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Tectonics

tudies of the structure of the earth began in Europe as part of a developing program of geology. Over time, we have discarded multiple theories about tectonic processes, but these are presented to show the progress of geological

understanding of the Earth. This progress has come with improvement in the tools that we use and in a better grounding of geologists in chemistry, physics and mathematics. The major physical properties of the earth can be measured with a high degree of precision. They include:

• size – 6,378,099 meters equatorial radius • shape – oblate spheroid, almost spherical • mean density – 5.517 gm/cm3 • gravity – 9.8017 gm/sec2 • moment of inertia – 0.331 mr2 • magnetic field – 8.09 x 1025 emu dipole moment • geothermal flux – 1 x 1028 erg/yr

The difference between rock densities at the surface and the total mass of the Earth is strikingly different. The average density of the Earth is 5.5 gm/cm3, but the surface rocks have densities of only 2.7 to 3.0 gm/cm3, so densities must be higher in the interior and the composition must change. This is confirmed by the moment of inertia since for a uniform Earth sphere the moment should be 0.4 mr2, the difference from the measured value and the density difference between crustal rocks and total mass all indicate that densities increase inward and that the mass is concentrated toward the center of the Earth.

Techniques of Investigation

eismic surveys combined with data from oil wells and the deep sea drilling program have been a powerful tool for analysis.

Seismic Studies

eismology is the basic tool for investigation of the interior of the Earth. Interpretation of earthquake and man-made shock wave passage through the Earth yields the most complete and accurate data about the structure and composition

of the rocks.

The release of earthquake energy is transmitted as seismic waves which can be recorded on a seismometer. Transmission of seismic energy through rocks is by homogeneous waves which travel equally in all directions. The main types of seismic waves are compressional waves (P-waves) in which the particle motion is along the direction of propagation, shear waves (S-waves) in which the particle motion is perpendicular to the direction of travel and primary waves or

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surface waves. Travel time of seismic energy is a function of the density of the rocks and their compressibility, so that from a large number of measurements and travel time plots, the densities of rock strata can be determined. More than 10 major earthquakes occur each year with each releasing more than a thousand times the energy of the Hiroshima atom bomb. Data from these Earthquakes are combined with geological information about surface rock outcrops and borehole data, laboratory experiments done on rocks at high pressures, and astronomical observations to give us a basis for interpreting the structure and composition of the deep interior of the Earth. Two terms are used in discussing the location of an earthquake. The focus is the three-dimensional position of the source of the earthquake, and the epicenter is the position on the surface above the focus. Earthquakes may be divided into groups based on depth of occurrence:

• shallow focus – above 70 km depth within the Earth

• intermediate – 70-300 km • deep focus – below 300 km

The focus of deeper earthquakes is displaced toward continents and island arcs, away from trenches, marking major underthrust fault zones – subduction zone in the plate tectonics idiom. If we map the deep and intermediate focus earthquakes, the distribution is limited and most are in the Pacific Seismic Surveys

eismic reflection technique is similar to the operation of a fathometer but

the energy is increased. When transmitted energy strikes a plane of abrupt change in density (layered bed boundaries or sediment-water interface), part of the energy is reflected. Measurement of travel time can give the depth to the interface, and the moving ship traces the bed contacts as a fathometer can trace the water-sea floor interface. An important parameter besides intensity is the frequency of the sound source. In general, lower frequencies improve penetration with loss of bed definition. Reflection profiling is used to determine structure and thickness of the sediments. Seismic refraction surveys are similar to the study of earthquake seismic energy. The energy source is a powerful explosion and the resulting transmission of energy through the strata is tracked to determine rock densities. Magnetic and Gravity Surveys

agnetism, gravity, and heat flow are properties of the Earth that can be measured at the surface. The magnetic field is dipolar, with an axis slightly offset from the axis of rotation. The field can be defined by its strength and direction

at any point on the surface. Magnetic techniques measure the relative intensity of the field at sea with a proton precession magnetometer.

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The magnetic field is related to Earth physics and interpretation of the magnetic history of the Earth has a profound bearing on interpretation of the history of the crust. The field is similar to one surrounding a two-poled bar magnet roughly aligned with the Earth's axis. The cause of the magnetic field has not been fully explained. In any theory, several features must be explained:

• the field has two poles located near the geographic poles • it shows irregular variations in both position and polarity • these variations bear no relation to the crust and therefore must have their origin deep within the Earth.

The most widely accepted view is that internal electric currents produce a magnetic field much like that formed around a wire transmitting a current. A core rich in iron and nickel would be a good electrical conductor, and a fluid outer part of such a core would allow mechanical motion of electrical charges. According to the dynamo hypothesis, the Earth's magnetic field results from core motions and rotation of the Earth affects both orientation and strength of the field. Magnetic Reversals and Anomalies

hen molten rock solidifies and cools, iron bearing minerals become oriented to the magnetic field of the Earth and are permanent magnets reflecting the field at the time the rock formed. The orientation is imprinted with the

