Many features our planet have presented scientist with contradiction and puzzles.The remains of warm-water coral reefs are found of the coast of british isle ; marine fossil accure high in the alps and the himalays ; and coal deposits that were formed in warm tropical climate are found in Northern Europe, Siberia, Northest north Amrica. Great mountains range divide the ocean, volcanoes border the coast of the pacific ocean, and deep ocean tranchess are found adjacent to long island arcs. No single coherent theory explained all this futures until the new technology and new scientific discoveries of the 1950s and early 1960s combined to trigger a complete reexamination of earth s history 3.1 Earth s Interior The most detail information we have about the onterior has come from roughly a century of recording and studying passage of seismic waves through the body earth. Geologist and geophysicists monitor recording stasion all over earth surface that measure the type, strength, and arrival time of seismic waves generates by earthquakes, volcanic eruptions and man made explotions. There are two basic kind of seismic waves : surface wave travel along earth surface, and body waves travel through earth interior. Most of the information we have about earth interior comes from the study of body waves. There are two kind of body waves. These are P-waves, or primary wave ( so called because they travel faster than any other seismic wave and are the first to arrive at a recording stasion ), and S-waves, or secondary wave ( so called because they travel more slowly than P-waves and are the second wave to arrive at a stasion). P-waves and S-waves produce different types of motion in the material they travel through. P waves, also known as compresional waves, alternately compress and stretch the material they pass through, causing and oscillilation in the same direction as they move. P-waves can travel through all three states of meter : solid, liquid, and gas ( sound propagates through the air and the ocean as a compresional wave ). S-waves, also known as shear waves, oscillate at right angles to their direction of ocean ( similar to a plucked string ). S-wave propagate only through solids. The motion generated in materials by these two type of waves is shown in figures 3.1 The velocity of seismic waves depend on the carateristic off the bacterial they travel through including it s chemistry, it s density, and changes in the physical state of the material ( solid, partially molten, or molten ) cause by variation in preasurre and temperature with depth. When seismic waves encounter boundaries seperating layer that have significanly different charateristics, their speed and direction of travel will change. They maybe reflected of a boundaries, refracted allong it, or transmitted through it. Detail modeling pf the path taken by sistemic wave and their expected travel times through earth, as a function of distance, has produce a simple earth model consisting of four major layers as ilustrated in figures 3.2 at the planet s center in the inner core. It is solid and nearly five times as dense as common surface rocks. The inner core is composed primarily of iron with laser amounts of other element that most likely include nickel, sulfur, and oxygen. Surrounding the inner core is a region of the same chemical composition that is at least partially if not totally molten called the outer core. The outer core would behave like a fluid even if as much as 30% of it were composed of suspend cristal of iron that had formed from the surrounding liquid. Material in the outer core flows rapidly compared to the layer above it, probabaly on the other off km/year ; this generates earth magnetic field. Recent studies have determined that the inner core rotate a few The base od the lithosphere corresponds roughly to the region in the mantle where temperatures reach 650 0C + 100 0 C. do not propagate through the outer core. more buoyant material rises toward and the surface while cooler. comprises about 70 % of earth volume. See figure 3. it reaches a maximum thickness of about 100 km at an age of 80 million years. In oceanic regions the lithosphere thickens with increasing age of the sea floor. It is called the mohorovicic discontinuity.2 The four major layers of earth determined by studying the passage of seismic waves through the interior. S-waves. known as the Moho. rigid surface shell called the listhosphere. thin surface layer called the crust.1 Particle motin in seismic waves. Altough the mantle is solid.1 for a comparison of these layers and their properties. which only travel through solid. This has led to the identification od a strong. some parts of it are weaker than other. There are two kind of crust. continental and oceanic. The velocity of these motion is generally on the order of cm/year. The boundarie between the crust and the mantle is a chemical boundary.thins off a degree per year faster that the mantle. denser material sinks. or flow. with an average thickness of about 7 km. rigid. Material in the mantle flows very slowly in response to variation in temperature the crate changes in density. (b) S-wave motion can be illustrated by shaking rope to transmit a delfection along its lenght. in response to an aplied force. . Vibration si perpendicular to the direction of propagation. It is last dense and cooler than the core and is composed of magnesium iron silicates ( rocky material rather than metallic like the core ). allumunium and silica. mantle rock begins to lose its strength. (a) P-wave motion can be ilustrated with