HKDSE Geography/M1/Plates

According to the plate tectonics theory, the lithosphere is cracked into many segments as they have been torn apart by internal forces constantly generated from the asthenosphere and exerted onto the lithosphere. Each piece is a tectonic plate or simply plate.

Plates have highly irregular shape, varying thickness and varying size. Plates where oceanic crust is dominant are called oceanic plates. Plates where continental crust is dominant are called continental plates. Some plates only carry oceanic crust, but not plates carry only continental crust.

Plate movement

(Unimportant background knowledge) 200 million years ago, all continents formed a huge landmass, a supercontinent called Panagaea. Convection currents in the asthenosphere caused Panagaea to split into different continents. They drifted apart, resulting in the present locations of the seven continents. This is called continental drift.

(This is important) The theory of plate tectonics was later developed. As the core of the earth is very hot, magma at the bottom of the mantle is heated. This magma has lower density, so it rises up to form a rising current. It is then cooled down and sinks to form a sinking current. It rises and falls repeatedly, forming convection currents. This is mantle convection.

Plates lie and float on the asthenosphere. Mantle convection causes plate movement. Plates move at several centimetres a year.

Plate boundaries

The narrow zone between two adjacent plates is called a plate boundary. Plate boundaries are classified into three types.

	Constructive plate boundaries (Divergent)	Destructive plate boundaries (Convergent)	Conservative plate boundaries
Convection currents which are	Rising/Diverging	Sinking/Converging	/
cause plates to	Diverge from each other	Converge/collide into each other	Slide past each other
and create	Tensional force	Compressional force	Shear/lateral force/stress

Slab pull, ridge push

At destructive plate boundaries, gravity pulls the denser plate towards the subduction zone. This is slab pull.

At constructive plate boundaries, magma wells up to new crust at the opening. It cools and solidifies, forming a ridge. The new crust pushes the two sides of the plate away from it. This is ridge push.