CE3A8 SMJ Geology for Engineers 1 Plate T ectonics The significance of plate tectonics in geological science can be likened to the significance of the theory of evolution in biological scien ce. In both cases, what at first had seeme d a dive rse set of obser va tions could sudden ly b e explained by a single , simple theor y . Interesti ngly for geologist s, most of the people who pioneered the plate tectonic revolution (which culminated in the 1960s) are still alive to comment on how they arrived at their ideas, whereas the biological pioneers like Charles Darwin are long dead. For engineers, plate tectonics is significant because it ultimately explains in large part why and where deformation of Earth’s surface occurs. It also explains the global distribution of earthquake and volcanic hazards, although of course predicting the time and location of individual natural disasters within these zones is much more tricky. Historical Background Before the 1960s, there was no generally accepted global theory to explain major features of the earth : the contin ent s and oceans , the mounta ins and valley s, the volcanoes and earthquakes. In the 1960s, a new theo ry emer ged that expl ained all thi s and more as the resu lt ofinteractions of moving pieces of the earth’s surface layer, henceforth to be known as tectonic plates. While the development of plate tectonics was a long time in coming — scientific evidence of continental mobility had been recognised since the early 20th century — its acceptance was rapid and nearly absolute. By the early 1970s, virtually all earth scientists accepted the new theory and textbooks were re-written. Much of the text of this handout is abstracted directly from Plate Te ctoni cs: An insider’ s history of the Modern Theory of the Earth(2003, N Oreskes (ed.), Westview). This book describes the new data sources and scient ific techni ques that precipita ted the plate tectoni cs revolu tion from a human angle. It opens with an explanation of how the forerunners of the theory raised questions that were finally answered 30 years later. Ther e follows a collection of essa ys from scien tists who pla yed key roles in dev elopin g the theory which tell the stories of their involvement in the extraordinary evolution of the theory. For details on the science behind plate tectonics, a good starting point is The Solid Earth: An intro duction to globalgeophysics (2003, CMR Fowler, Cambridge). Continental Drift Since the 16th century, cartographers have noticed the jigsaw-puzzle fit of the con- tinental edges. Since the 19th cen tury , geologists have known that some fossil plants and animals are extraordinarily similar across the globe, and that some sequences of rock formations in distant conti- nents are also striki ngly alike. Palaeoenvironmental data was puzzling too, with ancient glacial deposits discovered near the equator and warm climatic indicators (such as limestones, laterites and coals) near the poles. The theorie s of supercontinents and cont inen tal drift were suggeste d in the first part of the 20th centur y: the palaeon tologi cal and geolo gical pattern s and jigsaw-puzzle fit could be explained if the continents had migrated across earth’s surface, sometimes joining together, sometimes breaking apart. Alfre d Wegene r, a German meteorolo gist, was a partic ularly signifi can t figure . In his book ‘The origin s of continents and oceans’ (1915), he proposed that continents were not immobile but have shifted their position through time, and that previously all continents formed a single supercontinent, termed Pangaea. Pangaea reconstruction based on modern climatic data. See www.scotese.com for more information and plate reconstruction animations.
The significance of plate tectonics in geological science can be likened to the significance of the theory
of evolution in biological science. In both cases, what at first had seemed a diverse set of observationscould suddenly be explained by a single, simple theory. Interestingly for geologists, most of the peoplewho pioneered the plate tectonic revolution (which culminated in the 1960s) are still alive to commenton how they arrived at their ideas, whereas the biological pioneers like Charles Darwin are long dead.For engineers, plate tectonics is significant because it ultimately explains in large part why and wheredeformation of Earth’s surface occurs. It also explains the global distribution of earthquake and volcanichazards, although of course predicting the time and location of individual natural disasters within thesezones is much more tricky.