magnetic pattern of the Earth's field as the rock cools below 578° C (Curie point). Interpretation of remnant magnetism in rocks that can be removed to the laboratory for measurements of direction and dip of the field are used to give long term movements of the magnetic axis and changes in polarity. The rocks preserve a small residual field pointing in the direction of the Earth's field at the time that they solidified. These changes are used in formulating the basic tectonic theories of continental movement. So long as the rock is not heated above the Curie Point, the magnetism remains and can be measured with laboratory instruments. If a carefully oriented sample is measured, we can determine the direction to the magnetic pole, and from the magnetic dip determine the latitude at the time the rock formed. The polarity of a magnetic field is the orientation of its positive and negative ends. Because the rocks record the orientation, we can construct a history of the Earth's field by studying magnetic orientations in rocks from many different ages and places. Reversals of the field can be measured by the positive-negative anomaly pattern from a magnetic survey. When a reversal occurs, the north magnetic pole becomes the south magnetic pole and vice versa. During the past 65 million years, we have a record of about 130 reversals . These reversals give us another way to measure geological time, with a magnetic event time scale. The north orientation is labeled normal and a time of opposite orientation (south seeking) is called reverse. The reversals of polarity probably result from occasional instabilities of motion within the fluid outer core. During a reversal, the magnetic poles migrate and the strength of the field varies erratically, and it probably weakens to near zero for brief spells. The fossil record does not show an effect on terrestrial life.

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Irregularity of the magnetic field intensity indicates that the dipole is modified by differences in permeability and magnetization of the crustal rocks. The differences between the dipole model and the observed field are termed magnetic anomalies. An instrument (proton precession magnatometer) that can measure the Earth's magnetic field intensity is towed behind a ship to find submarines, ore bodies, or shipwrecks. The magnetometer measures total field intensity which is compared to "standard" field intensity. Density variations in the rocks of the crust, masses of metal, and anything that can affect the magnetic field will create variations that are called magnetic anomalies. Intrusive igneous rocks, especially with a high magnetic content, increase the intensity of the field and are recorded as areas of positive magnetic anomalies. The basement rocks are so much more magnetic than the sediments, that for practical purposes, we assume that the magnetometer is reading basement rock magnetism and shows either concentration of magnetic material or nearness to the surface when positive anomalies are encountered. Concentration of low magnetic materials, such a halite beds and diapirs will give low readings. The anomalies are mapped for mineral exploration and to interpret geological structures. The field intensity is also affected by the polarity recorded in the basalts of the ocean floor, and magnetic intensity surveys across the Atlantic oceanic ridge led to discovery of a pattern of positive and negative magnetic anomalies. These were a major factor in development of the theory of sea floor spreading and the successor theory of plate tectonics. Gravity Anomalies

he force of gravity varies over the Earth with variations from the spherical shape, and with elevation of the surface. Variations are also caused by density inhomogeneities in the crustal rocks. These are the variations that we look for

with a gravitometer - a delicate balance for measuring differences in mass attraction. The gravity field results from the distribution of mass within the Earth, and anomalies from the theoretical model provide data for interpretation of the underlying density structure. The major gravity component is the attraction between the Earth

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and a body at the surface - the force of gravity is directed toward the center and perpendicular to its surface, which is an equipotential surface. These values are affected by elevation, so measurements are corrected for altitude and the material between the measuring station and the geoid. Differences measured after corrections - gravity anomalies - are caused by the buried rocks of different density, or by geological structures. Gravity data indicate that the continental and oceanic blocks are in mass balance (isostatic equilibrium). By visualizing the rocks of the crust and ocean as floating on the denser rock of the upper mantle (average density 3.4 gm/cm3), it is easier to understand how the continents maintain their attitude above the oceans by isostatic adjustment. When an area is loaded or unloaded, vertical movements occur. A given block of crust elevated or depressed by tectonic forces is out of balance and tends to return to isostatic balance once the forces cease to act on it. While out of balance, the block is marked by gravity anomalies. Some important conclusions are obtained by simple qualitative studies.

• The lack of systematic differences in gravity anomalies between oceans and continents shows that the deficit in mass due to the water in the oceans is compensated by material of higher density in the oceanic crust.

• Large negative anomalies associated with deep-sea trenches are associated with corresponding deficiencies of mass and indicate active forces sufficient to overcome isostatic adjustment.

Interpretation and determination of crustal structure from gravity data is not simple. The same gravity distribution can be attributed to many different mass distributions and development of a definitive model from gravity data alone is questionable. Gravity measurements are combined with seismic and magnetic surveys, and the underlying structure determined from a synthesis of the data. Heat Flow

he measurement of heat flow records the amount of heat emerging from the Earth's interior. The rate of heat loss from the Earth is about 2.4 x 1020 calories per year. A condition of thermal equilibrium exists at the ocean bottom in deep

water, allowing fairly simple measurement with probes on sediment coring devices. On land, thermal equilibrium is difficult to achieve, and measurement is difficult and restricted to mines and boreholes where sufficient time and conditions allow the development of an equilibrium state. We can assume that at one stage in its origin, the Earth was hot with temperatures in the interior on the order of 3000° C and that it has been cooling since, with turbulent convective transfer of energy. Radiogenic and gravitational energy have contributed to the heat budget, but most of the available heat came from the Earth's planetary origin. The rate of loss by convection has been damped by the upper mantle and crustal layers in which viscosity is high enough to give a thick layer dominated by the slower process of conduction loss. The available heat flow data gives correlation with tectonic environments. There is low and uniform heat flow for shield and non-orogenic or older orogenic zones. Cenozoic orogenic areas have higher heat flow. Oceanic areas have low and

