The structure of the Earth and the three plate boundaries; the processes of slab pull and ridge push that move plates; and the contrasting primary and secondary impacts of, and responses to, a tectonic event in an AC and an LIDC.

What this dot point is asking

This is OCR GCSE Geography B (J384) Component 1, Our Natural World, within Global Hazards: "What are the causes and impacts of tectonic activity?" OCR expects you to describe the structure of the Earth, explain the three plate boundaries, explain the processes that move plates (slab pull and ridge push, with convection now de-emphasised in the specification), and compare the primary and secondary impacts and responses of a tectonic event in an AC (Advanced Country) with one in an LIDC (Lower Income Developing Country). The development contrast is the heart of this topic.

The structure of the Earth and plate boundaries

The Earth has four layers: a thin, solid crust (oceanic crust is thin and dense; continental crust is thicker and less dense), the semi-molten mantle beneath it, and a liquid outer core and solid inner core of iron and nickel. The crust is cracked into tectonic plates that move slowly over the mantle. Most hazards occur at the boundaries where plates meet.

What moves the plates

The specification stresses two main mechanisms.

• Slab pull. At destructive boundaries, the cold, dense oceanic plate sinks into the mantle under its own weight. As it sinks it drags the rest of the plate behind it. This is now regarded as the main driver of plate movement.
• Ridge push. At constructive boundaries, new crust forms hot and high at the ridge. As it cools and moves away it becomes denser and slides down the slope of the mantle, pushing the plate ahead of it.

Older textbooks describe convection currents in the mantle dragging the plates; the OCR specification now treats slab pull and ridge push as the key processes, so name them in your answers.

Contrasting impacts: AC versus LIDC

OCR builds this topic around a comparison between two levels of development.

• Primary impacts are immediate and direct: ground shaking collapses buildings and infrastructure, killing and injuring people.
• Secondary impacts follow: fires from broken gas pipes, tsunamis (if the event is undersea), landslides, disease from broken water and sewage systems, homelessness and economic loss.
• Immediate responses are rescue, medical aid, shelter and emergency aid; long-term responses are rebuilding, repairing services and improving preparedness.

When you write the comparison, pair a named AC event (where preparation limited deaths) with a named LIDC event (where poverty raised them), and use figures from both.

Try this

Q1. Describe the structure of the Earth. [3 marks]

• Cue. A thin solid crust, a semi-molten mantle, a liquid outer core and a solid inner core.

Q2. Suggest why an earthquake of the same magnitude causes fewer deaths in an AC than in an LIDC. [4 marks]

• Cue. Building codes and earthquake-resistant design, early-warning systems, and well-funded emergency services in the AC, against weak buildings and limited services in the LIDC.