a sudden push in the end of a streched spring. Its marily of granite-type rock. Andrija Mohorovicic. which is low in silica and high in iron.2 The Lithosphere and Asthenosphere Rocks that behave rigidly do not deform permanently when a force less than the breaking strength of the rock is applied. potassium. The crust has not drawn to scale. and is named fot its discoverer. Warmer. y Figure 3. At higher temperatures at these depths. It actual thickness at this scale would be roughly the thickness of the black lines used in the illustration. which consists og crust and upper mantle material fused together. Vibrati n is parallel to the direction of o propagation.2 and table 3. The next layer. Rocks that have ductile behavior will deform. the mantle. Earth outer most layer is the cold. 3. This increase in is ward rotation rate is through to becaused by motion in the fluid outer core. which has a high content of sodium. Oceanic crust is relatifely dense. It is the lithosphere that comprises the plates of plates tectonics. magnesium and calcium. It si more homogeneous both chemically and structurally than continental crust and is compesed primarily of basalt-type rock. Continental crust is a relatively light and averages about 40 km in thickness. y Figure 3. much slower than the flow in the liquid outer core. nickel 9.2. because of temperature gradients and density differences. sometimes called mesosphere. magnesium and iron Solid and Magnesium 3. Table 3. potassium and alumunium ~0 1000 Oceanic 0 10 7 (average) Solid Mantle Base of 2866 1100 3200 curst 2891 Outer core 2891 2258 3200 5149 Inner core 5149 .2 Layer Lithosphere Upper Layers of Earth. with increasing depth. The athenosphere behaves in a ductile manner.3 and are compared in table 3.5500 6371 y Figures 3.350 Solid ( ~1% melt) Ductile response Mesosphere 350 2891 (core mantle 2541 Solid.1 Layer Layers of Earth Depth Layers Range (km) Thickness (km) 40 (average) State Composition Density (g/cm3) Temperature (0C) Crust Continental 0 65 Solid Silicates rich 2.8 13. The athenosphere is often equated with a region of low seismic velocity called the low velocity zone (LVZ). response 0 to 100 150 (continental regions) Asthenosphere Base of lithosphere 350 200 . The increase in pressure. Table 3. Rigid.The lithosphere is underlain by a weak. nickel 12.1222 Solid Iron.2 5. Seismic waves travel more slowly through the athenosphere.1 4000 .2 ~0 1100 . The lithosphere and underlying asthenosphere are shown in figure 3. Mobile boundary) Silicates rich 3.6 mobile iron silicates Liquid Iron.67 in sodium. Based on Their Response to Applied Stress Depth Range (km) Layers Thickness (km) Characteristics 0 100 (oceanic regions) 0 150 Solid.9 12. deforming and flowing slowly when stressed. deformable region in the mantle called the asthenosphere where the temperature and pressure conditions lead to partial melting of the rock and loss of strength. indicating that it may be as mush as 1% melt.0 in calcium. partially molten asthenosphere. Its rides on the weak. moving upward in some regions and downward in others.3 The Listhosphere is formed from the fusion of crust and upper mantle. results in greater strength once more in lower mantle. where the material is once again solid but will convect slowly. for which he retained the earliar name Gondwanaland. Alfred L. Wegener and Frank B. Wegener s theory proveked considerable debate in the decade of the 1920s. separated from the southern portion formed in Africa. this model si called whole-mantle covection. Wegener s theory. There are currently two proposed models of mantle convection.3 History of a Theory The possible fit of the bulge of south america into the bight of africa was noted by the English scholar Francis Bacon (1561 1626). the Thethys. which he called Laurasia. hardens. Panthalassa. scientist were able examine. India.6). Taylor independently proposed that the continents were slowly moving about Earth s surface. In Hess s model the upward-moving mantle rock would carry hear with it toward the surface. periodically offset by long faults. Australia. 195 Ma (million years before the present). Harry H. Hess proposed that relatively old. and creates new sea floor and oceanic crust. the northern portion composed of North America and Eurasia. Hess (1906-69) of Princeton University promoted the concept that there are convection cells within Earth s mantle driven by heat from Earth s core and natural radioactivity in the mantle. that was the ancestral Pacific Ocean and a seconf small ocean. and de oceanic nse lithosphere descends back into Earth s interior at deep steep-sided ocean trenches. proposed the existence of a single supercontinent he called Pangaea (figure 3. cold. These surveys clearly demonstrated that the deep-ocean floot is not a flat. the French naturalist Georges de Buffon (1707 1788).5. and Antartica. and others in later years.4). The data from these surveys were used to construct the first phyiographyc map of the world oceans. the German scientist and explorer Alecander von Humboldt (1769 1859). One confined to the upper mantle above a depth of 700 Km and the other in the lower mantle. 