Describe and Explain Constructive and Destructive Plate Margins

Constructive Plate Margin * This is where two plates move apart be it any combination of crust type either sial or sima. At a sima and sima divergence such as the Eurasian Plate and the North American Plate, both crusts diverge leaving magma to rise up. This with aid from the cool ocean water solidifies and creates gentle sloping volcanos with basic basaltic lava with low viscosity. The type of volcanoes found at these margins are shield and fissure volcanoes. At this specific plate margin, at the point of diverging, a mid-Atlantic ridge occurs. This is where magma has risen through the gap and solidified hitting the cool water, forming in this case the longest mountain range in the entire world. Where magma fills the gap and solidifies, this is the newest crust of the ocean bed. Lateral transform faults occur at these plate boundaries. At these faults, nothing is created or destroyed in the lithosphere, but the plates transfer the strain by slipping/striking horizontally into the ridge created when separating apart. Also at this point, the submarine volcanoes form occasionally peaking above sea level. These volcanoes do erupt quite frequently in volcanicity terms, but eruptions are very weak. At a sial and sial divergence, it is exactly the same situation and theory behind the movement, both plates move away from each other. Here magma rises but the types of volcanoes differ here with more acidic lavas being apparent. At these continental divergences, massive rift valleys form. These valleys are formed when the lithosphere stretches, which causes fractures into sets of parallel faults. The land inbetween the faults collapses into deep wide valleys which are separated by upright blocks of land called horsts. The fallen blocks of lowland are called grabens.

Destructive Plate Margins * When an oceanic plate meets a continental plate, the collision is associated with the process of subduction. As the oceanic plate is denser than the lighter continental plate, it subducts beneath it. At the point of impact, a deep ocean trench is marked by the bending of the sea bed floor. Where the plates meet, the continental land is crumpled and pushed upwards, which causes the formation of fold mountains (for example the Andes). At around 100km beneath the continental plate, the oceanic will reach the Benioff Zone which is where the plate begins to melt. As the plate subducts deeper and deeper, the temperature rises and rises, and combining with friction causes the melting. Where there is tension in the plate further down, can be the cause of sudden earthquakes which release waves of seismic energy upwards. Once the plate melts, magma is created, which rises in huge plumes that can cause enormous volcanic eruptions on the surface. * Where an oceanic plate meets an oceanic plate, the faster or denser plate subducts beneath the other which again leads to the same process of oceanic trenching and melting. The magma plumes then rise forming cresents of submarine volcanoes along the plate boundary which eventually can lead to the formation of island arcs, which can be seen with the Marinas Islands. * Where a continental plate meets a continental plate, there plates are of lower density than the asthenosphere below them. Subduction does not occur at this meeting, but collisions are massive. Land lifts up and buckles creating high rise fold mountains. Volcanic activity does not occur either due to no subduction, but where thw plates meet, can result in huge eartquakes with a shallow focus.