Planet Earth Topic 3: Plate Tectonics. Roger N. Anderson. The mid-ocean spreading centers that host the hydrothermal vents and chemosynthetic organisms
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© Roger N. Anderson2Planet Earth Topic 3: Plate TectonicsRoger N. AndersonThe mid-ocean spreading centers that host the hydrothermal vents andchemosynthetic organisms consist of a continuous string of volcanoes that encircle theglobe (Figure 3-1). This mid-ocean ridge system produces new sea floor when thevolcanoes erupt. Lava pours onto the sea floor, cools and spreads away from the ridgecrest to become the conveyor belt upon which the continents ride, hence the name sea-floor spreading. This process forms new ocean floor, then moves this new lava awayfrom the mountain crest as ever newer lava replaces it.Figure 3-1. The global mid-ocean ridge system as seen on this bathymetry map created by scientists fromLamont Doherty Earth Observatory of Columbia. The topography of the ocean basins was determinedfrom satellite measurements of the ocean surface, that is deflected slightly to mirror the sea floortopography.The mid-ocean ridge spreading centers form the extensional component of thetheory of Plate Tectonics. But how is new sea floor continuously formed at the mid-oceanspreading centers such as at the Mid-Atlantic Ridge, that grows the Atlantic Ocean andseparates North and South America farther and farther from Europe and Africa withoutexpanding the Earth?
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© Roger N. Anderson3Figure 3-2. The Lithospheric Plates that define the surface dynamics of Planet Earth.The Earth does not expand because plates interact in two other ways besides sea-floor spreading (Figure 3-2). At the opposite end of the conveyor belt, deep-sea trenchessuch as that off the west coast of South America are the locations where old rock fromthe surface ÒsubductsÓ or plunges downward, back into the mantle. Subduction zonesmark the collision of two plates, the opposite of the extension occurring at mid-oceanspreading centers. At subduction zones, one lithospheric plate rides over the other,forcing the downgoing plate back into the mantle and the overriding plate up into the airto form the great mountain ranges of the continents. There are mountain ranges of similarsize at both of these boundaries, but the ocean ridges begin miles beneath the sea surfaceand rarely grow high enough to break through (like Iceland).A third type of lithospheric plate interaction occurs in Plate Tectonics when twoplates slide past each other. A long linear fracture in the earth called a transform fault isformed, such as the San Andreas Fault in California. There, the Pacific lithospheric plateis sliding to the northwest across the North American lithospheric plate.The theory of Plate Tectonics proposes that the Earth’s surface is broken into alarge mosaic of lithospheric plates that continuously move relative to each other in aprecisely determined way. Geometry explains the motion; mantle convection, the push ofmid-ocean ridge topography, and the pull of subduction zones control the speed, timing,and directions of plate movements.
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© Roger N. Anderson4Each of these three plate boundaries is characterized by a different kind of force:tension from extension at mid-ocean ridge spreading center boundaries; compressionfrom collision at subduction zones; and shearing from tearing at transform faults (figure3-3).Figure 3-3. The three types of Plate boundaries are shown at top, and where they occur within a crosssection of the Nazca and Pacific Plates sliced open from South America on the right to Tahiti on the left.Evidence for Plate TectonicsHow is it that we know that the Earth is covered with a mosaic of rigid plates, andthat these plates interact with each other only at their common boundaries. The discoveryat the foundation of Plate Tectonics is that the surface of the Earth is capped by rigidouter or lithospheric plates that are only a hundred kilometers or so thick. These platesÒfloatÓ on a softer, partially liquid upper mantle.Seismology provides the most compelling evidence for the existence of theselithospheric plates. Earthquakes predominantly occur only along plate boundaries(shallow earthquakes in black in figure 3-4), and not inside the interior of the plates.Simply observing where the earthquakes are delineates the plate boundaries. Deep (inred, Figure 3-4) and intermediate depth earthquakes (green in Figure 3-4) define thesubduction back into the mantle shown in Figure 3-3.The seismic data to prove the plate tectonic theory was provided to the geologicalcommunity beginning in the 1950s, when the United States was preoccupied with thedetection of Soviet nuclear bomb tests. The Eisenhower administration built a worldwidenetwork of 125 seismic stations to record ground shaking from nuclear tests, and thisnetwork also recorded every large earthquake that happened anywhere in the world. Bythe late 1960s the network had accumulated enough earthquake locations to plot them ona world map. It became clear that earthquakes do not occur randomly on the surface, butin linear belts that wrap around the Earth. This remarkable fact had escaped detectionuntil the seismic network was constructed.
