Volcanic and seismic events are major pieces of evidence towards proving that plate-tectonics theory is valid Discuss the extent to which you agree with this statement. [40 marks]

their equivalent-age mountain ranges in Great Britain are currently separated by the Atlantic Ocean, they form an essentially continuous mountain range when the continents are positioned next to each other as they were during the Paleozoic Era.Glacial EvidenceAnother geologic feature that matches up across continental joins is the deposits left by ancient ice sheets. These are similar to the deposits left by relatively recent (Pleistocene) glaciations in Canada, Scandinavia, and northern United States, among other places. In South America and Africa there are very thick glacial deposits of the same age (Permian-Carboniferous). The deposits match almost exactly when the continents are moved back together. As glacial ice moves, it cuts grooves and scratches in underlying rocks and produces folds and wrinkles in soft sediments. These features left behind in glacial deposits provide evidence of the direction the ice was moving during the glaciation. When Africa and South America are moved back together, the direction of ice movement on both continents is consistent, radiating outward from the center of the former ice sheet. It’s hard to imagine how such similar glacial features could have been created if the continents had not once been joined together. Africa and South America must have had similar climates during this period, colder than their present-day climates. This also suggests that they were not in their present equatorial locations. Figure 4.6 shows glacial deposits in a reconstruction of the southern continents in Pangaea, with grooves and scratches indicating ice movement outward in all directions from what was then the South Pole.During the Late Paleozoic Era, massive glaciers covered large continental areas of the Southern Hemisphere. Evidence for this glaciation includes layers of till (sediments deposited by glaciers) and striations (scratch marks) in the bedrock beneath the till. Fossils and sedimentary rocks of the same age from the Northern Hemisphere, however, give no indication of glaciation. Fossil plants found in coals indicate that the Northern Hemisphere had a tropical climate during the time the Southern Hemisphere was glaciated. All the Gondwana continents except Antarctica are currently located near the equator in subtropical to tropical climates. Mapping of glacial striations in bedrock in Australia, India, and South America indicates that the glaciers moved from the areas of the present-day oceans onto land. This would be highly unlikely because large continental glaciers (such as occurred on the Gondwana continents during the Late Paleozoic Era) flow outward from their central area of accumulation toward the sea. If the continents did not move during the past, one would have to explain how glaciers moved from the oceans onto land and how large-scale continental glaciers formed near the equator. But if the continents are reassembled as a single landmass with South Africa located at the South Pole, the direction of movement of Late Paleozoic continental glaciers makes sense (  Figure 2.5). Furthermore, this geographic arrangement places the northern continents nearer the tropics, which is consistent with the fossil and climatologic evidence from Laurasia.Fossil EvidenceIf Africa and South America were really joined together at one time, with the same climate and matching geologic features, then they also should have had the same plants and animals. To check this hypothesis, Wegener turned to the fossil record. It revealed that communities of plants and animals appear to have evolved together until the time of the splitting apart of Pangaea, after which they evolved separately. In seeking support for the continental drift hypothesis, Wegener pointed to specific fossil species found in matching areas across the continental joins. One example he used was an ancient fern, Glossopteris, whose fossil remains have been found in southern Africa, South America, Australia, India, and Antarctica (Fig. 4.7). Could the seeds of this plant have been carried by wind or water from one location to another? Probably not. The seeds of Glossopteris were large and heavy and could not have been carried very far by wind or water currents. This fern flourished in a cold climate; it would not have thrived in the warm present day climates of the continents where its fossil remains are found. This, too, is consistent with the idea that these continents were once joined together with similar, polar climates. There are other examples as well. The fossil remains of Mesosaurus, a small reptile from the Permian Period, are found both in southern Brazil and in South Africa (Fig. 4.8). The types of rocks in which the fossils are found are very similar. Mesosaurus did swim but was too small (about half a meter long, less than 2 ft) to swim all