tarbuck & lutgens 2008 plate tectonics the new paradigm

50 C H A P T E R 2 Plate Tectonics: A Scientific Revolution Unfolds

together. Furthermore, along great faults, which he namedtransform faults, plates slide past one another. In a broadsense, Wilson had presented what would later be called thetheory of plate tectonics, a topic we will consider next.

Once the key concepts of plate tectonics had been setforth, the hypothesis-testing phase moved forward veryquickly. Some of the evidence that these researchers uncov-ered to support the plate tectonics model will be presentedin this and other chapters. Much of the supporting evidencefor the plate tectonics model already existed. What this the-ory provided was a unified explanation for what seemed tobe numerous unrelated observations from the fields of geol-ogy, paleontology, geophysics, and oceanography amongothers.

By the end of the 1960s the tide of scientific opinion hadindeed turned! However, some opposition to plate tectonics

continued for at least a decade. Nevertheless, Wegener hadbeen vindicated and the revolution in geology was nearingan end.

Plate Tectonics: The New Paradigm

Plate Tectonics� Introduction

By 1968 the concepts of continental drift and seafloor spread-ing were united into a much more encompassing theoryknown as plate tectonics Plate tectonicscan be defined as the composite of a great variety of ideas thatexplain the observed motion of Earth’s outer shell through

1tekton = to build2.

The priority, or credit, for a scientific ideaor discovery is usually given to the re-searcher, or group of researchers, who firstpublish their findings in a scientific jour-nal. However, it is not uncommon for two,or even more researchers, to reach similarconclusions almost simultaneously. Twowell-known examples are the independentdiscoveries of organic evolution by CharlesDarwin and Alfred Wallace and develop-ment of the calculus by Isaac Newton andGottfried W. Leibniz. Similarly, some ofthe main ideas that led to the tectonics rev-olution in the Earth sciences were also dis-covered independently by more than onegroup of investigators.

Although the continental drift hypothe-sis is rightfully associated with the nameAlfred Wegener, he was not the first to sug-gest continental mobility. In fact, FrancisBacon, in 1620, pointed out the similaritiesof the outlines of Africa and South Ameri-ca; however, he did not develop this ideafurther. Nearly three centuries later, in1910, two years before Wegener made aformal presentation of his ideas, Americangeologist F. B. Taylor published the firstpaper to outline the concept we now callcontinental drift. Why, then, is Wegenercredited with this idea?

Because the papers authored by Taylorhad relatively little impact among the scien-tific community, Wegener was not aware ofTaylor’s work. Hence, it is believed that We-gener independently and simultaneouslyreached the same conclusion. More impor-tant, however, Wegener made great effortsthroughout his professional life to provide awide range of evidence to support his hy-

new ideas that were influential to the de-velopment of the theory of plate tectonics.Thus, historians associate the names Hessand Dietz with the discovery of seafloorspreading with occasional mention of con-tributions by Holmes.

Perhaps the most controversial issue ofscientific priority came in 1963 when FredVine and D. H. Matthews published theirpaper that linked the seafloor spreadinghypothesis with the newly discovereddata on magnetic reversals. However, ninemonths earlier a similar paper by Canadi-an geophysicist, L. W. Morley was not ac-cepted for publication. One reviewer ofMorley’s paper commented, “Such specu-lation makes interesting talk at cocktailparties, but is not the sort of thing thatought to be published under serious, scien-tific aegis.” Morley’s paper was eventuallypublished in 1964, but priority had alreadybeen established, and the idea becameknown as the Vine-Matthews hypothesis.In 1971, N. D. Watkins wrote of Morley’spaper, “The manuscript certainly had sub-stantial historical interest, ranking as prob-ably the most significant paper in the earthsciences to ever be denied publication.”

With the development of the theory ofplate tectonics came many other races forpriority by researchers from various com-peting institutions. Some of the new ideasthat unfolded from this body of work willbe presented in this and later chapters. Be-cause priority for scientific ideas is compli-cated by the frequency of independent andnearly simultaneous discoveries, it becameprudent for investigators to publish theirideas as quickly as possible.

pothesis. By contrast, Taylor appeared con-tent to simply state, “There are many bondsof union which show that Africa and SouthAmerica were once joined.” Further, where-as Taylor viewed continental drift as asomewhat speculative idea, Wegener wascertain that the continents had drifted. Ac-cording to H. W. Menard in his book, TheOcean of Truth, Taylor was uncomfortablehaving his ideas coupled with Wegener’shypothesis. Menard quotes Taylor as writ-ing, “Wegener was a young professor ofmeteorology. Some of his ideas are very dif-ferent from mine and he went much furtherin his speculation.”

