Does Plate Tectonic Theory Help our Understanding of the Distribution of Seismic and Volcanic Events?

Contributed by Joe Timmins

Plate tectonic theory is a scientific theory that describes the large scale motion of the Earths lithosphere. The theory of plate tectonics was initially developed by Wegener in 1912.  His report put forward the idea of continental drift whereby he proposed the continents were once connected in one supercontinent, Pangea. This was suggested after he noticed the continents have a jig-saw fit, evidenced by South America and Africa. Plates have moved apart since Pangea, and this continental drift is what leads to seismic and volcanic activity. The lithosphere is made up of 8 main plates, and numerous small ones, which float on the earth’s asthenosphere, a highly viscous region at the top of the earth’s mantle. As these plates move, they interact with each other leading to seismic and volcanic events. Thus, a developed understanding of plate margins and their interactions can help us to understand the distribution of such events. This is particularly true as the majority of seismic and volcanic events occur at plate margins.

Evidence supporting Wegener’s theory includes fossilised remains of the mesosaurus being discovered on the coasts of Brazil and Gabon (West Africa) and also the same fossilised pollen species and rock sediments on these coast lines. Wegener’s theory was initially criticised as Wegener could not explain how the supercontinent he proposed split into different ‘jig-saw’ pieces. However, plate tectonics theory was built upon, furthering our understanding of tectonic events. Sea floor spreading was discovered, the formation of fresh areas of oceanic crust which occurs through the upwelling of magma at mid-ocean ridges and its subsequent outward movement on either side. Sea floor spreading provides evidence for the existence of plates and plate boundaries since new rock was being created and destroyed. An example of sea floor spreading was shown in the Atlantic. Here, as the Eurasian and North American plates are moving apart, magma rises through a rift and cools on the surface creating new plate material and the mid-Atlantic  ridge, a ridge of volcanoes. This is a constructive plate boundary, a linear feature that exists between two tectonic plates that are moving away from each other. Recent great advancements in technology means we can also use advanced methods to develop our understanding of tectonics. Carbon dating means we can assess the age of oceanic crust, which increases as you get further away from the mid-Atlantic Ridge and evidence from paleomagnetism equally proves sea floor spreading. Palaeomagnetism occurs as metallic rich rocks align in the crust towards the poles before they harden, after hundreds of thousands of years these poles flip and new bands of rock align in the opposite direction. Therefore, each band of the opposing aligned elements in the crust represent several hundred thousand years of crust that was created in that time. Sea floor spreading and the creation of new oceanic crust means that a plate must be being destroyed somewhere else, which brings me onto subduction zones and the consequent distribution of seismic and volcanic events.

Subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate and sinks into the mantle as the plates converge. Subduction zones involve the oceanic lithosphere of one plate sliding beneath the continental lithosphere or oceanic lithosphere of another plate due to the higher density of the oceanic lithosphere. Deep sea exploration has proven areas such as the Pacific Ring of Fire is at a destructive margin.   The Pacific Ring of Fire has a high concentration of earthquakes and volcanoes due to deep ocean trenches (e.g Marianas Trench) running close by parallel to these boundaries that show evidence of plates subducted beneath them (destroyed). Here a denser oceanic plate would subduct a continental plate- the plate would melt inside the mantle creating a pool of magma which would rise through the cracks in the rock forming a volcano. This development in plate tectonic theory helps explain firstly why volcanoes are always found along plate boundaries which are constructive (due to rising magma) and now also at destructive plate boundaries – due to plate melting.

Thus, plate tectonic theory explains why seismic and tectonic events occur at plate boundaries, what before any understanding, were perceived to be imaginary lines. However, there are some issues with this statement. Firstly, mountain building accompanied by seismic events can occur at plate boundaries instead of volcanic events; an example of this is along the Eurasian/Indo-Australian plate boundary. Here there are no volcanoes, but instead there are high mountain ranges such as the Himalayas. Two continental plates of the same density meet, leading to fold mountains being created whereby the two plates converge upwards as neither plate is denser than the opposing one. Pressure builds and eventually the plates fault upwards (Fracturing), adding to the creation of the mountains Explaining another way plates can be destroyed. Sudden faulting explains the seismic activity along this boundary, such as in Bam, Iran in 2003.

Another issue with the proposed distribution of seismic and volcanic activities is that intra-plate volcanoes do not correspond with the theory that volcanoes are found along plate boundaries. This is the case for the volcanoes of Hawaii and Yellowstone for instance. This does not weaken my proposed distribution however, as Tuzo Wilson came up for an explanation of this with his Hawaiian hot spot theory. He suggested hot spots were formed by magma plumes in the mantle which created melting of the crust at a particular point forming a volcano. The plume was stationary and the crust moved over it, creating a series of volcanoes called the Emperor sea mountain chain. As the crust moves, the plume would no longer build a volcano there and instead a relic feature would be left on the crust. Some of these old volcanoes have transformed into coral reeds after being eroded by the wind and sea until submerged in the sea. From this, we can conclude that intra-plate hot spots actually strengthen the theory of plate tectonics and plate movement.

A final issue with the distribution of seismic and volcanic events is that there is evidence of volcanoes away from plate boundaries. This is evident in the UK, such as at Arthur’s Seat in Edinburgh (extinct volcano) and the Whin Sill Dyke in England. However, similarly to Hawaiian hotspots, these also proved plate tectonic theory as well. They indicate temporal change of the position of plate boundaries that have moved away due to plate movement. Evidence from sea floor spreading, hot spots, subduction and convection currents in the earth’s movement driving movement have proven this case.

Plate tectonic theory has proved that there is a general correlation of the location of seismic and volcanic locations and their proximity to a plate boundary.  However this information is rather dubious as plate tectonic theory cannot tell us where along a plate boundary an earthquake will occur which are thousands of kilometers long. Moreover, while plate tectonic theory can help us understand the distribution of seismic events; this information can also be of little use as the effects of an earthquake can be felt far away. For example, after the 2011 Virginia Earthquake, the effects were felt as far North as Quebec, despite it being unusual for Quebec to feel any seismic activity due to it being away from any plate boundaries. The usefulness of plate tectonic theory is also limited due to some people’s perception of the causes of earthquakes and volcanic events.  Poorly educated people in LEDC’s may still believe tectonic events are from Gods, and so plate tectonic theory would be incomprehensible to these people who may not even be aware of plate tectonics.

To conclude, plate tectonic theory that has developed relatively recently in tectonic terms, has improved our understanding of the distribution of tectonic and seismic events. Through the understanding of how tectonic plates move, scientists have been able to assess the regular distribution of earthquakes and volcanoes found along these pate boundaries.