Explain how plate tectonics help us understand the forces at work along plate boundaries

The main forces at work along plate boundaries are collision and separation. Plate tectonics tells us that the earth’s crust is split into sections called plates. The heat from the earth’s core generates slow moving convection current in the mantle. The convection currents drag the plates along, causing them to collide and separate.

Collision happens in three different situations; oceanic and continental collision, oceanic and oceanic collision, and continental and continental collision. At oceanic-continental collision, the heavier oceanic plate slides down into the mantle in a process known as subduction. As the oceanic plate gets subducted it is melted in the mantle. This causes the formation of explosive volcanoes on the land, e.g. around the Andes in Chile. The Japanese earthquake and tsunami in 2011 was caused by the collision of the Pacific plate and the North American plate. The oceanic plate was subducted under the continental one, producing an 8.9 magnitude earthquake. This resulted in a huge tsunami and many aftershocks in mainland Japan.

At oceanic-oceanic collision, subduction also occurs and this leads to the formation of volcanic island arcs, e.g. Japan and the Philippines. In Japan, the collision of the Pacific plate and the Philippines plate causes subduction and volcanic mountains, e.g. Mt Fuji and Mt Pinatubo. In the case of continental-continental collision, subduction does not happen as they are less dense than the mantle below. The continental plates will both be destroyed by uplifting to form fold mountains, e.g. the Himalayas. Shallow earthquakes may happen at these boundaries, e.g. the Pakistan earthquake in 2008.

Divergent plate boundaries are places where new crust is formed. This occurs at mid-ocean ridges, e.g. the Mid-Atlantic Ridge where the Eurasian plate is separating from the North American plate. At these boundaries, the convection currents in the mantle drag the plates apart. This splits the crust and allows magma to rise to the surface, where it creates a new ocean floor. This lava is cooled to form the igneous rock known as basalt. The island of Iceland is made of basalt and exists as a result of seafloor spreading. In other parts of the Mid-Atlantic Ridge large underwater volcanoes are found. These volcanoes are less explosive than those found at subduction zones as they contain less silica and thus the lava is less runny.

Transform plate boundaries are those that do not fall into the divergent or convergent category. Instead, the plates move side by side. In some cases, one plate moves slower than the other creating friction and occasionally earthquakes, e.g. San Andreas Fault in California. In other cases, the plates are moving side by side but in opposite directions. A transverse plate boundary, where the North American and Caribbean plates are moving in opposite directions, caused a strong earthquake in Haiti in 2010.