Historical Background Before the 1960s, there was no generally accepted global theory to explainmajor features of the earth: the continents and oceans, the mountains and valleys, the volcanoes andearthquakes. In the 1960s, a new theory emerged that explained all this and more as the result of
interactions of moving pieces of the earth’s surface layer, henceforth to be known as tectonic plates.While the development of plate tectonics was a long time in coming — scientific evidence of continentalmobility had been recognised since the early 20th century — its acceptance was rapid and nearly absolute.By the early 1970s, virtually all earth scientists accepted the new theory and textbooks were re-written.
Much of the text of this handout is abstracted directly from Plate Tectonics: An insider’s history of the
Modern Theory of the Earth (2003, N Oreskes (ed.), Westview). This book describes the new data sourcesand scientific techniques that precipitated the plate tectonics revolution from a human angle. It openswith an explanation of how the forerunners of the theory raised questions that were finally answered 30years later. There follows a collection of essays from scientists who played key roles in developing thetheory which tell the stories of their involvement in the extraordinary evolution of the theory. For detailson the science behind plate tectonics, a good starting point is The Solid Earth: An introduction to global
geophysics (2003, CMR Fowler, Cambridge).
Continental Drift Since the 16th century, cartographers have noticed the jigsaw-puzzle fit of the con-tinental edges. Since the 19th century, geologists have known that some fossil plants and animals areextraordinarily similar across the globe, and that some sequences of rock formations in distant conti-nents are also strikingly alike. Palaeoenvironmental data was puzzling too, with ancient glacial depositsdiscovered near the equator and warm climatic indicators (such as limestones, laterites and coals) nearthe poles. The theories of supercontinents and continental drift were suggested in the first part of the20th century: the palaeontological and geological patterns and jigsaw-puzzle fit could be explained if thecontinents had migrated across earth’s surface, sometimes joining together, sometimes breaking apart.Alfred Wegener, a German meteorologist, was a particularly significant figure. In his book ‘The originsof continents and oceans’ (1915), he proposed that continents were not immobile but have shifted theirposition through time, and that previously all continents formed a single supercontinent, termed Pangaea .
Pangaea reconstruction based on modern climaticdata. See www.scotese.com for more information
Initial Rejection Continental Drift was not accepted on the basis of the geological evidence, althoughpossible mechanisms for lateral movement of the continents were vigourously debated through the firsthalf of the 20th century. 19th century ideas such as Contraction Theory had explained the differencebetween continents and oceans in terms of mainly vertical motions as the young hot Earth cooled andcontracted, but these theories became untenable following discovery of radiogenic heating of Earth’s
interior. Ideas arising from detailed surveying of India by Sir George Everest and others, together withmeasurement of post-glacial rebound in NW Europe and N America, lead to the concept of isosasty,whereby the continental crust floats on a fluid substratum. Isostasy meant that vertical movements of the continents were accepted, but it was not clear whether lateral movements were also possible. Aparadox concerning material properties of the mantle presented a further stumbling block. The mantlemust be rigid because it transmits seismic waves, yet isostasy implies that the mantle behaves as a fluid.
From Palaeomagnetism to Seafloor Spreading When World War II broke out, arguments aboutcrustal mobility were put on hold as earth scientists applied their special knowledge and skills to surf forecasting, submarine navigation, anti-submarine warfare and other pressing issues of the day. After-wards, a group of British geophysicists who had worked on magnetism and warfare (mine-sweeping anddemagnetising ships) turned their attention to rock magnetism. Initially, they hoped to answer questions
about the origins of earth’s magnetic field. But they discovered something else entirely: rocks on landrecorded evidence that the position of the land masses relative to earth’s poles had changed over thecourse of geological time. Some of them began to think again about continental drift. Yet these data didnot immediately cause a stampede, for they were new and uncertain and people doubted their reliability.
