Alfred Wegner’s studies of geological, biological and climatological evidence in the early twentieth century led to the development of the theory of continental drift, which has since resulted in the formulation of plate tectonic theory. This states that the Earth’s crust is not a continuous skin but a series of rigid spherical caps upto 70km thick.; the plates move via slab pull dues and driven by convention currents in the semi solid upper mantle. This theory to some extent helps to explain how and why both volcanic and seismic events occurred – however there are seismic events that occur outside the limits of this theory (for example as a result of human activity) and come volcanic events – the apparent movements of hotspots for instance, cannot currently be explained by plate tectonics theory. An understanding of the plate boundaries as part of plate tectonics theory can, for example. Explain the location and nature of seismic events. At destructive plate margins, for example, extremely violent earthquakes occur as the less dense plate buckles as the More dense plate is sub ducted, this pressure may be released in a violent seismic event such as the pressure release in the north Pacific which led to the Japanese tsunami in 2011. This seismic even can therefore be understood through an application of plate tectonics theory; the movement of plates due to continental drift explains the earthquake. A similar application of plate tectonics theory can be used to understand seismic events at conservative plate margins. As two plates slide past each other, friction builds until tension is released in an earthquake as the two plates slide past. For example, earthquakes along the San Andreas Fault (such as the Loma Prieta Earthquake in 1989) occur as a result of this process. An understanding of plate tectonics theory can therefore be used to help us not only understand how, why and where seismic events take place, but can also be used to (to some extent) predict the nature of the earthquake. Understanding the causation of earthquakes and their likely severity, earthquakes at destructive margins typically have a deeper focus, for example, due to the nature of subduction. However many seismic events are triggered by factors outside of the plate tectonics theory. Fracking, mining and other industrial works, for example represent potential human causes f earthquakes which have nothing to do with the theory of plate tectonics. The Texas Railroad Commission for example, has linked oil and gas extraction to seismic activity, in a phenomenon that has come to be know as ‘man made quakes’. In this respect, plate tectonics theory does not help our understanding of seismic events, as further information is required in order to explain the earthquake. Although plate tectonics theory can help our understanding of seismic events, it should also be noted that seismic events, in turn, can help our understanding of plate tectonics theory. Observations of the depths of earthquake foci related that they got deeper as the distance from the plate margin increases along a subducting plate: Title?:

This seismic evidence not only allowed geologists to prove that subduction was occurring – since a deeper earthquake indicated that crust is being drawn down into the earth – but also led to the discovery of the Benioff Zone. In this respect, therefore, it is more accurate to argue that our understanding of seismic events can help with plate tectonics theory, rather than the other way around. An understanding of plate boundaries as part of the theory of plate tectonics can, however, explain volcanic activity to a very great extent – the nature of eruptions can, for example, be explained trough plate tectonics theory. At destructive plate boundaries, such as the boundary between the Eurasian Plate and the Philippine plates, the oceanic plate is subducted and melted, before resurfacing as the density if the molten magma becomes lower than that of the rock – the high silica content of the eruptions are violent and potentially highly destructive. An example of this in the 1991 eruption of Mt Pinatubo – due to the Philippine plate being subducted under the Eurasian Plate, the eruption was the second largest of the 20th Century measuring as a VEI 6. The theory of plate tectonics can therefore help us not only to understand the reasons for the nature of a volcanic eruption, but also helps us to understand the reasons for the nature of a volcanic eruption, but also helps us to understand how to prepare fr and respond to such a seismic event; in the 1991 Pinatobu eruption scientists from the USGS were able to monitor the volcano for months (understanding from plate tectonics theory that these are indications of prominent volcanic activity), leading to a more efficient and effective evacuation process that saved thousands of lives. Similarly, plate tectonics theory helps to explain eruptions at constructive plate margins, such as North American Plate moving away from the Eurasian Plate at the Mid Atlantic ridge. The magma rises up to fill the gap has a low silica content, making it basaltic, gas can easily escape from the lava, explaining why eruptions at constructive plate margins are typically gentle, continuous and significantly less destructive. Hotspots present something of a geological quandary in relation to how plate tectonics theory helps to explain volcanic activity. On the one hand, theory are formed by magma plumes as a result of superheating radioactive material in the mantle – the same process that generates convection currents to drive the movement of tectonic plates. Plate tectonics theory does not presently explain how the magma plumes are formed, however, highlighting that volcanic activity is not consistently explained by plate tectonics theory. On the other hand, as with the discovery of the Benioff zone, hotspots illustrate that it may be more accurate to argue that our understanding of volcanic and seismic events can help to explain the theory of plate tectonics, rather than the other way around; for example the Hawaiian seamount chain of nearly 80 volcanoes indicates that the Pacific plate is moving – as it moves over the hotspot, new islands are created through volcanic activity. However, the sudden change in direction in the Hawaiian-Empeorer seamount chain (45mya) is not consistantwith what Is known about the movement of the Pacific plate, indicating that the hotspot may have moved and tis therefore not stationary as first believed. This phenomenon is not explained but the theory of plate tectonics, highlighting that the theory requires further development before it can fully explain volcanic and seismic activity. Overall, therefore, it is correct to say that plate tectonics theory helps our understanding of seismic and volcanic events; it explains the location and nature of both occurrences. However, it should be noted that in some instances it can be more accurate to argue instead that seismic and volcanic events help our understanding of plate tectonics theory (as with the Benioff zone), and that in other cases more information is needed to complete the theory’s explanations – as with hotspots for example.