the way across the ocean. Fossil remains of specific types of earthworms also occur in areas that are now widely separated. How could they possibly have migrated across the oceans? The land masses in which they lived must once have been connected.Some of the most compelling evidence for continental drift comes from the fossil record. Fossils of the Glossopteris flora are found in equivalent Pennsylvanian- and Permian-aged coal deposits on all five Gondwana continents. The Glossopteris flora is characterized by the seed fern Glossopteris (  Figure 2.1) as well as by many other distinctive and easily identifiable plants. Pollen and spores of plants can be dispersed over great distances by wind, but  Glossopteris-type plants produced seeds that are too large to have been carried by winds. Even if the seeds had floated across the ocean, they probably would not have remained viable for any length of time in saltwater. The present-day climates of South America, Africa, India, Australia, and Antarctica range from tropical to polar and are much too diverse to support the type of plants in the Glossopteris flora. Wegener therefore reasoned that these continents must once have been joined so that these widely separated localities were all in the same latitudinal climatic belt (  Figure 2.6). The fossil remains of animals also provide strong evidence for continental drift. One of the best examples is Mesosaurus, a freshwater reptile whose fossils are found in Permian-aged rocks in certain regions of Brazil and South Africa and nowhere else in the world (  Figure 2.6). Because the physiologies of freshwater and marine animals are completely different, it is hard to imagine how a freshwater reptile could have swum across the Atlantic Ocean and found a freshwater environment nearly identical to its former habitat. Moreover, if  Mesosaurus could have swum across the ocean, its fossil remains should be widely dispersed. It is more logical to assume that Mesosaurus lived in lakes in what are now adjacent areas of South America and Africa but were then united into a single continent. Lystrosaurus and Cynognathus are both land-dwelling reptiles that lived during the Triassic Period; their fossils are found only on the present-day continental fragments of Gondwana (  Figure 2.6). Because they are both land animals, they certainly could not have swum across the oceans currently separating the Gondwana continents. Therefore, it is logical to assume that the continents must once have been connected. Recent discoveries of dinosaur fossils in Gondwana continents further solidifies the argument that these landmasses were close to each other during the Early Mesozoic Era. Notwithstanding all of the empirical evidence presented by Wegener and later by du Toit and others, most geologists simply refused to entertain the idea that continents might have moved during the past. The geologists were not necessarily being obstinate about accepting new ideas; rather, they found the evidence for continental drift inadequate and unconvincing. In part, this was because no one could provide a suitable mechanism to explain how continents could move over Earth’s surface. Interest in continental drift waned until new evidence from oceanographic research and studies of Earth’s magnetic field showed that the present-day ocean basins were not as old as the continents but were geologically young features that resulted from the breakup of PangaeaPolar Wandering CurvesA turning point occurred in the 1950s, through the study of paleomagnetism. When magma cools and solidifies into rock, it becomes magnetized and takes on the prevailing polarity—the north-south directionality—of the Earth’s magnetic field (Fig. 4.9). The paleomagnetic signature of a rock also provides useful information about the location of the Earth’s poles. Just as a free-swinging magnet today will point toward today’s magnetic north polle, so too does a rock’s paleomagnetism act as a pointer toward the Earth’s magnetic north pole at the time of rock formation. Paleomagnetism provides another useful piece of information about the Earth’s magnetic poles at the time of rock formation. The angle or magnetic inclination of a freely swinging bar magnet varies with latitude, because the bar magnet always points directly to the pole (refer to Fig. 3.14). For example, near the equator a magnet will lie relatively flat (horizontal). The closer it gets to the poles, the steeper will be its angle from the horizontal (that is, its inclination). When it is located exactly at the pole, the bar magnet will be vertical, pointing directly to the pole—it will be at the highest possible angle from the horizontal (90°). Because this angle varies systematically with latitude, it is always possible to figure out how far away you are from the pole, on the basis of magnetic inclination. The same is true of a rock’s paleomagnetic inclination, which thus provides a record of the geographic latitude where the rock formed. In