Another controversy concerning priori-ty came with the development of theseafloor spreading hypothesis. In 1960,Harry Hess of Princeton University wrote apaper that outlined his ideas on seafloorspreading. Rather than rushing it to publi-cation, Hess mailed copies of the manu-script to numerous colleagues, a commonpractice among researchers. In the mean-time, and apparently independently, RobertDietz of Scripps Institution of Oceanogra-phy published a similar paper in therespected journal Nature (1961), titled“Continents and Ocean Basin Evolution bySpreading of the Sea Floor.” When Dietzbecame aware of Hess’s earlier, althoughunpublished paper, he acknowledged pri-ority for the idea of seafloor spreading toHess. It is interesting to note that the basicideas in Hess’s paper actually appeared ina textbook authored by Arthur Holmes in1944. Therefore, priority for seafloor spread-ing may rightfully belong to Holmes. Nev-ertheless, Dietz and Hess both presented

BOX 2.3 � UNDERSTANDING EARTH

Priority in the Sciences

M02_TARB6927_09_SE_C02.QXD 1/11/07 1:24 PM Page 50

Plate Tectonics: The New Paradigm 51

the mechanisms of subduction and seafloor spreading, which,in turn, generate Earth’s major features, including continents,mountains, and ocean basins. The implications of plate tec-tonics are so far-reaching that this theory has become thebasis for viewing most geologic processes.

Earth’s Major PlatesAccording to the plate tectonics model, the uppermost man-tle, along with the overlying crust, behave as a strong, rigidlayer, known as the lithosphere

which is broken into pieces called plates (Figure2.19). Lithospheric plates are thinnest in the oceans wheretheir thickness may vary from as little as a few kilometers atthe oceanic ridges to 100 kilometers in the deep-oceanbasins. By contrast, continental lithosphere is generallyabout 100 kilometers thick but may be more than 250 kilo-meters thick below older portions of landmasses. The litho-sphere overlies a weaker region in the mantle known as the

a ball2,1lithos = stone, sphere =

asthenosphere The tem-perature/pressure regime in the upper asthenosphere issuch that the rocks there are very near their melting temper-atures. This results in a very weak zone that permits the lith-osphere to be effectively detached from the layers below.Thus, the weak rock within the upper asthenosphere allowsEarth’s rigid outer shell to move.

The lithosphere is broken into numerous segments,called lithospheric or tectonic plates, that are in motionwith respect to one another and are continually changing inshape and size. As shown in Figure 2.20, seven major litho-spheric plates are recognized. They are the North American,South American, Pacific, African, Eurasian, Australian-Indian,and Antarctic plates. The largest is the Pacific plate, whichencompasses a significant portion of the Pacific Oceanbasin. Notice from Figure 2.20 that most of the large platesinclude an entire continent plus a large area of ocean floor(for example, the South American plate). This is a major de-parture from Wegener’s continental drift hypothesis, whichproposed that the continents moved through the oceanfloor, not with it. Note also that none of the plates are de-fined entirely by the margins of a continent.

Intermediate-sized plates include the Caribbean, Nazca,Philippine, Arabian, Cocos, Scotia, and Juan de Fuca plates. Inaddition, there are over a dozen smaller plates that havebeen identified but are not shown in Figure 2.20.

One of the main tenets of the plate tectonic theory is thatplates move as coherent units relative to all other plates. Asplates move, the distance between two locations on thesame plate—New York and Denver, for example—remainsrelatively constant, whereas the distance between sites ondifferent plates, such as New York and London, graduallychanges. (Recently it has been shown that plates can suffersome internal deformation, particularly oceanic lithosphere.)

a ball2.1asthenos = weak, sphere =

FIGURE 2.18 An alternate hypothesis to continental drift was an ex-panding Earth. According to this model Earth was once only half itscurrent diameter and covered by a layer of continents. As Earth expandedthe continents split into their current configurations, while new seafloor“filled in” the spaces as they drifted apart.

