But even chaotic systems are subject to statistical predictions related to the size of the events that may be expected and to their average frequency. Development of the geological sciences has been marked by two seemingly contradictory but in fact complementary trends.
Earth scientists have become ever more aware of the great complexity of the Earth and its constituents; certain individual minerals, for example, are among the most complex inorganic substances known. At the same time, earth scientists are uncovering the principles that lie behind the Earth's bewildering diversity—unifying concepts that bring coherence to a welter of seemingly unrelated observations.
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One way to approach the planet's apparent complexity is to look upon the Earth as a system of interacting components. Viewed in its entirety, the earth system is driven by energy from two sources: 1 inside the Earth from primordial energy remaining from accretion and by radioactive decay of elements and 2 from the Sun Figure 1.
As it moves through a homogeneous system, energy tends to reorganize that system into components and then to drive physical and chemical interactions among the components. In this way the Earth has been organized into a system of interacting components on a variety of spatial scales. As with any classification, the boundaries between these components are not everywhere sharp; for instance, the Earth contains liquids in the form of magmas,.
The Earth can be organized into temporal as well as spatial components. Geologists demonstrated the great antiquity of the Earth in the eighteenth and nineteenth centuries by analyzing the layers, or strata, of rock formed from sedimentary deposits. Since then, geologists have continued to refine and elaborate the geological time scale through increasingly sophisticated dating and stratigraphic techniques. The atmosphere and oceans mix too rapidly to carry information about times long past except in their bulk compositions. But some of the rocks in the continents have remained virtually unchanged since their formation billions of years ago.
Thus, events that took place 3. Geologists examine these rocks for clues about conditions when the rocks were formed and changes they underwent thereafter. In this way geologists have been able to chart past changes in temperature, sea level, atmospheric and oceanic composition, volcanism, plate tectonics, the evolution of life, the Earth's magnetic field, sedimentation rates, and glaciation Plate 2.
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Earth scientists study this record to learn how the components of the Earth behaved and interacted in the past, to understand how they interact now, and to predict how they will interact in the future. Geology is a science in which time plays an especially crucial role. This dependence on time adds a unique dimension to geological phenomena.
Every rock, every mountain, every volcano is the product of a particular circumstance. Similarly, every period of earth history is in some way special because of the ever-changing combinations of variously interacting spatial and temporal components, including the remarkable consequences of evolving life. A further complicating factor is the broad range of scales over which geological events occur. The spatial components range from individual mineral grains to objects the size of the Earth and larger, as in the case of the magnetic field.
The temporal components range from seconds—the duration of an earthquake—to billions of years. Understanding geological processes therefore involves enormous. Until recently, the geological sciences dealt with the great complexity of the Earth largely by focusing on specific spatial, temporal, or compositional regimes. Subdisciplines studied phenomena that were largely compartmentalized, and influences from outside the domain of study were greatly simplified or ignored.
As a result, the various subdisciplines of the field—geology, geochemistry, geophysics, paleontology, geohydrology, geoengineering—were neatly pigeonholed and had little need or incentive to communicate or work with one another. During the past 25 years, that fragmentation of the geological sciences has been disappearing under the influence of several momentous developments. The result has been a profound change in the way geoscientists study the Earth. Many advances have simultaneously contributed to a general unification of the geological sciences, but three in particular stand out: the plate tectonic revolution, the enhanced ability to produce images of both the surface and the interior, and the increasing recognition of humanity as a geological agent.
In the s the geological sciences experienced a conceptual revolution that continues to affect the field today. Traditionally, most geologists analyzed the history of the Earth primarily in terms of vertical movements: mountains emerged from a buckling crust and were eroded, sea levels rose and fell, whole areas of continents were uplifted. But a quarter century ago a series of developments in marine geology and paleomagnetism—the record of the magnetic signals preserved in the rocks—resulted in a radically new picture of the Earth.
This new picture acknowledged the essential role of large-scale horizontal movements throughout the Earth's evolution as well as that of vertical movements. This concept of plate tectonics has endured through two decades of scientific scrutiny and is now regarded as an established fact, a situation nearly unthinkable in Scientists now know that the crust is composed of about a dozen major Plate 3 and several minor plates that constantly move and jostle each other in response to movements in the underlying mantle.
Where two plates converge, one may override the other, and the leading edge of the lower plate may melt as it reaches greater depths or may produce melting in the overlying mantle. This convergence creates the oceanic trenches and zones of coastal volcanoes seen on the western edge of South America, in the Southwest Pacific, and elsewhere. If neither plate sinks, collision creates a wrinkled mountain belt, such as the Himalaya or the Urals.
Where plates merely sideswipe each other, the boundary shears laterally, as happens along California's San Andreas fault. In each type of plate boundary zone, earthquakes occur when the plates bind, build up stress, and suddenly slip free. A plate may break apart; two observable examples are a rift across a volcanic center such as the Red Sea and a rift through the interior of the plate related to stresses on a distant plate boundary, as at Lake Baikal.
The plate tectonic model enables geoscientists to synthesize formerly separate and enigmatic facets of crustal processes. This ongoing synthesis helps explain the distribution and timing of mountain building, igneous activity, earthquakes, sedimentary basins, ore deposits, and other features of both practical and theoretical import. Plate tectonics also establishes a new view of earth history, in which the plates have moved with time, with consequent effects on the hydrosphere, atmosphere, and biosphere.
