Earth tide

Earth tide (also known as solid Earth tide, crustal tide, body tide, bodily tide or land tide) is the displacement of the solid earth's surface caused by the gravity of the Moon and Sun. Its main component has meter-level amplitude at periods of about 12 hours and longer. The largest body tide constituents are semi-diurnal, but there are also significant diurnal, semi-annual, and fortnightly contributions. Though the gravitational forcing causing earth tides and ocean tides is the same, the responses are quite different.

Tide raising force

Lunar tidal force: these images depict the Moon directly over 30° N (or 30° S) viewed from above the Northern Hemisphere, showing both sides of the planet. Red up, blue down.

Tideforcenw
Tideforcese

The larger of the periodic gravitational forces is from the Moon but that of the Sun is also important. The images here show lunar tidal force when the Moon appears directly over 30° N (or 30° S). This pattern remains fixed with the red area directed toward (or directly away from) the Moon. Red indicates upward pull, blue downward. If, for example the Moon is directly over 90° W (or 90° E), the red areas are centred on the western northern hemisphere, on upper right. Red up, blue down. If for example the Moon is directly over 90° W (90° E), the centre of the red area is 30° N, 90° W and 30° S, 90° E, and the centre of the bluish band follows the great circle equidistant from those points. At 30° latitude a strong peak occurs once per lunar day, giving a significant diurnal force at that latitude. Along the equator two equally sized peaks (and depressions) are equally sized, giving semi-diurnal force there.

Body tide

Sectorialvert
Vertical displacements of sectorial movement.
Red up, blue down.
Sectorialeastwest
East-west displacements of sectorial movement.
Red east, blue west.
Sectorialnorthsouth
North-south displacements of sectorial movement.
Red north, blue south.
Tesseralvert
Vertical displacements of tesseral movement.
Red up, blue down.
Tesseraleastwest
East-West displacements of tesseral movement.
Red east, blue west.
Tesseralnorthsouth
North-South displacements of tesseral movement.
Red north, blue south.
Zonalvert
Vertical displacements of zonal movement. Red up, blue down.

The Earth tide encompasses the entire body of the Earth and is unhindered by the thin crust and land masses of the surface, on scales that make the rigidity of rock irrelevant. Ocean tides are a consequence of the resonance of the same driving forces with water movement periods in ocean basins accumulated over many days, so that their amplitude and timing are quite different and vary over short distances of just a few hundred km. The oscillation periods of the Earth as a whole are not near the astronomical periods, so its flexing is due to the forces of the moment.

The tide components with a period near twelve hours have a lunar amplitude (Earth bulge/depression distances) that are a little more than twice the height of the solar amplitudes, as tabulated below. At new and full moon, the Sun and the Moon are aligned, and the lunar and the solar tidal maxima and minima (bulges and depressions) add together for the greatest tidal range at particular latitudes. At first- and third-quarter phases of the moon, lunar and solar tides are perpendicular, and the tidal range is at a minimum. The semi-diurnal tides go through one full cycle (a high and low tide) about once every 12 hours and one full cycle of maximum height (a spring and neap tide) about once every 14 days.

The development of a systematic theory of Earth tides was started by George H. Darwin in 1879,[1] and was then furthered by numerous authors, most notably by William Kaula in 1964.[2]

The semi-diurnal tide (one maximum every 12 or so hours) is primarily lunar (only S2 is purely solar) and gives rise to sectorial deformations which rise and fall at the same time along the same longitude.[3] Sectorial variations of vertical and east-west displacements are maximum at the equator and vanish at the poles. There are two cycles along each latitude, the bulges opposite one another, and the depressions similarly opposed. The diurnal tide is lunisolar, and gives rise to tesseral deformations. The vertical and east-west movement is maximum at 45° latitude and is zero on the equator and at the poles. Tesseral variation have one cycle per latitude, one bulge and one depression; the bulges are opposed (antipodal), that is to say the western part of the northern hemisphere and the eastern part of the southern hemisphere, for example, and similarly the depressions are opposed, the eastern part of the northern hemisphere and the western part of the southern hemisphere, in this case. Finally, fortnightly and semi-annual tides have zonal deformations (constant along a circle of latitude), as the Moon or Sun gravitation is directed alternately away from the northern and southern hemispheres due to tilt. There is zero vertical displacement at 35°16' latitude.

