Geodynamics

Geodynamics focuses on the processes that cause the formation of and changes to Earth's crust and mantle (the superheated region between the planet's core and crust). Geodynamicists study Earth's plates and the processes by which they move atop the mantle, causing seismic and volcanic activity. Geodynamicists also study crustal deformation, subduction, and postglacial rebound. These processes and phenomena are studied by analyzing rock samples and observing environmental changes on Earth's surface from orbiting satellites.

Basic Principles

The field of geodynamics entails the study of the deformation of Earth's crust as caused by the planet's internal processes and external elements. Geodynamicists study the movement of the massive plates at an area above the mantle known as the lithosphere (a superheated layer of molten rock between the core and the outer crust).

According to the theory of plate tectonics, these plates are in constant motion, occasionally coming into contact with each other and causing seismic activity. Scientists who focus on geodynamics also study the movement of molten rock (magma) from the mantle outward, through fissures in the tectonic plates and ultimately the outer crust, occasionally through volcanoes. Such activity creates new physical characteristics on the planet's surface, such as mountains and calderas (deep, bowl-shaped depressions that are formed after a volcano collapses into a depleted magma chamber following volcanic events).

In addition to the earth's internal geological processes, geodynamicists study the deformation of the planet's outer crust caused by glaciers and oceans. These massive bodies of water change the shape of the material beneath them and cause subduction (a process whereby the crust and the tectonic plate beneath it are pushed down under the weight of a glacier or ocean). When the weight either dissipates or moves from a subduction zone, the material returns to its original form (a concept known as elastic deformation).

Background and History

The scientific study of the Earth's geological, seismological, volcanic, and geodynamic activity is a relatively young field. For millennia, however, humanity has acknowledged such phenomena, taking into account the often catastrophic results of volcanoes, earthquakes, and floods. Not until the mid-eighteenth century, however, did scientists begin to consider the more gradual deformations that were taking place.

In 1785, for example, Scottish geologist James Hutton first suggested a connection between the major seismic and volcanic events that had previously been recorded and the changes in the earth's surface that occurred gradually. In time, scientists speculated that the earth's continents were moving from one another, although it was not until 1915 that German geophysicist and meteorologist Alfred Wegener formally introduced the theory of continental drift—the notion that in a period of hundreds of millions of years Earth's continents (once all conjoined) have been in motion.

Wegener's theory has since been disproved, but his concept shed light on why fossils of the same species were found thousands of miles away on other continents. His theory also formed the basis of the theory of plate tectonics. In the 1960s, scientists who placed seismographs near nuclear test sites noticed that volcanoes, earthquake zones, and other active geologic surface features seemed to be located along the fault lines between tectonic plates. Furthermore, scientists studying the crust beneath the ocean noticed that the magnetic material contained in the rock indicated historic reversals in the earth's polar magnetic field, providing further proof that the planet's lithosphere is always in motion. Plate tectonics rests at the heart of the field of geodynamics, helping create a framework for understanding the processes that continue to shape Earth's surface.

The Lithosphere and Magma Flow

Geodynamics focuses on Earth as a planet that is always developing. Because of the planet's internal and external forces, processes, and elements, the outer crust is subject to continual deformation. The central internal element of this deformation is the motion of tectonic plates.

As molten rock moves from Earth's interior, it cools and contracts. However, the core's heat gives the rock buoyancy, allowing the plates to float atop the mantle. These plates are always moving and, occasionally, contacting one another. Owing to its low density and gravitational forces, magma will move through these plates at fault zones on its way to and through the crust.

The lithosphere has varying degrees of density. In some areas, it is highly dense and thick, pulling down the outer crust and causing lower land elevations. Examples of such lower elevations include the plains of North America. In less dense areas of the lithosphere, however, the horizontal forces that draw rock outward, coupled with magma moving outward, makes this area of the lithosphere denser than the crust, leading to increased land elevations and mountain ranges. For example, scientists believe that the Andes and Himalayas were formed this way.

An important element in the deformation of Earth's crust is the amount of stress placed on the lithosphere. Factors contributing to lithospheric stress include the pressures caused by the flow of material from the mantle, the regional density of the lithosphere, gravity, and the level of friction placed on the lithosphere by mantle convection (the flow of hotter and less dense molten rock toward the lithosphere). This stress contributes significantly to the topography of the outer crust. The results are particularly evident along the plate boundaries, such as those found in North America and Western Europe, where larger ridges and mountains are formed.