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© Roger N. Anderson5But what controls earthquakes so precisely as to align them into belts? Moreover,when the locations of all known volcanoes are added to the same world map, they fallalong the same belts as the earthquakes, so much so that it is difficult to distinguishbetween the earthquakes (circles in Figure 3-4) and volcanoes (solid dots in Figure 3-4).It took a simple but elegant intellectual leap for geologists to recognize that these belts ofseismic (earthquake) and volcanic activity are the boundaries of interaction among a fewlarge and rigid plates that cover the surface of the Earth.Figure 3-4. Earthquakes from 1977 to 1992 broken into categories determined by their focal depth, withblack circles indicating depths from the surface to 70 km, green circles are earthquakes occurring from 70to 300km depth, and red circles are from 300 to 700 km. Over 10,000 large earthquakes with magnitudegreater than 5.5 are plotted. Notice that majority of the large earthquakes occurs at or close to major plateboundaries, at the same locations as the active volcanoes (solid black dots) (see Figure3-2).The surface motions of those conveyor belts are governed by very precise lawsbecause the lithosphere is made up of rigid, solid plates constrained to move on thesphere that is the EarthÕs surface. Plates were discovered because they are so rigid thatthey obey these strict rules of geometric motion. These patterns were noticed only afterobservations of the sea floor we complete enough to map the shape and form of the seafloor Ð only in the 1960Õs as part of the cold warÕs anti-submarine defenses.Geometry of Plate TectonicsTwo solids on a sphere can do only one of three things and remain rigid: pullapart from each other, collide, or slide across each other. Tear a sheet of paper in two,
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© Roger N. Anderson6then hold one piece in each hand. Any motion of the left- hand piece relative to the rightcan be described by one of these three motions if and only if they remain on the sameplane of motion. Sound familiar? Substitute the surface of a sphere, and these are thethree forms of plate boundaries described in the last section. The plates interact only attheir edges, not within their solid interiors.Plate Tectonics describes the relative motion of surface plates but does not dealdirectly with the forces within the mantle that push and pull the plates and create themotion in the first place. This will come later.The Earth is covered by 12 large, stable lithospheric plates. They move relative toeach other causing tectonic activity only at their boundaries where interactions with otherplates occur. Almost all the earthquakes and volcanoes on the surface of the earth happenat these precisely defined boundaries.The elegance of plate tectonics goes far beyond just describing where earthquakesand volcanoes occur. There are processes occurring at these boundaries that allow us todetermine the direction of motion of plates on the surface. For example, we can predictthat if the Pacific plate continues to move to the northwest relative to the North Americanplate as defined by the San Andreas Fault (Figure 3-5), San Francisco will collide withAlaska in 40 million years. You might say that this information is not of much use, butplate tectonics also allows us to infer that San Francisco used to be attached to Sonora,Mexico, and that gold mines found there might “have brothers” ripped off Sonora andcarried a thousand miles to the north.Figure 3-5. Plate boundaries along the western margin of North America show that if motions continue,about 50 million years from now, southern California will collide with Alaska.
