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This curious world we inhabit is more wonderful than convenient; more beautiful than it is useful; it is more to be admired and enjoyed than used. Henry David Thoreau
Introduction
You will recall from a previous chapter that there are three major layers (crust, mantle, core) within the earth that are identified on the basis of their different compositions (Fig. 1).
The uppermost mantle and crust can be subdivided vertically into two layers with contrasting mechanical (physical) properties. The outer layer, the lithosphere , is composed of the crust and uppermost mantle and forms a rigid outer shell down to a depth of approximately 100 km (63 miles). The underlying asthenosphere is composed of partially melted rocks in the upper mantle that acts in a plastic manner on long time scales. The asthenosphere extends from about 100 to 300 km (63- miles) depth. The theory of plate tectonics proposes that the lithosphere is divided into a series of plates that fit together like the pieces of a jigsaw puzzle.
Although plate tectonics is a relatively young idea in comparison with unifying theories from other sciences (e.g., law of gravity, theory of evolution), some of the basic observations that represent the foundation of the theory were made many centuries ago when the first maps of the Atlantic Ocean were drawn. Geographer Abraham Ortellus noted the similarity between the coastlines of Africa, Europe and the Americas in the third edition of his Thesaurus Geographicus , published in 1596. Ortellus, adapting Plato's story of the demise of Atlantis, suggested that America was “torn away” from Europe and Africa and that the “projecting parts of Europe and Africa” would fit the “recesses” of America.
Such observations were little more than idle speculation until Austrian climatologist Alfred Wegener used the fit of opposing coastlines as one of the pieces of evidence to support his hypothesis of continental drift. Continental drift proposed that the continents were once assembled together as a single supercontinent Wegener named Pangaea. Wegener was unable to suggest a suitable mechanism to explain the motion of the continents across Earth's surface and his hypothesis received
Figure 1. The outermost part of Earth is divided into two mechanical layers, the lithosphere and asthenosphere.
Continental Drift
The concept of continental drift was proposed by Alfred Wegener. Wegener suggested that the earth's continents once formed a single super-continent landmass that he named Pangaea (Fig. 3). He suggested that Pangaea split apart into its constituent continents about 200 million years ago and the continents "drifted" to their current positions.
Wegener’s principal observations were:
Figure 3. A reconstruction of the supercontinent Pangaea.
Figure 4. Fit between Africa and South America along continental shelf.
Figure 5. Distribution of key fossils between continents.
of the seafloor), geophysicists (magnetic properties of rocks), seismologists (earthquakes), and geochronologists (determination of the age of rocks).
The ocean floor varies considerably in depth and character. Beginning at the edge of the continents we can recognize four principal depth zones. The first depth level is the continental shelf , shallow ocean floor (0-150 meters; 0-500 feet ) immediately adjacent to continental land masses. Beyond the shelf, the ocean floor steps down to the second depth level, the deep ocean basins known as the abyssal plains often over 4 kilometers below sea level.
The ocean floor rises to a third level approaching the oceanic ridge system , a submarine mountain chain that can be traced around the world (Fig. 7). The ocean floor is relatively shallow, less than 3 km (nearly 2 miles) deep along the ridge system. The ocean ridge system dominates the floor of the Atlantic Ocean, occupying over half its width (Fig. 8). Scientists surveying the ocean floor discovered that heat flow was high along oceanic ridges and suggested that the ridge system was a source of volcanic activity. Volcanism can be observed first-
Figure 7. Oceanic ridge systems dominate the floor of the world's oceans. Three principal sections of the ridge system are recognized: Mid- Atlantic Ridge, East Pacific Rise, and Mid-Indian Ridge.
Figure 8. Principal topographic features of the floor of the southern Atlantic Ocean. The oceanic ridge occupies more than half the width of the ocean floor. Image modified from original at NOAA's National Geophysical Data Center.
hand where the oceanic ridge comes to the surface of the North Atlantic Ocean in Iceland.
The final depth level in the oceans is apparent in narrow, deep (> 7 km; 4 miles) oceanic trenches found along the margins of some continents (Fig. 9) such as South America or adjacent to volcanic island chains like the Aleutian Islands, Alaska. Despite the nearby volcanism, heat flow is relatively low along trenches. Geophysicists have long recognized that deep earthquakes are associated with trenches down to depths of 700 to 800 km (440-500 miles), far below the ocean floor. Shallow earthquakes are mainly located near trenches and oceanic ridges.
Earth’s magnetic field , originating from the partially molten rocks of the outer core , causes compass needles to point toward the magnetic poles. While the magnetic poles are found at high latitudes they are seldom coincident with the geographic poles. Just as we can define magnetic poles, it is also possible to generate a magnetic equator and lines of magnetic latitude. The orientation of the magnetic field varies with latitude and resembles a giant dipole magnet located in the Earth's interior (Fig. 10). The orientation of the magnetic field can be defined by its declination and inclination. Declination defines the orientation of the lines of magnetic force that stretch from one magnetic pole to another. The declination direction therefore points toward the magnetic poles. The
Figure 9. Distribution of ocean ridges and trenches on the sea floor. Oceanic ridges (white) form a network of submarine mountains on the seafloor. Oceanic ridges are often offset at fracture zones (shown here only for the southern Pacific Ocean). Trenches (red) are concentrated around the margins of the Pacific Ocean. Numbered trenches are: 1. Aleutian; 2. Kurile- Japan; 3. Mariana; 4. Philippine; 5. Bougainville; 6. Tonga-Kermadec; 7. Central America; 8. Peru-Chile; 9. Puerto Rico; 10. South Sandwich; 11. Java.
interior in the Southern Hemisphere during intervals of reverse polarity.
