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The Geophysics Portal
Age of oceanic lithosphere
Geophysics (/ˌdʒiːoʊˈfɪzɪks/) is a subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space environment, and the use of quantitative methods for their analysis. The term geophysics sometimes refers only to solid earth applications: Earth's shape; its gravitational and magnetic fields; its internal structure and composition; its dynamics and their surface expression in plate tectonics, the generation of magmas, volcanism and rock formation. However, modern geophysics organizations and pure scientists use a broader definition that includes the water cycle including snow and ice; fluid dynamics of the oceans and the atmosphere; electricity and magnetism in the ionosphere and magnetosphere and solar-terrestrial physics; and analogous problems associated with the Moon and other planets.
Although geophysics was only recognized as a separate discipline in the 19th century, its origins date back to ancient times. The first magnetic compasses were made from lodestones, while more modern magnetic compasses played an important role in the history of navigation. The first seismic instrument was built in 132 AD. Isaac Newton applied his theory of mechanics to the tides and the precession of the equinox; and instruments were developed to measure the Earth's shape, density and gravity field, as well as the components of the water cycle. In the 20th century, geophysical methods were developed for remote exploration of the solid Earth and the ocean, and geophysics played an essential role in the development of the theory of plate tectonics. (Full article...)
Selected general articles
The Schiehallion experiment was an 18th-century experiment to determine the mean density of the Earth. Funded by a grant from the Royal Society, it was conducted in the summer of 1774 around the Scottish mountain of Schiehallion, Perthshire. The experiment involved measuring the tiny deflection of the vertical due to the gravitational attraction of a nearby mountain. Schiehallion was considered the ideal location after a search for candidate mountains, thanks to its isolation and almost symmetrical shape.
The experiment had previously been considered, but rejected, by Isaac Newton as a practical demonstration of his theory of gravitation; however, a team of scientists, notably Nevil Maskelyne, the Astronomer Royal, was convinced that the effect would be detectable and undertook to conduct the experiment. The deflection angle depended on the relative densities and volumes of the Earth and the mountain: if the density and volume of Schiehallion could be ascertained, then so could the density of the Earth. Once this was known, it would in turn yield approximate values for those of the other planets, their moons, and the Sun, previously known only in terms of their relative ratios. (Full article...)
Earth's gravity measured by NASA GRACE mission, showing deviations from the theoretical gravity of an idealized, smooth Earth, the so-called Earth ellipsoid. Red shows the areas where gravity is stronger than the smooth, standard value, and blue reveals areas where gravity is weaker. (Animated version.)
The gravity of Earth, denoted by g, is the net acceleration that is imparted to objects due to the combined effect of gravitation (from mass distribution within Earth) and the centrifugal force (from the Earth's rotation).
In SI units this acceleration is measured in metres per second squared (in symbols, m/s2 or m·s−2) or equivalently in newtons per kilogram (N/kg or N·kg−1). Near Earth's surface, gravitational acceleration is approximately 9.81 m/s2 (32.2 ft/s2), which means that, ignoring the effects of air resistance, the speed of an object falling freely will increase by about 9.81 metres (32.2 ft) per second every second. This quantity is sometimes referred to informally as little g (in contrast, the gravitational constant G is referred to as big G). (Full article...)
Mean sea level (MSL) (often shortened to sea level) is an average level of the surface of one or more of Earth's bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datum – a standardised geodetic datum – that is used, for example, as a chart datum in cartography and marine navigation, or, in aviation, as the standard sea level at which atmospheric pressure is measured to calibrate altitude and, consequently, aircraft flight levels. A common and relatively straightforward mean sea-level standard is instead the midpoint between a mean low and mean high tide at a particular location.
Sea levels can be affected by many factors and are known to have varied greatly over geological time scales. However, 20th century and current millennium sea level rise is presumed to be caused by climate change, and careful measurement of variations in MSL can offer insights into ongoing climate change. (Full article...)
The Toronto Magnetic and Meteorological Observatory on the grounds of the University of Toronto
The Toronto Magnetic and Meteorological Observatory is a historical observatory located on the grounds of the University of Toronto, in Toronto, Ontario, Canada. The original building was constructed in 1840 as part of a worldwide research project run by Edward Sabine to determine the cause of fluctuations in magnetic declination. Measurements from the Toronto site demonstrated that sunspots were responsible for this effect on Earth's magnetic field. When this project concluded in 1853, the observatory was greatly expanded by the Canadian government and served as the country's primary meteorological station and official timekeeper for over fifty years. The observatory is considered the birthplace of Canadian astronomy. (Full article...)
Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite (e.g. the Moon) and the primary planet that it orbits (e.g. Earth). The acceleration causes a gradual recession of a satellite in a prograde orbit away from the primary, and a corresponding slowdown of the primary's rotation. The process eventually leads to tidal locking, usually of the smaller body first, and later the larger body. The Earth–Moon system is the best-studied case.
