Geography

Geography – Plate Boundaries

Question: “Plate boundaries are zones where crust is both created and destroyed”. Examine the above statement, with reference to examples you have studied.

Answer:

The study of plate tectonics shows us how crust is both created and destroyed at plate boundaries. The earth consists of seven large plates and several smaller plates. They float on thermal convection currents in the mantle which allow them to move. The earth’s core has a temperature of over 4,000 degrees C. The core is situated in the centre of the earth and is made up of solid/semi-molten nickle and iron. The mantle lies just above the core. It is extremely hot there which forces a convection current of molten magma to rise slowly upwards. This current cools down as it reaches the asthenosphere. It then moves in a sideways direction ans when it has cooled some more, it sinks back to the bottom to be reheated. This circular motion constantly repeats itself and this causes the plates to move

There are three types of plate boundaries: constructive, destructive and passive boundaries.

At constructive, or divergent boundaries, land is being created. The magma rises from the mantle towards the surface and pushes the plates apart. When the magma reaches the ocean floor, it is cooled by sea water to form new oceanic crust. The crust builds up to form underwater mountain ranges. These sometimes rise above the water, creating volcanic islands. This process is known as sea-floor spreading, a theory proposed by geologist Harry Hess in the 1960s, which suggests that the ocean floors are widening and new rock is being formed where the plates originally separated. This is happening where the American Plate moves apart from the Eurasian and African Plates, forming the Mid-Atlantic ridge, where volcanic islands such as Iceland are situated.

At destructive, or convergent boundaries, plates collide and land is either changed or destroyed. There are three types of plate convergences: oceanic-oceanic convergence, oceanic-continental convergence and continental-continental convergence.

At oceanic-oceanic convergence, the heavier plate is subducted under the younger, lighter rock and a long, deep, narrow ocean trench is formed. For example, the collision of the Pacific and Marinas plates forms the Marinas Trench near Japan.

The boundary of two colliding continental plates results in the formation of fold mountain ranges. This includes the Himalayas, where the Indian and Asian plates are colliding.

At oceanic-continental convergence, heavy oceanic crust is pushed into the mantle by lighter continental crust and fold mountain ranges are often formed here. For example, the Andes were formed when the Nazca Plate was pushed into the earth by the South American Plate. Fold mountains, volcanoes and earthquakes are commonly found at destructive plate boundaries.

At passive plate boundaries, land is neither created or destroyed. One plate slides past another horizontally. The line along which they slide is called a fault line. The plates can either slide past each other other in the same direction at different speeds. Earthquakes occur violently in these zones because it is so close to the surface of the earth’s crust. The San Andreas Fault in California is a result of the Pacific plate moving north eastwards 6 times that of the North American plate.

*Include a labelled diagram of each type of plate boundary.

Grade: 26-30 / 30