212 Lithosphere And Asthenosphere
It has long been recognized that for large-scale structures to attain isostatic equilibrium, the outermost shell of the Earth must be underlain by a weak layer Figure 2.35 Section from San Francisco, California to Lamar, Colorado based on seismic refraction data redrawn from Pakiser, 1963, by permission of the American Geophysical Union. Copyright 1963 American Geophysical Union . Figure 2.35 Section from San Francisco, California to Lamar, Colorado based on seismic refraction data redrawn from...
2116 Tests of isostasy
The state of isostatic compensation of a region can be assessed by making use of gravity anomalies. The isostatic anomaly, IA, is defined as the Bouguer anomaly minus the gravity anomaly of the subsurface compensation. Consider a broad, flat plateau of elevation h compensated by a root of thickness r. The terrain correction From the Airy criterion for isostatic equilibrium Substitution of this condition into the equation reveals that the isostatic anomaly is equal to the free-air anomaly over a...
2112 Airys hypothesis
Airy's hypothesis assumes that the outermost shell of the Earth is of a constant density and overlies a higher density layer. Surface topography is compensated by varying the thickness of the outer shell in such a way that its buoyancy balances the surface load. A simple analogy would be blocks of ice of varying thickness floating in water, with the thickest showing the greatest elevation above the surface. Thus mountain ranges would be underlain by a thick root, and ocean basins by a thinned...
2115 Isostatic rebound
The equilibrium flexural response of the lithosphere to loading is independent of the precise mechanical properties of the underlying asthenosphere as long as it facilitates flow. However, the reattainment of equilibrium after removal of the load, a phenomenon known as isostatic rebound, is controlled by the viscosity of the asthenosphere. Measurement of the rates of isostatic rebound provides a means of estimating the viscosity of the upper mantle. Fennoscandia represents an example of this...
Continental And Oceanic Crust
On the basis of information presented in this and following chapters, the major differences between continental and oceanic crust can be summarized as follows 1 Layering. The large-scale layering of the continental crust is ill defined and highly variable, reflecting a complex geologic history. In places there is a broad subdivision by the Conrad discontinuity, but this is not globally developed. By contrast, the layering of the majority of oceanic crust is well defined into three distinct...
213 Seismic waves
The strain energy released by an earthquake is transmitted through the Earth by several types of seismic wave Fig. 2.2 , which propagate by elastic deformation of the rock through which they travel. Waves penetrating the interior of the Earth are known as body waves, and consist of two types corresponding to the two possible ways of deforming a solid medium. P waves, also known as longitudinal or compressional waves, correspond to elastic deformation by compression dilation. They cause the...
212 Earthquake descriptors
Earthquakes are normally assumed to originate from a single point known as the focus or hypocenter Fig. 2.1 , which is invariably within about 700 km of the surface. In reality, however, most earthquakes are generated by movement along a fault plane, so the focal region may extend for several kilometers. The point on the Earth's surface vertically above the focus is the epicenter. The angle subtended at the center of the Earth by the epicenter and the point at which the seismic waves are...
2111 Introduction
The phenomenon of isostasy concerns the response of the outer shell of the Earth to the imposition and removal of large loads. This layer, although relatively strong, is unable to support the large stresses generated by, for example, the positive weight of a mountain range or the relative lack of weight of an ocean basin. For such features to exist on the Earth's surface, some form of compensating mechanism is required to avoid the large stresses that would otherwise be generated. Isostasy was...
Further Reading
Hallam, A. 1973 A Revolution in the Earth Sciences from continental drift to plate tectonics. Oxford University Press, Oxford, UK. LeGrand, H.E. 1988 Drifting Continents and Shifting Theories. Cambridge University Press, Cambridge, UK. Marvin, U.B. 1973 Continental Drift the evolution of a concept. Smithsonian Institution, Washington, DC. Oreskes, N. 1999 The Rejection of Continental Drift theory and method in American Earth Science. Oxford University Press, New York. Oreskes, N. ed. 2001 Plate...
26 Metamorphism Of Oceanic Crust
Many of the rocks sampled from the ocean basins show evidence of metamorphism, including abundant green-schist facies assemblages and alkali metasomatism In close proximity to such rocks, however, are found completely unaltered species. It is probable that this metamorphism is accomplished by the hydrothermal circulation of seawater within the oceanic crust. There is much evidence for the existence of such circulation, such as the presence of metalliferous deposits which probably formed by the...
25 Ophiolites
The study of oceanic lithosphere has been aided by investigations of characteristic rock sequences on land known as ophiolites literally snake rock, referring to the similarity of the color and texture to snakeskin see Nicolas, 1989, for a full treatment of this topic . Ophiolites usually occur in collisional orogens Section 10.4 , and their association of deep-sea sediments, basalts, gabbros, and ultramafic rocks suggests that they originated as oceanic lithosphere and were subsequently thrust...