The similar process of tidal deceleration occurs for satellites that have an orbital period that is shorter than the primary's rotational period, or that orbit in a retrograde direction. (Full article...)- The Chandler wobble or Chandler variation of latitude is a small deviation in the Earth's axis of rotation relative to the solid earth, which was discovered by and named after American astronomer Seth Carlo Chandler in 1891. It amounts to change of about 9 metres (30 ft) in the point at which the axis intersects the Earth's surface and has a period of 433 days. This wobble, which is a nutation, combines with another wobble with a period of one year, so that the total polar motion varies with a period of about 7 years.
The Chandler wobble is an example of the kind of motion that can occur for a freely rotating object that is not a sphere; this is called a free nutation. Somewhat confusingly, the direction of the Earth's rotation axis relative to the stars also varies with different periods, and these motions—caused by the tidal forces of the Moon and Sun—are also called nutations, except for the slowest, which are precessions of the equinoxes. (Full article...)
Mantle convection is the very slow creeping motion of Earth's solid silicate mantle caused by convection currents carrying heat from the interior to the planet's surface.
The Earth's surface lithosphere rides atop the asthenosphere and the two form the components of the upper mantle. The lithosphere is divided into a number of tectonic plates that are continuously being created or consumed at plate boundaries. Accretion occurs as mantle is added to the growing edges of a plate, associated with seafloor spreading. This hot added material cools down by conduction and convection of heat. At the consumption edges of the plate, the material has thermally contracted to become dense, and it sinks under its own weight in the process of subduction usually at an ocean trench. (Full article...)- Geodynamics is a subfield of geophysics dealing with dynamics of the Earth. It applies physics, chemistry and mathematics to the understanding of how mantle convection leads to plate tectonics and geologic phenomena such as seafloor spreading, mountain building, volcanoes, earthquakes, faulting and so on. It also attempts to probe the internal activity by measuring magnetic fields, gravity, and seismic waves, as well as the mineralogy of rocks and their isotopic composition. Methods of geodynamics are also applied to exploration of other planets. (Full article...)
- Radiocarbon dating (also referred to as carbon dating or carbon-14 dating) is a method for determining the age of an object containing organic material by using the properties of radiocarbon, a radioactive isotope of carbon.
The method was developed in the late 1940s at the University of Chicago by Willard Libby. It is based on the fact that radiocarbon (14
C) is constantly being created in the Earth's atmosphere by the interaction of cosmic rays with atmospheric nitrogen. The resulting 14
C combines with atmospheric oxygen to form radioactive carbon dioxide, which is incorporated into plants by photosynthesis; animals then acquire 14
C by eating the plants. When the animal or plant dies, it stops exchanging carbon with its environment, and thereafter the amount of 14
C it contains begins to decrease as the 14
C undergoes radioactive decay. Measuring the amount of 14
C in a sample from a dead plant or animal, such as a piece of wood or a fragment of bone, provides information that can be used to calculate when the animal or plant died. The older a sample is, the less 14
C there is to be detected, and because the half-life of 14
C (the period of time after which half of a given sample will have decayed) is about 5,730 years, the oldest dates that can be reliably measured by this process date to approximately 50,000 years ago, although special preparation methods occasionally make accurate analysis of older samples possible. Libby received the Nobel Prize in Chemistry for his work in 1960. (Full article...) - Mineral physics is the science of materials that compose the interior of planets, particularly the Earth. It overlaps with petrophysics, which focuses on whole-rock properties. It provides information that allows interpretation of surface measurements of seismic waves, gravity anomalies, geomagnetic fields and electromagnetic fields in terms of properties in the deep interior of the Earth. This information can be used to provide insights into plate tectonics, mantle convection, the geodynamo and related phenomena.
Laboratory work in mineral physics require high pressure measurements. The most common tool is a diamond anvil cell, which uses diamonds to put a small sample under pressure that can approach the conditions in the Earth's interior. (Full article...) 
Seismology (/saɪzˈmɒlədʒi, saɪs-/; from Ancient Greek σεισμός (seismós) meaning "earthquake" and -λογία (-logía) meaning "study of") is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other planet-like bodies. The field also includes studies of earthquake environmental effects such as tsunamis as well as diverse seismic sources such as volcanic, tectonic, glacial, fluvial, oceanic, atmospheric, and artificial processes such as explosions. A related field that uses geology to infer information regarding past earthquakes is paleoseismology. A recording of Earth motion as a function of time is called a seismogram. A seismologist is a scientist who does research in seismology. (Full article...)
Automatic ground penetrating Radar (upGPR) near Swiss Camp (Greenland)
Near-surface geophysics is the use of geophysical methods to investigate small-scale features in the shallow (tens of meters) subsurface. It is closely related to applied geophysics or exploration geophysics. Methods used include seismic refraction and reflection, gravity, magnetic, electric, and electromagnetic methods. Many of these methods were developed for oil and mineral exploration but are now used for a great variety of applications, including archaeology, environmental science, forensic science, military intelligence, geotechnical investigation, treasure hunting, and hydrogeology. In addition to the practical applications, near-surface geophysics includes the study of biogeochemical cycles. (Full article...)- Figure of the Earth is a term of art in geodesy that refers to the size and shape used to model Earth. The size and shape it refers to depend on context, including the precision needed for the model. The sphere is an approximation of the figure of the Earth that is satisfactory for many purposes. Several models with greater accuracy have been developed so that coordinate systems can serve the precise needs of navigation, surveying, cadastre, land use, and various other concerns. (Full article...)
- Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares the abundance of a naturally occurring radioactive isotope within the material to the abundance of its decay products, which form at a known constant rate of decay. The use of radiometric dating was first published in 1907 by Bertram Boltwood and is now the principal source of information about the absolute age of rocks and other geological features, including the age of fossilized life forms or the age of the Earth itself, and can also be used to date a wide range of natural and man-made materials.
Together with stratigraphic principles, radiometric dating methods are used in geochronology to establish the geologic time scale. Among the best-known techniques are radiocarbon dating, potassium–argon dating and uranium–lead dating. By allowing the establishment of geological timescales, it provides a significant source of information about the ages of fossils and the deduced rates of evolutionary change. Radiometric dating is also used to date archaeological materials, including ancient artifacts. (Full article...) - In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynamo.
In the space environment close to a planetary body, the magnetic field resembles a magnetic dipole. Farther out, field lines can be significantly distorted by the flow of electrically conducting plasma, as emitted from the Sun (i.e., the solar wind) or a nearby star. Planets having active magnetospheres, like the Earth, are capable of mitigating or blocking the effects of solar radiation or cosmic radiation, that also protects all living organisms from potentially detrimental and dangerous consequences. This is studied under the specialized scientific subjects of plasma physics, space physics and aeronomy. (Full article...) - The tidal force is a gravitational effect that stretches a body along the line towards the center of mass of another body due to a gradient (difference in strength) in gravitational field from the other body; it is responsible for diverse phenomena, including tides, tidal locking, breaking apart of celestial bodies and formation of ring systems within the Roche limit, and in extreme cases, spaghettification of objects. It arises because the gravitational field exerted on one body by another is not constant across its parts: the nearest side is attracted more strongly than the farthest side. It is this difference that causes a body to get stretched. Thus, the tidal force is also known as the differential force, as well as a secondary effect of the gravitational field.
In celestial mechanics, the expression tidal force can refer to a situation in which a body or material (for example, tidal water) is mainly under the gravitational influence of a second body (for example, the Earth), but is also perturbed by the gravitational effects of a third body (for example, the Moon). The perturbing force is sometimes in such cases called a tidal force (for example, the perturbing force on the Moon): it is the difference between the force exerted by the third body on the second and the force exerted by the third body on the first. (Full article...)
Atmospheric electricity is the study of electrical charges in the Earth's atmosphere (or that of another planet). The movement of charge between the Earth's surface, the atmosphere, and the ionosphere is known as the global atmospheric electrical circuit. Atmospheric electricity is an interdisciplinary topic with a long history, involving concepts from electrostatics, atmospheric physics, meteorology and Earth science.
Thunderstorms act as a giant battery in the atmosphere, charging up the electrosphere to about 400,000 volts with respect to the surface. This sets up an electric field throughout the atmosphere, which decreases with increase in altitude. Atmospheric ions created by cosmic rays and natural radioactivity move in the electric field, so a very small current flows through the atmosphere, even away from thunderstorms. Near the surface of the Earth, the magnitude of the field is on average around 100 V/m. (Full article...)- Geophysical survey is the systematic collection of geophysical data for spatial studies. Detection and analysis of the geophysical signals forms the core of Geophysical signal processing. The magnetic and gravitational fields emanating from the Earth's interior hold essential information concerning seismic activities and the internal structure. Hence, detection and analysis of the electric and Magnetic fields is very crucial. As the Electromagnetic and gravitational waves are multi-dimensional signals, all the 1-D transformation techniques can be extended for the analysis of these signals as well. Hence this article also discusses multi-dimensional signal processing techniques.
Geophysical surveys may use a great variety of sensing instruments, and data may be collected from above or below the Earth's surface or from aerial, orbital, or marine platforms. Geophysical surveys have many applications in geology, archaeology, mineral and energy exploration, oceanography, and engineering. Geophysical surveys are used in industry as well as for academic research. (Full article...)
In astronomy, axial precession is a gravity-induced, slow, and continuous change in the orientation of an astronomical body's rotational axis. In particular, it can refer to the gradual shift in the orientation of Earth's axis of rotation in a cycle of approximately 26,000 years. This is similar to the precession of a spinning top, with the axis tracing out a pair of cones joined at their apices. The term "precession" typically refers only to this largest part of the motion; other changes in the alignment of Earth's axis—nutation and polar motion—are much smaller in magnitude.
Earth's precession was historically called the precession of the equinoxes, because the equinoxes moved westward along the ecliptic relative to the fixed stars, opposite to the yearly motion of the Sun along the ecliptic. Historically,
the discovery of the precession of the equinoxes is usually attributed in the West to the 2nd-century-BC astronomer Hipparchus. With improvements in the ability to calculate the gravitational force between planets during the first half of the nineteenth century, it was recognized that the ecliptic itself moved slightly, which was named planetary precession, as early as 1863, while the dominant component was named lunisolar precession. Their combination was named general precession, instead of precession of the equinoxes. (Full article...)- The Hollow Moon hypothesis, or Spaceship Moon hypothesis, proposes that Earth's Moon is either wholly hollow or otherwise contains a substantial interior space. No scientific evidence exists to support the idea; seismic observations and other data collected since spacecraft began to orbit or land on the Moon indicate that it has a thin crust, extensive mantle and small, dense core, although overall it is much less dense than Earth.
