Evaluate how plate tectonics theory helps our understanding of the distribution of seismic and volcanic events

Evaluate how plate tectonics theory helps our understanding of the distribution of seismic and volcanic events.

Plate tectonics is a relatively new science. It’s partially explained in the sense that we still don’t know fully about how the plates came together, what they were before they were the seven continents we know today. We already know, or have a theory, about what our world was like a long time ago and so if we already know that the plates can pull and push against each other, then surely there must have been a different set up to the one we have now. There is a theory that we were once a supercontinent called Pangaea that began to break up around 200 million years ago. Using plate tectonics, we can explain, predict and see which type of seismic and volcanic events are related to plate tectonics, using more theories like Pangaea to explain them.
Covering our earth’s surface there are seven major plates. These plates cover the inside structure of our earth, which consists of our inner core, our outer core, the mantle and then the crust which is what the plates are made out of. Plate tectonics is the theory of these plates moving along the earth’s lithosphere. The lithosphere is in the upper mantle and the lower crust and is responsible for continental drift. This is also because of the convection current running underneath and pulling and pushing the plates apart and together, causing them to converge and destruct. There are two types of plates, oceanic and continental. Both of these plates are completely different and when faced with each other, both have different reactions depending on the type of plate margin.
Oceanic crust is younger, heavier and denser. It is made from Sima and subducts when colliding with a continental crust. Continental crust however is older but lighter, less dense and is made from Sial. This crust folds when met with oceanic or itself, creating fold mountains like the Rockies in the US or the Himalayas. There are three types of plate boundaries, Constructive, Destructive or Conservative. Constructive is when the two plates are pulling away from each other, an example would be sea floor spreading which is backed up by the mid-Atlantic ridge which is spreading every day and has a line of volcanoes along the middle of the ridge, creating new crust each time it spread further apart. As these two plates spread apart, new magma rises up to create new oceanic crust and basaltic rock.
Iceland is an amazing example of a constructive plate margin as Iceland is made from a pocket of magma, an island made from the magma rising over the mid-Atlantic ridge. How do we know this for certain? The pocket of magma that sits beneath Iceland is thought to be what created the island, as hot lava rose to the surface of the ocean, where it cooled and gradually accumulated into an island beginning about 70 million years ago. Also, if we use Wegener’s theory of the jigsaw puzzle, explaining how our continents used to be one, Pangaea, then it’s plain to see that Iceland can fit nowhere in that formation thus proving its nonexistence until relatively recently.
The destructive plate margin is when the plates collide with each other. Now, this can happen in three different ways. The first is when Oceanic and Continental push into each other and collide. When this happens, the oceanic plate sinks, or subducts back into the magma and it reheated and melted back into molten rock. As the lighter and larger plate, the continental folds on itself, causing Fold Mountains but also volcanoes. There are two types of volcanoes, shield and an acid dome and this plate margin causes acid dome volcanoes which are the most explosive and violent. As the oceanic plate subducts underneath the continental, it causes a judder and this is where the plates are slowly rubbing against each other, little cracks and faults causing the plates to stick and jolt. This is called the Benioff point and is where we get earthquakes due to this problem. Also we can get deep sea trenches here like the one in Chile/Peru because of the subduction of the plate.
The second is where two Oceanic plates meet and collide into eachother. The plate with the more power is the plate that will subduct under the other and this is how island arcs are made. An island arc is heavily volcanic, acid dome volcanoes and also prone to earthquakes because of the constant pushing of the subducting plate and the process of the juddering as explained before and the Benioff zone is put into place here as well. An example of an island arc would be Japan or places like Thailand. The third type of destructive plate margin is when two continental crusts meet and collide. This simply creates Fold Mountains on the earth’s crust but also below the crust as well. There is no volcanoes created here and only earthquakes which made from this, an example being the Alps.
The third and final type of plate boundary is the conservative plate margin which is simply the two plates, any two plates rubbing together. This doesn’t seem to be so chaotic but it is. When I talked earlier about the plates subducting and rubbing together, this is that process. The plates build up pressure as the force themselves together and it can be either way. They can even be moving in the same direction but it will still cause the same effect as they are probably moving at different speeds to each other. This means that the pressure will build and they will jolt, causing an earthquake. This can be found at the San Andreas Fault and at this margin, land is neither created nor destroyed, just the two plates moving sideways past each other, horizontally.