the 1950s, geophysicists studying paleomagnetic pole positions found evidence suggesting that the Earth’s magnetic poles had wandered all over the globe for at least the past several hundred million years. They plotted the pathways of the poles on maps and referred to the phenomenon as apparent polar wandering (Fig. 4.10). Geophysicists were puzzled by this evidence because they knew that the Earth’s magnetic poles and its axis of rotation are always close together. When it was discovered that the path of apparent polar wandering measured in North America differed from that  measured in Europe, geophysicists were even more puzzled. They knew that it was extremely unlikely that the magnetic poles had moved. Instead they concluded, somewhat reluctantly, that it must have been the continents themselves that had moved, carrying their magnetic rocks with them. Thus, the apparent polar wandering path of a continent, determined from the paleomagnetism of rocks of different ages, provides a historical record of the movement of that continent relative to the positions of the magnetic poles. Note from Figure 4.10 that the apparent polar wandering paths of Europe and North America are separate from 600 million years ago to about 50 million years ago. You might also notice that the shapes of the paths look rather similar. In fact, if you rotate Europe and North America from their present positions and reassemble them as a single continent, as if the Atlantic Ocean weren’t there, an interesting thing happens: the two paths overlap and fit together exactly. This indicates that Europe and North America were moving together as a single continent during, the several hundred million years represented by these paths. This was—and still is—a very powerful argument for the existence, long ago, of a supercontinent that eventually split into separately moving pieces.  How can the apparent wandering of the magnetic poles be best explained?As paleomagnetic research progressed during the 1950s, some unexpected results emerged. When geologists measured the paleomagnetism of geologically recent rocks,they found it was generally consistent with Earth’s current magnetic field. The paleomagnetism of ancient rocks,though, showed different orientations. For example, paleomagnetic studies of Silurian lava flows in North America indicated that the north magnetic pole was located in thewestern Pacific Ocean at that time, whereas the paleomagnetic evidence from Permian lava flows pointed to yet another location in Asia. When plotted on a map, thepaleomagnetic readings of numerous lava flows from all ages in North America trace the apparent movement of the magnetic pole (called  polar wandering) through time  (  Figure 2.8). This paleomagnetic evidence from a single continent could be interpreted in three ways: The continent remained fixed and the north magnetic pole moved; the north magnetic pole stood still and the continent moved; or both the continent and the north magnetic pole moved.Upon additional analysis, magnetic minerals from European Silurian and Permian lava flows pointed to a different magnetic pole location from those of the same age in North America (  Figure 2.8). Furthermore, analysis of lava flows from all continents indicated that each continent seemingly had its own series of magnetic poles. Does this reallymean there were different north magnetic poles for each continent? That would be highly unlikely and difficult to reconcile with the theory accounting for Earth’s magnetic field. The best explanation for such data is that the magnetic poles have remained near their present locations at the geographic north and south poles and the continents have moved. When the continental margins are fitted together so that the paleomagnetic data point to only one magnetic pole, we find, just as Wegener did, that the rock sequences and glacial deposits match and that the fossil evidence is consistent with the reconstructed paleogeography.Plageomagnetic bands and the spreading of the sea floorThis new understanding of apparent polar wandering helped revive the hypothesis of continental drift. But many scientists were still holding out for a final piece of evidence that would demonstrate conclusively that a supercontinent had actually split apart and seas had flowed into the widening rift. Specifically, they were trying to envision a mechanism whereby the crust could actually split open. Evidence concerning that mechanism finally appeared but not until the early 1960s—three decades after Wegener’s death. The clue was found by scientists who made a crucial discovery while studying the paleomagnetic properties of Atlantic seafloor rocks. When oceanographers surveyed the floor of the Atlantic Ocean with magnetometers, they were astonished to find that parts of the sea floor consist of magnetized rocks with alternating bands of normal and reversed polarities (Fig.4.11). The bands are hundreds of kilometers long. More important, they are exactly