Plate Tectonics in a nutshell:

The structure and physical properties of the Earth are the key to understanding plate tectonics.

The Earth’s core is as hot as the surface of the Sun. This causes slow convection currents in the plastic mantle which result in slow movements in the plates of the Earth’s crust. This is the start of the tectonic cycle. The map of the world shows the main plates and the direction in which they are moving.

The convection currents circulating in the mantle may take millions of years to rise to the surface. If currents of hot, molten rock rise under the thin oceanic crust they form ‘hot spots’ of intense volcanic activity. The Hawaiian islands lie above one of these hot spots today. In Hawaii, magma rises to the crust surface forming ‘shield volcanoes’; the ‘runny’ basalt lava flows quickly and far, giving shield volcanoes gentle slopes.

Under the thicker continental crust, rising convection currents push the crust up into a dome, causing tension and cracking. As the crust is pulled apart, large slabs of rock sink and rift valleys form. Volcanoes appear where the magma escapes from cracks in the rift valley. This is typical of the rift valleys in East Africa.

The Red Sea and the Gulf of Aden illustrate the next stage of the tectonic cycle when the floor of the rift valley can widen to form a linear sea. This region will gradually form a new ocean, separating Africa from Arabia.
The Atlantic is an example of an expanding young ocean. A submerged ocean ridge has formed down the centre of the Atlantic from the Arctic to the Antarctic (Diagram 4). This mid-Atlantic ridge  follows the boundary between the American plate and the African and Eurasian plates. A rift valley lying along the centre of the ridge, slowly generates new oceanic floor. So the mid-Atlantic ridge is called a constructive plate margin.

As the Atlantic Ocean gradually widens, the Pacific Ocean is slowly closing. The Pacific is a much larger and older ocean than the Atlantic. Over millions and millions of years, thick layers of sediment have collected on the ocean floor. These deposits are particularly thick near continents where rivers have carried silt into the ocean. As the oceanic crust is thin, the weight of sediment makes it sag. More sediment collects, and eventually the crust breaks. As new crust is being created at the ocean ridge, the old oceanic crust, near to the continent, is pushed down or subducted.

The deep depression in the crust where subduction occurs is called an ocean trench. Subduction of the ocean crust and wet sediment into the mantle creates magma which rises up to the surface. Some escapes as thick, explosive lava, forming volcanic islands in arcs which fringe the trench. There are island arcs which extend from the Aleutian Trench in the north-west Pacific to the Tonga Trench in the south-west Pacific. Some islands form larger island groups like Japan. Ocean trenches are part of a destructive plate margin, because ocean floor disappears into them and is destroyed.

As the Atlantic grows, the American continents are moving westwards. Along the western edge of South America, the Nazca Plate is being pushed against the American Plate. Here the ocean crust is being forced under the advancing landmass, pushing up marine sediments and creating the Andes Mountains. In North America the story is different. In California, the North American Plate and the Pacific Plate are moving sideways past each other along the San Andreas Fault. No land is being formed or destroyed along this boundary which is called a conservative plate margin.

A tectonic cycle ends when two continental land masses converge and the ocean between them disappears. Layers of  oceanic sediment are squeezed into tight folds forming high mountains. This is what happened when India moved to collide with Asia. The ancient Tethys Ocean disappeared and the Himalayan mountains were formed. The crust here is so thick that volcanic activity has stopped, although earthquakes are common.