Editing Geophysics (section)

== Physical phenomena ==
Geophysics is a highly interdisciplinary subject, and geophysicists contribute to every area of the [[Earth sciences]], while some geophysicists conduct research in the [[planetary sciences]]. To provide a more clear idea on what constitutes geophysics, this section describes phenomena that are studied in [[physics]] and how they relate to the Earth and its surroundings. Geophysicists also investigate the physical processes and properties of the Earth, its fluid layers, and magnetic field along with the near-Earth environment in the [[Solar System]], which includes other planetary bodies.

=== 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/>

=== Vibrations ===
{{Main|Seismology}}
[[Image:Pswaves.jpg|thumb|upright=1.3|Illustration of the deformations of a block by body waves and surface waves (see [[seismic wave]]) |alt=Deformed blocks with grids on surface.]]
[[Seismic wave]]s are vibrations that travel through the Earth's interior or along its surface.<ref>{{Cite web |date=2024-01-12 |title=Seismic wave {{!}} Earth's Interior Structure & Movement {{!}} Britannica |url=https://www.britannica.com/science/seismic-wave |access-date=2024-02-18 |website=www.britannica.com |language=en}}</ref> The entire Earth can also oscillate in forms that are called [[normal modes]] or [[seismic wave#Normal modes|free oscillations of the Earth]]. Ground motions from waves or normal modes are measured using [[seismograph]]s. If the waves come from a localized source such as an earthquake or explosion, measurements at more than one location can be used to locate the source. The locations of earthquakes provide information on plate tectonics and mantle convection.<ref name=Shearer>{{cite book|last=Shearer|first=Peter M.|title=Introduction to seismology|year=2009|publisher=Cambridge University Press|location=Cambridge|isbn=9780521708425|edition=2nd}}</ref><ref name=Stein>{{harvnb|Stein|Wysession|2003}}</ref>

Recording of [[seismic wave]]s from controlled sources provides information on the region that the waves travel through. If the density or composition of the rock changes, waves are reflected. Reflections recorded using [[Reflection seismology|Reflection Seismology]] can provide a wealth of information on the structure of the earth up to several kilometers deep and are used to increase our understanding of the geology as well as to explore for oil and gas.<ref name=Telford>{{harvnb|Telford|Geldart|Sheriff|1990}}</ref> Changes in the travel direction, called [[Seismic refraction|refraction]], can be used to infer the [[Earth's interior|deep structure of the Earth]].<ref name=Stein/>

Earthquakes pose a [[Earthquakes#Effects/impacts of earthquakes|risk to humans]]. Understanding their mechanisms, which depend on the type of earthquake (e.g., [[Intraplate earthquake|intraplate]] or [[Deep focus earthquake|deep focus]]), can lead to better estimates of earthquake risk and improvements in [[earthquake engineering]].<ref name=Bozorgnia2004>{{harvnb|Bozorgnia|Bertero|2004}}</ref>

=== Electricity ===
Although we mainly notice electricity during [[thunderstorms]], there is always a downward electric field near the surface that averages 120 [[volt]]s per meter.<ref name=Harrison>{{harvnb|Harrison|Carslaw|2003}}</ref> Relative to the solid Earth, the ionization of the planet's atmosphere is a result of the galactic [[cosmic rays]] penetrating it, which leaves it with a net positive charge.<ref>{{Cite web |last=Nicoll |first=Keri |date=April 2016 |title=Earth's electric atmosphere |url=https://www.metlink.org/wp-content/uploads/2020/11/PhysRev25_4_Nicoll.pdf |access-date=February 18, 2024 |website=metlink.org}}</ref> A current of about 1800 [[ampere]]s flows in the global circuit.<ref name=Harrison/> It flows downward from the [[ionosphere]] over most of the Earth and back upwards through thunderstorms. The flow is manifested by lightning below the clouds and [[sprite (lightning)|sprite]]s above.

