Plate Tectonics Theory

‘Evaluate how plate tectonics theory helps our understanding of the distribution of seismic and volcanic events’ In 1912, Alfred Wegener published his theory that a single super continent named Pangaea once existed about 300 million years ago. He proposed that Pangaea then later split into two continents of Laurasia in the north and Gondwanaland in the south and that today’s continents were the result of further splitting of these two land masses. Where the plates split are known as plate boundaries. Wegener’s theory of continental drift was supported by both geological and biological evidence that these areas were once joined.

The geological evidence included the rock sequences in Northern Scotland closely agreeing with those found in East Canada, indicating that they were laid down under the same conditions in one location as well as the obvious jig saw fitting appearance of today’s continents, in particular, the bulge of south America fitting into the indent below west Africa.

The biological evidence comprised of fossil findings linking different continents.

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Fossil brachiopods found in Indian limestones were comparable with similar fossils in Australia and the fossil remains of Mesosaurus’ were found in both South America and southern Africa. It is unlikely that the same reptile could have developed in both areas or that it could have migrated across the Atlantic.

Despite the evidence, Wegener’s theory was unable to explain how continental movement had occurred. However from the 1940’s additional evidence accumulated after the discovery of the mid-Atlantic ridge and huge oceanic trenches.

Examination of the ocean crust either side of the mid-Atlantic ridge suggested that sea-floor spreading was occurring. Magnetic surveys of the ocean floor in the 1950’s, showed regular patterns of paleomagnetic striping surrounding the ridges. It was discovered that when lava erupts on the ocean floor, magnetic domains within iron rich minerals in the lava are aligned with the magnetic field of the earth. This fixes as the lava cools and records the earths polarity at the time of their cooling.

As the polarity of the earth reverses every 400,000 years, bands of normal and reversed polarity rocks are mirrored on either side of the mid ocean ridges, suggesting that new rocks are being added equally on either side. Surveys also established the age of the rocks and found young ages for places on or near the ridges and much older ages for rock nearer to the continental masses, demonstrating that older crust is continually being pushed aside by new crust. The discovery of sea floor spreading led to the assumption that the earth must be getting bigger however this was not the case and the discovery of oceanic trenches allowed for the conclusion that plates must be being destroyed at different boundaries to accommodate the increase in their size at mid-oceanic ridges. Hot spots around the core of the earth generate thermal convection currents within the mantle which cause magma to rise towards the crust and then spread before cooling and sinking.

This circulation of magma is the driving force of plate movement. This movement has an effect on all the plates determining their type of boundary each with their own features and resulting volcanic and seismic events. Divergent boundaries occur along spreading centers where plates are moving apart and new crust is created by magma pushing up from the mantle resulting in oceanic ridges and rift valleys. Where two oceanic plates are moving apart they produce mid oceanic ridges with their form influenced by the rate at which the plates separate. Volcanic activity occurs along the ridge forming submarine volcanoes which sometimes rise above sea level accommodating fairly gentle sides and frequently gentle 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. Where two continental plates are spreading they produce rift valleys.

The brittle crust fractures at sections as it moves apart causing a normal fault where hanging wall falls down relative to the foot wall due to tensional stress. A feature of a rift valley is known as a ‘graben’ which forms when a block of rock falls between two faults and creates the valley floor and also a ‘horst’ which is formed when a block of rock is pushed up between two faults. This area is associated with volcanic activity as the crust is much thinner than in neighbouring areas. Convergent plate boundaries occur when two plates are moving towards each other. Where oceanic and continental crusts meet, the denser oceanic crust is forced under the lighter continental plate known as subduction.

The down warping of the oceanic plate forms a very deep ocean trench and the continental plate edge is affected by the reverse fault lines that cause folding of the plate to produce uplifted rock that forms Fold Mountains. As the oceanic crust descends, the increase in pressure can trigger major earthquakes along the line of the sub ducting plate. As it descends further the surroundings become hotter and additional heat from the friction causes the rock to melt in the benioff zone which begins to rise as plutons of magma. When they reach the surface they form composite explosive volcanoes.

Eruptions can also occur offshore producing volcanic islands referred to as island arcs. Where two types of the same plate meet they create collision zones by which the compression of the two plates results in the folding of the plate to form Fold Mountains. As there is little subduction, there is no volcanic activity however the movement of plates can trigger shallow-focus earthquakes. Conservative plate boundaries occur when two crustal plates slide past each other and the movement of the plates is parallel to the plate boundary. The movement of the plates creates stresses between the plate edges and as they rub past each other the release of friction triggers shallow focus earthquakes. However as there is no subduction, there is no volcanic activity. The best known example of a conservative boundary is the San Andreas Fault in California, where the pacific and North American plates move parallel to each other.

Volcanic activity that does not occur along any plate boundary can be the result of many a fault lines and hot spots beneath the crust. Alfred Wegener’s theory allowed us to gain insight into the potential creation of our tectonic plates and their boundaries. The evidence provided by wegener’s theory and the record of paleo-magnetism upon the ocean floor supported the idea that the tectonic plates are moving. Supported by the theory of convection currents, the movement of these plates helps our understanding of the distribution of seismic and volcanic events by allowing us to identify varying plate boundaries that create different features and as a result cause these events. This explains their distribution, as events such as these are located in areas above plate boundaries, apart from the odd one which can occur above many a fault lines or hot spots caused by the movement of plates.