layer of ice melts off the top of the iceberg, the remaining iceberg will rise. Similarly, Earth's lithosphere "floats" in the asthenosphere.{{sfn|Kearey|Klepeis|Vine|2009|p=43}}<ref name="Monroe1992">{{cite book |last1=Monroe |first1=James S. |title=Physical geology : exploring the Earth |date=1992 |publisher=West Pub. Co |location=St. Paul |isbn=0314921958 |page=305}}</ref>

===Continental collisions===
When continents collide, the continental crust may thicken at their edges in the collision. It is also very common for one of the plates to be underthrust beneath the other plate. The result is that the crust in the collision zone becomes as much as {{convert|80|km||sp=us}} thick,{{sfn|Kearey|Klepeis|Vine|2009|p=322}} versus {{convert|40|km||sp=us}} for average continental crust.{{sfn|Kearey|Klepeis|Vine|2009|p=19}} As noted [[#Airy|above]], the Airy hypothesis predicts that the resulting mountain roots will be about five times deeper than the height of the mountains, or 32&nbsp;km versus 8&nbsp;km. In other words, most of the thickened crust moves ''downwards'' rather than up, just as most of an iceberg is below the surface of the water.<!--[[WP:CALC]]-->

However, convergent plate margins are tectonically highly active, and their surface features are partially supported by dynamic horizontal stresses, so that they are not in complete isostatic equilibrium. These regions show the highest isostatic anomalies on the Earth's surface.{{sfn|Kearey|Klepeis|Vine|2009|p=48}}

=== Mid-ocean ridges ===
Mid-ocean ridges are explained by the Pratt hypothesis as overlying regions of unusually low density in the upper mantle.{{sfn|Kearey|Klepeis|Vine|2009|p=48}} This reflects thermal expansion from the higher temperatures present under the ridges.<ref>{{cite book |last1=Philpotts |first1=Anthony R. |last2=Ague |first2=Jay J. |title=Principles of igneous and metamorphic petrology |date=2009 |publisher=Cambridge University Press |location=Cambridge, UK |isbn=9780521880060 |edition=2nd |pages=6–10}}</ref>

=== Basin and Range ===
In the [[Basin and Range Province]] of western North America, the isostatic anomaly is small except near the Pacific coast, indicating that the region is generally near isostatic equilibrium. However, the depth to the base of the crust does not strongly correlate with the height of the terrain. This provides evidence (via the Pratt hypothesis) that the upper mantle in this region is inhomogeneous, with significant lateral variations in density.{{sfn|Kearey|Klepeis|Vine|2009|p=48}}

===Ice sheets===
{{Main|Post-glacial rebound}}
The formation of [[ice sheets]] can cause Earth's surface to sink. Conversely, isostatic post-glacial rebound is observed in areas once covered by ice sheets that have now melted, such as around the [[Baltic Sea]]<ref>{{cite journal |last1=Eronen |first1=Matti |last2=Gluckert |first2=Gunnar |last3=Hatakka |first3=Lassi |last4=Van de Plassche |first4=Orson |last5=Van der Plicht |first5=Johannes |last6=Rantala |first6=Pasi |date=28 June 2008 |title=Rates of Holocene isostatic uplift and relative sea-level lowering of the Baltic in SW Finland based on studies of isolation contacts |url=https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1502-3885.2001.tb00985.x |journal=[[Boreas (journal)|Boreas]] |volume=30 |issue=1 |pages=17–30 |doi=10.1111/j.1502-3885.2001.tb00985.x |s2cid=54582233 |access-date=15 November 2022}}</ref> and [[Hudson Bay]].<ref>{{cite journal |last1=Balestra |first1=Barbara |last2=Bertini |first2=Adele |last3=De Vernal |first3=Anne |last4=Monechi |first4=Simonetta |last5=Reale |first5=Viviana |date=1 October 2013 |title=Late Quaternary sea surface conditions in the Laurentian Fan: Evidence from coccolith and dinocyst assemblages |url=https://www.sciencedirect.com/science/article/abs/pii/S0031018213003234#! |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=387 |pages=200–210 |doi=10.1016/j.palaeo.2013.07.002 |bibcode=2013PPP...387..200B |access-date=15 November 2022|url-access=subscription }}</ref> As the ice retreats, the load on the [[lithosphere]] and [[asthenosphere]] is reduced and they ''rebound'' back towards their equilibrium levels.  In this way, it is possible to find former [[sea cliff]]s and associated [[wave-cut platform]]s hundreds of metres above present-day [[sea level]]. The rebound movements are so slow that the uplift caused by the ending of the last [[glacial period]] is still continuing.{{sfn|Kearey|Klepeis|Vine|2009|pp=45-46}}

In addition to the vertical movement of the land and sea, isostatic adjustment of the Earth also involves horizontal movements.<ref>{{cite journal |last1=James |first1=Thomas S. |last2=Morgan |first2=W. Jason |title=Horizontal motions due to post-glacial rebound |journal=Geophysical Research Letters |date=June 1990 |volume=17 |issue=7 |pages=957–960 |doi=10.1029/GL017i007p00957|bibcode=1990GeoRL..17..957J }}</ref> It can cause changes in Earth's [[Earth's gravity|gravitational field]]<ref>{{cite journal |last1=Alexander |first1=J. C. |title=Higher harmonic effects of the Earth's gravitational field from post-glacial rebound as observed by Lageos |journal=Geophysical Research Letters |date=November 1983 |volume=10 |issue=11 |pages=1085–1087 |doi=10.1029/GL010i011p01085|bibcode=1983GeoRL..10.1085A }}</ref> and [[Rotation of Earth|rotation rate]], [[polar wander]],<ref>{{cite journal |last1=Wahr |first1=John |author-link=John M. Wahr |last2=Dazhong |first2=Han |last3=Trupin |first3=Andrew |last4=Lindqvist |first4=Varna |title=Secular changes in rotation and gravity: Evidence of post-glacial rebound or of changes in polar ice? |journal=Advances in Space Research |date=November 1993 |volume=13 |issue=11 |pages=257–269 |doi=10.1016/0273-1177(93)90228-4|bibcode=1993AdSpR..13k.257W }}</ref> and [[earthquake]]s.<ref>{{cite book |last1=Davenport |first1=Colin A. |last2=Ringrose |first2=Philip S. |last3=Becker |first3=Amfried |last4=Hancock |first4=Paul |last5=Fenton |first5=Clark |title=Earthquakes at North-Atlantic Passive Margins: Neotectonics and Postglacial Rebound |chapter=Geological Investigations of Late and Post Glacial Earthquake Activity in Scotland |date=1989 |pages=175–194 |doi=10.1007/978-94-009-2311-9_11|isbn=978-94-010-7538-1 }}</ref>

===Lithosphere-asthenosphere boundary===
The hypothesis of isostasy is often used to determine the position of the [[lithosphere]]-[[asthenosphere]] boundary (LAB).<ref>{{cite journal |last1=Grinč |first1=M. |last2=Zeyen |first2=H. |last3=Bielik |first3=M. |year=2014 |title=Automatic 1D integrated geophysical modelling of lithospheric discontinuities: a case study from Carpathian-Pannonian Basin region |journal=Contributions to Geophysics and Geodesy |volume=44 |number=2 |pages=115–131 |doi=10.2478/congeo-2014-0007 |bibcode=2014CoGG...44..115G |s2cid=129497623 |url=https://journal.geo.sav.sk/cgg/article/download/109/104 |access-date=13 December 2021|doi-access=free }}</ref>