23 Composition Of The Earth
All bodies in the solar system are believed to have been formed by the condensation and accretion of the primitive interstellar material that made up the solar nebula. The composition of the Sun is the same as the average composition of this material. Gravitational energy was released during accretion, and together with the radioactive decay of short-lived radioactive nuclides eventually led to heating of the proto-Earth so that it differentiated into a radially symmetric body made up of a...
245 Oceanic layer 1
Layer 1 has been extensively sampled by coring and drilling. Seabed surface materials comprise unconsoli-dated deposits including terrigenous sediments carried into the deep oceans by turbidity currents, and pelagic deposits such as brown zeolite clays, calcareous and silicic oozes, and manganese nodules. These deep-sea sediments are frequently redistributed by bottom currents or contour currents, which are largely controlled by thermal and haline anomalies within the oceans. The dense, cold...
14 Impact Of Plate Tectonics
Plate tectonics is of very great significance as it represents the first theory that provides a unified explanation of the Earth's major surface features. As such it has enabled an unprecedented linking of many different aspects of geology, which had previously been considered independent and unrelated. A deeper understanding of geology has ensued from the interpretation of many branches of geology within the basic framework provided by plate tectonics. Thus, for example, explanations can be...
217 Ambiguity in focal mechanism solutions
It is apparent from Fig. 2.7 that the same distribution of compressional and dilational quadrants would be obtained if either nodal plane represented the actual fault plane. Thus, the same pattern of first motions would be obtained for sinistral motion along a north-south plane as for dextral motion along an east-west plane. Figure 2.8 Ambiguity in the focal mechanism solution of a thrust fault. Shaded areas represent regions of compressional first motions C , unshaded areas represent regions...
22 Velocity Structure Of The Earth
Knowledge of the internal layering of the Earth has been largely derived using the techniques of earthquake seismology. The shallower layers have been studied using local arrays of recorders, while the deeper layers have been investigated using global networks to detect seismic signals that have traversed the interior of the Earth. The continental crust was discovered by Andrija MohoroviCiC from studies of the seismic waves generated by the Croatia earthquake of 1909 Fig. 2.14 . Within a range...
3 Continental drift
3.2 Continental reconstructions 3.2.2 Geometric reconstructions of continents 3.2.3 The reconstruction of continents around the Atlantic 3.2.4 The reconstruction of Gondwana 3.6.6 Paleogeographic reconstructions 4 Sea floor spreading and transform faults 72 4.1 Sea floor spreading 73 4.1.3 Geomagnetic reversals 74 4.1.4 Sea floor spreading 77 4.1.7 Dating the ocean floor 84 4.2.3 Ridge jumps and transform 5.1 Plates and plate margins 92 5.2 Distribution of earthquakes 92 5.3 Relative plate...
218 Seismic tomography
Tomography is a technique whereby three-dimensional images are derived from the processing of the integrated properties of the medium that rays encounter along their paths through it. Tomography is perhaps best known in its medical applications, in which images of specific plane sections of the body are obtained using X-rays. Seismic tomography refers to the derivation of the three-dimensional velocity structure of the Earth from seismic waves. It is considerably more complex than medical...
13 Geosynclinal Theory
Prior to the acceptance of plate tectonics, the static model of the Earth encompassed the formation of tec-tonically active belts, which formed essentially by vertical movements, on the site of geosynclines. A review of the development of the geosyncline hypothesis and its explanation in terms of plate tectonics is provided by Mitchell amp Reading 1986 . Geosynclinal theory envisaged elongate, geographically fixed belts of deep subsidence and thick sediments as the precursors of mountain ranges...
216 Focal mechanism solutions of earthquakes
The seismic waves generated by earthquakes, when recorded at seismograph stations around the world, can be used to determine the nature of the faulting associated with the earthquake, to infer the orientation of the fault plane and to gain information on the state of stress of the lithosphere. The result of such an analysis is referred to as a focal mechanism solution or fault plane solution. The technique represents a very powerful method of analyzing movements of the lithosphere, in...
11 Continental Drift
Although the theory of the new global tectonics, or plate tectonics, has largely been developed since 1967, the history of ideas concerning a mobilist view of the Earth extends back considerably longer Rupke, 1970 Hallam, 1973a Vine, 1977 Frankel, 1988 . Ever since the coastlines of the continents around the Atlantic Ocean were first charted, people have been intrigued by the similarity of the coastlines of the Americas and of Europe and Africa. Possibly the first to note the similarity and...