CE3A8 SMJ Geology for Engineers 1 Plate Tectonics 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 observations could suddenly be explained by a single, simple theory. Interestingly for geologists, 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 continents and oceans, the mountains and valleys, the volcanoes and earthquakes. 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 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 Tectonics: An insider s history of the Modern Theory of the Earth (2003, N Oreskes (ed.), Westview). This book describes the new data sources and scientific techniques that precipitated the plate tectonics revolution 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. There follows a collection of essays from scientists who played key roles in developing 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 introduction to global geophysics (2003, CMR Fowler, Cambridge). Continental Drift Since the 16th century, cartographers have noticed the jigsaw-puzzle fit of the continental edges. Since the 19th century, geologists have known that some fossil plants and animals are extraordinarily similar across the globe, and that some sequences of rock formations in distant continents are also strikingly 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 theories of supercontinents and continental drift were suggested in the first part of the 20th century: the palaeontological and geological patterns and jigsaw-puzzle fit could be explained if the continents 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 origins 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.
CE3A8 SMJ Geology for Engineers 2 Initial Rejection Continental Drift was not accepted on the basis of the geological evidence, although possible mechanisms for lateral movement of the continents were vigourously debated through the first half of the 20th century. 19th century ideas such as Contraction Theory had explained the difference between continents and oceans in terms of mainly vertical motions as the young hot Earth cooled and contracted, 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 with measurement 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. A paradox concerning material properties of the mantle presented a further stumbling block. The mantle must 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 about crustal 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. Afterwards, a group of British geophysicists who had worked on magnetism and warfare (mine-sweeping and demagnetising 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 land recorded evidence that the position of the land masses relative to earth s poles had changed over the course of geological time. Some of them began to think again about continental drift. Yet these data did not 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 out of curiosity, partly because the US Navy hoped these measurements might suggest new means to hide or detect submarines. The results surprised everyone: a distinctive pattern in which some rocks were magnetised in concert with earth s current field and some in opposition to it. When plotted on paper in black and white, the pattern looked like zebra stripes. Scientists wondered what these magnetic stripes meant; noone at first connected the pattern with continental drift. Then, another group of scientists proved that over the course of geological history earth s magnetic field had reversed its polarity many times. Suddenly the meaning of the stripes became clear: the sea floor was splitting apart, or spreading,
CE3A8 SMJ Geology for Engineers 3 and new volcanic rocks were magnetised in alignment with earth s field each time they erupted at the sea floor. 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 a stampede to put together the pieces of the global story. There were several important lines of evidence and at first it was not entirely clear if they would fit together. If seafloor spreading at mid-ocean ridges was caused by convection currents rising from deep within very hot regions in the earth, then heat flow should be highest over these ridges, but scientists found some heat flow values at the ridges that were extremely low. This didn t seem to fit the big picture. Nor did the fact that heat flow over the continents was the same, on average, as over the ocean floors. For some scientists, these were reasons to remain unconvinced. But while heat flow measurements caused a certain amount of confusion, seismic data proved compelling. Advances in seismology were crucial to illuminating the big picture. For some time, seismologists had been mapping the distribution of global earthquakes and attempting to determine the nature of the motions associated 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 displayed a fabulous pattern outlining crustal blocks, and accurate determination of earthquakes slip directions proved that these blocks were moving in just the ways that global tectonics required. Global seismicity defines the plate boundaries. For more information on relationships between seismicity 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 the verge 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 into the earth at the boundaries between continents and oceans. Moreover, it was becoming clear that the geological arguments that had been put forward for continental drift more than 40 years earlier were probably largely correct. But those data had been criticised as qualitative and not verifiable or at least they looked unverifiable in retrospect. It remained to quantify the motions of the crustal blocks and to show that the motions calculated for any one block were consistent with the motions calculated for adjacent blocks. The blocks began to be referred to as plates flat, thin and rigid and the result was a theory called plate tectonics.
CE3A8 SMJ Geology for Engineers 4 From the Oceans to the Continents Continental drift was first proposed on the basis of geological evidence accumulated from fieldwork by geologists on the continents. In contrast, plate tectonics was developed largely on the basis of evidence from the sea floor, or earthquakes under it, collected mostly by geophysicists. When geologists realised what was happening, the most alert among them saw an opportunity 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 great San Andreas Fault suddenly could be explained, clearly and elegantly, by the new model. With every old understanding up for grabs and new understandings emerging daily, one of the 20th century s greatest scientific 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 geological arguments 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 was accepted without the evidence that Wegener sought. The geophysical data of plate tectonics heat flow, 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 actual observations of the motions themselves. It took another decade before such observations could be made through the development of satellite-based global positioning systems. However, because of their military applications, 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.
CE3A8 SMJ Geology for Engineers 5 Plate vs Crust The crust is a compositional layer. Crust = Light Scum. 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 atmosphere from the top of the plate. Plates have an equilibrium thickness (around 120 km) when the heatflux supplied to the bottom balances 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 3 types of plate boundary. Constructive Plate Boundaries. ridges. Oceanic plates are created by Seafloor Spreading at mid-ocean Destructive Plate Boundaries. So that Earth does not expand, oceanic plates are recycled (subducted) 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 continentocean 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 mantle beneath 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 composition.
CE3A8 SMJ Geology for Engineers 6 Young plate is thin. Since hot, convecting mantle has been drawn upwards to lie directly beneath the 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 thickness and average density increase with age, the plate sinks with respect to the mid-ocean ridge crest. The equilibrium plate thickness is 120 km, the equilibrium depth of oceanic abyssal plains is 6.5 km and 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 tend exponentially to equilibrium values. Processes at destructive plate boundaries Subduction. The old, cold, thick oceanic plate dives down into the mantle beneath either a continental 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 the melting temperature of the mantle, so magma begins to form. Volcanoes. The magma rises and adds to the overlying crust. The magma is more granitic in composition than that which forms oceanic crust. This compositional difference makes the magma more explosive, and a chain of volcanoes forms behind the oceanic trench and above the subducting plate. What drives Plate Tectonics? On the largest scale, Plate Tectonics is driven by the cooling of the planet. The cycle of oceanic plate creation by Seafloor Spreading and destruction by Subduction allows the Earth to lose heat much faster than it could if its surface were a single, rigid, spherical shell with all 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 rheological effect of lubricating the base of the plates as they slide over the mantle. On a more local scale, the characteristic 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 the mantle means that the density of the subducting plate increases, resulting in a force known as Slab Pull.