Since these displacements affect the vertical direction east-west and north-south variations are often tabulated in milliarcseconds for astronomical use. The vertical displacement is frequently tabulated in μgal, since the gradient of gravity is location dependent so that the distance conversion is only approximately 3 μgal per cm

Other Earth tide contributors

In coastal areas because the ocean tide is quite out of step with the Earth tide, at high ocean tide there is an excess (or at low tide a deficit) of water about what would be the gravitational equilibrium level and the adjacent ground falls (or rises) in response to the resulting differences in weight. Displacements caused by ocean tidal loading can exceed the displacements due to the Earth body tide. Sensitive instruments far inland often have to make similar corrections. Atmospheric loading and storm events may also be measurable, though the masses in movement are less weighty.

Tidal constituents

Principal tide constituents. The amplitudes may vary from those listed within several per cent.[4][5]

Semi-diurnal

Tidal constituent Period Vertical amplitude (mm) Horizontal amplitude (mm)
M2 12.421 hr 384.83 53.84
S2 (solar semi-diurnal) 12.000 hr 179.05 25.05
N2 12.658 hr 73.69 10.31
K2 11.967 hr 48.72 6.82

Diurnal

Tidal constituent Period Vertical amplitude (mm) Horizontal amplitude (mm)
K1 23.934 hr 191.78 32.01
O1 25.819 hr 158.11 22.05
P1 24.066 hr 70.88 10.36
φ1 23.804 hr 3.44 0.43
ψ1 23.869 hr 2.72 0.21
S1 (solar diurnal) 24.000 hr 1.65 0.25

Long term

Tidal constituent Period Vertical amplitude (mm) Horizontal amplitude (mm)
Mf 13.661 days 40.36 5.59
Mm (moon monthly) 27.555 days 21.33 2.96
Ssa (solar semi-annual) 0.50000 yr 18.79 2.60
Lunar node 18.613 yr 16.92 2.34
Sa (solar annual) 1.0000 yr 2.97 0.41

Effects

Volcanologists use the regular, predictable Earth tide movements to calibrate and test sensitive volcano deformation monitoring instruments. The tides may also trigger volcanic events. [6] [7] Seismologists have determined that microseismic events are correlated to tidal variations in Central Asia (north of the Himalayas). The semidiurnal amplitude of terrestrial tides can reach about 55 cm (22 in) at the equator which is important in GPS, VLBI, and SLR measurements.[8][9] Also to make precise astronomical angular measurements requires knowledge of the Earth's rate of rotation (length of day, precession, and nutation), which is influenced by Earth tides (so-called pole tide). Terrestrial tides also need to be taken in account in the case of some particle physics experiments. [10] For instance, at the CERN or SLAC, the very large particle accelerators were designed while taking terrestrial tides into account for proper operation. Among the effects that need to be taken into account are circumference deformation for circular accelerators and particle beam energy. [11] [12]

Body tides in planets and moons, as well as in binary stars and binary asteroids, play a key role in long-term dynamics of planetary systems. For example, it is due to body tides in the Moon that it is captured into the 1:1 spin-orbit resonance and is always showing us one side. Owing to the body tides in it, Mercury is trapped in the 3:2 spin-orbit resonance with the Sun. [13] For the same reason, it is believed that many of the exoplanets are captured in higher spin-orbit resonances with their host stars. [14]