There is argument that plate tectonics is such a new science that it doesn’t help us understand the whole distribution of seismic and volcanic eruptions. For example, hot spots are not at a plate boundary/margin but they create a volcano. A hotspot is a plume of magma which is underneath the crust as it bursts up through the plate and creates a shield volcano. A shield volcano is a low lying volcano which isn’t as violent as an acid dome. It produces a nonexplosive lava and unlike an acid dome, a shield volcano is used more for tourism and such. As the plate moves, the plume stays unmoving and so this creates more rock that makes an island. Hawaii is a good example of this, where it has a long structure because of the moving plate and the stationary plume. However, even though this is a developing science, it has given us more insight predicting the future of volcanic and seismic events.
As a developing science, its making more theories, exploring more ideas and testing them as well as the older ones. Wegener first began to make his theory about continental drift by looking at the continents and seeing jigsaw puzzle. He presented his theory in 1912 and look how far we have come in that time. Although he didn’t have any empirical evidence for his theory, it made him do more research and soon he found that the rock types on both the South American boarder by Brazil and the west African boarder were both the similar and also by finding fossils on both sides from the same reptile, especially with fossil plants too. This suggests that we were once a large continent and animals alike would roam from what we would separate as continents now. The geology matched up and soon also the mountain ranges did too as the Appalachian Mountains join to those in Europe.
This theory is still developing new ideas and these ideas can be put forward to explain new formations. Like, for example, we may be experiencing a new ocean in east Africa as it is an active continental rift zone, the plates pulling apart.
Even though continental drift and the theory of plates is widely accepted in this day and age, it doesn’t answer the question for all seismic and volcanic activity. You can get earthquakes a long way away from plate boundaries, actually still associated with them. So, in India, where you're creating the Himalayas and you’ve got a huge amount of compression, you can still get earthquakes hundreds of miles away from that plate boundary just because the compression and the crushing there is so immense. When an earthquake happens maybe at a plate boundary, the shockwaves can travel through the earths mantle and hit another country like the UK or New Zeland and these are some of the examples of places where we can get small earthquakes that have simply been sent through shockwaves.
Not only can we get earthquakes through no plate boundary activity but also just from human activity. In May 2008, in China, a deadly earthquake struck which registered 7.9 and left thousands of people dead. In the aftermath of the disaster, attention quickly turned to a nearby dam, just 3.4 miles from the epicenter of the earthquake. Many scientists believe the Zipingpu Dam reservoir may have helped trigger the massive quake.­ Less often, earthquakes are caused by natural occurrences like volcanic eruptions. But vibrations felt at the surface of the Earth can also result from Earth-shaking, man-made events like underground bomb testing and mine collapses -- and the filling and emptying of dam reservoirs.
If even these things can cause earthquakes, then its obvious that even if we understand it, plate tectonics cannot help us understand all seismic and volcanic activity. Take volcanic for example, Lahars which are not caused by plate tectonics. They are caused by snow melt from the glaciation on top of mountains, flooding and if Lava mixes with wet mud and soil. Lahars have the consistency, viscosity and approximate density of wet concrete: fluid when moving, solid at rest. An example of an Lahar could be In 1985, the volcano Nevado del Ruiz erupted in central Colombia. The lahars picked up speed in gullies and coursed into the six major rivers at the base of the volcano; they engulfed the town of Armero, killing more than 20,000 of its almost 29,000 inhabitants. Casualties in other towns, particularly Chinchiná, brought the overall death toll to 23,000.
These exceptions just go to show that not all activity on both parts come from plate boundaries and so even though it can tell us how and when these natural scenarios will unfold, how they are made and where they will be made, it cannot completely give us a heads up of all natural disasters. However, the fact that we can plan ahead, take into consideration that a volcano will be made there or an earthquake might happen somewhere else could help us prepare. Take California which is one a plate margin that is a passive plate boundary. This margin is doing something called ‘unzipping’ in which the plates are moving against each other but they haven’t yet jolted or fully ‘unzipped’ which means they will soon. As they understand this, America are now trying to pump water and oil into the fault line, lubricating it so that the jolt wont hit so hard and the earthquake wont be so destroying. This is what makes the understanding we have of plate tectonics so rewarding even if not all activity can be explained by it, the foundations of what we know are from this theory.

By Grace Dominy.