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uniform values of flow except for the crests of oceanic rises, where high heat flow occurs. In island arcs, higher heat flow is associated with zones of intermediate and deep earthquakes. Ocean Drilling

he first scientific drilling operations in the deep sea began in 1961 in 945 m water depth off southern California, drilling 1,315 m into the sea floor. Immediately, a second site in 3,558 m of water, known as the Experimental

Mohole, was drilled off Baha California. This hole penetrated 183 m of sediment and 13 m of basalt, failing to reach the Mohole but demonstrating the feasibility of recovering scientifically valuable cores at depths well beyond the reach of the piston corer. In 1964, four United States oceanographic institutions joined together as JOIDES, Joint Oceanographic Institutions for Deep Earth Sampling, proposing that the U. S. National Science Foundation (NSF) support drilling off Jacksonville, Florida. Six sites were continuously cored to sub-bottom depths of more than 1 km, revealing significant oceanographic changes on the east Florida margin since the Late Cretaceous. Well preserved planktic and benthic microfossils from the cores were instrumental in developing the biostratigraphic zonation schemes used today. JOIDES then initiated the Deep Sea Drilling Project (DSDP), which originally proposed 18-months of ocean drilling in the Atlantic and Pacific Oceans. The NSF funded modification of a drilling vessel under construction; it was modified specifically for scientific ocean drilling, core recovery and analysis. The resulting Glomar Challenger spent 15 years drilling the ocean basins and providing geologic data to solidify the theory of plate tectonics, to develop the discipline of paleoceanography, and to greatly advance scientific understanding of Earth history and processes. In the 1970's, other U.S. and international institutions joined JOIDES. In 1985, the Ocean Drilling Project (ODP) succeeded DSDP with dedication of a larger, more sophisticated drillship, the JOIDES Resolution. The ODP continues past its original 10-year mission. The scientific discoveries of DSDP and ODP have affected everything from oil and mineral exploration to predicting earthquakes and global-climate fluctuations. Yet those discoveries would not have been possible without such astonishing engineering feats as hole re-entry cones, advanced piston corers, and stabilization techniques that allow drilling in stormy Antarctic seas, which is further testimony to the interdisciplinary nature of the Earth sciences. Furthermore, these discoveries would not have been possible if the United States, Germany, France, Canada, Japan, the United Kingdom, and the European Science Foundation had not dedicated the monetary resources needed to undertake this level of scientific research.

Interior of the Earth

he interior of the Earth is not accessible beyond about 10 kilometers, so the internal structure is inferred from indirect evidence.

Seismic Discontinuities

ver the past 200 years, geologists have accumulated an impressive set of data describing the structure and morphology of the continents. In this century, we have learned much more about the features of the sea floor, and in

the last half of this century, we have developed a unifying theory describing the tectonics that produced these features.

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The natural sequence is to describe the tectonic processes in the ocean basins that form the features, the modification by erosion and deposition and finally the description of the geomorphic forms moving from ocean to land. The surface of the Earth is a complex result of internal forces, the materials of construction and the physical processes acting on the surface. The basic division into continents and ocean basins results from the difference in composition of the oceanic crust and the continental crust - the rocks of the upper 5 to 70 km of the earth. The theory of plate tectonics provides a framework for understanding the forces acting on the Earth's crust, and the resultant surface features. The solid earth is divided into three principal units:

• Core • Mantle • Crust

The composition and structure of the Earth has been deduced from seismic data and study of the solar system. The basic structure of the earth is a layered globe with increasing density of material inward. The core of the earth is composed of an inner solid core, and an outer liquid core. The next layer is the mantle, which forms most of the earth. The crust is that part of the Earth from the surface to less than 70 km inward. The base of the crust is marked by a seismic event called the Mohorovicic Discontinuity (Moho). Seismic data show a number of discontinuities in seismic wave propagation in the Earth's interior which occur when abrupt changes in rock density are encountered and the seismic waves are reflected and refracted. Studies of the topography, crustal thickness, and seismic wave transmission indicate that the crust is part of a rigid plate about 100 km thick (lithosphere) that includes the upper part of the mantle. This lies above a plastic or more fluid zone of the mantle called the asthenosphere. This is a low velocity zone to seismic wave transmission.

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Although the crust has igneous, sedimentary, and metamorphic rocks, more than 95 percent of the rock is igneous. The crust is not a layer of uniform thickness, but is characterized by irregularity. Basically, it is thinnest beneath the oceans and thickest under continents. Crustal Units

rustal rocks are divided into granitic type continental

crust (average density 2.8 gm/cm3 and basaltic type oceanic crust (average 3.0 gm/cm3). The continental granitic crust is 50 to 70 km thick, compared to about five to ten km for the oceanic crust. The difference in density between the two types of crust results in a physiographic difference in elevation of the ocean basins and the continents. If we average the elevations of individual kilometer squares of the earth's surface and construct a curve of percentage of elevation, two dominant levels emerge. These correspond to the average continental platform and the ocean floor, with the boundary between them being the continental slope; extreme highs and lows are the mountains and ocean trenches. Oceanic crust is remarkably similar in all oceans. Three layers have been measured in the ocean crust:

• Layer 1 is 0.1 to 1.0 kilometers of unconsolidated sediments.