References

  1. ^ G. H. Darwin, Philosophical Transactions of the Royal Society of London, 170, 447-530, 1879
  2. ^ W. M. Kaula, Reviews of Geophysics, 2, 661-684, 1964
  3. ^ Paul Melchior, "Earth Tides", Surveys in Geophysics, 1, pp. 275–303, March, 1974.
  4. ^ John Wahr, "Earth Tides", Global Earth Physics, A Handbook of Physical Constants, AGU Reference Shelf, 1, pp. 40–46, 1995.
  5. ^ Michael R. House, "Orbital forcing timescales: an introduction", Geological Society, London, Special Publications; 1995; v. 85; p. 1-18. http://sp.lyellcollection.org/cgi/content/abstract/85/1/1
  6. ^ Sottili G., Martino S., Palladino D.M., Paciello A., Bozzano F. (2007), Effects of tidal stresses on volcanic activity at Mount Etna, Italy, Geophys. Res. Lett., 34, L01311, doi:10.1029/2006GL028190, 2007.
  7. ^ Volcano watch, USGS.
  8. ^ IERS Conventions (2010). Gérard Petit and Brian Luzum (eds.). (IERS Technical Note ; 36) Frankfurt am Main: Verlag des Bundesamts für Kartographie und Geodäsie, 2010. 179 pp., ISBN 9783898889896, Sec. 7.1.1, "Effects of the solid Earth tides" [1]
  9. ^ User manual for the Bernese GNSS Software, Version 5.2 (November 2015), Astronomical Institute of the University of Bern. Section 10.1.2. "Solid Earth Tides, Solid and Ocean Pole Tides, and Permanent Tides" [2]
  10. ^ Accelerator on the move, but scientists compensate for tidal effects, Stanford online.
  11. ^ circumference deformation
  12. ^ particle beam energy affects
  13. ^ Noyelles, B.; Frouard, J.; Makarov, V. V.; & Efroimsky, M. (2014). "Spin-orbit evolution of Mercury revisited". Icarus. 241: 26–44. arXiv:1307.0136. Bibcode:2014Icar..241...26N. doi:10.1016/j.icarus.2014.05.045.CS1 maint: extra punctuation (link) CS1 maint: multiple names: authors list (link)
  14. ^ Makarov, V. V.; Berghea, C.; & Efroimsky, M. (2012). "Dynamical Evolution and Spin–Orbit Resonances of Potentially Habitable Exoplanets: The Case of GJ 581d". The Astrophysical Journal. 761 (2): 83. arXiv:1208.0814. Bibcode:2012ApJ...761...83M. doi:10.1088/0004-637X/761/2/83. 83.CS1 maint: extra punctuation (link) CS1 maint: multiple names: authors list (link)

Bibliography

  • McCully, James Greig, Beyond the Moon, A Conversational, Common Sense Guide to Understanding the Tides, World Scientific Publishing Co, Singapore, 2006.
  • Paul Melchior, Earth Tides, Pergamon Press, Oxford, 1983.
  • Wylie, Francis E, Tides and the Pull of the Moon, The Stephen Greene Press, Brattleboro, Vermont, 1979.
Atmospheric tide

Atmospheric tides are global-scale periodic oscillations of the atmosphere. In many ways they are analogous to ocean tides. Atmospheric tides can be excited by:

The regular day–night cycle in the Sun's heating of the atmosphere (insolation)

The gravitational field pull of the Moon

Non-linear interactions between tides and planetary waves.

Large-scale latent heat release due to deep convection in the tropics.

Bahama Banks

The Bahama Banks are the submerged carbonate platforms that make up much of the Bahama Archipelago. The term is usually applied in referring to either the Great Bahama Bank around Andros Island, or the Little Bahama Bank of Grand Bahama Island and Great Abaco, which are the largest of the platforms, and the Cay Sal Bank north of Cuba. The islands of these banks are politically part of the Bahamas. Other banks are the three banks of the Turks and Caicos Islands, namely the Caicos Bank of the Caicos Islands, the bank of the Turks Islands, and wholly submerged Mouchoir Bank. Further southeast are the equally wholly submerged Silver Bank and Navidad Bank north of the Dominican Republic.

