Constructive and Destructive plate margins are the two more interesting boundaries to look at. These two boundaries are completely different in the processes that occur and the effects as well as the landforms that appear.
Constructive Plate Boundaries
Constructive plate margins are also known as diverging plate boundaries; this is because the two plates move away from each other in opposite directions. As the two plates move away from each other there is a gap between the two plates and this is filled with new rocks, which have risen up from the magma below. This movement is usually found under the sea and is the main reason for sea floor spreading and this is because the cracks which have been left from the diverging plates needs filling in and the up swell of magma does this and the sea floor is then larger than if the plates had not diverged away from each other. This area where this has taken place is now called the “Mid Atlantic Ridge”. This is found through the middle of the Atlantic Ocean and it separated the continental plates of North and South America from the African plate and the Eurasian plate. At the ridge new crust is being formed in the centre due to the up welling of magma. This is then moved to the side of the ridge. As the ridge reaches the surface, for example in Iceland, volcanic activity helps to form new crust to replace the crust, which has moved from the ridge. Along the mid Atlantic ridge this is also taking place in Madera and the Canaries.
The landforms that are created at constructive plate boundaries include new crust, ridges, ridge volcanoes and earthquakes. The new crust at these plates is formed by the up swell of magma through the gap, which has been made by the diverging boundaries. This creates new crust on the earth’s surface. Deep-sea ridges are formed in the gaps between the diverging plates. The ridge forms as the magma rises. Volcanoes form on the diverging boundaries as the intense pressure leads to an eruption of magma. Finally the earthquakes form as the plates move as pressure causes the plates to shake and move. The landforms found at destructive plate boundaries include trenches, volcanoes and earthquakes. Trenches in the sea floor are causes in the area where the plates are subducted. The trench does get deeper the further the subducted plate sinks. Earthquakes are formed under the earths crust and are known as subducted earthquakes. They do not usually cause much damage, as they are so deep within the earths crust. Their shock is absorbed within the mantel.
Destructive Plate Boundaries
A destructive plate boundary however does not create sea floor it destroys it. There are two types of destructive plate boundaries. One involves ocean-ocean plate boundaries and the other ocean-continent plate boundaries when two oceanic crusts meet at their boundaries one margin is subducted underneath the other. For this to happen the less dense plate is pushed down and under the more dense plate. At the point where this takes place it is known as the subduction zone and at this point the earths crust is destroyed. It is destroyed by the plates being sunk the plate underneath continues to sink below the other plate. However it is melted by the magma by the time it has sunk 300 kilometres and by the time the subducted plate reaches 700 kilometers it has completely broken up due to the intense pressure and the heat. When looking at this the crust at these margins is getting smaller as the crust is being destroyed and therefore leads people to think that the crust is getting small and so is the world. The constructive margins are also made up of oceanic and continental crust. This has the same effect as the previous oceanic-oceanic plate; the less dense oceanic crust sinks below and is subducted under the continental plate. The oceanic plate takes the same form of destruction and the plate sinks and melts and then finally is completely broken up. As the oceanic crust is subducted a very deep trench is formed and this is a very steep hole in the sea floor. Some land forms, which are well known at destructive plate boundaries include, deep-sea trenches, volcanic arches and overhanging plates. An example of a deep-sea trench is the Marinas Trench, which has been left due to two destructive plates meeting. The Andean Island is an example of a volcanic arc and island arcs can be found in Japan.
Therefore we can see that there are two types of destructive margins and these are oceanic-oceanic and oceanic-continental. They occur as the two plates move towards each other and the less dense plate is subducted under the other. The subducted plate is broken up and melted below the surface. A constructive plate margin is opposite of this and the two plates involved are moving away from each other. This causes new material to rise in the gap made. As we can see from this the two margins mentioned here are opposites of each other but are both the result of plates being moved on the underlying magma below.
As mentioned before the constructive plate margins are the cause of sea floor spreading and the destructive plate margins are destroying the earths crust. There is a balance within the earths crust and this means that the rate of sea floor spreading is equal to the amount of crust being destroyed and therefore the earths curst and size is staying the same and not getting any bigger or any smaller.