The Hollow Moon concept is similar to the better-known Hollow Earth hypothesis, which was a recurring plot device in pre-spaceflight science fiction. The first discussion of a hollow Earth was by scientist Edmond Halley in 1692, while the first publication to mention a hollow Moon was not until H. G. Wells' 1901 novel The First Men in the Moon. (Full article...) 
Temperature profile of inner Earth, schematic view (estimated).
Geothermal gradient is the rate of temperature change with respect to increasing depth in Earth's interior. As a general rule, the crust temperature is rising with depth due to the heat flow from the much hotter mantle; away from tectonic plate boundaries, temperature rises in about 25–30 °C/km (72–87 °F/mi) of depth near the surface in most of the world. However, in some cases the temperature may drop with increasing depth, especially near the surface, a phenomenon known as inverse or negative geothermal gradient. Strictly speaking, geo-thermal necessarily refers to Earth but the concept may be applied to other planets. Some of the common SI units found in literature are °C/km, K/km and mK/m, they are all equivalent.
Earth's internal heat comes from a combination of residual heat from planetary accretion, heat produced through radioactive decay, latent heat from core crystallization, and possibly heat from other sources. The major heat-producing nuclides in Earth are potassium-40, uranium-238, uranium-235, and thorium-232. The inner core is thought to have temperatures in the range of 4000 to 7000 K, and the pressure at the centre of the planet is thought to be about 360 GPa (3.6 million atm). (The exact value depends on the density profile in the earth.) Because much of the heat is provided for by radioactive decay, scientists believe that early in the earth's history, before nuclides with short half-lives had been depleted, Earth's heat production would have been much higher. Heat production was twice that of present-day at approximately 3 billion years ago, resulting in larger temperature gradients within the earth, larger rates of mantle convection and plate tectonics, allowing the production of igneous rocks such as komatiites that are no longer formed. (Full article...)
The 2007–2008 Nazko earthquakes were a series of small volcanic earthquakes measuring less than 4.0 on the Richter magnitude scale. They took place in the sparsely populated Nazko area of the Central Interior of British Columbia, Canada starting on October 9, 2007 and ending on June 12, 2008. They occurred just west of Nazko Cone, a small tree-covered cinder cone that last erupted about 7,200 years ago.
No damage or casualties resulted from the Nazko earthquakes, which were too small to be felt by people, but local seismographs recorded them. The earthquake swarm occurred at the eastern end of a known volcanic zone called the Anahim Volcanic Belt. This is an east-west trending line of volcanic formations extending from the Central Coast to the Central Interior of British Columbia. (Full article...)
Earth's inner core is the innermost geologic layer of planet Earth. It is primarily a solid ball with a radius of about 1,220 km (760 mi), which is about 20% of Earth's radius or 70% of the Moon's radius.
There are no samples of Earth's core accessible for direct measurement, as there are for Earth's mantle. Information about Earth's core mostly comes from analysis of seismic waves and Earth's magnetic field. The inner core is believed to be composed of an iron–nickel alloy with some other elements. The temperature at the inner core's surface is estimated to be approximately 5,700 K (5,430 °C; 9,800 °F), which is about the temperature at the surface of the Sun. (Full article...)
Computer simulation of Earth's field in a period of normal polarity between reversals. The lines represent magnetic field lines, blue when the field points towards the center and yellow when away. The rotation axis of Earth is centered and vertical. The dense clusters of lines are within Earth's core.
Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magnetic field is generated by electric currents due to the motion of convection currents of a mixture of molten iron and nickel in Earth's outer core: these convection currents are caused by heat escaping from the core, a natural process called a geodynamo. The magnitude of Earth's magnetic field at its surface ranges from 25 to 65 μT (0.25 to 0.65 gauss). As an approximation, it is represented by a field of a magnetic dipole currently tilted at an angle of about 11 degrees with respect to Earth's rotational axis, as if there were an enormous bar magnet placed at that angle through the center of Earth. The North geomagnetic pole actually represents the South pole of Earth's magnetic field, and conversely the South geomagnetic pole corresponds to the north pole of Earth's magnetic field (because opposite magnetic poles attract and the north end of a magnet, like a compass needle, points toward Earth's South magnetic field, i.e., the North geomagnetic pole near the Geographic North Pole). As of 2015, the North geomagnetic pole was located on Ellesmere Island, Nunavut, Canada.
While the North and South magnetic poles are usually located near the geographic poles, they slowly and continuously move over geological time scales, but sufficiently slowly for ordinary compasses to remain useful for navigation. However, at irregular intervals averaging several hundred thousand years, Earth's field reverses and the North and South Magnetic Poles respectively, abruptly switch places. These reversals of the geomagnetic poles leave a record in rocks that are of value to paleomagnetists in calculating geomagnetic fields in the past. Such information in turn is helpful in studying the motions of continents and ocean floors in the process of plate tectonics. (Full article...)