• The second layer varies in thickness and is difficult to detect if it is thin. It averages 1.7 km in thickness and is generally assumed to be basaltic material. This layer can be traced into the abyssal hills.

• Layer 3 is called the oceanic layer, and it typifies the oceanic crust. The average thickness is 4.9 kilometers.

The widespread occurrence of layer 3 and the remarkable velocity stability in widely different parts of the ocean show that it is a characteristic feature of oceanic crust. The velocity measurements put it in a composition range of gabbro, basalts and similar materials. This composition has been confirmed by drill cores. The uniformity throughout the world indicates similar material underlying all oceans. On the assumption of spreading seafloor and renewal of the oceans, a model of composition of the layers can be made. At the mid-ocean ridge, splitting and spreading of the central rift valley occurs in response to rising convection in the mantle. Molten rock wells up through the fissures and forms rounded masses of pillow lava on the valley floor. The lava cools and solidifies to form a linear wall of volcanic basalt. The next split forms on the same line and forces the previous dike apart so that another feeder of magma is injected along the axis of the previous mass. These volcanic basalts form the oceanic crust. In time, layer 1 sediments are deposited over this material.

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The immediate supply of magma does not come directly from the asthenosphere but is from an intermediate chamber within the oceanic crust itself, which is refueled from the inner mantle. As plates move apart, the chamber walls are carried sideways and molten rock solidifies against the walls as they cool. Since these cool slowly, they form the coarsely crystalline rocks such as gabbros and peridotites. The crust is probably derived from the denser mantle of the earth. As the mantle differentiated, relatively light silicon, oxygen, aluminum, potassium, sodium, calcium, carbon, nitrogen, hydrogen, helium, and lesser amounts of other elements rose to the surface to form the crust, seawater, and atmosphere. For oceanic crust, the melting of upper mantle material and extrusion at the surface of the resulting magma seems straightforward. Continental crust origin has been more complex. Continental crust did not evolve in a stable form until about 3.9 to 4.1 billion years before the present. The oldest rocks dated are from northwestern Canada with ages about 3.8 to 3.9 billion years old. However detrital zircons found in younger rocks in Australia have been dated at 4.1 to 4.2 billion years. Only when the heat from radioactivity and meteorite impacts declined to the point where large chunks of crustal rock could be preserved did we develop a stable continental crust. Mantle Convection

onvection in the mantle is believed to be the fundamental process responsible for tectonic motion, but hot spot plumes discussed next, may also have a role. The convection may be within the mantle and carry the plates on a conveyor

belt. In the conveyor belt model, the rising limbs of the convecting cells in the mantle determine the positions of oceanic ridges. The convecting mantle would cause the lithosphere to split, and the moving mantle would carry the lithosphere laterally toward the subduction zone. The descending cell would mark the location of the trench and would drag the lithosphere down into the mantle. Movements in the asthenosphere are thought to be strongly coupled to the lithosphere.

The size and shape of the convection cells within the mantle are also a matter of debate. The principle models are:

• layered mantle convection, and • whole mantle convection.

In the first model, two separate convecting layers of mantle are envisioned. The upper layer is confined largely to the asthenosphere and lithosphere. Slabs of lithosphere are known to penetrate to depths of 700 km before becoming undetectable by their seismic activity, so a significant part of the upper mantle must be involved in convection. Below 700 km, the mantle is thought to convect independently of the upper mantle and probably at a very slow rate. The second model considers convection to involve the entire mantle. Heat for whole mantle convection is supplied from the outer core. The major difference in these models is the size of the convection cells. Another variety of mantle convection involves the rise of jet-like plumes of low-density material from the core-mantle boundary region.

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Tectonics

idespread acceptance of the plate tectonics model of the Earth has overshadowed earlier theories, but in the following section we will look at earlier concepts of a mobile Earth. The theories of continental drift and sea floor

spreading have been abandoned in favor of plate tectonics, but an examination of their concepts helps us see the development of the modern plate tectonic theory. Development of Concepts

eformation of rocks and mountain building episodes were recognized early in the developing science of geology. Early concepts and efforts to find underlying causes looked at many possible mechanisms. The extensive

compression shown by the major mountain ranges were attributed to a shrinking earth in which a wrinkled and contracted outer skin resulted in compressive mountains. We were a long way from knowing what lay under the oceans. Early theories envisioned a stable configuration of oceans and continents and were based on the geology of the continental masses. They did contribute the concept of convection cells within the earth which has carried into modern theory as a mechanism of movement. Although stable continent theories are gone, some of the mechanisms are valid as overprints to a Plate Tectonic Theory. Continental Drift and a World Adrift

n 1912, Alfred Wegner proposed that the present distribution of the continents resulted

from fragmentation of an original land mass, followed by the new continents moving further apart. Although he was not the first to suggest this, the concept was developed and spread under his influence. The general similarity of the Atlantic coastlines led to the idea that the Atlantic was an immense rift left behind by spreading continents and filled with magma from below. The mechanism of drift was ascribed to tidal forces and Coriolis acceleration. The jigsaw puzzle fit of the two sides was interesting, but more solid lines of evidence were marshaled to fit the criteria:

