Geodynamics

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.[1]

Overview

Geodynamics is generally concerned with processes that move materials throughout the Earth. In the Earth's interior, movement happens when rocks melt or deform and flow in response to a stress field.[2] This deformation may be brittle, elastic, or plastic, depending on the magnitude of the stress and the material's physical properties, especially the stress relaxation time scale. Rocks are structurally and compositionally heterogeneous and are subjected to variable stresses, so it is common to see different types of deformation in close spatial and temporal proximity.[3] When working with geological timescales and lengths, it is convenient to use the continuous medium approximation and equilibrium stress fields to consider the average response to average stress.[4]

Experts in geodynamics commonly use data from geodetic GPS, InSAR, and seismology, along with numerical models, to study the evolution of the Earth's lithosphere, mantle and core.

Work performed by geodynamicists may include:

Deformation of rocks

Rocks and other geological materials experience strain according to three distinct modes, elastic, plastic, and brittle depending on the properties of the material and the magnitude of the stress field. Stress is defined as the average force per unit area exerted on each part of the rock. Pressure is the part of stress that changes the volume of a solid; shear stress changes the shape. If there is no shear, the fluid is in hydrostatic equilibrium. Since, over long periods, rocks readily deform under pressure, the Earth is in hydrostatic equilibrium to a good approximation. The pressure on rock depends only on the weight of the rock above, and this depends on gravity and the density of the rock. In a body like the Moon, the density is almost constant, so a pressure profile is readily calculated. In the Earth, the compression of rocks with depth is significant, and an equation of state is needed to calculate changes in density of rock even when it is of uniform composition.[5]

Elastic

Elastic deformation is always reversible, which means that if the stress field associated with elastic deformation is removed, the material will return to its previous state. Materials only behave elastically when the relative arrangement along the axis being considered of material components (e.g. atoms or crystals) remains unchanged. This means that the magnitude of the stress cannot exceed the yield strength of a material, and the time scale of the stress cannot approach the relaxation time of the material. If stress exceeds the yield strength of a material, bonds begin to break (and reform), which can lead to ductile or brittle deformation.[6]

Ductile

Ductile or plastic deformation happens when the temperature of a system is high enough so that a significant fraction of the material microstates (figure 1) are unbound, which means that a large fraction of the chemical bonds are in the process of being broken and reformed. During ductile deformation, this process of atomic rearrangement redistributes stress and strain towards equilibrium faster than they can accumulate.[6] Examples include bending of the lithosphere under volcanic islands or sedimentary basins, and bending at oceanic trenches.[5] Ductile deformation happens when transport processes such as diffusion and advection that rely on chemical bonds to be broken and reformed redistribute strain about as fast as it accumulates.

Brittle

When strain localizes faster than these relaxation processes can redistribute it, brittle deformation occurs. The mechanism for brittle deformation involves a positive feedback between the accumulation or propagation of defects especially those produced by strain in areas of high strain, and the localization of strain along these dislocations and fractures. In other words, any fracture, however small, tends to focus strain at its leading edge, which causes the fracture to extend.[6]

In general, the mode of deformation is controlled not only by the amount of stress, but also by the distribution of strain and strain associated features. Whichever mode of deformation ultimately occurs is the result of a competition between processes that tend to localize strain, such as fracture propagation, and relaxational processes, such as annealing, that tend to delocalize strain.

Deformation structures

Structural geologists study the results of deformation, using observations of rock, especially the mode and geometry of deformation to reconstruct the stress field that affected the rock over time. Structural geology is an important complement to geodynamics because it provides the most direct source of data about the movements of the Earth. Different modes of deformation result in distinct geological structures, e.g. brittle fracture in rocks or ductile folding.