Meanwhile, American scientists had been measuring the magnetism of rocks on the sea floor, partly outof curiosity, partly because the US Navy hoped these measurements might suggest new means to hideor detect submarines. The results surprised everyone: a distinctive pattern in which some rocks weremagnetised in concert with earth’s current field and some in opposition to it. When plotted on paper inblack and white, the pattern looked like zebra stripes. Scientists wondered what these magnetic stripesmeant; noone at first connected the pattern with continental drift. Then, another group of scientistsproved that over the course of geological history earth’s magnetic field had reversed its polarity manytimes. Suddenly the meaning of the stripes became clear: the sea floor was splitting apart, or spreading,
and new volcanic rocks were magnetised in alignment with earth’s field each time they erupted at the seafloor. Once the idea was in place, it took only a few years to demonstrate that it was right.
Heat Flow and Seismology The confirmation of seafloor spreading led to a rush — some might say astampede — to put together the pieces of the global story. There were several important lines of evidenceand at first it was not entirely clear if they would fit together. If seafloor spreading at mid-ocean ridgeswas caused by convection currents rising from deep within very hot regions in the earth, then heat flowshould be highest over these ridges, but scientists found some heat flow values at the ridges that wereextremely low. This didn’t seem to fit the big picture. Nor did the fact that heat flow over the continentswas the same, on average, as over the ocean floors. For some scientists, these were reasons to remainunconvinced. But while heat flow measurements caused a certain amount of confusion, seismic dataproved compelling.
Advances in seismology were crucial to illuminating the big picture. For some time, seismologists had beenmapping the distribution of global earthquakes and attempting to determine the nature of the motionsassociated with them. But their data were often sparse, inaccurate or confusing. The development of the
worldwide standard seismograph network (WWSSN) to aid in detecting nuclear weapons tests came at just the right time to solve the problems of plate tectonics: accurate locations of earthquakes displayeda fabulous pattern outlining crustal blocks, and accurate determination of earthquakes’ slip directionsproved that these blocks were moving in just the ways that global tectonics required.
Global seismicity defines the plate boundaries. For more information on relationships betweenseismicity and plate tectonics, refer to the lectures on earthquakes later in the course.
The Plate Model By 1967 most geophysicists and oceanographers were either convinced or on theverge of being convinced that earth’s surface was divided into large blocks that were moving en masse:splitting apart at mid-ocean ridges, moving laterally across the ocean basins and then sinking back intothe earth at the boundaries between continents and oceans. Moreover, it was becoming clear that thegeological arguments that had been put forward for continental drift more than 40 years earlier wereprobably largely correct. But those data had been criticised as qualitative and not verifiable — or atleast they looked unverifiable in retrospect. It remained to quantify the motions of the crustal blocksand to show that the motions calculated for any one block were consistent with the motions calculatedfor adjacent blocks. The blocks began to be referred to as plates — flat, thin and rigid — and the resultwas a theory called plate tectonics.
From the Oceans to the Continents Continental drift was first proposed on the basis of geologicalevidence accumulated from fieldwork by geologists on the continents. In contrast, plate tectonics wasdeveloped largely on the basis of evidence from the sea floor, or earthquakes under it, collected mostlyby geophysicists. When geologists realised what was happening, the most alert among them saw anopportunity for a radical reinterpretation of geological history based on the new model of crustal mobility.Moreover, important geological features that had never been fully understood — like California’s greatSan Andreas Fault — suddenly could be explained, clearly and elegantly, by the new model. With everyold understanding up for grabs and new understandings emerging daily, one of the 20th century’s greatestscientific revolutions happened.
Continents Really Do Move Alfred Wegener died on the Greenland icecap trying to find proof of continental drift. By proof he meant observations of the continents actually moving today. The geologicalarguments for drift were all indirect: they were surprising facts that could be explained if the continents
had moved, but they were not actual observations of moving continents. Ironically, plate tectonics wasaccepted without the evidence that Wegener sought. The geophysical data of plate tectonics — heatflow, seismicity, palaeomagnetism — were in their own way also indirect. They were observations of phenomena that followed from crustal motions or perhaps helped drive them, but they were not actualobservations of the motions themselves. It took another decade before such observations could be madethrough the development of satellite-based global positioning systems. However, because of their militaryapplications, many of the data collected by these satellites remained classified until the 1990s. Finally,almost a century after Alfred Wegener first suggested it and 30 years after earth scientists accepted it,we now have direct evidence that the earth really does move.