A variety of electric methods are used in geophysical survey. Some measure [[spontaneous potential]], a potential that arises in the ground because of human-made or natural disturbances. [[Telluric current]]s flow in Earth and the oceans. They have two causes: [[electromagnetic induction]] by the time-varying, external-origin [[geomagnetic field]] and motion of conducting bodies (such as seawater) across the Earth's permanent magnetic field.<ref>{{harvnb|Lanzerotti|Gregori|1986}}</ref> The distribution of telluric current density can be used to detect variations in [[electrical resistivity]] of underground structures. Geophysicists can also provide the electric current themselves (see [[induced polarization]] and [[electrical resistivity tomography]]).

=== Electromagnetic waves ===
[[Electromagnetic waves]] occur in the ionosphere and magnetosphere as well as in [[Earth's outer core]]. [[Dawn chorus (electromagnetic)|Dawn chorus]] is believed to be caused by high-energy electrons that get caught in the [[Van Allen radiation belt]]. [[Whistler (radio)|Whistlers]] are produced by [[lightning]] strikes. [[Hiss (electromagnetic)|Hiss]] may be generated by both. [[Electromagnetic radiation|Electromagnetic]] waves may also be generated by earthquakes (see [[seismo-electromagnetics]]).

In the highly conductive liquid iron of the outer core, magnetic fields are generated by electric currents through electromagnetic induction. [[Alfvén wave]]s are [[magnetohydrodynamic]] waves in the [[magnetosphere]] or the Earth's core. In the core, they probably have little observable effect on the Earth's magnetic field, but slower waves such as magnetic [[Rossby wave]]s may be one source of [[geomagnetic secular variation]].<ref name="Merrill">{{harvnb|Merrill|McElhinny|McFadden|1998}}</ref>

Electromagnetic methods that are used for geophysical survey include [[transient electromagnetics]], [[magnetotellurics]], [[surface nuclear magnetic resonance]] and electromagnetic seabed logging.<ref>{{cite book |last1=Stéphane |first1=Sainson |title=Electromagnetic seabed logging : a new tool for geoscientists |date=2017 |publisher=Springer |isbn=978-3-319-45355-2}}</ref>

=== Magnetism ===
{{further|Earth's magnetic field|Aeromagnetic survey|Paleomagnetism}}
The Earth's magnetic field protects the Earth from the deadly [[solar wind]] and has long been used for navigation. It originates in the fluid motions of the outer core.<ref name=Merrill/> The magnetic field in the upper atmosphere gives rise to the [[Aurora (astronomy)|auroras]].<ref name=Kivelson>{{harvnb|Kivelson|Russell|1995}}</ref>

[[Image:Geomagnetisme.svg|thumb|right|upright=1.1|Earth's dipole axis (pink line) is tilted away from the rotational axis (blue line). |alt=Diagram with field lines, axes and magnet lines.]][[File:Geodynamo Between Reversals.gif|thumb|left|Computer simulation of the [[Earth's magnetic field]] in a period of normal polarity between [[Geomagnetic reversal|reversals]]<ref>{{cite news |title=Earth's Inconstant Magnetic Field |url=https://science.nasa.gov/science-news/science-at-nasa/2003/29dec_magneticfield |access-date=13 November 2018 |work=science@nasa |publisher=National Aeronautics and Space Administration |date=29 December 2003 |language=en}}</ref>]]
The Earth's field is roughly like a tilted [[dipole]], but it changes over time (a phenomenon called geomagnetic secular variation). Mostly the [[geomagnetic pole]] stays near the [[geographic pole]], but at random intervals averaging 440,000 to a million years or so, the polarity of the Earth's field reverses. These [[geomagnetic reversals]], analyzed within a [[Geomagnetic reversal#Geomagnetic polarity time scale|Geomagnetic Polarity Time Scale]], contain 184 polarity intervals in the last 83 million years, with change in frequency over time, with the most recent brief complete reversal of the [[Laschamp event]] occurring 41,000 years ago during the [[Glacial period#Last glacial period|last glacial period]]. Geologists observed [[Geomagnetic reversal#History|geomagnetic reversal recorded]] in volcanic rocks, through [[Magnetostratigraphy#Correlation and ages|magnetostratigraphy correlation]] (see [[natural remanent magnetization]]) and their signature can be seen as parallel linear magnetic anomaly stripes on the seafloor. These stripes provide quantitative information on [[seafloor spreading]], a part of plate tectonics. They are the basis of [[magnetostratigraphy]], which correlates magnetic reversals with other [[Stratigraphic section|stratigraphies]] to construct geologic time scales.<ref name=Opdyke>{{harvnb|Opdyke|Channell|1996}}</ref> In addition, the [[paleomagnetism|magnetization in rocks]] can be used to measure the motion of continents.<ref name=Merrill/>