NorthAmerican

plate

SouthAmerican

plate

Africanplate

Eurasianplate Eurasian

plate

Australian-Indianplate

Pacificplate

Philippineplate

Antarctic plate

NorthAmerican plate

Nazcaplate

Antarcticplate

Arabianplate

Cocosplate

Caribbeanplate

Scota plate

FIGURE 2.19 Illustration of some of Earth’s lithospheric plates.



Lithospheric plates move relative to each other at a veryslow but continuous rate that averages about 5 centimeters(2 inches) per year. This movement is ultimately driven bythe unequal distribution of heat within Earth. Hot materialfound deep in the mantle moves slowly upward and servesas one part of our planet’s internal convection system. Con-currently, cooler, denser slabs of oceanic lithosphere de-scend into the mantle, setting Earth’s rigid outer shell intomotion. Ultimately, the titanic, grinding movements ofEarth’s lithospheric plates generate earthquakes, create vol-canoes, and deform large masses of rock into mountains.

Plate BoundariesTectonic plates move as coherent units relative to all otherplates. Although the interiors of plates may experiencesome deformation, all major interactions among individualplates (and therefore most deformation) occur along theirboundaries. In fact, plate boundaries were first established byplotting the locations of earthquakes. Moreover, plates arebounded by three distinct types of boundaries, which aredifferentiated by the type of movement they exhibit. Theseboundaries are depicted at the bottom of Figure 2.20 and arebriefly described here:

1. Divergent boundaries (constructive margins)—wheretwo plates move apart, resulting in upwelling of mate-rial from the mantle to create new seafloor (Figure2.20A).

2. Convergent boundaries (destructive margins)—wheretwo plates move together, resulting in oceanic litho-sphere descending beneath an overriding plate, even-tually to be reabsorbed into the mantle, or possibly inthe collision of two continental blocks to create amountain system (Figure 2.20B).

3. Transform fault boundaries (conservative margins)—where two plates grind past each other without theproduction or destruction of lithosphere (Figure2.20C).

Each plate is bounded by a combination of these threetypes of plate margins. For example, the Juan de Fuca platehas a divergent zone on the west, a convergent boundary onthe east, and numerous transform faults, which offset seg-ments of the oceanic ridge (Figure 2.20). Although the totalsurface area of Earth does not change, individual plates maydiminish or grow in area depending on any imbalance be-tween the growth rate at divergent boundaries and the rateat which lithosphere is destroyed at convergent boundaries.The Antarctic and African plates are almost entirely bound-ed by divergent boundaries and hence are growing largerby adding new lithosphere to their margins. By contrast, thePacific plate is being consumed into the mantle along itsnorthern and western flanks faster than it is being replaced,and therefore is diminishing in size.

It is also important to note that plate boundaries are notfixed but move about. For example, the westward drift ofthe South American plate is causing it to override the Nazca

plate. As a result, the boundary that separates these platesis gradually being displaced as well. Moreover, since theAntarctic plate is surrounded by constructive margins andis growing larger, the divergent boundaries are migratingaway from the continent of Antarctica.

New plate boundaries can be created in response tochanges in the forces acting on these rigid slabs. For exam-ple, a relatively new divergent boundary is located in theRed Sea. Less than 20 million years ago the Arabian Pennin-sula began to rift away from Africa. At other locations,plates carrying continental crust are presently moving to-ward one another. Eventually these continents may collideand be sutured together. In this case, the boundary that onceseparated two plates disappears as the plates become one.The result of such a continental collision is a majestic moun-tain range such as the Himalayas.

In the following sections we will briefly summarize thenature of the three types of plate boundaries.

Divergent Boundaries

Plate Tectonics� Divergent Boundaries

Most divergent bound-aries are located along the crests of oceanic ridges and canbe thought of as constructive plate margins since this is wherenew oceanic lithosphere is generated (Figure 2.21). Diver-gent boundaries are also called spreading centers, becauseseafloor spreading occurs at these boundaries. Here, as theplates move away from the ridge axis, the fractures thatform are filled with molten rock that wells up from the hotmantle below. Gradually, this magma cools to produce newslivers of seafloor. In a continuous manner, adjacent platesspread apart and new oceanic lithosphere forms betweenthem. As we shall see later, divergent boundaries arenot confined to the ocean floor but can also form on thecontinents.