symmetrical on either side of a centerline that corresponds to the crest of the ridge running down the middle of the Atlantic Ocean. In other words, if you could fold the sea floor in half along the midocean ridge, the bands on either sidewould match exactly. The symmetrical patterns of magnetic reversals discovered in seafloor rocks mystified scientists at first. Then several groups of geophysicists, working independently, came up with the same explanation. They proposed that the sea floor had split apart along the midocean ridge and that the rocks on either side were moving away from one another. As the rocks spread apart, molten material from the mantle below welled up into the crack, solidifying into new volcanic rocks on the seafloor. When the molten rock solidified, it took on the magnetic field polarity of the Earth at that time. Over time the spreading sea floor functioned like a conveyor belt, carrying the newly magnetized bands of rock away from the centerline of the ridge in either direction. This process came to be known as seafloor spreading. Geologists also have demonstrated that the ages of seafloor rocks increase with distance from the ridge. The youngest rocks are found along the centreline ridge, where new molten material wells up (Fig. 4.12). When magnetic reversals occur, they are recorded in the newly formed rocks along the midocean ridge  The result is the formation of symmetrical bands of volcanic rock with alternating magnetic polarities. This final piece of evidence convinced the great majority of geologists that seafloor spreading indeed occurs. As it turned out, geophysicists—those who had most vigorously opposed Wegener’s ideas—ultimately provided the paleomagnetic evidence that supported continental driftInterest in continental drift revived during the 1950s as a result of evidence from paleomagnetic studies, a relatively new discipline at the time. Paleomagnetism is the remanent magnetism in ancient rocks recording the direction and intensity of Earth’s magnetic field at the time of the rock’s formation. Earth can be thought of as a giant dipole magnet in which the magnetic poles essentially coincide with the geographic poles (  Figure 2.7). This arrangement means that the strength of the magnetic field is not constant but varies, being weakest at the equator and strongest at the poles. Earth’s magnetic field is thought to result from the different rotation speeds of the outer core and mantle.  What is the Curie point and why is it important?When magma cools, the magnetic iron-bearing minerals align themselves with Earth’s magnetic field, recording bothits direction and its strength. The temperature at which iron-bearing minerals gain their magnetization is called theCurie point. As long as the rock is not subsequently heated above the Curie point, it will preserve that remanent magnetism. Thus an ancient lava flow provides a record of the orientation and strength of Earth’s magnetic field at the time the lava flow cooled. A renewed interest in oceanographic research led to extensive mapping of the ocean basins during the 1960s. Such mapping revealed an oceanic ridge system more than 65,000 km long, constituting the most extensive mountain range in theworld. Perhaps the best-known part of the ridge system is the Mid-Atlantic Ridge, which divides the Atlantic Ocean basin into two nearly equal parts (  Figure 2.10). As a result of the oceanographic research conductedduring the 1950s, Harry Hess of Princeton University proposed the theory of  seafloor spreading in 1962 to account for continental movement. He suggested that continents do not move across oceanic crust, but ratherthe continents and oceanic crust move together. Thus the theory of seafloor spreading answered a major objectionof the opponents of continental drift—namely, how could continents move through oceanic crust? In fact, the continents moved with the oceanic crust as part of a lithospheric system. Hess postulated that the seafloor separates at oceanic ridges, where new crust is formed by upwelling magma. Asthe magma cools, the newly formed oceanic crust moves laterally away from the ridge.As a mechanism to drive this system, Hess revived the idea (proposed in the 1930s and 1940s by Arthur Holmes and others) of thermal convection cells in the mantle; thatis, hot magma rises from the mantle, intrudes along fractures defining oceanic ridges, and thus forms new crust. Cold crust is subducted back into the mantle at oceanictrenches, where it is heated and recycled, thus completing a thermal convection cellMagnetic surveys of the oceanic crust revealed striped magnetic anomalies  (deviations from the average strength ofEarth’s magnetic field) in the rocks that are both parallel to and symmetric around the oceanic ridges (  Figure 2.11). Furthermore, the pattern of oceanic magnetic anomaliesmatches the pattern of magnetic reversals already known from studies of continental lava flows (  Figure 2.9). Whenmagma wells up and cools