To what extent can the theory of plate tectonics explain global distribution of seismic and volcanic activity?

During the 1950s continental drift became an interest as a result of new found evidence from paleomagnetic studies, which was relatively new at the time. The studies look at the magnetism in ancient rocks and record the intensity and direction of the Earth’s magnetic field at the approximate time of the rocks formation. These magnetic surveys of the oceanic crust found stripes of anomalies, where the strength of the magnetic field differed, in the rocks that are parallel and symmetric around ridges in the crust. Moreover, this pattern of anomalies is the same as that of magnetic reversals in continental lava flow studies, which also supports the idea that new oceanic crust forms along ridges and the seafloor laterally moves away from them.

Furthermore, the Earth has two types of poles; magnetic and geographic, which are not located in the same place. The geographical poles are fixed by the axis of the Earth’s rotation and are located at the earth’s most northern and southern points. However, a compass point’s to the magnetic north pole, where the Earth’s magnetic pull is the strongest. This arrangement means that the magnetic fields strength varies because it is strongest at the poles and weakest further away at the equator. The Earth’s magnetic field is thought to be due to the different rotation speeds of the mantle and outer core.

Earthquakes often occur in broad uneven belts along plate boundaries because most are related to plate motion. However, the earthquakes that do not occur along plate boundaries are associated with the reactivation of old fault lines, such as the September 2002 earthquake in the Midlands which measured 4.8 on the Richter scale. It is believed that it was caused by the movement along an old fault line called the Malvern lineament.  It has also been suggested that human activity could be the cause of some minor earthquakes due to the impacts of building projects; including the construction of large reservoirs in which the water puts pressure on the surface rock or in deep mines where they gradually start to cave in or sink.

Volcanoes form when magma rises through cracks or weaknesses in the Earth’s crust causing pressure to increase inside the Earth. Once the pressure is released such as by plate movement magma explodes to the surface, this is called a volcanic eruption. The lava then cools to form a new crust. Most volcanic activity is associated with plate tectonic processes and is mainly along plate margins. Therefore, volcanic activity occurs along oceanic ridges where the plates are moving apart, (an example of this is the mid- Atlantic ridge where Iceland was formed by volcanic activity) on or near subduction zones. This tends to be the most violent activity such as the line of volcanoes (the ‘ring of fire’) that surrounds the Pacific Ocean. Volcanic activity is also associated with rift valleys as the African rift valley has numerous volcanoes along it including Mount Kilimanjaro, as well as hot spots such as the one in the middle of the Pacific Ocean which gave rise to the Hawaiian Islands. However, the Hawaiian Islands do not fit in with the majority distribution pattern of volcanoes because they were formed in the Pacific Ocean, over 3200km from the nearest plate boundary. Some geologists believe that the seemingly stationary hotspots are due to plumes of magma that originates deep within the mantle while others propose they are moving very slowly and are created from a far less depth. Therefore, the theory of plate tectonics explains the distribution of most of the Earth volcanic activity because more activity occurs along plate margins.

The Earth’s crust is made up of over 50 plates, all of them move at different rates and directions. Most of them are in motion moving on average 5 to 10cm per year but some move faster up to 18cm per year. Furthermore, a reason behind the theory of plate tectonics is the idea that the temperature increases the closer you are towards the Earth’s core and the heat released within the mantle by radioactive decay of the elements helps to create convection currents. These hotter areas of mainly liquid magma produce a circulatory motion which causes the plates to move. These currents create high temperatures that cause tensional forces which pull the crust apart at ridges and rift zones. In areas of Continental crust rift valley systems can be made and if rifting continues new areas of land are created in the ocean floor. In Oceans this rifting creates mid-oceanic ridges and the spaces left by the plates being pulled apart are filled by basaltic lavas and dykes. Also at subduction zones the convection currents can pull slabs of Oceanic crust down into the mantle.

The theory of plate tectonics sufficiently describes seismic and volcanic activity that occurs along plate boundaries, practically localized movement and it is supported by several pieces of both geological and biological evidence, such as fossil remains of the Mesosauras reptile have been found in South America and Southern Africa. This is because it is very unlikely that this reptile could have migrated across the Atlantic or developed in both places. Furthermore, it provides an explanation for the distribution of majority of earthquakes and volcanoes so can explain the global distribution to a high extent. However, it does not take into account every single one and can be argued that the theory’s extent is weak because it does not provide an adequate explanation of the start of continental movements or the initial plate boundaries. It also assumes that the Earth is billions of years old and the time frame for the continents movements is dependent on various periods in of the earth’s history, which can be classified differently. On the other hand the theory of plate tectonics provides a basis for analyse of present events along plate boundaries and the prediction of future events including their size and scale. Overall, the theory of plate tectonics explains the global distribution of seismic and volcanic activity to a high extent because the majority of events occur along the plate boundaries.