Plate tectonics relates activity within the Earth that is directly associated with the movements of plates to characteristics and changes of the surface. For example, midplate hotspots, which are responsible for such features as the volcanic activity at Yellowstone Park and the Hawaiian Islands, are caused by plumes of hot material rising from within the mantle, perhaps from as deep as the core-mantle boundary.
Plate tectonics also offers a natural framework for geochemical cycles, which involve the transfer of elements among the various envelopes that form the earth system. The revolutionary theory of plate tectonics is comparable in power and elegance to the Copernican theory of the sixteenth century, to Newton's theory of gravitation in the seventeenth century, to the establishment of atomic theory at the beginning of the nineteenth century, to the theory of evolution in biology later in that century, and to the development of quantum mechanics and relativity in physics in the twentieth century.
Plate tectonic theory is the newest of these major advances in human understanding, and as a result many of its implications have yet to be worked out. The theory is better at explaining the large-scale features of the Earth than the smaller ones. It suggests the locations of some natural resources but not others. And although the source of the energy that drives the plates is assumed to be convection in the mantle, this is still unproven. These questions are fundamentally important, not only for understanding the functioning of the Earth but also for applying the theory to practical problems such as resource exploration or the prediction of natural hazards.
Research in the geological sciences is a data-intensive activity. Many advances in the discipline have occurred when new observations became available or when innovative technologies allowed the gathering of new kinds of data. At the same time, recently revised ideas about the Earth have spurred the development of original instruments and techniques specifically designed to test those ideas. Measurements of the Earth can today be made with unprecedented levels of accuracy and sensitivity.
Just a few decades ago, classical wet chemical methods typically allowed the detection of elements in natural materials at a level of about one part per thousand. Today, mass spectrometers, electron microprobes, and particle accelerators detect elements and isotopes at levels of one part per billion or better from sample areas of just a few microns. Such levels of sensitivity have tremendously increased our ability to date earth materials and analyze the changes they have undergone by measuring isotopic and elemental compositions. The traditional forms of gathering geological data are field observations and the preparation of geological maps, which remain a fundamental enterprise.
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Maps of the surface and near-surface are critical in locating and assessing fuels, minerals, and other resources; in analyzing environmental conditions and geological hazards; and in reconstructing the geological history of an area. Maps are the basic tools that geologists use in approaching specific problems Plate 4. The ability to use maps effectively is greatly enhanced by geographic information systems GIS , a method of analyzing and integrating digital spatial data of diverse geological and geographical nature.
Geologists have also developed methods for producing images of the Earth's interior.http://leondumoulin.nl/language/alternate/haircut-for-fine-straight.php
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Drilling can probe a few kilometers into the crust to produce measurements and samples of material; beyond a dozen or so kilometers, indirect geophysical methods must be used. Seismic waves, generated either by earthquakes or human activity, are the most effective means of producing images of the deep layers.
Geophysicists can detect variations in the velocity of reflected and refracted seismic waves from the surface to its core. These variations Plate 5 are caused by changes in the density of the materials within the Earth. Laboratory measurements of the physical and chemical properties of earth materials supplement seismic images to model the structure, composition, and dynamics of otherwise inaccessible portions of the Earth.
In addition, geochemists sample materials that originated in the crust and mantle and develop experiments that test the range of conditions that led to their formation; both produce further hypotheses about the nature of the Earth's interior. A new source of striking images began to be produced in the s with the launch of the first satellites designed to observe the Earth and return data to ground receivers.
Satellite monitoring offers an integrated and efficient method of gathering information on a global basis. Visual images are one form of remote sensing obtained from orbiting platforms, but satellites also monitor the Earth at nonvisible wavelengths, gauge the gravity and magnetic fields, measure the altitude of the land and sea precisely, and perform a number of other data-gathering operations. Global positioning systems use satellites to measure the location of receivers on the Earth to an accuracy of a few centimeters.
This technique has enabled earth scientists to measure the relative velocities of the plates, determine how the velocities change where plates come together, and assess the rigidity of plate interiors. In the future, global positioning systems may be used to measure changes in the ground surface before earthquakes, volcanic eruptions, or other natural hazards occur. These measurements may lead to reliable predictions of potential disasters and timely warnings to endangered populations. Observations of other planets by interplanetary probes also have shed light on the history and nature of the Earth.
The other planets in the solar system bear striking resemblances and notable dissimilarities to the Earth. Our neighbors in the solar system—the Moon, Venus, Mars, and Mercury—are analog of the Earth, frozen in different states of evolution.
Investigations of these planets on an integrated global scale have encouraged earth scientists to adopt a similar approach to our own. It was recognized in the nineteenth century that variations in the Earth's orbit would cause changes in incoming solar radiation that could be important in controlling ice ages.
Theoreticians first calculated how these variations would interact, and the study of deep-sea sediments has yielded persuasive evidence that the recurring ice ages are indeed closely associated with orbital cycles. The importance of orbital variations in controlling the climatic changes of the past 2-million-years has proved revolutionary.