Oceanic Ridges and Seafloor SpreadingAlong well-developed divergent plate boundaries, theseafloor is elevated, forming the oceanic ridge. The intercon-nected oceanic ridge system is the longest topographic fea-ture on Earth’s surface, exceeding 70,000 kilometers(43,000 miles) in length. Representing 20 percent of Earth’ssurface, the oceanic ridge system winds through all majorocean basins like the seam on a baseball. Although thecrest of the oceanic ridge is commonly 2 to 3 kilometershigher than the adjacent ocean basins, the term “ridge”may be misleading because this feature is not narrow buthas widths from 1000 to 4000 kilometers. Further, along theaxis of some ridge segments is a deep down-faulted struc-ture called a rift valley.

The mechanism that operates along the oceanic ridgesystem to create new seafloor is appropriately calledseafloor spreading. Typical rates of spreading average around

1di = apart, vergere = to move2


Convergent Boundaries 55

5 centimeters (2 inches) per year. This is roughly the samerate at which human fingernails grow. Comparatively slowspreading rates of 2 centimeters per year are found alongthe Mid-Atlantic Ridge, whereas spreading rates exceeding15 centimeters (6 inches) have been measured along sec-tions of the East Pacific Rise. Although these rates of litho-spheric production are slow on a human time scale, theyare nevertheless rapid enough to have generated all ofEarth’s ocean basins within the last 200 million years. Infact, none of the ocean floor that has been dated exceeds 180million years in age.

The primary reason for the elevated position of theoceanic ridge is that newly created oceanic crust is hot, andoccupies more volume, which makes it less dense thancooler rocks. As new lithosphere is formed along the ocean-ic ridge, it is slowly yet continually displaced away fromthe zone of upwelling along the ridge axis. Thus, it beginsto cool and contract, thereby increasing in density. Thisthermal contraction accounts for the greater ocean depthsthat exist away from the ridge crest. It takes about 80 mil-lion years before the cooling and contracting cease com-pletely. By this time, rock that was once part of the elevatedoceanic ridge system is located in the deep-ocean basin,where it may be buried by substantial accumulations ofsediment. In addition, cooling causes the mantle rocksbelow the oceanic crust to strengthen, thereby adding to theplate’s thickness. Stated another way, the thickness ofoceanic lithosphere is age-dependent. The older (cooler) itis, the greater its thickness.

Continental RiftingDivergent plate boundaries can alsodevelop within a continent, in whichcase the landmass may split into two ormore smaller segments, as Alfred We-gener had proposed for the breakup ofPangaea. The splitting of a continent isthought to begin with the formation ofan elongated depression called a con-tinental rift. A modern example of acontinental rift is the East African Rift.Whether this rift will develop into afull-fledged spreading center and even-tually split the continent of Africa is amatter of much speculation.

Nevertheless, the East African Riftrepresents the initial stage in the break-up of a continent (see Figure 13.20,page 366). Here tensional forces havestretched and thinned the continentalcrust. As a result, molten rock ascendsfrom the asthensophere and initiatesvolcanic activity at the surface (Figure2.22A). Large volcanic mountains suchas Kilimanjaro and Mount Kenya ex-emplify the extensive volcanic activitythat accompanies continental rifting.Research suggests that if tensional forces

are maintained, the rift valley will lengthen and deepen,eventually extending out to the margin of the plate, splittingit in two (Figure 2.22C). At this point, the rift becomes a nar-row sea with an outlet to the ocean, similar to the Red Sea.The Red Sea formed when the Arabian Peninsula rifted fromAfrica, an event that began about 20 million years ago. Con-sequently, the Red Sea provides oceanographers with a viewof how the Atlantic Ocean may have looked in its infancy.

Convergent Boundaries

Plate Tectonics� Convergent Boundaries

Although new lithosphere is constantly being produced atthe oceanic ridges, our planet is not growing larger—itstotal surface area remains constant. To balance the additionof newly created lithosphere, older, denser portions ofoceanic lithosphere descend into the mantle along conver-gent boundaries. Be-cause lithosphere is “destroyed” at convergent boundaries,they are also called destructive plate margins (Figure 2.23A).

Convergent plate margins occur where two plates movetoward each other and the leading edge of one is bent down-ward, allowing it to slide beneath the other. The surface ex-pression produced by the descending plate is a deep-oceantrench, such as the Peru–Chile trench (see Figure 13.9, page357). Trenches formed in this manner may be thousands of

1con = together, vergere = to move2

NorthAmerica

Africa

Europe

Magmachamber

LithosphereAsthenosphere

Riftvalleys

Lithosphere

Asthenosphere

Mid

-Atla

ntic

Ridge

Upwelling

FIGURE 2.21 Most divergent plate boundaries are situated along the crests of oceanic ridges.



kilometers long, 8 to 12 kilometers deep, and between 50 and100 kilometers wide.