Carbonate platform

A carbonate platform is a sedimentary body which possesses topographic relief, and is composed of autochthonic calcareous deposits. Platform growth is mediated by sessile organisms whose skeletons build up the reef or by organisms (usually microbes) which induce carbonate precipitation through their metabolism. Therefore, carbonate platforms can not grow up everywhere: they are not present in places where limiting factors to the life of reef-building organisms exist. Such limiting factors are, among others: light, water temperature, transparency and pH-Value. For example, carbonate sedimentation along the Atlantic South American coasts takes place everywhere but at the mouth of the Amazon River, because of the intense turbidity of the water there. Spectacular examples of present-day carbonate platforms are the Bahama Banks under which the platform is roughly 8 km thick, the Yucatan Peninsula which is up to 2 km thick, the Florida platform, the platform on which the Great Barrier Reef is growing, and the Maldive atolls. All these carbonate platforms and their associated reefs are confined to tropical latitudes. Today's reefs are built mainly by scleractinian corals, but in the distant past other organisms, like archaeocyatha (during the Cambrian) or extinct cnidaria (tabulata and rugosa) were important reef builders.

Earth radius

Earth radius is the distance from the center of Earth to a point on its surface. Its value ranges from 6,378 km (3,963 mi) at the equator to 6,357 km (3,950 mi) at a pole. Earth radius is a term of art in astronomy and geophysics and a unit of measurement in both. It is symbolized as R in astronomy. In other contexts, it is denoted or sometimes .

Earth's radius can be defined in different ways because Earth is not a perfect sphere. The surface to which a radius extends is commonly chosen to be on an ellipsoid representing the shape of Earth. Like the surface, what point gets used for the center of Earth is also a matter of definition and therefore contributes to the diverse ways of defining Earth's radius.

When only one radius is stated, the International Astronomical Union (IAU) prefers that it be the equatorial radius. The International Union of Geodesy and Geophysics (IUGG) gives three global average radii: the arithmetic mean of the radii of the ellipsoid (R1); the authalic radius, which is of a sphere having the same surface area as the ellipsoid (R2); and the volumetric radius, which is of a sphere having the same volume as the ellipsoid (R3). All three of those radii are about 6,371 kilometres (3,959 mi).

Many other ways to define Earth radius have been described. Some appear below. A few definitions yield values outside the range between polar radius and equatorial radius because they include local or geoidal topology or because they depend on abstract geometrical considerations.

Gravity

Gravity (from Latin gravitas, meaning 'weight'), or gravitation, is a natural phenomenon by which all things with mass or energy—including planets, stars, galaxies, and even light—are brought toward (or gravitate toward) one another. On Earth, gravity gives weight to physical objects, and the Moon's gravity causes the ocean tides. The gravitational attraction of the original gaseous matter present in the Universe caused it to begin coalescing, forming stars—and for the stars to group together into galaxies—so gravity is responsible for many of the large-scale structures in the Universe. Gravity has an infinite range, although its effects become increasingly weaker on farther objects.

Gravity is most accurately described by the general theory of relativity (proposed by Albert Einstein in 1915) which describes gravity not as a force, but as a consequence of the curvature of spacetime caused by the uneven distribution of mass. The most extreme example of this curvature of spacetime is a black hole, from which nothing—not even light—can escape once past the black hole's event horizon. However, for most applications, gravity is well approximated by Newton's law of universal gravitation, which describes gravity as a force which causes any two bodies to be attracted to each other, with the force proportional to the product of their masses and inversely proportional to the square of the distance between them.

Gravity is the weakest of the four fundamental interactions of physics, approximately 1038 times weaker than the strong interaction, 1036 times weaker than the electromagnetic force and 1029 times weaker than the weak interaction. As a consequence, it has no significant influence at the level of subatomic particles. In contrast, it is the dominant interaction at the macroscopic scale, and is the cause of the formation, shape and trajectory (orbit) of astronomical bodies.

The earliest instance of gravity in the Universe, possibly in the form of quantum gravity, supergravity or a gravitational singularity, along with ordinary space and time, developed during the Planck epoch (up to 10−43 seconds after the birth of the Universe), possibly from a primeval state, such as a false vacuum, quantum vacuum or virtual particle, in a currently unknown manner. Attempts to develop a theory of gravity consistent with quantum mechanics, a quantum gravity theory, which would allow gravity to be united in a common mathematical framework (a theory of everything) with the other three fundamental interactions of physics, are a current area of research.