• When the mantle melts, the magma rises and solidifies to form crust.
• The plate is defined in terms of temperature and hence strength.Plate = ‘Cold Skin’.
• Heat is supplied to the base of the plate by the convecting mantle, and heat is lost to the atmospherefrom the top of the plate.
•
Plates have an equilibrium thickness (around 120 km) when the heatflux supplied to the bottombalances the heatflux lost at the top.
• The Plate is composed of the crust and the part of the mantle rigidly attached to it.
Types of Plate Boundary
Earth’s surface is composed of a set of relatively rigid plates that ‘float’ on top of the mantle which,although solid, behaves as a fluid over geological time. Motion between the plates is concentrated at 3types of plate boundary.
• Constructive Plate Boundaries. Oceanic plates are created by Seafloor Spreading at mid-oceanridges.
• Destructive Plate Boundaries. So that Earth does not expand, oceanic plates are recycled ( sub-
ducted ) back into the mantle at oceanic trenches.
• Conservative Plate Boundaries. The plates slide past each other laterally.
Continents move with the oceans like parcels on a conveyor belt, so that there are 2 types of continent-ocean boundary.
• Passive Continental Margins. The continental plate is fixed rigidly to the oceanic plate.
• Active Continental Margins. The continental plate over-rides the subducting oceanic plate.
Processes at constructive plate boundaries
• Plate Spreading. As the oceanic plates spread apart at a mid-ocean ridge, the hot convecting mantlebeneath the plates is drawn upwards to fill the gap.
• Melting. The hot mantle melts as it rises because the confining pressure decreases.
• Formation of crust. The magma rises, cools and solidifies to form oceanic crust of basaltic/gabbroic
• Young plate is thin. Since hot, convecting mantle has been drawn upwards to lie directly beneaththe crust, the plate at the mid-ocean ridge is no thicker than the crust.
• Plate cools to equilibrium. As the plate spreads away from the mid-ocean ridge, it cools and thickens
towards its equilibrium thickness. The principle of isostasy means that because the plate’s thicknessand average density increase with age, the plate sinks with respect to the mid-ocean ridge crest. Theequilibrium plate thickness is ∼ 120 km, the equilibrium depth of oceanic abyssal plains is 6.5 kmand the characteristic time constant to reach these values is ∼ 60 Myr (million years). Solution of the heatflow equations shows that both plate thickening and associated seafloor subsidence tendexponentially to equilibrium values.
Processes at destructive plate boundaries
• Subduction. The old, cold, thick oceanic plate dives down into the mantle beneath either a conti-nental or another oceanic plate. Bending of the plate results in a deep trench.
• Water. Sea water subducted down into the mantle along with the oceanic plate decreases themelting temperature of the mantle, so magma begins to form.
• Volcanoes. The magma rises and adds to the overlying crust. The magma is more granitic incomposition than that which forms oceanic crust. This compositional difference makes the magmamore explosive, and a chain of volcanoes forms behind the oceanic trench and above the subductingplate.
What drives Plate Tectonics? On the largest scale, Plate Tectonics is driven by the cooling of theplanet. The cycle of oceanic plate creation by Seafloor Spreading and destruction by Subduction allowsthe Earth to lose heat much faster than it could if its surface were a single, rigid, spherical shell withall heat lost by conduction. Water is also tremendously important; water returned to the mantle via
Subduction both allows production of the granites that form the continents, and has the rheologicaleffect of ‘lubricating’ the base of the plates as they slide over the mantle. On a more local scale, thecharacteristic mid-ocean ridge topography helps plates ’slide away’ from the ridge (known as Ridge Push).Furthermore, high-pressure metamorphism of oceanic crustal rocks as they are subducted down into themantle means that the density of the subducting plate increases, resulting in a force known as Slab Pull.