Thermodynamics

The physical characteristics of rocks that control the rate and mode of strain, such as yield strength or viscosity, depend on the thermodynamic state of the rock and composition. The most important thermodynamic variables in this case are temperature and pressure. Both of these increase with depth, so to a first approximation the mode of deformation can be understood in terms of depth. Within the upper lithosphere, brittle deformation is common because under low pressure rocks have relatively low brittle strength, while at the same time low temperature reduces the likelihood of ductile flow. After the brittle-ductile transition zone, ductile deformation becomes dominant.[2] Elastic deformation happens when the time scale of stress is shorter than the relaxation time for the material. Seismic waves are a common example of this type of deformation. At temperatures high enough to melt rocks, the ductile shear strength approaches zero, which is why shear mode elastic deformation (S-Waves) will not propagate through melts.[7]

Dynamics of the Earth

The main motive force behind stress in the Earth is provided by thermal energy from radioisotope decay, friction, and residual heat.[8][9] Cooling at the surface and heat production within the Earth create a metastable thermal gradient from the hot core to the relatively cool lithosphere.[10] This thermal energy is converted into mechanical energy by thermal expansion. Deeper hotter and often have higher thermal expansion and lower density relative to overlying rocks. Conversely, rock that is cooled at the surface can become less buoyant than the rock below it. Eventually this can lead to a Rayleigh-Taylor instability (Figure 2), or interpenetration of rock on different sides of the buoyancy contrast.[2][11]

Model of the initiation of termination of a Rayleigh-Taylor instability in 2D
Figure 2 shows a Rayleigh-Taylor instability in 2D using the Shan-Chen model. The red fluid is initially located in a layer on top of the blue fluid, and is less buoyant than the blue fluid. After some time, a Rayleigh-Taylor instability occurs, and the red fluid penetrates the blue one.

Negative thermal buoyancy of the oceanic plates is the primary cause of subduction and plate tectonics,[12] while positive thermal buoyancy may lead to mantle plumes, which could explain intraplate volcanism.[13] The relative importance of heat production vs. heat loss for buoyant convection throughout the whole Earth remains uncertain and understanding the details of buoyant convection is a key focus of geodynamics.[2]

Methods

Geodynamics is a broad field which combines observations from many different types of geological study into a broad picture of the dynamics of Earth. Close to the surface of the Earth, data includes field observations, geodesy, radiometric dating, petrology, mineralogy, drilling boreholes and remote sensing techniques. However, beyond a few kilometers depth, most of these kinds of observations become impractical. Geologists studying the geodynamics of the mantle and core must rely entirely on remote sensing, especially seismology, and experimentally recreating the conditions found in the Earth in high pressure high temperature experiments.(see also Adams–Williamson equation).

Numerical modeling

Because of the complexity of geological systems, computer modeling is used to test theoretical predictions about geodynamics using data from these sources.

There are two main ways of geodynamic numerical modeling.[14]

  1. Modelling to reproduce a specific observation: This approach aims to answer what causes a specific state of a particular system.
  2. Modelling to produce basic fluid dynamics: This approach aims to answer how a specific system works in general.

Basic fluid dynamics modelling can further be subdivided into instantaneous studies, which aim to reproduce the instantaneous flow in a system due to a given buoyancy distribution, and time-dependent studies, which either aim to reproduce a possible evolution of a given initial condition over time or a statistical (quasi) steady-state of a given system.