Illustration of the dynamo mechanism that creates the Earth's magnetic field: convection currents of fluid metal in the Earth's outer core, driven by heat flow from the inner core, organized into rolls by the Coriolis force, create circulating electric currents, which generate the magnetic field.
In physics, the dynamo theory proposes a mechanism by which a celestial body such as Earth or a star generates a magnetic field. The dynamo theory describes the process through which a rotating, convecting, and electrically conducting fluid can maintain a magnetic field over astronomical time scales. A dynamo is thought to be the source of the Earth's magnetic field and the magnetic fields of Mercury and the Jovian planets. (Full article...)
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Selected geophysicist
Bust of Shen at the Beijing Ancient Observatory
Shen Kuo (Chinese: 沈括; 1031–1095) or Shen Gua, courtesy name Cunzhong (存中) and pseudonym Mengqi (now usually given as Mengxi) Weng (夢溪翁), was a Chinese polymathic scientist and statesman of the Song dynasty (960–1279). Excelling in many fields of study and statecraft, he was a mathematician, astronomer, antiquarian, meteorologist, geologist, entomologist, anatomist, climatologist, zoologist, botanist, pharmacologist, medical scientist, agronomist, archaeologist, ethnographer, cartographer, geographer, geophysicist, metallurgist, mineralogist, encyclopedist, military general, diplomat, hydraulic engineer, inventor, economist, academy chancellor, finance minister, governmental state inspector, philosopher, art critic, poet, and musician. He was the head official for the Bureau of Astronomy in the Song court, as well as an Assistant Minister of Imperial Hospitality. At court his political allegiance was to the Reformist faction known as the New Policies Group, headed by Chancellor Wang Anshi (1021–1085).
In his Dream Pool Essays or Dream Torrent Essays (夢溪筆談; Mengxi Bitan) of 1088, Shen was the first to describe the magnetic needle compass, which would be used for navigation (first described in Europe by Alexander Neckam in 1187). Shen discovered the concept of true north in terms of magnetic declination towards the north pole, with experimentation of suspended magnetic needles and "the improved meridian determined by Shen's [astronomical] measurement of the distance between the pole star and true north". This was the decisive step in human history to make compasses more useful for navigation, and may have been a concept unknown in Europe for another four hundred years (evidence of German sundials made circa 1450 show markings similar to Chinese geomancers' compasses in regard to declination). (Full article...)
William Gilbert (/ˈɡɪlbərt/; 24 May 1544? – 30 November 1603), also known as Gilberd, was an English physician, physicist and natural philosopher. He passionately rejected both the prevailing Aristotelian philosophy and the Scholastic method of university teaching. He is remembered today largely for his book De Magnete (1600).
A unit of magnetomotive force, also known as magnetic potential, was named the Gilbert in his honour. (Full article...)
Francis Birch (August 22, 1903 – January 30, 1992) was an American geophysicist. He is considered one of the founders of solid Earth geophysics. He is also known for his part in the atomic bombing of Hiroshima and Nagasaki.
During World War II, Birch participated in the Manhattan Project, working on the design and development of the gun-type nuclear weapon known as Little Boy. He oversaw its manufacture, and went to Tinian to supervise its assembly and loading into Enola Gay, the Boeing B-29 Superfortress tasked with dropping the bomb. (Full article...)
James Alfred Van Allen (September 7, 1914 – August 9, 2006) was an American space scientist at the University of Iowa. He was instrumental in establishing the field of magnetospheric research in space.
The Van Allen radiation belts were named after him, following his discovery using Geiger–Müller tube instruments on the 1958 satellites (Explorer 1, Explorer 3, and Pioneer 3) during the International Geophysical Year. Van Allen led the scientific community in putting scientific research instruments on space satellites. (Full article...)
Adam Marian Dziewoński (November 15, 1936 – March 1, 2016) was a Polish-American geophysicist who made seminal contributions to the determination of the large-scale structure of the Earth's interior and the nature of earthquakes using seismological methods. He spent most of his career at Harvard University, where he was the Frank B. Baird, Jr. Professor of Science. (Full article...)
Henry Cavendish FRS (/ˈkævəndɪʃ/; 10 October 1731 – 24 February 1810) was an English natural philosopher, scientist, and an important experimental and theoretical chemist and physicist. He is noted for his discovery of hydrogen, which he termed "inflammable air". He described the density of inflammable air, which formed water on combustion, in a 1766 paper, On Factitious Airs. Antoine Lavoisier later reproduced Cavendish's experiment and gave the element its name.
A notoriously shy man, Cavendish was nonetheless distinguished for great accuracy and precision in his researches into the composition of atmospheric air, the properties of different gases, the synthesis of water, the law governing electrical attraction and repulsion, a mechanical theory of heat, and calculations of the density (and hence the mass) of the Earth. His experiment to measure the density of the Earth has come to be known as the Cavendish experiment. (Full article...)