Subduction and Postglacial Rebound

Another important element on which geodynamics focuses is subduction. In this process, the lithosphere beneath the earth's oceans is pushed down, causing plate displacement. The space left by the subducting plate, known as the subduction zone, is filled by outward-flowing magma. When two plates separate beneath the ocean, the ocean floor becomes deformed, creating new mountains and ridges. When ocean plates come into contact with continental plates, as commonly occurs along the boundaries of the Pacific Ocean (the region known as the Ring of Fire), volcanic and seismic activity increases. Magma that flows into these subduction fields commonly contains water and certain volatile minerals, which cause explosive volcanic activity and, ultimately, significant deformation of the outer crust.

Although subduction causes this downward movement of the lithosphere (and the tectonic plates within it), the effects of subduction are not permanent. In fact, when the pressure above is removed, the lithosphere and the outer crust slowly return to their original positions, a phenomenon known as postglacial rebound. Scientists study this aspect of geodynamics by analyzing the layers of rock after the dissipation of ancient glaciers and by studying the deformation caused by modern glaciers. Postglacial rebound explains why the earth's surface is moving north at 0.035 inch per year. Earth's center of mass is located at the North Pole, and before the glaciation associated with the Ice Age, the outer crust was positioned relative to this center. Glaciers pushed the surface down and away from North Pole, but when the glaciers dissipated, the elasticity of the crust caused a gradual return of Earth's center of mass to that region.

Scientists study postglacial rebound by observing folds in the rock that were influenced by glaciers. In the Italian, French, and Swiss Alpine ranges, for example, scientists have unearthed significant evidence of postglacial rebound. Such evidence is available because this region is among several in the world in which the postglacial rebound from the last glacial maximum (a period about 30,000 years ago, in which ancient glaciers were at their thickest) had the most pronounced deforming effects, forming high peaks and other formations. The accentuated evidence of postglacial rebound located in this and other mountainous regions has enabled geologists and geophysicists to formulate theories about the rates at which rebound occurs and the effects of the last glacial maximum on the earth's lithosphere.

Computer Modeling

One of the most effective approaches to studying geodynamics is mathematical and computer modeling. Through the creation of general formulae, scientists can input data from a specific area to study such physical factors as temperature and pressure. In one 2011 study, for example, scientists sought to analyze the chemical and mineralogical composition of the Caribbean plate using an area in eastern Cuba as their point of reference. Using a basic pressure and temperature formula, and a software program to compile data on minerals, scientists were able to create a profile of the area's thermal history and, within this arena, the subduction that occurred there.

Modeling can compile data from samples around the world to create a general framework for understanding geodynamic processes. In the case of one study, information gleaned at fifty-six different subduction zones was compiled to create a more comprehensive illustration of plate geometry, ages, velocities, and other characteristics in general. Such models help scientists understand the temperatures and other forces that influence subduction zones.

Satellite Technology

In many cases, evidence of deformation, surface movement, and postglacial rebound is visible not from the ground but far above the earth. In 2009, the European Space Agency launched the Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) to study the planet's gravitational field and the ocean floor's topography. GOCE was utilized by geodynamicists to study the lithosphere's structure and densities. By recording gravitational data, scientists now are able to study subduction zones, postglacial rebound, and plate deformation more comprehensively.

GOCE was far from the only satellite used in the study of the deformation of the earth. National Aeronautics and Space Administration's (NASA) Gravity Recovery and Climate Experiment (GRACE) satellite, launched in 2002, provided time-lapsed data on postglacial recovery. Scientists also use the worldwide global positioning system to study tectonic motion. Furthermore, NASA and other organizations use satellites to study the planet's geodynamic activity using onboard radar, gradiometers, and other technologies to gather data about tectonic movement and deformation. By the 2020s, scientists routinely used Interferometric Synthetic Aperture Radar, commonly called InSAR, to study Earth's surface. InSAR uses orbiting satellites to collect radar images of Earth's surface. It can operate in bad weather and in darkness. With InSAR, a radar pulse is emitted from a satellite and scattered by Earth's surface. A satellite then records the data. Scientists have used InSAR to monitor volcanoes and to study deformation at Yellowstone National Park.