Hans-Georg Wenzel

Hans-Georg Wenzel (3 February, 1945 – 11 November, 1999), also known as George Wenzel, was a German geodesist, geophysicist and university lecturer. His most important field of work was physical geodesy, where he worked after his dissertation on earth tides with geophysical measurements up to global models of the earth gravity field.

Index of physics articles (E)

The index of physics articles is split into multiple pages due to its size.

To navigate by individual letter use the table of contents below.

Jim Berkland

James O. Berkland (July 31, 1930 – July 22, 2016) was a California geologist who controversially claimed to be able to predict earthquakes, including the 1989 Loma Prieta earthquake and 1994 Northridge Earthquake and who popularized the idea that some people are earthquake sensitive. He was profiled in a popular 2006 book as The Man Who Predicts Earthquakes. The book includes a chapter that notes "Many of Berkland's theories--based on tides, moons, disoriented pets, lost cats and dogs, and magnetic field changes--were factors in the great Indian Ocean quake-tsunami disaster on December 26, 2004." but neither his methods nor his predictions have been published in any scientific journals for peer review. Nevertheless his results have been disputed by peers, with other scientists going so far as calling him a crank and a clown.

Large low-shear-velocity provinces

Large low-shear-velocity provinces, LLSVPs, also called LLVPs or superplumes, are characteristic structures of parts of the lowermost mantle (the region surrounding the outer core) of the Earth. These provinces are characterized by slow shear wave velocities and were discovered by seismic tomography of the deep Earth. There are two main provinces: the African LLSVP and the Pacific LLSVP. Both extend laterally for thousands of kilometers and possibly up to 1000 km vertically from the core-mantle boundary. The Pacific LLSVP has specific dimensions of 3000 km across and 300 m higher than the surrounding ocean-floor, and is situated over four hotspots that suggest multiple mantle plumes underneath. These zones represent around 8% of the volume of the mantle (6% of the Earth). Other names for LLSVPs include superwells, thermo-chemical piles, or hidden reservoirs. Some of these names, however, are more interpretive of their geodynamical or geochemical effects, while many questions remain about their nature.

List of submarine volcanoes

A list of active and extinct submarine volcanoes and seamounts located under the world's oceans. There are estimated to be 40,000 to 55,000 seamounts in the global oceans. Almost all are not well-mapped and many may not have been identified at all. Most are unnamed and unexplored. This list is therefore confined to seamounts that are notable enough to have been named and/or explored.

Oceanic plateau

An oceanic or submarine plateau is a large, relatively flat elevation that is higher than the surrounding relief with one or more relatively steep sides.There are 184 oceanic plateaus covering an area of 18,486,600 km2 (7,137,700 sq mi), or about 5.11% of the oceans. The South Pacific region around Australia and New Zealand contains the greatest number of oceanic plateaus (see map).

Oceanic plateaus produced by large igneous provinces are often associated with hotspots, mantle plumes, and volcanic islands — such as Iceland, Hawaii, Cape Verde, and Kerguelen. The three largest plateaus, the Caribbean, Ontong Java, and Mid-Pacific Mountains, are located on thermal swells. Other oceanic plateaus, however, are made of rifted continental crust, for example Falkland Plateau, Lord Howe Rise, and parts of Kerguelen, Seychelles, and Arctic ridges.

Plateaus formed by large igneous provinces were formed by the equivalent of continental flood basalts such as the Deccan Traps in India and the Snake River Plain in the United States.

In contrast to continental flood basalts, most igneous oceanic plateaus erupt through young and thin (6–7 km (3.7–4.3 mi)) mafic or ultra-mafic crust and are therefore uncontaminated by felsic crust and representative for their mantle sources.