• If two separate continents were once united, it should be possible to recognize common features. • If the continents were in different positions on the earth's surface at an earlier date, then climatic zones should

have changed in time. Series of correspondences were found in the sediments, fossils, climate, earth movements, and igneous rock formations on opposite sides of the Atlantic Ocean. Similarities of geological history and tectonic structures supported the concept. However, every evidence presented was matched with counter arguments, giving viable alternate explanations. Since the theory of continental drift supposes that the positions of the continents relative to one another and their relationship to the axis of rotation of the earth have changed in time, a method that will fix the geographic position of sites within the continents in relation to the polar axis during the geological past will disclose whether motion has taken place since that time. This is provided by the

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measurement of paleomagnetism. If rock age and paleomagnetic orientation are determined, we can obtain pole positions for various geological ages which can be plotted for the rocks of a continent. These show a grouping for a particular age and a serial progression with time. This is an indication that the phenomenon is not local, and that major displacements of the crust relative to the pole have occurred. This may be explained by:

• expansion of the Earth • change in the axis of rotation • polar wandering • continental drift -- or other mobile continent models

Another possibility is questioning of the data -- whether the data are statistically valid and a true representation of what transpired. The major trends show sufficient consistency to make error in the main conclusion unlikely. The crux of the matter is whether there are explanations of the discrepancies between polar wandering curves which do not require the hypothesis of horizontal continental displacement. Expansion or contraction of the earth does not fit our knowledge of earth physics and geological history. Polar wandering (slippage of the crust as a single unit) is possible, but it does not fit the paleomagnetic data which show divergence in the polar wandering paths for different continents. When the paths are plotted for pole position using data from North America and Europe, there is a discrepancy and divergence of paths that is too great to attribute to errors in measurement. This divergence is also an argument against expansion or change in the axis of rotation. By the time of the annual meeting of the American Association of Petroleum Geologists in 1928, opponents had marshaled heavy arguments against the idea of continental drift. The most telling flaw was failure to propose a mechanism whereby continental crustal blocks would move through a basaltic ocean crust. A simple change in concept --sea floor spreading-- solved this, but first we went through a world war and about 30 years of theories replacing drifting continents. Convection currents (theory) in the earth's mantle led to several theories of compressive force origin. The rise of ocean surveys following W.W.II revised a concept of pull-apart tectonics and earth expansion. Convective processes were joined to hot radioactive decay areas in the earth in other concepts. Similarities of flora, fauna, structures, glaciation, and other lines of evidence for continental drift can be explained by other means, but taken together with the paleomagnetic data, there is support for the concept of mobile continents. A spreading sea floor model solved some of the problems of the continental drift idea. Thermal convection circulation in the earth's mantle provides a mechanism for lateral crustal movement. Short term stresses may produce rupture of rocks, but when subjected to shearing stress over periods of millions of years, the mantle and crust can behave plastically. Although the evidence for convection currents is indirect, their reality is strongly suggested by an increasing body of independent observations. A heavy influx of ocean basin data and a marriage of geology and geophysics led to a new approach to tectonics and Earth movements, the world of sea floor spreading. This was refined and revised into a plate tectonics theory and major geological wars were resolved by peace. Sea-Floor Spreading Convection theory and a new and different set of data led to the construction of a reasonable theory, sea floor spreading, to describe the evolution of ocean basins in a pattern that would fit the data. The sea-floor (oceanic crust) was assumed to be a hydrated form of mantle material. The major features of the sea floor were described as a direct result of a spreading and renewal of the sea floor crust by convective processes. The oceanic crust was assumed to be coupled to the convecting mantle material therefore moving with it. The continental crust was carried passively with the oceanic crust to areas of convergence where it is stabilized above the downward moving oceanic crust and mantle material. The oceanic rises mark divergence and sites of rising convection cells; oceanic trenches mark convergence and descending crust in this theory.

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Magnetic surveys across the oceanic rises show a striped pattern of negative and positive anomaly bands. This pattern has been interpreted as being related to reversals of the earth's magnetic field in the geological past. It was suggested that the anomalies could be caused by stripes of the ocean floor that were alternately magnetized normally and reversed -- thus enforcing or reducing the field. If the stripes represent rock that welled up along a spreading axis during successive periods when the earth's field was "normal" or "reversed", we have a spreading ocean floor acting like an immense magnetic tape recorder. As the earth's magnetic field changed with time, its direction became frozen in the newest lavas being formed. The whole sea floor slowly moved from the spreading center carrying its signature. Spreading from a central axis should produce bilateral symmetry and the anomaly record would match itself when reversed. The Moho is not found beneath the oceanic rises and seismic velocities are lower, which can be explained by the higher temperatures, fracturing of the material, and the convective process. If the rate of movement is several centimeters per year, the floors of the oceans would be renewed every 200 to 300 million years, and the young sea floors would account for the relatively thin sediment cover. There would be a process of progressive overlap toward the rise as the ocean crust migrated, and this has been confirmed by drilling into the sea floor. Two features of the sea floor spreading concept are unique from the other theories. The oceanic crust was not conceived as a separate layer so much as it was thought to be the exposed mantle surface -- the upper part of a convection cell whose properties were modified by serpentinization. The continental borders were sites of ocean crust-mantle movement downward and therefore of compressive shear between continental and oceanic crust. Trenches and earthquake activity fit this concept, but not the eastern margin of the Americas and the western Europe-Africa margin, and this led to a new theory.