3.4 Pangea in the early Jurassic. in detail and the deep-ocean floor. Hess s original model is correct with convections throughout the entire mantle from the base of the Lithosphere to the core-mantle boundary . proposed that the southern continents had once been joined into a single continent he called Gondwanaland. and for the first time. active volcanism along the axis of the ridge results in the extrusion of basaltic magma onto the sea floor where it cools. shown in figure 3. This hear would cause the overlying oceanic crust and shallow mantle rock to expand. Laurasia and Gondwanaland. first. Pangaea is composed of the two subcontinents. Eduard Suess. featureless environment but includes steep-sided ocean trenches and a series of mountain ranges. At the beginning of this century. an austrian geologist. South America.3.4 Evidence for a New Theory In the early 1950s a worldwide effort was made to survey the sea floor. There is one great ocean. thereby creating great mountain ranges or ridges on the sea floor (figure 3. . but most geologist agreed that it was not possible to move the continental rock masses through the rigid basaltic crust of the ocean basins. In a series of volumes published between 1885 and 1909. y Figure 3. often called continental drift. Follow the north-south ridge in the centre of the Atlantic ocean . which are small propellers mounted perpendicular to the ship s keel that can be used to move the ship sideways or rotate it. and geophyicists explored Earth s crust on land and under the oceans. These zones are found to correspond to the areas along the ridges.6 Seafloor spreading creates new crust at mid-ocean ridges and subducts old crust in deep-sea trenches.7).6). The ocean ridges where new crust is fo rmed are also known as spreading centers. Vertical cylindrical samples or cores.5 Evidence for Crustal Motion As oceanographers. One of the great challanges of deep-ocean drilling is keeping the drilling ship in a stable position at the surface. additional evidence to support the idea of seafloor spreading began to accumulate.5 Aphysiographyc chart of the world oceans y Figure 3.9). In the late summer of 1968 the specially constructed drilling ship Glomar Challanger went to sea on its first drilling expedition.5. Conspicuous trenches may be seen along the west coast of South America. it connects around Africa to the ridge in central Indian ocean. were obtained by drilling through the sediments that cover the ocean bottom and into the basaltic rock of the oceanic crust. Earthquakes that occur deeper than about 100 km can extend to depths as great as about 700 km. geologist. An epicenter is the point on Earth s surface directly above the earthquakes source. or spreading centres and the trenches or subduction zones. highest in the vicinity of the mid-ocean ridges where the crust is thinner and younger and decreasing as the distance from the ridge center and crustal age increases (figure 3. Epicenter of earthquakes were known to be distributed around Earth in relatively narrow and distict zones (figure 3.These mid-ocean ridges and deep trenches may be seen in figure 3. 3. This is accomplished by using a specialized propulsion system including bow and stern thrusters. This process is shown being driven by convection cells in mantle. most of the rising mantle material is turned aside under the rigid lithosphere. y Figure 3. This lateral movement of the lithosphere produces seafloor spreading (figure 3. . The deeper the earthquakes are. ocean trenches mark subduction zones where older lithsophere descends into the mantle. the more they are displaced away from the axis of the trench. Researchers sank probes into the sea floor to measure the heat from heat flow shows a regular pattern. beneath continets and island arcs. These deep events are ascociated with subduction at ocean trenches. This process required the development of a new technology and a new type of ship. Although the ascending magma breaks through the oceanic crust and solidifies to form new sea floor. seaward of the Aleutian Islands and along the eastern coast of Asia into the western South Pacific. with two opposite poles is called a dipole. When basaltic magma erupts on the sea floor along ocean ridges it cools and solidifies to form basaltic rock. In this manner. Particles of magnetic act like smal magnets. A magnetic field like Earth s. y Figure 3. each time a layer of volcanic material solidified. This temperature change with depth is used to compute the heat flow through the sediment.10 Deep ocean drilling technique.7 Earthquake epicenters. Acoustical guidence systems are used to maneuver the drilling ship over the bore hole and to guide the drill string back into the bore hole . 1961-67. The north magnetic pole has been moving at a rate of approximately 40 km per year and the south magnetic pole has been moving roughly 10 to 15 km per year. the magnetic signature ( magnetic strength and orientation ) of these particles is frozen into them creating a fossil magnetism that will remain unchange unless the rock is heated again to a temperature above the curie temperature. Nearly 170 magnetic reveals have been identified during the last 76 million years.12 ). Earth s gamiliar north (900 N) and south (900 S) geographic polse mark the acis about which it rotates.11 shows that are sediments closest to the ridge system are thin over the new crust while older crust farther away from the ridge system is more deeply buried. roughly 5800 C. the type of rock that makes up the oceanic crust. Figure 3. No ocean crust older than 180 million years was found and sediment age and thickness were shown to increase with distance from the ocean ridge system. The investigation of fossil magnetism in rocks is the science of paleomagnetism.50 away from Earth s axis of rotation (figure 3. Earth s surface at the magnetic equator and converge and dip toward Earth s surface at the magnetic poles ( Figure 3. it record the magnetic orientation and polarity of the