along a ridge summit, it records Earth’s magnetic field at that time as either normal or reversed. As new crust forms at the summit, the previously formed crust moves laterally away from the ridge. Thesemagnetic stripes represent times of normal and reversed polarity at oceanic ridges (where upwelling magma formsnew oceanic crust), conclusively confirming Hess’s theory of seafloor spreading.The seafloor spreading theory also confirms that ocean basins are geologically young features whose openings andclosings are partially responsible for continental movement (  Figure 2.12). Radiometric dating reveals that the oldestoceanic crust is somewhat less than 180 million years old, whereas the oldest continental crust is 3.96 billion years old.Although geologists do not universally accept the idea of thermal convection cells as a driving mechanism for plate movement, most accept that plates are created at oceanic ridges and destroyed at deep-sea trenches, regardless of the driving mechanism involved.For many geologists, the paleomagnetic data amassed in support of continental drift and seafloor spreading were convincing. Results from the Deep-Sea Drilling Project (seeChapter 12) confirmed the interpretations made from earlier paleomagnetic studies. Cores of deep-sea sediments and seismic profiles obtained by the Glomar Challenger and other research vessels have provided much of the data that support the seafloor spreading theory. According to this theory, oceanic crust is continuously forming at mid-oceanic ridges, moves away from these  ridges by seafloor spreading, and is consumed at subduction zones. If this is the case, then oceanic crust should beyoungest at the ridges and become progressively older with increasing distance away from them. Moreover, the age ofthe oceanic crust should be symmetrically distributed about the ridges. As we have just noted, paleomagnetic data con-firm these statements. Furthermore, fossils from sediments overlying the oceanic crust and radiometric dating of rocksfound on oceanic islands both substantiate this predicted age distribution.Sediments in the open ocean accumulate, on average, at a rate of less than 0.3 cm in 1000 years. If the ocean basins were as old as the continents, we would expect deep-seasediments to be several kilometers thick. However, data from numerous drill holes indicate that deep-sea sediments are at most only a few hundred meters thick and arethin or absent at oceanic ridges. Their near-absence at the ridges should come as no surprise because these are theareas where new crust is continuously produced by volcanism and seafloor spreading. Accordingly, sedimentshave had little time to accumulate at or very close to spreading ridges where the oceanic crust is young, but their thickness increases with distance away from theridges   Wegener began lecturing and writing scientific about continental drift in 1910. His theory was that contininents have not always been in their current positions and that they changed positions through “drifting” to where they are today. Wegener believed that continents had once been joined together in a ‘super continent’, Pangeam and then they split into fragmants to move locations. However despite the wealth of geological evidence that Wegner collected he was unable to explain the mechanisim by which this movement occurred. One of his main lines of argument was the ‘Jigsaw Model’. This concept arose out of comparing the continental shelves of different continents, with the best example being that of the Atlantic coasts of South America and Africa, which appeared to fit together as if they were in a jigsaw. Importantly this theory is more accurate with the use of the continental shelf line rather than the shore line as this section is affected by erosion which has changed the shape of the shoreline away from that of the shelf line. The edge of a continent is defined as being halfway down the continental slope, and when this fit between continents is made it provides conclusive evidence that the continents must have been joined in Pangaea. In the case of South America and Africa the “best-fit” position, which is the average gap or overlap between the two continents, is a mere 90km. The most significant overlapping areas consist of large volumes of sedimentary rocks which were formed due to the constructive plate boundary in which new land was created through magma rising to the surface and pushing these two continents apart.  