Plate Tectonics

he sea floor spreading concept was replaced with the plate tectonics model. The theory of plate tectonics is a

modification and reformulation that has utilized the concept of sea floor spreading but instituted changes in concept of the processes and interactions of blocks of the earth to fit the available data better. The theory holds that the Earth is divided into eight major plates (and about 20 minor) or spherical slabs of crust and mantle about 100 km thick that ride and move over a weak asthenosphere zone in the mantle. The margins are divergent or convergent zones. In the convergences, there is a subduction or collision of two plates. All of the major plates except the Pacific contain an embedded continent by which they are identified.

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The rocks of the ocean floor differ from the continental crust in both composition and age. Continental rocks have been dated as old as 3.5 billion years and ages of 1.5 billion are not uncommon. In contrast, the oldest ocean floor material is less than 200 million years old and the rocks of the oceanic rises are younger. This startling data is explained by the spreading sea floor and plate tectonics concepts. Oceanic crust is formed continuously from intrusion of mantle materials at the oceanic rises creating new, young ocean floor. As the plates move apart, the inflow of molten lava forms new basaltic sea floor. The other sides of the plate are shear fault zones of transcurrent and transform faulting. The older oceanic crust is drawn downward with the upper mantle as the lithosphere subducts in the deep ocean trench regions. This does not happen with the continental crust. Because of its lower density, it rides upward and the continents are progressively built up while the ocean floor is always in the process of being renewed and destroyed. The features and changes in the Earth that were proposed by adherents of continental drift and spreading sea floor are part of the plate tectonics theory -- the changes are in the mechanisms, plate definition and the boundaries, and the lithosphere concept. The driving force is still convection current circulation, but additional thought has been given as to how the plates are moved. About 200 million years ago, the continental land masses were joined as one major continent. There were movements prior to this which can be traced, but because of limited outcrops, the records are incomplete. The

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convergent movement is reflected in the record of major mountain building. The latest episode of plate movement resulted in the spreading apart of the continents to their present position -- a movement that is still in progress. We know that the changes in movement patterns occurred, but do not know the causes. If we use the 1000 meter contour on the continental slope to define the edges of the continental blocks, there is an extraordinary fit of South American to Africa and North America to Eurasia, India, Australia, and Antarctica fit to show the form of Pangaea. This fit is reinforced by using paleomagnetic data in the reconstruction, and we can see the fit of tectonic and stratigraphic trends that are older than 200 million years. The origin of today's oceans lies in the pattern of breakup and movement of the plates. Since the movements create strains and release at the boundaries, the plate boundaries are marked by earthquake activity. Earthquake distribution is not random and the zones of activity extending through the Mediterranean, Middle East, Northern India, around the Pacific, and along the ocean rises mark principal plate boundaries.

Differences in earthquake focus and intensity allow us to distinguish different types of boundaries. Under the median valley of the ocean rise where the plates diverge, earthquakes are relatively shallow and because of less crustal rigidity and states of tension, are of relatively low intensity. Where the plates slide past one another along transform faults -- as on the north coast of Turkey -- and the ocean rise offsets, the earthquakes are shallow to intermediate depths. On the rises, the intensities are not severe, but where the trace cuts through continental edges, major earthquakes can occur. Intermediate and deep focus earthquakes are restricted to convergent boundaries where subduction occurs.

• The Atlantic Ocean, the Caribbean Sea, and the Indian Ocean are new ocean basins. • The Mediterranean Sea is a remnant of the Tethys Sea and is still undergoing closure. • The Pacific Ocean is bounded by subduction zones and it is closing as the Atlantic grows. • The oldest oceanic crust is found in the northwestern corner of the Pacific because it is an ocean where crust has

been destroyed while new crust was forming in the other oceans. • The Americas have drifted west, while those continents around the Indian Ocean have moved north. • Antarctica has remained almost stationary, as has Eurasia, but Eurasia is undergoing clockwise rotation in which

Europe has moved north and China southward. • The last major event in this movement was the detachment of Australia from Antarctica in the Eocene - about 55

million years ago.

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Divergent Boundaries

ivergent plate boundaries are axes of spreading where a plate splits and is pulled apart. These boundaries are uplifted by upwelling heat and characterized by tensional stresses that produce block faulting, fractures and open fissures.

Basaltic magma derived from partial melting of the mantle is injected into the fissures and extruded to form new seafloor.

The initial rift zone in the disruption of a continent is marked by an introduction of oceanic crust between continental (forming the Atlantic Ocean) or oceanic crust blocks and a general uplift. These crustal boundaries are passive margins and tectonic activity is minor once separation is achieved. At the present time, East Africa is bowed upward in a broad arch which is splitting at the crest, forming a linear rift valley marked by volcanic activity and the formation of pillow lava. This is a first step in the divergent boundary. As the process continues, the rift floor will sink and oceanic crust will grow. There is already oceanic crust in the Afar region of the African rift valley. As the margins spread, the flow will cool and subside, forming an oceanic area like the Red Sea. The passive margins are relatively free of mountain chains and tectonic activity since they lie within a plate.