Convergent boundaries are also called subductionzones, because they are sites where lithosphere is descend-ing (being subducted) into the mantle. Subduction occursbecause the density of the descending tectonic plate isgreater than the density of the underlying asthenosphere. Ingeneral, oceanic lithosphere is more dense than the as-thenosphere whereas continental lithosphere is less denseand resists subduction. As a consequence, it is alwaysoceanic lithosphere that is subducted.

Slabs of oceanic lithosphere descend into the mantle atangles that vary from a few degrees to nearly vertical (90 de-

grees), but average about 45 degrees. Theangle at which oceanic lithosphere descendsdepends largely on its density. For example,when a spreading center is located near asubduction zone, the lithosphere is youngand, therefore, warm and buoyant. Hence,the angle of descent is small. This is the situ-ation along parts of the Peru–Chile trench.Low dip angles usually result in consider-able interaction between the descending slaband the overriding plate. Consequently,these regions experience great earthquakes.

As oceanic lithosphere ages (gets fartherfrom the spreading center), it gradually cools,which causes it to thicken and increase in den-sity. Once oceanic lithosphere is about 15 mil-lion years old, it becomes more dense than thesupporting asthenosphere and will sink whengiven the opportunity. In parts of the westernPacific, some oceanic lithosphere is more than180 million years old. This is the thickest andmost dense in today’s oceans. The subductingslabs in this region typically plunge into themantle at angles approaching 90 degrees.

Although all convergent zones have thesame basic characteristics, they are highlyvariable features. Each is controlled by thetype of crustal material involved and the tec-tonic setting. Convergent boundaries canform between two oceanic plates, one ocean-ic and one continental plate, or two continen-tal plates. All three situations are illustratedin Figure 2.23.

Oceanic–ContinentalConvergenceWhenever the leading edge of a plate cappedwith continental crust converges with a slabof oceanic lithosphere, the buoyant continen-tal block remains “floating,” while thedenser oceanic slab sinks into the mantle(Figure 2.23A). When a descending oceanicslab reaches a depth of about 100 kilometers,melting is triggered within the wedge of hot

asthenosphere that lies above it. But how does the subduc-tion of a cool slab of oceanic lithosphere cause mantle rockto melt? The answer lies in the fact that volatiles (mainlywater) act like salt does to melt ice. That is, “wet” rock, in ahigh-pressure environment, melts at substantially lowertemperatures than “dry” rock of the same composition.

Sediments and oceanic crust contain a large amount ofwater which is carried to great depths by a subducting plate.As the plate plunges downward, water is “squeezed” fromthe pore spaces as confining pressure increases. At evengreater depths, heat and pressure drive water from hydrat-ed (water-rich) minerals such as the amphiboles. At a depthof roughly 100 kilometers, the mantle is sufficiently hot that

A.

Upwarping

Continental crust

B.

Continental rift

D.

Rift valley

Mid-ocean ridge

Oceanic crust

Continentalcrust

C.

Linear sea

FIGURE 2.22 Continental rifting and the formation of a new ocean basin. A. Continental riftingis thought to occur where tensional forces stretch and thin the crust. As a result, molten rockascends from the asthenosphere and initiates volcanic activity at the surface. B. As the crust ispulled apart, large slabs of rock sink, generating a rift valley. C. Further spreading generates anarrow sea. D. Eventually, an expansive ocean basin and ridge system are created.

M02_TARB6927_09_SE_C02.QXD 1/11/07 1:24 PM 6

Convergent Boundaries 57

ment, these mantle-derived magmas mayascend through the crust and give rise to avolcanic eruption. However, much of thismolten rock never reaches the surface;rather, it solidifies at depth where it acts tothicken the crust.

Partial melting of mantle rock generatesmolten rock that has a basaltic composition,similar to what erupts on the Island ofHawaii. In a continental setting, however,basaltic magma typically melts and assimi-lates some of the crustal rocks through whichit ascends. The result is the formation of a sil-ica-rich (SiO2) magma. On occasions whensilica-rich magmas reach the surface, theyoften erupt explosively, generating largecolumns of volcanic arc and gases. A classicexample of such an eruption was the 1980eruption of Mount St. Helens. You will learnmore about the formation of magma and itsinfluence on the explosiveness of volcaniceruptions in Chapters 4 and 5.