Portrait of Gauss by Christian Albrecht Jensen (1840)
Johann Carl Friedrich Gauss (/ɡaʊs/; German: Gauß [kaʁl ˈfʁiːdʁɪç ˈɡaʊs] (
listen); Latin: Carolus Fridericus Gauss; 30 April 1777 – 23 February 1855) was a German mathematician and physicist who made significant contributions to many fields in mathematics and science. Sometimes referred to as the Princeps mathematicorum (Latin for '"the foremost of mathematicians"') and "the greatest mathematician since antiquity", Gauss had an exceptional influence in many fields of mathematics and science, and is ranked among history's most influential mathematicians. (Full article...)
Wallace "Wally" Smith Broecker (November 29, 1931 – February 18, 2019) was an American geochemist. He was the Newberry Professor in the Department of Earth and Environmental Sciences at Columbia University, a scientist at Columbia's Lamont-Doherty Earth Observatory and a sustainability fellow at Arizona State University. He developed the idea of a global "conveyor belt" linking the circulation of the global ocean and made major contributions to the science of the carbon cycle and the use of chemical tracers and isotope dating in oceanography. Broecker popularized the term "global warming". He received the Crafoord Prize and the Vetlesen Prize. (Full article...)
Richard Dixon Oldham FRS (/ˈoʊldəm/; 31 July 1858 – 15 July 1936) was a British geologist who made the first clear identification of the separate arrivals of P-waves, S-waves and surface waves on seismograms and the first clear evidence that the Earth has a central core. (Full article...)
John Tuzo Wilson CC OBE FRS FRSE FRSC (October 24, 1908 – April 15, 1993) was a Canadian geophysicist and geologist who achieved worldwide acclaim for his contributions to the theory of plate tectonics.
Plate tectonics is the scientific theory that the rigid outer layers of the Earth (crust and part of the upper mantle), the lithosphere, is broken up into around 13 pieces or "plates" that move independently over the weaker asthenosphere. Wilson maintained that the Hawaiian Islands were created as a tectonic plate (extending across much of the Pacific Ocean) shifted to the northwest over a fixed hotspot, spawning a long series of volcanoes. He also conceived of the transform fault, a major plate boundary where two plates move past each other horizontally (e.g., the San Andreas Fault). (Full article...)
Alfred Lothar Wegener (/ˈveɪɡənər/; German: [ˈʔalfʁeːt ˈveːɡənɐ]; 1 November 1880 – November 1930) was a German polar researcher, geophysicist and meteorologist.
During his lifetime he was primarily known for his achievements in meteorology and as a pioneer of polar research, but today he is most remembered as the originator of continental drift hypothesis by suggesting in 1912 that the continents are slowly drifting around the Earth (German: Kontinentalverschiebung). His hypothesis was controversial and widely rejected by mainstream geology until the 1950s, when numerous discoveries such as palaeomagnetism provided strong support for continental drift, and thereby a substantial basis for today's model of plate tectonics. Wegener was involved in several expeditions to Greenland to study polar air circulation before the existence of the jet stream was accepted. Expedition participants made many meteorological observations and were the first to overwinter on the inland Greenland ice sheet and the first to bore ice cores on a moving Arctic glacier. (Full article...)
Portrait by Joseph Karl Stieler (1843)
Friedrich Wilhelm Heinrich Alexander von Humboldt (14 September 1769 – 6 May 1859) was a German polymath, geographer, naturalist, explorer, and proponent of Romantic philosophy and science. He was the younger brother of the Prussian minister, philosopher, and linguist Wilhelm von Humboldt (1767–1835). Humboldt's quantitative work on botanical geography laid the foundation for the field of biogeography. Humboldt's advocacy of long-term systematic geophysical measurement laid the foundation for modern geomagnetic and meteorological monitoring.
Between 1799 and 1804, Humboldt travelled extensively in the Americas, exploring and describing them for the first time from a modern Western scientific point of view. His description of the journey was written up and published in several volumes over 21 years. Humboldt was one of the first people to propose that the lands bordering the Atlantic Ocean were once joined (South America and Africa in particular). (Full article...)
Portrait by Asta Nørregaard, 1900
Kristian Olaf Bernhard Birkeland (13 December 1867 – 15 June 1917) was a Norwegian scientist. He is best remembered for his theories of atmospheric electric currents that elucidated the nature of the aurora borealis. In order to fund his research on the aurorae, he invented the electromagnetic cannon and the Birkeland–Eyde process of fixing nitrogen from the air. Birkeland was nominated for the Nobel Prize seven times. (Full article...)
Patrick Maynard Stuart Blackett, Baron Blackett OM CH FRS (18 November 1897 – 13 July 1974) was a British experimental physicist known for his work on cloud chambers, cosmic rays, and paleomagnetism, winning the Nobel Prize for Physics in 1948. In 1925 he became the first person to prove that radioactivity could cause the nuclear transmutation of one chemical element to another. He also made a major contribution in World War II advising on military strategy and developing operational research. His left-wing views saw an outlet in third world development and in influencing policy in the Labour Government of the 1960s. (Full article...)
Andrija Mohorovičić (23 January 1857 – 18 December 1936) was a Croatian geophysicist. He is best known for the eponymous Mohorovičić discontinuity and is considered one of the founders of modern seismology. (Full article...)