The ocean rise looks like a sinuous mountain chain some 1000 km wide. The axis has straight crestal/median valley segments that are cut and offset by transform

faults normal to the median valley. This pattern results from the spreading process which adapts itself to the shapes of the retreating continents. Rifting of the median valley is related to the rising convection cell. The warmer convecting mantle creates a zone of upward pressure which forces the walls of the crest upward and outward and linear fissures allow lava flow which erupts to form small hills of pillow lava. In the crust below the eruption, the magma cools, forming a dike of volcanic rock (basalt). The next split pulls the dike apart and more magma is injected along the axis of the previous dike. Each of these injected dikes is given a magnetic polarity signature as it cools. Each successive injection adds younger material as the older dikes are carried away from the injection axis. The immediate magma source is an intermediate chamber within the oceanic crust along the axis. This chamber is refueled from within the inner mantle. As the plates move apart, the chamber walls are carried sideways and molten rock solidifies against the walls. These plutonic bodies cool slowly forming coarse grained basic igneous rocks. Transform Boundaries

he oceanic ridge is broken into segments as the crust moves along numerous shear fault lines normal to the axis (at 2 to 10 cm/yr). This

movement produces shallow earthquakes. Beyond the offset area, the two sides move together and fault shear becomes negligible. The fault has movement at the middle, but not the ends, and an opposite sense of direction of faulting on either side of the center – these are the

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characteristics of a transform fault. These faults are by definition restricted to areas of lateral spreading. Transform faults may also form boundaries where plates are offset by major lateral movement. Convergent Boundaries

late convergences, where plates collide, are tectonically active with complicated geological processes, including igneous activity, crustal deformation, and mountain building (orogeny). The collision and subduction of plates may

follow many patterns, but several basic responses can be related to modern tectonic situations. Oceanic crust meets oceanic crust in the Pacific and it seems to be a matter of chance as to which plate is subducted. Along the Tonga-Kermadec Trench, the Pacific plate moves under the Indo-Australian lithosphere but in the nearby Solomon basin along the same plate boundary, the Indo-Australian plate is subducting. At both sites, chains of volcanic islands form above the descending plate and the subduction zone is marked by earthquakes. When oceanic crust meets continental crust, the oceanic plate subducts. As the Nacza plate moves under South America, earthquakes and volcanic activity are associated with the Andes. Subduction destroys oceanic crust and reduces the oceanic area. The Marianas Trench is separated from Asia by the Philippine Sea which is a case of oceanic crust colliding with a marginal sea bordering a continent. Upwelling magma from the subducting oceanic crust is trapped between Asia and the Pacific. The marginal back-arc sea floor is active and local spreading centers force the crust toward the Pacific plate. A final ocean extinction occurs when continental blocks meet in collision. The collision of India and Asia 30 million years ago uplifted continental areas into the Himalayas and the approach of Africa to Europe will soon (less than 50 million years) eliminate the Mediterranean, Baltic, and Black Seas and then form another great mountain chain. When two plates collide in a zone of convergence and one plate (oceanic) passes under the other, a layer of sedimentary rock on the oceanic plate is scraped off and accumulates as an island arc or against a continental margin as exotic terrain. The lithosphere (oceanic crust - upper mantle) slab of some 100 km thickness subducts at an angle of some 30 degrees with melting of the surface due to friction and pressure. At depths of 100 to 300 km (asthenosphere layer), the lighter molten rocks force their way upward behind the subduction zone, forming a volcanic chain. The slab of subducted lithosphere moves downward, causing earthquakes until it finally breaks up at a depth of about 700 km. This whole sloping surface is an area of shallow to deep earthquake activity of major intensity.

Hot Spots

ot spots are a form of intraplate volcanism. Isolated hot-spot volcanoes result from the presence of mantle plumes – columns of magma rising in the upper mantle. If the overlying plate is weak, some of the magma breaks through to

form a volcano. Composition of the material in the eruption depends on the composition of the plate through which the magma rises; along with the composition of the magma source in the mantle. From studies of hotspots, the basalt is different from the basalt that forms from the upper mantle at spreading centers.

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The idea of hotspots was established in 1963 from observations concerning the geology of the Hawaiian Islands. It was known that the Islands are progressively older from the southeastern most island of Hawaii to the northwest. Mantle plumes (which are models and theories that have been postulated from indirect evidence) are columns of molten or hot rock that originate deep inside Earth's mantle and rise slowly toward the surface, lifting the crust and forming volcanoes. They may consist of hot mantle material rising as blobs rather than in a continuous column. They are thought of as having diameters of 100-200 km and rising at rates of perhaps 2 m per year. They have a variety of shapes and sizes, and they originate at various depths between 700 km and the core mantle boundary (2900 km). Melting occurs as the pressure drops when the material rises toward the surface. Their position appears to be relatively stationary, so the lithospheric plates drift over them. The plumes are therefore independent of the major tectonic elements of the crust and they rise up under continents and ocean plates alike. The plumes may form over regions of locally high concentrations of radioactive elements in the mantle (heat producing) or rise over anomalies in the outer core. Mantle plumes appear to be temporary features that form and ultimately fade and die with a typical life span on the order of 100 million years. Bathymetric data in the Pacific Ocean show an L-shaped chain of volcanic islands and submerged volcanoes (the Emperor-Hawaiian chain) that show a progression of ages when dated radiometrically. Dates run from 75 million years at the northwestern end of the chain to 40 million years at the bend, and still active volcanoes in Hawaii . From the distance and orientation, the direction of plate movement and rate can be determined. Hot spots occur under continents as well as beneath ocean basins. They are more easily detected in the oceans, perhaps because the magmas can more readily work their way up through the thinner oceanic lithosphere. The use of hot spots to mark plate motion depends upon a presumption that movement of the hot spots is slight. Other lines of study of plate motion and position of hot spots indicate that the hot spots do move, but this movement is very slow in relation to the movement of plates. A high concentration of hot spots is found in the African plate. Africa has apparently come to rest over a concentration of hot spots. This has developed uplift and a unique topography. Other plates (Antarctica, China, southeast Asia) which are moving slowly, have higher numbers of hot spots. In rapidly moving plates such as the North and South American plates, hot spots are rare. Hot spots not only mark the movement of plates, but they may also play a part in the movement of plates. When a continent comes to rest, the dome that swells up over a hot spot is subject to fracturing and producing a three-armed rift. These may initiate a zone of divergence. Typically, two arms of the rift open to form an ocean basin and the third arm fails and remains as a fissure in the continental landmass. By restoring the margins of the Atlantic Ocean to their Pangaea position, an abundance of three-armed rifts are revealed. The successful arms merged to form the mid ocean spreading zone and the unsuccessful ones remained as rifts extending into the continents.