See also

References

  1. ^ Ismail-Zadeh & Tackley 2010
  2. ^ a b c d Turcotte, D. L. and G. Schubert (2014). "Geodynamics."
  3. ^ Winters, J. D. (2001). "An introduction to igenous and metamorphic petrology."
  4. ^ Newman, W. I. (2012). "Continuum Mechanics in the Earth Sciences."
  5. ^ a b Turcotte & Schubert 2002
  6. ^ a b c Karato, Shun-ichiro (2008). "Deformation of Earth Materials: An Introduction to the Rheology of Solid Earth."
  7. ^ Faul, U. H., J. D. F. Gerald and I. Jackson (2004). "Shear wave attenuation and dispersion in melt-bearing olivine
  8. ^ Hager, B. H. and R. W. Clayton (1989). "Constraints on the structure of mantle convection using seismic observations, flow models, and the geoid." Fluid Mechanics of Astrophysics and Geophysics 4.
  9. ^ Stein, C. (1995). "Heat flow of the Earth."
  10. ^ Dziewonski, A. M. and D. L. Anderson (1981). "Preliminary reference Earth model." Physics of the Earth and Planetary Interiors 25(4): 297-356.
  11. ^ Ribe, N. M. (1998). "Spouting and planform selection in the Rayleigh–Taylor instability of miscible viscous fluids." Journal of Fluid Mechanics 377: 27-45.
  12. ^ Conrad, C. P. and C. Lithgow-Bertelloni (2004). "The temporal evolution of plate driving forces: Importance of “slab suction” versus “slab pull” during the Cenozoic." Journal of Geophysical Research 109(B10): 2156-2202.
  13. ^ Bourdon, B., N. M. Ribe, A. Stracke, A. E. Saal and S. P. Turner (2006). "Insights into the dynamics of mantle plumes from uranium-series geochemistry." Nature 444(7): 713-716.
  14. ^ Tackley, Paul J.; Xie, Shunxing; Nakagawa, Takashi; Hernlund, John W. (2005), "Numerical and laboratory studies of mantle convection: Philosophy, accomplishments, and thermochemical structure and evolution", Earth's Deep Mantle: Structure, Composition, and Evolution, American Geophysical Union, 160, pp. 83–99, Bibcode:2005GMS...160...83T, doi:10.1029/160gm07, ISBN 9780875904252
Bibliography

External links

Central Institution for Meteorology and Geodynamics

The Central Institution for Meteorology and Geodynamics (German: Zentralanstalt für Meteorologie und Geodynamik, ZAMG) is the national meteorological and geophysical service of Austria.

It is a subordinate agency of the Austrian Federal Ministry for Science and Research. The ZAMG headquarters are located in Vienna, with regional offices in Salzburg, Innsbruck, Graz and Klagenfurt.

ZAMG was founded in 1851 and is the oldest weather service in the world. Its task is not only to operate monitoring networks and to conduct research in various fields, but also to make the results available to the public.

Computational Infrastructure for Geodynamics

The Computational Infrastructure for Geodynamics (CIG) is a community-driven organization that advances Earth science by developing and disseminating software for geophysics and related fields. It is a National Science Foundation-sponsored collaborative effort to improve geodynamic modelling and develop, support, and disseminate open-source software for the geodynamics research and higher education communities.

CIG is located at the University of California, Davis, and is a member-governed consortium with 62 US institutional members and 15 international affiliates.

Dynamic topography

The term dynamic topography is used in geodynamics to refer to elevation differences caused by the flow within the Earth's mantle.

Eurasian Plate

The Eurasian Plate is a tectonic plate which includes most of the continent of Eurasia (a landmass consisting of the traditional continents of Europe and Asia), with the notable exceptions of the Indian subcontinent, the Arabian subcontinent, and the area east of the Chersky Range in East Siberia. It also includes oceanic crust extending westward to the Mid-Atlantic Ridge and northward to the Gakkel Ridge.

The eastern side is a boundary with the North American Plate to the north and a boundary with the Philippine Sea Plate to the south and possibly with the Okhotsk Plate and the Amurian Plate. The southerly side is a boundary with the African Plate to the west, the Arabian Plate in the middle and the Indo-Australian Plate to the east. The westerly side is a divergent boundary with the North American Plate forming the northernmost part of the Mid-Atlantic Ridge, which is straddled by Iceland. All of the volcanic eruptions in Iceland, such as the 1973 eruption of Eldfell, the 1783 eruption of Laki, and the 2010 eruption of Eyjafjallajökull, are caused by the North American and the Eurasian Plates moving apart, which is a result of divergent plate boundary forces.

The geodynamics of central Asia is dominated by the interaction between the Eurasian and Indian Plates. In this area, many subplates or crust blocks have been recognized, which form the Central Asian and the East Asian transit zones.