time period in which it cooled. At different times in Earth s history the present north and south magnetic poles have changed places. (a) epicenters of earthquakes with dephts less than 100 km outline regions of crustal movement. Figure 3. The most elegant proof for spreading came from a study of the magnetic evidence locked into the oceans floors. or curie point.The cores taken by the Glomar Challanger provided much of the data needed to establish the existence of seafloor spreading. When the temperature of the rock drops below a critical level called the curie temperature. During this time the magnetite grains in the rock become magnetized in a direction parallel to the existing magnetic field at that time and place. our present magnetic orientation has existed for 710.8 Recovering a heat flow probe from the deep-sea floor. The probe consist of a cylinder holding the measuring and recording instrument above a thin probe that penetrate the sediment. Figure 3. These particles are particularly abundant in basalt. (b) Epicenter of earthquakes with depths greater than 100 km are related to subduction.9 Heat flow through the pacific Ocean floor. y y y Figure 3. rock can preserve a record of the strength an orientation of Earth s magnetic field at the time of their formation. Values are shown against age of crust and distance from the ridge crest. Thermister (devices that measure temperature electronically) at the top and bottom of the probe measure the temperature difference over a fixed distance equal to the length of the probe.000 years.12). Most igneous rocks contain particles of a naturally magnetic iron mineral called magnetite. Earth also behaves as if it has a giant bar magnet embedded in its interior tilted roughly 11. Paleomagnetic studies of fossil magnetism in continental rocks provide evidence of relative motion between tectonic plates and Earth s magnetic poles through time. the direction of the magnetic was recorded in the new crust. there should be a symmetrical pattern of magnetic stripes centered at the ridges and becoming older away from the ridges ( Figure 3. Some plates. Vine anf Matthews proposed that if such were the case. When fred J.13 ). consist entirely of oceanic lithosphere. it is possible to create a plot of the apparent location of the north magnetic pole through time. the resulting maps revealed a pattern of stripes that ran parallel with the mid-ocean ridge ( Figure 3. Mathews of Cambridge University proposed that these stripes represented a recording of the polar reversalsod Earth s magnetic fiels. y y y Figure 3. it locked in the direction of the existing magnetic field. NP and SP indicate the north and south geographic poles respectively.11 Age and thickness Figure 3. but most plates like south american plate. Seefloor spreading moved this material off on either side of the ridge. Vine and Drummond H. Each time Earth s magnetic field reversed. created by seafloor spreading during the past 200 million years. producing the majority of the earthquake and volcanic activity that occurs on Earth. A freely suspended magnet will aling itself with these lines of magnetic force. A polar wandering curve records the motion of the plate with respect to a nearly stasionary north magnetic pole.13 ). or the last of 5% of Earth s history. 3. they interact with one another along their boundaries. the north seeking end of magnet pointing to the north magnetic pole and the south seeking end pointing to the south magnetic pole. Such as a polar wandering curve ( Figure 3. . consist of variable amount of both oceanic and continental lithosphere with a gadual transition from one to the other along the margins of contintents.13 Reversals in Earth s magnetic polarity cause the symmetricakky stripped pattern centered on the Mid-Atlantic Ridge Figure 3. By measuring the fossil magnetism in continental rocks of different ages that are all found on the same tectonic plate.15 ). to be replaced by more molten rock.14 The age of the ocean crust based on seafloor sprading magnetic patterns is shown in this color shaded image Figure 3. such as the pacific plate. Plates move away from each other along divergent plate boundaries. A polar wandering curve is evidence that the relative positions of the plate and the north magnetic pole have changed with time. When magnetometers were towed over the sia floor during the early 1960s. frozen into the sea floor. These are called transform boundaries and are marked by large faults called transform faults.6 Plate Tectonics The theory of plate tectonics incorporates the ideas of continental drift and seafloor spreading in a unified model.15 The position of the north pole million years before present. Another type of boundaries occur where two plates are neither converging nor diverging.12 Lines of magnetic force surround Earth and converge at the magnetic poles. Plates move toward one another and collide along convergent plate boundaries. but are simply sliding past one another. The present ocean basins are not old but new. As the plates move. As the molten basalt rose along the crack of the ridge system and solidified.y y Figure 3. 3. and continent continent convergent. As two paltes move away from the ridge crest they will simply slide past one another along the transformation fault. seafloor plateaus or parts of other continental landmasses.9 Convergent Boundaries Plates move toward each other along convergent plate boundaries. forming a rift valley along the length of the boundary. causing the thickness of the oceanic lithosphere to increase with age and distance from the ridge. the margin is called a leading or active margin. Subduction of downgoing plate typically creates and continues to occur along an ocean trench. Crustal fragments wwith properties and histories distinct from adjoining crust and added by collisions are known as terranes. These fossil faults. however.8 Tranform Boundaries The ocean ridge system is divided into segments that are separated by transform faults. 