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© Roger N. Anderson8But what if a spreading center does not separate two or three plates? How do wedetermine relative motion, or even more interestingly, how do we know what kind ofmotion is occurring along that boundary at all?It is possible to define the boundaries of plates by the type of tectonic activityassociated with each. But it is important to realize that the continent- ocean transition isan insignificant barrier compared to the thickness of a lithospheric plate. Therefore, acontinent-ocean boundary can occur well within any single plate, and in general suchedges of continents have little to do with plate boundaries! The continents are, however,large enough to weigh down plates that contain significant surface area of continent, butthe continental crust is insignificant compared to the dimensions of a plate (greater than100 km thick and often several thousand kilometers across). That is, plates withcontinents appear to move more slowly than those without, such as the Pacific plate; butthey still move by the same geometric laws.Plate tectonics is a powerful geological tool because it is predictive. One candetermine the type of plate boundary interactions by studying the focal mechanisms ofearthquakes, for example. North American-Pacific plate interaction can be determined bystudying the orientation of the fault planes along this boundary (Figure 3-7). Theseearthquake mechanisms have distinct orientations that tell in what direction the plates areconverging or diverging. They are of different form, depending upon whether the faultmotion causing the earthquake was tensional (a gravity, or normal, fault in which themotion is in the same direction as gravity’s pull), compressional (a thrust fault caused bycollision where motion appears to be against gravity), or strike-slip (as the name implies,one plate slides across the other). Sound familiar (Figure 3-3)? Earthquake focalmechanisms can be used equally well to determine the pole of rotation of Pacific-NorthAmerica motion, as transform faults determined North America-African plate motion inthe previous example.Figure 3-7. The topography of California is explained by the plate tectonic boundaries, but that of Nevadamust await further discussion of how continents work in the next chapter.
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© Roger N. Anderson9Seismology and Plate TectonicsWhen an earthquake happens, the Earth shakes with a precise pattern to itsvibrations Ð a pattern controlled by the type of earthquake that it is. The initial, outgoingwave of energy from the break along a fault is called a compressional wave (also calledthe primary or P-wave) because the particles in the Earth push and pull each other as thevibration passes directly away from the break (think of it as shaking exactly away from,then toward, the location of the earthquake like ripples on a pond moving away from apebble-impact). The second form of vibration is a shear wave (also known as thesecondary or S-wave), so called because the Earth transmits the shear vibrations by asideways motion, moving perpendicular to the expanding shear waves. This wave isimportant because solid must be in contact with solid for a shear wave to pass. If liquidexists anywhere within the Earth, shear waves will be absent for the portion of the wave’spath that crosses through that region.Waves going directly through the center of the Earth have very slow velocities asthey pass through the outer core because part of that path is through the liquid iron core.Not only do shear waves disappear there, but compressional waves are slowed by theÒsoftnessÓ of the liquid as well. That is, the time it takes for the P-wave to pass throughthe Earth appears to be slow because the liquid outer core absorbs all shear and somecompressional energy and thus slows down even the P-wave. But it then speeds up againin the solid iron inner core, before slowing again thru the outer liquid core on its wayback to the earths surface on the other side of the planet. The lithospheric plates that form surface layer of the Earth are fast transmitters ofshear waves, and thus rigid and solid. However, only a few hundred kilometers below thesurface, a slow velocity is observed everywhere from the partially liquid upper mantle.Lithospheric plates vary in thickness with their oceanic portions sometimes less than 100km thick and continental segments sometimes over 300 km thick.Low velocity seismic waves are found wherever there is molten rock, but notmuch is required Ð even 1 percent melt and 99% solid rock . Even that scant amount isenough to eliminate shear waves and slow down P-waves. It is this upper mantle ofpartially molten magma ÒslushÓ that provides lava to the surface at mid-ocean ridgespreading centers whenever extension pulls two plates apart and a crack appears in thelithosphere. The partially molten upper mantle is called the asthenosphere. Thelithosphere, made up of cold, completely solidified rock, is gravitationally unstable inthat it is heavier than the asthenosphere below. In fact, plate tectonics is often describedas a set of large conveyor belts, with new rock moving upward at mid-ocean ridges,outward and along the ocean basins and downward, back into the asthenosphere atsubduction zones.Seismologists use visual displays of earthquake focal mechanisms, calledballoons, that are really the simplest information one can get from an earthquake. Anearthquake literally shakes the entire Earth. But as we saw above, there is logic and apattern to this shaking. As the compressional wave passes any given location on theEarth, the ground either heaves up or sinks down as its first-motion response to thepassing compressional energy. But the earth moves either up or down in four quadrants,not in halves as one might expect.