GEOS-3

GEOS-3, or Geodynamics Experimental Ocean Satellite 3, or GEOS-C, was the third and final satellite as part of NASA's Geodetic Earth Orbiting Satellite/Geodynamics Experimental Ocean Satellite program (NGSP) to better understand and test satellite tracking systems. For GEOS missions 1 and 2, GEOS stands for Geodetic Earth Orbiting Satellite; this was changed to Geodynamics Experimental Ocean Satellite for GEOS-3.

Geodynamics of Venus

NASA's Magellan spacecraft mission discovered that Venus has a geologically young surface with a relatively uniform age of 500±200 Ma (million years). The age of Venus was revealed by the observation of over 900 impact craters on the surface of the planet. These impact craters are nearly uniformly distributed over the surface of Venus and less than 10% have been modified by plains of volcanism or deformation. These observations indicate that a catastrophic resurfacing event took place on Venus around 500 Ma, and was followed by a dramatic decline in resurfacing rate. The radar images from the Magellan missions revealed that the terrestrial style of plate tectonics is not active on Venus and the surface appears to be immobile at the present time. Despite these surface observations, there are numerous surface features that indicate an actively convecting interior. The Soviet Venera landings revealed that the surface of Venus is essentially basaltic in composition based on geochemical measurements and morphology of volcanic flows. The surface of Venus is dominated by patterns of basaltic volcanism, and by compressional and extensional tectonic deformation, such as the highly deformed tesserae terrain and the pancake like volcano-tectonic features known as coronae. The planet's surface can be broadly characterized by its low lying plains, which cover about 80% of the surface, 'continental' plateaus and volcanic swells. There is also an abundance of small and large shield volcanoes distributed over the planet's surface. Based on its surface features, it appears that Venus is tectonically and convectively alive but has a lithosphere that is static.

Geothermal gradient

Geothermal gradient is the rate of increasing temperature with respect to increasing depth in the Earth's interior. Away from tectonic plate boundaries, it is about 25–30 °C/km (72-87 °F/mi) of depth near the surface in most of the world. Strictly speaking, geo-thermal necessarily refers to the Earth but the concept may be applied to other planets.

The 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 isotopes in the Earth are potassium-40, uranium-238, uranium-235, and thorium-232. At the center of the planet, the temperature may be up to 7,000 K and the pressure could reach 360 GPa (3.6 million atm). Because much of the heat is provided by radioactive decay, scientists believe that early in Earth history, before isotopes 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.

Gulf of Lion

The Gulf of Lion (French: golfe du Lion, Spanish: golfo de León, Italian: Golfo del Leone, Occitan: golf del/dau Leon, Catalan: golf del Lleó, Medieval Latin: sinus Leonis, mare Leonis, Classical Latin: sinus Gallicus) is a wide embayment of the Mediterranean coastline of Languedoc-Roussillon and Provence in France, reaching from the border with Catalonia in the west to Toulon.

The chief port on the gulf is Marseille. Toulon is another important port. The fishing industry in the gulf is based on hake (Merluccius merluccius), being bottom-trawled, long-lined and gill-netted and currently declining from over-fishing.

Rivers that empty into the gulf include the Tech, Têt, Aude, Orb, Hérault, Vidourle, and the Rhône.

The continental shelf is exposed here as a wide coastal plain, and the offshore terrain slopes rapidly to the Mediterranean's abyssal plain. Much of the coastline is composed of lagoons and salt marsh.

This is the area of the cold, blustery winds called the Mistral and the Tramontane.

Isostasy

Isostasy (Greek ísos "equal", stásis "standstill") is the state of gravitational equilibrium between Earth's crust (or lithosphere) and mantle such that the crust "floats" at an elevation that depends on its thickness and density.