Measurement of magnetism in the rocks of the oceanic crust revealed stripes of high and low magnetic intensity corresponding with areas of normal and reverse polarity, respectively. These stripes are oriented parallel to adjacent oceanic ridges. Ocean floor rocks reveal a symmetrical pattern of magnetic polarity (stripes) on either side of oceanic ridges. For example, a sequence of magnetic reversals (switch from normal to reverse polarity and back) in the western Atlantic, offshore from the Carolinas, can be matched with a similar sequence in the eastern Atlantic off the coast of West Africa. These patterns were interpreted to suggest that new crust was divided in half as it formed along the oceanic ridges and each half moved in opposite directions away from the ridge (Fig.
Ages of igneous rocks on land can be determined using radioactive dating techniques (For a discussion of how geologists determine the absolute age of rocks, see Numerical Time section of the Geologic Time chapter). Ages can be matched with the history of magnetic reversals to identify the sequence and length of intervals of normal and reverse polarity. The patterns of polarity in rocks of the ocean floor were used to establish the ages of rocks in the ocean basins.
Figure 12. The orientation of Earth's magnetic field and the polarity of rocks of the ocean floor relative to the oceanic ridge. Above: A and C: Rocks with normal polarity (blue, green) form when the compass points to the magnetic North Pole (as it does today); B: conditions of reverse polarity (white) represent periods when the compass arrow points to the South Pole. Left: Alternation of normal and reverse magnetic fields produces a striped pattern of magnetism in the ocean floor rocks.
Analysis of rock samples from the ocean floor reveals that the oceanic crust is relatively young in comparison to the continents. The oldest oceanic rocks are less than 200 Myrs (million years) old. In contrast the maximum age of the continental crust has been established as four billion years (4,000 million).
The age of the ocean floor varies with location and is consistently youngest at the oceanic ridges and older along the ocean margins (Fig. 13). The age of the oceanic crust increases symmetrically with distance from the ridge system in the Atlantic Ocean. Oceanic rocks along the North American and African coastlines are approximately 180 Myrs old whereas rocks adjacent to the ridge may be less than one million years old. The absence of rocks older than 200 Myrs was interpreted to suggest that all of the older oceanic crust has been destroyed. This suggests Earth has a crustal recycling system that constantly creates young crust at oceanic ridges and destroys old crust elsewhere. The presence of the older oceanic floor along the trenches was used to infer that the oceanic lithosphere was being consumed at the trenches.
Scientists surveying the ocean floor learned that heat flow was greatest along the oceanic ridge system. The ridge system was
Figure 13. Age of oceanic crust: map. Ages range from young (less than one million years old) along the oceanic ridges (red color) to old (180 million years old) along the ocean margins (e.g., northwest Pacific Ocean). The difference in oldest ages in the northern and southern Atlantic oceans has been interpreted to show that the northern Atlantic Ocean began to form approximately 40 million years before the southern Atlantic Ocean. Original images from the NOAA's National Geophysical Data Center.
Plate Tectonics
The theory of plate tectonics proposes that the lithosphere is divided into eight major plates (North American, South American, Pacific, Nazca, Eurasian, African, Antarctic, and Indian-Australian) and several smaller plates (e.g., Arabian, Scotia, Juan de Fuca) that fit together like the pieces of a jigsaw puzzle (Fig. 15). These plates are mobile, moving in constant, slow motion measured in rates of centimeters per year. The movements of plates over millions of years resulted in the opening and closure of oceans and the formation and disassembly of continents. The theory links Earth’s internal processes to the distribution of continents and oceans; it is the big picture view of how Earth works.
Figure 15. Distribution of tectonic plates with type of plate boundary.
Plates are typically composed of both continental and oceanic lithosphere. For example, the South American plate contains the continent of South America and the southwestern Atlantic Ocean. Plate boundaries may occur along continental margins ( active margins ) that are characterized by volcanism and earthquakes. Continental margins that do not mark a plate boundary are known as passive margins and are free of volcanism and earthquakes. The Atlantic coastlines of North and South America are examples of passive margins.
Scientists had long recognized that volcanoes and earthquakes were present in greatest concentrations around the rim of the Pacific Ocean ( Ring of Fire ). Seismologists Kiyoo Wadati and Hugo Benioff noted that the focal depths of earthquakes became progressively deeper underlying ocean trenches (Fig. 16). Prior to the seafloor spreading hypothesis there was no obvious explanation for the presence of these Wadati-Benioff zones. Now it is widely accepted that earthquakes occur as one plate bends and fractures as it descends beneath another into the asthenosphere.