The volcanoes of the towering Andes arethe product of magma generated by the sub-duction of the Nazca plate beneath the SouthAmerican continent (see Figure 2.20). Moun-tains such as the Andes, which are producedin part by volcanic activity associated withthe subduction of oceanic lithosphere, arecalled continental volcanic arcs. The Cas-cade Range of Washington, Oregon, and Cal-ifornia is another volcanic arc which consistsof several well-known volcanic mountains,including Mount Rainier, Mount Shasta, andMount St. Helens (see Figure 5.19, p. 142).(This active volcanic arc also extends intoCanada, where it includes Mount Garibaldi,Mount Silverthrone, and others.)

Oceanic–Oceanic ConvergenceAn oceanic–oceanic convergent boundary hasmany features in common with oceanic–conti-nental plate margins. The differences aremainly attributable to the nature of the crustcapping the overriding plate. Where twooceanic slabs converge, one descends beneaththe other, initiating volcanic activity by thesame mechanism that operates at oceanic–continental plate boundaries. Water “squeezed”from the subducting slab of oceanic litho-sphere triggers melting in the hot wedge of

mantle rock that lies above. In this setting, volcanoes grow upfrom the ocean floor, rather than upon a continental platform.When subduction is sustained, it will eventually build a chainof volcanic structures that emerge as islands. The volcanicislands are spaced about 80 kilometers apart and are builtupon submerged ridges of volcanic material a few hundred

A.

Oceanic crust

Trench

Continentalvolcanic arc

Continental crust

Asthenosphere

Continentallithosphere

Melting

Oceanic crust

Trench

Continental crust

Melting

100 km

200 km


C.


Asthenosphere

Asthenosphere

Oceanic lithosphere

Volcanic island arc

B.

Suture

100 km

200 km

100 km

200 km

Collision mountains

Subducting oceanic lithosphere



FIGURE 2.23 Zones of plate convergence. A. Oceanic–continental B. Oceanic–oceanic C. Continental–continental.

the introduction of water leads to some melting. Thisprocess, called partial melting, generates as little as 10 per-cent molten material, which is intermixed with unmeltedmantle rock. Being less dense than the surrounding mantle,this hot mobile material gradually rises toward the surfaceas a teardrop-shaped structure. Depending on the environ-



kilometers wide. This newly formed land consisting of an arc-shaped chain of small volcanic islands is called a volcanic is-land arc, or simply an island arc (Figure 2.23B).

The Aleutian, Mariana, and Tonga islands are examplesof volcanic island arcs. Island arcs such as these are general-ly located 100 to 300 kilometers (60 to 200 miles) from adeep-ocean trench. Located adjacent to the island arcs justmentioned are the Aleutian trench, the Mariana trench, andthe Tonga trench (see Figure 1.17).

Most volcanic island arcs are located in the western Pacif-ic. Only two volcanic island arcs are located in the At-lantic—the Lesser Antilles arc adjacent to the Caribbean Seaand the Sandwich Islands in the South Atlantic. The LesserAntilles are a product of the subduction of the Atlantic be-neath the Caribbean plate. Located within this arc is the is-land of Martinique, where Mount Pelée erupted in 1902,destroying the town of St. Pierre and killing an estimated28,000 people; and the island of Montserrat, where volcanicactivity has occurred very recently.*

Relatively young island arcs are fairly simple structuresthat are underlain by deformed oceanic crust that is generallyless than 20 kilometers (12 miles) thick. Examples include thearcs of Tonga, the Aleutians, and the Lesser Antilles. By con-trast, older island arcs are more complex and are underlain bycrust that ranges in thickness from 20 to 35 kilometers. Exam-ples include the Japanese and Indonesian arcs, which arebuilt upon material generated by earlier episodes of subduc-tion or sometimes on a small piece of continental crust.

Continental–Continental ConvergenceAs you saw earlier, when an oceanic plate is subducted be-neath continental lithosphere, an Andean-type volcanic arcdevelops along the margin of the continent. However, if thesubducting plate also contains continental lithosphere, con-tinued subduction eventually brings the two continentalblocks together (Figure 2.23C). Whereas oceanic lithosphereis relatively dense and sinks into the asthenosphere, conti-nental lithosphere is buoyant, which prevents it from beingsubducted to any great depth. The result is a collision be-tween the two continental fragments (Figure 2.23C).