Patterns of Plate Movement

ost of the literature discusses the last 200 million years of plate movement that changed the worlds continents from a single land mass to the present configuration. We will discuss this, but also go back in time to describe some of M

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the earlier movements. In geology, the farther back we go, less information is available. Rock outcrops become scarce, fossils non-existent, and the rocks have been intensely folded and distorted, losing much of the original character. The continents are composed of lighter silicic material that rides passively on the moving plates. Except for the small amount of continentally derived sediment subducted back into the mantle, the less dense material is so buoyant that it cannot sink into the denser mantle material below. The continents move with the plates, sometimes colliding and sometimes splitting. They are much older than the ocean basins. Because continents are not consumed back into the mantle, they preserve records of plate movements in the early history of Earth – records in the form of ancient faults, old mountain belts, granitic batholiths, and sediments deposited along ancient continental margins. Ordovician mountain building resulted from the conversion of an Early Ordovician passive margin to a mid-Ordovician active margin by collision with a volcanic arc or microcontinent. The plate tectonic interpretation of similarities of mountains in North America and Europe suggests that they were separated until mid-Paleozoic time. During the Ordovician and Silurian periods, these two continents and several microcontinents were moving toward each other as the intervening ocean was being consumed by subduction on both sides. This pattern of assembly explains the exotic terranes that have been identified in the Appalachian region of North America. The land masses and microcontinents were fused together by continental collision mountain-building in Devonian time. The late Paleozoic was dominated by the collision of various continents to form the supercontinent of Pangaea. In the Pennsylvanian and Permian, the eastern and southern margins of North America collided with Africa and South America, producing a mountain belt from the Appalachians to the southern Rocky Mountains. By Permian time, the supercontinent was fully assembled and uplifted with marine seaways disappearing from the continental interiors. The end of the Paleozoic was a time of unusually widespread mountain building from the continental collisions. The Mesozoic brought the tectonic effects of the breakup of Pangaea. In eastern North America, the Atlantic Ocean began opening forming Triassic rift grabens. As the Atlantic opened and the North American plate moved westward over the Pacific, major subduction occurred on the western margin which included terrane collision. By the Jurassic, a volcanic arc complex had developed along the western margin, and exotic terranes continued to be accreted to the western margin. The speed of spreading in the Atlantic produced high ocean ridges that caused sea level rise and shallow sea invasion of continental blocks.

Summary

umerous lines of data and evidence lend support to the concept of a mobile lithosphere. Single items or several, and indeed all of the points can be refuted or explained in other ways. However, as in a job of police detection, it is the

fact of accordance and the weight of the evidence as each bit adds more support which gives credence to the theory of plate tectonics. As presented, the plate tectonics concept makes a neat package, providing a framework for interpreting geological processes that works. The basic movements of lithospheric plates and tectonic features resulting from the movement have been described in detail, but no explanation has been given for epirogenic events in the platforms, block faulting, and other non-marginal features. Geologists have described regularities and cyclic events in the history of the earth, and the concept of plate tectonics may be a random, incidental occurrence of tectonic activity. There is an inherent limitation of application of plate tectonics that is neglected by its adherents. Processes within the earth may generate various modes of tectonic behavior – one of which is the plate response. An unfortunate tendency has been selectivity in choice of data used,

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attributing all features to drift patterns, to base broad interpretations on limited data, and worst of all, to be woefully ignorant of basic geological knowledge in many of the model areas. Plate movement in time - images generated with http://www.odsn.de/odsn/services/paleomap/paleomap.html program.

Plate tectonics is not an answer to all problems, but it is accepted as the major tectonic process shaping the earth. The lithosphere – asthenosphere and mantle convection are accepted models. If mantle convection and a rising column – divergence occurs – this could drive a spreading sea floor plate tectonics system – it could also lead to uplift and epirogenic movements, block faulting, etc., without rifting and sea floor generation. An uplifted oceanic rise can be driven as described in the plate tectonics model, or may slide by gravity tectonics. The structural patterns are not completely answered and at its present stage, plate tectonics is a working hypothesis. It may be the answer, or part of the answer working in conjunction with other processes. The best approach is an open mind.