Plate Tectonics Theory

“EVALUATE HOW PLATE TECTONICS THEORY HELPS OUR UNDERSTANDING OF THE DISTRIBUTON OF SEISMIC AND VOLCANIC EVENTS”

In this essay I am going to look in detail at plate tectonic theory and how it can be used to explain seismic and volcanic events worldwide. However, I will also look at other potential causes of these events, and some evidence that does not follow the theory that all these events are caused by plate boundaries. I am going to look at a range of case studies from differing scales from all areas of the world in order to back up my points. These case studies will include Japan 2011, Koynanagar 1967, Lincoln 2008, Surtsey and the ‘Ring of Fire’. I hope after looking at these case studies in some greater detail I will be able to conclude to what extent plate tectonics theory explains seismic and volcanic events. Plate tectonics theory was something first introduced by German researcher Alfred Wegener in 1915, when he published his theory that the world once began as one continent, ‘Pangaea’, that split into two continents, and continued to until it reached the way it is today. This theory was based upon his discovery that the earth’s continents appear to fit together like a jigsaw through fossil patterns, geology and the rough shape of continents. For example, fossil brachiopods found in Indian limestone were comparable with similar fossils in Australia and the fossil remains of Mesosaurus’ were found in both South America and southern Africa. The geological evidence includes rock sequences in Northern Scotland being very similar to those found in East Canada, indicating that they were put down under the same conditions in one location as well as the jig saw fitting appearance of today’s continents, in particular, the bulge of south America fitting into the indent below west Africa. All of this evidence gave Wegener enough belief for his theory of plate tectonics and continental drift, however because he could not prove why and how it happened he found it hard to make others believe him. However, the discovery of the Mid Atlantic Ridge in the Atlantic Ocean, and huge trenches supported his theory as it supported that the sea floor was spreading, and being replaced by new material, shown through Magnetic surveys of the ocean floor in the 1950’s, because of regular patterns of paleomagnetic striping, where lava rises and cools it is fixed with the Earth’s polarity at the time. People believed the earth was increasing in size, however as this was not true it was discovered that the earth must be being destroyed in ways dependent upon the type of boundary which results in volcanic and seismic events. Where two oceanic plates are moving apart they produce mid oceanic ridges where their form is influenced by the rate at which the plates separate. Volcanic activity occurs along the ridge forming submarine volcanoes, which sometimes rise above sea level with gentle sides and frequent, moderate eruptions. An example of this is located in Surtsey, to the south of Iceland, and Iceland itself. As new crust forms and spreads, transform faults occur at right angles to the plate boundary due to shearing pressure. The parts of the spreading plates on either side of these fault lines may move at different rates causing shallow focus earthquakes. This supports the theory of plate tectonics for both seismic and volcanic events. Convergent plate boundaries are when two plates move towards each other, often causing subduction when denser oceanic crust is forced under lighter continental crust. This often results in volcanic activity, as oceanic crust, which is pushed down, melts down to mantle and through convection, rises and is forced out through faults in the continental crust causing volcanic activity. The largest example of this is the ‘ring of fire’ which is a collection of volcanoes around the north boundary of the Pacific Plate. It is home to 452 volcanoes, 75% of the worlds active volcanoes, and 90% of the worlds earthquakes occur here. As the plates crash together several shallow focus earthquakes cause devastation. One of the most recent examples of this was the 2011 Japanese earthquake and tsunami, caused entirely through plate tectonics on the Ring of Fire where subduction of the Pacific Plate and the Philippine Sea Plate occurs. This earthquake reached a level of 9.0 of magnitude, and was recorded as the costliest natural disaster in the world’s history, costing the world bank US$235 billion. Over 15,000 people were killed and 6000 injured showing the severity of the disaster. Some areas of Japan moved 2.4m closer to North America because of the earthquake and was believed to have shifted the earths axis by 10-25cm. Two days later, the Shinmoedake volcano in Kyushu erupted, which is believed to be linked to the movement in the lithosphere of the surrounding area causing cracks and faults. This shows how plate tectonics causes huge destruction, and is strong evidence for the argument that it is the main cause of seismic events. However, plate tectonics are not the only reason seismic and volcanic events occur. Earthquakes and eruptions caused by factors other than tectonics are known as induced earthquakes and eruptions. These can be caused by factors such as human activity, including mining, reservoir building and nuclear testing, intraplate earthquakes caused by ruptures in the interior of a plate or hotspots in the earth’s crust. In Lincoln in 2008, an earthquake caused by intraplate ruptures occurred, reaching 5.2 on the Richter scale. This earthquake was caused because of a rupture along a strike-slip fault. Because the earthquake occurred so far away from a plate boundary, it could not be caused by plate tectonic theory or plate boundaries, showing that earthquakes are not just limited to areas where these boundaries occur. This is the same for all North European seismic activity. Another cause of earthquakes here is glacial rebound, for example in Scotland, where the land is rising after the weight of ice pushed it down during the last ice age, causing very minor earthquakes, less than 3.5 on the Richter scale. Human activity can also be responsible for the production of earthquakes. In 1967 in Koynanagar in India, an earthquake with magnitude 6.5 hit very near to the site of the construction of the Konya dam. It claimed 180 lives and injured 1500 people. Effects from the earthquake were felt 230km away in Bombay with tremors and power outages. It is believed that this earthquake happened because of pressure put on the geology of the surrounding area through the dam’s construction and the production of waves from heavy industrial machinery. Volcanic activity can also occur that is not linked to plate tectonic theory. A volcano hotspot is a region on the Earth’s surface that has experienced volcanism for a long time. A good example of this is the Hawaiian Islands. Each of the islands in the long chain were created by the same volcano hot spot. The volcanoes build up like islands that rise above the surface of the ocean, and they are then carried away, creating extinct volcanoes and new volcanoes are created by the same hot spot at the same time. Others include the Azores hotspot, the Canary hotspot and the East Australia. About 30 km below the surface of the Earth is the mantle, molten rock that can seep out of the crust and form magma chambers, which find their way to the surface, creating volcanoes. The location of these hotspots is not dependent upon their location at all, so does not reply upon plate tectonics theory. Overall, I feel that plate tectonic theory can help to explain a lot of seismic and volcanic activity on the earth, however it cannot describe all of it. I feel it explains that large, highly destructive earthquakes and eruptions are caused by plate boundaries due to the enormous amounts of movement and pressure caused by these there, and human interaction or induced activity could not cause events of the same magnitude. However plate boundaries do not determine whether seismic and volcanic activity can occur, just to what extent it does. Areas such as the Ring of Fire prove this more than anywhere, due to the vast amount of volcanic and seismic activity that occurs here, and the highly destructive nature of this activity when compared to a small earthquake like the one in Lincoln, or in Koynanagar in India.