Such a collision occurred when the subcontinent of India“rammed” into Asia, producing the Himalayas—the mostspectacular mountain range on Earth (Figure 2.24). Duringthis collision, the continental crust buckled, fractured, andwas generally shortened and thickened. In addition to theHimalayas, several other major mountain systems, includ-ing the Alps, Appalachians, and Urals, formed during conti-nental collisions.

Prior to a continental collision, the landmasses involvedwere separated by an ocean basin (Figure 2.24A). As the con-tinental blocks converge, the intervening seafloor is subduct-ed beneath one of the plates. Subduction initiates partialmelting in the overlying mantle, which in turn results in thegrowth of a volcanic arc. Depending on the location of thesubduction zone, the volcanic arc could develop on either of

*More on these volcanic events is found in Chapter 5.

the converging landmasses, or if the subduction zone devel-oped several hundred kilometers seaward from the coast, avolcanic island arc would form. Eventually, as the interven-ing seafloor is consumed, these continental masses collide(Figure 2.24B). This folds and deforms the accumulation ofsediments and sedimentary rocks along the continental mar-gin as if they had been placed in a gigantic vise. The result isthe formation of a new mountain range composed of de-formed and metamorphosed sedimentary rocks, fragmentsof the volcanic arc, and often slivers of oceanic crust.

Transform Fault Boundaries

Plate Tectonics� Transform Fault Boundaries

The third type of plate boundary is the transformfault, where plates slide horizontally

past one another without the production or destruction oflithosphere (conservative plate margins). The nature of trans-form faults was discovered in 1965 by J. Tuzo Wilson of theUniversity of Toronto. Wilson suggested that these largefaults connect the global active belts (convergent bound-aries, divergent boundaries, and other transform faults) intoa continuous network that divides Earth’s outer shell intoseveral rigid plates. Thus, Wilson became the first to suggestthat Earth was made of individual plates, while at the sametime identifying the faults along which relative motion be-tween the plates is made possible.

Most transform faults join two segments of an oceanicridge (Figure 2.25). Here, they are part of prominent linearbreaks in the oceanic crust known as fracture zones, which in-clude both the active transform faults as well as their inactiveextentions into the plate interior. These fracture zones arepresent approximately every 100 kilometers along the trend ofa ridge axis. As shown in Figure 2.25, active transform faultslie only between the two offset ridge segments. Here seafloorproduced at one ridge axis moves in the opposite direction asseafloor produced at an opposing ridge segment. Thus, be-tween the ridge segments these adjacent slabs of oceanic crustare grinding past each other along the fault. Beyond the ridge

across, forma = form21trans =

Students Sometimes Ask . . .Someday will the continents come back together and form asingle landmass?

Yes, it is very likely that the continents will eventually comeback together, but not anytime soon. Since all of the continentsare on the same planetary body, there is only so far a continentcan travel before it collides with another landmass. Recent re-search suggests that a supercontinent may form about onceevery 500 million years or so. Since it has been about 200 millionyears since Pangaea broke up, we have only about 300 millionyears to go before the next supercontinent is assembled.


Transform Fault Boundaries 59

A.

IndiaContinental

shelfdeposits

Continentalcrust

Ocean basin

Continental volcanic arc

Tibet

MeltingAsthenosphere

Developingaccretionary

wedge

B.

C.

Himalayas

IndiaTibetanPlateau

Suture

Asthenosphere

Indiatoday

10 millionyears ago

38 millionyears ago

55 millionyears ago

71 millionyears ago

GangesPlain


FIGURE 2.24 The ongoing collision of India and Asia, starting about 45 million years ago,produced the majestic Himalayas. A. Converging plates generated a subduction zone, while partialmelting triggered by the subducting oceanic slab produced a continental volcanic arc. Sedimentsscraped from the subducting plate were added to the accretionary wedge. B. Position of India inrelation to Eurasia at various times. (Modified after Peter Molnar) C. Eventually the two landmassescollided, deforming and elevating the sediments that had been deposited along the continentalmargins. In addition, slices of the Indian crust were thrust up onto the Indian plate.

crests are the inactive zones, where the fractures are preservedas linear topographic scars. The trend of these fracture zonesroughly parallels the direction of plate motion at the time oftheir formation. Thus, these structures can be used to map thedirection of plate motion in the geologic past.