These plateaus often rise 2–3 km (1.2–1.9 mi) above the surrounding ocean floor and are more buoyant than oceanic crust. They therefore tend to withstand subduction, more-so when thick and when reaching subduction zones shortly after their formations. As a consequence, they tend to "dock" to continental margins and be preserved as accreted terranes. Such terranes are often better preserved than the exposed parts of continental flood basalts and are therefore a better record of large-scale volcanic eruptions throughout Earth's history. This "docking" also means that oceanic plateaus are important contributors to the growth of continental crust. Their formations often had a dramatic impact on global climate, such as the most recent plateaus formed, the three, large, Cretaceous oceanic plateaus in the Pacific and Indian Ocean: Ontong Java, Kerguelen, and Caribbean.

Physical oceanography

Physical oceanography is the study of physical conditions and physical processes within the ocean, especially the motions and physical properties of ocean waters.

Physical oceanography is one of several sub-domains into which oceanography is divided. Others include biological, chemical and geological oceanography.

Physical oceanography may be subdivided into descriptive and dynamical physical oceanography.Descriptive physical oceanography seeks to research the ocean through observations and complex numerical models, which describe the fluid motions as precisely as possible.

Dynamical physical oceanography focuses primarily upon the processes that govern the motion of fluids with emphasis upon theoretical research and numerical models. These are part of the large field of Geophysical Fluid Dynamics (GFD) that is shared together with meteorology. GFD is a sub field of Fluid dynamics describing flows occurring on spatial and temporal scales that are greatly influenced by the Coriolis force.

Pierre-Simon Laplace

Pierre-Simon, marquis de Laplace (; French: [pjɛʁ simɔ̃ laplas]; 23 March 1749 – 5 March 1827) was a French scholar whose work was important to the development of engineering, mathematics, statistics, physics, astronomy, and philosophy. He summarized and extended the work of his predecessors in his five-volume Mécanique Céleste (Celestial Mechanics) (1799–1825). This work translated the geometric study of classical mechanics to one based on calculus, opening up a broader range of problems. In statistics, the Bayesian interpretation of probability was developed mainly by Laplace.Laplace formulated Laplace's equation, and pioneered the Laplace transform which appears in many branches of mathematical physics, a field that he took a leading role in forming. The Laplacian differential operator, widely used in mathematics, is also named after him. He restated and developed the nebular hypothesis of the origin of the Solar System and was one of the first scientists to postulate the existence of black holes and the notion of gravitational collapse.

Laplace is remembered as one of the greatest scientists of all time. Sometimes referred to as the French Newton or Newton of France, he has been described as possessing a phenomenal natural mathematical faculty superior to that of any of his contemporaries.

He was Napoleon's examiner when Napoleon attended the École Militaire in Paris in 1784.

Laplace became a count of the Empire in 1806 and was named a marquis in 1817, after the Bourbon Restoration.

Tidal locking

Tidal locking (also called gravitational locking, captured rotation and spin-orbit locking), in the most well-known case, occurs when an orbiting astronomical body always has the same face toward the object it is orbiting. This is known as synchronous rotation: the tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner. For example, the same side of the Moon always faces the Earth, although there is some variability because the Moon's orbit is not perfectly circular. Usually, only the satellite is tidally locked to the larger body. However, if both the difference in mass between the two bodies and the distance between them are relatively small, each may be tidally locked to the other; this is the case for Pluto and Charon.

The effect arises between two bodies when their gravitational interaction slows a body's rotation until it becomes tidally locked. Over many millions of years, the interaction forces changes to their orbits and rotation rates as a result of energy exchange and heat dissipation. When one of the bodies reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit, it is said to be tidally locked. The object tends to stay in this state when leaving it would require adding energy back into the system. The object's orbit may migrate over time so as to undo the tidal lock, for example, if a giant planet perturbs the object.

Not every case of tidal locking involves synchronous rotation. With Mercury, for example, this tidally locked planet completes three rotations for every two revolutions around the Sun, a 3:2 spin-orbit resonance. In the special case where an orbit is nearly circular and the body's rotation axis is not significantly tilted, such as the Moon, tidal locking results in the same hemisphere of the revolving object constantly facing its partner.

