Plate tectonics explained
Geography · Key Stage 3 · The School
What moves the plates
Heat from the Earth's interior drives slow convection in the mantle, and the plates ride on it. Two other forces matter: RIDGE PUSH, where new crust at a mid-ocean ridge slides away downhill, and SLAB PULL, where a dense sinking plate drags the rest behind it. Slab pull is now thought to be the strongest of the three.
Three kinds of boundary
CONSTRUCTIVE: plates move apart, magma rises, new crust forms — Iceland and the Mid-Atlantic Ridge. DESTRUCTIVE: plates move together, the denser oceanic plate subducts, producing deep trenches, violent volcanoes and major earthquakes — Japan, the Andes. CONSERVATIVE: plates slide past each other, no new crust and no volcano, but severe earthquakes — the San Andreas Fault.
Reading a boundary
Given a boundary type you can predict the landforms and the hazards, which is exactly what the exam asks. Constructive: shield volcanoes, gentle eruptions, small quakes. Destructive: composite volcanoes, explosive eruptions, deep-focus quakes, fold mountains. Conservative: earthquakes only.
DESTRUCTIVE (Nazca under South America): one plate sinks, melts, and feeds explosive volcanoes — the Andes, plus deep, powerful earthquakes. CONSTRUCTIVE (Mid-Atlantic Ridge): plates part, magma rises, giving runny lava and shallow, mild quakes — Iceland. CONSERVATIVE (San Andreas): plates slide past, no volcanoes at all, but stress builds and releases as major earthquakes. Name the boundary and you have predicted the hazard.
Magnitude does not decide the toll. Two earthquakes of similar size can kill two hundred people or two hundred thousand, and the difference is human: building standards and whether they are enforced, the time of day, how deep the focus was, how far help has to travel, and how much money there is to prepare. The HAZARD is physical and the DISASTER is not. That distinction is most of the marks in a question about impacts.