=== Radioactivity ===
{{further|Radiometric dating}}
[[Image:Thorium decay chain from lead-212 to lead-208.svg|thumb|Example of a radioactive decay chain (see [[Radiometric dating]]) |alt=Diagram with compound balls representing nuclei and arrows.]]
[[Radioactive decay]] accounts for about 80% of the Earth's [[internal heat]], powering the geodynamo and plate tectonics.<ref name=Turcotte>{{harvnb|Turcotte|Schubert|2002}}</ref> The main heat-producing [[isotopes]] are [[Potassium|potassium-40]], [[Uranium|uranium-238]], uranium-235, and [[Thorium|thorium-232]].<ref>{{harvnb|Sanders|2003}}</ref>
Radioactive elements are used for [[radiometric dating]], the primary method for establishing an absolute time scale in [[geochronology]].

Unstable isotopes decay at predictable rates, and the decay rates of different [[isotope]]s cover several orders of magnitude, so radioactive decay can be used to accurately date both recent events and events in past [[Era (geology)|geologic eras]].<ref name=Renne>{{harvnb|Renne|Ludwig|Karner|2000}}</ref> Radiometric mapping using ground and airborne [[gamma spectrometry]] can be used to map the concentration and distribution of radioisotopes near the Earth's surface, which is useful for mapping lithology and alteration.<ref>{{cite web|title=Radiometrics|url=http://www.ga.gov.au/scientific-topics/disciplines/geophysics/radiometrics|website=Geoscience Australia|date=15 May 2014|publisher=Commonwealth of Australia|access-date=23 June 2014}}</ref><ref>{{cite web|title=Interpreting radiometrics|url=http://spatial.agric.wa.gov.au/geophysics/radiometrics.asp|archive-url=https://web.archive.org/web/20120321102348/http://spatial.agric.wa.gov.au/geophysics/radiometrics.asp |archive-date=21 March 2012|website=Natural Resource Management|publisher=Department of Agriculture and Food, Government of Western Australia|access-date=23 June 2014}}</ref>

=== Fluid dynamics ===
{{Main|Geophysical fluid dynamics}}
[[Fluid dynamics|Fluid motions]] occur in the magnetosphere, [[Earth's atmosphere|atmosphere]], ocean, mantle and core. Even the mantle, though it has an enormous [[viscosity]], flows like a fluid over long time intervals. This flow is reflected in phenomena such as [[isostasy]], [[post-glacial rebound]] and [[mantle plume]]s. The mantle flow drives plate tectonics and the flow in the Earth's core drives the geodynamo.<ref name=Merrill/>

Geophysical fluid dynamics is a primary tool in [[physical oceanography]] and [[meteorology]]. The rotation of the Earth has profound effects on the Earth's fluid dynamics, often due to the [[Coriolis effect]]. In the atmosphere, it gives rise to large-scale patterns like [[Rossby waves]] and determines the basic circulation patterns of storms. In the ocean, they drive large-scale circulation patterns as well as [[Kelvin waves]] and [[Ekman spirals]] at the ocean surface.<ref name=Pedlosky>{{harvnb|Pedlosky|1987}}</ref> In the Earth's core, the circulation of the molten iron is structured by [[Taylor columns]].<ref name=Merrill/>

Waves and other phenomena in the magnetosphere can be modeled using [[magnetohydrodynamics]].