Other Scientific Disciplines

In many cases, geodynamics is greatly aided by the application of other scientific disciplines. Wegener's view of continental drift, for example, was born in part by paleontology, as Wegener (and others) observed that fossils of the same species could be found on opposite sides of the globe. His observations led to the claim that at one point the continents were connected. Wegener's theories led to the widely accepted geodynamic concept of plate tectonics.

Geophysicists frequently look to prehistoric eras for evidence of the earth's geodynamic properties. One example of this interdisciplinary approach is a 2006 study of the Tibetan plateau. Scientists used paleoclimatologic information to study the collision of the two plates in this region of East Asia. Based on an analysis of unearthed deposits (dating back to the Eocene period, about fifty million years ago), geophysicists determined that the plateau had been elevated by more than 4 kilometers (2.5 miles) as long as thirty-five million years ago.

Relevant Groups and Organizations

One of the key government agencies involved in the study of the earth's geodynamic phenomena is the US Geological Survey (USGS). USGS scientists conduct analytical studies of geodynamic activity in the United States and its territories around the world. The USGS frequently partners with private and public organizations and scientists on large-scale research projects.

In addition to supporting the USGS, the US government provides funding for private research. The National Science Foundation offers a wide range of scientific grants, including funding for geodynamic research. The recipients of such grants are often affiliated with either a private research organization or a university.

The field of geodynamics requires a great deal of theory, mathematical computation, and extensive data collection, calling for the expertise of university researchers and faculties. At some educational institutions, geodynamics research is part of geology and earth science course curricula. However, some universities (such as Michigan State University) form groups of geodynamics-oriented researchers to collaborate and develop scientific papers, books, and other media on the subject. University professors and researchers commonly collaborate with peers at other institutions, sharing data and findings to support theories and research efforts.

Energy companies are always seeking new oil and gas deposits. Such findings are usually unearthed in subduction zones and other areas where seismic, geodynamic, and volcanic activity is prevalent. For this reason, the petroleum industry often calls upon full-time geophysicists and geodynamics consultants to help them find such deposits. These professionals largely use seismic equipment and data to survey a given area and locate new deposits.

Implications and Future Prospects

The study of geodynamics is critical to understanding volcanic and seismic activity, as well as the forces that created (and continue to reshape) the earth's surface profile. Some of the most significant developments in geodynamics research (such as plate tectonics) have been introduced only in the last few decades of the twentieth century. Much of this science has relied on the collection and analysis of minerals or the visual study of rock formations beneath the many strata (layers) of the earth's surface. Scientists studying geodynamics also record seismic waves, a process that helps estimations of crustal density and magma viscosity.

Although these scientific approaches remain important, the study of geodynamics has been greatly aided by the introduction of new technologies. Satellite systems, for example, have enabled geophysicists to survey the earth's deformations, magnetic fields, and other key characteristics. Updates to computer systems are enabling scientists to continually gather larger and larger amounts of data and create comprehensive models of the processes occurring far beneath Earth's surface.

Geodynamics has been further aided by the advent of the Internet. Scientists from around the world can now connect with one another through a vast array of networks to share theories, data, and other information about advances in geodynamics and changes in the earth's internal and surface areas. The Internet also enables scientists located anywhere in the world to access remote observatories and seismic stations in real time. The speed by which information is collated and shared and the volume of data compiled through the use of twenty-first century technologies continues to benefit the ever-evolving field of geodynamics.

Principal Terms

elastic deformation: the process by which material beneath a heavy object, such as a glacier, is deformed and returned to its original form after the weight is lifted

last glacial maximum: the prehistoric period (approximately 30,000 years ago) in which the glaciers covering Earth were at their thickest

lithosphere: the layer of large plates believed to be floating on molten rock beneath the earth's outer crust

magma: molten rock pushed outward from the earth's core

mantle: the superheated layer of molten rock located between Earth's core and its outer crust

plate tectonics: the theory stating that beneath the earth's outer crust there exists a series of plates in constant motion and through which magma flows

postglacial rebound: process by which the earth's crust slowly returns to its original position after a glacier dissipates or moves away from the subduction zone

subduction: geodynamic process whereby an extremely heavy object located on Earth's outer crust pushes down on the crust and the tectonic plates underneath

Bibliography

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