This concept is invoked to explain how different topographic heights can exist at Earth's surface. When a certain area of Earth's crust reaches the state of isostasy, it is said to be in isostatic equilibrium. Isostasy does not upset equilibrium but instead restores it (a negative feedback). It is generally accepted that Earth is a dynamic system that responds to loads in many different ways. However, isostasy provides an important 'view' of the processes that are happening in areas that are experiencing vertical movement. Certain areas (such as the Himalayas) are not in isostatic equilibrium, which has forced researchers to identify other reasons to explain their topographic heights (in the case of the Himalayas, which are still rising, by proposing that their elevation is being supported by the force of the impacting Indian Plate; the Basin and Range Province of the Western US is another example of a region not in isostatic equilibrium.)

Although originally defined in terms of continental crust and mantle, it has subsequently been interpreted in terms of lithosphere and asthenosphere, particularly with respect to oceanic island volcanoes such as the Hawaiian Islands.

In the simplest example, isostasy is the principle of buoyancy wherein an object immersed in a fluid is buoyed with a force equal to the weight of the displaced fluid. On a geological scale, isostasy can be observed where Earth's strong crust or lithosphere exerts stress on the weaker mantle or asthenosphere, which, over geological time, flows laterally such that the load is accommodated by height adjustments.

The general term 'isostasy' was coined in 1882 by the American geologist Clarence Dutton.

LAGEOS

LAGEOS, Laser Geodynamics Satellite or Laser Geometric Environmental Observation Survey, are a series of two scientific research satellites designed to provide an orbiting laser ranging benchmark for geodynamical studies of the Earth. Each satellite is a high-density passive laser reflector in a very stable medium Earth orbit (MEO).

Mantle convection

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 plates that are continuously being created and consumed at their opposite 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.This subducted material sinks through the Earth's interior. Some subducted material appears to reach the lower mantle, while in other regions, this material is impeded from sinking further, possibly due to a phase transition from spinel to silicate perovskite and magnesiowustite, an endothermic reaction.The subducted oceanic crust triggers volcanism, although the basic mechanisms are varied. Volcanism may occur due to processes that add buoyancy to partially melted mantle, which would cause upward flow of the partial melt due to decrease in its density. Secondary convection may cause surface volcanism as a consequence of intraplate extension and mantle plumes.Mantle convection causes tectonic plates to move around the Earth's surface. It seems to have been much more active during the Hadean period, resulting in gravitational sorting of heavier molten iron, nickel, and sulphides to the core and lighter silicate minerals to the mantle.

National Observatory of Athens

The National Observatory of Athens (NOA; Greek: Εθνικό Αστεροσκοπείο Αθηνών) is a research institute in Athens, Greece. Founded in 1842, it is the oldest research foundation in Greece, as it was the first scientific research institute built after Greece became independent in 1829, and one of the oldest research institutes in Southern Europe.

Nutation

Nutation (from Latin nūtātiō, "nodding, swaying") is a rocking, swaying, or nodding motion in the axis of rotation of a largely axially symmetric object, such as a gyroscope, planet, or bullet in flight, or as an intended behaviour of a mechanism. In an appropriate reference frame it can be defined as a change in the second Euler angle. If it is not caused by forces external to the body, it is called free nutation or Euler nutation. A pure nutation is a movement of a rotational axis such that the first Euler angle is constant. In spacecraft dynamics, precession (a change in the first Euler angle) is sometimes referred to as nutation.

Paraná and Etendeka traps

The Paraná-Etendeka traps (or Paraná and Etendeka Plateau; or Paraná and Etendeka Province) comprise a large igneous province that includes both the main Paraná traps (in Paraná Basin, a South American geological basin) as well as the smaller severed portions of the flood basalts at the Etendeka traps (in northwest Namibia and southwest Angola). The original basalt flows occurred 128 to 138 million years ago. The province had a post-flow surface area of 1.5 x 106 km² (580,000 miles²) and an original volume projected to be in excess of 2.3 x 106 km³.