However, in this case the exact same portion of the body does not always face the partner on all orbits. There can be some shifting due to variations in the locked body's orbital velocity and the inclination of its rotation axis.

Tidal triggering of earthquakes

Tidal triggering of earthquakes is the idea that tidal forces may induce seismicity.

In connection with earthquakes, syzygy refers to the idea that the combined tidal effects of the sun and moon – either directly as earth tides in the crust itself, or indirectly by hydrostatic loading due to ocean tides – should be able to trigger earthquakes in rock that is already stressed to the point of fracturing, and therefore a higher proportion of earthquakes should occur at times of maximal tidal stress, such as at the new and full moons.

Previously, scientists have searched for such a correlation for over a century, but with the exception of volcanic areas (including mid-ocean spreading ridges) the results have been mixed. It has been suggested that some negative results are due to failure to account for tidal phase and fault orientation (dip), while "many studies reporting positive correlations suffer from a lack of statistical rigor." One systematic investigation found "no evidence for an increase in seismicity during intervals of large tidal range but there is clear evidence for small but significant increase in earthquake rates near low tide"; it did not find an increase of earthquakes near peak spring tides. Seismicity is favored at low tides, particularly for reverse faults, because unloading unclamps the fault, reducing friction. Ocean loading has no effect at all on strike-slip faults.Research work has shown a robust correlation between small tidally induced forces and non-volcanic tremor activity.

Volcanologists use the regular, predictable Earth tide movements to calibrate and test sensitive volcano deformation monitoring instruments. The tides may also trigger volcanic events.

Tide

Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun, and the rotation of the Earth.

Tide tables can be used for any given locale to find the predicted times and amplitude (or "tidal range"). The predictions are influenced by many factors including the alignment of the Sun and Moon, the phase and amplitude of the tide (pattern of tides in the deep ocean), the amphidromic systems of the oceans, and the shape of the coastline and near-shore bathymetry (see Timing). They are however only predictions, the actual time and height of the tide is affected by wind and atmospheric pressure. Many shorelines experience semi-diurnal tides—two nearly equal high and low tides each day. Other locations have a diurnal tide—one high and low tide each day. A "mixed tide"—two uneven magnitude tides a day—is a third regular category.Tides vary on timescales ranging from hours to years due to a number of factors, which determine the lunitidal interval. To make accurate records, tide gauges at fixed stations measure water level over time. Gauges ignore variations caused by waves with periods shorter than minutes. These data are compared to the reference (or datum) level usually called mean sea level.While tides are usually the largest source of short-term sea-level fluctuations, sea levels are also subject to forces such as wind and barometric pressure changes, resulting in storm surges, especially in shallow seas and near coasts.

Tidal phenomena are not limited to the oceans, but can occur in other systems whenever a gravitational field that varies in time and space is present. For example, the shape of the solid part of the Earth is affected slightly by Earth tide, though this is not as easily seen as the water tidal movements.

Undersea mountain range

Undersea mountain ranges are mountain ranges that are mostly or entirely underwater, and specifically under the surface of an ocean. If originated from current tectonic forces, they are often referred to as a mid-ocean ridge. In contrast, if formed by past above-water volcanism, they are known as a seamount chain. The largest and best known undersea mountain range is a mid-ocean ridge, the Mid-Atlantic Ridge. It has been observed that, "similar to those on land, the undersea mountain ranges are the loci of frequent volcanic and earthquake activity".

United States Antarctic Program

The United States Antarctic Program (or USAP; formerly known as the United States Antarctic Research Program or USARP and the United States Antarctic Service or USAS) is an organization of the United States government which has presence in the continent of Antarctica. Founded in 1959, the USAP manages all U.S. scientific research and related logistics in Antarctica as well as aboard ships in the Southern Ocean.

Wave base

The wave base, in physical oceanography, is the maximum depth at which a water wave's passage causes significant water motion. For water depths deeper than the wave base, bottom sediments and the seafloor are no longer stirred by the wave motion above.

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