=== Heat flow ===
{{Main|Geothermal gradient}}
[[File:Convection-snapshot.png|thumb|upright=1.4|A model of [[thermal convection]] in the [[Earth's mantle]]. The thin red columns are [[mantle plumes]]. |alt=Pseudocolor image in vertical profile.]]
The Earth is cooling, and the resulting [[heat flow]] generates the Earth's magnetic field through the [[geodynamo]] and plate tectonics through [[mantle convection]].<ref name=Davies>{{harvnb|Davies|2001}}</ref> The main sources of heat are: primordial heat due to Earth's cooling and [[radioactivity]] in the planets upper crust.<ref>{{Cite web |title=What is "Heat Flow"? |url=https://www.smu.edu/dedman/academics/departments/Earth-Sciences/Research/GeothermalLab/DataMaps/GeothermalMapofNorthAmerica/What-is-Heat-Flow |access-date=2024-02-18 |website=www.smu.edu |language=en}}</ref> There is also some contributions from [[phase transitions]]. Heat is mostly carried to the surface by [[thermal convection]], although there are two thermal boundary layers – the [[core–mantle boundary]] and the [[lithosphere]] – in which heat is transported by [[Conduction (heat)|conduction]].<ref name=Fowler>{{harvnb|Fowler|2005}}</ref> Some heat is carried up from the bottom of the [[Mantle (geology)|mantle]] by [[mantle plumes]]. The heat flow at the Earth's surface is about {{math| 4.2 × 10<sup>13</sup> W}}, and it is a potential source of [[Geothermal energy|geothermal]] energy.<ref name=Pollack>{{harvnb|Pollack|Hurter|Johnson|1993}}</ref>

=== Mineral physics ===
{{Main|Mineral physics}}
The physical properties of minerals must be understood to infer the composition of the Earth's interior from [[seismology]], the [[geothermal gradient]] and other sources of information. Mineral physicists study the [[elasticity (physics)|elastic]] properties of minerals; their high-pressure [[phase diagrams]], melting points and [[equations of state]] at high pressure; and the [[Rheology|rheological properties]] of rocks, or their ability to flow. Deformation of rocks by [[creep (deformation)|creep]] make flow possible, although over short times the rocks are brittle. The [[viscosity]] of rocks is affected by temperature and pressure, and in turn, determines the rates at which tectonic plates move.<ref name=Poirier/>

Water is a very complex substance and its unique properties are essential for life.<ref name=Sadava>{{harvnb|Sadava|Heller|Hillis|Berenbaum|2009}}</ref> Its physical properties shape the [[hydrosphere]] and are an essential part of the [[water cycle]] and [[climate]]. Its thermodynamic properties determine [[evaporation]] and the thermal gradient in the [[atmosphere]]. The many types of [[precipitation (meteorology)|precipitation]] involve a complex mixture of processes such as [[coalescence (physics)|coalescence]], [[supercooling]] and [[supersaturation]].<ref>{{harvnb|Sirvatka|2003}}</ref> Some precipitated water becomes [[groundwater]], and groundwater flow includes phenomena such as [[percolation]], while the [[conductivity (electrolytic)|conductivity]] of water makes electrical and electromagnetic methods useful for tracking groundwater flow. Physical properties of water such as [[salinity]] have a large effect on its motion in the oceans.<ref name=Pedlosky/>

The many phases of ice form the [[cryosphere]] and come in forms like [[ice sheet]]s, [[glacier]]s, [[sea ice]], freshwater ice, snow, and frozen ground (or [[permafrost]]).<ref>{{harvnb|CFG|2011}}</ref>