Geodynamics
Encyclopedia
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.
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:
of rocks in response to stresses. Stress is defined as the average force per unit area exerted on each part of the rock. The deformation can be measured as strain, a change in length normalized by the total length of the body. If the deformation is elastic
, the rock can spring back to its original shape after the stress is released. In elastic solids, the strain is proportional to the stress.
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. Since it is not of uniform composition, information from seismology
is also needed to determine the elastic properties of deep rock (see also Adams–Williamson equation
).
As long as the deformation is small, the equations of elasticity
can be used to describe how a solid deforms under stress. Equations for bending
are used to calculate the effect on the lithosphere
of adding or removing loads. Examples include bending of the lithosphere under volcanic islands or sedimentary basin
s, and bending at oceanic trenches.
Geophysics
Geophysics is the physics of the Earth and its environment in space; also the study of the Earth using quantitative physical methods. The term geophysics sometimes refers only to the geological applications: Earth's shape; its gravitational and magnetic fields; its internal structure and...
dealing with dynamics of the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
. It applies physics, chemistry and mathematics to the understanding of how mantle convection
Mantle convection
Mantle convection is the slow creeping motion of Earth's rocky mantle caused by convection currents carrying heat from the interior of the Earth to the surface. The Earth's surface lithosphere, which rides atop the asthenosphere , is divided into a number of plates that are continuously being...
leads to plate tectonics
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
and geologic phenomena such as seafloor spreading
Seafloor spreading
Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. Seafloor spreading helps explain continental drift in the theory of plate tectonics....
, 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
Mineralogy
Mineralogy is the study of chemistry, crystal structure, and physical properties of minerals. Specific studies within mineralogy include the processes of mineral origin and formation, classification of minerals, their geographical distribution, as well as their utilization.-History:Early writing...
of rocks and their isotopic composition
Isotope geochemistry
Isotope geochemistry is an aspect of geology based upon study of the relative and absolute concentrations of the elements and their isotopes in the Earth. Variations in the abundance of these isotopes, typically measured with an isotope ratio mass spectrometer or an accelerator mass spectrometer,...
. Methods of geodynamics are also applied to exploration of other planets.
Overview
Experts in geodynamics commonly use data from geodeticGeodetic system
Geodetic systems or geodetic data are used in geodesy, navigation, surveying by cartographers and satellite navigation systems to translate positions indicated on their products to their real position on earth....
GPS, InSAR
Insar
Insar is a town and the administrative center of Insarsky District of the Republic of Mordovia, Russia, located southwest of Saransk at the confluence of the Issa and Insarka Rivers. Population:...
, and seismology
Seismology
Seismology 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 effects, such as tsunamis as well as diverse seismic sources such as volcanic, tectonic, oceanic,...
, along with numerical models, to study the evolution of the Earth's lithosphere
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...
, mantle and core
Outer core
The outer core of the Earth is a liquid layer about 2,266 kilometers thick composed of iron and nickel which lies above the Earth's solid inner core and below its mantle. Its outer boundary lies beneath the Earth's surface...
.
Work performed by geodynamicists may include:
- Modeling brittle and ductile deformation of geologic materials
- Predicting patterns of continental accretionAccretion (geology)Accretion is a process by which material is added to a tectonic plate or a landmass. This material may be sediment, volcanic arcs, seamounts or other igneous features.-Description:...
and breakup of continentContinentA continent is one of several very large landmasses on Earth. They are generally identified by convention rather than any strict criteria, with seven regions commonly regarded as continents—they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia.Plate tectonics is...
s and supercontinentSupercontinentIn geology, a supercontinent is a landmass comprising more than one continental core, or craton. The assembly of cratons and accreted terranes that form Eurasia qualifies as a supercontinent today.-History:...
s - Observing surface deformation and relaxation due to ice sheets and post-glacial reboundPost-glacial reboundPost-glacial rebound is the rise of land masses that were depressed by the huge weight of ice sheets during the last glacial period, through a process known as isostasy...
, and making related conjectures about the viscosityViscosityViscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
of the mantle - Finding and understanding the driving mechanisms behind plate tectonicsPlate tectonicsPlate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
.
Elasticity
A lot of geodynamics deals with the deformationDeformation (mechanics)
Deformation in continuum mechanics is the transformation of a body from a reference configuration to a current configuration. A configuration is a set containing the positions of all particles of the body...
of rocks in response to stresses. Stress is defined as the average force per unit area exerted on each part of the rock. The deformation can be measured as strain, a change in length normalized by the total length of the body. If the deformation is elastic
Elasticity (physics)
In physics, elasticity is the physical property of a material that returns to its original shape after the stress that made it deform or distort is removed. The relative amount of deformation is called the strain....
, the rock can spring back to its original shape after the stress is released. In elastic solids, the strain is proportional to the stress.
Pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
is the part of stress that changes the volume of a solid; shear stress
Shear stress
A shear stress, denoted \tau\, , is defined as the component of stress coplanar with a material cross section. Shear stress arises from the force vector component parallel to the cross section...
changes the shape. If there is no shear, the fluid is in hydrostatic equilibrium
Hydrostatic equilibrium
Hydrostatic equilibrium or hydrostatic balance is the condition in fluid mechanics where a volume of a fluid is at rest or at constant velocity. This occurs when compression due to gravity is balanced by a pressure gradient force...
. 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
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
, 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
Equation of state
In physics and thermodynamics, an equation of state is a relation between state variables. More specifically, an equation of state is a thermodynamic equation describing the state of matter under a given set of physical conditions...
is needed to calculate changes in density of rock even when it is of uniform composition. Since it is not of uniform composition, information from seismology
Seismology
Seismology 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 effects, such as tsunamis as well as diverse seismic sources such as volcanic, tectonic, oceanic,...
is also needed to determine the elastic properties of deep rock (see also Adams–Williamson equation
Adams–Williamson equation
The Adams–Williamson equation, named after L. H. Adams and E. D. Williamson, is a relation between the velocities of seismic waves and the density of the Earth's interior. Given the average density of rocks at the Earth's surface and profiles of the P-wave and S-wave speeds as function of depth,...
).
As long as the deformation is small, the equations of elasticity
Elasticity
Elasticity may refer to:*Elasticity , continuum mechanics of bodies that deform reversibly under stressNumerous uses are derived from this physical sense of the term, which is inherently mathematical, such as used in Engineering, Chemistry, Construction and variously in Economics:*Elasticity , the...
can be used to describe how a solid deforms under stress. Equations for bending
Bending
In engineering mechanics, bending characterizes the behavior of a slender structural element subjected to an external load applied perpendicularly to a longitudinal axis of the element. The structural element is assumed to be such that at least one of its dimensions is a small fraction, typically...
are used to calculate the effect on the lithosphere
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...
of adding or removing loads. Examples include bending of the lithosphere under volcanic islands or sedimentary basin
Sedimentary basin
The term sedimentary basin is used to refer to any geographical feature exhibiting subsidence and consequent infilling by sedimentation. As the sediments are buried, they are subjected to increasing pressure and begin the process of lithification...
s, and bending at oceanic trenches.