Discuss the degree to which the theory of plate tectonics is supported by seismic and volcanic activity

 

The shift and contact of the rigid plates that make up the earth, causes earthquakes, volcanic activity, the presence of mountains and other geographical occurrences. It can therefore be understood that seismic and volcanic activity will occur at plate margins. This essay will evaluate the extent to which this is true, and thus how far the theory of plate tectonics is supported by such phenomena.

Volcanoes are openings in the earth’s crust through which lava, ash and gases erupt. Lava is taken as proof of the magma that comprises the mantle beneath the earth’s crust.  The majority of volcanoes are found at constructive and destructive plate margins (this can be seen in image 2, where the triangles represent areas that are volcanically active). At divergent margins the most common type of volcano is a fissure volcano, these occur when an elongated crack in the crust allows lava to escape. They are often found around spreading ridges where tension is pulling the crust apart. Most divergent plate boundaries are at the bottom of the oceans, therefore most volcanic activity is submarine, causing seafloor spreading. Examples of this form of volcano are mid-ocean ridges and where the mid-oceanic ridge is above sea-level, volcanic islands are formed, for example, Iceland.

At many destructive plate margins, volcanoes can be attributed to the subduction of an oceanic plate beneath a continental plate (this process can be seen in image 5). The rock that makes up the oceanic crust, is formed in a water rich environment, some of this water is incorporated into the rock and carried down into the mantle as subduction occurs. The heat and pressure of the mantle causes the rocks to release their water which mixes with the mantle forming magma. Magma has a lower density than the mantle causing it to rise to the surface causing a volcano, typical examples of this are Mount Etna and the ring of fire.  A similar process occurs when a oceanic crust meets oceanic crust, however, the volcanoes formed beneath the ocean often form islands, such as those found in the western pacific, these form island arcs, so named because of the trend they follow.

On the other hand, not all volcanoes are found at plate boundaries (as can be seen in map 2 above). For example, Hawaiian islands which are completely volcanic in their origin, were formed in the middle of the Pacific Ocean, at least 3,200 km from the nearest plate boundary. It is believed that these islands were created as the plate moved over a hot spot. Hot spots seem to be stationary and are thought, by some geologists, to be caused by plumes of magma originating deep within the mantle, others believe they occur at much less depth, and are moving slowly. Another hypothesis suggests that the volcanic activity is not caused by the temperature but by a certain level of thinning in the lithosphere due to its extension, that allows the rising of melt from shallow depths.  

As volcanic material erupts through the oceanic crust, over time it may build up to form an island. These islands will however, become part of the plate and gradually move away from the heat source. New volcanoes will form over the hot spot, and a new island is created, this can lead to an island chain, if the plate is moving in one direction over the hot spot such as the Hawaiian chain (an illustration of how such a chain might occur can be found above in figure 6). Islands will form in a cluster if the plate is rotating around a hot spot. The island located closest to the hot spot is most volcanically active.

An earthquake is a sudden release of energy in the earth’s crust that creates seismic waves. The majority of earthquakes are found at plate margins (this can be seen on map 3, where coloured dots represent earthquakes of varying magnitude). Earthquakes can result in tsunamis if they displace enough ocean floor, liquefaction, landslides and avalanches. They can also have a large human effect, depending upon the population density surrounding the epicentre.

Tectonic earthquakes occur anywhere in the earth where there is sufficient stored elastic strain energy to force two resisting plates to move against each other, thus releasing the pressure. The sides of a fault only move past each other smoothly and aseismically if there are no irregularities that could cause friction. It is unusual for fault lines not to have irregularities that lead to what is described as stick-slip behaviour. Once the fault has locked, the plates are continually pressed against each other by convection currents in the mantle, leading to a build up of stored energy around the fault surface. This continues until the stress has risen sufficiently to break through the asperity, suddenly allowing sliding over the locked portion of the fault, releasing the stored energy. This energy is released as a combination of seismic waves, frictional heating of the fault surface, and cracking of the rock, resulting in an earthquake.

There are three main types of fault that may cause an earthquake: normal, thrust and strike-slip (an illustration of this can be seen in image 7). Normal and thrust faulting are examples of dip-slip, where the displacement along the fault is vertical. Normal faults occur mainly in areas where the crust is being extended such as a divergent boundary. Earthquakes associated with normal faults are generally less than magnitude 7, are often submarine and present a minor hazard due to their distance from human habitation. Thrust faults occur in areas where the crust is being shortened such as at a convergent boundary. These are the most powerful earthquakes where the submergence of oceanic crust creates deep to shallow focus earthquakes (shallow 0-70km deep, mid 70-300km deep and deep 300-700km deep), they are associated with the most powerful earthquakes, including almost all of those of magnitude 8 or more. Strike-slip faults are found at conservative plate margins and it is common for a series of faults to mark where the crust had failed. An example of this is the St Andres Fault; it is a broad shatter zone of interrelated faults. Strike-slip faults, particularly continental transforms can produce major earthquakes up to about magnitude 8 (image 8 shows where earthquakes can occur at various plate margins).  Many earthquakes are caused by movement on faults that have components of both dip-slip and strike-slip; this is known as oblique slip.

Like volcanoes, it is also possible for earthquakes to occur away from plate boundaries. Where plate boundaries occur within continental crust, faults are spread out over a much larger area than the plate boundary itself. In the case of the San Andreas Fault, many earthquakes occur away from the plate boundary and are related to strains developed some distance away, caused by major irregularities in the fault. Far from plate boundaries complex strata can result in deep fault zones which can result in earthquakes in apparently stable regions. Earthquakes in China and central Asia occur along lines of weakness, related to the collision of India with the Eurasian plate over 50 million years ago.

Ninety percent of all earthquakes occur along plate margins and by far the greater amount of energy released as seismic waves comes from subduction zones. Furthermore, the depths of earthquakes are consistent with the idea of oceanic recycling by subduction and sea-floor spreading. The 10 percent of earthquakes that do not occur at plate boundaries are sufficiently explained by extended lines of weakness and the theory of their cause is sufficiently supported by necessary evidence. The majority of volcanoes also occur at plate boundaries, due mainly to subduction of an oceanic plate, but those that do not, are adequately explained by the presence of hot spots. In conclusion, though not strictly located at plate margins, the distribution of volcanic and seismic activity across the globe supports the theory of plate tectonics.

Bibliography:

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http://eqseis.geosc.psu.edu/~cammon/HTML/Classes/IntroQuakes/Notes/plate_tect01.html

http://www.infoplease.com/ipa/A0001765.html

http://pubs.usgs.gov/gip/volc/tectonics.html

http://earthquake.usgs.gov/learn/topics/plate_tectonics/rift_man.php