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© Roger N. Anderson10The planet cannot move only in two halves like an orange sliced along the faultthat caused the earthquake in the first place. Consider the San Andreas Fault: If half theglobe west of California heaved up and slid to the north after the San Franciscoearthquake in 1906 and the other half of the planet east of California sank and moved tothe south, the rotation of the Earth would be disrupted. It would be as if one were to slicea tennis ball in half, then glue it back together with the two halves offset. Throwing thetennis ball would cause it to carom wildly off to one side. The Earth cannot do the same.It must spin smoothly in its orbit. The secret of how the Earth accommodates the splittingmotion of an earthquake is found in that familiar axiom: For every force in one direction,there must be an equal force in the opposite direction (one of Newton’s laws of motion).If the Pacific moves to the north, then the Indian Ocean on the opposite quadrant of theglobe must move counter to it to balance the net force on the Earth. Otherwise, ourrotation would be disrupted. This means that the Earth must move in quadrants during anearthquake, rather than in halves, and this is just what we observe when studying the firstmotions of all earthquakes.Consider a gravity, or normal, fault where the Earth breaks along a slope and therock slides downhill in the direction of the gravitational pull. Tensional forces cause suchfaulting. Somewhere on the opposite side of the globe, the Earth must move in theopposite direction to counteract this force. There are now many thousand more than theoriginal 125 seismographic stations around the world that have instruments that recordthe motion of earthquakes. By recording whether the first motion of the ground waseither up or down after an earthquake, the mechanism or type of earthquake can bedetermined, even if the earthquake happened hundreds of miles underground. A normalfault will cause all stations near the downthrown side of the fault to record downwardfirst motions. Stations on the other side of the fault will record upward first motions. Byexamining enough seismograms from the four quadrants of the planet centered on thelocation, or epicenter of the earthquake, the two regions that moved up can be mapped.Ninety degrees from these quadrants will be two regions that moved down. By projectingthe spherical earth onto a plane, a balloon, or focal mechanism can be drawn. The patternof the balloon allows one to determine the type of earthquake that occurred, even if theevent was located thousands of miles to sea under the ocean.What if a mysterious earthquake occurred last night in the center of the IndianOcean? Visual inspection of the fault is impossible, yet we can determine not only whattype of event it was, but also the orientation of the quadrant boundaries will show thetype of plate boundary on which the earthquake occurred.By determining the focal mechanism ÒballoonsÓ from the earthquakes occurringunder Japan, we can describe the physical processes occurring during subduction. Notonly do we see the Pacific plate hanging beneath Japan, as outlined in the locations of theearthquakes themselves, but the focal mechanisms on the upper plane of the plate are allcompressional or thrust earthquake balloons. Those on the bottom plane are all normalfault balloons (Figure 3-8). The plate is first bending under Japan, then unbending againafter passing into the mantle. Such forces can be replicated by sliding a sheet of paperover the edge of a table. Because the paper is elastic, first it bends then unbends as itpasses over the edge of the table. The balloons have allowed us to see not only the forcespresent for hundreds of miles beneath Japan, but the elasticity of the solid lithosphericplates as well.
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© Roger N. Anderson11Figure 3-8. Small circles are all the earthquakes to occur beneath Japan (marked by the black bar at thesurface) between 1970 and 1990. In the lower right are the composite focal mechanism balloons for all theearthquakes that occurred along the two bands marking the upper and lower boundaries of the subductingPacific plate. A black center indicates compressional or thrust earthquakes. A white center indicatesextensional or normal earthquakes. The plate is unbending!Earthquakes have been precisely recorded only since the late 1950s, and plates,though they move slowly: a few centimeters per year or about six feet in the averageperson’s lifetime. So plate tectonic theory would not be very powerful unless it could beextended back in time long before the 1950Õs. Earthquakes recorded in human historytake us back sometimes thousands of years. They go back even further than that when wesee the seismic definition of a plate plunging 250 km or more into the mantle beneath asubduction zone like Japan though, because we know that at a rate of 10 cm/yr, 2.5million years of that plate’s motion is outlined to us along that path (Figure 3-8). But weneed an even longer-lived geological recording of the past motions of plates, and themagnetization of the Earth provides just such a record.
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