3. The direction og motion og the plates on either side of a transformation fault is determined by the direction of seafloor spreading in each plate and is opposite the sense of diplacement of the ridge segments. When a plate boundary is located along a continental margin. Most divergent boundaries are located along the axis of the mid-ocean ridge system.7 Divergent Boundaries A series if successive divergent boundaries were responsible for the breakup of Pangaea. Three possible combination of cnverging lithosphere occur : ocean continental. Difference in the age and temperature of the planet accros a transform boundary can create significant and rapid changes in the elevation of the sea floor. This is caused by the subduction or sinking of one plate into the manle beneath the other plate. fuses to the base of the crust and begins to behave rigidly. In the early stages of a volcanic eruption. These are also frequently referred to as atlantic style margin since they found on both sides of the atlantic ocean as well as around antartica. As the two plates diverge. seamont volcanoes. Faulting along the boundary will create rifting in the continental crust. Terranes may beb pieces of island arc systems. the artic ocean and in the indian ocean. 3. Active continental margins are often marked by ocean trenches where oceanic lithosphere is subducted beneath the edge of the continent. The magma solidifies to form a new ocean crust along the edges of each diverging plate.10 Continental Margin When a continent rifts and moves away from a spreading center the resultant continental margin is known as a trailing or passive margin.3. ocean-ocean convergent.center. . baswaltic magma can flow rapidly onto the seafloor in relatively flat flows called sheet flows that may extend several kilometers away from their source. From a continuous linear feature called a fracture zone. Plates move apart at different rates along the length of the ridge system. producing the individual continents and ocean basins.alongwith the active transform fault between the two segmenys of ridge crest. the mantle rock immidiately beneath the crust cools. The thickness of oceanic lithosphere increases at a rate proportional to the square roof age. production of heat is continous.12 Hot Spots Scattered around the earth are appoximately forty specific fixed areas of isolated volcanic activity known as hot spots. As an oceanic plate moves over a hot spot. and then slide down the side of the elevated ridge. They are found under continents and ocean. this model proposes that the heat is lost at varying rates that are related to the movement of the continents. The spreading rate affects the physical structure of divergent plate boundaries. cool and subside. By the late Cretaceous. in the center of plates and at the mid-ocean ridges.11 Motion of the plates The mechanism that dries the plates is still not fully understood. successive and usually nonexplosive. The rate at which plate moves away from the axis of the ridge is commonly known as the half spreading rate while the rate at which the two plate move away from each other is called full spreading rate or simply the spreading rate. pushing the rest of the plate away from the axis of the ridge. The actual movement of the plates may be due to a combination of forces including slab pll. About 200 million years ago. the continents were all joined in a single land-mass we call Pangaea. Pangaea began to break apart into Laurasia and Gondwana. 3. . 3. The sinking lithosphere may create tension in the rest of the plate away from the ridge.13 The breakup of Pangaea Figure 3. Twenty million years ago. coalesce and compress. ridge push and convection in the mantle as envisioned by Harry Hess. Plates could be forced apart at ridges by formation of new crust. As the continent shift. in addition to the contraction and increasing density of the cooling plate and the weight of the seamount. Int heaearly Triassic when the firsl mammals and dinosaurus appeared.30 traces the recent plate movements that led to the configuration of the continents and oceans as we know them today. Subsidence occurs when the plate supporting the seamounts slides down and away from the bulge of the hot spot. the North atlantic Ocean was opening and the caribbean sea was beginning to form. the sea level drops. arabia moved away from Africa to form the Gulf of Aden anf the new and still opening Red Sea Although . 94 million years ago. This possible driving force it salde ridge push . This motion. When sufficient heat accumulates under a continental mass.3. the sea level rises and covers their borderlands. the rifting process begins unstitching the continents and as the continents move apart. this proposed mechanism is called slab pull . eruption can be produce a linear series of peaks or seamounts with the yougest peak above the hot spot and the seamounts increasing in age as the distance from the hot spot increases. depresses the mantle and carries the seamounts below the surface. India was continuing to move toward asia and the Tethys Ocean was being consumed from the north as the Indian Ocean was expanding from the south.