Evaluate how plate tectonics theory helps our understanding of the distribution of seismic and volcanic events (40 Marks)

Plate tectonics is the understanding of how the planets differing oceanic and continental crust fit together and indeed how our earth works. Plate tectonics are an essential condition in order to have life - without it the world would be a flat marshland with little or no chance for complex life. After Alfred Weigner proved his thesis, in the 1900s, our understanding of plate tectonics has come a long way and indeed this has helped us to understand the disruption of seismic and volcanic events. 

In modern geological and earth sciences there is a common understanding that the earth is broken up into different continental and oceanic crusts - in simpler terms one big jig saw puzzle. This plates are constantly moving due to the convection currents produced by the realize of heat from the earths core, this rises up into the lithosphere and due to heat rising and then cooling moves the plates around the earth. This movement causes the earths plates to move, however where these plates meet can differ massively in its form. Firstly, we have Divergent/constructive plate margins this is where the two plates are moving away from one another and new plate is produced, a key example of this is the Mid Atlantic Ridge. We also have destructive/convergent plate margins this is where two plates come together, and itself can change depending on what type of crust is meeting. For instance if two continental crust meet one can get fold mountains where the collision takes place, furthermore when one continental and one oceanic plate meet there is a subduction zone, this is where the oceanic plate due to its relative denser composition sinks beneath the continental crust. The final type of plate margin is a conservative plate margin, this is where two plates slide past one another E.G. San Andreas Fault, California. Of course if new plate is made at a constructive plate margin it must indeed be destroyed somewhere else, this happens at destructive or convergent plate margins. This understanding of how the world works helps us massively in understanding the disruption of seismic events. This constant change has caused massive global evolution over the milliner. The earth was once one super continent, Pangea, and this can become evident from the way South America and Africa fit together almost perfectly. 

Firstly, at Convergent plate margins this is where two plates meet and as I previously said one plate is normally sub ducted under the other, when it comes to oceanic vs. continental the oceanic is always sub ducted. We know understand that when a plate is sub-ducted under the other the crust is heated by the earths magma in turn producing its own. This magma then rises and forms a Volcano. This tends to be a composite or acidic dome volcano, this is because the magma produced at the convergent plate margin tends to be more acidic and indeed more explosive in nature. It is more viscous in nature therefore does not flow as easily. This causes more violent eruptions normally resulting in pyroplastic flows which are massively destructive.  The reason for its different composition to the more basic lava produced at a conservative plate margin, is that older sea floor has been melted which tends to be more acidic in nature. An example of this is Monseratt which is part of the British empire in the commonwealth. I am now able to use my knowledge of plate tectonics to understand the disruption of volcanic activity. The Benioff zone which is present along the margin where the two plates are being sub-ducted is an area where seismic activity is common. When one plate is being drawn under the other it often causes friction and gets stuck, when this pressure is realized a seismic event is often felt. For example The Tsunami of 2004 where the Indo Australian plate is being sub-ducted under the Eurasian plate. From this previous knowledge along side monitoring of subduction zones, including watching ground deformation and GPS imaging for instance the constant monitoring that happens on Monseratt after the 1994 disaster, we are able to see when and where a volcanic event and in some respect seismic events are going to take place. Although, predicting where the epicenter and when the earthquake is going to happen has proved very difficult and still needs work. However, from our basic knowledge we do know seismic events are likely to occur at subduction zones. 

Furthermore, our knowledge of the other two types of plate margins has also enabled us to understand the disruption of seismic and volcanic events. We now know that at a conservative plate margin two plates are slipping past each other in opposite directions, it produces or destroys no crust so there is no volcanic activity. However, we are able to understand that at these margins some of the worst seismic events in history have taken place E.G. San Francisco 1904 the city was brought too its knees as the North American and Eurasian plate slipped causing a earthquake measuring 7.8 on the Richter scale. These two plates that are moving past each other often get stuck, the pressure builds up over time eventually realizing itself as an earthquake. This can then often lead to further unzipping of the margin as the plates try and get back in line according to Reeds elastic rebound theory. So from our knowledge of plate tectonics and conservative plate margins we are able to understand the distribution of seismic activity along their margin. 

The final type of plate margin is the constructive/divergent plate margin, this is where the two plates are moving away from each other, magma rises through the gap left and forms new crust. This is happening constantly at the Mid Atlantic Ridge, the sea floor is spreading at a rate of about 4mm a year and can be proved by poloermagnetism which is where we can sea where the earths magnetic north has changed over time. Conservative plate margins have huge amounts of seismic and volcanic activity. Due to the MAR not being straight, it has to bend in order to do so seismic activity must take place, this is because different parts of the margin will be moving at different rates often leading to these scars scrapping one another and creating seismic activity, although this tends to of a much lower magnitude then the other margins. There also tends to be large amount of volcanic activity at these locations, the magma is far more basic and less acidic, this means it is far less viscous and is able to flow often resulting in low flat volcanoes called Shield volcanoes. This again shows how our knowledge of plate tectonics has allowed us to understand where these volcanic and seismic activities take place and a key example of this would be Iceland which is covered in low flat shield volcanoes and Fissures which are splits in the landscape produced by the MAR. 

However, there is indeed an argument that even though we have a greater knowledge of plate tectonics we cannot know everything. Hot spots are areas in the lithosphere where a superheated plum of molten magma rises closer to the surface. This causes massive amounts of volcanic activity and indeed small earthquakes as the magma rises through the ground. These hot spots are often very hard to explain, and even though we have a greater knowledge of plate tectonics then we did 100 years ago hotspots still remain a anomaly.  These hotspots often create island arcs, this is as the plates are being moved by the convection currents but the hotspot still remains in the same location a key example of this is Mauna Loa Hawaii. Hotspots can also be responsible for Caldera formation, Caldera are often sunken magma chambers that have some of the most explosive power in the volcanic arsenal. However, we are still unsure on how and why these hotspots are formed, indeed we know how they create such features as discussed however we do not know their origin. 

Furthermore, our understanding of plate tectonics has not come along to fully explain the human impact on seismic events. For instance the building of a Dam in Scheuan China was said to have caused the tragic earthquake of May 2008 which measured 7.8. It killed over 70,000 people and could have been due to the Dam placing stress on the fault line, however our theory on plate tectonics did not go far enough to stop this happening. After all, if plate tectonics had come far enough the Dam may have never been made or indeed people would have been relocated. Mining also has played a role in shaping seismic activity, and yet again the theory of plate tectonics does not explain why this is the case for example the Australian town of Newcastle was flattened in seconds after 100,000s of tones of coal had been removed over the year...However we still ask ourselves why? Further along the line of seismic activity, our understanding of how to predict future events is rather lacking. We understand our the plates are moving around the world however are are still unable to predict when it is going to happen. Seismic events seem to appear random with no harmony. For instance Little Park Field is a town in California which underwent massive seismic activity in the late 80s, Scientist and geologist alike went to park-field to try and get an understanding of prediction, however the earthquake came 20 years late and any research that has been made was proved invalid as the earthquake prediction had not worked. We are still using primitive ideas on how to predict an earthquake, for instance using snakes in the 21st century! Earthquakes appear to be random in their distribution, we know we they happen just not when! 

To conclude, plate tectonics theory does indeed help us to understand where seismic and volcanic events are going to take place. However, there are some factors that it does not explain, Hotspots and Seismic randomness these factors appear to be anomalies to the trend. This suggest to me that our knowledge of plate tectonics only needs further development to truly understand all of earths mysteries. We have come along way since the condemning of the great and correct! Geologist Alfred Weigner and perhaps we only have further to go...