Ridge push

Ridge push (also known as gravitational sliding) or sliding plate force is a proposed driving force for plate motion in plate tectonics that occurs at mid-ocean ridges as the result of the rigid lithosphere sliding down the hot, raised asthenosphere below mid-ocean ridges. Although it is called ridge push, the term is somewhat misleading; it is actually a body force that acts throughout an ocean plate, not just at the ridge, as a result of gravitational pull. The name comes from earlier models of plate tectonics in which ridge push was primarily ascribed to upwelling magma at mid-ocean ridges pushing or wedging the plates apart.

Sean Solomon

Sean Carl Solomon (born October 24,1945) is the director of the Lamont-Doherty Earth Observatory of Columbia University, where he is also the William B. Ransford Professor of Earth and Planetary Science. Before moving to Columbia in 2012, he was the director of the Department of Terrestrial Magnetism at the Carnegie Institute in Washington, D.C. His research area is in geophysics, including the fields of planetary geology, seismology, marine geophysics, and geodynamics. Solomon is the principal investigator on the NASA MESSENGER mission to Mercury. He is also a team member on the Gravity Recovery and Interior Laboratory mission and the Plume-Lithosphere Undersea Melt Experiment (PLUME).

Slab pull

Slab pull is that part of the motion of a tectonic plate caused by its subduction. In 1975 Forsyth and Uyeda showed using inverse theory methods that of the many likely driving forces of plates slab pull was the strongest. Plate motion is partly driven by the weight of cold, dense plates sinking into the mantle at oceanic trenches. This force and slab suction account for almost all of the force driving plate tectonics. The ridge push at rifts contributes only 5 to 10%.

Carlson et al. (1983) in Lallemandet al. (2005) defined the slab pull force as:

Where:

K is 4.2g (gravitational acceleration = 9.81 m/s2) according to McNutt (1984);
Δρ = 80 kg/m3 is the mean density difference between the slab and the surrounding asthenosphere;
L is the slab length calculated only for the part above 670 km (the upper/lower mantle boundary);
A is the slab age in Ma at the trench.

The slab pull force manifests itself between two extreme forms:

Between these two examples there is the evolution of the Farallon Plate: from the huge slab width with the Nevada, the Sevier and Laramide orogenies; the Mid-Tertiary ignimbrite flare-up and later left as Juan de Fuca and Cocos plates, the Basin and Range Province under extension, with slab break off, smaller slab width, more edges and mantle return flow.

Some early models of plate tectonics envisioned the plates riding on top of convection cells like conveyor belts. However, most scientists working today believe that the asthenosphere does not directly cause motion by the friction of such basal forces. The North American Plate is nowhere being subducted, yet it is in motion. Likewise the African, Eurasian and Antarctic Plates. Ridge push is thought responsible for the motion of these plates.

The subducting slabs around the Pacific Ring of Fire cool down the Earth and its Core-mantle boundary. Around the African Plate upwelling mantle plumes from the Core-mantle boundary produce rifting including the African and Ethiopian rift valleys.

True polar wander

True polar wander is a solid-body rotation of a planet or moon with respect to its spin axis, causing the geographic locations of the north and south poles to change, or "wander". If a body is not totally rigid (as is the case of the earth), then in a stable state, the largest moment of inertia axis will be aligned with the spin axis, with the smaller two moments of inertia axes lying in the plane of the equator. If the body is not in this steady state, true polar wander will occur: the planet or moon will rotate as a rigid body to realign the largest moment of inertia axis with the spin axis. (See Polhode#Description.)

If the body is near the steady state but with the angular momentum not exactly lined up with the largest moment of inertia axis, the pole position will oscillate. Weather and water movements can also induce small changes. These subjects are covered in the article Polar motion.

W. Jason Morgan

William Jason Morgan (born October 10, 1935) is an American geophysicist who has made seminal contributions to the theory of plate tectonics and geodynamics. He retired as the Knox Taylor Professor emeritus of geology and professor of geosciences at Princeton University. He currently serves as a visiting scholar in the Department of Earth and Planetary Sciences at Harvard University.

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