Definition Of Geodynamics
Geodynamics is a subfield of geophysics that deals with the dynamics of the Earth. It involves the application of physics, chemistry and mathematics to the understanding of how mantle convection leads to plate tectonics and geologic phenomena such as mountain building, seafloor spreading, volcanoes, faulting, earthquakes and so on.
It also attempts to study the internal activities 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 in the exploration of other planets.
Geodynamics can simply be referred to as the processes by which mantle convection shapes and reshapes the Earth and other rocky planets.
Brief Geologic History and Geodynamics of the Earth
The Earth formed through a hot accretion process. Almost simultaneously, the core and the mantle were separated from each other. At the final stages of the accretion process, the outer layer approximately 2000 km thick was molten, thus representing a magma ocean.
This magma ocean produced the primary crust of the Earth. Surface waters were precipitated from the atmosphere and released from the crystallizing magma ocean. The plate tectonic processes started at around 4.3 to 4 Ga BP.
In the Archean, the overall tectonic mechanism was quite specific, due to substantially higher mantle temperature and thicker oceanic crust. The normal plate tectonics acted during the Proterozoic and Phanerozoic with the periodic assembly of continents, which are known as supercontinent cycles.
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. Cooling at the surface and heat production within the Earth create a metastable thermal gradient from the hot core to the relatively cool lithosphere. 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, or interpenetration of rock on different sides of the buoyancy contrast.
Negative thermal buoyancy of the oceanic plates is the primary cause of subduction and plate tectonics, while positive thermal buoyancy may lead to mantle plumes, which could explain intraplate volcanism. 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.
Numerical Modeling In Geodynamics:
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.
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:
a.) Instantaneous studies, which aim to reproduce the instantaneous flow in a system due to a given buoyancy distribution.
b.) 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.
A Brief Overview On Geodynamics:
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.
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. 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.
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:
- Modeling brittle and ductile deformation of geologic materials.
- Predicting patterns of continental accretion and breakup of continents and supercontinents.
- Observing surface deformation and relaxation due to ice sheets and post-glacial rebound, and making related.
- conjectures about the viscosity of the mantle.
- Finding and understanding the driving mechanisms behind plate tectonics.
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