Eugene Parker in 2018 at the launch of the solar probe that bears his name
Eugene Newman Parker (born June 10, 1927) is an American solar astrophysicist who—in the mid-1950s—developed the theory of the supersonic solar wind and predicted the Parker spiral shape of the solar magnetic field in the outer Solar System. In 1987, Parker proposed that the solar corona might be heated by myriad tiny "nanoflares", miniature brightenings resembling solar flares that would occur all over the surface of the Sun.
Parker spent four years at the University of Utah and has been at the University of Chicago since 1955, where he has held positions in the physics department, the astronomy and astrophysics department, and the Enrico Fermi Institute. Parker was elected to the National Academy of Sciences in 1967. In 2017, NASA renamed its Solar Probe Plus to Parker Solar Probe in his honor, marking the first time NASA had named a spacecraft after a living person. In 2018, the American Physical Society awarded him the Medal for Exceptional Achievement in Research. (Full article...)
Benjamin Franklin by Joseph Duplessis, 1778
Benjamin Franklin FRS FRSA FRSE (January 17, 1706 [O.S. January 6, 1706] – April 17, 1790) was an American polymath who was active as a writer, scientist, inventor, statesman, diplomat, printer, publisher and political philosopher. Among the leading intellectuals of his time, Franklin was one of the Founding Fathers of the United States, a drafter and signer of the United States Declaration of Independence, and the first United States postmaster general. As a scientist, he was a major figure in the American Enlightenment and the history of physics for his discoveries and theories regarding electricity. As an inventor, he is known for the lightning rod, bifocals, and the Franklin stove, among other inventions. He founded many civic organizations, including the Library Company, Philadelphia's first fire department, and the University of Pennsylvania. Franklin earned the title of "The First American" for his early and indefatigable campaigning for colonial unity, initially as an author and spokesman in London for several colonies. As the first United States ambassador to France, he exemplified the emerging American nation. Franklin was foundational in defining the American ethos as a marriage of the practical values of thrift, hard work, education, community spirit, self-governing institutions, and opposition to authoritarianism both political and religious, with the scientific and tolerant values of the Enlightenment. In the words of historian Henry Steele Commager, "In Franklin could be merged the virtues of Puritanism without its defects, the illumination of the Enlightenment without its heat." Franklin has been called "the most accomplished American of his age and the most influential in inventing the type of society America would become."
Franklin became a successful newspaper editor and printer in Philadelphia, the leading city in the colonies, publishing the Pennsylvania Gazette at age 23. He became wealthy publishing this and Poor Richard's Almanack, which he authored under the pseudonym "Richard Saunders". After 1767, he was associated with the Pennsylvania Chronicle, a newspaper that was known for its revolutionary sentiments and criticisms of the policies of the British Parliament and the Crown. (Full article...)
Charles Francis Richter (/ˈrɪktər/); April 26, 1900 – September 30, 1985) was an American seismologist and physicist.
Richter is most famous as the creator of the Richter magnitude scale, which, until the development of the moment magnitude scale in 1979, quantified the size of earthquakes. Inspired by Kiyoo Wadati's 1928 paper on shallow and deep earthquakes, Richter first used the scale in 1935 after developing it in collaboration with Beno Gutenberg; both worked at the California Institute of Technology. (Full article...)- Richard Doell (1923 – March 6, 2008) was a distinguished American scientist known for developing the time scale for geomagnetic reversals with Allan V. Cox and Brent Dalrymple. This work was a major step in the development of plate tectonics. Doell shared the Vetlesen Prize with Cox and Dalrymple. (Full article...)
A stamp of Zhang Heng issued by China Post in 1955
Zhang Heng (Chinese: 張衡; AD 78–139), formerly romanized as Chang Heng, was a Chinese polymathic scientist and statesman who lived during the Han dynasty. Educated in the capital cities of Luoyang and Chang'an, he achieved success as an astronomer, mathematician, seismologist, hydraulic engineer, inventor, geographer, cartographer, ethnographer, artist, poet, philosopher, politician, and literary scholar.
Zhang Heng began his career as a minor civil servant in Nanyang. Eventually, he became Chief Astronomer, Prefect of the Majors for Official Carriages, and then Palace Attendant at the imperial court. His uncompromising stance on historical and calendrical issues led to his becoming a controversial figure, preventing him from rising to the status of Grand Historian. His political rivalry with the palace eunuchs during the reign of Emperor Shun (r. 125–144) led to his decision to retire from the central court to serve as an administrator of Hejian Kingdom in present-day Hebei. Zhang returned home to Nanyang for a short time, before being recalled to serve in the capital once more in 138. He died there a year later, in 139. (Full article...)
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Selected images
Computer simulation of Earth's field in a period of normal polarity between reversals. The lines represent magnetic field lines, blue when the field points towards the center and yellow when away. The rotation axis of Earth is centered and vertical. The dense clusters of lines are within Earth's core. (from Earth's magnetic field)
Variations in virtual axial dipole moment since the last reversal. (from Earth's magnetic field)
A model of thermal convection in the Earth's mantle. The thin red columns are mantle plumes.