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Evaluate how plate Tectonic Theory Helps Our Understanding of Seismic and Volcanic Events [40 marks] Alfred Wegner’s studies of geological, biological and climatological evidence in the early twentieth century led to the development of the theory of continental drift, which has since resulted in the formulation of plate tectonic theory. This states that the Earth’s crust is not a continuous skin but a series of rigid spherical caps upto 70km thick.; the plates move via slab pull dues and driven by convention currents in the semi solid upper mantle. This theory to some extent helps to explain how and why both volcanic and seismic events occurred – however there are seismic events that occur outside the limits of this theory (for example as a result of human activity) and come volcanic events – the apparent movements of hotspots for instance, cannot currently be explained by plate tectonics theory. An understanding of the plate boundaries as part of plate tectonics theory can, for example. Explain the location and nature of seismic events. At destructive plate margins, for example, extremely violent earthquakes occur as the less dense plate buckles as the More dense plate is sub ducted, this pressure may be released in a violent seismic event such as the pressure release in the north Pacific which led to the Japanese tsunami in 2011. This seismic even can therefore be understood through an application of plate tectonics theory; the movement of plates due to continental drift explains the earthquake. A similar application of plate tectonics theory can be used to understand seismic events at conservative plate margins. As two plates slide past each other, friction builds until tension is released in an earthquake as the two plates slide past. For example, earthquakes along the San Andreas Fault (such as the Loma Prieta Earthquake in 1989) occur as a result of this process. An understanding of plate tectonics theory can therefore be used to help us not only understand how, why and where seismic events take place, but can also be used to (to some extent) predict the nature of the earthquake. Understanding the causation of earthquakes and their likely severity, earthquakes at destructive margins typically have a deeper focus, for example, due to the nature of subduction. However many seismic events are triggered by factors outside of the plate tectonics theory. Fracking, mining and other industrial works, for example represent potential human causes f earthquakes which have nothing to do with the theory of plate tectonics. The Texas Railroad Commission for example, has linked oil and gas extraction to seismic activity, in a phenomenon that has come to be know as ‘man made quakes’. In this respect, plate tectonics theory does not help our understanding of seismic events, as further information is required in order to explain the earthquake. Although plate tectonics theory can help our understanding of seismic events, it should also be noted that seismic events, in turn, can help our understanding of plate tectonics theory. Observations of the depths of earthquake foci related that they got deeper as the distance from the plate margin increases along a subducting plate:

This seismic evidence not only allowed geologists to prove that subduction was occurring – since a deeper earthquake indicated that crust is being drawn down into the earth – but also led to the discovery of the Benioff Zone. In this respect, therefore, it is more accurate to argue that our understanding of seismic events can help with plate tectonics theory, rather than the other way around. An understanding of plate boundaries as part of the theory of plate tectonics can, however, explain volcanic activity to a very great extent – the nature of eruptions can, for example, be explained trough plate tectonics theory. At destructive plate boundaries, such as the boundary between the Eurasian Plate and the Philippine plates, the oceanic plate is subducted and melted, before resurfacing as the density if the molten magma becomes lower than that of the rock – the high silica content of the eruptions are violent and potentially highly destructive. An example of this in the 1991 eruption of Mt Pinatubo – due to the Philippine plate being subducted under the Eurasian Plate, the eruption was the second largest of the 20th Century measuring as a VEI 6. The theory of plate tectonics can therefore help us not only to understand the reasons for the nature of a volcanic eruption, but also helps us to understand the reasons for the nature of a volcanic eruption, but also helps us to understand how to prepare fr and respond to such a seismic event; in the 1991 Pinatobu eruption scientists from the USGS were able to monitor the volcano for months (understanding from plate tectonics theory that these are indications of prominent volcanic activity), leading to a more efficient and effective evacuation process that saved thousands of lives. Similarly, plate tectonics theory helps to explain eruptions at constructive plate margins, such as North American Plate moving away from the Eurasian Plate at the Mid Atlantic ridge. The magma rises up to fill the gap has a low silica content, making it basaltic, gas can easily escape from the lava, explaining why eruptions at constructive plate margins are typically gentle, continuous and significantly less destructive. Hotspots present something of a geological quandary in relation to how plate tectonics theory helps to explain volcanic activity. On the one hand, theory are formed by magma plumes as a result of superheating radioactive material in the mantle – the same process that generates convection currents to drive the movement of tectonic plates. Plate tectonics theory does not presently explain how the magma plumes are formed, however, highlighting that volcanic activity is not consistently explained by plate tectonics theory. On the other hand, as with the discovery of the Benioff zone, hotspots illustrate that it may be more accurate to argue that our understanding of volcanic and seismic events can help to explain the theory of plate tectonics, rather than the other way around; for example the Hawaiian seamount chain of nearly 80 volcanoes indicates that the Pacific plate is moving – as it moves over the hotspot, new islands are created through volcanic activity. However, the sudden change in direction in the Hawaiian-Empeorer seamount chain (45mya) is not consistantwith what Is known about the movement of the Pacific plate, indicating that the hotspot may have moved and tis therefore not stationary as first believed. This phenomenon is not explained but the theory of plate tectonics, highlighting that the theory requires further development before it can fully explain volcanic and seismic activity. Overall, therefore, it is correct to say that plate tectonics theory helps our understanding of seismic and volcanic events; it explains the location and nature of both occurrences. However, it should be noted that in some instances it can be more accurate to argue instead that seismic and volcanic events help our understanding of plate tectonics theory (as with the Benioff zone), and that in other cases more information is needed to complete the theory’s explanations – as with hotspots for example.