However, this remarkable fit does not just occur with Africa and South America with most continents fitting together for example Australia and Antartica suggesting that the Pangea model applied to all of the world’s current continents. One would expect that if these continents were originally joined together there would be both geological, through the similarities in rock sequences and mountain ranges, and fossil evidence to endorse this concept. Although rocks are constantly being formed by using radiometric dating we can match rock sequences for example there is a formation match which is paricualrily strong between rocks roughly 550 million years old in north east Braxil and West Africa. The specific similiraties between these rock structures suggests that these two land masses were joined together 550 million years ago. This relationship is also exhibited in the similarities between the Godwana (the southernmost of the two super continents that became parts of Pangaea) continents where Marine, nonmarine and glacial rock sequeneces of Pennsulvanian to Jurassic ages are almost identical on all 5 Gondwana continents strongly indicating tat they were all once joined together. The trends of several mountain ranges also support the concept of ‘continental drift’. These mountain ranges seemingly end at the coastline of one continent only to apparently continue on another continent across the ocean. The folded Appalachian Mountains of North America, for example, trend north eastward through the eastern United States and Canada and terminate abruptly at the Newfoundland coastline. Mountain ranges of the same age and deformational style are found in eastern Greenland, the Caledonides of Ireland, Great Britain, and Norway. Younger parts of the Appalachian Mountains also line up with a belt of similar age in Africa and Europe. Using the continental drift hypthesis and repositioning the continents to the layout of Pangeae it is possible to see continuations of mountain ranges across now independent continents providing evidence to further enhance the validity of plate tectonics. Wegener belieced that if the continents really were joined in the Paleocoic Era then there should be fossil evidence to support this conclusion as these regions would have experienced the same geological and climatic conditions and hence should have had the same organisms inhabiting them. The fossil evidence revealed that the plant and animal communities appear to have evolved together until the splitting of Pangaea after which they evolved separately. The primary example Wegener used was the ancient fern, Glossopteris, the fossils of which have been found in South America, India, Southern Africa and Antartica. Given that there is no realistic way in which the seeds of Glossopteris could have travelled these distances, whilst leaving no evidence on other continents, means that these regions must have been connected in the past. These hypothesis is enforced by the fact that the present day climates of present South America, Africa, India, Australia, and Antarctica range from tropical to polar and are much too diverse to support the type of plants in the Glossopteris flora. As a result, Wegener reasoned that these continents must once have been joined so that these widely separated localities were all in the same latitudinal climatic belt. Another example is the fresh water reptile, Mesosaurus, which was also found in southern Brazil and South Africa, both in Permian-aged rocks, and given that it was less than ½ metre long and was unable to swim in salty water it is inconceivable how it would have travelled across Atlantic Ocean to inhabit these two areas. In both examples the rock structure in which the fossils were found was very similar giving further evidence to support the plate tectonics theory.   The deposits left by glacial ice sheets provide further evidence towards proving the plate tectonics theory. As glacial ice moves, it striates the underlying rocks and produces folds and wrinkles in soft sediments whilst depositing layers of till. These features left behind in glacial deposits provide evidence of the direction the ice was moving during the glaciation. For instance in South America and Africa there are very thick glacial till deposits both from the Permian-Carboniferous age. The deposits match almost exactly when the continents are moved back together. Using Africa and South America as an example, when they are moved back together, the direction of ice movement on both continents is consistent, radiating outward from the center of the former ice sheet. The extent of this evidence implies two lines of evidence both supporting plate tectonics. Firstly due to the distance between Africa and South America it is highly unlikely that a glacier could have existed that spanned the whole Atlantic Ocean and secondly, the practically identical features suggests that both South America and Africa were at the same equatorial locations, in order to have similar climates as such a glacier could not form in present conditions, hence they must have been once joined together in Gondawana as it is consistent with the findings that have been discussed. It’s hard to imagine how such similar glacial features could have been created if the continents had not once been joined together. Africa and South America must have had similar climates during this period, colder than their present-day climates. This also suggests that they were not in their present equatorial locations.