Example of a radioactive decay chain from lead-212 (212Pb) to lead-208 (208Pb) . Each parent nuclide spontaneously decays into a daughter nuclide (the decay product) via an α decay or a β− decay. The final decay product, lead-208 (208Pb), is stable and can no longer undergo spontaneous radioactive decay. (from Radiometric dating)
Estimated declination contours by year, 1590 to 1990 (click to see variation). (from Earth's magnetic field)
World map showing the position of the Moho. (from Structure of Earth)
World map showing frequency of lightning strikes, in flashes per km² per year (equal-area projection). Lightning strikes most frequently in the Democratic Republic of the Congo. Combined 1995–2003 data from the Optical Transient Detector and 1998–2003 data from the Lightning Imaging Sensor. (from Atmospheric electricity)
An artist's rendering of the structure of a magnetosphere. 1) Bow shock. 2) Magnetosheath. 3) Magnetopause. 4) Magnetosphere. 5) Northern tail lobe. 6) Southern tail lobe. 7) Plasmasphere. (from Earth's magnetic field)
Background: a set of traces from magnetic observatories showing a magnetic storm in 2000.
Globe: map showing locations of observatories and contour lines giving horizontal magnetic intensity in μ T. (from Earth's magnetic field)
Apatite crystals are widely used in fission track dating. (from Radiometric dating)
Ale's Stones at Kåseberga, around ten kilometres south east of Ystad, Sweden were dated at 56 CE using the carbon-14 method on organic material found at the site. (from Radiometric dating)
A concordia diagram as used in uranium–lead dating, with data from the Pfunze Belt, Zimbabwe. All the samples show loss of lead isotopes, but the intercept of the errorchron (straight line through the sample points) and the concordia (curve) shows the correct age of the rock. (from Radiometric dating)
Common coordinate systems used for representing the Earth's magnetic field. (from Earth's magnetic field)
Computer simulation of the Earth's magnetic field in a period of normal polarity between reversals. (from Geophysics)
Geomagnetic polarity during the late Cenozoic Era. Dark areas denote periods where the polarity matches today's polarity, light areas denote periods where that polarity is reversed. (from Earth's magnetic field)
Lightning sequence (Duration: 0.32 seconds) (from Atmospheric electricity)
Example of a quadrupole field. This can also be constructed by moving two dipoles together. (from Earth's magnetic field)
A model of short-wavelength features of Earth's magnetic field, attributed to lithospheric anomalies (from Earth's magnetic field)
The movement of Earth's North Magnetic Pole across the Canadian arctic. (from Earth's magnetic field)
Thermal ionization mass spectrometer used in radiometric dating. (from Radiometric dating)
A depiction of atmospheric electricity in a Martian dust storm, which has been suggested as a possible explanation for enigmatic chemistry results from Mars (see also Viking lander biological experiments) (from Atmospheric electricity)
Strength of the axial dipole component of Earth's magnetic field from 1600 to 2020. (from Earth's magnetic field)
Schematic representation of spherical harmonics on a sphere and their nodal lines. Pℓ m is equal to 0 along m great circles passing through the poles, and along ℓ-m circles of equal latitude. The function changes sign each ℓtime it crosses one of these lines. (from Earth's magnetic field)
Age of the sea floor. Much of the dating information comes from magnetic anomalies.
A schematic illustrating the relationship between motion of conducting fluid, organized into rolls by the Coriolis force, and the magnetic field the motion generates. (from Earth's magnetic field)
Animation of tsunami triggered by the 2004 Indian Ocean earthquake (from Seismology)
Relationship between Earth's poles. A1 and A2 are the geographic poles; B1 and B2 are the geomagnetic poles; C1 (south) and C2 (north) are the magnetic poles. (from Earth's magnetic field)
Lu-Hf isochrons plotted of meteorite samples. The age is calculated from the slope of the isochron (line) and the original composition from the intercept of the isochron with the y-axis. (from Radiometric dating)
Structure of Earth (from Structure of Earth)
Installation for a temporary seismic station, north Iceland highland. (from Seismology)
Cloud to ground lightning. Typically, lightning discharges 30,000 amperes, at up to 100 million volts, and emits light, radio waves, x-rays and even gamma rays. Plasma temperatures in lightning can approach 28,000 kelvins. (from Atmospheric electricity)
_The_role_of_phosphates_for_the_Lu%E2%80%93Hf_chronology_of_meteorites.gif)
Subcategories
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Did you know ...?
- ... that a world map by Gerardus Mercator attributes the Earth's magnetic field to a magnetic mountain (pictured) above the Arctic Circle?
- .. that tectonic subsidence, the sinking of the Earth's crust, can be caused by extension, cooling, or loading?
- ... that tectonic plates in the Earth's crust are not completely rigid but can be deformed by the melting of an ice cap?
- ... that substorms were first described in 1964?
In the news
- 5 December 2019: Earthquakes on the Cascadia Fault may trigger earthquakes on the San Andreas Fault (Nature)
- 10 December 2019/ 20 January 2020: New evidence suggests Earth's magnetic field was present 3.7 billion years ago (Nature) - or perhaps even 4.2 billion years ago (Science)
- 13 January 2020: Brazil opens "spectacular" research base in Antarctica (Science)
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