In another role, transform faults provide the means bywhich the oceanic crust created at ridge crests can be trans-ported to a site of destruction: the deep-ocean trenches.Figure 2.26 illustrates this situation. Notice that the Juan deFuca plate moves in a southeasterly direction, eventuallybeing subducted under the west coast of the United States.The southern end of this plate is bounded by the Mendocino

fault. This transform fault boundary connects the Juan deFuca ridge to the Cascadia subduction zone (Figure 2.26).Therefore, it facilitates the movement of the crustal materialcreated at the ridge crest to its destination beneath the NorthAmerican continent (Figure 2.26).

Although most transform faults are located within theocean basins, a few cut through continental crust. Two exam-ples are the earthquake-prone San Andreas Fault of Califor-nia and the Alpine Fault of New Zealand. Notice in Figure2.26 that the San Andreas Fault connects a spreading centerlocated in the Gulf of California to the Cascadia subductionzone and the Mendocino fault located along the northwest



coast of the United States. Along the San Andreas Fault, thePacific plate is moving toward the northwest, past the NorthAmerican plate. If this movement continues, that part of Cal-ifornia west of the fault zone, including the Baja Peninsula,will eventually become an island off the West Coast of theUnited States and Canada. It could eventually reach Alaska.However, a more immediate concern is the earthquake activ-ity triggered by movements along this fault system.

Testing the Plate Tectonics ModelWith the development of the theory of plate tectonics, re-searchers from all of the Earth sciences began testing this newmodel of how Earth works. Some of the evidence supportingcontinental drift and seafloor spreading has already been pre-sented. In addition, some of the evidence that was instrumen-

tal in solidifying the support for thisnew idea follows. Note that much of thisevidence was not new; rather, it wasnew interpretations of already existingdata that swayed the tide of opinion.

Evidence from OceanDrillingSome of the most convincing evidenceconfirming seafloor spreading has comefrom drilling directly into the oceanfloor. From 1968 until 1983, the sourceof these important data was the DeepSea Drilling Project, an internationalprogram sponsored by several majoroceanographic institutions and the Na-tional Science Foundation. The primarygoal was to gather firsthand informa-tion about the age of the ocean basinsand processes that formed them. To ac-complish this, a new drilling ship, theGlomar Challenger, was built.

Operations began in August 1968 inthe South Atlantic. At several sites holeswere drilled through the entire thick-ness of sediments to the basaltic rockbelow. An important objective was togather samples of sediment from justabove the igneous crust as a means ofdating the seafloor at each site.* Be-cause sedimentation begins immediate-ly after the oceanic crust forms, remainsof microorganisms found in the oldestsediments—those resting directly onthe crust—can be used to date the oceanfloor at that site.

When the oldest sediment from eachdrill site was plotted against its distancefrom the ridge crest, the plot demonstrat-ed that the age of the sediment increased

with increasing distance from the ridge. This finding support-ed the seafloor-spreading hypothesis, which predicted theyoungest oceanic crust would be found at the ridge crest andthe oldest oceanic crust would be at the continental margins.

The data from the Deep Sea Drilling Project also rein-forced the idea that the ocean basins are geologically youth-ful because no seafloor with an age in excess of 180 millionyears was found. By comparison, continental crust that ex-ceeds 4 billion years in age has been dated.

The thickness of ocean-floor sediments provided addi-tional verification of seafloor spreading. Drill cores from theGlomar Challenger revealed that sediments are almost entire-ly absent on the ridge crest and the sediment thickness withincreasing distance from the ridge. Because the ridge crest is

Fracture zone

Transform fault(active)

Inactivezone

Inactivezone

SouthAmerica

Africa

Asthenosphere

Oceanic crust

Oceaniclithosphere

KEY

Spreading centers

Fracture zones

Transform faults

M i d - A t l a n t i cR i d g e

FIGURE 2.25 Diagram illustrating a transform fault boundary offsetting segments of the Mid-Atlantic Ridge.

*Radiometric dates of the ocean crust itself are unreliable because of the alteration ofbasalt by seawater.


tarbuck & lutgens 2008 plate tectonics the new paradigm

Documents

plate tectonics model

thetheory of plate tectonics

tectonics revolution

wegener hadbeen

namealfred wegener

concepts of continental

great faults

hynew ideas