Fig. 5.1 shows a cross-section through the Earth at a destructive plate boundary. The Earth's lithosphere is divided into large tectonic plates that move sl...

(a) The type of plate boundary shown in Fig. 5.1 is a destructive (convergent) plate boundary. [1]

At a destructive boundary, two tectonic plates move towards each other. This contrasts with a constructive boundary (plates move apart) and a conservative boundary (plates slide past each other).

(b) What happens at a destructive plate boundary: [4]

1. Two tectonic plates move towards each other, driven by convection currents in the mantle beneath them. [1]
2. Where an oceanic plate meets a continental plate, the denser oceanic plate is forced beneath the less dense continental plate in a process called subduction. Oceanic crust is denser because it is made of basalt, while continental crust is made of lighter granite. [1]
3. The oceanic plate descends into the mantle at an angle, creating a region called the subduction zone. As it descends, increasing pressure and temperature begin to alter and eventually melt the plate material. [1]
4. At the surface, a deep ocean trench forms at the point where the oceanic plate bends downward beneath the continental plate. On the overriding continental plate, intense compression causes the rock to buckle and fold upward, forming fold mountains. [1]

(c) How volcanic eruptions occur at destructive plate boundaries: [4]

1. As the oceanic plate is subducted deeper into the mantle, it reaches zones where temperatures exceed 1000 degrees C and pressures are enormous. [1]
2. Friction between the two plates generates additional heat, and the high temperature causes the descending oceanic plate and overlying mantle material to melt, forming a body of molten rock called magma. Water trapped in the oceanic plate lowers the melting point of the surrounding rock, aiding this melting process. [1]
3. The magma is less dense than the solid rock surrounding it, so buoyancy forces cause it to rise through cracks, faults and zones of weakness in the overlying continental plate. [1]
4. When the rising magma reaches the Earth's surface, it erupts as lava, along with volcanic ash, rock fragments and gases (water vapour, CO₂, SO₂). Over successive eruptions, this material builds up to form a composite volcano with steep sides. [1]