The ocean floor was being pulled or pushed into the mantle were it was heated to form magma which in turn generated volcanoes. The destruction of the oceanic lithosphere caused earthquakes down to depths of 700 to 800 km (440-500 miles), explaining the presence of the deepest earthquakes adjacent to oceanic trenches. The term subduction zone was coined to refer to locations marked by Wadati-Benioff zones where the oceanic lithosphere is consumed adjacent to a trench.
Plate tectonics theory (Fig. 17) joined continental drift to sea- floor spreading to propose:
Figure 16. Inclined zone of earthquake foci adjacent to oceanic trench slopes downward under the overriding plate. The distribution of foci define the Wadati- Benioff zone.
Today satellite technology is used to determine the current rates of plate motion. Satellites anchored in space can record tiny movements of fixed sites on Earth, thus constraining the motions of plates (Fig. 19). Rates of seafloor spreading range from a little as 1-2 centimeters per year along the oceanic ridge in the northern Atlantic Ocean to more than 15 cm/yr along the East Pacific Rise spreading center. Current seafloor spreading rates are approximately five times higher for the East Pacific Rise than the Mid-Atlantic Ridge. Spreading rates changed through time but consistently higher rates in the Pacific )cean basin can account for the contrast in size of the Atlantic and Pacific Oceans. The Pacific Ocean floor would be even wider if oceanic crust were not consumed at subduction zones along much of its margin.
Figure 18. Relative ages and locations of Hawaiian Islands relative to hot spot (mantle plume). Kauai has traveled from the location of the plume to its present site over the last five million years. A submarine volcano, Loihi, is forming over the current position of the plume.
Figure 19. Directions and rates of plate motions (centimeters per year) along oceanic ridge systems. Spreading rates in the Pacific Ocean are nearly five times faster than in the Atlantic.
Divergent Plate Boundaries
Ocean ridges and subduction zones are boundaries between plates of lithosphere. A gap is created when oceanic lithosphere separates along the oceanic ridge. The gap is filled by magma that rises from the asthenosphere. The magma cools and solidifies to create new oceanic lithosphere.
Think about it...
Figure 20. Locations of divergent plate boundaries and sense of plate motion indicated by arrows. Map courtesy of NOAA National Geophysical Data Center.
million years) but the Pacific is much wider than the Atlantic because it is spreading 2 to 3 times as fast (Figs. 20, 22).
Convergent Plate Boundaries
Ocean ridges and subduction zones are boundaries between plates of lithosphere. Oceanic lithosphere is destroyed at subduction zones where lithosphere descends into the mantle beneath trenches. This older lithosphere melts to form magma. The magma rises through the overlying lithosphere and may form volcanoes at Earth's surface.
Convergent boundaries come in three varieties depending upon the type of lithosphere that is juxtaposed across a subduction zone.
Oceanic Plate vs. Oceanic Plate Convergence The older of the two plates descends into the subduction zone when plates of oceanic lithosphere collide along a trench. The descending plate carries water-filled sediments from the ocean floor downward into the mantle. The presence of water alters
Think about it... Print the blank map of the plates found in the Plate Motions exercise the end of the chapter. Identify the divergent plate boundaries and label them young or mature. Can you identify some of the pairs of locations that are getting further apart because they lie on opposite sides of a divergent boundary?
the physical and chemical conditions necessary for melting and causes magma to form. The magma rises up through the over- riding oceanic plate, reaching the surface as a volcano. As the volcano grows, it may rise above sea level to form an island.
Trenches often lie adjacent to chains of islands ( island arcs ) formed by magma from the subducted plate. The Aleutian Islands (Fig. 23) off the tip of Alaska were formed by magma generated when the Pacific Plate descended below some oceanic lithosphere on the margin of the North American Plate. Current volcanic activity on the island of Montserrat in the Caribbean is the result of subduction of the South American Plate below an island arc that marks the edge of the Caribbean Plate (Fig. 24).
Oceanic Plate vs. Continental Plate Convergence When oceanic lithosphere collides with continental lithosphere, the oceanic plate will descend into the subduction zone (Fig. 25). Oceanic lithosphere is denser than continental lithosphere and is therefore consumed preferentially. Continental lithosphere is almost never destroyed in subduction zones.
The Nazca Plate dives below South America in a subduction zone that lies along the western margin of the continent. Convergence between these plates has resulted in the formation of the Andes Mountains (the second highest mountain range on Earth), extensive volcanism, and widespread earthquake activity (Fig. 23). The largest earthquakes are concentrated along subduction zones.
Continental Plate vs. Continental Plate Convergence The tallest mountains in the world were formed (and continue to grow) as a result of continental collision. The Himalayan mountains mark the boundary between the Indian and Eurasian
Figure 23. Locations of convergent plate boundaries and sense of plate motion indicated by arrows. Map courtesy of NOAA National Geophysical Data Center.
Figure 24. Features associated with a convergent plate boundary where two oceanic plates collide. The plate with the older (cooler, more dense) crust descends into the subduction zone. A chain of volcanic islands (island arc) forms on the overriding plate.
